Steel having excellent properties of workability by machine tools and, after a hardening thermal treatment, excellent mechanical properties and process for the production thereof

Abstract

 A steel having excellent workability properties by machine-tools and, after a hardening thermal treatment, excellent mechanical properties and almost non existent size variations and internal tensions, comprising an amount lower than 0.25% by weight of carbon; 2,5 to 4,5% by weight of nickel; 2,5 to 4,55 by weight of molybdenum; an amount lower than 0.25% by weight of vanadium and amounts comprised between 0 and 1,1% by weight of at least one of the elements selected from manganese, silicon and chromium; the remaining up to 100 being iron, impurities excluded.

Description

[0001] The present invention relates to a steel having excellent properties of workability by machine-tools and, after a hardening thermal treatment, excellent mechanical properties and almost non existent size variations and internal tensions, and process for the production thereof.

[0002] More particularly, the present invention relates to a steel suitable for being used in the production of large size mold units for molding plastic materials, having, after a hardening thermal treatment, excellent mechanical characteristics, such that tensile strength (Rm) and ultimate yielding load (Rs) have a Rs/Rm ratio of about 0,9, ultimate tensile elongation, and necking down and contraction coefficient (Z); a Brinell hardness (HB) comprised between 340 and 450, and almost non existent size variations and internal tensions.

[0003] The present invention also relates to the process for the production of said steel.

[0004] As is known, in molding plastic materials, steel mold. units are utilized that have been already hardened and tempered for a Brinell hardness (HB) of 310-340 on the surface, generally of rather large sizes, as for instance 1500 mm x 2000 mm of required length.

[0005] Such units are submitted to mechanical workings for the excavation of the shape necessary to obtain the relevant matrix or punch.

[0006] Because of obvious reasons of hardenability of the material, its values of hardness and, therefore, its mechanical characteristics decrease according to the depth of the excavation, to the extent that molds having deep excavations need, at the end of the working, to be submitted to a thermal treatment, not to jeopardize the work life of said mold and the related total life cycle cost.

[0007] As is known, thermal re-treatment carried out on pre-worked artifacts is quite a risky operation, as it requires the carrying out of a hardening treatment at a temperature of about 850°C in oil-water, followed by a subsequent treatment of tempering, to obtain the surface hardness desired.

[0008] As the artifact has very different sections, the tempering treatment involves different cooling gradients and therefore very high internal tensile stresses, due to changes in the phases of the material on different times. Such internal stresses lead often to the break of the artifact, with remarkable economic and logistic damages due to the costs of the material, the mechanical works carried on and the subsequent thermal treatment which the artifact has been submitted to.

[0009] Object of the present invention is to obviate the above drawbacks.

[0010] More particularly, object of the present invention is to provide a steel suitable for the production of large size molds, needing deep mechanical work excavation.

[0011] A further object of the present invention is to provide a steel suitable for the production of large size molds for molding plastic materials that need deep mechanical work excavations essentially free from internal tensile stresses.

[0012] Still another object of the present invention is to provide a steel suitable for the production of large size molds for molding plastic materials, having excellent mechanical features, a HB hardness suitable to be easily workable by machine tools and to be brought to its desired end form, and size variations and internal stresses almost non existent after the hardening thermal treatment.

[0013] In its more general aspect, the present invention allows to obtain these and still other objects that will be highlighted by the following description by means of a steel having a carbon content not higher than 0.25% by weight and a content of nickel an molybdenum higher that 2% by weight for each element.

[0014] Small amounts of at least a trace element selected from among manganese, silicon and chrome may be added. The contemporary presence of at least one of such trace element, in well defined ratios between each other and with respect to the other elements contained in steel, permits to realize a synergetic action that allow to improve toughness, tensile strength (Rm), Rs/Rm ratio and hardening.

[0015] Therefore, object of the present invention is a steel having excellent mechanical properties and of workability on machine tools and size variations and internal tension almost non existent, after a thermal hardening treatment, containing an amount lower than 0,25% by weight of carbon; 2.5 to 4.5 % by weight of nickel; 2.5 to 4.5% of molybdenum; an amount lower than 0.25% by weight of vanadium, and amounts comprised between 0 and 1.1% by weight of at least one of the elements selected from manganese, silicon and chromium; the remaining up to 100 being iron, impurities excluded.

[0016] Preferably, the steel of the present invention comprises:

Carbon	0.05 to 0.25% by weight;
Nickel	2.50 to 4.50 by weight;
Molybdenum	2.50 to 4.50% by weight;
Vanadium	0.05 to 0.25% by weight;
Manganese	0.10 to 1.10% by weight;
Silicon	0.10 to 1,10% by weight;
Chrome	0.10 to 1.10% by weight;

the remaining up to 100 being iron, impurities excluded.

[0017] Still more preferably, the steel of the present invention comprises:

Carbon	0.10 to 0.20% by weight;
Nickel	2.50 to 3.50 by weight;
Molybdenum	2.50 to 3.50% by weight;
Vanadium	0.07 to 0.10 % by weight;
Manganese	0.50 to 1.00% by weight;
Silicon	0.20 to 0,30% by weight;
Chrome	0.15 to 0.30% by weight;

the remaining up to 100 being iron, impurities excluded.

[0018] The steel of the present invention is characterized by a hardening mechanism without phase transformation with an optimum calibration of properties, based on treatment temperature; an optimum workability of the solute and a good workability of the finished product; a greater total life cycle cost, and optimum polishing and photo-cutting characteristics.

[0019] Thanks to these characteristics, it is particularly suitable for use in the production of large molds for plastic materials, wherein high mechanical characteristics extremely uniform throughout the artifact are required.

[0020] The steel of the present invention having the above reported composition is first submitted to a melting treatment in order to remove possible structural alterations due to fabrication processes. Such treatment includes heating steel to temperatures comprised between 870 and 900°C, with a stay time at such temperatures necessary to melt possible constituents, preferably 2-4 hours, and subsequent quenching thereof.

[0021] The steel, that is supplied either in the molten state or the state of forged mold blocks, after the melting treatment at a temperature comprised between 870 and 900°C, is submitted to a tempering thermal treatment, to attenuate the internal tensions caused by quenching and to bring Brinell hardness value to an interval comprised between 300 and 340 HB, so as to become easily workable with machine tools, and take on the final form desired. Such tempering thermal treatment is carried out by heating to about 400°C and subsequent staying at such temperature for 10-14 hours, followed by a spontaneous air cooling until the ambient temperature is reached.

[0022] After the mechanical working according to the design, the artifact is submitted to a thermal hardening treatment at temperatures comprised between 520 and 600°C and subsequent staying at such temperatures for a period of time comprised between 5 and 15 hours, followed by a spontaneous air cooling that increases both hardness and mechanical characteristics, ensuring high toughness and almost non existent size variations and internal tensions.

[0023] In order to better clarify the present invention, and to put the same in practice, the following example is reported by way of illustrating, exemplifying but non-limiting example.

[0024] A steel was prepared having the following composition:

Carbon	0.15 % by weight;
Nickel	3% by weight;
Molybdenum	3% by weight;
Vanadium	0.08 % by weight;
Manganese	0.70% by weight;
Silicon	0,25 % by weight;

Chrome

0.20% by weight;

the remaining up to 100 being iron, impurities excluded.

[0025] Steel was first submitted to a melting treatment by heating to temperatures of 880-890°C for 3 hours and subsequent quenching, followed by a tempering at a temperature of about 400°C to obtain a Brinell hardness (HB) of 320.

[0026] With such steel a cubic molded block was obtained, that had a side of 200 mm, and in its inside a cubic cavity was obtained having a side of 100 mm, centered on a face.

[0027] The mold was hardened by treating it at 520-600°C for 10 hours, followed by a spontaneous cooling at ambient temperature.

[0028] After such treatment, the mod has the following characteristics depending on the treatment temperature:

PROPERTY	STANDARD	UNIT	VALUES
Ultimate yielding load (Rs)	UNI EN 10002	MPa	820-1200
Ultimate load (Rm)	UNI EN 10002	MPa	1200-1400
Ultimate elongation (A)	UNI EN 10002	%	≥ 15
Necking down coefficient (Z)	UNI EN 10002	%	≥ 45
Brinell hardness (HB)	UNI EN 10003		360-450
Rockwell hardness (HRC)	UNI EN 10109		37-45
Charpy-V strength	UNI EN 10045	Joule	≥ 25

Dimensional variations:
External sides	0.11% mean increase
Internal sides	0.07 % mean increase
Thickness	0.14% mean increase
Height	0.16% mean increase
Mold plane	0.11% mean increase
Depth	0,2% mean side increase

[0029] The mold was molten again to bring it to the original Brinell hardness of 320 and an opening was obtained by milling that had a depth of 80 mm and a width of 34 mm, on each side, in correspondence of the smaller thickness part, as shown in the attached figure which shows a perspective view of the mold.

[0030] After a hardening treatment carried out on the same conditions as above, the following size variations were ascertained:

External sides (A)	0.03 % mean increase
Internal sides (B)	0.26 % mean increase
Aperture width (C)	0,11% mean restriction
Thickness (D)	0.22 % mean increase
Height (E)	0.03 % mean increase
Mold plane (F)	0.24 % mean increase
Depth (G)	0,13 % mean side increase

Claims

1. A steel having excellent workability properties by machine-tools, and after a hardening thermal treatment, excellent mechanical. properties and almost non existent size variations and internal tensions, of the type comprising C-Ni-Mo, characterized in that carbon content is not higher than 0.25% by weight and that of nickel and molybdenum is higher than 2% by weight for each element.

2. The steel according to claim 1, characterized in that it also comprises small amounts of at least another trace element selected from among manganese, silicon and chrome.

3. The steel according to claim 1 or 2, characterized in that it comprises an amount lower than 0.25% by weight of carbon; 2.5 to 4.5% by weight of nickel; 2.5 to 4.5% by weight of molybdenum; an amount lower. than 0.25% by weight of vanadium and amounts comprised between 0 and 1.1% of at least one of the elements selected among manganese, silicon and chrome; the remaining up to 100 being iron, impurities excluded.

4. The steel according to any of the preceding claims, characterized in that it comprises:

Carbon	0.05 to 0.25% by weight;
Nickel	2.50 to 4.50 by weight;
Molybdenum	2.50 to 4.50% by weight;
Vanadium	0.05 to 0.25% by weight;
Manganese	0.10 to 1.10% by weight;
Silicon	0.10 to 1,10% by weight;
Chrome	0.10 to 1.10% by weight;

the remaining up to 100 being iron, impurities excluded.

5. The steel according to any of the preceding claims, characterized in that it comprises:

Carbon	0.10 to 0.20% by weight;
Nickel	2.50 to 3.50 by weight;
Molybdenum	2.50 to 3.50% by weight;
Vanadium	0.07 to 0.10 % by weight;
Manganese	0.50 to 1.00% by weight;
Silicon	0.20 to 0,30% by weight;
Chrome	0.15 to 0.30% by weight;

the remaining up to 100 being iron, impurities excluded.

6. Use of the steel according to any of the preceding claims for the production of large-size mold units for molding plastic materials.

7. A process for the production of a mold unit for molding plastic materials, characterized in that it comprises:

• submitting steel of any of the preceding claims 1-5 to a thermal melting treatment at temperatures comprised between 870 and 900°C, followed by a thermal tempering treatment at about 400°C, to obtain Brinell hardness values (HB) comprised between 300 and 340;

• forming the mold by mechanical working; and

• submitting the formed mold to a hardening thermal treatment at temperatures comprised between 520 and 600°C followed by a spontaneous ambient temperature cooling, to obtain an increase in both hardness and mechanical characteristics, such as high toughness and almost non existent size variations and internal tensions.

8. The process according to claim 7, characterized in that the melting thermal treatment includes heating at temperatures comprised between 870 and 900°C, a staying period at such temperatures of 2-4 hours, followed by quenching.

9. The process according to claims 7 and 8, characterized in that the tempering thermal treatment includes heating up to about 400°C, a staying period at such temperature of about 10-14 hours, and spontaneous air cooling at ambient temperature.

10. The process according to claim 7, 8 or 9, characterized in that the hardening treatment is carried out by heating at temperatures comprised between 520 - 600°C, a staying period of 5-15 hours at such temperature, and spontaneous air cooling at ambient temperature.

Drawing