CN111411208A - A heat treatment method for reducing the precipitation of reticulated cementite in hypereutectoid rails - Google Patents

Abstract

The invention relates to the technical field of heat treatment of steel rails, in particular to a heat treatment method for reducing the precipitation of reticulated cementite in hypereutectoid steel rails. The heat treatment method provided by the present invention includes the following steps: heating the rolled or heat-treated hypereutectoid rail to above 900° C., and keeping the temperature to obtain austenitized steel rail; firstly cooling the austenitized steel rail by first cooling Speed cooling to an isothermal temperature for 30-50s; then cool down to a final cooling temperature below 400°C at a second cooling rate; then naturally cool to room temperature; the isothermal temperature is 600-630°C; the first cooling rate and the second cooling rate The two cooling rates are independently 8 to 10°C/s. The results of the examples show that the heat treatment method provided by the present invention can effectively reduce the precipitation of network cementite of the hypereutectoid rail, and improve the mechanical properties of the hypereutectoid rail.

Description

A heat treatment method for reducing the precipitation of reticulated cementite in hypereutectoid rails

technical field

The invention relates to the technical field of heat treatment of steel rails, in particular to a heat treatment method for reducing the precipitation of reticulated cementite of a hypereutectoid steel rail.

Background technique

As an important part of railway transportation, the performance of steel rails is particularly important to ensure the safety of traffic and the efficiency of railway operation. With economic development and the continuous increase of heavy-haul railway traffic, the contact conditions between the wheel and rail are getting worse, and the damages such as side grinding and peeling off of the rail are becoming more and more serious, and the traditional pearlitic rail can no longer meet the current use requirements. The hypereutectoid rail was first developed by Japanese researchers. It has higher carbon content and cementite density, and a larger carbon content can enable it to obtain high strength and hardness after heat treatment, and the increase of cementite density. It improves its anti-rolling contact fatigue performance and wear resistance, and can be used on large-capacity, heavy-duty lines and small-radius lines. However, with the increase of carbon content, it will lead to the precipitation of network cementite at the grain boundary, and microcracks are easy to form at the cementite and continue to expand along the continuous network, which has a bad influence on the mechanical properties of the rail.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a heat treatment method for reducing the precipitation of reticulated cementite in hypereutectoid rails, and the heat treatment method provided by the present invention can effectively reduce the precipitation of harmful microstructure reticulated cementite in the hypereutectoid rail, improve Mechanical properties of hypereutectoid rails.

In order to achieve the above-mentioned purpose of the invention, the present invention provides the following technical solutions:

The invention provides a heat treatment method for reducing the precipitation of hypereutectoid rail network cementite, comprising the following steps:

The rolled or heat-treated hypereutectoid rail is heated to above 900°C, and heat preservation is performed to obtain austenitized rail;

The austenitized steel rail is first cooled to an isothermal temperature at a first cooling rate for 30-50 s; then cooled to a final cooling temperature below 400°C at a second cooling rate; and then naturally cooled to room temperature; the isothermal temperature is 600-630°C; the first cooling rate and the second cooling rate are independently 8-10°C/s.

Preferably, the first cooling rate and the second cooling rate are the same.

Preferably, the carbon content of the as-rolled or heat-treated hypereutectoid rail is 0.94-1.00 wt.%.

Preferably, in terms of mass percentage, the chemical composition of the as-rolled or heat-treated hypereutectoid rail includes: C0.94-1.00%, Si 0.45-0.80%, Mn 0.75-1.25%, P≤0.015%, S≤ 0.010%, Cr+Nb+Ni0.625%, the balance is iron.

The invention provides a heat treatment method for reducing the precipitation of network cementite in a hypereutectoid steel rail, which comprises the following steps: heating the hypereutectoid steel rail in a rolled state or heat treatment state to a temperature above 900° C. for heat preservation to obtain an austenitized steel rail ; Cool the austenitized steel rail to an isothermal temperature at a first cooling rate for 30 to 50 s; then cool it to a final cooling temperature below 400°C with a second cooling rate; then naturally cool to room temperature; the isothermal temperature is 600-630°C; the first cooling rate and the second cooling rate are independently 8-10°C/s. In the present invention, the rolled or heat-treated hypereutectoid rail is first heated to above 900°C, so that the carbides with uneven distribution and particle size in the raw materials are completely solid-solubilized and fully austenitized; Cooling to 600-630°C at a speed of /s, and isothermal at this temperature for 30-60s, can effectively refine the pearlite lamella spacing and avoid the appearance of reticulated cementite; then cool at a speed of 8-10°C/s To the final cooling temperature below 400 ℃. The results of the examples show that the heat treatment method provided by the present invention can effectively reduce the precipitation of the network cementite of the hypereutectoid rail and improve the mechanical properties of the hypereutectoid rail. The hardness is 396-415HB, the tensile strength is 1312-1437MPa, and the yield The strength is 800-911 MPa, the elongation is 12.24-16.80%, and the area shrinkage is 20.79-25.70%.

In addition, the heat treatment method provided by the present invention is simple and easy to operate, and is suitable for industrial application.

Description of drawings

1 is a schematic diagram of a heat treatment method according to an embodiment of the present invention;

Figure 2-a is the metallographic structure diagram of the sample prepared in Comparative Example 1;

Figure 2-b is the metallographic structure diagram of the sample prepared in Comparative Example 2;

Figure 2-c is the metallographic structure diagram of the sample prepared in Comparative Example 3;

Figure 2-d is the metallographic structure diagram of the sample prepared in Comparative Example 4;

Figure 2-e is the metallographic structure diagram of the sample prepared in Example 1;

Figure 2-f is the metallographic structure diagram of the sample prepared in Example 2;

Figure 2-g is the metallographic structure diagram of the sample prepared in Example 3;

Figure 3-a is the microscopic SEM topography of the tensile fracture of Comparative Example 1;

Figure 3-b shows the microscopic SEM morphology of the tensile fracture of Comparative Example 2;

Figure 3-c is a microscopic SEM image of the tensile fracture of Comparative Example 3;

Figure 3-d is the microscopic SEM topography of the tensile fracture of Comparative Example 4;

Fig. 3-e is the microscopic SEM topography of the tensile fracture of Example 1;

Fig. 3-f is the microscopic SEM topography of the tensile fracture of Example 2;

FIG. 3-g is a microscopic SEM topography of the tensile fracture of Example 3. FIG.

Detailed ways

The invention provides a heat treatment method for reducing the precipitation of hypereutectoid rail network cementite, comprising the following steps:

The rolled or heat-treated hypereutectoid rail is heated to above 900°C, and heat preservation is performed to obtain austenitized rail;

The austenitized steel rail is first cooled to an isothermal temperature at a first cooling rate for 30-50 s; then cooled to a final cooling temperature below 400°C at a second cooling rate; and then naturally cooled to room temperature; the isothermal temperature is 600-630°C; the first cooling rate and the second cooling rate are independently 8-10°C/s.

The present invention heats the hypereutectoid rail in the as-rolled or heat-treated state to a temperature above 900°C, preferably 900°C. The heat preservation is performed to obtain an austenitized rail. In the present invention, the carbon content of the as-rolled or heat-treated hypereutectoid rail is preferably 0.94-1.00 wt.%, more preferably 0.95-0.96 wt.%. The present invention has no particular limitation on the specific composition of the hypereutectoid rail in the rolled state or heat treatment state, and a hypereutectoid rail well known in the art may be used. In a specific embodiment of the present invention, the chemical composition of the rolled or heat-treated hypereutectoid rail includes: C 0.94-1.00%, Si 0.45-0.80%, Mn 0.75-1.25%, P ≤0.015%, S≤0.010%, Cr+Nb+Ni 0.625%, the balance is iron. In the present invention, the initial temperature of the as-rolled or heat-treated hypereutectoid rail is preferably room temperature, and the heating rate from the room temperature of the rolled or heat-treated hypereutectoid rail to 900°C is preferably 10°C/ s. The present invention can fully austenitize the hypereutectoid rail in the rolled state or heat treatment state by keeping the temperature above 900°C. In a specific embodiment of the present invention, the holding time above 900°C is preferably 900s.

After the austenitized steel rail is obtained, the present invention firstly cools the austenitized steel rail to an isothermal temperature at a first cooling rate for 30 to 50 s; Cool naturally to room temperature.

In the present invention, the first cooling rate is 8 to 10°C/s, preferably 8 to 9°C/s. In the present invention, the isothermal temperature is 600-630°C, preferably 600-610°C; the isothermal time is 30-50s, preferably 30-40s. The method is firstly cooled to 600-630°C at a speed of 8-10°C/s, and isothermal at this temperature for 30-50s, which can refine the pearlite lamella spacing and avoid the appearance of reticulated cementite.

In the present invention, the second cooling rate is 8 to 10°C/s, preferably 8 to 9°C/s. The present invention cools to a final cooling temperature below 400°C at a speed of 8-10°C/s, and the function of cooling to a temperature below this temperature is to avoid the growth of pearlite.

In the present invention, the first cooling rate and the second cooling rate are preferably the same, and the functions are to avoid the precipitation of reticulated cementite and refine the lamellar spacing of pearlite, thereby improving the mechanical properties of the hypereutectoid steel. In the present invention, the cooling process is preferably performed in a quenching cooling device, and the cooling medium used is preferably a water-mist mixture.

In a specific embodiment of the present invention, a schematic diagram of the heat treatment method is shown in FIG. 1 , and the detailed heat treatment process is shown in Embodiments 1-3.

In the hypereutectoid steel rail obtained by the heat treatment method of the present invention, the interlayer spacing of pearlite sheets is 110-134.5 nm, the thickness of cementite sheets is 17.6-25.8 nm, the percentage of cementite is 44.22-48.13%, and the grain boundary Precipitation of reticulated cementite was not evidently found. The hypereutectoid rail obtained by the invention has a tensile strength of 1312-1437 MPa, a yield strength of 800-911 MPa, an elongation rate of 12.24-16.80%, a reduction in area of 20.79-25.70%, and a hardness of 396-415HB.

The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some, but not all, embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

In terms of weight percentage, the rolled hypereutectoid rail or the heat-treated hypereutectoid rail is adopted in the examples, and the chemical composition is: C 0.94～1.00%, Si 0.45～0.80%, Mn 0.75～1.25%, P≤0.015 %, S≤0.010%, Cr+Nb+Ni 0.625%, the balance is iron.

Example 1

The as-rolled hypereutectoid rail (temperature is room temperature) is heated to 900°C, and kept for 900±30s until fully austenitized to obtain austenitized rail;

Cool the austenitized rail to 600±12°C at a rate of 7.8±0.5°C/s, isothermally for 30±4s; then continue to cool to 400±15°C at a rate of 7.8±0.5°C/s, and finally cool naturally to room temperature to obtain a hypereutectoid rail.

Example 2

The as-rolled hypereutectoid rail (at room temperature) is heated to 900°C, and kept for 900±25s until fully austenitized to obtain austenitized rail;

Cool the austenitized steel rail to 600±5°C at a rate of 9.6±0.3°C/s, isothermally for 30±2s; then continue to cool to 400±25°C at a rate of 9.6±0.3°C/s, and finally cool naturally to room temperature to obtain a hypereutectoid rail.

Example 3

The as-rolled hypereutectoid rail (temperature is room temperature) is heated to 900°C, and kept for 900±30s until fully austenitized to obtain austenitized rail;

Cool the austenitized rail to 630±16°C at a speed of 8.2±0.6°C/s, isothermally for 32±2s; then continue to cool to 400±6°C at a speed of 8.2±0.6°C/s, and finally cool naturally to room temperature to obtain a hypereutectoid rail.

Comparative Example 1

Take the as-rolled hypereutectoid rail as Comparative Example 1.

Comparative Example 2

Take the heat-treated hypereutectoid rail as Comparative Example 2.

Comparative Example 3

Heat the rolled hypereutectoid rail (at room temperature) to 900°C, and keep the temperature for 900±36s until fully austenitized to obtain austenitized rail;

Cool the austenitized rail to 630±18°C at a rate of 4.9±0.3°C/s, isothermally for 30±6s; then continue to cool to 400±2°C at a rate of 4.9±0.3°C/s, and finally cool naturally to room temperature to obtain a hypereutectoid rail.

Comparative Example 4

Heat the rolled hypereutectoid rail (at room temperature) to 900°C, and keep the temperature for 900±27s until fully austenitized to obtain austenitized rail;

Cool the austenitized rail to 630±16°C at a rate of 5±0.6°C/s, isothermally for 60±2s; then continue to cool to 400±18°C at a rate of 5±0.6°C/s, and finally cool naturally to room temperature to obtain a hypereutectoid rail.

Test Example 1

After the samples prepared in Examples 1 to 3 and Comparative Examples 1 to 4 were polished with sandpaper, they were corroded with 3% nitric acid, and then the microstructure was observed with a metallographic microscope. The results are shown in Figures 2-a to 2- g. Figure 2-a is Comparative Example 1, Figure 2-b is Comparative Example 2, Figure 2-c is Comparative Example 3, Figure 2-d is Comparative Example 4, Figure 2-e is Example 1, and Figure 2-f For Example 2, Figure 2-g is for Example 3. It can be seen from Figures 2-a to 2-g that the as-rolled structure of Comparative Example 1 is composed of pearlite structure with large lamellar spacing, and a clear lamellar structure can be seen under the microscope, and at the same time the grain boundary can be seen. The distribution of the reticulated cementite is shown by the arrow; although the pearlite structure of the hypereutectoid rail of Comparative Example 2 heat treated in the factory is refined compared with the rolled state, it is still coarse and can still be seen under the microscope. Clear lamellar spacing, and there is still a small amount of reticular cementite precipitation at the grain boundary; the pearlite lamellar spacing in Comparative Examples 3 and 4 is still relatively coarse, and pearlite lamellae can still be seen in some areas under the microscope Layer spacing. Compared with Comparative Examples 1-4, the pearlite lamellar spacing of Examples 1-3 is obviously refined, and the lamellar structure of pearlite can hardly be seen under the microscope. In addition, no obvious reticular infiltration can be observed at the grain boundary. Carbon precipitation.

The lamellar spacing of pearlite in the SEM photos of Comparative Examples 3-4 and Examples 1-3 was counted by the truncation method, and the average value was obtained, and the cementite content was measured by software. The results are shown in Table 1.

Table 1 Pearlite interlamellar spacing and cementite content

It can be seen from Table 1 that as the isothermal time increases from 30s to 60s, the phase transition time becomes longer, the spacing between pearlite lamellae increases, and the cementite content decreases. Under the premise of constant isothermal temperature and isothermal time, the increase of cooling rate will increase the degree of supercooling, the driving force of phase transition will increase, and the spacing of pearlite lamellae will decrease. However, when the cooling rate reaches a certain value, the interlayer spacing increases slightly. This is because the pearlite phase changes into a diffusion-type phase transition. Although the increase in the cooling rate increases the degree of supercooling, it reduces the diffusion rate of carbon atoms. , and the combined effect of the two finally resulted in a small increase in the spacing of pearlite lamellae.

Test case 2

According to GB/T228.1-2010 "Tensile Test of Metal Materials Part 1: Test Method at Room Temperature", the room temperature tensile test was carried out on the GNT300 electronic universal testing machine, and the tensile rate was 0.6mm/min. Test Example 1 The tensile strength, yield strength, elongation and area shrinkage of the samples prepared from ~3 and Comparative Examples 1 to 4; Test Method" to measure the Brinell hardness of each sample. The test results are shown in Table 2.

Table 2 Mechanical properties test results

It can be seen from Table 2 that the hardness of the hypereutectoid rail after heat treatment is higher than that of the rolled state and the heat treated state. The isothermal temperature is greatly improved. This is because the isothermal temperature is low, the pearlite lamellar spacing is more refined, and the percentage of hard phase cementite is also large; The mean range of hardness change of eutectoid rail is "broken line", with a peak at 8°C/s cooling rate, and the hardness at 10°C/s cooling rate is higher than that at 5°C/s cooling rate. This hardness change is similar to that of pearlite flakes. The changes in the interlayer spacing are consistent, indicating that the refined pearlite lamella spacing can effectively improve the hardness; by comparing the hardnesses corresponding to the two isothermal times in the 600°C isothermal, it can be seen that the longer the isothermal time, the lower the hardness of the hypereutectoid rail. , this is because the long isothermal time leads to the growth of pearlite, which increases the lamellar spacing; Example 1 reaches the maximum hardness of 415HB, which is 36.1% higher than that of the rolled state and 20.3% higher than that of the heat-treated state.

Test case 3

The microscopic SEM morphology of the tensile fractures of the samples prepared in Examples 1 to 3 and Comparative Examples 1 to 4 are shown in Figures 3-a to 3-g. Among them, Figure 3-a is the microscopic SEM morphology of the tensile fracture of Comparative Example 1, Figure 3-b is the microscopic SEM morphology of the tensile fracture of Comparative Example 2, and Figure 3-c is the tensile fracture of Comparative Example 3. Microscopic SEM topography, Figure 3-d is the microscopic SEM topography of the tensile fracture of Comparative Example 4, Figure 3-e is the microscopic SEM topography of the tensile fracture of Example 1, and Figure 3-f is Example 2 The microscopic SEM morphology of the tensile fracture, Figure 3-g is the microscopic SEM morphology of the tensile fracture in Example 3. Comparative examples 1 to 4 (Figs. 3-a, 3-b, 3-c, 3-d) are all composed of cleavage planes, river patterns, tearing edges, and some secondary cracks. This type of fracture is obvious. The cleavage fracture is a brittle fracture. However, in Example 3 (Fig. 3-g), the cleavage plane became significantly smaller, the tearing edge became shorter and the number increased, and a certain number of dimples were observed, but the size of the dimples was small and the depth was shallow, so the fracture The method belongs to quasi-cleavage fracture and still belongs to brittle fracture. Example 1 (Fig. 3-e) and Example 2 (Fig. 3-f) showed a large number of tear dimples on the micro-section, the surface was honeycomb, and compared with Example 3, the size and depth of the dimples were different. Increase, this kind of fracture belongs to ductile fracture. It can be seen from Figures 3-a to 3-g that the fracture in the rolling state is a brittle fracture, while the fracture in the heat treatment state is gradually transformed from a brittle fracture to a ductile fracture.

It can be seen from the above examples and comparative examples that the microstructure of the hypereutectoid rail after heat treatment has obvious changes compared with the rolled state. With the decrease of isothermal temperature and isothermal time, the spacing between pearlite lamellae gradually decreases; as the cooling rate increases, the spacing between pearlite lamellae first decreases greatly and then increases slightly; the mechanical properties of hypereutectoid rails after heat treatment The performance is improved compared with the rolled state and the heat treatment state; the lower the isothermal temperature, the higher the hardness and tensile strength; the shorter the isothermal time, the higher the hardness and tensile strength. The as-rolled fracture is a brittle fracture, while the heat-treated fracture is a ductile fracture. As the cooling rate increases, the isothermal temperature decreases, and the isothermal time decreases, the fracture type transitions from brittle fracture to ductile fracture. It is indicated that the heat treatment method provided by the present invention can effectively refine the lamellar spacing of pearlite, and at the same time avoid the appearance of reticulated cementite at the grain boundary, which is beneficial to improve the mechanical properties of the hypereutectoid rail.

The above are only the preferred embodiments of the present invention. It should be pointed out that for those skilled in the art, without departing from the principles of the present invention, several improvements and modifications can be made. It should be regarded as the protection scope of the present invention.

Claims (4)

1. A heat treatment method for reducing hypereutectoid steel rail reticular cementite precipitation comprises the following steps:

heating the rolled or heat-treated hypereutectoid steel rail to over 900 ℃, and preserving heat to obtain an austenitized steel rail;

cooling the austenitized steel rail to an isothermal temperature at a first cooling speed for 30-50 s; then cooling to the final cooling temperature below 400 ℃ at a second cooling speed; naturally cooling to room temperature; the isothermal temperature is 600-630 ℃; the first cooling speed and the second cooling speed are 8-10 ℃/s independently.

2. The thermal processing method of claim 1, wherein said first cooling rate and said second cooling rate are the same.

3. The heat treatment method according to claim 1, wherein the carbon content of the as-rolled or as-heat treated hypereutectoid steel rail is 0.94 to 1.00 wt.%.

4. A heat treatment method according to claim 1 or 3, characterized in that the chemical composition of said as-rolled or as-heat treated hypereutectoid steel rail comprises, in mass%: 0.94-1.00% of C, 0.45-0.80% of Si, 0.75-1.25% of Mn, less than or equal to 0.015% of P, less than or equal to 0.010% of S, 0.625% of Cr + Nb + Ni and the balance of iron.

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