JPH10176244A - High strength bainitic rail having low thermal expansion coefficient and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a high strength bainitic rail excellent in wear resistance and damaging resistance and having a low thermal expansion coefficient equal to or below that of a pearlite type heat treated rail ant to provide a method for producing the same. SOLUTION: This high strength bainitic rail contains, by mass, 0.20 to 0.50% C, 0.10 to 0.30% Si, 0.50 to 2.50% Mn, <=0.035% P, <=0.035% S, 0.50 to 1.50% Mo, 0.30 to 2.00% Cr, 0.05 to 0.15% Nb, and the balance iron with inevitable impurities. In this case, the componental compsn. satisfies the inequality of -2.5C+2.2Si+1.6Mn+0.6Cr+0.5Mo-11.6(Nb+V)+9<=11.0 (where the elemental symbols in the inequality denote the respective mass%), the metallic structure of the rail is formed of a bainitic structure homogeneous all over the surface, the hardness of the rail is regulated to >=400Hv in all positions of the top part and the corner parts of the head part, and it has a low thermal expansion coefficient.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength bainite rail having a low coefficient of thermal expansion equal to or lower than that of a conventional pearlitic heat-treated rail.

[0002]

2. Description of the Prior Art Conventionally, pearlite steel is used for rail steel used under high load from the viewpoint of abrasion resistance and anti-rolling fatigue characteristics, and only high strength mainly by miniaturization of pearlite lamellar is used. Have been oriented. However, in recent years, rail use conditions have become more and more severe along with higher speed and higher axle weight in rail transport. Especially in the United States and Canada, high axle heavy railways that transport ore,
Schelling damage at the gauge corner of the outer curve and flaking damage on the top surface of the inner track have been regarded as problems.

A high-strength bainite rail having a bainite structure is disclosed as a damage-resistant rail that dramatically improves such shelling resistance and flaking resistance, which is completely different from conventional fine pearlite steel. Such bainite rails have increased damage resistance by giving a difference in hardness between the top and the corner of the head,
They are roughly classified into those with increased damage resistance due to the promotion of wear.
The former example is disclosed in JP-A-2-282448, JP-A-6-336614, and JP-A-7-34133, and the latter example is disclosed in JP-A-5-218717 and JP-A-8-83133. -92696.

[0004]

However, since all such bainite rails have a low C content and contain a large amount of alloying elements such as Mn, Cr, and Mo, they cannot be used with conventional pearlite-type heat-treated rails. There is a problem that the coefficient of thermal expansion is higher than that. In particular, when such a high-strength bainite rail is actually laid, there are the following problems.

[0005] (1) Since there are many places where the temperature can reach 80 ° C. or more in summer, problems such as buckling (curving) of the rail due to expansion due to heat are likely to occur.

(2) Since there are many places where the temperature can reach -30 ° C. in winter, the amount of shrinkage due to low temperature cannot be ignored, and the rails are broken due to shrinkage while being restrained by the sleepers. Tends to occur.

(3) If the gap between the rail joints is widened to prevent buckling (bending) of the rail due to summer expansion, damage occurs due to an increase in the impact of the wheel on the rail end. Cheap.

The bainite rails described in JP-A-5-21871 and JP-A-8-92696, in which damage resistance is enhanced by promoting wear, were used under high axle load conditions such as overseas mining railways. In this case, there is also a problem that the wear of the rail head is remarkably promoted and the life of the rail is extremely shortened.

As described above, in addition to the wear resistance and damage resistance required under a high-curve section such as a sharply curved section under high-speed operation conditions or an overseas mining railway, the coefficient of thermal expansion of a high-strength bainite rail is also high. Although the reduction of the noise is an important issue, there is no example in the prior art in which a systematic study was performed from such a viewpoint.

The present invention has been made in view of such problems, and has high wear resistance and damage resistance as compared with the conventional pearlite-type heat-treated rail, and has a low thermal expansion coefficient equal to or less than that of the pearlite-type heat-treated rail. A bainite rail and a method for manufacturing the same are provided.

[0011]

Means for Solving the Problems The invention of claim 1 is:
(A) In mass%, C: 0.20 to 0.50% Si: 0.10 to 0.30% Mn: 0.50 to 2.50% P: 0.035% or less S: 0.035% Mo: 0.50 to 1.50% Cr: 0.30 to 2.00% Nb: 0.05 to 0.15%, with the balance being a component composition consisting of iron and inevitable impurities. , (B) the component composition satisfies the following inequality, -2.5C + 2.2Si + 1.6Mn + 0.6Cr + 0.5Mo-11.6 (Nb + V) + 9 ≦ 11.0 (where the element symbols in the formula are (C) a low heat in which the metal structure of the rail is a bainite structure that is uniform over the entire surface, and (d) the hardness of the rail is Hv 400 or more at any position of the top and the corner of the head. It is a high-strength bainite rail having an expansion coefficient.

The reasons for limiting the hardness, chemical components and production conditions in the present invention will be described. (1) Hardness (abrasion resistance, damage resistance) Abrasion resistance If the hardness is Hv400 or more, the abrasion amount is equal to or less than that of the high-strength fine pearlite rail. It is most desirable to evaluate the wear resistance by the amount of wear when actually laid, but it is also effective to use a Nishihara type abrasion tester to simulate the contact conditions of the actual laid rail by a comparative test . With this test method, the wear resistance can be evaluated in a short period of time.

Therefore, in order to examine these points in detail, a wear test was performed using 0.3C-0.5Cr-0.3Mo-0.1Nb steel. The hardness of the steel was changed by changing the amount of Mn added. table
The steel used in the wear test (0.31C-0.35Si-Mn-0.5Cr-0.3
Mo-0.10Nb steel).

[0014]

[Table 1]

The abrasion test is performed using a Nishihara type abrasion test piece having an outer diameter of 30 mm and a width of 8 mm under a contact load of 250 kg, a sliding rate of -10%, and a dry condition without a lubricant. It was measured. The outer diameter is used for measuring the coefficient of thermal expansion.
Use a round bar specimen of 5 mm and length of 50 mm,
The average value of ° C was determined.

FIG. 1 is a graph showing changes in wear loss and coefficient of thermal expansion accompanying an increase in hardness (Vickers hardness Hv) prepared from the results of this wear test. In the figure, values of the conventional pearlite-type heat treatment rail are also added. With increasing hardness with increasing Mn, wear loss decreases. When compared at the same hardness, bainite steel wears more than pearlite steel. However, by increasing the hardness to Hv400, wear resistance equivalent to that of a normal pearlite-type heat-treated rail can be obtained. On the other hand, the coefficient of thermal expansion continuously increases with an increase in hardness with an increase in Mn.
At 400, it is 1.08 times the pearlitic heat treatment rail.

As shown in FIG. 1, wear loss is affected by hardness and microstructure. The wear loss decreases with increasing hardness, and at the same hardness, the bainite structure has a larger wear loss than the pearlite structure. It is less than or equal to that of the pearlite rail, and higher wear resistance than that of the high-strength fine pearlite rail can be obtained.

Damage Resistance If the hardness of the bainite structure is Hv 275 or more, damage resistance equal to or higher than that of a high-strength fine pearlite rail can be obtained.

Among various types of damage, flaking damage accompanied by plastic deformation is one of the damages whose suppression is most important. Flaking damage is considered to be a phenomenon in which the plastic flow and the fatigue strength of the matrix influence in a complex manner, and it is important to suppress the plastic flow that the plastic flow hardly occurs and the fatigue strength is high.

It is most desirable to evaluate the flaking damage by actual laying, but it is also effective to use a Nishihara-type abrasion tester to simulate the occurrence of flaking damage by a comparative test. Using this test method, it is possible to evaluate flaking damage resistance in a short period of time.

FIG. 3 is a diagram schematically showing a test piece shape and a test method in the flaking damage test. Using the steel whose components are shown in Table 1, using the sample shown in FIG.
g, a sliding rate of -20%, and an oil lubrication condition were examined for flaking resistance.

FIG. 2 is a diagram showing the relationship between the hardness and the life of flaking generated based on the results of the flaking damage test. When compared at the same hardness, bainite steel having a high 0.2% proof stress has a longer flaking generation life than high-strength rail steel having a fine pearlite structure. Therefore, if the bainite structure has a hardness of Hv 275 or more, flaking damage resistance superior to that of the currently used high-strength fine pearlite rail steel can be obtained.

Therefore, in the case of the bainite structure, the hardness is H
If it is v400 or more, not only the above-mentioned wear resistance but also damage resistance can be obtained which are equal to or higher than those of the currently used high-strength fine pearlite rail. Therefore, in the present invention, the hardness is set to Hv400 or more at any position of the top and the corner of the head. Also, if the hardness of a part of the crown decreases, the wear resistance and damage resistance relatively decrease, so that the head needs to have uniform hardness.

(2) Chemical Components Next, the coefficient of thermal expansion was examined. Table 2 shows the components of the steel used in the test. Here, 0.3C-0.5Cr-0.3Mo-0.1Nb
For -0.05V steel, set three levels of Si content,
At the i level, the strength was changed by the amount of Mn added. The measurement of the coefficient of thermal expansion is the same as described above.

[0025]

[Table 2]

FIG. 4 is a diagram showing the relationship between the hardness and the coefficient of thermal expansion prepared from the obtained results. Although the coefficient of thermal expansion increases with an increase in hardness with an increase in the amount of Mn, when compared with the same hardness, the lower the Si content, the lower the coefficient of thermal expansion.

Similarly, the case where the Mo level was changed was examined. Table 3 shows the components of the steel used in the test. Here, the Mo amount was changed by three levels in the 0.2C-0.28Si-0.8Cr-0.1Nb-0.03V steel, and the strength was changed by the amount of Mn added at each Mo level.

[0028]

[Table 3]

FIG. 5 is a diagram showing the relationship between the hardness and the coefficient of thermal expansion prepared from the obtained results. Although the coefficient of thermal expansion increases with an increase in hardness with an increase in the amount of Mn, when compared with the same hardness, the higher the Mo content, the lower the coefficient of thermal expansion.

Based on these test results, the influence of the component elements on the coefficient of thermal expansion was studied diligently. As a result, the influence of the component elements on the coefficient of thermal expansion in the bainite structure could be arranged as follows. The coefficient of thermal expansion in the bainite structure has a good correlation with the parameter A represented by the following equation.

A = -2.5C + 2.2Si + 1.6Mn + 0.6Cr + 0.5Mo-11.6 (Nb + V) +9 Here, each element symbol in the formula represents each mass%.

FIG. 6 is a diagram showing the relationship between this parameter A and the coefficient of thermal expansion. The parameter A and the coefficient of thermal expansion are on substantially the same straight line. By setting the value of the parameter A to 11 or less, a low coefficient of thermal expansion (11 × 10 ⁻⁶ or less) equal to or less than that of the high-strength fine pearlite rail can be obtained.

The reasons for limiting the composition of each component will be described below. C has a negative coefficient in the parameter A, and is an essential element for securing a low coefficient of thermal expansion, which is an important gist of the present invention. Further, it is an essential element for securing hardness, and if it is less than 0.20%, it is difficult to secure hardness and wear resistance as rail steel. On the other hand, if it exceeds 0.50%, a uniform bainite structure cannot be obtained on the rail head, and martensite is generated, and damage originating from the martensite occurs. Therefore, C is limited to 0.20 to 0.50%.

Although Si is an effective element as a deoxidizing agent, its effect is not recognized at less than 0.10%. As shown in FIG. 4, in order to ensure a low coefficient of thermal expansion in a high-strength bainite structure, a lower one is desirable, so the upper limit is set to 0.30%. Therefore, Si is
Limited to 0.10 to 0.30%.

Mn is an element that contributes to increasing the strength of the rail by lowering the bainite transformation temperature and increasing the hardenability. However, if the content is less than 0.50%, the effect is small, and if it exceeds 2.5%, a martensitic structure is liable to occur due to micro-segregation of the steel, and hardening or embrittlement occurs during heat treatment and welding, resulting in deterioration of the material. Therefore, Mn
Is limited to 0.50 to 2.5%.

Mo is an important element that promotes bainite transformation, and as shown in FIG. 5 of the present invention, can achieve a significant increase in hardness while keeping the coefficient of thermal expansion low. To effectively utilize the effect, 0.50% or more is necessary. However, if it exceeds 1.50%, a martensitic structure due to micro segregation of steel is easily generated, and hardening and embrittlement occur during heat treatment and welding. It is not preferable because the material is deteriorated. Therefore, Mo is limited to 0.50 to 1.50%.

Since P degrades toughness, P
Limited to 35% or less.

S is mainly present in the steel in the form of inclusions, but if it exceeds 0.035%, the amount of these inclusions increases significantly, causing material deterioration due to embrittlement.
% Or less.

Cr is an important element that promotes bainite transformation, and is a very important element for achieving high strength with a microstructure as a bainite structure like the steel of the present invention. If it is less than 0.30%, the bainite hardenability is low and the microstructure does not become a uniform bainite structure. Meanwhile, 2.
If it exceeds 00%, martensite is likely to be generated, which is not preferable. Therefore, Cr is limited to 0.30 to 2.00%.

Nb has a negative coefficient in the parameter A and is an essential element for securing a low coefficient of thermal expansion, which is an important gist of the present invention. Furthermore, since it promotes bainite transformation and also precipitates after rolling in association with C in bainite, the hardness is increased by precipitation strengthening up to the inside of the head, wear resistance is improved, and the life of the rail is extended. It is valid. It is also effective in improving delayed fracture resistance. However, this effect is not effective when the addition is less than 0.05%, and the effect is saturated even when the addition exceeds 0.15%. Therefore, Nb is 0.05-0.1
Limited to 5%.

According to a second aspect of the present invention, the composition according to the first aspect further comprises one or more of Ni: 0.10 to 1.00% and V: 0.01 to 0.10%. Claim 1 characterized by the following:
A high-strength bainite rail having the described low coefficient of thermal expansion.

According to the present invention, N is added to the first aspect of the invention.
i or V is added. The reasons for limiting the individual component compositions are described below.

Ni is an element effective for promoting the bainite transformation and increasing the strength. However, the effect is not recognized below the above-mentioned claim, and the effect is saturated when added beyond the claim. I will. Therefore, it is effective to add within the above-mentioned claims. Therefore, Ni is 0.10
Limited to ~ 1.0%.

V has a negative coefficient in the parameter A, and is an essential element for securing a low coefficient of thermal expansion, which is an important gist of the present invention. Furthermore, since it precipitates after rolling in association with C in bainite, it is effective for increasing the hardness by improving precipitation hardening to the inside of the head, improving wear resistance, and extending the life of the rail. However, this effect is 0.
Addition of less than 01% is not effective, and addition of more than 0.10% saturates the effect. Therefore,
V is limited to 0.01 to 0.10%.

The invention according to claim 3 is characterized in that (a) mass%
C: 0.20 to 0.50% Si: 0.10 to 0.30% Mn: 0.50 to 2.50% P: 0.035% or less S: 0.035% or less Mo: 0.35% 50 to 1.50% Cr: 0.30 to 2.00% Nb: 0.05 to 0.15%, with the balance being iron and unavoidable impurities. Using steel whose composition satisfies the following inequality, -2.5C + 2.2Si + 1.6Mn + 0.6Cr + 0.5Mo-11.6 (Nb + V) + 9 ≦ 11.0 (However, the element symbols in the formula are each mass (C) The steel is hot-rolled under the condition of a rolling finish temperature of 750 to 1000 ° C. to form a rail, and (d) a temperature from the bainite transformation start point or higher to 400 ° C. or lower is obtained. Method of manufacturing high-strength bainite rail with low coefficient of thermal expansion that allows the entire body to be transformed into bainite by cooling at a cooling rate of 5 ° C./s or less. It is.

This invention and the next invention of claim 4 are inventions of a manufacturing method, and both will be described together.

According to a fourth aspect of the present invention, the composition of the third aspect further comprises one or more of Ni: 0.10 to 1.00% and V: 0.01 to 0.1%. A method for manufacturing a high-strength bainite rail having a low coefficient of thermal expansion according to claim 3.

In these inventions, claim 1 or claim 2
A high-strength bainite rail is manufactured by using steel having the same composition as that of the invention of (1). Hereinafter, the reasons for limiting the manufacturing conditions will be described.

First, when the rolling finishing temperature is lower than 750 ° C., ferrite is easily formed from unrecrystallized austenite, and the strength is significantly reduced. If the rolling finish temperature exceeds 1000 ° C., austenite grains become coarse, and it becomes difficult to secure toughness after hot rolling. Therefore, the rolling finishing temperature is set to 750
The temperature is set to not lower than 1000C and not higher than 1000C.

With respect to the cooling rate, in the case of the component range of the present application, a bainite structure can be obtained even by air cooling, and the required strength can be secured. Therefore, there is no problem even if accelerated cooling is not performed. When accelerated cooling is used, high hardness can be obtained with the same components,
Abrasion resistance and damage resistance are further improved. However, when the cooling rate exceeds 5 ° C./s, martensite is formed and the toughness is significantly reduced. Therefore, the cooling rate is allowed to cool to 5 ° C./s or less.

The reason why the whole is transformed into bainite is that the appearance of a structure other than the bainite structure partially impairs the abrasion resistance and loses the correlation between the parameter A and the coefficient of thermal expansion. This is to prevent a desired low coefficient of thermal expansion from being obtained by preparing the component composition.

[0052]

BEST MODE FOR CARRYING OUT THE INVENTION The steel used for the high-strength bainite rail of the present invention can be easily produced by a usual converter steelmaking method or an electric furnace steelmaking method. It goes without saying that the component composition may be contained as ordinary impurities other than those specified in the present invention.

Rolling can be performed by any method as long as it is a normal rail rolling method. After rolling, ordinary air cooling may be used. Needless to say, it is possible to avoid cooling that is close to slow cooling in which a structure other than the bainite structure appears partially. With normal air cooling, there is no problem because a bainite structure can be obtained.

[0054]

【Example】

(Example 1) 1250 test steels having the components shown in Table 4 were used.
After the rolling was completed at 850 ° C., except for No. 1, it was allowed to cool. No. 1 was accelerated and cooled at 2 ° C./s after rolling. With respect to the evaluation methods of wear resistance, damage resistance, and coefficient of thermal expansion, the same methods as in the above (1) were used. These test results are added to Table 4.

[0055]

[Table 4]

In this table, No. 1 is a conventional heat-treated pearlite rail, and the other is a bainite type rail. Although the flaking generation life is superior to the heat-treated pearlite rail in any of the bainite-type rails, No. 7 and 8 which do not satisfy the hardness requirements of the present invention have more wear resistance than the heat-treated pearlite rail. Inferior.

Nos. 2, 3 and 4 satisfy the condition of hardness of the present invention, but the parameter A ≦ 11.0 (−2.5C + 2.2Si + 1.6Mn) which is the condition of the present invention relating to the coefficient of thermal expansion. + 0.6Cr
+ 0.5Mo-11.6 (Nb + V) + 9 ≦ 11.0)
Higher coefficient of thermal expansion than heat-treated pearlite rail. On the other hand, Nos. 5, 6, 9, 10, and 11 all have better abrasion resistance, damage resistance, and low coefficient of thermal expansion than the heat-treated pearlite rail because they satisfy the conditions of the present invention. .

Example 2 A test steel having the components shown in Table 5 was heated to 1270 ° C., and after rolling at 920 ° C., 1.
Accelerated cooling was performed at 5 ° C / s to 350 ° C. With respect to the evaluation methods of wear resistance, damage resistance, and coefficient of thermal expansion, the same methods as in the above (1) were used. These test results are added in Table 5.

[0059]

[Table 5]

In this table, No. 12 satisfies the hardness condition of the present invention, and therefore has better wear resistance and damage resistance than the heat-treated pearlite rail, but the present invention relates to the coefficient of thermal expansion of the present invention. Parameter A ≦ 11.0 (-2.5C + 2.2Si +
Since 1.6Mn + 0.6Cr + 0.5Mo-11.6 (Nb + V) + 9 ≦ 11) is not satisfied, the coefficient of thermal expansion is higher than that of the heat-treated pearlite rail.

Nos. 13 and 14 satisfy the condition of the present invention relating to the coefficient of thermal expansion, but do not satisfy the condition of the hardness of the present invention, so that the abrasion resistance is poor. On the other hand, No. 15 to 19
Since all of them satisfy the conditions of the present invention, they have better wear resistance, damage resistance, and lower coefficient of thermal expansion than the heat-treated pearlite rail.

Example 3 A test steel having the components shown in Table 6 was heated to 1250 ° C., rolled at 790 ° C., and allowed to cool or accelerated to 400 ° C. Regarding the methods for evaluating wear resistance, damage resistance, and coefficient of thermal expansion, see the above (1),
The same technique was used. These test results are added to Table 6.

[0063]

[Table 6]

In this table, Nos. 20 to 28 all satisfy the conditions of the present invention, and therefore have better wear resistance, damage resistance, and lower coefficient of thermal expansion than the heat-treated pearlite rail regardless of the cooling conditions. ing.

[0065]

According to the present invention, by performing heat treatment by air cooling or on-line after hot rolling, it has excellent wear resistance and damage resistance as compared with the conventional pearlite-type heat-treated rail, and is equal to or less than the pearlite-type heat-treated rail. A high-strength bainite rail having a low coefficient of thermal expansion is obtained.

[Brief description of the drawings]

FIG. 1 is a graph showing changes in wear loss and coefficient of thermal expansion with an increase in hardness.

FIG. 2 is a diagram showing the relationship between hardness and the life of flaking.

FIG. 3 is a diagram schematically illustrating a test piece shape and a test method in a flaking damage test.

FIG. 4 is a diagram showing a relationship between hardness and coefficient of thermal expansion. (Si
Level effect)

FIG. 5 is a diagram showing a relationship between hardness and coefficient of thermal expansion. (Mo
Level effect)

FIG. 6 is a diagram showing a relationship between a parameter A of the invention and a coefficient of thermal expansion.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/58 C22C 38/58

Claims (4)

[Claims] 1. (a) In mass%, C: 0.20 to 0.50% Si: 0.10 to 0.30% Mn: 0.50 to 2.50% P: 0.035% or less S : 0.035% or less Mo: 0.50 to 1.50% Cr: 0.30 to 2.00% Nb: 0.05 to 0.15%, with the balance being iron and unavoidable impurities (B) the component composition satisfies the following inequality: -2.5C + 2.2Si + 1.6Mn + 0.6Cr + 0.5Mo-11.6 (Nb + V) + 9 ≦ 11.0 (wherein (C) The metal structure of the rail is a bainite structure that is uniform over the entire surface, and (d) the hardness of the rail is at any of the top and the corner of the head. A high-strength bainite rail having a low coefficient of thermal expansion of Hv400 or more. 2. The composition according to claim 1, further comprising one or more of Ni: 0.10 to 1.00% and V: 0.01 to 0.10%. Item 1
A high-strength bainite rail having the low coefficient of thermal expansion described. 3. (a) In mass%, C: 0.20 to 0.50% Si: 0.10 to 0.30% Mn: 0.50 to 2.50% P: 0.035% or less S : 0.035% or less Mo: 0.50 to 1.50% Cr: 0.30 to 2.00% Nb: 0.05 to 0.15%, with the balance being iron and unavoidable impurities (B) using a steel having the following composition, and satisfying the following inequality: -2.5C + 2.2Si + 1.6Mn + 0.6Cr + 0.5Mo-11.6 (Nb + V) + 9 ≦ 11.0 (however, (C) The steel is hot-rolled under the condition of a rolling finish temperature of 750 to 1000 ° C. to form a rail, and (d) the bainite transformation is started. Cooling from the temperature above the point to 400 ° C or less at a cooling rate of 5 ° C / s or less from the natural cooling, a high thermal expansion coefficient with a low coefficient of thermal expansion that transforms the whole into bainite. Method of manufacturing a degree bay night rail. 4. The composition according to claim 3, further comprising at least one of Ni: 0.10 to 1.00% and V: 0.01 to 0.1%. A method for producing a high-strength bainite rail having a low coefficient of thermal expansion.