Steels for tyre rims and monobloc tyres for railways and the like

Description

[0001] This invention relates to two types of steel, utilizable the one for the fabrication of tyre rims, the other one for the fabrication of monobloc tyres for railways, tramways and underground railways.

[0002] It is known that one of the more critical problems in the field of rolling stock is constituted by the phenomenon of tyre fluting, which is a phenomenon of local deterioration of the rolling surface of rim, which, following localized wear, transforms in time its circular starting profile into a multi-side polygonal profile (examples 14-20).

[0003] During the running of the vehicle, such deterioration becomes a source of noise and, in order to restrain the sound emission within the limits provided for by the law, it is necessary to perform frequent profile grindings on the traditional materials used at present, in order to restore the original circular profile, which involves high costs and consumptions.

[0004] Object of this invention is to obviate the aforementioned drawbacks.

[0005] In its more general aspect, this invention allows to reach this object through the utilization, for the fabrication of rims for railway tyres and monobloc railway tyres, of two types of C-Mn-Mo steels in which small amounts of elements (oligoelements) have been added comprising B, Zr, Nb, V and Al. The contemporaneous presence of such oligoelements, in well defined ratios relatively to one another and to the other elements comprised in the steels, allows to realize a synergetical activity which allows to obtain a good hardening, necessary to reach a remarkable hardness and wear resistance, higher than those of the traditional steels used at present for railway tyres, preserving at the same time toughness.

[0006] The compositions of such C-Mn-Mo micro-alloyed steels, the one being preferably suitable for the fabrication of railway tyre rims, the other one being preferibly suitable for the fabrication of monobloc railway rims, are different only as concerns the C content, which is 0.51-0.53% in the first case and 0.43-0.45% in the second case.

[0007] Such compositions are shown in the following table, where, besides oligoelements, also all the other elements comprised are indicated, stressing for some of them the extreme minimum and maximum percent contents foreseen.

MICROALLOYED STEEL FOR TYRE RIMS		MICROALLOYED STEEL FOR MONOBLOC TYRES
C	0.51 - 0.53	0.43 - 0.45
S	≦0.005	≦0.005
P	≦0.015	≦0.015
Mn	0.73 - 0.78	0.73 - 0.78
Cr	≦0.10	≦0.10
Ni	0.20 - 0.25	0.20 - 0.25
Mo	0.40 - 0.45	0.40 - 0.45
Cu	≦0.25	≦0.25
Si	0.30 - 0.35	0.30 - 0.35
Sn	≦0.020	≦0.020
As	≦0.025	≦0.025
Ti	≦0.010	≦0.010
B	0.00100 - 0.0018	0.0010 - 0.0018
Zr	0.015 - 0.020	0.015 - 0.020
Nb	0.025 - 0.030	0.025 - 0.030
V	0.03 - 0.05	0.03 - 0.05

[0008] In the case of the micro-alloyed steel for railway tyre rims, the following mechanical properties are ensured which refer to two different steel grades, obtained with two different conditions of heat treatment and which allow to satisfy different application fields.

	N grade steel	T grade steel
Ultimate tensile strength (N/mm2)	820.920	920-1040
Yield point (N/mm2)	≧550	≧700
Elongation (%)	≧14	≧14
Striction (%)	≧35	≧35
CHARPY U resilience temp. = 20°C (Joule)	≧15	≧20

[0009] N grade steel undergoes a NORMALIZATION by heating at 870-900°C and following maintenance for 2-4 hours at such temperature, followed by spontaneous air cooling up to room temperature. T grade steel undergoes instead HARDENING AND TEMPERING comprising:

• HARDENING through heating at 870-900°C, and following maintenance for 2-4 hours at such temperature, followed by water cooling up to room temperature;
• TEMPERING through heating at 580-620°C, and following maintenance for 10-14 hours at such temperature, followed by spontaneous air cooling up to room temperature.

[0010] The high temperature of the T grade ensures that high toughnesses are reached and reduces the problems of surface hardness decline during shoe braking. The excellent hardening capacity of the material ensures also limited hardness declines in the section, promoting resistance to wear throughout the tyre rim life.

[0011] Both for N grade steel and T grade steel, the treatment methods are so regulated as to guarantee a good homogeneity of the hardness of the rolling surface.

[0012] Instead, in the case of micro-alloyed steel for monobloc railway tyres, the following mechanical properties are ensured in the tyre plate and the rim crown.

	Side plate	Rim crown
Ultimate tensile strength (N/mm2)	≦850	940-1020
Yield point (N/mm2)		≧700
Elongation (%)	≧15	≧14
Striction (%)	≧45	≧40
CHARPY U resilience temp. = 20°C (Joule)		≧20

[0013] Said monobloc tyres undergo a differential hardening treatment in a suitable plant having known characteristics. Such heat treatment comprises the following steps:

• NORMALIZATION by heating at 870-930°C and following maintenance for 2-4 hours at such temperature, followed by spontaneous air cooling up to room temperature.
• ROUGH MACHINING;
• AUSTENIZITING by heating at 870-900°C and following maintenance for 2-4 hours at such temperature, followed by cooling in the differential hardening plant;
• TEMPERING through heating at 580-540°C, and following maintenance for 8-12 hours at such temperature, followed by spontaneous air cooling up to room temperature.

[0014] The tempering temperature of 500-540°C ensure, also in this case, that high toughnesses are reached and reduces the problems of surface hardness decline during shoe braking.

[0015] Also the conditions that promote wear resistance throughout the life of the monobloc tyre and the good homogeneity of the rolling surface hardness are ensured.

Claims

1. Steels for tyre rims and monobloc tyres for railways, tramways and underground railways, characterized in that they comprise two types of C-Mn-Co steels micro-alloyed with B, Zr, Nb, V and Al.

2. Steels according to claim 1, characterized in that they have the following percent chemical composition:

MICROALLOYED STEEL FOR TYRE RIMS		MICROALLOYED STEEL FOR MONOBLOC TYRES
C	0.51 - 0.53	0.43 - 0.45
S	≦0.005	≦0.005
P	≦0.015	≦0.015
Mn	0.73 - 0.78	0.73 - 0.78
Cr	≦0.10	≦0.10
Ni	0.20 - 0.25	0.20 - 0.25
Mo	0.40 - 0.45	0.40 - 0.45
Cu	≦0.25	≦0.25
Si	0.30 - 0.35	0.30 - 0.35
Sn	≦0.020	≦0.020
As	≦0.025	≦0.025
Ti	≦0.010	≦0.010
B	0.00100 - 0.0018	0.0010 - 0.0018
Zr	0.015 - 0.020	0.015 - 0.020
Nb	0.025 - 0.030	0.025 - 0.030
V	0.03 - 0.05	0.03 - 0.05

3. Steel according to claims 1 and 2, characterized in that it has, depending on the treatment undergone, two finishing grades, N grade and T grade, having the following mechanical properties:

	N grade steel	T grade steel
Ultimate tensile strength (N/mm2)	820.920	920-1040
Yield point (N/mm2)	≧550	≧700
Elongation (%)	≧14	≧14
Striction (%)	≧35	≧35
CHARPY U resilience temp. = 20°C (Joule)	≧15	≧20

4. Steel for monobloc tyres according to claims 1 and 2, characterized in that it has in the side plate and the rim crown the following mechanical properties:

	Side plate	Rim crown
Ultimate tensile strength (N/mm2)	≦850	940-1020
Yield point (N/mm2)		≧700
Elongation (%)	≧15	≧14
Striction (%)	≧45	≧40
CHARPY U resilience temp. = 20°C (Joule)		≧20

5. Steel for tyre rims according to claim 4, characterized in that said mechanical properties of N grade steel are reached through NORMALIZATION by heating at 870-900°C and following maintenance for 2-4 hours at such temperature, followed by spontaneous air cooling up to room temperature, and in that said mechanical properties of T grade steel are reached through HARDENING AND TEMPERING comprising HARDENING through heating at 870-900°C, and following maintenance for 2-4 hours at such temperature, followed by spontaneous water cooling up to room temperature; and TEMPERING through heating at 580-620°C, and following maintenance for 10-14 hours at such temperature, followed by spontaneous air cooling up to room temperature.

6. Steel for monobloc tyres according to claim 4, characterized in that said mechanical properties of the side plate and the rim crown are reached through a heat treatment comprising the following steps:

- NORMALIZATION by heating at 870-930°C and following maintenance for 2-4 hours at such temperature, followed by spontaneous air cooling up to room temperature.

- ROUGH MACHINING;

- AUSTENIZITING by heating at 870-900°C and following maintenance for 2-4 hours at such temperature, followed by cooling in the differential hardening plant;

- TEMPERING through heating at 580-540°C, and following maintenance for 8-12 hours at such temperature, followed by spontaneous air cooling up to room temperature.