Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/04—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rails
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
  • B21B1/08—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling structural sections, i.e. work of special cross-section, e.g. angle steel
  • B21B1/085—Rail sections
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/009—Pearlite

Definitions

• This invention relates to methods for producing pearlitic steel rails having a high content of carbon .
• the present steel rails are excellent in both wear resistance and ductility and may be used in railroads for carrying heavy loads .
• grain refinement of the pearlite structure is effective in simultaneously improving both ductility and toughness of the steel rail.
• a decrease in the temperature and an increase in the amount of the reduction rate of the hot rolling process are carried out.
• a reheat treatment at a low temperature after rolling is carried out.
• an acceleration of pearlite transformation from austenite grains by the use of seed transformation is carried out.
• This method is one where a fine-grained pearlite structure is obtained by pearlite transformation caused by rapid cooling after reheat treatment of a steel rail at a low temperature following a rolling process.
• This method improves both the ductility and the toughness of the pearlitic steel rail.
• the carbon rails in which has increased the carbon content in order to improve the wear resistance, causes a decrease in the ductility and toughness of the pearlite structure after the accelerated cooling process. This problem is due to the fact that coarse carbides remain insoluble in the austenite grains when the reheat treatment at a low temperature is carried out.
• the reheat process also introduces economic problems since it generally increases the production cost and decreases the productivity.
• Japanese Laid-open Patent Hei 07-173530 discloses a production method for steel rails with high ductility where three or more consecutive passes of rolling at set intervals of time from one pass to next pass is carried out in the finish rolling process of high carbon content steel rails;
• Japanese Laid-open Patent Hei 2001-234238 discloses a production method for steel rails with a high wear resistance and a high toughness where two or more consecutive passes of rolling at set intervals of time from one pass to the next pass is performed, then continuous rolling and rapid cooling are sequentially carried out in the finish rolling process of high carbon content steel rails;
• Japanese Laid-open Patent Hei 2002-226915 discloses a production method for steel rails with a high wear resistance and a high toughness where cooling is allowed between passes of rolling (inter-stand) , and continuous rolling and rapid cooling are sequentially carried out in the finish rolling process of high-carbon rails.
• An object of the present invention is to provide a method for manufacturing a rail that is excellent in both ductility and wear resistance by producing a pearlite of fine-grain structure and high hardness.
• One embodiment of the invention relates to a method for producing a steel rail having a high content of carbon, comprising: finish rolling the rail in two consecutive passes, with a reduction rate per pass for a cross-section of the rail of 2-30%, wherein conditions of the finish rolling satisfy ttie following relationship: S ⁇ CPTl; wherein the CPTl is the value expressed by the following expression 1
• CPTl 800 / (C x T) (expression 1) wherein S is the maximum rolling interval time (seconds) , and (C * T) is defined as follows; C is the carbon content of the steel, wherein the carbon content is more than 0.85 mass%, but less than or equal to 1.40 mass%, based on the total mass of the steel, and T is the maximum surface temperature (degree C) of a rail head.
• This method produces a steel rail with a high content of carbon that is excellent in wear resistance and ductility.
• Another embodiment of the invention relates to a method for producing a steel rail with a high content of carbon, comprising: finish rolling the rail in three or more passes, with a reduction rate per pass for a cross-section of the rail of 2-30%, wherein conditions of the finish rolling satisfy the following relationship: S ___ CPT2, wherein the CPT2 is the value expressed by the following expression 2,
• S is the maximum rolling interval time (seconds)
• C X T X P is defined as follows;
• C is the carbon content of the steel rail, wherein the carbon content is more than 0.85 mass%, but less than or equal to 1.40 mass%, based on the total mass of the steel, and T is the maximum surface temperature (degree C) of a rail head, and P is the number of passes, which is 3 or more.
• the rail of the present invention in addition to the carbon, further comprises at least one element in the following list: Si, Mn, Cr, Mo, B, Co, Cu, Ni, Ti, Mg, Ca, Al, Zr, N, V, Nb.
• the balance of the rail comprises Fe.
• the rail further optionally comprises impurities, which may be unavoidable.
• the methods of the present invention further comprise: immediately after finish rolling, cooling the surface of the rail head at a cooling rate of 2-30 °C/second until the surface temperature reaches 950-750 °C.
• the methods further comprise cooling the surface of the rail head at a cooling rate of 2-30 ° C/second until the surface temperature reaches at least 600 °C, and then allowing the rail to further cool at room temperature (e.g., approximately 45°F to 95°F, preferably 65°F to 85°F) .
• the methods of the present invention further comprise: after the finish rolling process, when the temperature of the rail head is more than 700 °C, cooling the surface of the rail head at a cooling rate of 2-30 ° C/second until the surface temperature reaches at least 600 °C, and then allowing the rail to further cool at room temperature (e.g., approximately 45°F to 95°F, preferably 65°F to 85°F).
• room temperature e.g., approximately 45°F to 95°F, preferably 65°F to 85°F.
• V, Nb and N are controlled so as to be within a range defined based from an expression concerning each additional amount of V, Nb and N in order to inhibit the growth of austenite grain caused after the continuous rolling,
• FIG.l Shows the relationship between the maximum temperature of the rail head (°C) and the multiple of S, C, T (SX CXT), where S is the maximum interval time of rolling (seconds), C is the carbon content of the steel (mass%) , and T is the maximum temperature of rail head (°C) .
• FIG.2 Shows the relationship between the carbon content (mass%) and the multiple of S, C, T (S X C X T) , where S is the maximum interval time of rolling (seconds) , C is the carbon content of the steel (mass%) , and T is the maximum temperature of the rail head (°C).
• FIG.3 Shows the relationship between the maximum interval time of rolling (second) and the multiple of S, C, T (SxCxT) , where S is the maximum interval time of rolling (seconds) , C is the carbon content of the steel, and T is the maximum temperature of the rail head (°C).
• FIG.4 Shows the relationship between the number of rollings (times) and the multiple of S, C, T, P (SxC TxP) , where S is the maximum interval time of rolling (seconds) , C is the carbon content of the steel (mass%) , T is the maximum temperature of the rail head (°C), and P is the numbers of rollings (times).
• FIG.5 An illustration explaining the different portions of the rail.
• 1 is the top of rail head, and 2 is the head corner.
• FIG.6 Shows the portion of the rail where the specimen for the tensile test is taken.
• FIG.7 Shows the relationship between the carbon content and the total elongation value of the rail.
• the present inventors analyzed factors that cause the pearlite structure to be coarse, which is the reason why ductility is not improved. This analysis was performed by studying the combinations of carbon content, the surface temperature at the rail head, the reduction rate of the cross-section of the rail, and the interval time of rolling. After various experiments, it was found that the grain size of austenite structure turns coarse after continuous hot rolling if the maximum interval time during continuous rolling exceeds a certain value.
• the present inventors investigated why the grain size of austenite become coarse if the maximum interval time of rolling increases. It was found that the growth of grains of austenite structure have a positive correlation with the carbon content of the steel and the maximum surface temperature of the rail head during continuous finish rolling. In addition, it was also found that there is a positive correlation between the growth of grains of austenite structure and the number of passes of rolling, such as when the number of passes of rolling is 3 or more.
• the present inventors carried out an analysis of multiple correlations on the relationship between the optimal interval time of rolling for inhibiting the grain size of austenite from becoming coarse, the carbon content, the maximum surface temperature of the rail head during continuous finish-rolling, and the number of rolling passes.
• the result was that the growth of austenite grain at the interval of rolling is inhibited and a fine-grained austenite structure is obtained if the maximum interval time of continuous rolling is equal to or less than the value calculated by particular equations. If the number of rolling passes is 2, the equation is one based on the carbon content and the maximum surface temperature of the rail head. However, if the number of rolling passes is 3 or more, the equation is one based on the carbon content, the maximum surface temperature of the rail head, and the number of passes of rolling.
• the present inventors also investigated a method for inhibiting the growth of austenite grain caused after continuous rolling by controlling precipitation. It was found that the precipitation of V-carbide, V-Nitride, V-carbonitride, Nb-carbide and Nb-carbonitride generated during continuous rolling causes pinning of austenite grains, which inhibits the growth of austenite grain. In addition, the present inventors investigated the conditions where the precipitation of V-carbide, V-Nitride, V-carbonitride, Nb-carbide and Nb-carbonitride during the continuous rolling can be fully controlled.
• the present inventors further investigated a method of inhibiting the growth of austenite grain after (finishing) continuous rolling by applying rapid cooling immediately after the rolling.
• the result is that the growth of austenite grain after rolling can be inhibited if the surface of the rail head is cooled down rapidly within a predetermined range of temperature and at a predetermined cooling rate immediately after completing the continuous rolling.
• the present inventors further investigated a method of obtaining pearlite structure excellent in wear resistance and ductility from a fine-grained austenite structure.
• the result was that a pearlite structure having high toughness and fine-grained structure can be obtained by rapidly cooling the surface of the rail head having an austenite phase at a predetermined temperature range and at a predetermined cooling rate.
• the obtained pearlite structure of the rail head retains wear resistance and ductility.
• C carbon
• the reason for the limitation on the chemical composition of steel rails: C (carbon) is an element for expediting pearlite transformation and ensuring wear resistance. If the C content is 0.85 mass% or less, it is difficult to ensure a volume ratio of cementite in the pearlite structure, which makes it difficult to ensure wear resistance in use for railroads carrying heavy loads. On the other hand, if the C content exceeds 1.40 mass%, a large amount of pro-eutectoid cementite is generated on the old austenite grain boundary, which lowers the wear resistance and the ductility. In view of this, the C content is limited to the range from more than 0.85 to 1.40 mass%. Preferably, a lower limit of C content of 0.95 mass% can highly improve the wear resistance, which greatly improves the life-span of the rail.
• At least one of Si, Mn, Cr, Mo, B, Co, Cu, Ni, Ti, Mg, Ca, Al, Zr, N, V and/or Nb can be further added when needed for improving the hardness (strength) of the pearlite structure, for improving ductility of the pearlite structure, for preventing a heat affected zone, for instance a welding zone, from softening, and for controlling the section hardness distribution inside the rail head.
• Si is an important element as an oxygen scavenger and as an element for increasing the hardness (strength) of the rail head through solid-solution strengthening with a ferrite phase in the pearlite structure.
• Si is an element for inhibiting generation of a pro-eutectoid cementite structure in a hyper-eutectoid steel to prevent the lowering of ductility. If the Si content is less than 0.05 mass%, these good effects cannot be significantly expected, and if the Si content is more than 2.00 mass%, the weldability is degraded because of generation of oxide and/or generation of a great deal of surface flaws during hot rolling. In addition, the hardenability is drastically increased and a martensite structure is generated which is detrimental to the wear resistance and the ductility of the rail. Thus, the Si content is limited to the range of from 0.05 to 2.00 mass%.
• Mn is an element for increasing the hardenability and for improving the wear resistance by decreasing the pearlite lamellar spacing to ensure the hardness of the pearlite structure. If the Mn content is less than 0.05 mass%, these effects cannot be significantly expected, which makes it difficult to ensure the wear resistance necessary for the rail. If the Mn content is more "than 2.00%, the hardenability is drastically increased and a martensite structure is generated which is detrimental to the wear resistance and the ductility of the rail. Therefore, the Mn content is limited to the range of from 0.05 to 2.00 mass%.
• Cr is an element capable of increasing the equilibrium "transformation temperature, which leads to decrease of the pearlite lamellar spacing to provide high hardness (strength) .
• Cr is also capable of strengthening the cementite phase, which leads to increased hardness (strength) of the pearlite to provide improved wear resistance. If the Cr content is less than 0.05 mass%, these effects cannot be significantly expected. If the Cr content is more than 2.00%, the hardenability is drastically increased and a martensite structure is largely generated which degrades the wear resistance and the ductility of the rail. Therefore, the Cr content is limited to the range of from 0.05 to 2.00 mass %.
• Mo is an element capable of increasing the equilibrium
• B boron
• B is an element for forming iron-carbon boride on the grain boundary of austenite, increasing the fineness of the generated pro-eutectoid cementite structure, making the pearlite transformation temperature less dependent on the cooling rate and making hardness distribution of the rail head uniform, which prevents degradation of rail ductility and provides a longer life. If the B content is less than 0.0001 mass%, these effects cannot be significantly expected, i.e. there cannot be expected improvement with respect to generation of pro-eutectoid cementite structure and/or rail head hardness distribution uniformity.
• Co is an element for making solid-solution with ferrite in the pearlite structure, which improves the hardness (strength) of the pearlite structure by solid-solution strengthening. Co is also an element for increasing the transformation energy of pearlite, which improves the ductility by refining the grain of the pearlite structure. Also, Co is an element for improving wear resistance by refining the grain of ferrite which is formed by wheel contact on the rail head.
• Co content is less than 0.003 mass%, these effects cannot be significantly expected. If the Co content is more than 2.00 mass%, the ductility of the ferrite phase in the pearlite structure is drastically lowered, which causes spalling damage on the rolling contact surface and lowers the surface damage resistance of the rail. Therefore, the Co content is limited to the range of from 0.003 to 2.00 mass%.
• Cu is an element for making solid-solution with ferrite in the pearlite structure, which improves the hardness (strength) of the pearlite structure by solid-solution strengthening. If the Cu content is less than 0.01 mass%, these effects cannot be readily expected.
• the Cu content is more than 1.00 mass%, the hardenability is drastically increased and a martensite structure is generated which is detrimental to the wear resistance of the rail. Further, the ductility of the ferrite phase in the pearlite structure is drastically lowered, which degrades the ductility of the rail. Therefore, the Cu content is limited to the range from 0.01 to 1.00 mass %.
• Ni is an element for preventing the creation of brittleness during hot rolling caused by adding Cu and for increasing the hardness (strength) of the pearlitic steel by solid-solution strengthening with ferrite. It is also an element for inhibiting softening in heat-affected zones, for instance welding zones, by precipitation strengthening (fine precipitation of Ni 3 Ti, intermetallic compound) . If the Ni content is less than 0.01 mass%, these effects cannot be readily expected. If the Ni content is more than 1.00 mass%, the ductility of the ferrite phase is drastically lowered, which causes on the rolling contact surface and lowers the surface damage resistance of the rail. Therefore, the Ni content is limited to the range of from 0.01 to 1.00 mass% .
• Ti is a element effective in preventing creation of brittleness of the welded joint portion by increasing the fineness of the structure of heat affected zones which are heated up to the austenite region taking advantage of the insolubility of Ti nitride and Ti carbide precipitated in reheating during welding. If the Ti content is less than 0.0050 mass%, these effects cannot be readily expected. If the Ti content is more than 0.0500 mass%, coarse Ti nitride and Ti carbide are generated, which greatly lowers the ductility and the fatigue-damage resistance of the rail. Thus, the Ti content is limited to the range of from 0.0050 to 0.0500 mass%.
• Mg is an element effective in improving the ductility of the pearlite structure by forming fine oxide bonding with 0 (oxygen) , S (sulfur) or Al, inhibiting grain growth of crystal grains during reheating for rail rolling and for improving the fineness of the austenite grain.
• Mg is also an element effective in improving the ductility of the pearlite structure by finely dispersing MnS with MgO and/or MgS, forming Mn depleted zones around MnS, expediting the generation of pearlite transformation, and increasing the fineness of the pearlite block size as a result. If the Mg content is less than 0.0005 mass%, these effects cannot be readily expected.
• the Mg content is more than 0.0200 mass%, coarse Mg oxide is generated, which greatly lowers the ductility and the fatigue-damage resistance of the rail.
• the Mg content is limited to the range of from 0.0005 to 0.0200 mass %.
• Ca is an element effective in improving the ductility of the pearlite structure by forming sulfide CaS (Ca has a strong bonding force with S) , finely dispersing MnS with CaS, forming Mn depleted zone around MnS, expediting the generation of pearlite transformation, and increasing the fineness of the pearlite block size as a result. If the Ca content is less than 0.0005 mass%, these effects cannot be expected. If the Ca content is more than 0.0150 mass%, coarse Ca oxide is generated, which lowers the ductility and the fatigue-damage resistance of the rail. Thus, the Ca content is limited to the range of from 0.0005 to 0.0150 mass % -
• Al is an important element as an oxygen scavenger.
• Al is also an element for shifting the eutectoid transformation temperature toward the side of a higher temperature and for shifting the amount of eutectoid carbon toward the higher side.
• Al is also an element effective in inhibiting the generation of pro-eutectoid cementite structure and in highly strengthening the pearlite structure. If the Al content is less than 0.0100 mass%, these effects cannot be expected. If the Al content is more than l.OO mass%, it becomes difficult to dissolve into the steel, which causes generation of coarse aluminum-type inclusions which can be a source of fatigue-damage and lower the ductility and the fatigue-damage resistance of the rail. Also, oxide is formed at welding, which degrades weldability drastically. In view of above, the Al content is limited to the range of from 0.0100 to 1.00 mass % .
• Zr is an element for inhibiting the formation of a segregation zone in the central region of the billet (bloom, slab) and thereby inhibiting generation of pro-eutectoid cementite structures generated in the segregation region of the rail. This is made by increasing the percentage of equiaxed crystals (grains) in the solidification structure, since Zr0 2 inclusions have a good lattice match, and become solidification cores of the high carbon content steel rail of which the primary crystal is ⁇ -Fe, which enables to form high equi-axed crystal rate in the solidification structure. If the Zr content is less than 0.0001 mass%, the number of Zr0 2 inclusions are not enough to work as solidification cores.
• N enables the inhibition of grain growth of austenite grain by precipitating V nitride, V-carbonitride and/or Nb-carbonitride during continuous rolling.
• N is also an element effective in increasing both the ductility and the hardness (strength) of the pearlite structure by precipitating V nitride, V-carbonitride and/or Nb-carbonitride during the cooling process after continuous rolling. Further N is an element effective in preventing heat affected zones of welded joint parts from softening by precipitating V nitride, V-carbonitride and/or Nb-carbonitride in the heat affected zones which is reheated at a temperature range below the Acl point.
• N is an element effective in improving the ductility of the pearlite structure by forming segregation on the austenite grain boundary, which expedites pearlite transformation from the austenite grain boundary and increases the fineness of the pearlite block size. If the N content is less than 0.0060 mass%, the effects mentioned above are very weak. If the N content is more than O.0200 mass%, it becomes difficult to dissolve N into the steel to make a solid-solution, which generates bubbles which can be a source of fatigue damage. In view of this, the N content is limited to the range of from 0.0060 to 0.0200 mass%. Usually, steel rail initially includes N as impurity by a maximum of 0.0050 mass%. Consequently,- N should be added in amounts sufficient to provide N in amounts within the range of from 0.0060 to 0.0200 mass% to expect the above effects.
• V enables the inhibition of grain growth of austenite grain by precipitating V carbide, V nitride, and/or V-carbonitride during continuous rolling.
• V is also an element effective in increasing both the ductility and the hardness (strength) of the pearlite structure through precipitation-hardening by precipitating V carbide, V nitride, and/or V-carbonitride during the cooling process after continuous rolling.
• Further V is an element effective in preventing heat affected zones of welded joint parts from softening by precipitating V carbide, V nitride, and/or V-carbonitride at relatively a high temperature range in the heat affected zones, which are reheated at a temperature range below the Acl point.
• V content is less than 0.005 mass%, these effects cannot be significantly expected, i.e. no significant improvement in the ductility and the hardness of the pearlite structure will be achieved. If the V content is more than 0.500 mass%, coarse V carbide, V nitride, and/or V-carbonitride, which can be sources of atigue-damage, generate and the ductility and the fatigue damage resistance of the rail are degraded. Thus, the V content is limited to the range of from 0.005 to 0.500 mass%.
• Nb enables the inhibition of grain growth of an austenite grain by precipitating Nb carbide, and/or Nb-carbonitride during continuous rolling.
• Nb is also an element effective in increasing both the ductility and the hardness (strength) of the pearlite structure through precipitation-hardening by precipitating Nb carbide, and/or Nb-carbonitride during the cooling process after continuous rolling.
• Nb is an element effective in preventing heat affected zones of welded joint parts from softening by precipitating Nb carbide, and/or Nb-carbonitride at temperatures ranging from low to high in the heat affected zones, which are reheated at a temperature range below the Acl point.
• the Nb content is less than 0.002 mass%, these effects cannot be significantly expected, i.e. no significant improvement in the ductility and the hardness of the pearlite structure can be expected. If the Nb content is more than 0.050 mass%, coarse Nb carbide, and/or Nb-carbonitride, which can be sources of fatigue-damage, generate and the ductility and the fatigue damage resistance of the rail are degraded. Thus, the Nb content is limited to the range of from 0.002 to 0.050 mass %. [ 0048 ]
• V, Nb or N The reason for the limitation on the added amount of V, Nb or N, which enables the inhibition of the grain growth of austenite grain after rolling is as follows. Concerning the above-mentioned V, Nb and N, it is preferable to add these elements in amounts such that (expression 3) below is satisfied. The reason why the added amount of V, Nb or N is limited to the range calculated based on the (expression 3) below concerning V mass%, Nb mass% and N mass% is now explained. The reason is that in continuous rolling of high carbon content steel rail, methods for inhibiting grain growth of austenite grain after rolling by controlling precipitations have been studied.
• V, Nb and N needed to sufficiently inhibit the growth of austenite grain was experimentally investigated. The investigation indicated that the contribution rate by unit amount (mass%) of V, Nb and N (N is added to expedite formation of V-nitride, V-,
• Nb-carbonitride Nb-carbonitride
• PC V(mass%) + 10xNb(mass%) + 5xN(mass%) (expression 3)
• PC V(mass%) + 10xNb(mass%) + 5xN(mass%) (expression 3)
• N is added in order to expedite the formation of precipitation V-nitride, V-carbonitride, and Nb-carbonitride.
• Addition of N alone does not form the above precipitations, i.e. there is no effect of inhibiting the growth of the austenite grain. Consequently, in order to inhibit the growth of the austenite grain, addition of V alone, Nb alone, or addition of a combination of V and Nb, V and N, Nb and N, or V, Nb and N can be made.
• N alone in other words, when neither V nor Nb are added, the value of PC is regarded as 0 (zero) mass%.
• steel rail usually contains N as an impurity in an amount of about 0.0050 mass% at maximum.
• N is added so that the N content becomes equal to or more than 0.0060 mass%. Therefore, in the calculation of the (expression 3) above, the N content is assumed to be 0 (zero) mass% if the N content is less than 0.0060 mass% .
• the reason for the limitation of the cross-section reduction rate per pass is as follows.
• the cross-section reduction rate per pass of the rail in the finish rolling is limited to the range of from 2 to 30%. If the cross-section reduction rate of the rail is more than 30%, a great amount of heat is generated, which largely increases the temperature of the rail head surface. This causes the austenite grain of the rail head to become coarse, which makes it difficult to ensure the ductility of the rail. In addition, it also becomes difficult to ensure the formability during rail rolling.
• the cross-section reduction rate per pass in the finish rolling is less than 2%, it is not possible to obtain the necessary strain to re-crystallize the austenite grain of the rail head. Therefore, the austenite grain is not fine-grain, which fails to ensure the ductility of the rail.
• the cross-section reduction rate per pass in the finish rolling is limited to the range from 2 to 30%.
• the reason for the limitation of the maximum interval time of rolling is as follows.
• the maximum interval time of rolling (S in seconds) is limited to a time equal to or less than the value calculated from the following two expressions (expression (1) and expression (2) below) .
• the experiment involving two passes of continuous rolling with a 2-30% cross-sectional reduction rate per pass is carried out with respect to high carbon content steel rail while changing the conditions of maximum rolling interval time (S) , the carbon content of the steel (C, mass%) , and the maximum surface temperature of the rail head (T, °C) and the ductility (total elongation value) of the steel rail was checked by a tensile test.
• steel containing the same chemical composition is rolled with the conditions of one pass, a rolling temperature of 950 °C and a cross-section reduction rate of
• FIG.l shows the results of a continuous rolling experiment.
• the experimental conditions were: carbon content (C, mass%) of the steel was 1.0 mass%, the cross-section reduction rate was 2-30% per pass, the maximum rolling interval time (S, seconds) was 0.8 seconds, the number of passes was 2, and the maximum surface temperature of the rail head was changed (T, °C) .
• the vertical axis represents (S C T) and the horizontal axis represents the maximum surface temperature of the rail head (T, °C) .
• FIG.2 shows the result of another continuous rolling experiment.
• the experimental conditions were: the carbon content (C, mass.) of the steel was changed, the cross-section reduction rate was 2-30% per pass, the maximum rolling interval time (S, seconds) was 0.8 seconds, the number of passes was 2, and the maximum surface temperature of the rail head was 950 °C.
• the vertical axis represents (S C T) and the horizontal axis represents the carbon content (C, mass%) .
• FIG.3 shows the results of another continuous rolling experiment.
• the experimental conditions were: the carbon content (C, mass%)of the steel was 1.0 mass%, the cross-section reduction rate was 2-30% per pass, the maximum rolling interval time (S, seconds) was changed, the number of passes was 2, and the maximum surface temperature of the rail head was 950 °C.
• the vertical axis represents (SxCxT) and the horizontal axis represents the maximum rolling interval time (S, seconds).
• FIG.4 shows the results of another continuous rolling experiment.
• the experimental conditions were: the carbon content (C, mass%)of the steel was 1.0 mass%, the cross-section reduction rate was 2-30% per pass, the maximum rolling interval time (S, seconds) was 0.5 seconds, the number of passes (P, times) was changed (3-6 passes), and the maximum surface temperature of the rail head (T, °C) was 950 °C.
• steel containing the same chemical composition was rolled with the conditions of one pass, a rolling temperature of 950 °C and a cross-section reduction rate of 10% per pass, and the ductility (total elongation value) was checked in the same manner.
• the vertical axis represents (SxCxT P) and the horizontal axis represents the number of passes (P, times) in the continuous finish rolling.
• the present inventors have studied the operation conditions of continuous rolling to ensure the ductility (total elongation value) using the correlations described above.
• C carbon content of the steel
• P passes
• the maximum rolling interval time (S, seconds) and the maximum surface temperature of the rail head (T, °C) are controlled.
• the maximum rolling interval time (S, seconds) and the ductility (total elongation value) have a correlation.
• the maximum rolling interval time S has to be less than or equal to the value CPTl calculated from the following (expression 1) consisting of C (mass%) of the carbon content of the steel and T (°C) of the maximum surface temperature of a rail head during the rolling, and if the number of passes is 3 or more, the maximum rolling interval time S has to be less than or equal to the value CPT2 calculated from the following (expression 2) consisting of C (mass%) of the carbon content of the steel, T (°C) of the maximum surface temperature of a rail head during the rolling and P (number of times) of the number of passes.
• CPTl 800/ (C x T) (expression 1)
• CPT2 2400/ (C x T x P) (expression 2) S(sec) ⁇ CPTl, CPT2
• the rolling interval time means the time that a blank (billet, bloom, slab) needs to travel from one rolling stand (pass) to next rolling stand (pass) , wherein each of the rolling stands is required to be operated with the reduction rate of 2% or more. In other words, if a particular rolling stand in the continuous finish rolling process is operated with the reduction rate less than 2%, the particular stand cannot be taken into account for determining the rolling interval time, but rather be ignored.
• the maximum rolling interval time means the longest time among the rolling interval times. In the case of 3 passes (3 rolling stands), for example, if the time A taken between first pass and second pass is longer than the time B taken between second pass and third pass, then the time A is the Maximum rolling interval time.
• the surface temperature of the rail head (T, °C) is the surface temperature of the rail head measured between each consecutive pass.
• the maximum surface temperature of the rail head is the highest temperature among those measured.
• the reason for the limitation of the condition on the rapid cooling of the rail head immediately after hot rolling is as follows. If the cooling rate for cooling the rail head immediately after hot rolling is less than 2°C/sec, the austenite grains become coarse during the cooling, which degrades the ductility of the rail head.
• the cooling rate for cooling the rail head immediately after hot rolling is more than 30°C/sec, a large amount of heat recuperation from inside the rail head generates after the rapid cooling, which raises the temperature of the surface of the rail head to form coarse austenite grains and leads to degradation of the ductility. Therefore, the cooling rate for the rail head immediately after hot rolling is limited to the range of 2-30°C/sec. [0063] As for the temperature range within which the rapid cooling is applied, if the rapid cooling is terminated at a temperature of more than 950 °C, austenite grains may significantly grow depending on the carbon content of the steel, which causes coarse grains of austenite and degrades the ductility of the rail head.
• the rapid cooling is still applied after the temperature reaches below 750 °C, a large amount of heat returning from inside the rail head may generate depending on the rate of cooling, which raises the temperature of the surface of the rail head and generates coarse austenite grains, which lower the ductility.
• the temperature range within which the rapid cooling is applied is limited to the range of 950-750 °C.
• the rapid cooling rate As for the range of the rapid cooling rate, if the rapid cooling rate of the surface of the rail head is less than 2° C/second, no improvement on hardness of the rail head can be seen. Besides, pro-eutectoid cementite may be generated depending on the carbon content and/or alloy elements, which degrades the ductility. And, if the rapid cooling rate is more than 30°C/second, a martensite structure is generated in the present composition system, which significantly degrades the ductility of the rail head. Thus, the rapid cooling rate is limited to a range of 2-30°C/second.
• the rapid cooling As for the temperature to which the rapid cooling is terminated, if the rapid cooling is terminated at a temperature of more than 600 °C, a large amount of heat returning from inside the rail is generated. As a result, the temperature rise causes pearlite transformation, which leads to the failure of hardening the pearlite structure, i.e., failure of ensuring wear resistance. This also causes the pearlite structure to become coarse, which degrades the ductility of the rail head surface. Therefore, the rapid cooling has to be performed until the temperature reaches at least 600 °C. There is no limitation on the lower temperature. However, 400 °C is the practical lower limit considering the requirements of ensuring the hardness of the rail head surface and preventing the martensite structure from being formed in the segregation region inside the rail head.
• FIG.5 shows the names of the parts of a rail. Shown are the top of the rail head 1 and the head corner 2.
• the "surface temperature of the rail head” of the present invention described herein refers to the surface temperature at the top of head 1 and the head corners 2, 2 in FIG.5.
• austenite grain can be fine-grained at the rolling and the ductility of the rail is improved.
• the temperatures relating to the heat treatments performed immediately after or after continuous hot rolling, such as the rapid cooling process refer to the temperature of surface of the top of head 1 and the head corners 2,2, or the temperature of the region within a depth of 5mm from the head surface. By controlling the temperature of this region, a fine-grained pearlite structure having an excellent wear resistance can be obtained.
• coolant used for cooling is not limited. However, air, mist, and a mixture of air and mist are preferable to ensure controlled cooling.
• the metal structure of the rail head produced by the present invention should preferably be a pearlite structure.
• a slight amount of pro-eutectoid ferrite structure, pro-eutectoid cementite structure and bainite structure may generate in the pearlite structure depending on the selection of chemical composition and/or selection of rapid cooling conditions. However, a slight amount of these structures in the pearlite structure do not significantly affect the fatigue strength or the ductility.
• a rail head produced using the present invention can therefore include a slight amount of pro-eutectoid ferrite structure, pro-eutectoid cementite structure and bainite structure .
• TABLE 1 shows the chemical composition of the tested steel rails.
• TABLE 2 shows the elements (carbon content, PC value) , hot rolling conditions, heat treatment conditions, micro-structures, hardnesses and total elongation values of tensile test of the rails produced by the methods of the invention from the tested steel rails.
• TABLE 3 shows the elements (carbon content, PC value) , hot rolling conditions, heat treatment conditions, micro-structures, hardnesses and total elongation values of tensile test of the rails produced by conventional methods from the tested steel rails.
• the rails for the examples are as follows: (1) Rails produced by the methods of the invention (26 rails listed in TABLE 2 denoted by Nos. 1-26) . Rails denoted Nos. 1-4, and 6-15: chemical composition are shown in TABLE 1 and the hot rolling conditions and heat treatment conditions are shown in TABLE 2. Rails denoted Nos. 5, and 16-26: chemical composition are shown in TABLE 1 and the hot rolling conditions, reheat treatment conditions and PC values are shown in TABLE 2. (2) Rails produced by methods for comparison (18 rails listed in TABLE 3 denoted by Nos. 27-44). Rails denoted 27-44: chemical composition are shown in TABLE 1 and the hot rolling conditions are shown in TABLE 3.
• Test Machine all-purpose miniature tensile test machine Form of the specimen: Similar to JIS No.4,
• Length of parallel part 25mm
• Diameter of parallel part 6mm
• TABLE 1 shows the chemical composition of steel for tested rail examples .
• TABLE 2 shows the conditions of hot rolling, cooling after rolling and heat treatment after rolling and the properties of the heads of the rails produced by the method of the present invention.
• TABLE 3 shows the conditions of hot rolling, cooling after rolling and heat treatment after rolling and the properties of the heads of the rails produced by conventional methods.
• TABLE 2 and rail 32 in TABLE 3 are the same in composition as steel G in TABLE 1, whose carbon content is 1.10 mass%.
• the value of the total elongation on the tensile test of rail 10 in TABLE 2 is 2.0% higher than that of rail 32 in TABLE 3, the former and the latter being 11.7% and 9.7% respectively.
• the difference comes from the fact that the maximum interval time of rolling of rail 10 in TABLE 2 is controlled less than expression (2) value and this control makes the austenite structure fine-grained.
• the control of the maximum interval time of the rolling must meet expressions (1) or (2) above, and with continuous rolling this improves the ductility of the rails.
• control of the PC value within the range of 0.04-0.30 improves the ductility of the rails.
• FIG.7 shows the relationship ' between the carbon content and the value of the total elongation on the tensile test, which have the most influence on the ductility.
• rails 1-4 and 6-15 produced by the methods of the present invention have improved ductility on the head of the rails in any amount of carbon content because the maximum interval time of the rolling is controlled.
• rails 5 and 16-26 have further improved ductility on the head of rails in any amount of carbon content because not only the maximum interval time of rolling but also the PC value, which is calculated by the equation based on V, Nb, N, is controlled within the required range of 0.04-0.30 in rails 5 and 16-26. Therefore, not only is the austenite structure made fine-grained but also the growth of the austenite grains is inhibited.
• rails 1-26 have improved wear resistance and ductility.
• the mechanisms are as follows:
• the maximum interval time of rolling is controlled to be less than the value calculated by the equation based on the carbon content of steel, the maximum head temperature of the rails during continuous rolling and the number of rolling passes and rapid cooling is carried out immediately after rolling.
• These controls provide a fine-grain austenite structure.
• the heat treatment process after rolling at a suitable temperature range and at a suitable cooling rate is carried out. These controls inhibit the generation of pro-eutectoid cementite and martensite, which are detrimental to the ductility of the rails.

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