A heat treatment method for improving the low-temperature impact toughness of steel

Abstract

The present invention provides a heat treatment method for improving the low-temperature impact toughness of steel, comprising the following steps: S1. Preheating treatment: heating the steel to a temperature range of 840-880 °C, holding for 2-3 h, and then cooling in oil to room temperature; then heating to 560 °C, holding for 8-10 h, and then cooling in oil to room temperature; S2. Final heat treatment: heating the steel in the furnace to 840 °C, holding for 0.5-1 h, then isothermally treating in the temperature range of Ac1-Ac3 while cooling in the furnace, continuing to hold for 0.5-1 h, and then water-cooling to room temperature; finally heating to 580-600 °C, holding for 2 h, and then air-cooling to room temperature. Through the combined action of preheating treatment and final heat treatment, it helps the steel absorb more energy during the impact process, thereby improving the low-temperature impact toughness of the steel.

Description

Technical Field

[0001] The present invention relates to the technical field of marine shafting steel, and particularly relates to a heat treatment method for improving the low-temperature impact toughness of steel. Background Art

[0002] With the gradual depletion of conventional exploitable energy sources, the development and utilization of marine resources has become an inevitable trend in future energy utilization. Polar and deep-sea regions are rich in energy reserves and have received increasing attention. At the same time, with the gradual opening of the Arctic shipping route, the exploitation of marine resources has gradually developed from shallow seas to deep-sea regions and polar alpine regions, which will greatly promote the demand and development of high-ice-class icebreaking ships. The natural conditions in the polar regions are harsh, with challenges such as low-temperature tests, sea ice obstacles, iceberg attacks, blizzard attacks, fragile ecological environments, polar night disturbances, and poor visibility, and higher requirements for the performance of related equipment and materials. The ultra-low temperature environment in the polar regions poses demands for key technologies such as low-temperature resistance, corrosion resistance, wear resistance, weldability, and high toughness of materials.

[0003] Therefore, special low-temperature steel is usually used for high-ice-class shafting steel, and it must have comprehensive properties such as sufficient low-temperature toughness, strength, and fatigue strength. Improving the low-temperature toughness of shafting materials is the key point for the mechanical properties of marine shafting materials to meet the standards. Therefore, a heat treatment method for improving the low-temperature toughness of shafting materials is needed. Summary of the Invention

[0004] The main purpose of the present invention is to provide a heat treatment method for improving the low-temperature toughness of steel. By using this method, the low-temperature toughness of steel, especially shafting steel, can be greatly improved to meet the mechanical property requirements of ultra-low icebreaking ship shafting steel.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0006] A heat treatment method for improving the low-temperature impact toughness of steel, comprising the following steps:

[0007] S1. Preheating treatment: Heat the steel to a temperature range of 840 - 880 °C, hold for 2 - 3 h, and then cool it in oil to room temperature; then heat it to 560 °C, hold for 8 - 10 h, and then cool it in oil to room temperature;

[0008] S2. Final heat treatment: Heat the steel in the furnace to 840 °C, hold for 0.5 - 1 h, then isothermally treat it at a temperature within the Ac1 - Ac3 range while cooling in the furnace, continue to hold for 0.5 - 1 h, and then cool it in water to room temperature; finally, heat it to 580 - 600 °C, hold for 2 h, and then air-cool it to room temperature.

[0009] The present invention creatively proposes an isothermal quenching process of preheating treatment plus final heat treatment. During the isothermal process, the grains are refined and part of the α-phase is retained. High-temperature tempering transforms the retained austenite in the steel into martensite, and finally a room-temperature structure mainly composed of the α-phase is obtained. Multiple carbides pin the displacement of grain boundaries, and the interaction between high dislocation density and grain boundaries effectively hinders the movement of dislocations. Through the above combined action, it is helpful to absorb more energy during the impact process, thereby improving the impact toughness of the steel.

[0010] Further, the isothermal treatment temperature in the step S2 is 730 - 770 °C. During the final heat treatment process, isothermal treatment is carried out at a temperature slightly higher than medium temperature within the Ac1 - Ac3 range, that is, 730 - 770 °C.

[0011] Further, the steel is a steel for shafting, and its chemical composition in mass percentage includes: C 0.3 - 0.4%, Si 0.17 - 0.37%, Mn 0.5 - 0.8%, Cr 1.0 - 1.7%, Ni 1.2 - 1.6%, Mo 0.2 - 0.5%, Cu ≤ 0.25%, S ≤ 0.025%, P ≤ 0.025%, and the rest are iron elements and inevitable impurities.

[0012] Further, the steel is a steel for shafting, and its chemical composition in mass percentage includes: C 0.36%, Si 0.27%, Mn 0.61%, Cr 1.63%, Ni 1.59%, Mo 0.25%, Cu 0.10%, S ≤ 0.025%, P ≤ 0.025%, and the rest are iron elements and inevitable impurities.

[0013] The alloy element ratio of the steel is reasonably selected to improve the hardenability and tempering stability, and at the same time improve the strength and toughness of the steel. Combined with the heat treatment process described in the present invention, the best low-temperature impact toughness can be obtained.

[0014] The heat treatment method described in the present invention has at least the following beneficial effects compared with the prior art:

[0015] 1. Compared with other high-toughness shafting materials, the Ni content with moderate composition in the present invention improves the low-temperature impact toughness while reducing the production cost of the steel on the premise of ensuring the yield strength and fatigue strength.

[0016] 2. The heat treatment process proposed in the present invention includes two steps of preheating treatment and final heat treatment, both of which are indispensable. Through the synergistic effect of preheating treatment and final heat treatment, a composite structure of martensite and ferrite is obtained, effectively inhibiting the formation and propagation of microcracks, and thus improving the low-temperature impact toughness, with higher comprehensive performance. Description of the Drawings

[0017] To more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the accompanying drawings required for the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other relevant drawings can also be obtained based on these drawings.

[0018] Figure 1 It is the tissue morphology photo (left) of the sample in Example 1 under a scanning electron microscope and the impact fracture surface photo at -40°C (right);

[0019] Figure 2 It is the tissue morphology photo (left) of the sample in Example 2 under a scanning electron microscope and the impact fracture surface photo at -40°C (right);

[0020] Figure 3 It is the tissue morphology photo (left) of the sample in Example 3 under a scanning electron microscope and the impact fracture surface photo at -40°C (right);

[0021] Figure 4 It is the tissue morphology photo (left) of the sample in Comparative Example 1 under a scanning electron microscope and the impact fracture surface photo at -40°C (right);

[0022] Figure 5 It is the optical microscope tissue photos of the samples in Example 1, 2, 3 and Comparative Example 1. Among them, a: the sample of Example 1; b: the sample of Example 2; c: the sample of Example 3; d: the sample of Comparative Example 1;

[0023] Figure 6 It is the IPF and KAM diagrams of Example 2 and Comparative Example 1. Among them, (a) and (b) are the IPF and KAM diagrams of Comparative Example 1 respectively, and (c) and (d) are the IPF and KAM diagrams of Example 2 respectively.

[0024] The realization of the purpose of the present application, functional characteristics and advantages will be further described in conjunction with the embodiments with reference to the accompanying drawings. Detailed implementation manners

[0025] The following will clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0026] In addition, the technical solutions between various embodiments can be combined with each other, but it must be based on the ability of those of ordinary skill in the art to implement. When the combination of technical solutions results in contradictions or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0027] The present invention provides a heat treatment method for improving the low-temperature impact toughness of steel, comprising the following steps:

[0028] S1. Preheating treatment: Gradually heat the steel to a temperature range of 840 - 880 °C, hold for 2 - 3 h, and then cool it in oil to room temperature; then gradually heat it to 560 °C, hold for 8 - 10 h, and then cool it in oil to room temperature;

[0029] S2. Final heat treatment: Rapidly heat the steel in the furnace to 840 °C, hold for 0.5 - 1 h, and then slowly cool it in the furnace to a temperature within the Ac1 - Ac3 range for isothermal treatment. Preferably, slowly cool it in the furnace to 730 - 770 °C for isothermal treatment, continue to hold for 0.5 - 1 h, and then cool it in water to room temperature; finally, heat it to 580 - 600 °C, hold for 2 h, and then air-cool it to room temperature.

[0030] In some embodiments, the steel is steel for shaft systems, and its chemical composition in mass percentage includes: C 0.3 - 0.4%, Si 0.17 - 0.37%, Mn 0.5 - 0.8%, Cr 1.0 - 1.7%, Ni 1.2 - 1.6%, Mo 0.2 - 0.5%, Cu ≤ 0.25%, S ≤ 0.025%, P ≤ 0.025%, and the rest are iron elements and inevitable impurities.

[0031] In some embodiments, the steel is steel for shaft systems, and its chemical composition in mass percentage includes: C 0.36%, Si 0.27%, Mn 0.61%, Cr 1.63%, Ni 1.59%, Mo 0.25%, Cu 0.10%, S ≤ 0.025%, P ≤ 0.025%, and the rest are iron elements and inevitable impurities.

[0032] In the present invention, the elemental composition of the steel is reasonably selected. Among them, a suitable C content is selected to balance the low-temperature toughness while increasing the tensile strength and increasing the cold brittleness of the steel; a suitable Si content is selected to significantly improve the tensile strength and yield strength of the steel, and combine with Mo and Cr elements to improve the oxidation resistance of the steel; a suitable Mn content is selected, which can dissolve into ferrite, improve the strength of the steel, play a role in solution strengthening, and can improve the hardenability, and is also beneficial to desulfurization of the steel; a suitable Cr content is selected to improve the hardenability and tempering stability, and at the same time can improve the strength, hardness and wear resistance of the steel; a suitable Ni content is selected to improve the plasticity and toughness of the steel, and at the same time improve the strength of the steel; a suitable Mo content is selected to improve the hardenability and tempering stability, effectively inhibit the segregation of harmful elements in the steel, refine the grains, and is beneficial to alleviating the temper brittleness at high temperature; ensuring that the Cu content is less than 0.25 wt% can improve the strength and toughness of the steel; the P element and S element both have an adverse effect on the low-temperature toughness of the steel. Therefore, the contents of P and S are limited to not more than 0.025 wt%. By compounding the above element contents and cooperating with the heat treatment method described in this solution, the low-temperature impact toughness of the steel is significantly improved.

[0033] Those skilled in the art should be aware that the Ac1 temperature is the temperature at which pearlite transforms into austenite when the steel is heated, and the Ac3 temperature is the final temperature at which the steel transforms into austenite when heated.

[0034] It should be noted that forging the raw material along the axial direction after melting is beneficial to the forming of the shaft and the improvement of the impact toughness and fatigue strength along the axial direction. In the preheat treatment process of this solution, the steel is heated above Ac1 and held for a sufficient time. The Si element in the steel increases the chemical potential of ferrite, promotes the diffusion of C in ferrite into the austenite interior to obtain austenite structure, and after tempering at 560 °C, it is mainly tempered sorbite and a small amount of retained austenite structure. The presence of Ni element expands the austenite phase region. The measured Ac1 and Ac3 temperatures of the shaft steel in the above mass range differ by about 100 °C. During the final heat treatment process, isothermal treatment is carried out at a medium-high temperature within the Ac1 - Ac3 range for 0.5 - 1 h to make the grains crystallize and increase the grain boundaries and phase interfaces. At the same time, a part of ferrite is retained, and the retained ferrite promotes the martensite transformation. Subsequently, it is cooled under the condition of a cooling rate greater than the critical cooling rate (water cooling at a rate greater than 10 °C / s and less than 100 °C / s), and the austenite transforms into martensite structure. The tempering temperature is 580 - 600 °C to transform the retained austenite into tempered martensite, and finally obtain ferrite and a small amount of martensite structure. Thereby reducing the martensite lath structure and making the Cr-rich M 23Carbides such as C6 are evenly dispersed and not overly coarsened. Ni inhibits the formation of the brittle phase cementite, thereby reducing the nuclei of induced cracks during plastic deformation. After multiple tests, it is found that the dislocation density reaches a maximum value after tempering at 600 °C. Finally, the martensite and ferrite composite structure can effectively inhibit the formation and propagation of microcracks and further improve the low-temperature toughness.

[0035] The technical solutions of the present invention will be further described in detail below in conjunction with specific embodiments. It should be understood that the following embodiments are only used to explain the present invention and are not used to limit the present invention.

[0036] In all the examples and comparative examples of the present invention, the mechanical properties of the prepared steel are tested by using V-notch specimens to detect the impact toughness. The impact samples are detected in accordance with the requirements of GB / T 229-2020 "Metallic materials - Charpy pendulum impact test method"; the room-temperature tensile samples are in accordance with GB / T 228.1-202 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature".

[0037] Example 1

[0038] A heat treatment method for improving the low-temperature impact toughness of steel includes the following steps:

[0039] S1. Preheating treatment: Gradually heat the steel to within 840 °C, hold for 3 h, and then air-cool to room temperature; then gradually heat to 560 °C, hold for 8 h, and then air-cool to room temperature;

[0040] S2. Final heat treatment: Rapidly heat the steel in the furnace to 840 °C, hold for 0.5 h, then slowly cool in the furnace to 770 °C for isothermal treatment, continue to hold for 0.5 h, and then water-cool to room temperature; finally heat to 600 °C, hold for 2 h, and then air-cool to room temperature.

[0041] The chemical composition of the steel used in this example is as follows in mass percentage: C 0.36%, Si 0.27%, Mn 0.61%, Cr 1.63%, Ni 1.59%, Mo 0.25%, Cu 0.10%, and the rest are iron elements and inevitable impurities. The measured Ac1 temperature of the steel used in the thermal simulation experiment is 690 °C, and the Ac3 temperature is 793 °C.

[0042] After the above process, the samples prepared in Example 1 are detected and their mechanical properties are tested. The microstructure of the samples in Example 1 is observed by using a scanning electron microscope (SEM). As Figure 1 shown on the left, the austenite grain boundaries can be clearly seen. The structure is tempered troostite, but the acicular morphology of martensite has not completely disappeared, and the acicular contour can still be observed; Figure 1 The right is a photo of the impact fracture morphology at -40 °C. The width of the protruding shear lip area is about 1706 μm. The wider the shear lip area, the better the impact toughness.

[0043] Example 2

[0044] This example uses the same parameters and processes as Example 1, except that the isothermal treatment temperature in the final heat treatment step is 750 °C.

[0045] The samples prepared in Example 2 were tested for detection and mechanical properties. A scanning electron microscope (SEM) was used to observe the microstructure of the samples in Example 2. As Figure 2 shown on the left, the tempered sorbite structure is fine and uniform, where the large particles may be the second phase, and the striped area is the pearlite lamella; the width of the protruding shear lip area in the right figure is about 1712 μm at the widest point.

[0046] Example 3

[0047] This example uses the same parameters and processes as Example 1, except that the isothermal treatment temperature in the final heat treatment step is 730 °C.

[0048] The samples prepared in Example 3 were tested for detection and mechanical properties. The test results are shown in Table 1. A scanning electron microscope (SEM) was used to observe the microstructure of the samples in Example 3. As Figure 3 shown on the left, the number of martensite needle-like morphologies slightly decreases; the right figure shows quasi-cleavage fracture, and the width of the shear lip area is measured to be about 1460 μm at the widest point.

[0049] Comparative Example 1

[0050] This comparative example uses the same steel as Example 1. The heat treatment method only includes preheating treatment: the steel is heated to 840 °C in a gradient manner, held for 3 h, and then oil-cooled to room temperature; then it is heated to 560 °C in a gradient manner, held for 8 h, and then oil-cooled to room temperature.

[0051] After the above process, the samples prepared in Comparative Example 1 were tested for detection and mechanical properties. The average value of the low-temperature impact energy at -40 °C was only 50 J, the tensile strength was 804.5 Mpa, the yield strength was 936.0 Mpa, and the elongation after fracture was 18.1%.

[0052] A scanning electron microscope (SEM) was used to observe the microstructure of the samples in Comparative Example 1. As Figure 4 shown on the left, the carbide size is larger than that in Example 1, and the austenite grains are coarse; Figure 4 the right figure is a photo of the impact fracture morphology at -40 °C. It can be seen that the width of the lower shear lip area is relatively thin, about 180 μm, and the boundary with the river pattern in the radiation area is clearly visible, indicating poor toughness.

[0053] Comparative Example 2

[0054] The chemical composition of the steel used in this comparative example, in mass percentage, includes: C 0.33%, Si 0.30%, Mn 0.64%, Cr 1.07%, Ni 0.34%, Mo 0.48%, and the rest are iron elements and inevitable impurities. The chemical composition of the steel is shown in Table 2. The specific parameters of the heat treatment method are the same as those in Example 1, including only preheating treatment and two steps of preheating treatment and final heat treatment. The mechanical properties after heat treatment stage S1 and S2 are tested.

[0055] After preheating treatment S1, the yield strength of the specimen under normal temperature tension is 704 Mpa, and the tensile strength is 841 Mpa; the low-temperature impact absorption energy at -40 °C is 35.2 J. After treatment S2, the yield strength is increased to 1066 Mpa, and the tensile strength is increased to 918.5 Mpa; the low-temperature impact absorption energy at -40 °C is increased to 48 J. However, the low-temperature impact performance at -60 °C decreases after S2. The detailed mechanical properties are shown in Table 3.

[0056] Comparative Example 3

[0057] The chemical composition of the steel used in this comparative example, in mass percentage, includes: C 0.35%, Si 0.30%, Mn 0.92%, Cr 1.29%, Ni 0.55%, Mo 0.27%, and the rest are iron elements and inevitable impurities, referring to Table 2. The heat treatment method is the same as that in Comparative Example 2, and the mechanical properties after heat treatment stage S1 and S2 are tested.

[0058] After heat treatment S2, while the tensile performance is improved, the low-temperature impact performance is also improved, but its low-temperature impact energy at -40 °C is only 25.4 J, which is significantly lower than the impact absorption energy of Examples 1-3.

[0059] Comparative Example 4

[0060] The chemical composition of the steel used in this comparative example, in mass percentage, includes: C 0.44%, Si 0.24%, Mn 1.15%, Cr 0.22%, Ni 0.36%, Mo 0.07%, and the rest are iron elements and inevitable impurities, referring to Table 2. The heat treatment method is the same as that in Comparative Example 2 and Comparative Example 3, and the mechanical properties after heat treatment stage S1 and S2 are tested.

[0061] After treatment S2 in this comparative example, the tensile strength can be increased to 988 Mpa, and the yield strength can be increased to 894.5 Mpa. The low-temperature impact performance is also improved, but the impact performance decreases severely at lower temperatures. Similarly, its strength and toughness are far inferior to those of Examples 1-3.

[0062] Table 1 Mechanical properties of Examples 1-3 (normal temperature tension and low-temperature impact)

[0063]

[0064] Note: A KV is the impact energy of V-notch specimen, R m is the tensile strength, R p0.2 is the yield strength, A is the elongation after fracture, and Z is the reduction of area.

[0065] Table 2 Chemical compositions of the steels in Comparative Examples 2-4 in mass percentage

[0066] Serial number C Si Mn Cr Ni Mo Fe and impurities Comparative example 2 0.33 0.30 0.64 1.07 0.34 0.48 Balance Comparative example 3 0.35 0.30 0.92 1.29 0.55 0.27 Balance Comparative example 4 0.41 0.24 1.15 0.22 0.36 0.07 Balance

[0067] Table 3 Mechanical properties of Comparative Examples 1-4 (tensile at room temperature and impact at low temperature)

[0068]

[0069] As can be seen from Table 1, the average value of the low-temperature impact absorption energy of the steel after being treated by the present solution in Example 1 is 121.4 J at -20°C, 114.2 J at -40°C, and 116.0 J at -60°C. The average value of the low-temperature impact absorption energy of the steel after being treated by the present solution in Example 2 is 130.5 J at -20°C, 100.6 J at -40°C, and 104.3 J at -60°C. The average value of the low-temperature impact absorption energy of the steel after being treated by the present solution in Example 3 is 119.0 J at -20°C, 114.8 J at -40°C, and 116.1 J at -60°C. The sample of Comparative Example 1 was only preheated and not subjected to final heat treatment, and its low-temperature impact toughness at -40°C was only 50 J / cm 2 , which is significantly lower than the samples in Examples 1-3. The steels used in Comparative Examples 2-4 are not within the range of the chemical compositions described in the present solution. After heat treatment according to the present invention, the low-temperature impact absorption energy above -40°C and the room-temperature tensile properties have been improved to varying degrees, indicating that the heat treatment method adopted in the present solution can significantly improve the mechanical properties and low-temperature impact toughness of the steel, especially the low-temperature toughness, which is significantly improved after the heat treatment method of the present invention, and is very helpful for improving the comprehensive performance of the low-temperature marine shafting materials. However, the low-temperature impact performance of Comparative Examples 2-4 is significantly lower than that of Examples 1-3. It shows that the steels with the raw material components in Examples 1-3 of the present solution have the best low-temperature impact toughness and still have good low-temperature impact toughness at -60°C. It should be noted that Comparative Examples 2-4 are only for exemplary display. When the other components are not within the range described in the present solution, the measured low-temperature impact toughness of the samples does not reach the best effect, which will not be elaborated here.

[0070] Refer to Figure 5Optical micrographs of the metallographic structures of Examples 1, 2, 3 and Comparative Example 1. In the figures, a, b, c, and d are all tempered sorbite. Figure 5 (d) shows that the structure in Comparative Example 1 contains ferrite and cementite. As can be seen from the figure, the structure of Example 2 is the most uniform. At the same time, the electron backscatter diffraction method (EBSD) was used to compare the grain size statistics and stress distribution of Comparative Example 1 and Example 2, as Figure 6 shown. (a) and (b) are the IPF and KAM maps of Comparative Example 1, respectively, and (c) and (d) are the IPF and KAM maps of Example 2, respectively. Comparing the two figures (a) and (c), it is obvious that the grain size of Example 2 in figure (c) is smaller; after statistics, there are a total of 6205 grains on this surface, and the equivalent circle diameter ranges from 2.14 to 24.08 μm, and most of the sizes are in the range of 2.14 to 6.53 μm. The original austenite grain boundaries are clearly visible, and the martensite grains are diffusely distributed therein. In the (a) figure of Comparative Example 1, 1654 grains were counted, and the equivalent circle diameter ranges from 2.1 to 62.4 μm, and most of the sizes are in the range of 4.55 to 23.8 μm. KAM reflects the degree of local deformation. The green and blue distribution ranges in the (d) figure of Example 2 are more than those in the (b) figure of Comparative Example 2, indicating that Example 2 treated by S2 has less residual stress distributed in the grains and is not easily fractured when resisting impact. This also proves that the low-temperature impact performance and room-temperature tensile performance of Example 2 are superior to those of Comparative Example 1.

[0071] In summary, the heat treatment process of the present invention adopts the synergistic effect of two steps of preheating treatment and final heat treatment to obtain a composite structure of martensite and ferrite, effectively inhibiting the formation and propagation of microcracks, and further improving the low-temperature impact toughness; further, selecting the appropriate steel chemical composition in this solution and cooperating with the heat treatment process of this solution can obtain the best low-temperature impact toughness while maintaining good mechanical properties.

[0072] The above are only the preferred embodiments of the present invention, and do not limit the patent scope of the present invention accordingly. Any equivalent structural transformation made by using the content of the specification of the present invention under the inventive concept of the present invention, or direct / indirect application in other related technical fields is included in the patent protection scope of the present invention.

Claims

1. A heat treatment method for improving the low-temperature impact toughness of steel, characterized in that, It includes the following steps: S1. Preheating treatment: Heat the steel to a temperature range of 840 - 880 °C, hold for 2 - 3 h, and then cool it in oil to room temperature; then heat it to 560 °C, hold for 8 - 10 h, and then cool it in oil to room temperature; S2. Final heat treatment: Heat the steel in the furnace to 840 °C, hold for 0.5 - 1 h, then isothermally treat it at a temperature within the Ac1 - Ac3 range while cooling in the furnace, continue to hold for 0.5 - 1 h, and then cool it in water to room temperature; finally, heat it to the tempering temperature of 580 - 600 °C, hold for 2 h, and then air-cool it to room temperature; The steel is a steel for shafting, and its chemical composition by mass percentage includes: C 0.3 - 0.4%, Si 0.17 - 0.37%, Mn 0.5 - 0.8%, Cr 1.0 - 1.7%, Ni 1.2 - 1.6%, Mo 0.2 - 0.5%, Cu ≤ 0.25%, S ≤ 0.025%, P ≤ 0.025%, and the rest are iron elements and inevitable impurities.

2. The heat treatment method for improving the low-temperature impact toughness of steel according to claim 1, characterized in that The isothermal treatment temperature in step S2 is 730 - 770 °C.

3. A heat treatment method for improving the low-temperature impact toughness of steel according to claim 1, characterized in that, The steel is a steel for shafting, and its chemical composition by mass percentage includes: C 0.36%, Si 0.27%, Mn 0.61%, Cr 1.63%, Ni 1.59%, Mo 0.25%, Cu 0.10%, S ≤ 0.025%, P ≤ 0.025%, and the rest are iron elements and inevitable impurities.

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