Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

• the present invention concerns non-alloy free-cutting steels with improved machinability. It is known that elements such as lead and bismuth; as elements and/or as a mixture with tellurium and selenium, behave as embrittling liquid metals when they are dispersed as inclusions in a metallic matrix and heated above their respective melting temperatures.
• a liquid-metal embrittling agent is a metal, either element or alloy, with a relatively low melting temperature such that it is liquid at the temperature prevailing at the root of a microcrack, in the steel matrix, formed either at an inclusion in the steel or propagated within the steel from a starting point at the point of contact between the steel matrix and the cutting-edge of a machine tool.
• a liquid-metal embrittling agent must also have a value of surface free energy (surface tension) which is relatively low in the liquid state in order to give the agent the necessary ability to "wet” a relatively large area along the grain boundaries or at the limits of the various phases present.
• tellurium is generally used in combination with lead and bismuth.
• steels with controlled sulphur contents and/or resulphurised steels from the data in the literature it can be shown that a Te/S ratio of 0.2 is the most efficacious to attenuate the anistropy of mechanical properties caused by the presence of non-spheroidal sulphides.
• tellurium is twofold, it being a spheroidiser and it is also an embrittler for on combination with lead, and especially with bismuth, it lowers their surface free energy (or the value of surface tension) at their melting temperatures, thus enhancing their embrittling effect.
• selenium is only used together with lead and bismuth additions to produce effects similar to those indicated above, but its very high price excludes it as a commercially viable element.
• bismuth Unlike lead and tellurium, there is no toxicity problem with bismuth. Because of this reason, bismuth is most interesting from an industrial point of view, for as it is known that bismuth has the ability to improve the machinability of steels, it can replace lead and tellurium in steels with improved machining characteristics.
• bismuth additions in steel are present as particles which are insoluble in the metal matrix and which have dimensions normally less than 5 microns.
• the melting point of bismuth is relatively low (271 °C) and its surface free energy at temperatures near the melting point is also relatively low (375 erg/cm 2 ).
• Sulphur present in the steel as sulphides therefore has a positive effect on the machinability of a steel, this effect varying according to their shape, size and orientation.
• the sulphides referred to are those of manganese and not iron sulphide which forms a eutectic phase with iron which melts at 988 °C.
• This eutectic phase is the cause of hot-shortness, i.e. the well-known phenomenon of brittleness during hot-rolling.
• the manganese content (in weight percent) of the steel must be more than three times that of sulphur in order to ensure the formation of manganese sulphides which do not form eutectic phases with iron and which have melting temperature above typical hot-rolling and hot-forging temperatures.
• Sulphide inclusion morphologies can be divided into the following three main categories;
• bismuth has a marked tendency to "wet" grain boundaries and interphase limits.
• microcracks During a machining operation, the force applied at the point of contact between steel and tool causes microcracks to be formed which generally propagate along grain boundaries and interphase limits. The lower the energy required to propagate the microcrack, the better is the machinability.
• Bismuth acts as an embrittling liquid metal and lowers the energy of microcrack propagation. In fact, at typical machining temperatures it melts and immediately migrates to the roots of microcracks, thus embrittling the matrix and facilitating chip fracture.
• the dimensions of the bismuth inclusions must be very small, indicatively around 5 microns, in order to multiply the points where it is available in the steel microstructure.
• Sulphur also is employed in the production of improved-machinability steels for it combines with manganese present in the steel to form manganese sulphide inclusions, all of which are potential microcrack formers during the period when the chip separates away from the matrix.
• the shape of the sulphides depends essentially on the solubility of sulphur in the liquid steel.
• manganese sulphides which have a low solubility, tend to precipitate out at relatively high temperatures as multiphase particles which also contain oxides and silicates (type 1 sulphides).
• deoxidation is controlled by limited aluminium additions of the order of parts per million in order to obtain a low residual free oxygen level (around 40 parts per million). Sulphides are obtained which are more soluble and which precipitate out at the primary grain boundaries (type 2 sulphides) at the solidification temperature.
• the solubility of manganese sulphide is lower than in the previous case, and therefore it tends to precipitate out at temperatures which are higher than those for type 1 sulphides; the inclusions formed are single phase and usually more in number than type 1 and type 2 sulphides owing to the numerous nucleation sites created by aluminium oxides or by oxides of other metals used to completely deoxidise the steel.
• the manganese sulphide inclusions are also deformed.
• Spheroidal type 1 sulphides are the most suitable for improving machinability, for they are better distributed within the matrix and, after hot-rolling, also have a lenticular shape which makes them embrittlement sites and microcrack formers that diminish the cutting forces required for chip fracture.
• lenticular sulphides Compared to type 2 and 3 sulphides (lamellar or stringers), the presence of lenticular sulphides is the cause of a clear-cut improvement in the value of the machinability index.
• the need to determine the value of the machinability index of a given product using "industrial" methods and time-limits usually means basing this value on a limited number of tests.
• the principal factors that influence steel machinability are: the properties of the steel to be worked (chemical composition, mechanical properties, hardness, structural homogeneity and type of metallographic structure), characteristics of the tool used for machining (type of material, geometric shape, tool angles, hardness, degree of sharpness of the cutting edge) and machining parameters (type of machining, cutting speed, depth of cut, feed, type of cooling medium,).
• a reference steel is chosen to which is assigned an arbitrary value of 100 for the machining index.
• Other steels are compared to the reference steel in identical test conditions.
• tool flank wear has been adopted as the variable for the measurement of the machinability index.
• tool flank wear is meant the maximum depth of wear (expressed in millimetres) on the flank of the tool used for lathe-turning.
• the purpose of the present invention is to realize free-cutting steels, containing sulphides having suitable lengths, shape factors and degrees of spheroidisation, which may be used by the engineering industry in general and the automotive industry in particular and which have better machinability than the steels presently known in the art.
• Another purpose of the present invention is to realise a non-alloy free-cutting steel which is not harmful neither for those who produce it in the steelworks nor for those who employ it in successive working operations.
• Yet another purpose of the present invention is to realise a non-alloy free-cutting steel with improved machinability characteristics in an inexpensive manner without employing complex and costly technological means.
• ⁇ /w is termed the "spheroidisation index" relating to the sulphides present in the steel.
• mean statistical value used above and hereinafter is meant the determination of the value on a number of samples which is statistically significant, i.e. at least 1 sample per 10 tonnes of steel produced with a minimum number of 3 samples.
• the mean statistical value of the spheroidisation index and the shape factor are determined metallographically as follows. Each of the above samples is divided along its longitudinal axis. On one of the sections obtained in this manner, 10 fields, equidistant between the sample axis and one of the longitudinal edges, are selected for examination at a magnification of x200 with an image processor which permits the sulphide morphology to be expressed in quantitative terms.
• Tests were performed at various cutting speeds within a pre-selected range. Each test lasted for 20 minutes (hence the term "v20" used to indicate this test) and the depth of tool flank wear was measured at the end of the test using appropriate optical instrumentation.
• bismuth as an element and/or as an alloy and/or as a mixture with one or more elements insoluble in liquid steel or having spheroidizing effects on sulphides, in proportions between 0.05% and 0.30% in weight improves the value of the machinability index up to 30% for free-cutting steels which have values of the spheroidisation index, shape factor and sulphide lengths within the above-cited limits.
• bismuth to carbon free-cutting steels, as an element and/or as an alloy and/or as a mixture with one or more elements insoluble in liquid steel or having spheroidizing effects on sulphides, in appropriate quantities which in any case are included in the range from 0.05% to 0.30% in weight, in the liquid-steel ladle or to the tundish or to the mould of a continuous casting machine permits bismuth to be well distributed in the metallic matrix as particles the dimensions of which must be less than or equal to 5 microns, as has been stated earlier in the introductory part.
• free-cutting steels non-alloy steels produced with sulphur additions between 0.10% and 0.40% in weight (limiting values included).
• the sulphides according to claim 1 improve machinability when compared to traditional techniques, while ensuring freedom from hot-shortness and also improve the toughness of the steel, especially in the direction transverse to the rolling direction.
• non-alloy free-cutting steel of the present invention can be subjected to numerous variations and modifications without affecting its innovative nature.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Treatment Of Steel In Its Molten State (AREA)
• Heat Treatment Of Steel (AREA)

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Citations (8)

• 1997
  • 1997-12-01 IT ITMI972661 patent/IT1296821B1/it active IP Right Grant
• 1998
  • 1998-09-10 EP EP98203028A patent/EP0919636A1/de not_active Withdrawn

Patent Citations (9)

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