A microalloy bainitic steel with high resistance to fatigue and to fretting fatigue

Description

Field of the invention

[0001] The present invention refers to a microalloy bainitic steel with high resistance to fatigue and to fretting fatigue.

[0002] More particularly, the present invention refers to a microalloy bainitic steel with high resistance to fatigue and to fretting fatigue being particularly suitable to be used for the manufacturing of wheels for railway, tramway and/or metro fields.

Description of the prior art

[0003] It is known that the wheel is one of the most important components making the rolling stock. During the running of the vehicle, the wheel is submitted to complex phenomena such as ratchetting, fretting fatigue wear friction, etc. intervening on the wheel/rail interface. In fact, during the wheel/rail contact on a thin layer of material placed immediately under the contact area, great stresses occur due to the severe load conditions. These stresses give rise both to the first stages of fretting fatigue failures and to wearing.

[0004] Fatigue failures are very dangerous as they can cause the detachment of small portions of wheel, with serious consequences both on suspensions, on the axle-box bearings of the vehicle and on the railway infrastructure.

[0005] According to the working conditions, fretting fatigue failures can be spallings due to ratchetting phenomena or to heat loads or spallings due to shellings or deep shellings.

[0006] In the first case, spallings are induced by cold ratchetting. It is known that the wheel/rail contact develops in cyclic elastic-plastic conditions with localized ratchetting and the sum of ratchetting deriving from various cycles may give rise to local failures for damage accumulation.

[0007] In the second case, spallings are due to the detachment of small portions of wheels with cratering forming on the rolling surface in conditions of high axle loads or in the presence of macrodefects at depths comprised between 6 and 30 mm under the rolling surface.

[0008] Steels with high resistance levels are known on the market, they successfully remove the drawbacks due to wearing phenomena, but the problems connected to fretting fatigue failures which constitute an ever higher percentage of the damage phenomena of wheels and rails are still present.

Summary of the invention

[0009] The object of the present invention is to remove the above-mentioned drawbacks.

[0010] More particularly, the object of the present invention is to provide a steel suitable for the manufacturing of wheels for the railway, tramway and/or metro fields having a high resistance to fretting fatigue failures.

[0011] A further object of the present invention is to provide a steel suitable for the manufacturing of wheels for the railway, tramway and/or metro fields having high mechanical resistance and toughness features combined with high resistance features to wearing and fretting fatigue.

[0012] According to the present invention these and other objects, that will be clear from the following description, are obtained by a steel comprising:

• C in quantities comprised between 0.15 and 0.35% by weight for each element;
• Si and Mn in quantities comprised between 0.50 and 1.50% by weight for each element;
• Cr, Ni and Mo in quantities comprised between 0.10 and 1.00% by weight for each element, and
• small quantities of trace elements with microalloying properties.

[0013] The presence of these trace elements in well defined ratios among them and compared with the other steel elements helps the steel hardening and protects its toughness. These trace elements with microalloying properties are Nb, Zr, Al and Ti in quantities not higher than 0.06% by weight and B, Ca and N in quantities not higher than 0.007% by weight.

[0014] Therefore, the object of the present invention is a microalloy bainitic steel with high resistance to fatigue and to fretting fatigue containing:

• C in quantities comprised between 0.15 and 0.35% by weight;
• Si and Mn in quantities comprised between 0.50 and 1.50% by weight for each element;
• Cr, Ni and Mo in quantities comprised between 0.10 and 1.00% by weight for each element;
• Nb, Zr, Al and Ti in quantities not higher than 0.06% by weight for each element and,
• B, Ca and N in quantities not higher than 0.007% by weight for each element;
• The remaining part up to 100 being Fe without impurities.

[0015] The steel of the present invention preferably comprises:

Carbon	0.15 - 0.35% by weight
Silicon	0.60 - 1.20% by weight
Manganese	0.60 - 1.50% by weight
Chromium	0.10-1.00% by weight
Nickel	0.10 - 1.00% by weight
Molybdenum	0.10 - 0.90% by weight
Vanadium	0.01 - 0.15% by weight
Niobium	0.01 - 0.05% by weight
Zirconium	0.005 - 0.06% by weight
Aluminium	0.005 - 0.06% by weight
Titanium	0.005 - 0.06% by weight
Boron	0.0005 - 0.005% by weight
Calcium	0.0002 - 0.005% by weight
Nitrogen	0.003 - 0.007% by weight

[0016] The remaining part up to 100 being iron without impurities.

[0017] After the mechanical machining of the steel of the present invention for the manufacturing of wheels for the railway, tramway and/or metro fields, the product is submitted to austenitization and quenching in water. This process consists in the heating at a temperature comprised between 850 and 950° C, a stay at this temperature for one hour for about each 40 mm of thickness of the processed product and in a quick cooling in water until room temperature is reached.

[0018] The product is then submitted to two thermal tempering processes in order to attenuate internal stresses caused by the abrupt cooling.

[0019] The first thermal tempering process is carried out by heating at a variable temperature comprised between 500 and 700°C according to the required features for a stay time at this temperature of about one hour for each 20 mm of thickness of the processed product, followed by a spontaneous air cooling until room temperature is reached.

[0020] The second thermal tempering process is carried out by heating at a temperature lower than 20°C compared to the one of the first thermal tempering process for a stay time at this temperature of about one hour for each 20 mm of thickness of the processed product followed by a spontaneous air cooling for at least 25 minutes and in any case being such to cool the rail wheel rolling until room temperature is reached.

[0021] The cooling of the rail wheel rolling can be carried out at a different speed from the one used for the cooling of the web in order to give different properties to each part of the wheel.

[0022] The steel of the present invention is characterized by an excellent balancing between tensile and toughness features. Toughness values are very high at any temperature level.

[0023] A clear demonstration of said feature balancing is due to the fact that the ductile/brittle transition temperature, defined by the F.A.T.T. value corresponding to 50% is placed at - 40°C even though the tensile stress values are very high.

[0024] In order to better understand the present invention and put it into practice the following illustrative and non limitative example is given.

Example

[0025] Some castings have been manufactured according to the following objective analysis:

Carbon	0.27 - 0.32% by weight
Silicon	0.60 - 0.80% by weight
Manganese	0.90 - 1.10% by weight
Chromium	0.20 - 0.50% by weight
Nickel	0.20 - 0.50% by weight
Molybdenum	0.20 - 0.50% by weight
Vanadium	0.020 - 0.050% by weight
Niobium	0.020 - 0.040% by weight
Zirconium	0.015 - 0.045% by weight
Aluminium	0.015 -0.055% by weight
Titanium	0.010 - 0.040% by weight
Boron	0.0010 - 0.040% by weight
Calcium	0.002 - 0.003% by weight
Nitrogen	0.003 - 0.004% by weight

[0026] The remaining part up to 100 being iron without impurities.

[0027] With the steels thus obtained some rail wheels have been manufactured according to the prior arts each of them having a rolling diameter of 915 mm.

[0028] Each wheel has been submitted to austenitization and quenching by heating at a temperature of about 900°C for a stay time at this temperature of about 3 hours and a quick cooling in water until room temperature was reached.

[0029] Each wheel was then submitted to two subsequent thermal tempering processes the first of which was carried out by heating at a temperature of about 520°C - 600°C for a stay time at this temperature of about six hours and a spontaneous air cooling until room temperature was reached; the second one was carried out by heating at a temperature of 20°C lower than the one of the first process for a stay time at such temperature of about six hours and a spontaneous air cooling for 60 minutes until room temperature was reached.

[0030] The rail wheels thus obtained had the following average mechanical features:

ROLLING SURFACE
PROPERTIES	STANDARD	UNIT	VALUE
Monotonic yield	UNI EN10002	Mpa	1,090
Tensile stress	UNI EN10002	Mpa	1,210
Elongation at break	UNI EN10002	%	16
Reduction coeff.	UNI EN 10002	%	55

WEB
PROPERTIES	STANDARD	UNIT	VALUE
Monotonic yield	UNI EN10002	Mpa	1,090
Tensile stress	UNI EN10002	Mpa	1,210
Elongation at break	UNI EN10002	%	16
Reduction coeff.	UNI EN10002	%	55

Claims

1. A microalloy bainitic steel with high resistance to fatigue and to fretting fatigue comprising:

- C in quantities comprised between 0.15 and 0.35% by weight

- Si and Mn in quantities comprised between 0.50 and 1.50% by weight for each element;

- Cr, Ni and Mo in quantities comprised between 0.10 and 1.00% by weight for each element and,

- small quantities of trace elements with microalloying properties.

2. The steel according to claim 1, characterized in that the trace elements with microalloying properties are Nb, Zr, Al and Ti in quantities not higher than 0.06% by weight and B, Ca and N in quantities not higher than 0.007% by weight.

3. The steel according to claims 1 or 2 characterized in that it contains:

- C in quantities comprised between 0.15 and 0.35% by weight

- Si and Mn in quantities comprised between 0.50 and 1.50% by weight for each element;

- Cr, Ni and Mo in quantities comprised between 0.10 and 1.00% by weight for each element;

- Nb, Zr, Al and Ti in quantities not higher than 0.06% by weight for each element and,

- B, Ca and N in quantities not higher than 0.007% by weight for each element;

- the remaining part up to 100 being Fe without impurities.

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

Carbon	0.15 - 0.35% by weight
Silicon	0.60 - 1.20% by weight
Manganese	0.60 - 1.50% by weight
Chromium	0.10-1.00% by weight
Nickel	0.10 - 1.00% by weight
Molybdenum	0.10 - 0.90% by weight
Vanadium	0.01 - 0.15% by weight
Niobium	0.01 - 0.05% by weight
Zirconium	0.005 - 0.06% by weight
Aluminium	0.005 - 0.06% by weight
Titanium	0.005 - 0.06% by weight
Boron	0.0005 - 0.005% by weight
Calcium	0.0002 - 0.005% by weight
Nitrogen	0.003 - 0.007% by weight

The remaining part up to 100 being iron without impurities.

5. A method for the manufacturing of wheels for the railway, tramway and/or metro fields with high resistance to fatigue and fretting fatigue, characterized in that it consists in manufacturing the wheel by mechanical machining of the steel of any of the previous claims and in submitting it to an austenitization and quenching in water and subsequently to two thermal tempering processes.

6. The method according to claim 5, characterized in that the austenitization and quenching in water consists in a heating at a temperature comprised between 850°C and 950°C in a stay at such a temperature for one hour for about each 40 mm of thickness of the processed product and in a quick cooling in water until room temperature is reached.

7. The method according to claim 5 or 6, characterized in that the first thermal tempering process consists in a heating at a variable temperature comprised between 500 and 700°C according to the required features, a stay at this temperature for about one hour for each 20 mm of thickness of the processed product and a spontaneous air cooling until room temperature is reached.

8. The method according to any of the previous claims from 5 to 7, characterized in that the second thermal tempering treatment is carried out by heating at a temperature lower than 20°C compared to the one of the first thermal tempering process for a stay time at this temperature of about one hour for each 20 mm of thickness of the processed product followed by a spontaneous air cooling for at least 25 minutes and in any case being such to cool the rail wheel rolling until room temperature is reached.

9. The method according to claim 8, characterized in that the cooling of the wheel rolling is carried out at a different speed from the one used for the cooling of the web.