USE OF A STEEL FOR CUTTING TOOL HOLDERS

Abstract: The invention relates to a steel containing in weight-%: 0.3-0.5 C, from traces to max. 1.5 Si, 0.2-1.5 Mn, max. 0.03 P, preferably max. 0.025 P, 0.01-0.2 S, 4-7 Cr, from traces to max. 1 Ni, 0.5-2.0 Mo, which can completely or partly be replaced by the double amount W, from traces to max. 1 Co, 0.2-1.5 V, from traces to max. 0.5 Nb, from traces to a total of max. 0.2 % of rare earth metals, balance essentially iron, impurities and accessory elements in normal amounts, as a material for cutting tool holders.

Description: USE OF A STEEL FOR CUTTING TOOL HOLDERS TECHNICAL FIELD The invention relates to the use of a steel with a specific composition, as a material for cutting tool holders. BACKGROUND OF THE INVENTION A cutting tool holder is the body on or in which the tool bit, that is active during the niachining operation, is attached. Cutter- and drill bodies are typical cutting tool holders, which are provided with active machining carbide elements. The material in such cutting tool holders is usually a steel which is referred to as holder steel in this technical field. A number of requirements are raised upon holder steels: - Cutting tool holders, such as cutter- and drill bodies, often have a complicated shape, and the most niachining of the tool holder body is made in the soft-annealed condition of the steel. Nevertheless, it must be possible to air-harden the material without significant changes of the dimensions during the hardening operation. - Some cutting tool holders are tough-hardened, while those surfaces against which the carbide bits are attached are induction-hardened. Therefore h must be possible for the material to be induction-hardened. - Machining by cutting is performed at ever increasing machining rates, which results in the cutting tool holder becoming very hot. The material therefor has to have good hot-hardness. - Some types of cutting tool holders, such as certain drill bodies with carbide tips which are attached by soldering, are PVD-coated subsequent to the hardening operation in order that the cuttings of the borings shall not wear out the drilling spiral in the drill body. Therefore, it must be possible to PVD-coated the material without any significant hardness reduction. - Some types of cutting tool holders, such as cutter bodies, are subjected to high pulsating loads when they are in use. The material therefor must have good mechanical features, including a good toughness and fatigue strength. - Many types of cutting tool holders have a very complicated shape. Small threaded holes, and long, narrow borings frequently occur. Therefore the material shall have a good machinability, particularly when high speed steel tools are used. Low or medium alloyed tool steels are used as materials for cutting tool holders. The composition of a number of typical holder steels are listed in the table below. Besides the element mentioned in the table, which refer to weight- %, the steels only contain iron, and impurities and accessory elements. None of the known holder steels satisfies the above specified requirements in a completely satisfactory way. DISCLOSURE OF THE INVENTION The invention suggests a steel alloy intended to be used as a material for cutting tool holders, which satisfies the said requirements more satisfactorily than the steels of prior art. The composition of the steel is given in the appending claims. The invention also relates to cutting tool holders made of the steel. In the following, the importance of the individual elements and their mutual interaction will be explained. All percentages, which relate to the chemical composition of the steel, refer to weight- %. Carbon shall exist in an amount of at least 0.3%, preferably at least 0.35%, suitably at least 0.37%, in order that the steel shall obtain a desired hardness and strength. The carbon content must not exceed 0.5%, preferably not exceed 0.45%, and suitably not exceed 0.41%. At higher carbon contents, the steel can be too hard and brittle. Typically, the steel contains 0.39%C. Silicon may exist in amounts from a trace amount to a maximum of 1.5%, but preferably the steel should contain at least 0.40% Si. The silicon exists in the steel in a dissolved state, but can also exist as silicon-calcium oxides, which in their turn are preferably modified by means of sulphur, which in the form of sulphides may cover the oxide and make it essentially plastic, wherein the said inclusions can function as a lubricating film when the steel is being machined. Preferably, the steel should not contain more than 1.2% Si. Preferred ranges are 0.7-0.9% Si or 0.6-0.8% Si. A typical (nominal) Si content is 0.7%. Manganese shall exist in an amount of at least 0.2% in order to improve the tempering resistance of the steel and in order to prevent redbrittleness of the steel through the formation of maganese sulphides, when the steel contains higher amounts of sulphur. The steel however, shall not contain more than 1.5% maganese, preferably max. 1.0% Mn. A particularly preferred range is 0.3-0.5% Mn. An optimal maganese content is 0.4%. Sulphur shall exist in an amount of at least 0.01% in order to give the steel an adequate machinability. The steel shall not contain more than at most 0.2% S. If the sulphur content is higher, there is a risk that redbrittleness may occur, which can not be completely compensated by a correspondingly high content of maganese. Preferably, the steel should not contain more than max. 0.05% sulphur. A preferred content of sulphur lies in the range 0.01-0.03% S. A typical (nominal) sulphur content is 0.02%. Chromium shall exist in the steel in an amount between 4 and 7% in order to give the steel a good hardenability. A preferred range is 4.5-5.5% Cr; typically 5.0% Cr. Nickel is not a critical element in the steel, but can be tolerated in an amount up to 1%, preferably max. 0.5%. Molybdenum improves the hardenability of the steel and also its tempering resistance and hence its hot hardness and shall therefor exist in an amount of at least 0.5%; max. 2.0%. A preferred range is 1.2% Mo, preferably 1.2-1.6% Mo. Typically, the steel contains 1.4% Mo. In principal, molybdenum can be replaced completely or partly by the double amount of tungsten. Tungsten, however, is an expensive alloy metal and it also complicates the handling of return scrap. Therefore, tungsten should be avoided in amounts higher than amounts recognised as impurities. Cobalt, for the same reason as tungsten, should not exist in the steel but can be tolerated in amounts up to max. 1.0%, preferably max. 0.05%. Vanadium is favourable for the tempering resistance and the wear strength of the steel and shall exist in the steel in an amount of at least 0.2%, but not exceed 1.5%. Preferably, the vanadium content should lie between 0.6-1.3%, suitably between 0.8- 1.1%; typically the content ofvanadiumis θ.95%. Possibly, the steel can also contain oxygen and calcium in functional amounts, more particularly 50-100 ppm oxygen and 5-75 ppm calcium in order to interact to form calcium oxides, which are modified by means of sulphur as has been mentioned in the foregoing. Niobium forms primary carbonitrides which are difficult to dissolve and shall not exist in amounts above 0.5%. Preferably, niobium should not exist in amounts above impurity level. Also titanium, zirconium, aluminium and other strong carbide and/or nidrideformers are impurities which are not desired and therefore shall not exist in amounts above impurity level. Rare earth metals, such as cerium, lanthanum and others can possibly be added to the steel in order to afford the steel isotropic features, optimal machinability, good mechanical features, and a good hot- workability. The total content of the rare earth metals can amount to max. 0.4%, preferably max. 0.2%. The nominal (typical) composition has the following specification: 0.37-0.41 C, 0.40- 1.20 Si, 0.30-0.50 Mn, max. 0.025 P, 0.010-0.030 S, 5.00-5.30 Cr, max. 0.25 Ni, 1.25- 1.50 Mo, max. 0.20 W, max. 0.20 Co, 0.90-1.00 V, max. 0.005 Ti, max. 0.030 Nb, max. 0.25 Cu, max. 0.020 Al 5-50 ppm Ca, 60-90 ppm O, balance iron. The steel is hardened from an austenitizing temperature between 860 and 1100°C, preferably between 960 and 1050°C, wherein the hardening temperature is chosen within said range depending on the desired hardness. When the steel is hardened from a temperature within the lower part of the range 960 - 1050°C, more particularly from a temperature of 960°C or slightly above that temperature, it is possible to achieve a hardness of 48 HRC while it is possible to achieve a hardness of 54 HRC if the steel is hardened from a temperature within the upper part of the range 960 - 1050°C, i.e. at or near 1050°C, prior to tempering. Tempering can be performed either as low temperature tempering from a temperature between 180- 250°C, or as high temperature tempering from a temperature between 550-600°C in order to provide high hardness in combination with good toughness. The drawing shows typical tempering curves for a steel according to the invention after hardening from different temperatures between 960 and 1025°C. The invention shall be explained more in detail in the following with reference to performed experiments. PERFORMED EXPERIMENTS A great number of steel heats having compositions according to Table 1 were manufactured. The given contents of the elements of the compositions are mean values of measurements made on different places in the ingots that were manufactured. In Table 1, the composition of reference material, SS 2242, has also been included. The contents of the reference material are nominal contents. The content of phosphorus, sulphur, aluminium, nitrogen, calcium, and oxygen have not been mentioned. For all the materials, the balance is iron and impurities which can exist in normal amounts in addition to those impurities or accessory elements that are mentioned in the table. In Table 1, results from machinability tests which were performed on material in the soft annealed state are also shown. The mentioned values refer to the periphery speed of the drill (mean value) in which the drill is rotated so that the total length of 1000 mm shall be achieved before the drill is worn out. Also, the number of borings that are possible to drill at a drilling rate of 30 m/min, before the drill is worn out, are stated in the table. Table 1

Claims: CLAIMS 1. Use of a steel containing in weight-%: 0.3-0.5 C, from traces to max. 1.5 Si, 0.2-1.5 Mn, max. 0.03 P, preferably max. 0.025 P, 0.01-0.2 S, 4-7 Cr, from traces to max. 1 Ni, 0.5-2.0 Mo, which can completely or partly be replaced by the double amount W, from traces to max. 1 Co, 0.2-1.5 V, from traces to max. 0.5 Nb, from traces to total max. 0.2% of rare earth metals, balance essentially iron, impurities and accessory elements in normal amounts, as a material for cutting tool holders. 2. Use according to claim 1, wherein the steel contains 0.35-0.45 C. 3. Use according to claim 2, wherein the steel contains 0.37-0.41 C. 4. Use according to claim 1, wherein the steel contains 0.4-1.2 Si. 5. Use according to claim 4, wherein the steel contains 0.7-0.9 Si. 6. Use according to claim 4, wherein the steel contains 0.6-0.8 Si. 7. Use according to claim 1, wherein the steel contains 0.2-1.0 Mn. 8. Use according to claim 7, wherein the steel contains 0.3-0.5 Mn. 9. Use according to claim 1, wherein the steel contains 0.01-0.05, preferably 0.01-0.03 S. 10. Use according to claim 1, wherein the steel contains 4.5-5-5 Cr. 11. Use according to claim 1, wherein the steel contains 1-2 Mo. 12. Use according to claim 11, wherein the steel contains 1.2-1.6 Mo. 13. Use according to claim 1, wherein the steel contains 0.6-1.3 V, preferably 0.8-1.1 V. 14. Use according to claim 1, wherein the steel contains 50-100 ppm oxygen and 5-75 ppm Ca. 15. Use according to claim 1, wherein the steel contains a total of max. 0.4% of rare earth metals. 16. Cutting tool holder consisting of a steel with a composition according to any of the preceding claims.

Inventors: GUNNARSSON STAFFAN, RYDELL BO, GUNNARSSON, STAFFAN, RYDELL, BO
Applicants: GUNNARSSON STAFFAN, RYDELL BO, UDDEHOLM TOOLING AB, GUNNARSSON, STAFFAN, RYDELL, BO, UDDEHOLM TOOLING AKTIEBOLAG
Application year: 1997
Main classification: C22C38
IPC codes: B23B27, C22C38