GB2359095A

GB2359095A - Stainless steel

Info

Publication number: GB2359095A
Application number: GB0003163A
Authority: GB
Inventors: Rattan Jindal
Original assignee: Jindal Strips Ltd
Current assignee: Jindal Strips Ltd
Priority date: 2000-02-14
Filing date: 2000-02-14
Publication date: 2001-08-15
Also published as: GB0003163D0

Abstract

A stainless steel comprises (in % by weight) C 0.03-0.08, Mn 7.00-10.25, Si 0.10-0.75, Cr 14.25-16.50, Ni 2.25-4.75, Cu 0.90-2.00, N 0.01-0.20, Fe 65.37-75.44 and impurities 0.02-0.10 (e.g. P 0.01-0.07 and S 0.01-0.03). The steel is made by melting a charge of ferro-chromium, ferro-nickel, ferro-manganese, ferro-silicon, copper and iron into an electric arc furnace and then refining the melt in an argon-oxygen decarburisation converter by injecting a mixture of oxygen, argon and nitrogen. The steel is suitable for making consumable goods such as cutlery, saucepans and other culinary accessories.

Description

IMPROVED STAINLESS STEEL ALLOYS AND PROCESS FOR THE PREPARATION THEREOF The present invention relates to in general to novel alloys. More particularly, the invention relates to improved grades of stainless steel alloys and to a process for the preparation thereof. Still more particularly, the invention relates to such improved stainless steel in the form of cold-rolled sheets having specific application to the fabrication of consumer durables such as cutlery, milk pails, containers and culinary accessories and utensils including saucepans, skillets, woks, stirrers, spatulas, cooking spoons, ladles, measuring spoons and the like. Conventionally, steel including stainless steel is classified in grades depending on the end purpose for which the steel is intended to be put. These grades are determined according to standards laid down by the American Iron & Steel Institute (AISI) which is the universally recognised authority for this purpose. Each grade identifies the percentage ranges within which the various metallic and non-metallic elements of the alloy constituting that particular grade may be present. These elements include carbon, manganese, silicon, chromium, nickel, molybdenum, nitrogen, phosphorus and sulphur, with the last two being present as unavoidable impurities. The field of application of stainless steel is so vast as to be virtually limitless. For instance, in heavy industrial areas, stainless steel finds application in architecture, chemical processing apparatus, pharmaceutical production equipment, petrochemical refinery vessels and reactors, the transportation industry, environmental safeguards and environmental protection equipment and pulp and paper processing apparatus. In lighter industry, stainless steel is invaluable in the domestic range of white goods, for example washing machines, refrigerators, deep freezers, cooking ranges, food processors, kitchen appliances and so on. Nevertheless, all these industrial and hi-tech uses apart, stainless steel nowadays finds one very special application in the domestic area. This is its application to the production of consumer durables. such as cutlery, tableware, food containers, and utensils, particularly but not exclusively, utensils for culinary purposes. AISI - 304 stainless steel presented a major disadvantage of expense because of its high nickel content. This unwelcome increase in the cost of the raw material for the alloy in turn reflected itself in an increase in the cost of the ultimate steel product. In the circumstances, it has since that time been an ongoing effort with steel manufacturers to develop a stainless steel alloy which cuts down on the expense factor while not affecting adversely the quality of the stainless steel in so far as its application towards to the fabrication of utensils and consumer durables is concerned. Such approaches have been oriented towards the replacement, in part, of the quantum of imported nickel in conventional stainless steel alloys by one or more alternative alloying components.

A fundamental object of the present invention, therefore, is to provide an improved stainless steel alloy having specific application to the fabrication of utensils and consumer durables which is substantially less expensive to produce than hitherto known conventional stainless steel, for example that identified as grade AISI - 304.

A further object of the invention is the provision of a less expensive stainless steel alloy which nevertheless evinces mechanical properties equivalent to or better than conventional stainless steel alloys such as the AISI-304 alloy.

Yet another object of the invention resides in a process for the preparation of such a less expensive stainless steel alloy possessing improved or at least equivalent mechanical properties. The research carried out towards achieving the objects of the present invention has been directed to the production not only of a stainless steel alloy having a substantially reduced content of the nickel and, to a lesser extent, of chromium as compared with conventional stainless steel such as grade AISI-304 but also to a versatile spectrum of alloys applicable to the manufacture of specific steel utensils.

In effect, the invention lies in the partial substitution of the expensive nickel content of the known alloy by three elements, viz., manganese, copper and nitrogen, thereby bringing about a significant reduction in production costs. It has In respect of stainless steel suitable for domestic consumer durable production, two metallic elements in particular play an extremely significant role. The first of these elements is chromium the presence of which is essential in order to impart the stainless character to the steel. The second is nickel which controls formability of the steel. The greater the percentage of nickel present, the greater the formability of the alloy.

Where a shallow article such as a knife or spoon or ladle is drawn from an alloy melt, a high degree of formability of the alloy is not a criterion. The requirement of formability increases with the depth of the utensil that is to be drawn. Thus, a medium depth utensil such as a saucepan or a frying pan will require the steel to display a moderate degree of formability, greater than that for a knife or ladle. However, where deep vessels such as milk pails, buckets or woks are produced, the stainless steel must possess the greatest possible degree of formability. In fact, the greater the -depth of the utensil being drawn, the more formability the steel is required to evince and this requirement in turn translates itself into a greater percentage of nickel needed for the steel.

Nickel is an item which, as far as India is concerned, is almost entirely imported from abroad. Until the late 1980's, imported nickel was not terribly expensive. Hence, manufacturers could afford to employ the AISI - 304 alloy as a standard for the manufacture of all types of consumer durables and utensils irrespective of whether their production involved shallow drawing, medium drawing or deep drawing of the steel. Cost, till that time, was not a deterrent.

Then without warning, the worldwide cost of nickel per metric ton went up by several hundred percent. Suddenly, the employment of the conventional

AISI - 304 Grade Stainless Steel - Mechanical Properties Tensile strength, Mpa (minimum) 515 Yield strength, Mpa (minimum) 205 Percentage elongation in 50 mm gauge length (minimum) 40 Maximum hardness: Brinnel Hardness No. 201 Rockwell B (Rb): 92

Carbon : 0.03% to 0.08 Manganese : 7.00 % to 8.00 Silicon 0.10% to 0.75 Chromium : 15.50 % to 16.50 Nickel 4.25% to 4.75 Copper : 0.90% to 1.10 Nitrogen : 0.01 % to 0.20 Impurities 0.02% to 0.10 Iron 72.19% to 68.52%. According to a still further embodiment, the present invention further provides an improved stainless steel alloy particularly suitable for the formation of cold-rolled sheets employed in the fabrication of shallow to medium depth utensils and other consumer durables which comprises on a percentage by weight basis the following composition:

Carbon : 0.03 % to 0.08 Manganese : 8.00% to 0.25 Silicon : 0.10% to 0.75 Chromium 14.25% to 15.25 Nickel : 2.25% to 2.75 Copper : 1.60% to 2.00 Nitrogen 0.01 % to 0.20 Impurities : 0.02% to 0.10 Iron 73.74% to 68.62%. The impurities present in the alloy of the present invention are essentially phosphorus and sulphur which are present therein in the following amounts by weight:

Phosphorus : 0.01 % to 0.07 Sulphur 0.01 % to 0.03 % The present invention also provides a process for the production of an improved stainless steel alloy particularly suitable for the production of consumer durables been found that by dramatically increasing the content of manganese in the conventional alloy, there is imparted to resulting stainless steel an austenitic non magnetic structure. Thereafter, the presence of nitrogen, which in conventional stainless steel alloys is generally considered an undesirable impurity, is found to display an affinity for manganese whereby the solubility of nitrogen within the alloy melt is improved. The dissolved nitrogen, which is present as an interstitial element in the alloy, acts as an austenitic stabiliser to assist in retaining the non magnetic austenitic structure of the alloy.

In addition, the preferred form in which stainless steel is usually employed for the production of domestic utensils is as cold-rolled sheets. In this connection, it has been found that the introduction into the alloy of copper as a replacement for part of the nickel and chromium content helps increase formability of the alloy sheet. With the above-mentioned objects in mind and based on the findings stated herein, the present invention in its broadest novelty provides an improved stainless steel alloy particularly suitable for the production of consumer durables such as cutlery, milk pails, containers, culinary accessories and utensils which comprises on a percentage by weight basis the following composition:

Carbon : 0.03% to 0.08 Manganese : 7.00 % to 10.25 Silicon : 0.10% to 0.75 Chromium : 14.25 % to 16.50 Nickel : 2.25% to 4.75 Copper : 0.90% to. 2.00 Nitrogen : 0.01 % to 0.20 Impurities 0.02% to 0.10 Iron : 75.44% to 65.37%. According to a preferred embodiment, the present invention further provides an improved stainless steel alloy having enhanced formability particularly suitable for the production of deep drawn utensils and other consumer durables which comprises on a percentage by weight basis the following composition: manganese. The latter improves the solubility of nitrogen within the alloy melt whereby the dissolved nitrogen is located as an interstitial element in the resulting alloy and acts as an austenitic stabiliser.

The impurities present in the molten charge include sulphur, phosphorus, hydrogen and oxygen. Strictly speaking, the presence of carbon in excess of. maximum tolerable level would amount to an impurity also. However, a certain amount of carbon in the alloy melt is desirable in order to improve the strength of the final stainless steel product.

Upon injection of the gaseous mixture, carbon and hydrogen present in the melt are substantially converted to carbon monoxide and water vapour respectively with the carbon monoxide so produced being in turn converted to carbon dioxide. The water vapour and carbon dioxide are allowed to escape into the atmosphere along with any unconverted elemental hydrogen. Sulphur and other undesirable elements are oxidised to form the slag which is then removed. From a practical point of view, of course, it is not possible to remove these impurities totally but it is sufficient if they are at least reduced to insignificant levels which will not adversely affect the resulting stainless steel alloy. Thus, tolerable levels by weight of phosphorus and sulphur in the alloy are:

Phosphorus 0.01 % to 0.07 Sulphur : 0.01 % to 0.03 %. Conveniently, the charge is fed to an electric arc furnace where it is heated to molten state by striking an arc between the graphite electrodes of the furnace and the charge itself. The temperature of the furnace is raised to about 1550 C. in order to render the charged raw material completely molten. Thereafter, refining the molten charge to remove undesirable impurities is conveniently carried out in an argon-oxygen decarburisation convertor.

On extraction of the refined molten steel alloy from the decarburisation convertor, the alloy may be cast it into any preferred physical form by conventional casting methods. The physical form which the cast alloy can take includes slabs, bars, blooms, coils or sheets.

The present invention will now be described in greater detail by means of the following non-limitative Examples: such as cutlery, milk pails, containers, culinary accessories and utensils which comprises subjecting a charge composed on a percentage by weight basis of-

Ferro-chromium alloy : 23.75 % to 27.50 Ferro-nickel alloy 8.0 % to 17.0 Ferromanganese alloy : 10.0 % to 14.6 Ferro-silicon alloy : 0.15 % to 1.0 Copper : 0.9 % to 2.0 Iron 57.2 % to 37.9 to heat at a temperature of approximately 1500 C. until the charge is molten, injecting into the molten charge a gaseous mixture which reacts exothermically with metallic impurities in the melt to convert them into slag and with the non metallic impurities to convert them substantially into gaseous compounds which are allowed to escape, separating the slag thus formed from the residual molten metal and recovering the thus refined alloy having on a percentage by weight basis the following composition:

Carbon 0.03 % to 0.08 Manganese 7.00% to 10.25 Silicon : 0.10% to 0.75 Chromium : 14.25% to 16.50 Nickel : 2.25% to 4.75 Copper : 0.90 % to 2.00 Nitrogen : 0.01 % to 0.20% Impurities . : 0.02% to 0.10 Iron 75.44% to 65.37%. According to a preferred feature, the raw materials constituting said charge comprise ferro-chromium alloy containing at least 60 % chromium, ferro-nickel alloy containing at least 28 % nickel, ferromanganese alloy containing at least 70 manganese, ferro-silicon alloy having a content of at least 70 % silicon, metallic copper and iron in the form of ferrous scrap.

The preferred gaseous mixture injected into the molten charge comprises a mixture of oxygen, argon and nitrogen. The employment of nitrogen as part of the gaseous mixture has an advantage in that nitrogen displays an affinity for necessary to remove them from the hot metal or at least reduce them to insignificant levels which will not adversely affect the ultimate stainless steel alloy. In order to effect the removal from the molten charge, the. latter has to be subjected to a subsequent step of refining. This is done by tapping the liquid hot metal out from the electric arc furnace into a refractory lined ladle from where it is transferred to an argon-oxygen decarburisation convertor for refining it. In fact, such refinement achieves the dual purpose of removing the impurities present in the molten mass [or at least reducing them to tolerable limits] and of retaining the metallic elemental content of the hot metal within the predetermined ranges for the desired alloy.

A gaseous mixture of oxygen, argon and nitrogen is blown into the transferred hot metal within the convertor. The oxygen present in the gaseous mixture reacts exothermically with the carbon present in the molten mass to produce carbon monoxide and at the same time generate a high degree of thermal energy. This exothermic reaction brings about an agitation of the contents of the hot metal which is subjected to a stirring action. The other impurities present, such as sulphur and phosphorus, on coming into contact with the oxygen of the injected gaseous mixture are simultaneously oxidised. The oxidised compounds form a slag which rises upward and floats on the surface of the molten metal. Any hydrogen present in elemental form is vented as a gas or is likewise oxidised and escapes as water vapour or steam.

The slag formed as a result of the oxidation mentioned is then removed and the refined hot metal consisting essentially of the molten alloy in its desired composition is transferred from the decarburisation convertor to a further refractory lined vessel from where its contents are cast into any preferred physical forms of slabs, bars, blooms, coils or sheets by conventional casting methods. EXAMPLE 2 Preparation of Improved Grade JS -203 Stainless Steel Alloy An alternative embodiment of the alloy of the present invention having a composition particularly suitable for the formation of cold-rolled sheets employed EXAMPLE 1 Preparation of Improved Grade JS - 201 Stainless Steel Alloy A preferred embodiment of the alloy of the present invention having a composition which evinces enhanced formability and is particularly adaptable for the production of deep drawn utensils and other consumer durables has been identified as stainless steel grade JS - 201, the "JS" standing for uJindal Strips". For the production of one metric ton of stainless steel alloy JS - 201, raw materials comprising ferro-chromium alloy containing at least 60 % chromium, ferro-nickel alloy containing at least 28 % nickel, ferromanganese alloy containing at least 70 % manganese, ferro-silicon alloy having a content of at least 70 % silicon, metallic copper and iron in the form of ferrous scrap are charged to an electric arc furnace in the following amounts:

Ferro-chromium alloy : 260 kg. Ferro-nickel alloy 125 kg. Ferromanganese alloy 120 kg. Ferro-silicon alloy 50 kg: Copper : 11 kg. Ferrous scrap : 484 kg. The raw material thus charged is then heated by electrical energy generated by the striking of an arc between the graphite electrodes of the furnace and the charge itself. The temperature of the furnace is increased to approximately 1550 C. at which stage, the solid charge is completely melted down to a homogeneous liquid known as "hot metal". Understandably, the hot metal thus obtained contains, in addition to its metallic content, a number of other substances naturally occurring within the ferrous scrap and ferrous alloys employed as raw material. These substances include carbon, sulphur, phosphorus, hydrogen and oxygen and constitute impurities from the point of view of the final alloy product. The presence of such impurities is thus undesirable. Accordingly, it becomes

gauge length (minimum) 40 40 40 Maximum hardness: Brinnel Hardness No. 201 217 217 Rockwell B (Rb): 92 95 95 It must be clarified that as far as tensile strength, yield strength and percentage elongation is concerned, this Table presents only the minimum qualifying characteristics of stainless steel alloys essential for forming the utensils and other consumer durables mentioned. As far as hardness is concerned, the Table shows the maximum hardness achieved. The stainless steel alloy of the present invention achieves a face-centred cubic structure and a formability at least equivalent to that of conventional stainless steels such as AISI-304. The most important economic advantage of the invention lies in its successful replacement of the costly nickel content of conventional stainless steel alloys by a combination of cheaper alloying elements which impart equivalent characteristics of austenitic stabilisation without adversely affecting the quality of the resulting alloy. This makes the steel of the present invention considerably less expensive to manufacture than conventional alloys. Furthermore, by the skillful adjustment of the alloying elements of the replacement combination employed, the invention is capable of providing a versatile alloy which allows itself to be used in the formation of a wide variety of items extending from shallow drawn to deep drawn steel utensils and which evinces a hardness greater than that of conventional stainless steel alloys.

The description presented herein must be interpreted as being only illustrative of the present invention and must not be construed as limiting the invention in any way to only those embodiments described. Other embodiments not specifically described but falling within the scope and spirit of the invention, as would be apparent to any person skilled in the art, must also be deemed to be included herein. . in the fabrication of shallow to medium depth utensils and consumer durables has been identified as stainless steel grade JS - 203.

For the production of one metric ton of such stainless steel alloy JS - 203, raw materials comprising ferro-chromium ore containing at least 60 % chromium, ferro-nickel ore containing at least 28 % nickel, ferromanganese ore containing at least 70% manganese, ferro-silicon having a content of at least 70 % silicon, metallic copper and iron in the form of ferrous scrap are charged to an electric arc furnace in the following amounts:

Ferro-chromium alloy 245 kg. Ferro-nickel alloy : 85 kg. Ferromanganese alloy : 132 kg. Ferro-silicon 'alloy : 50 kg. Copper 14 kg. Ferrous scrap 474 kg. Thereafter, production of the desired stainless steel alloy JS - 203 follows in accordance with the procedure as described in Example 1 hereinabove.

Analytical studies of the alloys JS - 201 and JS - 203 produced according to the present invention have established that their mechanical properties compare very favourably with those of the conventional stainless steel alloy AISI - 304. Thus, in terms of tensile strength, yield strength and percentage elongation in a 50 mm gauge length, the properties of the three grades of alloys are the same but in terms of hardness, the figures for alloys JS - 201 and JS - 203 of the present invention exceed those of the conventional alloy. This can be observed from the comparative Table given hereafter:

TABLE Mechanical property AISI - 304 JS - 201 JS - 203 Tensile strength, Mpa (minimum) 515 515 515 Yield strength, Mpa (minimum) 205 205 205 Percentage elongation in 50 mm 3. An improved stainless steel alloy particularly suitable for the formation of cold-rolled sheets employed in the fabrication of shallow to medium depth utensils and other consumer durables which comprises on a percentage by weight basis the following composition:

Carbon : 0.03 % to 0.08 Manganese : 7.00% to 10.25 Silicon 0.10% to 0.75 Chromium : 14.25% to 16.50 Nickel 2.25% to 4.75 Copper : 0.90% to 2.00 Nitrogen : 0.01 % to 0.20 Impurities : 0.02% to 0.10 Iron : 75.44% to 65.37%. 4. An improved stainless steel alloy as claimed in any of claims 1 to 3 wherein said impurities comprise phosphorus and sulphur.

5. An improved stainless steel alloy as claimed in claim 4 wherein phosphorus and sulphur are present in the following amounts by weight:

Phosphorus : 0.01 % to 0.07 Sulphur : 0.01 % to 0.03 6. An improved stainless steel alloy as claimed in any of claims 1 to 5 which possesses a minimum tensile stress of 515 Mpa, a minimum yield strength of 205 Mpa, a minimum percentage elongation in 50 mm gauge length of 40 and a maximum hardness expressed in Brinnel Hardness of 217 and in Rockwell B of 95.

7. An improved stainless steel alloy substantially as herein described.

Claims

CLAIMS: 1. An improved stainless steel alloy particularly suitable for the production of consumer durables such as cutlery, milk pails, containers, culinary accessories and utensils which comprises on a percentage by weight basis the following composition:

Carbon 0.03 % to 0.08 Manganese 7.00% to 10.25 Silicon : 0.10% to 0.75 Chromium : 14.25% to 16.50 Nickel 2.25 % to 4.75 Copper 0.90% to 2.00 Nitrogen : 0.01 % to 0.20 Impurities : 0.02% to 0.10 Iron 75.44% to 65.37%. 2. An improved stainless steel alloy having enhanced formability particularly suitable for the production of deep drawn utensils and other consumer durables which comprises on a percentage by weight basis the following composition:

Carbon 0.03 % to 0.08 Manganese : 7.00% to 8.00 Silicon 0.10% to 0.75 Chromium : 15.50% to 16.50 Nickel 4.25% to 4.75 Copper 0.90% to 1.10% Nitrogen 0.01 % to 0.20 Impurities 0.02% to 0.10 Iron 72.19% to 68.52%. ferro-nickel alloy containing at least 28 % nickel, ferromanganese alloy containing at least 70 % manganese, ferro-silicon alloy having a content of at least 70 % silicon, metallic copper and iron in the form of ferrous scrap. 10. A process as claimed in claim 8 or 9 wherein the gaseous mixture injected into the molten charge comprises a mixture of oxygen, argon and nitrogen. 11. A process as claimed in any of claims 8 to 10 wherein the manganese present in the molten charge improves the solubility therein of the injected nitrogen whereby, for reason of its affinity for manganese, the dissolved nitrogen locates itself as an interstitial element in the resulting alloy where it acts as an austenitic stabiliser. 12. A process as claimed in any of claims 8 to 11 wherein hydrogen and excess carbon present as impurities in the molten charge are substantially converted by reaction with the oxygen in the injected gaseous mixture to water vapour and carbon monoxide respectively with the carbon monoxide so produced being in turn converted to carbon dioxide, said water vapour and carbon dioxide being allowed to escape into the atmosphere along with any unconverted elemental hydrogen while sulphur and other undesirable elements are oxidised to form the slag which is removed. 13. A process as claimed in any of claims 8 to 12 wherein said charge is heated to molten state in an electric arc furnace and refining the molten charge is carried out in an argon-oxygen decarburisation convertor. 14. A process for the production of an improved stainless steel alloy particularly suitable for the production of consumer durables such as cutlery, milk pails, containers, culinary accessories and utensils substantially as herein described with reference to the foregoing Examples. 8. A process for the production of an improved stainless steel alloy particularly suitable for the production of consumer durables such as cutlery, milk pails, containers, culinary accessories and utensils which comprises subjecting a charge composed on a percentage by weight basis of:

Ferro-chromium alloy 23.75 % to 27.50 Ferro-nickel alloy : 8.0 % to 17.0 Ferromanganese alloy : 10.0 % to 14.6 Ferro-silicon alloy 0.15 % to 1.0 Copper 0.9 % to 2.0 Iron : 57.2 % to 37.9 to heat at a temperature of approximately 1500 C. until the charge is molten, injecting into the molten charge a gaseous mixture which reacts exothermically with metallic impurities in the melt to convert them. into slag and with the non-metallic impurities to convert them substantially into gaseous compounds which are allowed to escape, separating the slag thus formed from the residual molten metal and recovering the thus refined alloy having on a percentage by weight basis the following composition:

Carbon 0.03% to 0.08 Manganese 7.00% to 10.25 Silicon 0.10% to 0.75 Chromium : 14.25% to 16.50 Nickel : 2.25% to 4.75 Copper 0.90% to 2.00 Nitrogen : 0.01 % to 0.20 Impurities 0.02% to 0.10 Iron : 75.44% to 65.37%. 9. A process as claimed in claim 8 wherein the raw materials constituting said charge comprise ferro-chromium alloy containing at least 60 % chromium,