EP0882808B1 - Steel for big-sized moulds and process for manufacturing the steel - Google Patents

Description

Mold makers and plastic processors driven by the demands of the automotive industry and industry in general, through the development of "Plastic" technical applications in the sector automotive increasingly important and by the challenges growing technical and economic, have more and more need a steel for molds to obtain molds with a uniform hardness over its thickness (from 30 at 35 Hrc), thickness up to 700 mm or even more.

Vehicle exit times, always more and more shorter, also pose problems in terms of concerns the management of changes in real time, machinability and weldability.

EP-A-431 557 discloses steel for large molds having a composition similar to that of invention and departure from steel invention by the lowest possible boron content.

The subject of the present invention is a method of manufacture of steel for large molds which makes it possible to obtain a steel solving these difficulties.

This process is characterized in that the steel is obtained by melting in an electric furnace, followed by a secondary metallurgical operation in a heating pocket, this operation itself being followed by degassing under vacuum of less than 1 torr, or even 0.2 torr, of a mixture having the following weight composition: Preferred range Wide fork Manganese 1.50 - 2% 1 to 3% Silicon 0.050 - 0.150% ≤ 0.400% Phosphorus ≤ 0.008% ≤ 0.015% Chromium 2 to 2.40% 1.50 to 3.5% Molybdenum 0.35 - 0.50% 0.25 to 1% Niobium 0.100 - 0.150% from 0.100 to 0.250% Titanium or Zirconium if necessary 100 to 200ppm 100 to 300ppm Copper ≤ 0.100% ≤ 0.300% Nickel ≤ 0.200% ≤ 0.300% Nitrogen ≤ 30 ppm ≤ 80 ppm Oxygen ≤ 20 ppm ≤ 80 ppm Calcium 5 to 20 ppm ≤ 30 ppm Boron 20 to 30 ppm 15 to 50 ppm Carbon 0.15 to 0.20% 0.10 to 0.25% Sulfur ≤ 0.005% ≤ 0.050% Aluminum ≤ 40 ppm ≤ 250 ppm

The complement being iron and impurities characteristics of steel making.

The ingot obtained after may be submitted degassing at a reflow by consumable electrode under vacuum or under slag, the ingot serving as an electrode consumable.

You can also submit the ingot to a first remelting under vacuum and then a second remelting under slag.

The invention also relates to a steel for molds characterized in that it has the following weight composition. Preferred range Wide fork Manganese 1.50 - 2% 1 to 3% Silicon 0.050 - 0.150% ≤ 0.400% Phosphorus ≤ 0.008% ≤ 0.015% Chromium 2 to 2.40% 1.50 to 3.5% Molybdenum 0.35 - 0.50% 0.25 to 1% Niobium 0.100 - 0.150% 0.100 - 0.250% Titanium or Zirconium if necessary 100 to 200ppm 100 to 300ppm Copper ≤ 0.100% ≤ 0.300% Nickel ≤ 0.200% ≤ 0.300% Nitrogen ≤ 30 ppm ≤ 80 ppm Oxygen ≤ 20 ppm ≤ 80 ppm Calcium 5 to 20 ppm ≤ 30 ppm Boron 20 to 30 ppm 15 to 50 ppm Carbon 0.15 to 0.20% 0.10 to 0.25% Sulfur ≤ 0.005% ≤ 0.050% Aluminum ≤ 40 ppm ≤ 250 ppm

The complement being iron and impurities characteristics of steel making.

In the steel according to the invention the manganese sulfides are perfectly distributed globular and the globular oxides are preferably encapsulated by calcium sulfides. This steel has a double hardening, a primary hardening by hardening solid solution by insertion of Boron during austenitization and precipitation during the quenching of boro-carbides type M23 (B.C) 6; these very fine precipitates germinate during quenching very energetic at the grain boundaries of austenite due to their face-centered cubic structure of parameter a = 10.6 A °. These boro-carbides are in orientation relationship and in coherence with the austenite of one of the two grains. This primary hardening is followed by a hardening secondary due to a dispersion mainly of carbides, nitrides, fine carbo-nitrides of Niobium homogeneously precipitated during an income. The Niobium, an essential dispersoid element, introduced into the part of the production of the steel according to the invention intervenes in the control of grain size, both during reheating and during phenomena recrystallization; it increases the hardenability of the steel according to the invention and causes hardening by precipitation. The role of Niobium combined with that of Boron is fundamental in the development of steel according to the invention and for obtaining the characteristics mechanicals mentioned below.

The steel according to the invention has a peening ability chemical to machining, electro-erosive, excellent; he has a quality polishing ability (grain> 1200 followed by a diamond polishing of 8µ or even 3µ ) .It can be nitrided, hardness> 60 Rc.

Résistance à la traction ≥ 950 N/mm²

Limite élastique 0,2 % ≥ 800 N/mm²

Allongement sens épaisseur ≃ 10 % mini 5%

Résilience - valeur en K U sens épaisseur ≃ 10 Joules mini 5 Joules

Dureté Brinell en  = 10mm / 3.000Kg - 290 à 330 -

The mechanical characteristics obtained on the product treated by quenching and precipitation between 400 ° C and 600 ° C are:

Tensile strength ≥ 950 N / mm ²

0.2% elastic limit ≥ 800 N / mm ²

Elongation thickness direction ≃ 10% minimum 5%

Resilience - value in KU thickness direction ≃ 10 Joules minimum 5 Joules

Brinell hardness in  = 10mm / 3.000Kg - 290 to 330 -

This steel faces two fundamental properties for molds: polishability and granulability in large part linked to the homogeneity of its structure and level of its inclusive cleanliness.

Its shaping is preferably carried out by thermo-mechanical transformation such as forging or rolling or molding in all cases.

Claims (6)

1. A method for producing steel for a large mould
   characterised in that it is obtained by melting a mixture in an electric furnace, followed by a secondary metallurgical operation in a heating ladle, and said operation being followed of vacuum degassing at a pressure of less than 1 torr, and even 0,2 torr, of a mixture having the following weighted composition: Manganese 1 to 3 % Silicon ≤ 0.400 % Phosphorus ≤ 0.015 % Chromium 1.50 to 3.5 % Molybdenum 0.25 to 1% Niobium 0.100- 0.250% Titanium and/or zirconium if necessary 100 to 300 ppm Copper ≤ 0.300 % Nickel ≤ 0.300 % Nitrogen ≤ 80 ppm Oxygen ≤ 80 ppm Calcium ≤ 30 ppm Boron 15 to 50 ppm Carbon 0.10 to 0.25 % Sulphur ≤ 0.050 % Aluminium ≤ 250 ppm as the balance is iron and impurities characteristic of the production of steel.
2. A method according to claim 1,
   characterised in that the ingot obtained by degassing is subjected to a remelting process by consumable electrode under vacuum or under slag, the ingot is used as the consumable electrode.
3. A method according to claim 2,
   characterised in that the ingot obtained by degassing is subjected to a first remelting process by consumable electrode under vacuum and to a second remelting process by consumable electrode under slag, wherein the ingot is used as the consumable electrode.
4. A method according to one of the preceding claims,
   characterised in that the shaping is executed by thermo-mechanical transformation.
5. A steel for a large mould,
   characterised in that it has the following weighted composition: Manganese 1 to 3 % Silicon ≤ 0.400% Phosphorus ≤ 0.015% Chromium 1.50 to 3.5 % Molybdenum 0.25 to 1 % Niobium 0.100- 0.250% Titanium and/or zirconium if necessary 100 to 300 ppm Copper ≤ 0.300 % Nickel ≤ 0.300 % Nitrogen ≤ 80 ppm Oxygen ≤ 80 ppm Calcium ≤ 30 ppm Boron 15 to 50 ppm Carbon 0.10 to 0.25 % Sulphur ≤ 0,0250 % Aluminium ≤ 250 ppm as the balance is iron and impurities characteristic of the production of steel.
6. A steel according to claim 5,
   characterised in that it has the following weighted composition: Manganese 1.50-2% Silicon 0.050- 0.150 % Phosphorus ≤ 0.008 % Chromium 2 to 2.40 % Molybdenum 0.35- 0.50 % Niobium 0.100-0.150 % Titanium and/or zirconium if necessary 100 to 200 ppm Copper ≤ 0.100 % Nickel ≤0.200 % Nitrogen ≤ 30 ppm Oxygen ≤ 20 ppm Calcium 5 to 20 ppm Boron 20 to 30 ppm Carbon 0.15 to 0.20 % Sulphur ≤ 0.005 % Aluminium ≤ 40 ppm as the balance is iron and impurities characteristic of the production of steel.