CLIAMS:We Claim:
1. High toughness hypoeutectoid rail composition comprising
C: 0.40-0.60 wt%;
Mn: 1.0-1.6 wt%;
Si: 0.20-0.80wt%;
S: upto 0.030wt%;
P: upto 0.030wt%;
Cr: 0.40-1.0wt%;
Nb: 0.01-0.05wt%;
Cu:0.20-0.50 wy.%;
Ni:0.20-0.50 wt %;
and balance is iron.

2. High toughness hypoeutectoid rail composition comprising
C: 0.40-0.60 wt%;
Mn: 1.0-1.6 wt%;
Si: 0.20-0.80wt%;
S: upto 0.030wt%;
P: upto 0.030wt%;
Cr: 0.40-1.0wt%;
V: 0.04-0.20wt%;
Nb: 0.01-0.05wt%;
and
Balance is iron.

3. High toughness hypoeutectoid rail composition as claimed in claim 2 comprising:
Cu 0.20-0.50 wt% and Ni 0.20 -0.50 wt %.

4. High toughness hypoeutectoid rail as claimed in anyone of claims 1 to 3 comprising selectively:
UTS : 880-1100 N/mm2
% El : 13 (Min)
YS : 560 (Min.) N/mm2
Hardness : 280 (Min) BHN
Fracture Toughness at -20oC : 35 (Min) MPavm

5. High toughness hypoeutectoid rail as claimed in anyone of claims 1 to 4 having
Fatigue Strength: 330-350MPa; and
CRI: 1.2-1.6.

6. High toughness hypoeutectoid rail as claimed in anyone of claims 1 to 5 wherein alloying elements are selected in such a way that corrosion resistance properties are also increased.

7. High toughness hypoeutectoid rail as claimed in anyone of claims 1 to 6 wherein lower carbon equivalent is maintained in steel grades produced to favour enhancing welding properties.

8. A process for the production of high toughness hypoeutectoid rails as claimed in anyone of claims 1 to 7 comprising the steps of
(i) Providing the selective heat composition;
(ii) Continuously casting of degassed steel continuous bloom caster
(iii) Cutting the blooms into desired length depending upon the different sections or profiles to be rolled;
(iv) Controlled cooling said blooms to ambient temperature and charging into reheating furnace;
(v) Reheating the blooms in reheat furnace and soaking at 1200-1300 oC for 4-6 hours;
(vi) Descaling the blooms just prior to rolling;
(vii) Hot rolling of blooms in desired number of different passes to produce the rail, maintaining finishing temperature between 900-1000oC; and
(viii) Cooling the rails in air after finishing.

9. A process for the production of high toughness hypoeutectoid rails as claimed in claim 8 comprising the steps of
(i) Making the rail steel by providing BF hot metal in a BOF converter and passing Steel through LF and RH to remove gas inclusions, adding different alloying elements and maintaining suitable superheat so to achieve the composition as claimed in claims 1 or 2;
(ii) Continuously casting said degassed steel into a four or three strand continuous bloom caster
(iii) Cutting the blooms into desired length of 5-5.8 m depending upon the different sections or profiles to be rolled;
(iv) Slowly cooling said blooms to ambient temperature and charging into reheating furnace;
(v) Reheating the blooms in reheat furnace and soaking at 1200-1300 oC for 4-6 hours;
(vi) Descaling the blooms just prior to rolling;
(vii) Hot rolling of blooms in desired number of different passes to produce the rail, maintaining finishing temperature between 900-1000oC; and
(viii) Cooling the rails in air after finishing.

10. A process as claimed in claim 9, wherein said degassing time in RH (Ruhrstahl Heraeus) degasser is maintained from 16 to 24 min and preferably 22 min.

Dated this the 3rd day of March, 2014
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)

,TagSPECI:FIELD OF THE INVENTION

The present invention relates to high toughness hypoeutectoid rail steel grades and process of manufacture thereof. More particularly, the present invention is directed to high toughness hypoeutectoid rail with high corrosion and wear resistance wherein Carbon is maintained below 0.6% level to increase the elongation and toughness and the reduction in strength due to reduction in carbon is compensated by adding some alloying elements wherein solid solution strengtheners like Mn, Si are added and Cr, V Nb, Ni, Cu, Mo are added in different combination to increase its strength and other mechanical properties. The alloying elements are selectively provided in such a way that corrosion resistance properties and fracture toughness are increased significantly. Advantageously, the high toughness hypoeutectoid rail steel according to the present invention is also having lower carbon equivalent of such steel grades resulting in enhanced welding properties favourig production of rails suitable for high axle load application.

BACKGROUND OF THE INVENTION

Indian railway is carrying 20.32 t axle load on most of the BG routes and it is planning to universalize 22.82t axle load for BG routes and Construct dedicated freight corridors (DFC) fit for 32.5 t axle loads. Feeder route required for connecting existing route to DFC are to be for minimum 25t axle load. So there is need to improve in metallurgy of rail to carry 32.5 t axle load which in terms of having property greater resistance against rail wear caused by wheel interaction as well as longer life (GMT). Applicants’ manufacturing facility at Bhilai Steel Plant is the sole supplier of rails to Indian railways. Its two sections under C-Mn rail category in R 52 & R 60 Kg are capable to withstand a maximum axle load of 29.12 & 33.79 t respectively. Increasing cross section weight to 68 Kg can increase the load bearing capacity to 38.1 t. The axle load has always shown an increasing trend over the years.

The rails produced by the applicants are also applied to use in coastal/ corrosion prone areas. There is the problem of discharge of human excrement onto the rails, which exerts severe corrosive attack. So property requisites for a modern-day rail are good resistance to fracture, adequate high cycle fatigue resistance to counter in-service problems like shelling (Incidentally, shelling is a fatigue-type of failure associated with initiation and propagation of sub-surface cracks culminating in spalling of rail pieces from the running surface of a rail head), must exhibit low fatigue crack growth rates, must have excellent wear resistance to endure all that heavy and frequent train movements, must possess sufficient resistance to plastic flow to counter in-service problems such as rail head shoulder bulging (distortion of rail cross-sectional profile) and must show corrosion resistance.

In order to address the above issues, RDCIS in recent times in close association with BSP and RDSO has embarked on a mission to develop new grades of special rail steels to cater to the diverse requirements/ applications of Indian Railways. As a result, three new categories of rails have been developed over the recent years and few are in laboratory scale.
(i) Microalloyed pearlitic rails with low alloying additions of Nb or V, high YS/UTS ratio and excellent resistance to in-service plastic flow and deformation. BSP has also taken initiative to develop 110 UTS rails by alloying with Cr-V-Nb.
(ii) Marine-weathering pearlitic rails with low alloying additions of Cu & Mo and with corrosion resistance derived from the development of a protective, adherent rust layer for during heavy-haul service in coastal/ corrosion-prone environments. RDSO, IIT Kanpur, RDCIS & BSP jointly developed a new class of corrosion resistant rail by alloying Nickel-Chromium-Copper. Both rails have shown excellent weather resistant properties and are commercially produced and laid at coastal regions of Indian Railway.
(iii) Corrosion resistant high strength rail (Cr – Cu) to sustain stringent operating conditions. New alloy design is being worked out at RDCIS laboratory to propose a rail under this category.

Schetky, le May and Dilewijns reported a medium carbon Cr-V-Ni-Cu rail composition which combines the good performance of Cr-V rail steels with the additional strengthening and corrosion resistance afforded by addition of copper. The superior properties are related to a carbide precipitation hardening in a very fine pearlite matrix, which exhibits very little free ferrite. Chrome hardens the steel alloy and gives greater wear and corrosion resistance. The vanadium also makes the alloy harder and gives greater resistance to wear and impact. D V Chervyakov et al have reported presence of vanadium up to 0.02-0.07wt% in rail steels which increases wear resistance and reliability of railroads. American Vanadium company has reported the relative wear of the vanadium steel rails is practically one half that of the carbon rails. N A Fomin, A I Tkachenko, V A Palyanichka, M S Gordienko and N G Nikulin have reported use of vanadium up to 0.03-0.06 in rail steels. It has been reported elsewhere (Ferroalloys & Alloying Additives Handbook), that the wear resistance of rail steels is improved considerably by the addition of 0.08-0.12 wt% vanadium. Wear resistant rails of this type are used in curves, switches and other points to experience severe service.

It has been observed through experimentation and trial that Just below the eutectoid temperature, relatively thick layers of both ferrite and cementite phases are produced, which is also known as coarse pearlite. With decreasing temperature, the layers become progressively thinner. The thin-layered structure produced in the vicinity of 540° C is called fine pearlite. Standard rails have coarse lamella spacing whereas premium rails would have finer lamella spacing. The fine-grained pearlitic rail steels are tougher, harder and stronger than the coarse-grained pearlitic rail steels. The tensile strength, hardness and toughness increase as distance between the lamellae decreases, which can be achieved by transforming austenite at the lowest temperature, without forming bainite and martensite.

There has been therefore a need in the related field of developing steel grade for producing rail sections suitable for high axle load rail transportation which would have high strength, toughness, hardness, corrosion and wear resistance with good ductility and weldability by adopting selective alloy design and process parameters.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to providing to high toughness hypoeutectoid rail with high corrosion and wear resistance suitable for high axle load rail transportation and process for its production.

A further object of the present invention is directed to high toughness hypoeutectoid rail wherein level of carbon would be maintained below 0.60 to increase the elongation and toughness.

A still further object of the present invention is directed to high toughness hypoeutectoid rail wherein reduction in strength due to reduction in carbon was compensated by adding some alloying elements.

A still further object of the present invention is directed to high toughness hypoeutectoid rail wherein alloying elements are selectively provided in such a way that corrosion resistance properties are also increased.

A still further object of the present invention is directed to high toughness hypoeutectoid rail wherein wherein to regain strength lost by the reduction in C content solid solution strengtheners like Mn, Si were added and other alloying elements like Cr, V Nb, Ni, Cu, Mo would be added in different combination to increase its strength and other mechanical properties.

A still further object of the present invention is directed to high toughness hypoeutectoid rail wherein lower carbon equivalent of selected grades will also enhance welding properties.

A still further object of the present invention is directed to high toughness hypoeutectoid rail wherein High fracture toughness in rail will reduce the chance of sudden failure.

SUMMARY OF THE INVENTION

Thus according to the basic aspect of the present invention there is provided high toughness hypoeutectoid rail composition comprising
C: 0.40-0.60 wt%;
Mn: 1.0-1.6 wt%;
Si: 0.20-0.80wt%;
S: upto 0.030wt%;
P: upto 0.030wt%;
Cr: 0.40-1.0wt%;
Nb: 0.01-0.05wt%;
Cu:0.20-0.50 wy.%;
Ni:0.20-0.50 wt %;
and balance is iron.

A further aspect of the present invention is directed to provide high toughness hypoeutectoid rail composition comprising
C: 0.40-0.60 wt%;
Mn: 1.0-1.6 wt%;
Si: 0.20-0.80wt%;
S: upto 0.030wt%;
P: upto 0.030wt%;
Cr: 0.40-1.0wt%;
V: 0.04-0.20wt%;
Nb: 0.01-0.05wt%;
and balance is iron.

A still further aspect of the present invention is directed to said high toughness hypoeutectoid rail composition comprising:
Cu 0.20-0.50 wt% and Ni 0.20 -0.50 wt %.

A still further aspect of the present invention is directed to said high toughness hypoeutectoid rail comprising selectively:
UTS : 880-1100 N/mm2 ;
% El : 13 (Min);
YS : 560 (Min.) N/mm2;
Hardness : 280 (Min) BHN;
Fracture Toughness at -20oC : 35 (Min) MPavm.

Yet another aspect of the present invention is directed to said high toughness hypoeutectoid rail having
Fatigue Strength: 330-350MPa; and CRI: 1.2-1.6.

A still further aspect of the present invention is directed to said high toughness hypoeutectoid rail wherein alloying elements are selected in such a way that corrosion resistance properties are also increased.

A still further aspect of the present invention is directed to said high toughness hypoeutectoid rail wherein lower carbon equivalent is maintained in steel grades produced to favour enhancing welding properties.

Yet another aspect of the present invention is directed to a process for the production of high toughness hypoeutectoid rails as described above, comprising the steps of
(i) Providing the selective heat composition;
(ii) Continuously casting of degassed steel continuous bloom caster
(iii) Cutting the blooms into desired length depending upon the different sections or profiles to be rolled;
(iv) Controlled cooling said blooms to ambient temperature and charging into reheating furnace;
(v) Reheating the blooms in reheat furnace and soaking at 1200-1300 oC for 4-6 hours;
(vi) Descaling the blooms just prior to rolling;
(vii) Hot rolling of blooms in desired number of different passes to produce the rail, maintaining finishing temperature between 900-1000oC; and
(viii) Cooling the rails in air after finishing.

A still further aspect of the present invention is directed to said process for the production of high toughness hypoeutectoid rails comprising the steps of
(i) Making the rail steel by providing BF hot metal in a BOF converter and passing Steel through LF and RH to remove gas inclusions, adding different alloying elements and maintaining suitable superheat so to achieve the composition as given above;
(ii) Continuously casting said degassed steel into a four or three strand continuous bloom caster
(iii) Cutting the blooms into desired length of 5-5.8 m depending upon the different sections or profiles to be rolled;
(iv) Slowly cooling said blooms to ambient temperature and charging into reheating furnace;
(v) Reheating the blooms in reheat furnace and soaking at 1200-1300 oC for 4-6 hours;
(vi) Descaling the blooms just prior to rolling;
(vii) Hot rolling of blooms in desired number of different passes to produce the rail, maintaining finishing temperature between 900-1000oC; and
(viii) Cooling the rails in air after finishing.

A still further aspect of the present invention is directed to said process wherein said degassing time in RH (Ruhrstahl Heraeus) degasser is maintained from 16 to 24 min and preferably 22 min.

The objects and advantages of the present invention are described hereunder in greater details with reference to the following accompanying non limiting illustrative drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: shows Structure of (a) BCC and (b) FCC;
Figure 2: shows growth and structure of pearlite: (a) redistribution of carbon and iron, and (b) photomicrograph of the pearlite lamellae;
Figure 3: shows typical Time-Temperature-Transformation diagram of eutectoid steel;
Figure 4 : shows flow chart for production of HTCR rail according to the present invention;
Figure 5 : shows graphically the effect of carbon equivalent on hardness;
Figure 6 : shows graphically the effect of carbon equivalent on impact values;
Figure 7 : shows that impact strength found to be deteriorated with increase in tensile strength;
Figure 8 : shows graphically the effect of Nb content on CVN values;
Figure 9 : shows graphically the effect of V content on CVN values;
Figure 10 : shows graphically that the Grades having high fracture toughness produced according to the present invention have also shown high impact strength.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The present invention is directed to high toughness hypoeutectoid rail with high corrosion and wear resistance wherein Carbon is maintained below 0.6% level to increase the elongation and toughness and the reduction in strength due to reduction in carbon was compensated by adding some alloying elements wherein solid solution strengtheners like Mn, Si were added and Cr, V Nb, Ni, Cu, Mo were added in different combination to increase its strength and other mechanical properties.

The design of the selective alloy composition for the high toughness hypoeutectoid steel grade targeted according to the present invention is arrived at based on the following considerations:

It is well known that Steel is an iron-carbon alloy containing various additions of alloying elements with a carbon content ranging between 0.02 to 2.04 %. Carbon is the most cost-effective alloying element to strengthen iron, which is soft in its pure form. It does this by preventing dislocations in the iron atom crystal lattice from sliding past one another.

Immediately after solidification at about 1538° C, pure iron forms a body–centered cubic (BCC) arrangement of iron atoms known as the d-iron phase as shown in accompanying Figure 1(a) and on further cooling, at between 1390 and 910° C, iron transforms to a face–centered cubic (FCC) structure known as austenite or ?-iron as shown in accompanying Figure 1(b). As cooling continues below 910°C, iron transforms back to the BCC structure known as a-ferrite.

The addition of carbon to iron even by a small amount lowers its melting point, affects the temperature at which it changes its crystal structure and leads to the formation of iron carbide also known as cementite (Fe3C) at lower temperature. The iron-carbon phase diagram represents the effect of carbon. This diagram shows the temperature and composition limits within which austenite, ferrite, cementite and the eutectoid mixture pearlite phases exist. The eutectoid reaction (0.83wt%C), which is important to the manufacturing of standard pearlitic rail, occurs in this case at 723oC and is as follows:

? (0.83%C) = a (0.0022%C) + Fe3C (6.67%C)

Fully pearlitic rail steel

In the iron-carbon system there is a eutectoid point at approximately 0.83 wt% C and 723°C. A eutectoid reaction describes the phase transformation of a single solid into two others, both with compositions differing from the original. At a temperature above 723°C, the rail steel alloy is composed entirely of austenite. As the alloy is cooled, no changes occur until the eutectoid temperature (723°C) is reached. Upon slowly crossing this temperature, the austenitic phase decomposes via a diffusion transformation to produce pearlite which is a lamellar structure containing layers of ferrite (0.022%C) and cementite (6.67%C).

As previously discussed, pearlite or bainite microstructures may be obtained depending on the cooling rate, which is a function of temperature and time. One convenient method of describing the non-equilibrium transformation of austenite during cooling is the isothermal transformation diagram, referred to more commonly as the time-temperature-transformation diagram (TTT diagram), an example of which is shown in accompanying Figure 3.

This diagram shows the results of transforming austenite isothermally at temperatures below the eutectoid line (723°C). A horizontal line marks the Ms temperature, which is the temperature at which martensite begins to form during cooling. Above that line, two solid “C” shaped curve lines mark the beginning and end of the transformations with the product of those transformations being ferrite and cementite. To the left of the “begin” transformation curve only austenite is present, whereas to the right of the “completion” curve only pearlite exists. Between these curves, the austenite is in the process of transforming to pearlite and consequently both microconstituents are present. The inspection of this diagram also reveals that at a few degrees below the eutectoid line, the transformation from austenite to pearlite takes a long time to begin and to be completed.

For instance, the transformation from austenite to pearlite takes around 180 seconds at 675°C whereas it takes only 8 seconds at 570°C.

Just below the eutectoid temperature, relatively thick layers of both ferrite and cementite phases are produced, which is also known as coarse pearlite. With decreasing temperature, the layers become progressively thinner. The thin-layered structure produced in the vicinity of 540° C is called fine pearlite. Standard rails have coarse lamella spacing whereas premium rails have finer lamella spacing. The fine-grained pearlitic rail steels are tougher, harder and stronger than the coarse-grained pearlitic rail steels. The tensile strength, hardness and toughness increase as distance between the lamellae decreases, which can be achieved by transforming austenite at the lowest temperature, without forming bainite and martensite.

Additions of alloying elements

Steels are defined as iron-carbon alloys that contain less than 2%Carbon. However, rail steels are not pure iron-carbon alloys in practice but contain also other alloying elements such as manganese, together with small amounts of impurities such as sulphur, phosphorus and other residual elements. The addition of alloying elements to a Fe-C alloy has a strong influence on the temperature range of ? and a phases. A particular alloying element may either expand the ? phase field (known as an austenite stabiliser) or contract this field (known as a ferrite stabiliser). In rail steels, alloying elements such as carbon and manganese are austenite stabiliser whereas silicon and molybdenum are ferrite stabilisers. It is important to point out that the austenite field can be completely restricted by elements such as titanium, tungsten and chromium, which can eliminate, in certain circumstances, the possibility of generating important rail steel microstructures i.e. pearlite and bainite.

Effect of alloying elements on rail steel

Rail Steel contains few elements as essential components while there are few, added to have some specialized properties. Here is a list of all possible elements which may present in rail steel.

Standard elements present
C, Mn, S, P, Si, O, H, Al

Optional elements
Cr, V, Nb, W, Ti, Ni, Cu, Mo

Carbon, C

C is an essential element for forming cementite in pearlite. Added in a range of 0.6-0.9 %. Strength is hardly obtained if C is less than 0.6 % while at more than 0.9 % grain boundaries are enriched by cementite making steel fracture prone. C finds its place in rail steel as interstitial element in ferrite (Body Centered Cubic structure) and constituting member of compound Fe3C (Orthorhombic structure)

Manganese, Mn

? Mn decreases the pearlite transformation temperature thereby decreasing interlamellar spacing.
? Generally added in a range of 0.4-1.5 %
? If it is less than 0.4 the said property is hardly achievable while at more than 1.5 % possibility of formation of martensite during welding and heat treatment increases.

Silicon, Si

? Si is added in rail steel as deoxidiser but it also participates in strengthening ferrite structure of pearlite by solid solution hardening
? Si is added in a range of 0.2-0.8 %
? Minimum 0.2 % of Si to be contained in steel to completely deoxidise the steel while at more than 0.8 % the probability of having oxide inclusion in steel increases because of higher affinity of Si for oxygen.

Sulphur, S & Phosphorus, P

? S participates in formation of A type inclusion while P degrades ductility. Both elements are deleterious to rail steel from mechanical properties point of view.
? S remains in a range of 0.005-0.015 %
? P remains in a range 0.015-0.030 %
? Achieving low sulphur specially below 0.010% in rail steel is beneficial from A type inclusion level point of view making steel more fracture resistant. However, lowering sulphur makes steel more costly.

Vanadium, V

? Vanadium is one of the well-known microalloying elements, it is characterized by strong effect on the structure and properties of steel when present in minute quantity, generally well less than 0.15%.
? It increases strength and toughness, primarily through a combination of grain refinement and precipitation strengthening, both of which depend on the formation of carbide and nitride (or carbonitride) particles.
? Wear resistant rails of this type are used in curves, switches and other points to experience severe service
Niobium, Nb

? In hot-rolled steels the beneficial effect of Nb begins during reheating, while the element is partially in high-temperature solid solution and partially still precipitated as fine carbonitrides. The residual precipitates help maintain a fine austenitic grain size before rolling.
? One-half the amount of niobium (0.012% Nb ( 0.025 % V) is needed to produce the same strength increase brought about by vanadium additions
? Normally added 0.03 % max in rail steel

Chromium, Cr

? Cr further increases strength of rail steel by solid solution hardening.
? Added in a range of 0.2-1.5 % in rail steel
? If added more than 1.5 % then probability of martensite formation increases.
? Normally added in combination with other strengthening element like V, Nb, Mo etc.

Nickel, Ni

? An important and widely used constituent of alloy steels, nickel is best known as a solid solution strengthener, a mild hardenability agent and, most important, as a means of promoting high toughness, especially at low temperatures.
? It increases strength without compromising ductility
? When Cu is added in rail steel then Ni is preferably added to combat Cu-induced attack on rail steel.
? Should not be added more than 1 % in rail steel as it increases hardenability and shifts the pearlitic structure of rail steel.

Copper, Cu

? Copper is added to steel to increase corrosion resistance.
? It also increases strength through precipitation hardening if present in concentrations greater than 0.75%.
? Copper produces these beneficial effects even though copper and iron have only very limited mutual solubility in both liquid and solid states.
? When Cu is added in rail steel then Ni (1/3 or ½ of Cu) is preferably added to combat Cu-induced attack on rail steel.
? Cu should not be added more than 1 % in rail steel

Molybdenum, Mo & Tungsten, W

? Mo & W are strong carbide formers strengthens rail steel
? It also increase strength by solid solution hardening
? Added not exceeding, Mo:0.25% & W :0.5 %
? Mo increases pitting and crevice corrosion resistance in marine atmospheres.

In order to regain strength lost by the reduction in C content solid solution strengthers like Mn, Si were added. Cr, V Nb, Ni, Cu, Mo are added in different combination to increase its strength and other mechanical properties
Vanadium is known for its ability to retard grain growth at elevated temperatures and for its beneficial affinity for carbon and nitrogen. Vanadium promotes fine grain size, increases hardenability and improves wear resistance through the precipitation of its carbides and nitrides. Thus, use is made of these effects in a large variety of steels including rail steels. Vanadium is well-known micro alloying element, which is chatacterised by its disproportionately strong effect on the structure and properties of steel when present in minute quantities (generally less than 0.15%). It increases strength and improves toughness, primarily through a combination of grain refinement and precipitation strengthening both of which depends on the formation of carbides and nitrides or carbonitride particles. By forming very fine precipitates during hot rolling, the vanadium retards the recovery and recrystallization of austenite. Effect of vanadium on transformation temperature are seen on the microstructure (generally with regard to pearlite content and bainite morphology), dislocation density and to some extent, grain size, as well. Vanadium as a carbide, nitride or carbonitride former is the most soluble in austenite, therefore, its effects on austenite recrystallization are weak at high and intermediate rolling temperatures, where the carbonitrides revert to solid solution, but are more pronounced at lower temperatures, where carbonitrides precipitate. Vanadium’s strengthening effects become more pronounced toward the end of and after hot rolling, at temperatures approaching 7000C, when precipitation of vanadium carbonitride begins, continuing well into the ferrite region in low carbon steels.

The present invention is thus directed to development of high toughness hypoeutectoid rail and process of manufacture for the same with following properties:

UTS : 880-1100 N/mm2
% El : 13 (Min)
YS : 560 (Min.) N/mm2
Hardness : 280 (Min) BHN
Fracture Toughness at -20oC : 35 (Min) MPavm

Based on the infrastructural requirements of Indian railways, development of following three rail steel grades were proposed:

HTCR1 (Low carbon microalloyed rail): Chemistry of 72 UTS rails taken as base.
HTCR2 (Low carbon NCC rail): NCC rail taken as base.
HTCR3 (Low C Cu-Mo rail): Cu-Mo rail taken as base

The composition and process steps for production of the stated steel grades for high toughness rail according to the present invention are illustrated by way of the following example:

Example:
HTCR1, 2 & 3 were selected for laboratory heat making and rolling. RDCIS has made & rolled 8 different grades of rail steel. Three heats have been made under HTCR 1, 4 heats under HTCR 2 category & one heat under HTCR 3 category. In order to regain strength lost by the reduction in C content solid solution strengtheners like Mn, Si were added. Cr, V Nb, Ni, Cu, Mo were added in different combination to increase its strength and other mechanical properties.

All heats were made at Melting and Solidification lab of RDCIS. Heats were made in air induction furnace of 50 Kg capacity. Two ingots in dimensions 110x100x350 mm were casted from each heat. Casted heats were allowed to air cooled and stripped at room temperature. About 25 mm from top is sliced to remove the defects associated with pipe. Chemistries of all heats are mentioned in Table 1.

Table 1: Chemistry of laboratory heats (wt%).
Grades C Mn Si P S Cr Mo V Nb Cu Ni
HTCR1 0.40-0.60 1.0-1.6 0.20-0.80 0.030 0.030 0.40-1.0 0.04-0.20 0.01-0.05
HTCR2 0.40-0.60 1.0-1.6 0.20-0.80 0.030 0.030 0.40-1.0 0.04-0.20 0.01-0.05 0.20-0.50 0.20-0.50
HTCR3 0.40-0.60 1.0-1.6 0.20-0.80 0.030 0.030 0.10-0.50 0.20-0.50
90 UTS 0.60-0.80 0.90-1.30 0.20-0.60 0.030 0.030

Each ingot was reheated in reheating furnace. Ingots were soaked at 1100oC for about two hours. Fully soaked ingots were discharged and rolled in two hi experimental rolling mill. All ingots were rolled into 25mm thick plate keeping reduction ratio similar to actual rail rolling. All plates were carefully tagged with identification number and allowed to cool in air.

Samples for the respective tests fabricated in central workshop. All samples were tested at respective machines at RDCIS, BSP & RSP. Tensile tests are validated by retesting at BSP.

Mechanical testing
Mechanical testing of samples of all grades were carried out. Tensile test was carried out at RDCIS and BSP while all other tests were carried out at RDCIS only. Test result is compiled in Table 2.
? HTCR 1 & 2 grades have YS, UTS & % El values more than the required values. However % El for HTCR2 is slightly less (11.2%).
? 283, 298, 287 & 252 BHN hardness was achieved in HTCR 1A ,1B, 2A & 3A grades respectively which are higher than the required value of 280 BHN except for 3A.
? Fracture toughness of all grades except 1B was found higher than 90 UTS rail. It was expected also as lower carbon concentration leads to higher fracture toughness.
? It was inferred with the test results that higher Cr concentration in 1A & 1B was responsible for increasing the UTS above 1100 MPa. Higher strength and hardness was also achieved in 2A with lower elongation. 3A could not match with the required values hence was discarded from consideration. It had happened due to very low carbon (0.44%) and absence of strong strengthening element. It can be compared with 1A in which carbon concentration was kept at 0.42% and UTS of 1100 could be achieved. It was possible due to presence of Cr. The response of other elements like V, Nb & Si could not be ignored. They have definitely improved the mechanical properties. 1A, 1B, 1C were added almost equivalent amount of V, Nb & Si to see the effect of C & Cr on mechanical properties.
? Grade 2A was derived from similar grade being rolled in BSP popularly known as NCC rail. Ni, Cr & Cu concentration in experimental grade 2A was kept similar to NCC grade but Carbon was reduced in a view to increase fracture toughness. Small amount of Nb was also added to compensate the strength loss due to reduction in carbon concentration. All values except elongation were found as per requirement. %El was found slightly lower (11.8%).
? It has been reported in literature that higher amount of Cr leads to lower value of fracture toughness. It has also been noticed that Arcellor Mittal has limited the use of Cr to 0.5 % in their rail steel due to this effect only.
? Based on literature and results found, grade 1C was made by reducing Cr concentration to 0.4 % keeping carbon to 0.49 %. As expected YS & UTS value came down but remained within the specified limit. Highest hardness out of five grades (301 BHN) was achieved with a fracture toughness value of 37 MPavm.

Table 2 : Mechanical properties of lab heats
Rail Type YS, MPa UTS, MPa %El Fracture Toughness, MPavm Fatigue Strength, MPa Hardness, BHN CRI
HTCR 1A 825.5 1101.0 14.90 37.77 330.0 283.0 1.400
HTCR 1B 824.0 1141.0 12.80 32.86 330.0 298.0 1.330
HTCR 1C 653.5 1033 15.8 35.99 330.0 301.5 1.260
HTCR 2A 753.5 1038.5 11.8 36.66 330.0 287.0 1.610
HTCR 2B 722.5 998.5 14.1 31.76 350.0 292.3 1.470
HTCR 2C 742.5 1083.5 11.6 29.70 350.0 292.8 1.510
HTCR 2D 772 1136 12.2 27.76 350.0 313.8 1.430
HTCR 3A 560 870.5 17.5 40.22 300.0 252.0 2.590
90 UTS 460 880 >10 29 280 260 1.0

Plant Scale heat making and rolling

One grade out of 8 made at laboratory scale was selected for plant scale production. Protocol for its plant scale production was made. One heat as per decided chemistry shown in table 3 was made in SMS II of BSP and continuous casted into 24 nos of blooms. 4 Nos of blooms were selected for rolling in Rail & Structural Mill, BSP and properties were investigated.

The rail steel is made in a BOF converter. Steel is passed through LF & RH to remove gas inclusions, adding different alloying elements and maintaining suitable superheat. The degassed steel is continuously casted into a four or three strand continuous bloom caster. The blooms are cut into a length of 5-5.8 m length depending upon the different sections or profiles being rolled. The blooms are slowly cooled to ambient temperature and charged into reheating furnace. These blooms are soaked at 1200-1300oC for 4-6 hours. Blooms are descaled just prior to rolling and rolled through 14 number of different passes. Finishing temperature are maintained between 900-1000oC. After finishing the rails are cooled in air. The composition of steel grade produced by plant heat is given in following Table 3 and Mechanical properties of plant heat is produced in table 4. Accompanying Figure 4 shows the flow chart illustrating the steps involved in producing the high toughness hypoeutectoid steel grades and rail sections produced thereof.

Table 3 : Chemistry of Plant heat (wt%)

Grades C Mn Si P S Cr Nb Cu Ni
HTCR2A 0.40-0.60 1.0-1.6 0.20-0.80 0.030 0.030 0.40-1.0 0.01-0.05 0.20-0.50 0.20-0.50

Table 4 : Mechanical properties of plant heat
Grade YS, MPa UTS, MPa %El Hardness
BHN Fracture Toughness, MPavm
HTCR 2A 600 970 16.27 300 40

On analyzing the above experimental results, it has been observed that hardness of alloyed rails depends on carbon concentration and other alloying elements. All alloying elements added in these steels have tendency to increase hardness as clearly evident from accompanying Figure 5. Impact strength is also found to be seriously affected by increasing allying content of steel. The trend of charpy impact values vs carbon equivalent is shown in Figure 6. Strong correlation was observed between tensile strength and charpy impact values as shown in Figure 7. It was observed that increase in tensile strength comes with compromise with charpy values. Nb content was found to be affecting CVN values as shown in Figure 8, while there was no effect of vanadium on CVN values which is apparent from Figure 9. Fracture toughness was found directly related with CVN values. Higher the fracture toughness higher is the CVN value as shown in Figure 10.

It is thus possible by way of the present invention to provide high toughness hypoeutectoid rail and process of manufacture for the same having carbon content less that 0.6% and wherein in order to regain strength lost by the reduction in C content solid solution strengtheners like Mn, Si were added and alloying elements Cr, V Nb, Ni, Cu, Mo are added in different combination to increase its strength, hardness, toughness with good ductility and corrosion resistance favouring application of such steel grade in producing rails for high axle load railway transportation.

We Claim:
1. High toughness hypoeutectoid rail composition comprising
C: 0.40-0.60 wt%;
Mn: 1.0-1.6 wt%;
Si: 0.20-0.80wt%;
S: upto 0.030wt%;
P: upto 0.030wt%;
Cr: 0.40-1.0wt%;
Nb: 0.01-0.05wt%;
Cu:0.20-0.50 wy.%;
Ni:0.20-0.50 wt %;
and balance is iron.

2. High toughness hypoeutectoid rail composition comprising
C: 0.40-0.60 wt%;
Mn: 1.0-1.6 wt%;
Si: 0.20-0.80wt%;
S: upto 0.030wt%;
P: upto 0.030wt%;
Cr: 0.40-1.0wt%;
V: 0.04-0.20wt%;
Nb: 0.01-0.05wt%;
and
Balance is iron.

3. High toughness hypoeutectoid rail composition as claimed in claim 2 comprising:
Cu 0.20-0.50 wt% and Ni 0.20 -0.50 wt %.

4. High toughness hypoeutectoid rail as claimed in anyone of claims 1 to 3 comprising selectively:
UTS : 880-1100 N/mm2
% El : 13 (Min)
YS : 560 (Min.) N/mm2
Hardness : 280 (Min) BHN
Fracture Toughness at -20oC : 35 (Min) MPavm

5. High toughness hypoeutectoid rail as claimed in anyone of claims 1 to 4 having
Fatigue Strength: 330-350MPa; and
CRI: 1.2-1.6.

6. High toughness hypoeutectoid rail as claimed in anyone of claims 1 to 5 wherein alloying elements are selected in such a way that corrosion resistance properties are also increased.

7. High toughness hypoeutectoid rail as claimed in anyone of claims 1 to 6 wherein lower carbon equivalent is maintained in steel grades produced to favour enhancing welding properties.

8. A process for the production of high toughness hypoeutectoid rails as claimed in anyone of claims 1 to 7 comprising the steps of
(i) Providing the selective heat composition;
(ii) Continuously casting of degassed steel continuous bloom caster
(iii) Cutting the blooms into desired length depending upon the different sections or profiles to be rolled;
(iv) Controlled cooling said blooms to ambient temperature and charging into reheating furnace;
(v) Reheating the blooms in reheat furnace and soaking at 1200-1300 oC for 4-6 hours;
(vi) Descaling the blooms just prior to rolling;
(vii) Hot rolling of blooms in desired number of different passes to produce the rail, maintaining finishing temperature between 900-1000oC; and
(viii) Cooling the rails in air after finishing.

9. A process for the production of high toughness hypoeutectoid rails as claimed in claim 8 comprising the steps of
(i) Making the rail steel by providing BF hot metal in a BOF converter and passing Steel through LF and RH to remove gas inclusions, adding different alloying elements and maintaining suitable superheat so to achieve the composition as claimed in claims 1 or 2;
(ii) Continuously casting said degassed steel into a four or three strand continuous bloom caster
(iii) Cutting the blooms into desired length of 5-5.8 m depending upon the different sections or profiles to be rolled;
(iv) Slowly cooling said blooms to ambient temperature and charging into reheating furnace;
(v) Reheating the blooms in reheat furnace and soaking at 1200-1300 oC for 4-6 hours;
(vi) Descaling the blooms just prior to rolling;
(vii) Hot rolling of blooms in desired number of different passes to produce the rail, maintaining finishing temperature between 900-1000oC; and
(viii) Cooling the rails in air after finishing.

10. A process as claimed in claim 9, wherein said degassing time in RH (Ruhrstahl Heraeus) degasser is maintained from 16 to 24 min and preferably 22 min.

Dated this the 3rd day of March, 2014
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)

ABSTRACT
TITLE: HIGH TOUGHNESS HYPOEUTECTOID RAIL AND A PROCESS FOR ITS PRODUCTION.

The present invention relates to high toughness hypoeutectoid rail with high corrosion and wear resistance and a process for its production wherein Carbon is maintained below 0.6% level to increase the elongation and toughness and the reduction in strength due to reduction in carbon is compensated by adding some alloying elements wherein solid solution strengtheners like Mn, Si are added and Cr, V Nb, Ni, Cu, Mo are added in different combination to increase its strength and other mechanical properties. The alloying elements are selectively provided in such a way that corrosion resistance properties and fracture toughness are increased significantly. Advantageously, the high toughness hypoeutectoid rail steel according to the present invention is also having lower carbon equivalent of such steel grades resulting in enhanced welding properties favourig production of rails suitable for high axle load application.