JPS5919173B2 - Manufacturing method of weldable low-alloy heat-treated hard-headed rail - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a low-alloy heat-treated hard-headed rail with excellent weldability.

In recent years, railway transportation has become faster and heavier, with the Shinkansen in Japan being an example of the former, and mining railways overseas being an example of the latter.

In these railways, damage to the top of the rail head and wear on the side of the rail head are important problems, and high-strength rails that can withstand harsh loading conditions are required. On the other hand, in order to prevent damage to rail joints and reduce rail maintenance costs, most railways currently use long rails by welding, and even high-strength rails have good welding properties and welded part quality. Homogeneity is an essential requirement. However, the high-strength rails manufactured and used to date have been studied only from the viewpoint of improving their chemical composition and manufacturing method, and have not been studied solely from the perspective of improving weldability, that is, the heat-affected zone of the rail weld. Weldability was poor because no consideration was given to preventing deterioration of material properties due to welding, such as hardening, embrittlement, or softening. By the way, the proposed high-strength rails can be divided into as-rolled alloy steel rails and heat-treated carbon steel hard-headed rails. ，
After hot-rolling steel to which alloying elements such as V are added into rails, it is naturally cooled to undergo pearlite transformation, resulting in a 100 to 120
It gave a tensile strength of kj1 mm2, and it was necessary to add a large amount of alloying elements in order to obtain high strength at a fairly slow cooling rate of air cooling from the austenite state after rolling. When this rail is welded to make a long rail, the cooling rate of the weld joint and heat-affected zone is much faster than the cooling rate after hot rolling, so martensite is partially generated and the base metal rail This results in significant hardening and embrittlement compared to the conventional method, causing breakage of the rail at welded parts, uneven wear, and deterioration of the trackbed, which is a major problem in practice. Furthermore, in order to prevent or eliminate the formation of martensite, treatments such as slow cooling during welding or post-heating have been carried out, but these methods significantly reduce welding efficiency and are therefore not practical. . Furthermore, since flash butt welding and gas pressure welding, which are often performed in rail welding, do not involve fusion welding, the alloying elements oxidize at the joint end faces, resulting in insufficient joint strength and problems such as breakage during transportation after welding. is also occurring. Next, referring to the latter heat-treated hard-headed rail, the rail head is hardened by heat treatment such as quenching and tempering or slack quenching to harden the rail head surface layer, giving it high strength. Its tensile strength is approximately 120 kg//ILm" or more.

However, these rails are different from the currently commonly used as-rolled high carbon steel rails (which, by the way, have a tensile strength of 70 to 90
kg/RIL) is heated to an austenitic state and then cooled very rapidly for quenching or slack quenching. Because the cooling rate is slow compared to the cooling rate of the weld, softening of the welded part is unavoidable, and local deformation and wear of the rail at the softened part, resulting in deterioration of the track bed, have become important problems. In order to prevent this softening, a method has been adopted in which the welded part is immediately reheated or rapidly cooled after welding, but this also reduces welding efficiency and is hardly practical. As mentioned above, although the current high-strength rails do have high strength and good properties for the base material, the strength of the welded parts of the rails relative to the base material (non-welded parts), which is essential for modern railways, is Since no measures have been taken to prevent the deterioration of the structure, when the high-strength rail is welded, the welded part has the disadvantage of hardening, embrittlement, or softening compared to the base material rail. ,
Many problems remain in terms of welding work, practical use, and rail maintenance.

That is, for as-rolled alloy steel rails, alloying elements are selected based on the cooling rate during natural cooling after hot rolling, while for heat-treated carbon steel hard-headed rails, alloying elements are selected based on the cooling rate during heat treatment based on the components of current high carbon steel. Due to the selected speed, many problems occurred in both cases in welding. The present invention
In order to solve the above-mentioned problems with current high-strength rails, we added alloying elements selected from the viewpoint of adding strength to high-carbon steel and preventing material deterioration of the welded parts during rail welding. When heat treating the head, by making the cooling conditions equivalent to the plug cooling speed during rail welding, welding is easy and welding efficiency is high.
Furthermore, the present invention provides a low-alloy hard-head heat-treated rail with excellent weldability, in which the strength, structure, and other material properties of the welded part are not significantly different from those of the base metal even as welded.

That is, in the present invention, C: 0.55 to 0.85%, Si: 0.5%
0-1.20%, Mn: 0.80-1.50%, Al
: 0.005 to 0.050% or the above C, Sj so that even if the steel ingot becomes dog-shaped or the rail cross section becomes large, a fine pearlite structure will be formed when the rail head is heat treated or the rail is welded. , Mn: 0.5 in addition to Al
0~1.20%, Cr: 0-20~O-90%, N
b: 0.004 to less than 0.010, further Mn+Cr
``After rolling a steel containing 1.60% or less and the balance consisting of iron and unavoidable impurities, the head surface layer of the rail is heated to a temperature of 850°C or higher to austenite,
By rapidly cooling between 800°C and 550°C for 50 to 400 seconds with gas or gas-liquid cooling,
Preferably, the structure of the surface layer of the head part up to a depth of 10 mff1 from the head surface is made into fine pearlite, and the tensile strength at room temperature is 120k9/Mm" or more, and the surface hardness of the top of the rail head is HB33O or more. This is a method for manufacturing a low-alloy heat-treated hard-headed rail with excellent weldability.The present invention will be described in detail below.

Low-alloy steel having the above-mentioned composition is melted in a converter, electric furnace, etc. and is rail-rolled.

Among these chemical components, C is an essential component for high strength and pearlite structure formation, and is an element that improves wear resistance, but if it is less than 0.55%, it will produce low carbon bainite during heat treatment. to reduce wear resistance, 0.85
If it exceeds 0.55% to 0.8%, pro-eutectoid cementite is generated at the austenite grain boundaries, or martinsite is generated in the micro-segregation areas of the welded part of the heat-treated layer, resulting in hardening and embrittlement.
It was limited to 5%. Si also strengthens the ferrite in pearlitic steel, increasing its strength, and at the same time increasing its damage resistance. Furthermore, it has a small effect on the start time and temperature of pearlitic transformation, and when considering waxing and welding, it is effective in controlling the cooling rate. Although it is an element that makes it extremely easy to
If it is less than 1.2%, the effect will be small, and if it exceeds 1.20%, it will cause embrittlement and reduce weld bondability, so it was limited to 0.50 to 1.20 so.

Mn is an element that retards pearlite transformation, and by changing the amount added, it is possible to control the start of pearlite transformation and control the strength, but if Cr and Nb are not used together, it is less than 0.80%. Then the effect is small <1.5
If it exceeds 0%, it tends to cause hydrogen embrittlement and martensite formation due to segregation, so it is limited to 0.80 to 1.50%.
A7 is added for deoxidation and has the effect of homogenizing the steel quality and preventing the formation of silicate inclusions that reduce fatigue strength.
O-005% to 0.050% was limited because O-005% to 0.050% would cause embrittlement if the content exceeded O-005%. The reason for adding Cr and Nb and the reason for limiting the range are as follows.

When Mn is not used in combination with Cr and Nb, as the steel ingot or rail cross section becomes larger, the microscopic segregation of Mn becomes stronger and fine martensite is generated in the heat affected zone during heat treatment of the rail head or during welding. and causes embrittlement. Therefore, in this case, reduce Mn, add Cr to compensate for the decrease in strength due to the decrease in Mn, and use Nb to refine the crystal grains during casting and heat treatment to generate martensite due to micro segregation. It is effective to prevent this. When reducing Mn, Cr can control pearlite transformation and strength in place of Mn, and can prevent the formation of fine martenquite without micron segregation.
If it is less than 0.20%, sufficient effect cannot be obtained;
If it exceeds 0%, the bondability during welding will deteriorate;
It was set at 20-0.90%. In this case, in order to reduce the amount of Mn added, limit it to 0.50% to 1.20%, and if Mn + Cr exceeds 1.60%, fine martensite will form in the weld even if Mn is reduced. To do this, Mn
The upper limit of +Cr was set to 1.60%. When Mn and Cr are used together, the addition of Nb is more effective; Nb refines the crystal grains during solidification of the steel ingot, prevents micro-segregation of Mn, and further suppresses the growth of austenite grains during heating in the austenite region. It is also effective in improving ductility. Another effect is that the pearlite transformation completion time is greatly shortened, which is effective in preventing the formation of harmful martensite during heat treatment or cooling during welding.
Furthermore, when the heat-treated hardened portion of this rail is welded, if the cooling time is within the limited range of the present invention, it has the effect of extremely minimizing changes in hardness. These effects are based on the fine Nb carbides precipitated in this rail steel,
If the content is less than 0.004%, an effective amount of carbide cannot be obtained, and if the content is more than 0.010%, huge carbides are formed, reducing toughness and fatigue strength.
It was limited to less than 0.010%. As mentioned above, reducing Mn and adding Cr and Nb is effective when the cross section of the steel ingot or rail is large; on the other hand, it is effective when the cross section of the steel ingot is small or when the slab is cooled rapidly. In the case of casting, it is often necessary to add Cr and Nb, but these limitations do not limit the casting method of the present rail steel.

Next, the rail head surface layer preferably 101 from the head surface!
In order to harden to the depth of W, after austenitizing, rapid cooling is performed, but in order to uniformly austenite the surface layer, it is necessary to heat to a temperature of 850° C. or higher, which is the Ac3 transformation point or higher.

Further, the rapid cooling is performed by cooling from 800° C. to 550° C. in 50 seconds to 400 seconds to perform slack quenching. If the cooling time is less than 50 seconds, martensite will be formed, and if it is more than 400 seconds, a fine pearlite structure and 120
Since it is not possible to obtain a tensile strength of 800 kg/In or more,
The time required for cooling from °C to 550 °C was limited to 50 to 400 seconds. This cooling time is 40kg/m~68kg/m
It is equal to the natural cooling time when flash butt welding a rail of Some kind of forced cooling means is required to achieve cooling inside the machine, but this is achieved by jetting a gas such as air, inert gas, or a gas-liquid mixture with a small amount of liquid from a nozzle. Since cooling with liquid alone would be difficult to control, we limited ourselves to rapid cooling using these gases. Furthermore, the temperature in this case indicates the temperature of the rail surface. The structure after cooling was limited to a fine pearlite structure, which is 12
If the strength is 0 kg/RILTIL2 or more, this is because the structure is significantly superior in wear resistance and damage resistance compared to other marten, site, low carbon bainite, tempered martensite, etc. In this case, a fine pearlite structure is essential. The tensile strength of the heat-treated hardened layer was set to be at least 120k9/WLm2 in order to have the same or higher strength as the current high-strength rail, but this cannot be increased as necessary if the structure is maintained as fine pearlite. No problem. In order to easily judge the strength, the strength can be regulated by the hardness of the top of the rail, and for this purpose, the hardness of the top of the rail was set to be HB33O or higher. The depth of the hardened layer of the rail head should have a tensile strength of 120 kg ALW" or more at a depth of at least 10 mrtt from the top surface and the side surface of the head, and a hardened layer with a tensile strength of 10 kg or more must be obtained. Alternatively, hardening the entire cross section of the rail is not a good idea because rail wear and damage is limited to the rail head surface and rail manufacturing is extremely difficult.The column and bottom of this rail are not heat treated and are rolled. However, the tensile strength is approximately 100 kg/Rnn2, and there is no problem in practical use.For the reasons stated above, the rail of the present invention has a tensile strength of approximately 100 kg/Rnn2. Although it is possible to obtain equivalent strength and structure, examples will be described below.
The chemical composition and tensile properties of the hardened layer at the head of the base rail are shown in comparison with the inventive steel and the conventional steel.

The cooling time from SOO° C. to 550° C. 1 during heat treatment of the steels A and B of the present invention was 100 seconds for rail A and 120 seconds for rail B. Conventional steel C is an as-rolled Si-Cr alloy steel rail with a fine pearlite structure, and conventional steel D is a heat-treated carbon steel hard-headed rail, in which the head of the carbon steel rail is heated by induction heating and then subjected to slack quenching. It also has a fine pearlite structure. The rails A and B of the present invention are 1
It has a tensile strength (σB) of 20kii2 or more, and a reduction of area (ψ) of 40% or more, and has good ductility despite its high strength.

Conventional steel C has a C quantity {a little less, but the Ss and Mn contents are A
Although it is equivalent to steel and has a high Cr content of 1.08%, its tensile strength is low at 102 kgAni, and its drawing strength is only the same as steel A and B. Further, despite the small amount of alloy, Steel D has a tensile strength of 123 ky/Mm2, which is equivalent to the steel of the present invention, and a high yield ratio, which is the result of heat treatment called slack quenching. Figure 1 (hardness was measured at 5 points from the crown surface on the center secondary section) shows the change in hardness of the flash butt welds of these rails, and the steels A and B of the present invention Although a softened part is observed at about 20 points from the center, the width and degree of softening is small, and the hardness near the center of the weld is about 320 Hv or more, which is due to the hardness of the left and right base material rails. There is not much difference in Hv, about 370, and the structure of the welded part also shows a fine pearlite structure due to continuous cooling, and it is clear that the material change of the welded part of the steel of the present invention is extremely small. On the other hand, comparative steel C has a base metal hardness of H
V is about 300, whereas the soluble part is hardened to a level exceeding Hv4OO, and the structure is also dotted with martensite, making it extremely hardened and brittle. If used as is, the welded part will be This resulted in early breakage of the rails or the turning of the trackbed into mud. In order to eliminate this hardening and embrittlement, it is necessary to perform slow cooling or post-heat tempering after welding, which significantly reduces welding efficiency and causes poor bonding properties due to the large amount of Cr. , it was difficult to select appropriate welding conditions. On the other hand, for comparison steel D, the base metal hardness is 350 Hv, but in the welded part it softens to the base metal hardness before heat treatment at Hv3lO or less, and the width is extremely wide at about 70r111L, and the effect of heat treatment is Totally lost. This is because the cooling rate during heat treatment is extremely fast compared to the cooling rate of the welded part. In this softened area, local wear and deformation of the rail occurs, resulting in the problem of mud on the trackbed. As described above, in the present invention, the steel composition and heat treatment conditions are specified from the viewpoint of both increasing the strength of the rail and preventing deterioration of material properties due to welding. Although it is not much different from existing high-strength rails, the material at the welded part is the same as the base material, and the change in material quality is extremely small. This is a characteristic that the rail of the present invention is significantly superior to in that conventional steel suffers from hardening, embrittlement, or softening at the welded portion.

[Brief explanation of the drawing]

FIG. 1 is a chart showing changes in hardness of flash butt welded parts of the rail of the present invention and the comparative rail.

Claims (1)

[Claims] 1 C: 0.55-0.85%, Si: 0.50-1.
20%, Mn: 0.80-1.50%, Al: 0.00
The head surface layer of a rail made by rail-rolling steel containing 5 to 0.05% and the remainder consisting of iron and unavoidable impurities is heated to a temperature of 850°C or higher to austenite, and then cooled by gas or gas-liquid cooling. 50 between 800 and 550℃
Welding characterized by cooling for ~400 seconds, turning the structure of the surface layer of the rail head into fine pearlite, making its tensile strength 120 kg/mm^2 or more, and the surface hardness of the top of the rail head H_B330 or more. A method for manufacturing low-alloy heat-treated hard-headed rails. 2C: 0.55-0.85%, Si: 0.50-1.
20%, Mn: 0.50-1.20%, Al: 0.00
5-0.05%, Cr: 0.20-0.90%, Nb0
．． 004 to less than 0.010% However, Mn+Cr: 1.6
The head surface layer of a rail made of rail-rolled steel containing 0% or less and the remainder consisting of iron and unavoidable impurities is heated to a temperature of 85%.
After heating to 0°C or higher to austenite, the temperature is 50 to 40°C between 800 and 550°C by gas or gas-liquid cooling.
Weldability characterized by cooling in 0 seconds, turning the structure of the surface layer of the rail head into fine pearlite, making its tensile strength 120 kg/mm^2 or more, and the surface hardness of the top of the rail head H_B330 or more. Manufacturing method for low-alloy heat-treated hard-headed rails.