Title of Invention

METHOD FOR PRODUCING STEEL FOR LARGE MOLDS

Abstract

The present invention concerns a method for producing steel for large molds. According to the invention, this steel is obtained by being melted in an electric furnace followed by a secondary metallurgical operation in a heating ladle, this operation being followed by a degassing under vacuum of less than I torr, in fact 0.2 torrs of a mixture having the following composition: Manganese 1 at 3%, silicon < 0.400%, phosphorus< 0.015%, chromium 1.50 at 3.5%, molybdenum 0.25 at 1%, niobium 0.100 -0.250%, 100 to 300ppm titanium or zirconium if necessary, copper < 0.300%, nickel < 0.300%, nitrogen < 80 ppm, oxygen < 80 ppm, calcium < 30 ppm, 15 to 50 ppm boron, 0.10 to 0.25% carbon, sulfur< 0.050%, aluminum < 250 ppm.

Full Text

« Method for producing steel for large molds » FIELD OF THE INVENTION Mold makers and plastic material transformers, forced by the needs of the motor industry and industry in general by the development of « plastic » technical applications in the ever-expanding motor vehicle sector and increasing technical-economic challenges, are increasingly requiring a steel for molds making it possible to obtain molds having a homogeneous hardness concerning thickness (from 30 to 35 Hrc), a thickness possibly extending up to 700 mm or even more. BACKGROUND OF THE INVENTION The outgoing production times for a vehicle, always becoming shorter and shorter, also pose problems as regards the management of real time modifications, as well as mach inability and weld ability, SUMMARY OF THE INVENTION The present invention concerns a method for producing a steel for large molds making it possible to obtain a steel resolving these difficulties. This method is characterized in that the steel is obtained by being melted in an electric furnace followed by a secondary metallurgical operation in a heating (ladle, this operation being followed by an under vacuum degassing of less than 1 torr, in fact 0.2 torrs, of a mixture having the following weighted composition : It is subsequently possible to submit the ingot obtained after degassing to a remelting process by a consumable electrode under vacuum or under slag, the ingot being used as a consumable electrode. It is also possible to submit the ingot to a first vacuum remitting and then to a second slag remelting process. The invention also concerns a steel for molds, wherein it has the following weighted composition ; In the steel of the invention, the manganese sulfides appear in the form of fully distributed globes and the globular oxides are preferably encapsulated by calcium sulfides. This steel posses double hardening, a primary hardening by hardening the solid solution by inserting boron during austenitization and precipitation during the quench hardening of M23 type boro-carbides (B.C) 6, These extremely fine precipitates germinate during the extremely energetic quench hardening to the austerity grain boundaries owing to their cubic structure with centered parameter faces a=; 10 6 A°. These boro-carbides are in orientation relation and in coherence with the austenite of one of the two grains. This primary hardening is followed by a secondary hardening due to a dispersion mainly of carbides, nitrides, fine niobium carbo-nitrides precipitated homogeneously during an annealing. The niobium, an essential dispersion element, introduced as part of producing the steel of the invention, takes part in the control] of the size of the grains and also at the time of reheating during recrystallization plainsmen. It increases the harden ability of the steel of the invention and provokes a secondary hardening. The role of the niobium, allied to that of the boron, is fundamental in manufacturing the steel of the invention and for obtaining the mechanical characteristics mentioned below. The steel of the invention possesses excellent aptitude to chemical granulation on machining and is electro erosive. It is also suitable for quality polishing (grain >I200 followed by a grain diamond polishing of 8 μ or indeed 3 μ ). It can be nitride-hardened, hardness >60 Re, The mechanical characteristics obtained for the product treated by hardening and precipitation between 400""C and 600°C are the following : Resistance to traction > 950 N/mm2 Elastic limit 0,2% > 800 N/mm2 Elongation, thickness direction ~ 10% min 5% Resilience - value in KU, thickness direction ~ 10 min Joules : 5 Joules Brinell hardness in 0 = 10 mm / 3.000Kg - 290 to 330 - This steel meets two fundamental properties for molds ; for the most pan polishability and granulability linked to the homogeneity of its structure and as regards its inclusionary property. For all shaping cases, its shaping is preferably carried out by thermo-mechanical transformation, such as forging, rolling or molding. WE CLAIM: 1 A method for producing a steel for large molds, wherein it is obtained by melting it in an electric furnace followed by a secondary metallurgical operation in a heating ladle, this operation being followed by a vacuum degassing of less than 1 torr. in fact 0.2 torrs. of a mixture having the following weighted composition : Manganese 1 to 3% Silicon Phosphorus Chromium 1.50 lo 3.5% Molybdenum 0.25 to 1% Niobium 0.100-0.250% Titanium or zirconium 100 to 300 ppm if necessary Copper Nickel Nitrogen Oxygen Calcium Boron 15 to 50 ppm Carbon 0.10 to 0,25% Sulfur Aluminum The complement is iron and the impurities characteristic of the production of steel. 2. The method according to claim 1, wherein the ingot obtained is subjected by degassing to a remelting by consumable electrode under vacuum or under slag, the ingot used as an electrode, 3. The method according to claim 2, wherein the ingot obtained after degassing is subjected to a first remelting by a consumable electrode under vacuum and to a second remelting by a consumable electrode under slag. 4. The method according to one of the preceding claims, wherein shaping is carried out by thermo-mechanical transformation, 5. A steel for large molds, wherein it has the following composition : Manganese 1 to 3% Silicon Phosphorus Chromium 1.50 to 3,5% Molybdenum 0,25 to !% Niobium 0.100-0,250% Titanium or zirconium 100 to 300 ppm if necessary Copper Nickel Nitrogen Oxygen Calcium Boron 15 to 50 ppm Carbon 0,10 to 0.25% Sulfur Aluminum The complement is iron and the impurities characteristic of the production of steel. 6. The steel for large molds according to claim 5, wherein it has the following composition : Manganese I SO - 2% Silicon 0.050-0.150% Phosphorus Chromium 2 to 2.40% Molybdenum 0.35-0.50% Niobium 0.100-0,150% Titanium or zirconium 100 to 200 ppm if necessary Copper 5 0.100% Nickel Nitrogen Oxygen Calcium 5 to 20 ppm Boron 20 to 30 ppm Carbon 0,15 to 0,20% Sulfur Aluminum The complement is iron and the impurities characteristic of the production of steel.

Documents:

1203-mas-1998 abstract.pdf

1203-mas-1998 claims-duplicate.pdf

1203-mas-1998 claims.pdf

1203-mas-1998 correspondence-others.pdf

1203-mas-1998 correspondence-po.pdf

1203-mas-1998 description (complete)-duplicate.pdf

1203-mas-1998 description (complete).pdf

1203-mas-1998 form-19.pdf

1203-mas-1998 form-2.pdf

1203-mas-1998 form-26.pdf

1203-mas-1998 form-4.pdf

1203-mas-1998 form-6.pdf

1203-mas-1998 others.pdf

1203-mas-1998 petition.pdf