JP6270730B2 - Rail steel with an excellent combination of wear resistance, rolling contact fatigue resistance and weldability - Google Patents

Description

  The present invention is a rail steel, in particular a rail steel for use as a grooved rail such as that used for road tracks, for example, a combination of excellent properties, in particular wear and rolling contact fatigue. The present invention relates to a rail steel that has resistance against both (rolling contact fatigue) and can be welded and repaired without requiring high-temperature preheating.

  The cost effectiveness of rail transport has become an important issue in recent years. The replacement of embedded rails in the road rail network causes disruption of service not only for rail traffic but also for other users of the shared infrastructure. This is because such a network is introduced in the city center. Road rail networks are often characterized by very packed track radii that inevitably experience side wear at a high rate and may therefore be a factor in determining rail life. Thus, an important first requirement of a cost effective rail steel grade for road surface applications is that it can be weld repaired without the need for high temperature preheating that damages the surrounding polymer in which the rail is embedded. However, even with this property, the factor that determines the life of the rail is the rate of vertical head wear, and therefore the second important requirement for a cost-effective rail steel grade is its resistance. Abrasion. Furthermore, the increase in passenger numbers means that rail traffic becomes more intense over the years, causing further wear on the rails. Further improvements are needed to make the rail material properties more tolerant and resistant to damage due to increased stress and imposed stress cycles. Advances in rail metallurgy and heat treatment technology have improved pearlite microstructure and increased wear resistance, but rail metallurgy continues to be further improved due to the need to significantly reduce life cycle costs.

  Another rail degradation mechanism often found in road tracks and subway networks is railhead corrugation. Although the occurrence of corrugation is affected by various system characteristics, it is widely accepted that increasing the hardness and yield strength of rail steel slows the occurrence and progression of corrugation. Thus, a third requirement for cost effective rail steel for road tracks and subway networks is an increase in hardness and yield strength.

  In the linear and gently curved sections of railways where experienced wear rates are generally low, the rail life and associated maintenance costs are also very close to the rail head surface or rail head surface. Rolling Contact Fatigue (RCF) is affected by the need to control crack initiation and growth. RCF can occur in a variety of forms, but is usually referred to as head check (HC), gauge corner cracking (GCC), or squat defects. Thus, a fourth requirement for cost effective rail steel for road tracks and subway networks is its resistance to rolling contact fatigue (RCF) initiation.

  Optimization of wheel and rail profiles to minimize damage stress and the use of normal polishing to maintain the desired profile while simultaneously removing the remaining damaged surface layer is RCF and corrugation It has become a proven management method for trajectories affected by. However, the cost of rail polishing is high and takes up the time available to operate a regular service. Therefore, development of more cost effective metallurgical solutions remains desirable.

In addition to improved resistance to wear, RCF, and corrugation, a further requirement for the design of new rail steel is that it can be repeatedly welded in place as a mitigation measure for the high side wear rates that occur due to the tight curvature of the road rail network. It is. Low preheat weld repair techniques, such as those described in Tata Steel's British Patent 2443494, provide a proven method for repeated weld repair of high carbon steel steel. However, the use of this technique has two important metallurgical upper limits for martensite start (M _s ) temperatures below 200 ° C. and martensite finish (M _f ) temperatures below 50 ° C., preferably much lower. Impose requirements. Thus, the composition of the new rail steel not only meets the requirements for resistance to wear, RCF, and corrugation, but also occurs over a range where transformation to martensite prevents transformation completion when using low preheat weld repair techniques. It needs to be designed to guarantee that. An important success factor of such weld repair processes is to bring precipitate separation from initial crack propagation due to fatigue, all within the molten metal, molten metal / parent rail contact surface, or heat affected zone, There are no hard or brittle microstructures or initial cracks.

British Patent No. 2443494

  Accordingly, it is an object of the present invention to provide a high strength rail that is highly resistant to wear, rolling contact fatigue, and corrugation while retaining the ability to be repeatedly weld repaired.

  It is also an object to provide a high-strength rail that can easily and reliably be repaired in-situ on the side by a welding process.

  It is also an object to provide a high strength rail having a hardness of at least 330 HV, a tensile strength of at least 1000 MPa and a yield strength of at least 600 MPa.

One or more objectives of the present invention are high strength pearlitic rail steels (in weight percent) having an excellent combination of wear properties, rolling contact fatigue resistance and weld repairability:
0.70% to 0.85% carbon,
0.65% to 1.00% silicon,
1.1% to 1.4% manganese,
0.07% to 0.15% vanadium,
Up to 0.008% nitrogen,
Up to 0.025% phosphorus,
0.008 to 0.030% sulfur,
Up to 2.5 ppm hydrogen,
Up to 0.10% chromium,
Up to 0.010% aluminum,
Up to 20 ppm oxygen,
This was achieved with rail steel containing a residue consisting of iron and inevitable impurities.

  The effectiveness of the chemical composition of the steel according to the present invention is best shown from an explanation of the reasons for the addition of various elements and a comparison of the important properties of the groove rail steel.

Carbon is the most cost effective strengthening alloying element in rail steel. This is because carbon provides the most cost-effective addition and achieves hardness and strength in fully pearlitic steel. In one embodiment, the maximum value of carbon is 0.8%. This reduces the risk of cementite network formation at the grain boundary. More preferably, the range of carbon content is 0.735% to 0.785%. This range provides an optimal balance between the volume fraction of hard cementite and the prevention of the detrimental network formation of friable cementite at the grain boundaries. Carbon is also a powerful hardener that promotes lower transformation temperatures and therefore finer interlayer spacing. The high volume fraction and fine interlayer spacing of hard cementite provides wear resistance and contributes to the increased RCF resistance of the composition included in one embodiment of the present invention. Furthermore, it is essential to lower the martensite start (Ms) temperature of the steel steel to ensure a strong weld, as demonstrated by the Tata Steel low preheat weld repair process ¹ . The indicated range of carbon is essential to achieve this goal. The following widely accepted method for calculating the M _s temperature clearly confirms the effectiveness of carbon in reducing the magnitude of this parameter. In fact, carbon is more effective than 13-17 times in reducing the M _s temperature as compared to manganese. According to Andrews (J. Iron & Steel Inst., 183 (1965), pp. 721-727), the M _s temperature (° C.) is given by 539-423 ×% carbon-30.4 ×% Mn (formula 1a) According to Steven and Haynes (J. Iron & Steel Inst., 183 (1956), pp. 349-359), it is given by 561-474 ×% carbon−33 ×% Mn (formula 1b). Both regression equations provide slightly different values for M _s . In the present invention, an average value of these two expressions is used as an approximate value of actual M _s . M _f is therefore determined from _{M s} by subtracting 0.99 ° C. from _{M s.}
M _s (° C.) = 0.5 × (M _{s, Andrews} + M _s , _{Stevens & Haynes} ) <Formula 1>
M _f (° C.) = M _s −150 <Formula 2>

The martensite start (M _s ) temperature of the steel is preferably below about 160 ° C. to ensure a strong weld.

In the present invention, the addition of silicon is an essential part of the steel steel design for manipulating the resulting microstructure and properties, and deliberate alloying addition as in most other rail steels. It does not reflect the manufacturing process path. Silicon is often used as a deoxidizing element, so the addition of silicon is usually intended only for that purpose. Since the present invention refers to a wide range of eutectoid compositions, the microstructure contains little proeutectoid ferrite. Instead, it is mainly the pearlite interlayer spacing that determines the resulting properties. Traditionally, pearlite microstructure improvement in groove rail compositions has been achieved by using accelerated cooling. The novelty of the approach in the present invention is that a three-dimensional object whose behavior at the wheel / rail contact surface is governed by the characteristics of two components of pearlite, ferrite and cementite truss rather than the bulk properties of hardness and tensile strength. Is to process the pearlite microstructure. As a result, the novelty in the present invention lies in the use of silicon to strengthen pearlite ferrite by solid solution strengthening, which in turn provides increased resistance to ratchet, wear, and rolling contact fatigue. In order to achieve the required mechanical properties, wear resistance and RCF resistance for the steel according to the invention, a minimum silicon content of 0.65% is essential. Silicon addition has only a limited effect on the hardenability of the steel, which is reflected in the formula for calculating the M _s temperature, but silicon addition up to 1.0% is M _s. It has been observed that it contributes slightly to the decrease in temperature. It has been found that a silicon content of 0.65% to 0.80% provides a good balance of required mechanical properties without adversely affecting weld repairability.

  Manganese is an important alloying element in all groove steels to provide the necessary hardenability to ensure relatively fine interlayer spacing after natural or accelerated cooling of such steel. This purpose is still valid for the present invention. In the present invention, which does not rely on accelerated cooling because of its properties, a higher manganese content is considered desirable in order to provide sufficient curability to achieve a pearlite microstructure with fine interlayer spacing. A manganese content of less than 1.1% Mn has been found to be sufficient to achieve the desired curability at a selected carbon content, while levels above 1.4% are particularly high for manganese. The increased risk of martensite formation in the separation zone was considered unacceptable. Higher levels of manganese are also considered undesirable from a welding standpoint because of the increased risk of hard and friable martensite formation. In a preferred embodiment, the manganese content is up to 1.35%. A preferred minimum value for manganese is 1.20% or even 1.25%.

The effectiveness of vanadium as a precipitation strengthening alloying element was utilized in the present invention to strengthen pearlite ferrite and thereby increase its resistance to ratchet, wear, and rolling contact fatigue. Vanadium forms vanadium carbide or vanadium nitride depending on the amount and temperature of nitrogen present in the steel. It is therefore necessary to examine the vanadium addition level along with the amount of nitrogen in the steel. This is because the effectiveness of precipitation strengthening in eutectoid pearlite steel decreases with increasing nitrogen levels, and therefore the higher the temperature, the rougher the vanadium nitride precipitates. Furthermore, such high temperature precipitates do not strengthen the pearlite ferrite and do not leave enough vanadium in the solution to increase curability and achieve finer interlayer spacing. The addition of vanadium to the eutectoid steel steel, does not exert much influence on the M _s temperature. The inventors have found that the proportion of vanadium that precipitates as carbide is almost maximal when the nitrogen content is limited to 0.003%, which decreases in proportion to the increase in nitrogen content. . Knowledge of these metallurgical principles was applied in an innovative way to arrive at ideal contents of 0.08% V and 0.003% nitrogen. Therefore, for cost-effective manufacturability reasons, a minimum nitrogen content of 0.003% is considered a practical lower limit, while an upper limit of 0.007% maximizes the benefits from expensive vanadium additions. Considered desirable to guarantee. However, although not desirable from a cost effectiveness point of view, higher nitrogen content can be tolerated if the vanadium content increases proportionally.

  In one embodiment of the invention, the minimum amount of nitrogen is 0.003% in conjunction with a minimum vanadium content of 0.07%. Preferably, nitrogen is at most 0.007%, while the corresponding value for vanadium is at least 0.07% and at most 0.12%. These maximum contents can be exceeded, but they are non-ideal and not economically interesting.

  Due to the addition of silicon, manganese and vanadium, the target properties are achieved with high consistency not only near the surface but also in the head bulk. This consistency is difficult to achieve with heat treated rails, while in steels according to the present invention, this consistency is achieved with as-hot rolled rails. Higher strength and / or yield strength and / or hardness can be accelerated from this steel with in-line or off-line heat treatment equipment using forced air, water, air mist or polymer quenchant This can be achieved by subjecting to cooling.

  The wear resistance obtained with steels according to the invention in the as-rolled state, accelerated cooling conditions or heat treatment conditions reduces the need to apply precautionary high-cost surface hardening for rails placed on sharp curves It's like

  Preferably the steel steel has a phosphorus content of at most 0.015%. The sulfur value should be between 0.008 and 0.030%. This is because it forms MnS inclusions. These inclusions act as sinks for any residual hydrogen that may be present in the steel. This hydrogen can cause shatter cracks that initiate fatigue cracks at the head (also known as tache ovals) under high stress from the wheels. Can be an eater. The addition of at least 0.008% sulfur prevents the adverse effects of hydrogen, while the maximum value of 0.03% is selected to avoid structural embrittlement. Preferably, the maximum value is 0.025%. Boron is not an essential alloying element, but can be used to improve the properties of steels according to the present invention, and amounts up to about 60 ppm can be used. Boron is a strong promoter for the formation of microstructural components such as bainite or martensite, especially when nitrogen in steel is combined with titanium. Otherwise, BN precipitates can be formed. In the steel according to the invention, it is important that the microstructure is substantially pearlite, preferably completely pearlite, and the amount of bainite or martensite microstructure components is kept as low as possible, preferably not present. . Preferably, boron is not present as an alloying element in the steel according to the invention, but can be present as an inevitable impurity. A boron content of less than 0.0005% (ie less than 5 ppm) is generally considered invalid as an alloying element and is therefore considered an impurity in this application.

  The maximum recommended level of inevitable impurities is based on EN136674-1: 2003, according to which the maximum limits are Mo 0.02%, Ni 0.10%, Sn-0.03%, Sb-0.020%, Ti-0. 0.025% and Nb-0.01%.

The wear resistance of the steel obtained from the present invention has been demonstrated using a proven comparative “twin disk” test procedure. Tested using laboratory twin-disc equipment similar to that described in 'Wear', 162-164 (1993), Microstructure and wear resistance of pearlitic rail steels, Albert J. Perez-Unzueta & John H. Beynon ⁵ I do. This facility simulates the forces that occur when a wheel rolls on a rail and slides. These assessments are not part of the formal rail qualification procedure, but have been found to provide good indicators of the relative in-service performance of different rail steel compositions. The test conditions for the abrasion test include 560 MPa contact stress and 25% slip, while those for RCF utilize a higher contact stress of 900 MPa, 5% slip and water lubrication. The results are shown in FIG. 1, in which the wear rate (mg / m slip) is plotted against the hardness (HV).

  It is clear that the wear rate decreases as a function of hardness and there is little measurable further decrease in wear rate above a hardness level of 330 Hv30 (about 313 HB). Accordingly, one of the objects of the present invention was to achieve wear resistance equal to at least a hardness level of 330 HV30. The optimized composition of the present invention in both laboratory and commercial castings achieved the desired wear resistance. The inventors have found that a balanced chemical composition results in very wear-resistant pearlite as a result of solid solution strengthening by silicon addition and pearlite ferrite strengthening by vanadium carbide very finely dispersed in pearlite ferrite lath. I found it. Furthermore, by limiting the nitrogen to less than 0.007%, we enhance the strength, hardness, and wear resistance of the optimized composition of the present invention by improving the pearlite interlayer spacing. Utilized the well-known strong curable effect of vanadium in solution.

  Although the road surface track is less susceptible to rolling contact fatigue (RCF) due to the low operating speed of the track surface in the embedded track, the strengthening of pearlite ferrite resulting from the composition of the present invention also improves the resistance of the steel to RCF. did. At least a 20% improvement in resistance to RCF initiation was demonstrated by the composition of the present invention and a standard R260 grade comparative laboratory twin disk test.

  Many factors together bring about these improvements. First, the volume fraction of hard cementite in the microstructure by increasing the carbon content for typical low carbon rail steel grades for groove rails while remaining in the low eutectoid region of the iron-carbon phase diagram. Will increase. The carbon content does not enter the hypereutectoid range of the composition, thus avoiding the risk of the detrimental network formation of friable cementite at the grain boundaries under relatively slow cooling experienced by the rail during manufacture. As a further precaution, the intentional addition of higher silicon and vanadium compositions is aimed at preventing particle boundary cementite in the separated section of the rail section. These additions also have a second equally important function. Silicon is a solid solution strengthener that increases the strength of pearlite ferrite, thereby increasing the resistance of pearlite to both wear and RCF initiation. Similarly, the precipitation of fine vanadium carbide within pearlite ferrite increases its strength, particularly proof stress, thereby increasing its resistance to both wear and RCF. A further feature of the composition design is to limit the nitrogen content to prevent premature formation of relatively coarse precipitates of vanadium nitride. This is because they are not very effective in increasing the strength of pearlite ferrite. This ensures that the vanadium additive remains in solution in the austenite and lowers the temperature, resulting in a finer precipitate. Part of the vanadium also remains in the solution, thereby acting as a hardenability agent to improve the pearlite spacing. Thus, the specific design of the composition claimed in this embodiment utilizes the various properties of the individual elements to produce a microstructure with a highly desirable combination of wear and RCF resistance. The mechanical properties of steels according to the invention and their resistance to wear and RCF initiation are better than most normal heat-treated pearlite rail grades, which are in accordance with European standards for groove rails (EN 14811: 2006 + A1: 2009) Similar to the hardest heat treatment grade included (Grade R340GHT). Although not essential for the current intended application of recessed street driving track rails, further improvements to tensile properties and resistance to wear, plastic deformation, and RCF can be followed up using a hot rolling or reheating stage. It could be obtained by subjecting the composition of the present invention to accelerated cooling after heat treatment.

Another equally important additional property of the steels according to the invention is that they can be welded repaired without the need for high temperature preheating. The patented Tata Steel weld repair process specifies a low preheat temperature of about 60 ° C to 80 ° C. The basic principle on which this process is based is to avoid completing the transformation to martensite within the heat affected zone formed by the deposited weld bead. Thus, in the present invention, there are two difficult goals in the design of the steel composition: firstly meeting the property requirements described in the previous paragraph, and secondly the martensite start (M _s ) transformation. The temperature and martensite finish (M _f ) transformation temperature was that temperature during which the transformation to martensite was not completed during the weld repair process. As a result, the M _f temperature should be below about 60 ° C., preferably much lower than this temperature to maximize the volume of unconverted retained austenite, which is the weld / parent metal. It is important to prevent the formation of initial cracks in the heat affected zone formed by the deposition of contact surfaces or weld beads. In general, the M _f temperature is believed to be about 150 ° C. below the M _s temperature that can be calculated using equations (1), (1a), (1b) and (2) above. The M _s and M _f temperatures of the various rail steels available for the minimum required hardness of the grade are shown in FIG. Temperature, paragraph

The carbon and manganese concentrations used in the calculation are midpoint values in the range specified by EN 14811: 2006 + A1: 2009. The M _s temperature is the upper limit value of the represented range, and the M _f temperature is the lower limit value.

The steel of the present invention, referred to as “invention” in FIG. 2, has the lowest M _f temperature and can therefore retain the highest rate of austenite and is therefore most resistant to the formation of initial cracks. it is obvious. In contrast, other groove rail steel grades have an undesirably high M _f temperature, which means that the risk of martensitic transformation completion and crack formation during weld repair is very high.

  The excellent wear resistance of the rail ensures that the rail takes time to wear in the vertical direction. The weld repairability and the fact that the steel according to the invention does not require a heat treatment to achieve its properties allows the rail to be repaired in situ, thus eliminating the need to remove the rail from the street Guarantee that can be repaired overnight. This is less inconvenient for road construction and city traffic. Such a combination of properties and rail chemistry that can be achieved by manipulation of the microstructure means that the rail is not only cost effective, but also provides a more environmentally friendly solution. This is because the rail can be easily repaired and does not need to be frequently replaced with a new rail. In addition, careful compositional design also eliminates the need for heat treatment steps during rail manufacturing, making rail products more environmentally friendly compared to steel steels that derive their properties from heat treatment after rolling the rails. Guarantee.

  The steel according to the present invention is suitable for purposes such as crane rails or flat bottom rails, but this rail steel is used for the manufacture of groove rails that benefit from the important characteristics of combined wear resistance and weld repairability. Was found to be very suitable.

Laboratory castings of steels C1-C4 and steel steel A of the present invention were produced as 60 kg ingots. Castings C1-C4 are pre-casts, which on the one hand have the requirements to achieve the required hardness, tensile properties and resulting wear resistance and weld repairability using a low preheating process. It was created to establish a balance of requirements that sometimes conflicted with the requirement of a sufficiently low _Ms temperature to ensure. The results obtained from these studies were best for laboratory castings, mainly of the composition of casting A. The ingot was rolled into a 30 mm thick plate and subjected to natural air cooling to accurately simulate the cooling conditions at the head of the rolled rail. A 300 ton commercial BOS casting (steel B) was made from the steel of the present invention based on the chemistry of laboratory casting A and was subsequently cast into a 355 × 305 mm bloom section. The bloom was rolled into various rail sections and cooled in a rail cooling bank under standard cooling conditions for normal rolled commercial rail grades. The entire rail length was manufactured without internal or surface fracture defects. The rails were tested as hot rolled.

  The chemical compositions of steels A and B are shown in Table 1. Comparative examples C1-C4 are also shown in Table 1.

  The hardness of steels A and B was found to be 330-335HV30. By selecting steel with a narrow chemistry window according to the present invention, we have both excellent wear resistance and RCF resistance, compatible with the performance of heat treated Grade 350HT, It has been found that it exhibits similar mechanical properties. Compared to certain grades, the steel of the present invention does not require heat treatment to obtain the desired properties.

The abrasion test result of the steel steel obtained from this invention is represented. It represents the M _s and M _f temperatures of the various rail steels available for the required minimum hardness of the grade.

Claims (11)

Steel for producing rails, having a hardness of at least 330 HV30, a tensile strength of at least 1000 MPa and a yield strength of at least 600 MPa (in mass%)
0.70% to 0.85% carbon,
0.65% to 1.00% silicon,
1.1% to 1.4% manganese,
0.07% to 0.15% vanadium,
Up to 0.008% nitrogen,
-0.025% phosphorus,
0.008% to 0.030% sulfur,
Up to 2.5 ppm hydrogen,
・ Up to 0.10% chromium,
Up to 0.010% aluminum,
Up to 20 ppm oxygen,
・ Iron and inevitable impurities,
Consists of
The martensite start transformation temperature (M _s ) determined by Equation 1 is lower than 175 ° C., and the martensite finish transformation temperature (M _f ) according to Equation 2 is lower than 25 ° C.,
M _s (° C.) = 0.5 × (M _{s, Andrews} + M _{s, Stevens & Haynes} ) <Formula 1>
M _{s, Andrews} (° C.) = 539-423 ×% carbon-30.4 ×% Mn <Formula 1a>
M _{s, Stevens & Haynes} (° C.) = 561-474 ×% carbon−33 ×% Mn
<Formula 1b>
M _f (° C.) = M _s −150 <Formula 2> Steel steel characterized by the following.   The steel as claimed in claim 1, comprising a carbon content of at least 0.735%.   Steel according to claim 1 or 2, comprising a manganese content of at least 1.20%.   4. Steel according to any one of claims 1 to 3, comprising a silicon content of at least 0.75%. 5. Steel according to any one of claims 1 to 4, comprising a combination of at least 0.08 % vanadium content and a maximum 0.005% nitrogen content. 5. Steel according to any one of claims 1 to 4, comprising a combination of at least 0.10 % vanadium content and a maximum nitrogen content of 0.007%.   The steel according to any one of claims 1 to 6, wherein the heat affected zone of the weld repair bead comprises at least 50% by volume of retained austenite.   Steel steel according to any one of claims 1 to 7, comprising a silicon content of at most 0.80%.   9. Steel according to any one of claims 1 to 8, comprising a manganese content of at most 1.35%.   Steel steel according to any one of the preceding claims, comprising a maximum carbon content of 0.8%.   11. A rail, characterized in that it is made from steel steel according to any one of the preceding claims having a hardness of at least 330 HV30, a tensile strength of at least 1000 MPa and a yield strength of at least 600 MPa.

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