JPH0718326A - Production of highly strong and tough rail by on-line heat treatment - Google Patents

Abstract

PURPOSE:To produce a highly strong and tough rail having high performance by specifically on-line heat treatment a steel material having specific composition composed of C, Si, Mn, P, S, Cr, Nb, Ni and Fe after it is specifically hot-rolled. CONSTITUTION:The steel material having composition composed of, by wt%, 0.60 to 0.85% C, 0.1 to 0.8% Si, 0.5 to 1.5% Mn, <=0.035% P, <=0.035% S, 0.2 to 0.8% Cr, 0.01 to 0.05% Nb, 0.1 to 0.4% Ni and the balance Fe with inevitable impurities is hot-rolled at finish rolling temperature of 800 to 1000 deg.C. The hot rolled material is on-line heat treated to cool the material acceleratively at a cooling rate of 1 to 3 deg.C/sec from >= pearlitic transformation temp. to 400 deg.C or below. Thus, highly strong and tough rail whose tensile strength is 120kg/mm<2> or more and 2mm U-notch Charpy absorption energy 2.0kg f.m or more is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a rail having high strength and high toughness by heat-treating it online after hot rolling.

[0002]

2. Description of the Related Art Conventionally, rails have been designed only to have high strength from the viewpoint of emphasis on wear resistance and rolling contact fatigue, but in recent years, rail usage conditions have been The rails are becoming more and more severe, and there is a demand for rails having excellent toughness that is free from damage even under such severe operating conditions. The toughness of ordinary rail is 2mmU notch Charpy absorbed energy +2
As low as 1.0 kgf ・ m or less at 0 ° C, 0.5 kgf ・ m
It is also difficult to increase.

As a means for improving the strength and toughness of rails, there is a heat treatment method. This heat treatment method includes (1) an offline heat treatment method in which, after the rolling is finished, the material is once cooled and then reheated to an Ac ₃ point or more for accelerated cooling (JP-A-63-128123). (2) An Ac ₃ point or more after the rolling is completed. On-line heat treatment method in which accelerated cooling is carried out as it is at a stage at a temperature of
23244).

[0004]

However, when these methods are applied to the manufacture of rails, there are the following problems, respectively. (1) is a method of transforming steel at a low temperature by accelerating cooling to refine the structure, and also has the effect of refining the structure by repeating the transformation. Therefore, a rail having high strength and high toughness can be obtained. However, reheating is performed after cooling, which is not preferable from the viewpoint of thermal efficiency.

Further, in (2), since the accelerated cooling is performed as it is after the completion of rolling, the thermal efficiency is good. However, with conventional rails, the toughness is not improved although the strength is improved by accelerated cooling.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a method for manufacturing a high-performance rail having excellent toughness while ensuring desired strength by online heat treatment.

[0007]

According to the present invention, in order to solve the above problems, C: 0.60 to 0.8 by weight%.
5%, Si: 0.1 to 0.8%, Mn: 0.5 to 1.5
%, P: 0.035% or less, S: 0.035% or less, C
r: 0.2 to 0.8%, Nb: 0.01 to 0.05%,
Ni: A steel material containing 0.1 to 0.4% with the balance being Fe and inevitable impurities, and a rolling finishing temperature of 1000.
~ 800 ℃ hot rolling, then 1 ~ 3 ℃ from the temperature before the start of pearlite transformation to 400 ℃ or less
Provided is a method for manufacturing a high-strength and high-toughness rail by online heat treatment, which is characterized by accelerated cooling at a cooling rate of / sec.

In order to solve the above problems, the present inventors have
First, the conditions of the rolling finishing temperature and the cooling rate were examined using a steel having a composition centered on 0.78C-0.02Nb-0.1Ni steel.

The composition used in this case is shown in Table 1. Also,
Table 2 shows the manufacturing conditions and characteristics when rails were manufactured using the steel materials having the compositions shown in Table 1. FIG. 1 shows B steel (0.
78% C-0.51% Si-0.84% Mn-0.10.
% Ni-0.44% Cr-0.022% Nb) to 128
The relationship between the rolling finish temperature and the toughness in the case of heating to 0 ° C., rolling while varying the finishing temperature in the range of 760 to 1040 ° C., and then performing accelerated cooling at a cooling rate of 1.8 ° C./sec is shown.
When the rolling finishing temperature is lower than 800 ° C, _U E ₂₀ decreases, the value also largely changes depending on the temperature and is unstable, and the strength also remarkably decreases to 120 kg / mm ² or less. 80
At 0 ℃, _U E ₂₀ is as high as 2.6 kgf ・ m and its strength is 12
The value is 0 kg / mm ² or more, and the value is stable. On the other hand, if the temperature exceeds 1000 ° C, the _U E ₂₀ will be 2.0 kgf · m or less and the toughness will decrease.

This is because, when the rolling finishing temperature is 800 ° C. or lower, pearlite transformation starts before accelerated cooling, a fine pearlite structure effective for wear resistance and high strength cannot be obtained, and it exceeds 1000 ° C. And the austenite grains before the pearlite transformation become coarse, and the structure is not sufficiently refined even in the subsequent accelerated cooling, and it is difficult to secure the strength and toughness.

Therefore, the rolling finishing temperature is 800 ° C. or higher and 10
By setting the temperature to 00 ° C. or less, it is possible to obtain a high toughness of 2 mm U notch Charpy absorbed energy at + 20 ° C. of 2.0 kgf · m or more.

[0012]

[Table 1]

[0013]

[Table 2]

FIG. 2 shows the B steel in Table 1 (0.78% C-0.5).
1% Si-0.84% Mn-0.10% Ni-0.44
% Cr-0.022% Nb) was heated to 1280 ° C. and rolled at a finishing temperature of 920 ° C., and then cooled at an air cooling rate (0.
4 ° C./sec) to 4.0 ° C./sec shows the relationship between cooling rate, strength, and toughness. Cooling rate is 1 ℃
If it is less than / sec, the toughness is low, but as the cooling rate increases, both strength and toughness improve. However, 3 ° C / sec
If it exceeds, both the strength and the toughness are remarkably reduced. That is, by setting the cooling rate to 1 ° C./sec or more and 3 ° C./sec or less, the tensile strength is 120 kg / mm ² or more and 2 m
mU notch Charpy absorbed energy at + 20 ° C 2.
It can be seen that high strength and high toughness of 0 kgf · m or more can be obtained. This is because if the cooling rate during pearlite transformation is less than 1 ° C / sec, the degree of supercooling is low and the pearlite structure is not sufficiently refined, and the desired strength and toughness cannot be achieved, and conversely, it exceeds 3 ° C / sec. And abnormal structures such as bainite and martensite are formed on the surface of the rail head, and the strength and toughness are significantly reduced. Therefore, the cooling rate from the temperature before the start of pearlite transformation to 400 ° C. or less at which the pearlite transformation ends is set to 1 to 3 ° C./sec.

Next, the steels A to J shown in Table 1 are 1280
920 ℃ which is heated to ℃ and satisfies the above rolling and cooling conditions.
After the rolling was completed at 1, the composition range was examined under the condition of accelerated cooling at 2 ° C./sec.

In FIG. 3, 0.78% C-0.51% Si-
N in 0.84% Mn-0.1% Ni-0.44% Cr steel
The relationship between the Nb addition amount and the strength and toughness when b is added from 0.0% to 0.069% is shown. With the addition of Nb, the strength gradually decreases and the toughness improves. Nb is 0.01%
If it is less than 0.05%, the toughness is low, and if it exceeds 0.05%, the effect of improving the toughness is reduced. From this, 0.78% C-
0.51% Si-0.84% Mn-0.10% Ni-
Under the condition of 0.44% Cr, Nb is 0.01% or more and 0.0
Tensile strength of 120 kg / mm by setting it to 5% or less
² or more, 2mm U notch Charpy absorbed energy +
It can be seen that a high-strength and high-toughness rail having a strength of 2.0 kgf · m or more at 20 ° C. can be obtained.

In FIG. 4, 0.78% C-0.51% Si-
0.84% Mn-0.44% Cr-0.02Nb Steel with N
The relationship between the amount of Ni added and the strength and toughness when i is added from 0.0% to 0.61% is shown. With the addition of Ni, the strength gradually increases and the toughness also improves. If Ni is less than 0.1%, the toughness is low, and if it exceeds 0.4%, the effect of improving strength and toughness is hardly seen. From this, 0.78% C
-0.51% Si-0.84% Mn-0.44% Cr-
Under the condition of 0.02% Nb, Ni is 0.1% or more and 0.4% or more.
By setting the following, tensile strength is 120 kg / mm ² or more, ² mm U notch Charpy absorbed energy is +20
It can be seen that high strength and high toughness of 2.0 kgf · m or more at 0 ° C. can be obtained.

From the above results, Nb is added by 0.01% or more and 0.05% or less, Ni is added by 0.1% or more and 0.4% or less, and the rolling finishing temperature is 800 ° C or more and 1000 ° C or less. Hot rolling, and then the cooling rate from the temperature before the start of pearlite transformation to 400 ° C or less is 1 to 3 ° C /
Tensile strength 120kg by accelerated cooling in sec
/ Mm ² or more, ² mmU notch Charpy absorbed energy at + 20 ° C. can obtain high strength and high toughness of 2.0 kgf · m or more.

Next, the reasons for limiting the content of each component will be described. (1) C: C is 0.6 to secure the strength as eutectoid steel.
The content is required to be 0% or more, but if it exceeds 0.85%, pro-eutectoid cementite is generated at the grain boundaries and causes embrittlement of the material, which is not preferable. Therefore, the amount of C is specified in the range of 0.60 to 0.85%.

(2) Si: Si is required to be 0.1% or more for deoxidation at the time of steel making, and it forms a solid solution with ferrite in pearlite and contributes to strengthening. However, if it exceeds 0.8%, the rate of increase in strength decreases. Therefore, the amount of Si is specified in the range of 0.1 to 0.8%.

(3) Mn: Mn is an element that contributes to the high strength of the rail by lowering the pearlite transformation temperature and enhancing the hardenability. However, if it is less than 0.5%, the desired strength cannot be ensured. On the contrary, if it exceeds 1.5%, a martensitic structure due to the microsegregation of the steel is apt to occur, which causes hardening or embrittlement during heat treatment and welding. This is not preferable because it causes material deterioration. Therefore, the Mn content is 0.5 to 1.
Specify in the range of 5%.

(4) P: Since P deteriorates toughness, it is specified to be 0.035% or less. (5) S: S is present mainly in the form of inclusions in steel and significantly deteriorates toughness, so it is specified to be 0.035% or less.

(6) Cr: Cr narrows the lamellar spacing of pearlite and contributes to high strength, but if it is less than 0.2%, its effect is low, and if it exceeds 0.8%, martensite is mixed during accelerated cooling. May cause Therefore, the Cr amount is specified in the range of 0.2 to 0.8%.

(7) Nb: Nb has the effect of suppressing recrystallization, and has the effect of reducing the size of pearlite colonies obtained after accelerated cooling and improving toughness. However, as shown in FIG. 3 described above, if it is less than 0.01%, its effect is small, and if it exceeds 0.05%, the effect of improving toughness is saturated. Therefore, the Nb amount is 0.01 to 0.05
Specify in the range of%.

(8) Ni: Ni strengthens ferrite in pearlite and contributes to high strength, and at the same time improves toughness. However, as shown in FIG. 4 described above, if it is less than 0.1%, its effect is small, and if it exceeds 0.4%, it is disadvantageous from the economical viewpoint. Therefore, the Ni content is 0.1 to
Specify in the range of 0.4%.

A steel material having the above composition, that is, 0.01% to 0.05% Nb and 0.1% Ni.
% Or more and 0.4% or less, hot rolling is performed so that the rolling finishing temperature is 800 ° C. or more and 1000 ° C. or less, and the pearlite transformation temperature range is 1 ° C./sec or more and 3 ° C. /
It is possible to achieve high strength and high toughness at a tensile strength of 120 kg / mm ² or more and a ² mm U-notch Charpy absorbed energy of 2.0 kgf · m or more at + 20 ° C. only by accelerating cooling at sec or less.

[0027]

【Example】

(Example 1) For two steel types, K steel and L steel, each having a composition within the scope of the present invention shown in Table 3, rolling finish temperatures were set to 750 to 750.
By changing the cooling rate in the range of 1050 ° C. and the cooling rate in the range of 0.4 to 4.5 ° C./s, the tensile properties and the impact value at + 20 ° C. in the 2 mm _U notch Charpy test (2 mm _U E ₊₂₀
Is represented). The results are shown in Table 4.

[0028]

[Table 3]

[0029]

[Table 4]

Since Condition 1 in Table 4 does not satisfy both the rolling finish temperature and the cooling rate, TS = 108.3 kg / mm
² , which is a low value of ² mm _U E ₊₂₀ = 1.3 kgf · m. Condition 2 is that the cooling rate is within the claimed range, but the rolling finishing temperature is not satisfied, so TS = 112.8 k
g / mm ² , 2 mm _U E ₊₂₀ = 1.5 kgf · m, which is a low value. Since conditions 3 and 4 both satisfy the rolling finish temperature and cooling rate, TS ≧ 120.0 kg / mm
² , which is a good value of ² mm _U E ₊₂₀ ≧ 2.0 kgf · m. Condition 5 is that the rolling finishing temperature satisfies the condition but does not satisfy the cooling rate, so TS = 11.
9.1 kg / mm ² , 2 mm _U E ₊₂₀ = 1.8 kgf ・
It shows a low value of m. Condition 6 satisfies both the rolling finish temperature and the cooling rate, so TS ≧ 120.0 kg /
mm ² , 2 mm _U E ₊₂₀ ≧ 2.0 kgf · m, which is a good value. Condition 7 is 2 mm _U because the rolling finishing temperature satisfies the condition but does not satisfy the cooling rate.
E ₊₂₀ = 1.9 kgf · m, which is a slightly low value.
Since conditions 8 and 9 satisfy both the rolling finish temperature and the cooling rate, TS ≧ 120.0 kg / mm ² , 2 mm _U E ₊₂₀
A good value of ≧ 2.0 kgf · m is shown. Condition 10
Indicates that the cooling rate satisfies the conditions but does not satisfy the rolling finishing temperature. Therefore, TS = 105.5 kg / mm ² ,
It shows a low value of 2 mm _U E ₊₂₀ = 1.5 kgf · m. (Example 2) A steel material having a composition of M to W shown in Table 5 was manufactured under the conditions of a finish rolling temperature of 920 ° C and a cooling rate of 1.8 to 1.9 ° C / s which satisfy the conditions of the present invention shown in Table 6. The tensile properties and the value of 2 mm _U E ₊₂₀ at that time are also shown in Table 6.

[0031]

[Table 5]

[0032]

[Table 6]

Since M steel and N steel do not contain Ni, the M steel shows a low value of ² mm _U E ₊₂₀ = 1.8 kgf · m and the N steel shows a TS value of 118.9 kg / mm ² . .
Since P steel and Q steel do not contain Nb, it is 2 m for P steel.
m _U E ₊₂₀ = 1.6 kgf · m, 2 mm _U E _{+2 for} Q steel
The value is as low as 1.8 kgf · m. Also, Nb
S steel that does not meet the requirements of TS is 117.3 kg /
The value is as low as mm ² .

On the other hand, O steel satisfying the present invention,
TS ≧ 120.0 kg / mm for R steel, T steel, U steel and V steel
² , which is a good value of ² mm _U E ₊₂₀ ≧ 2.0 kgf · m. For S steel, the steel composition of the present invention is C
Although u was added, cracking occurred during rolling.

[0035]

According to the present invention, there is provided a method for producing a high-performance rail having excellent toughness while ensuring desired strength by online heat treatment. Specifically, tensile strength 1
It is possible to manufacture a high-strength and high-toughness rail having a 20 mm / mm ² or more and a ² mm U notch Charpy absorbed energy of 2.0 kgf · m or more at + 20 ° C.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between rolling finish temperature and toughness.

FIG. 2 is a diagram showing a relationship between a cooling rate, strength, and toughness.

FIG. 3 is a diagram showing the relationship between the amount of Nb added and the strength and toughness.

FIG. 4 is a diagram showing the relationship between the amount of Ni added and the strength and toughness.

[Procedure amendment]

[Submission date] September 30, 1994

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0003

[Correction method] Change

[Correction content]

As a means for improving the strength and toughness of rails, there is a heat treatment method. This heat treatment method includes (1) an offline heat treatment method in which after the completion of rolling, the material is once cooled and then reheated to an Ac ₃ point or more to perform accelerated cooling (Japanese Patent Application No. 63-128123) (2) After the rolling, Ac ₃ points or more online heat treatment method involving directly accelerated cooling at a stage in the temperature (JP publicly Akira 63-
23244).

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0008

[Correction method] Change

[Correction content]

In order to solve the above problems, the present inventors have
First, the conditions of the rolling finishing temperature and the cooling rate were examined using a material having a composition centered on 0.78 % C-0.02 % Nb-0.1 % Ni.

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0017

[Correction method] Change

[Correction content]

In FIG. 4, 0.78% C-0.51% Si-
The relationship between the amount of Ni added and the strength and toughness when 0.8% to 0.61% of Ni is added to 0.84% Mn-0.44% Cr-0.02 % Nb steel is shown. With the addition of Ni, the strength gradually increases and the toughness also improves. If Ni is less than 0.1%, the toughness is low, and if it exceeds 0.4%, the effect of improving strength and toughness is hardly seen. From this, 0.78%
C-0.51% Si-0.84% Mn-0.44% Cr
Under the condition of -0.02% Nb, Ni is 0.1% or more and 0.4
%, The tensile strength is 120 kg / mm ²
Above, 2mmU notch Charpy absorbed energy +2
It can be seen that it is possible to obtain high strength and high toughness of 2.0 kgf · m or more at 0 ° C.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0034

[Correction method] Change

[Correction content]

On the other hand, O steel satisfying the present invention,
TS ≧ 120.0 kg / mm for R steel, T steel, U steel and V steel
² , which is a good value of ² mm _U E ₊₂₀ ≧ 2.0 kgf · m. Regarding W steel, the steel composition of the present invention is C
Although u was added, cracking occurred during rolling.

Claims (2)

[Claims] 1. C: 0.60 to 0.85% by weight,
Si: 0.1-0.8%, Mn: 0.5-1.5%,
P: 0.035% or less, S: 0.035% or less, Cr:
0.2-0.8%, Nb: 0.01-0.05%, N
i: a steel material containing 0.1 to 0.4% and the balance being Fe and inevitable impurities, and the rolling finishing temperature is 800 to 1
Hot rolling to 000 ℃, then 1 to 3 ℃ / from the temperature before the start of pearlite transformation to 400 ℃ or less
A method for producing a high-strength and high-toughness rail by online heat treatment, characterized by accelerating cooling at a cooling rate of sec. 2. The rail has a tensile strength of 120 kg / m.
The method for producing a high-strength and high-toughness rail by online heat treatment according to claim 1, wherein the absorbed energy of ² mm U notch Charpy is 2.0 kgf · m or more at + 20 ° C.