JP2007277716A - High-strength perlitic rail with excellent delayed-fracture resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength perlitic rail which is inexpensive and, despite this, has a tensile strength of 1,200 MPa or higher and excellent delayed-fracture resistance. <P>SOLUTION: The rail contains, in terms of mass%, 0.6-1.0% C, 0.1-1.5% Si, 0.4-2.0% Mn, up to 0.035% P, and 0.0005-0.010% S, the remainder being iron and unavoidable impurities. The rail has a tensile strength of 1,200 MPa or higher. In a lengthwise-direction section of at least the head part of the rail, A-type inclusions have a major-axis dimension of ≤250 μm and the number of A-type inclusions having a major-axis dimension of 1 to 250 μm is <25 per mm<SP>2</SP>of the area examined. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a high-strength pearlite rail having excellent delayed fracture resistance having a tensile strength of 1200 MPa or more.

  In high-axle heavy railways, such as mining railways that mainly transport ores, etc., which have large loads of trains and wagons, the load applied to the axles of freight cars is much larger than that of passenger cars, and the use environment of the rails is also severe. It is. Conventionally, steel having a pearlite structure has been used as a rail used in such an environment from the viewpoint of emphasizing wear resistance. However, in recent years, the load on freight cars has been further increased in order to improve the efficiency of transportation by rail, and the use environment of rails has become increasingly severe. There is a demand for improvement in fatigue resistance and fatigue damage resistance.

In order to meet such demands, from the viewpoint of emphasizing wear resistance and fatigue damage resistance, increasing the strength of the rail is directed, and as shown in Patent Document 1, the tensile strength is 120 kg / mm ² (1200 MPa). The above high-strength pearlite rails have been proposed. However, it is known that a high strength steel having a tensile strength of 1200 MPa or more has a high risk of delayed fracture, and the technology of Patent Document 1 is insufficient in delayed fracture resistance even though the strength is high.

  As techniques for improving delayed fracture characteristics of high-strength pearlite steel, for example, Patent Documents 2 and 3 disclose techniques for improving delayed fracture resistance by high-strength pearlite steel by wire drawing. However, when this technique is applied to a rail, there arises a problem that the manufacturing cost increases due to the strong wire drawing.

It is known that it is effective to control the form and amount of the A-based inclusion as a technique for improving delayed fracture resistance other than the above. Patent Documents 4 to 7 disclose control of the form and amount of the A-based inclusion in the rail steel. However, Patent Documents 4 to 7 are intended to improve the toughness and ductility of the rail. For example, in Patent Document 5, the size of A-based inclusions is 0.1 to 20 μm and the number of A-based inclusions is 1 mm ^2. A method of improving the toughness and ductility of the rail by controlling to 25 to 11,000 is disclosed, and although the toughness and ductility are improved, the delayed fracture resistance is not taken into consideration, and the good delay resistance is not necessarily considered. Destructive properties are not always obtained. Here, the A-type inclusion is an A-type inclusion defined in JIS G0555 Annex 1.
Japanese Patent Laid-Open No. 7-18326 Japanese Patent No. 3648192 JP-A-5-287450 JP 2000-328190 A JP-A-6-279928 Japanese Patent No. 3323272 JP-A-6-279929

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a high-strength pearlite rail that is inexpensive but has a tensile strength of 1200 MPa or more and excellent delayed fracture resistance.

In order to solve the above problems, the present invention provides the following (1) to (3).
(1) By mass%, C: 0.6 to 1.0%, Si: 0.1 to 1.5%, Mn: 0.4 to 2.0%, P: 0.035% or less, S: It contains 0.0005 to 0.010%, the balance is made of Fe and inevitable impurities, the tensile strength is 1200 MPa or more, and the size of the A-based inclusion is 250 μm in the rail head longitudinal section. A high-strength pearlite rail excellent in delayed fracture resistance, characterized in that there are less than 25 A-type inclusions having a size of 1 μm or more and 250 μm or less per 1 mm ^{2 of} test area.
(2) The high-strength pearlite rail excellent in delayed fracture resistance according to (1) above, wherein the amount of hydrogen in the rail is 2 ppm or less on a mass basis.
(3) In mass%, V: 0.5% or less, Cr: 1.5% or less, Cu: 1% or less, Ni: 1% or less, Nb: 0.05% or less, Mo: 1% or less W: One high-strength pearlite rail having excellent delayed fracture resistance according to (1) or (2) above, containing one or more selected from 1% or less.

In order to solve the above-mentioned problems, the present inventors have optimized the composition of the components and investigated the rail in which the form and amount of the A-based inclusions and the amount of hydrogen in the steel are changed. If the size of the inclusions is less than 1 μm, it is almost spherical, so the effect on delayed fracture resistance is not significant, but if it is greater than 1 μm, it will elongate and the effect on delayed fracture resistance will increase. Therefore, it has been found that by controlling the number of A-type inclusions having a size of 1 μm or more, the delayed fracture resistance is improved over conventional hypoeutectoid, eutectoid and hypereutectoid pearlite rails. It was also found that the delayed fracture resistance is further improved by limiting the hydrogen in the steel that causes delayed fracture resistance. In the present invention, based on such knowledge, the components in the rail are defined in a specific range, and in the longitudinal section of the rail head, the size of the A-based inclusion is 250 μm or less, and further 1 μm or more. , And control so that there are less than 25 A-type inclusions having a size of 250 μm or less per 1 mm ^{2 of the} test area, thereby having excellent delayed fracture resistance while having a tensile strength of 1200 MPa or more. Perlite rails can be realized. In addition to this, by setting the amount of hydrogen in the steel to 2 ppm or less, the delayed fracture resistance is further improved.

  According to the present invention, the tensile strength is 1200 MPa or more, and by controlling the size and number of the A-based inclusions in the steel, there is no need to perform costly strong wire drawing, and delayed fracture resistance Therefore, it is possible to provide a high-strength pearlite rail excellent in delayed fracture resistance while being inexpensive.

  Hereinafter, the present invention will be specifically described.

First, chemical components will be described.
The rail of the present invention is mass%, C: 0.6 to 1.0%, Si: 0.1 to 1.5%, Mn: 0.4 to 2.0%, P: 0.035% or less , S: 0.0005 to 0.010%, with the balance being composed of Fe and inevitable impurities. If necessary, V: 0.5% or less, Cr: 1.5% or less, Cu: 1% or less, Ni: 1% or less, Nb: 0.05% or less, Mo: 1% or less , W: 1 type or 2 types or more selected from 1% or less. Moreover, it is preferable that the amount of hydrogen in steel is 2 ppm or less on a mass basis.

C: 0.6 to 1.0%
C forms cementite in the pearlite structure and is an essential element for securing the strength of the rail, and the strength improves as the content increases. When the C content is less than 0.6%, it is difficult to obtain excellent strength as compared with the conventional heat-treated pearlite steel rail. On the other hand, if it exceeds 1.0%, pro-eutectoid cementite is generated at the austenite grain boundary during transformation after hot rolling, and the delayed fracture characteristics are remarkably lowered. Therefore, the C content is set to 0.6 to 1.0%. Preferably, the C content is 0.6 to 0.9%.

Si: 0.1 to 1.5%
Si is an element added as a deoxidizer, and for that purpose, it is necessary to contain 0.1% or more. Further, since Si has an effect of improving strength by solid solution strengthening to ferrite in pearlite, it is positively added. However, if the amount of Si exceeds 1.5%, a large amount of oxide inclusions are generated due to the high bonding strength with oxygen of Si, and the delayed fracture resistance is deteriorated. Therefore, the Si content is set to 0.1 to 1.5%. Preferably, the Si content is 0.2 to 1.2%. More preferably, the Si content is 0.2 to 0.9%.

Mn: 0.4 to 2.0%
Mn is an element that contributes to increasing the strength and ductility of the rail by reducing the pearlite transformation temperature and reducing the lamellar spacing of the pearlite structure. However, if the content is less than 0.4%, a sufficient effect cannot be obtained. If the content exceeds 2.0%, a martensitic structure due to microsegregation of steel tends to occur, and hardening and embrittlement occur during heat treatment and welding. The resulting material deteriorates. Therefore, the Mn content is set to 0.4 to 2.0%. Preferably, the Mn content is 0.4 to 1.5%.

P: 0.035% or less The inclusion of P exceeding 0.035% deteriorates ductility. Therefore, the P content is 0.035% or less. Preferably, the P content is 0.020%.

S: 0.0005 to 0.010%
S is present in steel mainly in the form of A-based inclusions, but when the content exceeds 0.010%, the amount of inclusions increases remarkably, and at the same time, coarse inclusions are produced, so that the resistance to delay is increased. Causes degradation of fracture characteristics. On the other hand, if it is less than 0.0005%, the cost of the rail is increased. Therefore, the S content is set to 0.0005 to 0.010%. Preferably, the S content is 0.0005 to 0.008%. More preferably, it is 0.0005 to 0.006%.

V: 0.5% or less V is precipitated as carbonitride during and after rolling, functions as a hydrogen trap site, and improves delayed fracture characteristics. Therefore, V is added as necessary. In order to obtain the effect, the V content is preferably 0.005% or more. However, if added over 0.5%, a large amount of coarse carbonitride precipitates, resulting in a decrease in delayed fracture resistance. Therefore, when adding V, the content is made 0.5% or less.

Cr: 1.5% or less Cr is an element for further strengthening by solid solution strengthening, and is added as necessary. In order to obtain the effect, the Cr content is preferably 0.2% or more. However, if its content exceeds 1.5%, the hardenability becomes high, martensite is generated, and the ductility is lowered. Therefore, when adding Cr, the content is made 1.5% or less.

Cu: 1% or less Cu, like Cr, is an element for further strengthening by solid solution strengthening, and is added as necessary. In order to obtain the effect, the Cu content is preferably 0.005% or more. However, if the content exceeds 1%, Cu cracking occurs. Therefore, when adding Cu, the content is made 1% or less.

Ni: 1% or less Ni is an element for increasing the strength without deteriorating ductility, and is added as necessary. Moreover, in order to suppress Cu cracking by adding Cu together, it is desirable to add Ni when Cu is added. In order to obtain the effect, the Ni content is preferably 0.005% or more. However, if the content exceeds 1%, the hardenability is increased, martensite is generated, and the ductility is lowered. Therefore, when adding Ni, the content is made 1% or less.

Nb: 0.05% or less Nb precipitates as carbonitride during and after rolling, functions as a hydrogen trap site, and improves delayed fracture characteristics. Therefore, Nb is added as necessary. In order to obtain the effect, the Nb content is preferably 0.005% or more. However, if added over 0.05%, a large amount of coarse carbonitride precipitates, resulting in a decrease in delayed fracture resistance. Therefore, when adding Nb, the content is made 0.05% or less. Preferably, the Nb content is 0.005 to 0.03%.

Mo: 1% or less, W: 1% or less Mo and W are precipitated as carbides during and after rolling, function as hydrogen trap sites, improve delayed fracture characteristics, and by solid solution strengthening Since further increase in strength can be achieved, it is added as necessary. In order to obtain the effect, the Mo and W contents are each preferably 0.005% or more. However, when both Mo and W are added in excess of 1%, martensite is generated and ductility is lowered. Therefore, in both cases of adding Mo and adding W, the content is made 1% or less. More preferably, Mo is 0.25% or less and W is 0.50% or less.

Hydrogen content in steel: 2 ppm or less Hydrogen is an element that causes delayed fracture. If the amount of hydrogen in steel exceeds 2 ppm, a large amount of hydrogen accumulates at the inclusion interface, and delayed fracture tends to occur. Therefore, it is preferable to limit the amount of hydrogen in steel to 2 ppm or less.

  The balance is Fe and inevitable impurities. Here, examples of impurities include P, N, O, etc. P is allowed to 0.035% as described above, N is allowed to be 0.005%, and O is allowed to be 0.004%. Further, in the present invention, Al and Ti mixed as impurities are allowed to be 0.0010% each. That is, since Al and Ti form oxides, the amount of inclusions in the steel increases, the delayed fracture resistance decreases, and the fatigue damage resistance, which is a basic characteristic of rails, decreases. It is necessary to control Ti to be 0.0010% or less.

Next, the tensile strength, the size and number of A-based inclusions will be described.
Tensile strength: 1200 MPa or more When the tensile strength is less than 1200 MPa, the delayed fracture resistance of the rail is good, but the wear resistance and fatigue resistance equivalent to those of a conventional pearlite rail cannot be obtained. Therefore, the tensile strength is set to 1200 MPa or more.

Size of A-type inclusions: 250 μm or less in the longitudinal section of the rail head portion If the size of A-type inclusions exceeds 250 μm, coarse inclusions are generated in the rail, so the delayed fracture resistance is reduced. . Therefore, the size of the A-based inclusion is 250 μm or less in the rail head longitudinal section.

Number of A-type inclusions: In the longitudinal section of the rail head, the number of A-type inclusions of 1 μm or more and 250 μm or less is less than 25 per 1 mm ^{2 of} test area. The number of A-type inclusions of 1 μm or more and 250 μm or less is covered When the number of inspection areas is 25 or more per 1 mm ² , coarse A-based inclusions increase, and the delayed fracture resistance of the rail is significantly deteriorated. Therefore, the number of A-based inclusions in the rail is less than 25 per 1 mm ^{2 of the} test area. Preferably, the number is less than 20 per 1 mm ^{2 of} test area, more preferably less than 6 per 1 mm ^{2 of} test area.
In addition, the system inclusions of less than 1 μm are spheroidized, and even if they exist, the delayed fracture resistance does not deteriorate. Therefore, in the present invention, the number of A-type inclusions (250 μm) that are 1 μm or more is defined as less than 25 as described above.

Next, the manufacturing method of the pearlite rail based on this invention is demonstrated.
First, steel is melted in a converter or electric furnace, and after secondary refining such as degassing as necessary, the composition of the steel is adjusted to be within the above preferred range, and continuously cast into bloom. The bloom immediately after continuous casting was gradually cooled at a cooling rate of 0.5 ° C./s or less over 40 to 150 hours, and then this bloom was heated to 1200 to 1350 ° C. in a heating furnace and hot-rolled to obtain a rail and To do. At this time, the rolling end temperature is preferably 900 to 1000 ° C., and the cooling rate after rolling is preferably 1 to 5 ° C./s.

  Next, the size and number of A-type inclusions defined in the present invention, the method for measuring the amount of hydrogen in steel, and the evaluation method for delayed fracture sensitivity will be described.

-Inclusion Dimension Measurement 12.7 mm x 19.1 mm rail length as shown in Fig. 1 starting from a 12.7 mm depth position from the rail head surface layer and a 5 mm position from the center in the rail width direction. A sample with the directional cross section as the observation surface is taken and the surface to be examined is mirror finished. The range of 5 mm × 10 mm (test area 50 mm ² ) at the center of the test piece was observed with a microscope magnification of 500 times, and sulfide-based nonmetallic inclusions were observed without etching, and the length of the A-based inclusions Measure the sides. The length of the long side is the size of the A-based inclusion. In addition, A-type inclusion means the A-type inclusion defined in JISG0555 appendix 1.

・ Measurement of the number of inclusions Similar to the measurement of inclusion dimensions, take a specimen from the rail head and finish the mirror finish on the test surface. A 5 mm × 10 mm (test area 50 mm ² ) range in the center of the test piece was observed with a microscope magnification of 500 times, and sulfide-based nonmetallic inclusions were observed without etching, and the size (long side The number of A-type inclusions having a length of 1 μm to 250 μm is measured. This number is converted into the number of A-based inclusions per 1 mm ² .

・ Measurement of hydrogen content in steel A test with a cross-sectional area of 5 mm x 5 mm and a length of 100 mm in the longitudinal direction of the rail head centering on the position of 25.4 mm from the surface of the rail head shown in Fig. A piece is taken and the amount of hydrogen in the steel is measured in accordance with an inert gas dissolution method-thermal conductivity method (JIS Z 2614).

-Delayed fracture test From the rail head surface layer shown in FIG. 3, a test piece having the dimensions shown in FIG. The collected specimens are finished with ▽▽▽ except for the threaded part and R part, and emery polished for parallel parts up to # 600. The test piece is mounted on an SSRT (Slow Strain Rate Technique) test apparatus, and an SSRT test is performed at 25 ° C. in the air at a strain rate of 3.3 × 10 ⁻⁶ / s. get the growth _{E 0.} Similarly to the test for elongation E ₀ in the atmosphere, this test piece was mounted on an SSRT test apparatus and a strain rate of 3.3 × 10 ⁻⁶ / s in a 20% aqueous ammonium thiocyanate solution at 25 ° C. in performed SSRT test, obtaining elongation _{E 1} of the specimen in an aqueous solution. Delayed fracture susceptibility (DF), which serves as an index for evaluating delayed fracture characteristics, is obtained by changing the values of E ₀ and E ₁ obtained as described above to DF = 100 × (1−E ₁ / E ₀ ). Substitute and calculate.

・ Tensile test A circle of 12.7 mm (0.5 inch) in diameter described in ASTM E8-04 with the central axis at a position 12.7 mm from the surface of the rail head shown in FIG. 5 and 12.7 mm from the head side. Bar specimens were collected and subjected to a tensile test with a gauge length of 25.4 mm (1 inch).

  Examples of the present invention will be specifically described below.

Example 1
Steel No. 1 having chemical components shown in Table 1. 1-1 to 1-7 are heated to 1250 ° C., hot-rolled and finished at 900 ° C., and then cooled at a cooling rate of 2 ° C./s to obtain rail No. 1-7. 1-1 to 1-7 were produced. This rail No. For 1-1 to 1-7, the size and number of A-based inclusions and the amount of hydrogen in steel were measured according to the measurement method described above, and the delayed fracture susceptibility was evaluated. In addition, steel No. which is a heat-treated pearlite steel having a C content of 0.68% is used. 1-1 manufactured by rail No. 1-1. Based on the delayed fracture characteristics of 1-1, rail No. When the delayed fracture susceptibility is improved by 10% or more (lower value) than 1-1, it was determined that the delayed fracture resistance was improved, and the delayed fracture susceptibility was evaluated. For example, in Table 1, rail No. The delayed fracture susceptibility improvement margin of 1-2 is 100 × {1− (84.2 / 85.0)} = 0.9%. In addition, rail No. 1-1 is a steel No. 1-1. 1-1, rail No. 1-2 is a steel No. 1-2. 1-2. Similarly, the rail No. 1-3 to 1-7 are steel Nos. It is manufactured using steel corresponding to 1-3 to 1-7, respectively.

  The test results are described in Table 2. FIG. 6 is a graph showing the relationship between the amount of S on the horizontal axis and the number of A-type inclusions and delayed fracture sensitivity improvement on the vertical axis. Rail No. The increase / decrease with respect to the delayed fracture sensitivity of 1-1 is shown. Further, FIG. 7 is a graph showing the relationship between the amount of S on the horizontal axis and the size of the A-type inclusions and the delayed fracture sensitivity improvement allowance on the vertical axis. And rail No. which is a conventional material. The increase / decrease with respect to the delayed fracture sensitivity of 1-1 is shown.

  As shown in FIGS. 6 and 7, the number of A-based inclusions is less than 20 and the size of A-based inclusions is 1 μm or more and 250 μm or less, so that the rail No. 1-4 to 1-7 are rail Nos. Which are conventional materials. It was found that the delayed fracture susceptibility was improved by 10% or more compared to 1-1. Therefore, the rail No. As shown in Table 2, 1-4 to 1-7 were confirmed to have excellent delayed fracture resistance while having a high tensile strength of 1200 MPa or more.

(Example 2)
Steel No. 1 having the chemical composition shown in Table 3. 2-1 to 2-15 were heated to 1250 ° C., hot-rolled and finished at 900 ° C., and then cooled at a cooling rate of 2 ° C./s to obtain rail No. 2-1 to 2-15 were produced. This rail No. For 2-1 to 2-15, in the same manner as in Example 1, the size and number of the A-based inclusions and the amount of hydrogen in the steel were measured, and further, the delayed fracture sensitivity and the delayed fracture sensitivity improvement allowance were evaluated. did. In addition, evaluation of the delayed fracture susceptibility improvement allowance was made for steel No. which is a heat-treated pearlite steel having a C content of 0.68%. Rail No. 2 manufactured according to 2-1. Based on the delayed fracture susceptibility of 2-1, rail no. It was determined that the delayed fracture resistance was improved when the delayed fracture sensitivity improvement margin was 10% or more than 2-1. In addition, rail No. 2-1. No. 2-1, rail no. 2-2 is a steel No. 2-2. 2-2, and similarly, rail No. 2-3 to 2-15 are steel Nos. It is manufactured using steel corresponding to 2-3 to 2-15.

  The test results are described in Table 4. From this result, the rail no. 2-7 to 2-13 are one or more selected from V, Cr, Cu, Ni, Nb, Mo, and W after controlling the composition of C, Si, Mn, P, and S within an appropriate range. Rail No., which is a comparative material, contains two or more components within the proper range, and the size and number of A-based inclusions, as well as the amount of hydrogen in steel and the contents of Al and Ti as impurities are within the proper range. . It has been found that the delayed fracture resistance of the rail can be further improved as compared with 2-2 to 2-6 and 2-14 and 2-15. Therefore, the rail No. As shown in Table 4, 2-7 to 2-13 were confirmed to have excellent delayed fracture resistance while having a high tensile strength of 1200 MPa or more.

  The present invention provides an excellent rail that contributes to the extension of the life of a rail of a high-axle heavy railway and the prevention of a railway accident, and provides an industrially beneficial effect.

The figure which shows the sampling position and dimension used for the inclusion dimension measurement and the inclusion number measurement. The figure which shows the collection position of the sample used for the measurement of the amount of hydrogen in steel. The figure which shows the collection position of a SSRT test piece. The figure which shows the shape and dimension of the test piece used for the SSRT test. The figure which shows the collection position of a tensile test piece. The graph which shows the influence of the amount of S on the number of A type inclusions in this invention material and a comparative material, and the delay fracture sensitivity improvement allowance. The graph which shows the influence of the amount of S on the magnitude | size of the A-type inclusion in this invention material and a comparison material, and the delay fracture sensitivity improvement allowance.

Claims (3)

In mass%, C: 0.6 to 1.0%, Si: 0.1 to 1.5%, Mn: 0.4 to 2.0%, P: 0.035% or less, S: 0.0005 -0.010% is contained, the balance is Fe and inevitable impurities, the tensile strength is 1200 MPa or more, and the size of the A-based inclusion is 250 μm or less in the longitudinal section of the rail head A high-strength pearlite rail excellent in delayed fracture resistance, characterized in that there are less than 25 A-type inclusions having a size of 1 μm or more and 250 μm or less per 1 mm ^{2 of the} test area.   The high-strength pearlite rail having excellent delayed fracture resistance according to claim 1, wherein the amount of hydrogen in the rail is 2 ppm or less on a mass basis.   Further, V: 0.5% or less, Cr: 1.5% or less, Cu: 1% or less, Ni: 1% or less, Nb: 0.05% or less, Mo: 1% or less, W: The high-strength pearlite rail excellent in delayed fracture resistance according to claim 1 or 2, comprising one or more selected from 1% or less.

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