Preparation method of medium-carbon low-alloy steel with high strength and heterogeneous structure

Abstract

A preparation method of a high-strength medium-carbon low-alloy steel with a heterogeneous structure, obtaining martensite and ferrite with uniform lamellar thickness through warm rolling deformation to explore the effect of stress strengthening induced by heterogeneous structure deformation in medium-carbon low-alloy steel. In the present invention, by adjusting the rolling temperature and then performing water quenching without subsequent tempering, the tensile strength of the medium-carbon low-alloy steel is significantly higher than that of the same material using a complete austenitization process. The obtained medium-carbon low-alloy steel has a uniform lamellar martensite and ferrite microstructure. While retaining most of the martensite structure, the stress induced by heterogeneous structure deformation generated during the deformation process according to its microstructural characteristics greatly improves the strength of the material. The present invention saves production costs, significantly improves the strength, greatly promotes industrial applications, and the test results in the tensile experiment confirm the key role of the stress induced by heterogeneous structure deformation in improving the strength of medium-carbon low-alloy steel.

Description

Technical Field

[0001] The present invention relates to the field of material preparation, and particularly to a method for preparing a high-strength heterogeneous structure medium-carbon low-alloy steel. Background Art

[0002] Medium-carbon low-alloy steels are widely used in manufacturing high-strength, large cross-sectional size, and complex load-bearing or transmission components in heavy machinery, such as steam turbine shafts, blades, transmission shafts, and harmonic reducer flexs. Usually, these components serve in harsh environments with extreme conditions such as high pressure and alternating loads. Excellent comprehensive mechanical properties are the fundamental guarantee for service safety. For medium-carbon low-alloy steels, different microstructures such as tempered martensite, tempered troostite, and tempered sorbite can be obtained by adjusting the tempering process. However, simply adjusting the tempering temperature or tempering time cannot significantly improve the strength. Therefore, how to design a reasonable microstructure through the introduction of new processes to obtain excellent high-strength properties has always been the goal pursued by researchers in related fields.

[0003] Recently, a large number of studies have shown that the construction of heterogeneous structure microstructures can significantly improve the strength of metal materials. Heterogeneous structure microstructures include gradient structures, non-uniform lamellar structures, harmonic structures, dual-phase steels, nanostructures, etc. Heterogeneous structure materials can be defined as materials with significant inhomogeneities from one region to another. This inhomogeneity can be in the microstructure, crystal structure, or composition, with the grain size varying in the range of micrometers to millimeters, and the geometric structure of the grains can also change, forming very different material systems. The reason why these heterogeneous structure materials can improve the material strength is that hard-soft interfaces are introduced into the material, inducing heterogeneous structure deformation-induced stress strengthening during the material deformation process.

[0004] In the prior art, annealing is usually carried out after rolling. The article "Enhanced mechanical properties of dual phase steel via cross rolling and intercritical annealing" published by Soleimani et al. in 《Materials Science and Engineering: A》(Materials Science and Engineering A, 2021.140778) describes the effects of rolling and intercritical annealing on the microstructure and properties of dual-phase steel. Although a good combination of strength and plasticity is achieved, its tensile strength is only 470 MPa, which cannot meet the application requirements of industrial fields with higher strength requirements. So far, rolling deformation has mainly been applied to low-carbon steels, dual-phase steels, etc. with a tensile strength below 1000 MPa. For medium-carbon alloy steels, conventional heat treatment processes are adjusted to improve their comprehensive mechanical properties, but few people study their deformation in the two-phase region. Summary of the Invention

[0005] To overcome the deficiency of low strength in medium-carbon low-alloy steel in the existing technology, the present invention proposes a method for preparing a high-strength heterogeneous structure medium-carbon low-alloy steel.

[0006] The specific process of the present invention is as follows:

[0007] Step 1, normalizing treatment:

[0008] Heat the heating furnace to 800°C - 850°C at a heating rate of 10°C / min. Put the medium-carbon low-alloy steel into the furnace and keep it warm for 1 h to fully austenitize it.

[0009] After the heat preservation is completed, air-cool it to obtain a medium-carbon low-alloy steel with a homogenized pearlite-like structure.

[0010] Step 2, rolling treatment:

[0011] Put the obtained medium-carbon low-alloy steel plate into a box-type resistance furnace at 750 - 770°C and keep it warm for 30 min. Perform room-temperature synchronous rolling on the medium-carbon low-alloy steel plate after heat preservation. The reduction per pass is 10%; the final reduction is 20 - 80%. After each pass of rolling is completed, transfer the medium-carbon low-alloy steel plate to the box-type resistance furnace and keep it warm for 3 min to compensate for the temperature drop. The transfer process of the medium-carbon low-alloy steel plate to the box-type resistance furnace is within 10 s. After rolling is completed, water-cool the alloy steel plate to room temperature to complete the rolling treatment of the medium-carbon low-alloy steel plate; no subsequent tempering treatment is carried out. The roll diameter of the small two-high rolling mill for synchronous rolling is 120 mm, and the roll length is 250 mm. The rotational speeds of the upper and lower rolls of this small two-high rolling mill are both 10 r / min.

[0012] The preparation of the medium-carbon low-alloy steel plate is completed.

[0013] The tensile strength of the medium-carbon low-alloy steel plate is 1850 - 2600 MPa, the yield strength is 1470 - 2010 MPa, and the total elongation is 3.2 - 8.0%.

[0014] The present invention obtains martensite and ferrite with uniform lamellar thickness through warm rolling deformation to explore the effect of heterogeneous structure deformation-induced stress strengthening in medium-carbon low-alloy steel.

[0015] The present invention adjusts the rolling temperature, then performs water-cooling quenching, and does not perform subsequent tempering, so that the tensile strength of the medium-carbon low-alloy steel greatly exceeds the strength of the same material using a complete austenitization process.

[0016] The medium carbon low alloy steel treated by the present invention has uniform lamellar martensite and ferrite microstructures, and greatly improves the strength of the material due to the heterogeneous structural deformation induced stress generated during the deformation process according to its microscopic characteristics while retaining most of the martensite structure.

[0017] Compared with the prior art, the present invention has the following significant advantages:

[0018] 1. Compared with conventional hot rolling, the present invention adopts rolling at a lower temperature, and no subsequent tempering heat treatment is performed after rolling and quenching, which has the effect of reducing energy consumption and saving production costs.

[0019] 2. The medium carbon low alloy steel obtained by rolling and quenching of the present invention has a tensile strength of 2600MPa compared with the performance of standard technical treatment. However, only conventional quenching heat treatment not only has no strengthening effect, but makes its strength lower and has almost no plasticity. The reason is that the great heterogeneity of the microstructure makes the deformation of ferrite and martensite at room temperature uncoordinated, and martensite cannot withstand sufficient deformation. In the rolling and quenching process of the present invention, ferrite and martensite are jointly subjected to a large deformation amount. On the one hand, the ferrite and martensite structures are refined, and on the other hand, the ferrite strength is also improved. Finally, in the tensile deformation process, due to the difference between the ferrite and martensite structures, heterogeneous structure deformation-induced stress strengthening will occur. The above factors work together to significantly improve the strength, which can greatly promote industrial applications.

[0020] 3. The heterogeneous structure generated by the present invention has been tested for the deformation induced stress of the heterogeneous structure in the tensile experiment, which shows that the deformation induced stress of the heterogeneous structure has a great contribution to the entire deformation stage. The results further confirm the key role of the deformation induced stress of the heterogeneous structure in improving the strength of medium-carbon low-alloy steel.

[0021] Compared with the performance of the medium carbon low alloy steel treated by the standard technology, the strength of the medium carbon low alloy steel treated by the present invention is greatly improved, which can greatly promote industrial application.

[0022] Table 1 Comparison of mechanical properties of medium carbon low alloy steel obtained by the present invention and that after treatment by standard technology

[0023] Technical requirements Tensile strength (MPa) Yield strength (MPa) Total elongation (%) Standard requirements ≥980 ≥835 ≥10 The present invention 1850-2600 1470-2010 3.2-8.0 BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a cyclic loading and unloading test diagram of medium carbon low alloy steel after normalizing at 800℃ for 1h, keeping at 750℃ for 30min, and 20% rolling and water cooling.

[0025] Figure 2It is a cyclic loading and unloading test diagram of medium-carbon low-alloy steel after normalizing at 800°C for 1 h, holding at 760°C for 30 min, and water cooling after 40% rolling.

[0026] Figure 3 It is a cyclic loading and unloading test diagram of medium-carbon low-alloy steel after normalizing at 850°C for 1 h, holding at 770°C for 30 min, and water cooling after 60% rolling.

[0027] Figure 4 It is a cyclic loading and unloading test diagram of medium-carbon low-alloy steel after normalizing at 850°C for 1 h, holding at 770°C for 30 min, and water cooling after 80% rolling.

[0028] Figure 5 It is a scanning electron microscope image of medium-carbon low-alloy steel after normalizing at 850°C for 1 h, holding at 770°C for 30 min, and water cooling after 80% rolling.

[0029] Figure 6 In the prior art, it is a cyclic loading and unloading test diagram of stainless steel after 70% rolling and annealing at 1000°C for 2 min by Xun Xiaowei et al.

[0030] Figure 7 It is a flow chart of the present invention. Detailed implementation mode

[0031] Example 1

[0032] This example is a method for preparing heterogeneous structure medium-carbon low-alloy steel by using a box-type resistance furnace for heat treatment, and it is prepared by using a box-type resistance furnace and a Φ400*350 rolling mill. No protective atmosphere is passed during the holding stage in the preparation.

[0033] In this example, the thickness of the medium-carbon low-alloy steel plate used is 5 mm, and its composition is 0.4 wt% C, 0.24 wt% Si, 0.628 wt% Mn, 0.0133 wt% P, 0.0027 wt% S, 1.34 wt% Ni, 0.869 wt% Cr, 0.182 wt% Mo, and the balance is Fe and unavoidable impurities.

[0034] The specific process is as follows:

[0035] Step 1, normalizing treatment:

[0036] Heat the heating furnace to 800°C at a heating rate of 10°C / min. Put the medium-carbon low-alloy steel into the furnace and hold for 1 h to fully austenitize it.

[0037] After the holding is completed, air-cool to obtain medium-carbon low-alloy steel with a homogenized pearlite-like structure.

[0038] Step 2, rolling treatment:

[0039] Put the obtained medium-carbon low-alloy steel plate into a box-type resistance furnace at 750 °C and keep it warm for 30 min. Then, subject the heat-insulated medium-carbon low-alloy steel plate to room-temperature synchronous rolling using a small two-high rolling mill with a roll diameter of 120 mm and a roll length of 250 mm; the rotational speeds of both the upper and lower pressing rolls in this small two-high rolling mill are 10 revolutions per minute, and the reduction rate per single-pass rolling is 10%. Immediately after the end of each pass of rolling, put the medium-carbon low-alloy steel plate into the box-type resistance furnace to keep it warm for 3 min to compensate for the temperature drop, and the rolling transfer process is within 10 s. The final reduction rate of this experiment is 20%, and a total of 2 passes of rolling are carried out, and the final thickness of the alloy steel plate is 4 mm. After the rolling is completed, water-cool the alloy steel plate to room temperature to complete the rolling treatment of the medium-carbon low-alloy steel plate, and no subsequent tempering treatment is carried out.

[0040] Table 2 Comparison of mechanical properties of the medium-carbon low-alloy steel obtained in this example and after standard technical treatment

[0041] Technical requirements Tensile strength (MPa) Yield strength (MPa) Total elongation (%) Standard requirements ≥980 ≥835 ≥10 This embodiment 1850 1470 3.2

[0042] The cyclic loading and unloading test diagram of the medium-carbon low-alloy steel obtained by the above method in this example. As Figure 1 shown, the dotted line represents the true stress-true strain curve of the medium-carbon low-alloy steel. It can be seen that the tensile strength is 2300 MPa, the yield strength is 1650 MPa, and the elongation is 3.2%. The other line is the cyclic loading and unloading curve. The larger the area surrounded by the loading and unloading closed loop, the greater the proportion of deformation-induced strengthening of the heterogeneous structure, and the greater the contribution value to the strength of the medium-carbon low-alloy steel. For the experimental steel processed by this example, through measurement, the proportion of ferrite is 27%, and the maximum ratio of deformation-induced strengthening of the heterogeneous structure during the deformation process can reach 47%. This shows that the deformation-induced strengthening of the heterogeneous structure plays a role in enhancing the material strength.

[0043] Appendix Figure 6 is the cyclic loading and unloading test diagram after rolling the stainless steel by 70% and annealing at 1000 °C for 2 min in a preparation method of a heterogeneous laminated structure duplex stainless steel proposed by Xun Xiaowei et al. in CN201910412901.4. It can be seen from the figure that the tensile strength reaches 1245 MPa and the total elongation is 12.5%, which is far lower than the mechanical properties in this example. This is because there is no martensite in the structure after high-temperature annealing after rolling, so the strength is much lower than that of this example. In this example, through rolling and quenching, on the one hand, the ferrite in the structure is refined, and on the other hand, the strength of the ferrite is also improved. Finally, during the tensile deformation process, due to the difference between the ferrite and martensite tissues, deformation-induced stress strengthening of the heterogeneous structure will occur. The above factors work together, resulting in a significant increase in the strength of the medium-carbon low-alloy steel, which can greatly promote industrial applications.

[0044] Example 2

[0045] This embodiment is a method for preparing medium-carbon low-alloy steel with a heterogeneous structure using a box-type resistance furnace for heat treatment.

[0046] First of all, this embodiment uses a box-type resistance furnace and a Φ400*350 rolling mill, and no protective atmosphere is passed during the heat preservation stage.

[0047] In this embodiment, the thickness of the medium-carbon low-alloy steel plate used is 5 mm, and its components are 0.4 wt% C, 0.24 wt% Si, 0.628 wt% Mn, 0.0133 wt% P, 0.0027 wt% S, 1.34 wt% Ni, 0.869 wt% Cr, 0.182 wt% Mo, and the balance is Fe and unavoidable impurities.

[0048] The specific process is as follows:

[0049] Step 1, normalizing treatment:

[0050] Heat the heating furnace to 800 °C at a heating rate of 10 °C / min. Put the medium-carbon low-alloy steel into the furnace and keep it warm for 1 h to fully austenitize it.

[0051] After the heat preservation is over, air-cool it to obtain medium-carbon low-alloy steel with a homogenized pearlite-like structure.

[0052] Step 2, rolling treatment:

[0053] Put the obtained medium-carbon low-alloy steel plate into a box-type resistance furnace at 760 °C and keep it warm for 30 min. Then, subject the heat-preserved medium-carbon low-alloy steel plate to room-temperature synchronous rolling through a small two-high rolling mill with a roll diameter of 120 mm and a roll width of 250 mm; the rotational speeds of the upper and lower pressure rolls in this small two-high rolling mill are both 10 revolutions per minute, and the reduction rate per single pass is 10%. Immediately after each pass of rolling, put the medium-carbon low-alloy steel plate into the box-type resistance furnace and keep it warm for 3 min to compensate for the temperature drop, and the rolling transfer process is within 10 s. The final reduction rate of this experiment is 40%, and a total of 4 passes are rolled, and the final thickness of the alloy steel plate is 3 mm. After the rolling is over, water-cool the alloy steel plate to room temperature to complete the rolling treatment of the medium-carbon low-alloy steel plate, and no subsequent tempering treatment is carried out.

[0054] Table 3 Comparison of the mechanical properties of the medium-carbon low-alloy steel obtained in this embodiment and after standard technical treatment

[0055] Technical requirements Tensile strength (MPa) Yield strength (MPa) Total elongation (%) Standard requirements ≥980 ≥835 ≥10 This embodiment 2080 1680 4.2

[0056] Cyclic loading and unloading test diagram of the medium-carbon low-alloy steel obtained in this embodiment through the above method. As Figure 2As shown, the dotted line represents the true stress-true strain curve of medium-carbon low-alloy steel. It can be seen that the tensile strength is 2380 MPa, the yield strength is 1680 MPa, and the elongation is 4.2%. Another line is the cyclic loading-unloading curve. The larger the area surrounded by the loading-unloading closed loop, the greater the proportion of deformation-induced strengthening of the heterogeneous structure and the greater the contribution value to the strength of medium-carbon low-alloy steel. For the experimental steel processed in this example, through measurement, the proportion of ferrite is 15%, and the maximum ratio of deformation-induced strengthening of the heterogeneous structure during the deformation process can reach 43.4%. This shows that the deformation-induced strengthening of the heterogeneous structure plays a role in enhancing the material strength.

[0057] Appendix Figure 6 This is the cyclic loading-unloading test diagram of stainless steel rolled by Xun Xiaowei et al. by 70% and then annealed at 1000°C for 2 minutes. It can be seen from the figure that the tensile strength reaches 1245 MPa and the total elongation is 12.5%, which is far lower than the mechanical properties in this example. This is because after high-temperature annealing after rolling, there is no martensite in the structure, so the strength is far lower than that in this example. In this example, the rolling reduction rate and the quenching temperature are increased. On the one hand, the ferrite in the structure is refined, improving the strength of the ferrite. On the other hand, the increase in the quenching temperature results in more martensite structure. Finally, during the tensile deformation process, due to the difference between the ferrite and martensite structures, deformation-induced stress strengthening of the heterogeneous structure will occur. The above factors act together, resulting in a significant increase in the strength of medium-carbon low-alloy steel, which can greatly promote industrial applications.

[0058] Example 3

[0059] This example is a method for preparing heterogeneous structure medium-carbon low-alloy steel using a box-type resistance furnace for heat treatment.

[0060] First, this example uses a box-type resistance furnace and a Φ400*350 rolling mill, and no protective atmosphere is passed during the heat preservation stage.

[0061] In this example, the thickness of the medium-carbon low-alloy steel plate used is 5 mm, and its composition is 0.4 wt% C, 0.24 wt% Si, 0.628 wt% Mn, 0.0133 wt% P, 0.0027 wt% S, 1.34 wt% Ni, 0.869 wt% Cr, 0.182 wt% Mo, and the balance is Fe and unavoidable impurities.

[0062] The specific process is as follows:

[0063] Step 1, normalizing treatment:

[0064] Heat the heating furnace to 850°C at a heating rate of 10°C / min. Place the medium-carbon low-alloy steel in the furnace and hold for 1 h to fully austenitize it. After the holding is completed, air cool to obtain a medium-carbon low-alloy steel with a homogenized pearlite-like structure.

[0065] Step 2, rolling treatment:

[0066] Place the obtained medium-carbon low-alloy steel plate in a box-type resistance furnace at 770°C and hold for 30 min. Then, subject the heat-insulated medium-carbon low-alloy steel plate to synchronous rolling at room temperature using a small two-high rolling mill with a roll diameter of 120 mm and a roll length of 250 mm; the rotational speeds of the upper and lower work rolls in this small two-high rolling mill are both 10 revolutions per minute, and the reduction ratio per single pass is 10%. Immediately after each pass of rolling, place the medium-carbon low-alloy steel plate in the box-type resistance furnace and hold for 3 min to compensate for the temperature drop, and the rolling transfer process is within 10 s. The final reduction ratio in this experiment is 60%, and a total of 6 passes of rolling are performed, with the final thickness of the alloy steel plate being 2 mm. After rolling is completed, water cool the alloy steel plate to room temperature to complete the rolling treatment of the medium-carbon low-alloy steel plate, and no subsequent tempering treatment is carried out.

[0067] Table 4 Comparison of mechanical properties of the medium-carbon low-alloy steel obtained in this example and after standard technical treatment

[0068] Technical requirements Tensile strength (MPa) Yield strength (MPa) Total elongation (%) Standard requirements ≥980 ≥835 ≥10 This embodiment 2400 1690 8.0

[0069] Cyclic loading and unloading test diagram of the medium-carbon low-alloy steel obtained in this example by the above method. As Figure 3 shown, the dashed line represents the true stress-true strain curve of the medium-carbon low-alloy steel. It can be seen that the tensile strength is 2400 MPa, the yield strength is 1690 MPa, and the elongation is 5.7%. The other line is the cyclic loading and unloading curve. The larger the area surrounded by the loading and unloading closed loop, the greater the proportion of deformation-induced strengthening of the heterogeneous structure, and the greater the contribution value to the strength of the medium-carbon low-alloy steel. For the experimental steel treated in this example, through measurement, the proportion of ferrite is 10%, and the maximum ratio of deformation-induced strengthening of the heterogeneous structure during the deformation process can reach 41%. This shows that the deformation-induced strengthening of the heterogeneous structure plays a role in enhancing the strength of the material.

[0070] Appendix Figure 6This is the cyclic loading and unloading test diagram of stainless steel after 70% rolling and annealing at 1000°C for 2 minutes by Xun Xiaowei et al. It can be seen from the figure that the tensile strength reaches 1245 MPa and the total elongation is 12.5%, which is much lower than the mechanical properties in this embodiment. This is because after high-temperature annealing after rolling, there is no martensite in the structure, so the strength is much lower than that in this embodiment. In this embodiment, by increasing the rolling reduction rate and the quenching temperature, on the one hand, the ferrite in the structure is refined, improving the strength of the ferrite, and on the other hand, the increase in the quenching temperature results in more martensite structure. Finally, during the tensile deformation process, due to the difference between the ferrite and martensite structures, heterogeneous structure deformation-induced stress strengthening occurs. The combined action of the above factors leads to a significant increase in strength, which can greatly promote industrial applications.

[0071] Example 4

[0072] This embodiment is a method for preparing a heterogeneous structure medium-carbon low-alloy steel using a box-type resistance furnace for heat treatment.

[0073] First of all, this embodiment uses a box-type resistance furnace and a Φ400*350 rolling mill, and no protective atmosphere is passed during the heat preservation stage.

[0074] In this embodiment, the medium-carbon low-alloy steel plate used has a thickness of 5 mm, and its composition is 0.4 wt% C, 0.24 wt% Si, 0.628 wt% Mn, 0.0133 wt% P, 0.0027 wt% S, 1.34 wt% Ni, 0.869 wt% Cr, 0.182 wt% Mo, and the balance is Fe and inevitable impurities.

[0075] The specific process is as follows:

[0076] Step 1, normalizing treatment:

[0077] The heating furnace is heated to 850°C at a heating rate of 10°C / min. The medium-carbon low-alloy steel is placed in the furnace and kept warm for 1 h to fully austenitize it. After the heat preservation is completed, it is air-cooled to obtain a medium-carbon low-alloy steel with a homogenized pearlite-like structure.

[0078] Step 2, rolling treatment:

[0079] Put the obtained medium-carbon low-alloy steel plate into a box-type resistance furnace at 770 °C and keep it warm for 30 min. Then, perform room-temperature synchronous rolling on the heat-insulated medium-carbon low-alloy steel plate using a small two-high rolling mill with a roll diameter of 120 mm and a roll length of 250 mm; the rotational speeds of the upper and lower working rolls in this small two-high rolling mill are both 10 revolutions per minute, and the reduction rate per single-pass rolling is 10%. Immediately after the end of each pass of rolling, put the medium-carbon low-alloy steel plate into the box-type resistance furnace to keep it warm for 3 min to compensate for the temperature drop, and the rolling transfer process is within 10 s. The final reduction rate of this experiment is 80%, and a total of 8 passes of rolling are performed, and the final thickness of the alloy steel plate is 1 mm. After the rolling is completed, cool the alloy steel plate to room temperature in water to complete the rolling treatment of the medium-carbon low-alloy steel plate, and no subsequent tempering treatment is carried out.

[0080] Table 5 Comparison of mechanical properties between the medium-carbon low-alloy steel obtained in this example and that after standard technical treatment

[0081] Technical requirements Tensile strength (MPa) Yield strength (MPa) Total elongation (%) Standard requirements ≥980 ≥835 ≥10 This embodiment 2600 2010 5.6

[0082] Cyclic loading and unloading test diagram of the medium-carbon low-alloy steel obtained by the above method in this example. As Figure 4 shown, the dotted line represents the true stress-true strain curve of the medium-carbon low-alloy steel. It can be seen that the tensile strength is 2600 MPa, the yield strength is 2010 MPa, and the elongation is 3.6%. The other line is the cyclic loading and unloading curve. The larger the area surrounded by the loading and unloading closed loop, the greater the proportion of deformation-induced strengthening of the heterogeneous structure, and the greater the contribution value to the strength of the medium-carbon low-alloy steel. For the experimental steel processed by this example, through measurement, the lamellar thickness of ferrite and martensite is about 190 nm, which indicates that the deformation-induced strengthening of the heterogeneous structure plays a role in enhancing the material strength, as Figure 5 shown.

[0083] Appendix Figure 6 is the cyclic loading and unloading test diagram of stainless steel rolled by Xiaowei Xun et al. by 70% and then annealed at 1000 °C for 2 min. It can be seen from the figure that the tensile strength reaches 1245 MPa and the total elongation is 12.5%, which is far lower than the mechanical properties in this example. This is because there is no martensite in the structure after high-temperature annealing after rolling, so the strength is much lower than that in this example. In this example, by increasing the rolling reduction rate, the ferrite in the structure is refined, and the strength of the ferrite is improved. Finally, during the tensile deformation process, due to the difference between the ferrite and martensite structures, deformation-induced stress strengthening of the heterogeneous structure will occur. The combined action of the above factors results in a significant increase in strength, which can greatly promote industrial applications.

[0084] Compared with the properties of the medium-carbon low-alloy steel processed by the method of the present invention and those of the standard technology, the strength is greatly improved, which can greatly promote industrial applications.

[0085] Table 6 Process parameters of each embodiment

[0086]

Claims

1. A preparation method of a high-strength medium-carbon low-alloy steel in a heterogeneous structure, characterized in that, The specific process is as follows: Step 1, normalizing treatment: Heat the heating furnace at a heating rate of 10 °C / min to 800 °C - 850 °C; put the medium-carbon low-alloy steel into the furnace and keep it warm for 1 h to fully austenitize it; After the heat preservation, air cool it to obtain a medium-carbon low-alloy steel with a homogenized pearlite-like structure; Step 2, rolling treatment: The rolling treatment process is 2 - 8 passes of rolling; Put the obtained medium-carbon low-alloy steel plate into a box-type resistance furnace at 750 - 770 °C and keep it warm for 30 min; perform room-temperature synchronous rolling on the medium-carbon low-alloy steel plate after heat preservation; the reduction rate per pass is 10%; the final reduction rate is 20 - 80%; after each pass of rolling, transfer the medium-carbon low-alloy steel plate to the box-type resistance furnace to keep it warm for 3 min to compensate for the temperature drop; after rolling, water cool the alloy steel plate to room temperature to complete the rolling treatment of the medium-carbon low-alloy steel plate; no subsequent tempering treatment is carried out.

2. The preparation method of the high-strength heterogeneous structure medium-carbon low-alloy steel according to claim 1, characterized in that, The roll diameter of the small two-high rolling mill for implementing the synchronous rolling is 120 mm and the roll length is 250 mm.

3. The preparation method of the high-strength heterogeneous structure medium-carbon low-alloy steel according to claim 2, wherein The rotational speeds of both the upper roll and the lower roll of the small two-high rolling mill are 10 r / min.

4. The preparation method of the high-strength heterogeneous structure medium-carbon low-alloy steel according to claim 1, characterized in that, The transfer process of transferring the medium-carbon low-alloy steel plate to the box-type resistance furnace is within 10 s.

5. The preparation method of the high-strength heterogeneous structure medium-carbon low-alloy steel according to claim 1, characterized in that, The tensile strength of the medium-carbon low-alloy steel plate is 1850 - 2600 MPa, the yield strength is 1470 - 2010 MPa, and the total elongation is 3.2 - 8.0%.

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