JP2002226915A - Manufacturing method of rail with high wear resistance and high toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a rail of high toughness in which the growth of re-crystallized grain γduring an after rolling is suppressed, a pearlite structure of the grain is obtained, and the toughness is provided in steel representing pearlite structure with high carbon content of excellent strength and wear resistance. SOLUTION: A steel slab containing, by mass, 0.6-1.20% C is subjected to rough rolling, and the intermediate rolling is performed by a reverse rolling mill at the surface temperature between 1,000 and 1,050 deg.C. The steel slab is subjected to the cooling immediately after the rolling of each pass of the intermediate rolling so that the temperature of the surface of a rail head and the surface of the bottom center is dropped by 50-100 deg.C. Next, the finish rolling is performed by a continuous rolling mill at the surface temperature between 850 and 1,000 deg.C with at least two passes of reduction ratio per pass of 5-30% and <=10 seconds between rolling passes. After the rolling, the steel slab is cooled to 800-950 deg.C at the cooling speed of 0.5-50 deg.C/s on the rail surface, and subjected to the natural or accelerated cooling.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a high toughness rail which is used for railways and other industrial machines and which has high toughness and high toughness and has a high carbon pearlite structure. It is.

[0002]

2. Description of the Related Art High carbon steel having a pearlitic metal structure is used as a structural material because of its high strength and good wear resistance. Rails are particularly frequently used in response to the trend.

[0003] As a method for producing such a steel material, for example, Japanese Patent Application Laid-Open No. 55-276 discloses that "steel of a specific component which easily exhibits a pearlite structure is cooled from a heating temperature of the Ac3 point or higher to 450-600 ° C. A method for producing a hard rail which is subjected to isothermal transformation at a temperature to generate a fine pearlite structure "and JP-A-58-221229," C: 0.65
-0.85%, Mn: 0.5-2.5%, quenched Mn steel rail with high temperature heat and improved wear resistance by making the structure of rail or rail head fine pearlite. Heat treatment method for rails "
No. 33322 discloses that a rolling rail of a specific component capable of stably obtaining a pearlite structure is immersed in a molten salt bath at a specific temperature from a temperature of not less than the Ar3 point, and Hv is reduced to about 10 mm below the surface of the rail top. Many techniques are known, as disclosed in "Method of heat treatment of rail exhibiting fine pearlite structure having hardness of>350".

However, although the strength and wear resistance of pearlite steel can be easily obtained as required standard products by the addition of alloying elements, the toughness is remarkably lower than that of steel mainly composed of ferrite. In the case of rail steel, a room temperature test value in a JIS No. 3 U-notch Charpy test is about 10 to 20 N · m. When such a low toughness steel is used as a structural member in a field where a repeated load or vibration is applied, there is a problem that low stress brittle fracture is caused from minute initial defects and fatigue cracks.

It is generally said that the improvement of the toughness of steel is achieved by refining the metal structure, that is, by reducing the austenite structure and transgranular transformation. The grain refinement of the austenite structure is achieved by, for example, low-temperature heating during rolling, a combination of controlled rolling and heat treatment as disclosed in JP-A-63-277721, or low-temperature reheating after rolling. Have been. However, in the rail manufacturing method, it is difficult to apply low-temperature heating during rolling, low-temperature rolling in controlled rolling, and large rolling under reduced pressure from the viewpoint of ensuring formability. Improvements are being made. However, this method also has problems such as high production cost and low productivity in the course of recent development of labor-saving and productivity-improving technologies for steel products, and early development of these methods is desired.

[0006]

SUMMARY OF THE INVENTION The present invention is intended to solve the above-mentioned problems, and overcomes the problem of controlled rolling that has been dependent on low temperatures or large pressures in forming rails.
An object of the present invention is to provide a method for performing controlled rolling specific to eutectoid steel to improve the toughness of eutectoid carbon steel such as rail steel.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted a number of experiments on steel components and their production methods in order to obtain a fine-grained pearlite structure and produce steel with improved toughness.
High-carbon steel, which is close to eutectoid carbon steel, was found to recrystallize immediately after rolling at a relatively low temperature and with a small rolling reduction during processing in the austenitic state. It was found that granules were obtained, and as a result, a fine-grained pearlite structure was obtained.
However, it has also been found that the finely recrystallized austenite grains immediately after rolling have a large grain growth between rolling passes and have an effect that offsets the effect of rolling in each pass.

The present invention has been made based on such knowledge, and the gist thereof is as follows. (1) After roughly rolling a slab containing C: 0.6 to 1.20% by mass, intermediate rolling by a reverse rolling mill is performed at a surface temperature of 1000 to 1150 ° C, and each of the intermediate rolling is performed. Immediately after the rolling of the pass, cooling is performed so that the temperature of the rail head surface and the bottom central surface decreases by 50 to 100 ° C, and then the finish rolling by a continuous rolling mill is performed to a surface temperature of 850 to 1000 ° C.
Between two or more passes with a cross-sectional reduction rate of 5 to 30% per pass and a rolling pass of 10 seconds or less,
After rolling, the rail is cooled to 800 to 950 ° C. at a cooling rate of 0.5 to 50 ° C./s, and then left to cool. Production method. (2) The composition of the billet is% by mass, and C: 0.6 to 1.20.
%, Si: 0.10 to 1.20%, Mn: 0.40
(1) The method for producing a high wear-resistant and high-toughness rail exhibiting a pearlite metal structure according to the above (1), characterized in that the rail contains up to 1.50%, with the balance being Fe and inevitable impurities. (3) The composition of the billet is mass%, and Cr: 0.05 to
2.00%, Mo: 0.01 to 0.30%, Co:
The method for producing a high abrasion-resistant and high-toughness rail exhibiting a pearlite metal structure according to the above (2), which comprises 0.10 to 2.00% of one or more kinds. (4) The composition of the billet is mass%, and Cu: 0.05 to
2.00%, Ni: 0.05 to 2.00%, one or two kinds of which are high in wear resistance and exhibit a pearlite metal structure according to the above (2) or (3).
Manufacturing method of high toughness rail. (5) The composition of the billet is expressed by mass% and V: 0.01 to
0.30%, Nb: 0.002 to 0.050%, T
i: 0.005 to 0.100%, Ca: 0.0005 to
0.0100%, Mg: 0.0005 to 0.0100%
The method for producing a high wear-resistant and high-toughness rail exhibiting a pearlite metal structure according to any one of the above (2) to (4), characterized by containing one or more of the following. (6) After cooling the rail surface to 800 to 950 ° C., continuously cool from 700 ° C. or higher to 500 ° C. at 2 to 15 ° C./s, and then allow to cool. A method for producing a high wear-resistant and high-toughness rail exhibiting a pearlite metal structure according to any one of (1) to (5).

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. First, the reason why the steel components are limited as described above in the present invention will be described. C is 0.60% as an effective component for forming a pearlite structure and ensuring abrasion resistance.
The above content is necessary. However, when the content is higher than 1.20%, a large amount of cementite structure is precipitated to increase the hardness, but the ductility is reduced, and the toughness which is the object of the present invention is significantly reduced.

The present invention is based on the knowledge of the austenite recrystallization behavior peculiar to steel containing at least the carbon near the eutectoid point as described above. There is no hindrance in the range in which pearlite is exhibited. For this reason, the following alloy elements can be appropriately added for the purpose of improving strength, ductility, and toughness.

Si is an element which increases the hardness (strength) of the rail head by solid solution hardening into the ferrite phase in the pearlite structure, but if its content is less than 0.10%, its effect cannot be expected sufficiently. If it exceeds .20%, a large number of surface flaws are generated during hot rolling, and the weldability is reduced due to the formation of oxides.
Limited to.

Mn lowers the pearlite transformation temperature,
It is an element that contributes to high strength by enhancing hardenability and further suppresses the formation of a pro-eutectoid cementite structure. However, if its content is less than 0.40%, its effect is small, and it is required for the rail head. It is difficult to secure sufficient hardness. If the content exceeds 1.50%, the hardenability is remarkably increased, and a martensite structure is easily generated, and segregation is promoted, and a pro-eutectoid cementite structure harmful to rail toughness is easily generated in the segregated portion. , Mn amount is 0.40 to 1.50
%.

[0013] Cr is an element that raises the equilibrium transformation point of pearlite and consequently makes the pearlite structure finer and contributes to higher strength, and at the same time, improves the wear resistance by strengthening the cementite phase in the pearlite structure. However, when the content is less than 0.05%, the effect is small.
%, A large amount of martensite structure is generated to lower the toughness of the rail. Therefore, the Cr content is limited to 0.05 to 2.00%.

Mo is an element that, like Cr, raises the equilibrium transformation point of pearlite and consequently refines the pearlite structure, thereby contributing to higher strength and improving wear resistance, but less than 0.01%. In addition, the effect is small, and excessive addition exceeding 0.30% promotes segregation, further reduces the pearlite transformation speed, generates a martensite structure in the segregated portion, and reduces the toughness of the rail. The amount of Mo was limited to 0.01 to 0.30%.

Co is an element that increases the transformation energy of pearlite to improve the strength by making the pearlite structure finer, but its effect cannot be expected if it is less than 0.10%, and 2.00%. However, the effect reaches the saturation range even if an excessive amount of addition is performed, so the Co content is limited to 0.10 to 2.00%.

Cu is an element which improves the strength without impairing the toughness of the pearlite steel, and its effect is 0.05 to 2.0.
The largest amount is in the range of 00%, and if it exceeds 2.00%, red heat embrittlement is likely to occur.
To 2.00%.

Ni is an element that improves the ductility and toughness of the pearlite steel and at the same time increases the strength of the pearlite steel by solid solution strengthening. %, No further effect can be expected. Therefore, the amount of Ni was limited to 0.05 to 2.00%.

In the heat treatment of the rail head, V forms carbides and nitrides in the inside of the rail head, which has a lower cooling rate than the surface of the rail head, and precipitates on the ferrite ground in the pearlite structure. Although it is an element for improving the hardness inside the portion, if it is less than 0.01%, it becomes difficult to form carbides and nitrides, and it becomes difficult to precipitate and harden the pearlite structure inside the rail head. Further, if more than 0.30% is added, no further effect can be expected.
Limited to 1 to 0.30%.

Nb, like V, increases the strength by precipitation hardening with Nb carbide and Nb nitride, and further suppresses the growth of crystal grains during heat treatment at a high temperature to reduce austenite grains. The effect of suppressing austenite grain growth acts up to a temperature range higher than V (around 1200 ° C.), and improves the ductility and toughness of the pearlite structure. The effect cannot be expected if it is less than 0.002%, and no further effect can be expected even if it is added in excess of 0.050%. Therefore, the amount of Nb is 0.002-
Limited to 0.050%.

In the reheating at the time of rail rolling, Ti
Utilizing the fact that precipitated Ti carbides and Ti nitrides do not dissolve, it is an effective component for refining austenite crystal grains during rolling and heating and improving ductility and toughness of pearlite structure. However, if the content is less than 0.005%, the effect is small, and if it is added more than 0.100%, coarse Ti carbides and Ti nitrides are formed and become a starting point of fatigue damage during use of the rail, and cracks are generated. Therefore, the amount of Ti is limited to 0.005 to 0.100%.

Ca has a strong binding force with S, which is an unavoidable impurity, and forms a sulfide as CaS.
S finely disperses MnS, forms a rare band of Mn around MnS, and contributes to the generation of pearlite transformation, thereby improving the ductility and toughness of the pearlite structure by reducing the pearlite block size. It is an effective element to make it. However, if it is less than 0.0005%, the effect is weak, and if it exceeds 0.0100%, a coarse oxide of Ca is generated to deteriorate rail ductility and toughness. %.

Mg combines with O or S or Al to form a fine oxide, suppresses the growth of crystal grains during reheating during rail rolling, refines austenite grains, and achieves a pearlite structure. It is an element effective for improving the ductility and toughness of steel. Further, MgO and MgS finely disperse MnS, form a thin band of Mn around MnS, and contribute to the generation of pearlite transformation. As a result, the pearlite block size is reduced, thereby improving the ductility of the pearlite structure. And an element effective for improving toughness. However, if less than 0.0005%, the effect is weak, and 0.01%.
If added in excess of 00%, a coarse oxide of Mg is formed to deteriorate the ductility and toughness of the rail.
005 to 0.0100%.

Next, each process condition of the present invention will be described. In the rolling of rail steel, the rolling reduction per pass in the intermediate rolling stage and the finish rolling stage after performing the rough rolling of the slab is usually 5 to 30 in terms of cross-sectional reduction rate from the viewpoint of ensuring the formability of the rail. % And the finishing temperature is on the order of 1000 ° C. On the contrary,
Recently, controlled rolling for the purpose of improving ductility and toughness has been performed at a lower temperature. In general, in the case of controlled rolling of steel mainly composed of ferrite, the rolling temperature is lowered to the unrecrystallized region of austenite, and the strain is introduced into the processed austenite to increase the ferrite nucleus and control rolling to obtain fine-grained ferrite. The law has been adopted.

However, in the case of pearlite steel, it was found that due to eutectoid transformation, the growth rate of pearlite was large, the austenite intragranular transformation nuclei did not work effectively, and it was difficult to obtain substantially fine pearlite. Therefore, it was found that obtaining fine-grained austenite was necessary for reducing pearlite.

From this point of view, a detailed study of the recrystallization behavior of austenite of a high carbon steel revealed that: 1) recrystallization to a lower temperature and a lower workability than a low carbon steel; The time required for complete recrystallization is very small,
That is, recrystallization is completed immediately after rolling. 3) When processing is continuously performed (about 10 seconds or less) even under a small pressure, recrystallization is repeated each time, and grain growth until the next processing is suppressed, or If the temperature after rolling is lowered even between passes of 10 seconds or more, grain growth is suppressed,
It was found that fine-grained recrystallized austenite grains were obtained.

Based on these findings, the optimum processing condition range was found. The reasons for limiting the conditions are described below. 1
In the case of intermediate rolling using a reverse rolling mill at a temperature of 000 to 1150 ° C., fine-grained austenite is obtained by recrystallization according to the reduction in area after each rolling pass. Specifically, it takes 20 to 60 seconds, during which the grain growth is remarkable. Therefore, the grain growth can be suppressed by performing cooling such that the temperature of the rail head surface and the bottom central surface decreases by 50 to 100 ° C. immediately after rolling in each pass, and the accumulation of fine austenite grains in each pass can be obtained. The previous austenite grains can be refined. Although the number of passes is not particularly specified, the pass is usually performed in two to three passes in view of the required area reduction rate for each pass. The cooling method is not particularly limited, but a method of spraying a liquid such as water or a mixed gas containing a mist is preferable in order to secure a required cooling rate.

The final rolling temperature is 850-100.
The range of 0 ° C. is optimal, and if it is lower than 850 ° C., austenite is in an unrecrystallized state, and as described above, it is not effective for miniaturization of pearlite. On the other hand, when the temperature exceeds 1000 ° C., the growth of austenite grains after rolling is large and becomes coarse austenite of mixed grains at the time of pearlite transformation, which is not effective in refining pearlite. Normally, the surface temperature rises due to reheating from the inside of the rail between the intermediate rolling and the finish rolling. Therefore, it is preferable to appropriately adjust the finishing temperature by cooling or cooling.

The rolling reduction per pass is 5 to 30.
% Is optimal. If it is less than 5%, it is not possible to introduce a strain effective for causing recrystallization, and if it exceeds 30%, it is effective for recrystallization. It is not effective because the number of rolling passes cannot be sufficiently taken from the total cross-sectional reduction amount and rail forming becomes difficult.

The time between passes must be 10 seconds or less. Austenite grains at high temperatures are liable to cause coarsening and mixing of crystal grains due to coalescence of adjacent grains, so-called grain growth. When the distance between rolling mills is large even in normal reverse rolling or continuous rolling, the time between passes is 20 to
It becomes as long as about 60 seconds, during which recovery of strain, recrystallization and further grain growth of the rolled austenite grains occur. In the high carbon component system of the present invention, recrystallization is completed immediately after rolling, so that grain growth occurs during the inter-pass time as described above, and the effect of recrystallization to reduce fine grains is reduced. That is, if the time between passes exceeds 10 seconds, the effect of grain growth between passes becomes so large that it cannot be overlooked, the effect of austenite grain refinement by rolling recrystallization is reduced, and the object cannot be achieved. Further, as described above, continuous rolling of at least two passes or more is necessary from the viewpoint of grain refinement by repetition of recrystallization.

After the completion of the above rolling, the reason for cooling to 800 to 950 ° C. at a cooling rate of 0.5 to 50 ° C./s on the rail surface will be described. Although it has been described earlier that the effect of austenite grain growth between rolling passes exceeds 10 seconds after rolling, the effect cannot be overlooked, but austenite grain growth after the completion of rolling also takes a similar behavior.
At this time, the grain growth can be suppressed by lowering the temperature of austenite as in the case of the reverse intermediate rolling described above. Therefore, the cooling rate on the rail surface is 0.5
By cooling to 800 to 950 ° C at ~ 50 ° C / s,
It is necessary to avoid the effect of temperature on grain growth. The cooling method in this case is also not particularly limited, but a method of spraying a liquid such as water or a mixed gas containing a mist is preferable in order to secure a required cooling rate.

When the cooling or the strength is to be further improved, accelerated cooling is performed. The reason for limiting accelerated cooling will be described.
Cooling start temperature is in the austenitic region of steel, at least 70
The temperature is required to be 0 ° C. or higher. If the temperature is lower than 0 ° C., effective transformation strengthening cannot be performed. Further, the cooling rate is required to be 2 to 15 ° C./s in a temperature range related to the transformation of the steel, that is, from a temperature of 700 or more to 500 ° C .; You can only get the transformation enhancement that is not. On the other hand, when the temperature exceeds 15 ° C./s, an abnormal structure such as bainite or martensite is mixed, and wear resistance and toughness are significantly impaired. The cooling method in this case is also not particularly limited, but from the viewpoint of controllability of the cooling rate, a method of blowing a gas such as air or a mixed gas containing mist is preferable.

[0032]

EXAMPLE After a test steel consisting of the chemical components shown in Table 1 was rough-rolled, it was subjected to three-pass intermediate rolling and finish rolling under the conditions shown in Table 2, and was cooled immediately after rolling under the conditions shown in Table 3. Then, it was allowed to cool or heat-treated and cooled to increase the strength under the conditions shown in Table 4. The cooling during the intermediate rolling shown in Table 2 and the cooling after the rolling shown in Table 3 were performed by a method of spraying water, and the heat treatment and cooling shown in Table 4 were performed by a method of blowing air.

Table 5 shows the case where rails were manufactured by combining the steel components, rolling conditions, cooling conditions immediately after rolling, and heat treatment cooling conditions for increasing the strength shown in Tables 1 to 4.
3 shows the mechanical properties of the rail steel according to the method of the present invention and the comparative method. In the method of the present invention, the strength of the rail changes depending on the steel composition and the cooling conditions, but the ductility value and the toughness value are significantly higher than those of the comparative method.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

[Table 4]

[0038]

[Table 5]

[0039]

As described above, according to the method of the present invention, a fine pearlite structure can be obtained, and a rail having improved toughness in addition to wear resistance can be manufactured.

Continued on the front page F term (reference) 4K032 AA06 AA07 AA08 AA09 AA10 AA11 AA12 AA14 AA15 AA16 AA19 AA22 AA23 AA24 AA31 AA32 AA35 AA36 BA00 CC04 CD01 CD02 CD03

Claims (6)

[Claims] 1. After roughly rolling a slab containing C: 0.6 to 1.20% by mass, intermediate rolling by a reverse rolling mill is performed at a surface temperature of 1000 to 1150 ° C., and said intermediate rolling is performed. Immediately after rolling in each pass, cooling is performed so that the temperature of the rail head surface and the bottom central surface decreases by 50 to 100 ° C., and then the finish rolling by a continuous rolling mill is performed to a surface temperature of 850 to 1
Cross section reduction rate of 5-30% per pass between 000 ° C
Rolling is performed in two or more passes and the interval between the rolling passes is 10 seconds or less, and after rolling, the cooling rate on the rail surface is 0.5 to 50.
A method for producing a high wear-resistant and high-toughness rail exhibiting a pearlite metal structure, characterized in that the rail is cooled to 800 to 950 ° C. at a temperature of 800 ° C./s and then left to cool. 2. The composition of a steel slab is in mass%, containing: C: 0.6 to 1.20%, Si: 0.10 to 1.20%, Mn: 0.40 to 1.50%, and the balance 2. The method for producing a highly wear-resistant and high-toughness rail exhibiting a pearlite metal structure according to claim 1, wherein the steel comprises Fe and unavoidable impurities. 3. The steel slab is further composed of one of the following components in mass%: Cr: 0.05 to 2.00%, Mo: 0.01 to 0.30%, Co: 0.10 to 2.00%. 3. The method for producing a high wear-resistant and high-toughness rail exhibiting a pearlite metal structure according to claim 2, comprising two or more types. 4. The steel slab is characterized in that the slab further contains one or two of Cu: 0.05 to 2.00% and Ni: 0.05 to 2.00% by mass. Claim 2
Or a method for producing a highly wear-resistant and high-toughness rail exhibiting the pearlite metal structure according to 3. 5. The composition of a billet in mass%, V: 0.01 to 0.30%, Nb: 0.002 to 0.050%, Ti: 0.005 to 0.100%, Ca: The pearlite metallographic structure according to any one of claims 2 to 4, comprising one or more of 0.0005 to 0.0100% and Mg: 0.0005 to 0.0100%. A method for manufacturing a high wear-resistant and high-toughness rail exhibiting. 6. The method according to claim 1, wherein after cooling the rail surface to 800 to 950.degree. C., cooling is continued at a temperature of from 700.degree. C. or more to 500.degree. C. at 2 to 15.degree. Item 6. A method for producing a highly wear-resistant and high-toughness rail exhibiting a pearlite metal structure according to any one of Items 1 to 5.