EP0739993B2 - Steel and process for manufacturing workpieces with high abrasion resistance - Google Patents

Abstract

An abrasion-resistant steel contains (by wt.%) 0.24-0.3 (pref. 0.24-0.27) C, 0-2 (pref. 0-1 Si) 0-2 (pref. 0-1) Al, 0-2 (pref. 0.3-1.6) Mn, 0-4 (pref. 0-2) Ni, 0-3 (pref. 0.5-1.8) Cr, 0-0.6 Mo and 0-1.2 W. The steel may also contain 0.005-0.005% B, at least one of Nb, V, Zr and Ti in amt. less than 0.3% and at least one of Se, Te, Ca, Bi and Pb in amt. below 0.1%, the rest being Fe. The following relationships also apply, 4.6C + 1.05Mn + 0.54Ni + 0.66(Mo + W/2) + 0.5Cr + k ≥ 1.6, where k = 0 if the steel contains less than 0.0005% B and k = 0.5 if the steel contains more than 0.005% B and 0.6% ≤ (Al+Si) ≤ 2%. To improve their abrasion resistance sheets of the steel are heated to above austenisation temp. (e.g. 900 degrees C) then cooled to 450 degrees C at more than 1 degrees C/s, further cooled to 200 degrees C over 50 s to 60 minutes (pref. 100 s to 30 minutes) and finally returned to ambient (below 250 degrees C) in less than 3 h. This gives the steel a martensitic or martensitic-bainitic structure contg. 5-15% austenite and a hardness of 400-500 HB.

Description

The present invention relates to steels for the manufacture of parts with high resistance to abrasion.

In the mineral industry, equipment such as, for example, chutes, screens, blades of attack, subjected to abrasive wear very severe. To manufacture this equipment, we use parts, and in particular sheets, of steel with high abrasion resistance, quenched, of hardness between about 550HB and 600HB. These steels contain 0.35% to 0.5% carbon, and 0.5% to 3% of at least one alloying element such as manganese, chromium, nickel or molybdenum. These steels have the disadvantage of being very difficult to cut and weld, they have a low forming capacity and are, in general, fragile.

The object of the present invention is to remedy this disadvantage in proposing a steel to make parts, and in particular sheet metal, easily weldable and cuttable, and having a abrasion resistance comparable to that of the parts according to the prior art.

For this purpose, the subject of the invention is a steel whose chemical composition comprises, by weight: 0.24% ≤ C ≤ 0.3% 0% ≤Si≤2% 0% ≤Al≤2% 0% ≤Mn≤2% 0% ≤Ni≤4% 0% <Cr ≤ 3% 0% ≤ Mo ≤ 0.6% 0% ≤W≤1,2% optionally from 0.0005% to 0.005% boron, optionally at least one element selected from Nb, V, Zr and Ti, in contents of less than 0.3%, optionally at least one element selected from Se, Te, Ca, Bi and Pb in contents of less than 0.1%, the balance being iron and impurities resulting from the preparation,
the chemical composition further satisfying the following relationships: 0.6% ≤ Al + Si ≤ 2% and 4.6xC + 1.05xMn + 0.54xNi + 0.66x (Mo + W / 2) + 0.5xCr + K ≥ 1.6 with
K = 0 if the steel contains less than 0.0005% boron resulting from residual impurities,
K = 0.5 if the steel contains more than 0.0005% boron
steel having a martensitic or martensite-bainitic structure, said structure containing between 5% and 15% austenite, and a hardness between 400HB and 500HB.

Preferably, the chemical composition of the steel comprises: 0.24% ≤C ≤0.27% 0% ≤Si ≤1% 0% ≤Al ≤1% 0.3% ≤Mn ≤1.6% 0% ≤Ni ≤2% 0.5% ≤Cr ≤1.8% and satisfies relationships: 0.15% ≤Mo + W / 2 ≤0.45% and 0.6% ≤ Si + Al ≤1%

More preferably, the chemical composition may further satisfy the relationship: 4.6 x C + 1.05 x Mn + 0.54 x Ni + 0.66 x (Mo + W / 2) + 0.5 x Cr + K ≤ 3.7
with:
K = 0 if the steel contains less than 0.0005% boron resulting from residual impurities
K = 0.5 if the steel contains more than 0.0005% boron.

• on approvisionne une pièce en acier conforme à l'invention,
• on austénitise la pièce par chauffage au dessus de Ac₃, puis on la refroidit jusqu'à la température ambiante de telle sorte que, en tout point de la pièce, la vitesse de refroidissement entre la température d'austénitisation et 450°C soit supérieure à 1°C/s, et le temps de passage de la température de 450°C à la température de 200°C soit comprise entre 50s et 60mn, et de préférence comprise entre 100s et 30mn,
• éventuellement, on effectue un revenu à température inférieure à 250°C pendant un temps inférieur à 3 heures.

The invention also relates to a method of manufacturing a high-abrasion steel part according to which:

• supplying a steel part according to the invention,
• the room is austenitized by heating above Ac ₃ , then it is cooled to room temperature so that, at any point in the room, the cooling rate between the austenitization temperature and 450 ° C. is greater than at 1 ° C./s, and the time of passage from the temperature of 450 ° C. to the temperature of 200 ° C. is between 50s and 60 min, and preferably between 100 s and 30 min,
• optionally, an income is made at a temperature below 250 ° C for a time less than 3 hours.

To cool the room to room temperature, since austenitization temperature, it is possible to soak the piece with oil, it is particularly the case when the piece is a sheet of thickness between 10mm and 100mm.

To cool the room to room temperature, since austenitization temperature, it is also possible to soak the piece in the air. It is especially the case when the piece is a sheet of thickness between 2mm and 20mm.

The invention finally relates to an abrasion resistant part made of steel according to the invention having a martensitic structure or martensito-bainitic containing between 5% and 15% of austenite, and having a hardness between 400HB and 500HB.

Parts according to the invention, or manufactured by the process according to the invention, can be used for the manufacture of parts wear and tear for equipment intended, in particular, for the operation of quarries and mines, public works, cement works, iron and steel to the tileries, brickworks or agriculture.

The invention will now be described in more detail but non-limiting way.

• plus de 0,24% de carbone pour permettre d'obtenir une dureté suffisante, nécessaire à une bonne résistance à l'abrasion, mais moins de 0,3% et, de préférence, moins de 0,27% pour obtenir une bonne soudabilité, une bonne découpabilité, une bonne aptitude au pliage et une ténacité satisfaisante ;
• de 0% à 2% de silicium et de 0% à 2% d'aluminium, la somme des teneurs en aluminium et en silicium étant supérieure à 0,6% pour ralentir la précipitation des carbures au cours du traitement thermique, mais inférieure à 2%, et de préférence, inférieure à 1% afin de faciliter l'élaboration de l'acier et de ne pas détériorer sa ténacité ;
• de 0% à 2% et, de préférence de 0,3% à 1,6% de manganèse, de 0% à 4% et, de préférence de 0% à 2% de nickel, de 0% à 3% et, de préférence de 0,5% à 1,8% de chrome, de 0% à 0,6% de molybdène, de 0% à 1,2% de tungstène, la somme de la teneur en molybdène et de la moitié de la teneur en tungstène étant de préférence comprise entre 0,15% et 0,45%, de façon à obtenir une trempabilité suffisante, mais pas trop importante, pour permettre d'obtenir une structure principalement martensitique ou martensito-bainitique contenant une proportion notable d'austénite retenue ; le chrome, le molybdène et le tungstène ayant, de plus, l'avantage de permettre la formation de carbures favorables à la résistance à l'abrasion;
• éventuellement, de 0,0005% à 0,005% de bore pour ajuster la trempabilité;
• éventuellement, au moins un élément pris parmi Nb, V, Zr et Ti pour obtenir un durcissement par précipitation qui ne détériore pas la soudabilité ;
• éventuellement, au moins un élément pris parmi Se, Te, Ca, Bi et Pb pour améliorer l'aptitude à l'usinage;

le reste étant du fer et des impuretés résultant de l'élaboration. The steel according to the invention contains, by weight:

• more than 0.24% carbon to provide sufficient hardness necessary for good abrasion resistance, but less than 0.3% and preferably less than 0.27% to obtain good weldability good cutability, good bendability and satisfactory toughness;
• from 0% to 2% of silicon and from 0% to 2% of aluminum, the sum of aluminum and silicon contents being greater than 0.6% to slow the precipitation of carbides during the heat treatment, but less than 2%, and preferably less than 1% in order to facilitate the development of the steel and not to deteriorate its toughness;
• from 0% to 2% and preferably from 0.3% to 1.6% of manganese, from 0% to 4% and, preferably from 0% to 2% of nickel, from 0% to 3% and, preferably from 0.5% to 1.8% of chromium, from 0% to 0.6% of molybdenum, from 0% to 1.2% of tungsten, the sum of the molybdenum content and from half of the tungsten content is preferably between 0.15% and 0.45%, so as to obtain sufficient quenching, but not too important, to obtain a predominantly martensitic or martensito-bainitic structure containing a significant proportion of retained austenite; chromium, molybdenum and tungsten further having the advantage of allowing the formation of carbides favorable to abrasion resistance;
• optionally from 0.0005% to 0.005% boron to adjust the quenchability;
• optionally, at least one element selected from Nb, V, Zr and Ti to obtain a precipitation hardening which does not deteriorate the weldability;
• optionally, at least one element selected from Se, Te, Ca, Bi and Pb to improve the machinability;

the rest being iron and impurities resulting from the elaboration.

For hardenability to be sufficient, it is preferable that the chemical composition of the steel satisfies the relationship: A = 4.6 x C + 1.05 x Mn + 0.54 x Ni + 0.66 x (Mo + W / 2) + 0.5 x Cr + K ≥ 1.6 with,
K = 0 if the steel contains less than 0.0005% of boron resulting from residual impurities, that is to say if there has been no addition of boron
K = 0.5 if the steel contains more than 0.0005% boron, that is, if boron has been added.

But, so that the hardenability is not too important not to deteriorate the processability, especially by welding or cutting thermal, it is also preferable, that A remains lower than or equal to 3.7.

This steel, because of its relatively low carbon content, has good weldability and good cutting ability by thermal means, but its resistance to abrasion depends on its micrographic structure and therefore the heat treatment to which it is subjected.

The inventors have found that when this steel has a structure consisting mainly of martensite or a mixture of martensite and bainite (martensito-bainitic structure) and from 5% to 15% retained austenite enriched in carbon, so as to have a hardness between 450HB and 500HB, its resistance to abrasion was very comparable to that of martensitic steels of hardness greater than 550HB and its ability to form by cold deformation plastic was much better. The good resistance to abrasion results from that, under the action of abrasive particles, the retained austenite is transformed locally in very hard martensite while benefiting from the ability to deforming the metal in a significant way. In addition, the presence of a fine dispersion of chromium and molybdenum carbides in the constituent martensitic improves wear resistance.

The inventors have also found that, to obtain this structure, it was necessary to austenitize the steel by heating above Ac ₃ , then cool to room temperature so that the cooling rate between the austenitization temperature and 450 ° C is greater than 1 ° C / s, and the time of passage of the temperature from 450 ° C to the temperature of 200 ° C is between 50s and 60mn, and preferably between 100s and 30mn. This heat treatment may optionally be supplemented by an income at a temperature below 250 ° C for a time of less than 3 hours.

In order to manufacture an abrasion-resistant part, a steel according to the invention is produced, it is cast in the form of a half product which is then shaped by hot plastic deformation, for example by rolling or forging. The part thus obtained is then austenitized by heating above Ac ₃ , and then cooled to room temperature so that, at any point, the cooling rate between the austenitization temperature and 450 ° C is greater than 1 ° C / s, and the time of passage of the temperature from 450 ° C to the temperature of 200 ° is between 50s and 60mn, and preferably between 100s and 30mn. Optionally, the part is subjected to an income at a temperature below 250 ° C for a time less than 3 hours.

The slow-speed traverse of the 450 ° C / 200 ° C range is intended to effect of allowing the retention of metastable austenite, while promoting the formation of fine chromium and molybdenum carbides, distributed homogeneous in the martensitic or martensito-bainitic component.

When the massiveness of the room lends itself, to cool the room up to room temperature, since the austenitization temperature, can soak the piece in oil. This is particularly the case when the room is a sheet of thickness between 10mm and 100mm.

In the same way, when the massiveness of the room lends itself to it, to cool the room to room temperature, since the temperature austenitization, it is also possible to soak the piece in the air. It is notably the case when the piece is a sheet of thickness between 2mm and 20mm.

This gives a piece, and in particular a sheet, resistant to abrasion, made of steel according to the invention having a structure martensitic or martensito-bainitic containing between 5% and 15% austenite, and having a hardness between 400HB and 500HB.

By way of example, sheets have been manufactured with steels A and B according to the invention, and with the steels C and D according to the prior art.

The compositions of these steels were, in thousandths of a% by weight: VS Yes al mn Or Cr MB B AT 247 817 63 1290 495 726 328 2.8 B 251 263 704 1305 439 715 342 2.6 VS 254 310 65 1329 445 702 351 2.6 D 415 307 62 1285 293 712 349 2.7

The characteristics of the sheets TA1, TA2, and TB, in accordance with the invention, and the plates TA3, TC and TD, given for comparison, were: sheet metal steel thickness. mm austenitization ° C temper income ° C hardness after HB treatment hardness in HB abraded layer abrasion resistance index TA1 AT 35 900 oil 200 463 525 97 TA2 AT 5 900 air 200 455 526 105 TB B 35 900 oil 200 466 529 102 TA3 AT 35 900 water 200 476 492 70 TC VS 35 900 oil 200 468 495 79 TD D 35 900 water 200 552 561 100

The sheet TD, according to the prior art, has a structure completely martensitic, a hardness higher than 550HB and an index of abrasion resistance of 100; but because of the carbon content of steel, it is difficult to weld.

The sheet TA3, made of steel according to the invention, has been soaked in water, which gives it a structure different from that which is required by the invention, and has an abrasion resistance index of 70, significantly lower than those of plates TA1, TA2 and TB in accordance with the invention, which are comparable to that of the reference sheet TD.

The plates TA1 and TB are also distinguished from the sheet metal TD by their folding ability; The plates TA1 and TB can be folded on a radius equal to 6 times the thickness, while the TD sheet can not be bent on a radius less than 15 times the thickness.

Due to their properties, parts in general, and sheet metal in particular, according to the invention, are particularly suitable for manufacture of any type of wear part subject to severe abrasion and incorporated, in particular, in equipment for handling products in bulk in all types of industries. By way of example, these parts can to be attack blades and reinforcement pads under blades of buckets of loaders / carriers or shovels, guide-chain plates excavators and draglines, racks, crowns of training, sprockets, side shields of crushers to percussion or jaws, sieve grids for use in the work public, in quarry or gravel pit development; funds and squeegees of chain conveyors, shields of hoppers or corridors, scales of helical descenders, combs of sludge packs of classifiers, parts of slaughter or transport machinery, cyclone skirts for mining or coal mining; of the shields of hoppers or skips, clamshell blades, funds of coke-cars, vibrating extractors, chamber armor shot blasting, guiding or shifting plates for the iron and steel industry; of the knives of disintegrators, blades and blender bottoms, funds mill extruders, molds, turbo-collector parts, dosing hoppers for tileries or brickworks; tools and teeth decompacting or desoldering, wood tongs, knives, choppers, beaters, hammers, smoothing plates, pieces of diggers or cleaners, log rails, subsoilers, plagues of dethatches for agriculture or work soil.

Claims (9)

1. Steel for the fabrication of pieces having a high resistance to wear, characterized in that its chemical composition comprises by weight: 0.24% ≤C ≤0.3% 0% ≤Si ≤2% 0% ≤Al ≤2% 0% ≤Mn ≤2% 0% ≤Ni ≤4% 0% ≤Cr ≤3% 0% ≤Mo ≤0.6% 0% ≤W ≤1.2% possibly from 0.0005 % to 0.005 % boron in order to adjust hardenability, possibly at least one element selected from the group comprising Nb, V, Zr and Ti in concentration of less than 0.3 %, possibly at least one element selected from the group comprising Se, Te, Ca, Bi and Pb in concentrations of less than 0.1 %, the rest being of iron and impurities resulting from production,
   the chemical composition satisfying in addition the following relations: 0.6% ≤ Al + Si ≤2% and, in order to ensure sufficient hardenability, 4.6 x C + 1.05 x Mn + 0.54 x Ni + 0.66 x (Mo + W/2) + 0.5 x Cr + K ≥ 1.6 with
   k = 0 if the steel contains less than 0.0005 % of boron resulting from residual impurities,
   K = 0.5 if the steel contains more than 0.0005 % boron
   the steel having a martensitic structure or martensitic-baintitic, said structure containing between 5 % and 15 % austenite and a hardness of between 400 HB and 500 HB.
2. Steel according to Claim 1, wherein its chemical composition comprises by weight: 0.24% ≤C ≤0.27% 0% ≤Si ≤1% 0% ≤Al ≤1% 0.3% ≤Mn ≤1.6% 0% ≤Ni ≤2% 0.5% ≤ Cr ≤1.8% its chemical composition in addition satisfying the relations: 0.15% ≤ Mo + W/2 ≤ 0.45 % 0.6% ≤ Si + Al ≤1%
3. Steel according to Claim 1 or Claim 2, wherein its chemical composition satisfies, in addition, the relation: 4.6 x C + 1.05 x Mn + 0.54 x Ni + 0.66 x (Mo + W/2) + 0.5 x Cr + K ≤3.7 with:
   K = 0 if the steel contains less than 0.0005 % boron resulting from residual impurities,
   K = 0.5 if the steel contains more than 0.0005 % boron.
4. A method for manufacturing a part made of steel that is highly resistant to wear, characterized in that:

   a steel part is provided whose chemical composition is defined by any one of Claims 1 to 3;

   the part is austentizcd by heat below Ac₃ then cooled to room temperature such that at every point on the part, the cooling rate between the austenitizing temperature and 450°C is higher than 1°C/s and the passage time from the temperature of 450°C to a temperature of 200 °C is between 50 seconds and 60 minutes and preferably between 100 seconds and 30 minutes;

   tempering at a temperature of less than 250°C may be done over a time of less than 3 hours.
5. The method according to Claim 4, characterized in that for cooling the part to ambient temperature from the temperature of austenitization the part is oil quenched.
6. The method according to Claim 5, wherein the part is a sheet having a thickness of between 10 mm and 100 mm.
7. The method according to Claim 4, wherein, in order to cool the part to ambient temperature form the of austenitization the part is air quenched.
8. The method according to Claim 7, wherein the part is a sheet having a thickness of between 2 mm and 20 mm.
9. A wear-resistant part, characterized in that it is comprised of a steel according to one of Claims 1 to 3.