Two-stage variable temperature composite heat treatment method and its application for medium-low carbon low alloy bainite steel parts

Abstract

The present invention relates to the technical field of heat treatment of medium-low carbon low alloy bainitic steel, and specifically provides a two-stage variable temperature composite heat treatment method and application of medium-low carbon low alloy bainitic steel parts. This low-temperature phase transformation and high-temperature carbonization two-stage variable temperature composite process method utilizes the process of bainite phase transformation at low temperature to obtain fine lath bainite structure, and carbon atom homogenization at high temperature to improve the stability of untransformed austenite, so that the steel parts obtain lath bainite, lath martensite / austenite islands and film-like residual austenite complex phase structure, with high strength, high toughness and good mechanical properties. At the same time, the use of high-temperature carbonization process technology can shorten the process time and improve production efficiency without affecting the performance of the steel parts, solving the problem of long process time for bainitic steel.

Description

Technical Field

[0001] The present invention relates to the technical field of heat treatment of medium-low carbon low alloy bainite steel, and specifically provides a two-stage variable temperature composite heat treatment method and application of medium-low carbon low alloy bainite steel parts. Background Art

[0002] Mining machinery operates in a complex and harsh environment. For example, concrete mixing blades and concrete conveying elbows are subject to both high-speed direct wear from sand, gravel, and soil, as well as high-force impacts from these impacts. As buildings grow taller, higher demands are placed on mining machinery components like these elbows. To withstand both high-speed direct wear and high-force impacts, manufacturing mining machinery components with high strength and toughness has long been a goal. Heat treatment technology is key to achieving these high-strength and high-toughness characteristics.

[0003] A Chinese invention patent application (application number 202210735380.8) discloses a wear-resistant lining containing ceramic particles and its preparation method. Ceramic particles are added to the steel material, and the ceramic particles act as a "hard phase" while the steel matrix acts as a "tough phase" to solve the strength and toughness constraints of existing wear-resistant liners. However, this preparation method requires that the molten steel be cooled to a certain temperature before adding the ceramic particles during smelting, and the molten steel needs to be stirred in the mold, making the preparation process complicated and difficult to operate.

[0004] A Chinese invention patent application (application number 202110226740.7) discloses a mixer blade and a method for its preparation. By casting a high-chromium cast iron wear-resistant material onto a tough substrate of low-alloy structural steel, the goal of "high strength and high toughness" is achieved. However, this preparation method requires casting the high-chromium cast iron wear-resistant material onto the low-alloy structural steel under negative pressure, placing extremely stringent requirements on the casting process and equipment.

[0005] A Chinese invention patent (application number 201210077144.8) discloses a polymer-bainite composite elbow for pump trucks and its preparation method. By heat-shrinkably coating the outer surface of a single bainite elbow with a polymer layer, the invention addresses the problem of insufficient toughness in existing bainite steel elbows. However, this invention requires physically adding the polymer layer to the prepared elbow, increasing manufacturing costs, lengthening the manufacturing process, and reducing production efficiency.

[0006] Professor Sir Harry Bhadeshia of the University of Cambridge has developed and applied nanobainite steel. After austenitizing high-carbon, low-alloy steel, it is isothermally treated at a relatively low bainite transformation temperature (150°C to 400°C) for a long period (e.g., 5-14 days) to produce components primarily composed of nanobainite. These components exhibit high strength and fracture toughness, but the long isothermal treatment required (up to 10 days) within the bainite isothermal transformation temperature range reduces production efficiency.

[0007] In view of this, the present invention is proposed. Summary of the Invention

[0008] The first object of the present invention is to provide a two-stage temperature-variable composite heat treatment method for medium-low carbon low alloy bainite steel parts.

[0009] The second object of the present invention is to provide a medium-low carbon low alloy bainite steel part obtained by the above method.

[0010] The third object of the present invention is to provide applications of the above method or medium-low carbon low alloy bainite steel parts.

[0011] In order to achieve the above purpose, the present invention adopts the following technical solutions:

[0012] The present invention provides a two-stage variable temperature composite heat treatment method for medium-low carbon low alloy bainite steel parts, comprising the following steps:

[0013] S1: After conventional normalizing treatment of steel parts, the steel structure is ferrite and pearlite.

[0014] S2: After S1, the steel piece is subjected to a two-stage variable temperature composite heat treatment, which includes:

[0015] S2-1: Heating and heat preservation treatment. At this time, the steel structure is completely transformed from ferrite and pearlite structure after normalizing treatment to austenite structure.

[0016] S2-2: First-stage controlled cooling treatment, after S2-1, the steel piece is cooled to the first low-temperature holding temperature T iso1 (℃), at this time the steel part does not undergo structural transformation.

[0017] S2-3: First-level low-temperature insulation, after S2-2, the steel parts are kept at the first low-temperature insulation temperature T iso1 (℃) Keep warm for a period of time t iso1 (min), so that the steel part forms a fine structure of bainitic ferrite.

[0018] S2-4: Secondary heating treatment, heating the steel piece to the second high temperature holding temperature T after S2-3 iso2 (℃), at this time no structural transformation occurs in the steel.

[0019] S2-5: Secondary high temperature insulation, after S2-4, the steel parts are kept at the second high temperature insulation temperature T iso2 (℃) Keep warm for a period of time t iso2 (min), so that the carbon atoms in the steel are evenly distributed in the untransformed austenite structure.

[0020] S2-6: Slow cooling treatment: After S2-5, the steel piece is subjected to a slow cooling treatment, at which time the untransformed austenite portion of the steel piece in S2-5 is transformed into martensite.

[0021] S3: After S2, the steel piece is subjected to a medium-low temperature tempering treatment.

[0022] The present invention provides a medium-low carbon low alloy bainite steel part obtained by treating the above-mentioned two-stage temperature-variable composite heat treatment method.

[0023] The present invention provides the above-mentioned two-stage variable temperature composite heat treatment method or the application of medium-low carbon low alloy bainite steel parts in manufacturing blades, liners and elbows in mining machinery.

[0024] Compared with the prior art, the technical effects of the present invention are:

[0025] The two-stage variable temperature composite heat treatment method of the present invention adopts a low-temperature phase transformation and high-temperature carbonization process. At low temperature, the bainite phase transformation occurs to obtain a fine lath bainite structure, and at high temperature, carbon atoms are homogenized to improve the stability of untransformed austenite, so that the steel parts obtain a lath bainite, lath martensite / austenite island and film-like residual austenite complex structure. At the same time, the high-temperature carbonization process technology is adopted to shorten the process time and improve production efficiency without affecting the performance of the steel parts, thus solving the problem of long process time of bainitic steel. The steel parts obtained by the above technology have high strength, high toughness and good mechanical property matching characteristics. At the same time, the microstructure of the steel parts contains austenite and carbide particles, which hinder the diffusion of hydrogen atoms, can improve its resistance to hydrogen-induced delayed fracture, prevent hydrogen embrittlement, and avoid sudden accidents. Steel parts with such characteristics are particularly suitable for manufacturing mining machinery components such as concrete mixing blades or concrete conveying elbows. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The various technical features of the present invention and the relationships between them are further explained below with reference to the accompanying drawings. The accompanying drawings are exemplary, and some technical features are not shown in actual proportion. In addition, some drawings may omit technical features that are commonly used in the technical field to which the present invention belongs and are not essential for understanding and implementing the present invention, or additional technical features that are not essential for understanding and implementing the present invention may be shown. In other words, the combination of the various technical features shown in the accompanying drawings is not intended to limit the present invention. In addition, throughout the present invention, the same figure numerals refer to the same content. The specific description of the drawings is as follows:

[0027] Figure 1 is the continuous cooling transformation curve of the medium-low carbon low alloy steel part 1, wherein A is austenite, B is bainite, M is martensite, F is ferrite, P is pearlite, Ac1 is the austenite start transformation temperature, Ac3 is the austenite end transformation temperature, Ms is the martensite start transformation temperature, Mf is the martensite end transformation temperature, and the same below;

[0028] Figure 2 It is the isothermal transformation curve of medium-low carbon low alloy steel material 1;

[0029] Figure 3 It is the continuous cooling transformation curve of medium-low carbon low alloy steel 2 material;

[0030] Figure 4 It is the isothermal transformation curve of medium-low carbon low alloy steel 2 material;

[0031] Figure 5 It is the continuous cooling transformation curve of medium-low carbon low alloy steel 3 material;

[0032] Figure 6 It is the isothermal transformation curve of medium-low carbon low alloy steel 3 material;

[0033] Figure 7 It is the continuous cooling transformation curve of medium-low carbon low alloy steel 4 material;

[0034] Figure 8 It is the isothermal transformation curve of medium-low carbon low alloy steel 4 material;

[0035] Figure 9 It is the continuous cooling transformation curve of medium-low carbon low alloy bainitic steel 5;

[0036] Figure 10 It is the isothermal transformation curve of medium-low carbon low alloy bainitic steel 5;

[0037] Figure 11 It is a conventional quenching and tempering heat treatment process;

[0038] Figure 12This is a SEM scanning microstructure photograph of a steel part after conventional quenching and tempering heat treatment test;

[0039] Figure 13 It is a single-stage high-temperature constant-temperature heat treatment process;

[0040] Figure 14 This is the SEM scanning microstructure photo of the steel after single-stage high-temperature constant-temperature heat treatment test;

[0041] Figure 15 It is a single-stage low-temperature constant-temperature heat treatment process;

[0042] Figure 16 This is the SEM scanning microstructure photo of the steel after the single-stage low-temperature constant-temperature heat treatment test;

[0043] Figure 17 It is a two-stage high-temperature-then-low-temperature variable temperature composite heat treatment process;

[0044] Figure 18 This is the SEM scanning microstructure photo of the steel after the double-stage high-temperature-then-low-temperature composite heat treatment test;

[0045] Figure 19 It is a two-stage low-to-high temperature composite heat treatment process;

[0046] Figure 20 This is a SEM scanning microstructure photograph of a steel part after a two-stage low-to-high temperature composite heat treatment test. DETAILED DESCRIPTION

[0047] Unless otherwise defined, all technical and scientific terms used in this application are the same as those generally understood by those skilled in the art to which this application belongs. In the event of any inconsistency, the meaning described in the full text of this application or the meaning derived from the content recorded in the full text of this application shall prevail. In addition, the terms used in this description are only for the purpose of describing the embodiments of the present application and are not intended to limit this application. Below, the specific embodiments of the application are described in detail.

[0048] The medium-low carbon low alloy bainitic steel in the present invention refers to a steel having a mass percentage of the carbon element between 0.18% and 0.42%, a total mass percentage of alloying elements such as Mn, Si, Cr, V, and Ti not exceeding 6%, and the remainder being Fe and other unavoidable impurities. In a preferred embodiment, the composition of the medium-low carbon low alloy bainitic steel comprises, by mass percentage, 0.18% to 0.42% C, 0.8% to 4% Mn, 0.6% to 3% Si, 0.2% to 3% Cr, and at least one of 0.02% to 0.22% V and 0.02% to 0.15% Ti, with the remainder being Fe and other unavoidable impurities; at the same time, the total content of Mn and Cr is not less than 2.5% and not more than 4.2%, and the total content of Mn, Si, Cr, V, and Ti is not more than 6%. For example, the components of medium-low carbon low alloy bainitic steel parts include C, Mn, Si, Cr and V, and the rest are Fe and other inevitable impurities; the components of medium-low carbon low alloy bainitic steel parts include C, Mn, Si, Cr and Ti, and the rest are Fe and other inevitable impurities; the components of medium-low carbon low alloy bainitic steel parts include C, Mn, Si, Cr, Ti and V, and the rest are Fe and other inevitable impurities.

[0049] In the steel of the present invention, adding 0.18% to 0.42% of C can improve the hardenability and hardenability of the steel, reduce the starting transformation temperature of bainite and martensite, and thus obtain a fine structure of bainite semi-strip structure, ensure the strength of the steel, and improve the thermal stability and mechanical stability of the retained austenite; by controlling the Si content, the precipitation of carbides is delayed or avoided, ensuring that there are no carbides composed of Fe and C in the process of the first-level controlled cooling treatment-first-level low-temperature insulation-second-level heating treatment-second-level high-temperature insulation composite quenching, forming a carbon-free lath structure. By limiting the content of Mn and Cr, when the sum of the Mn and Cr contents is too low, high-temperature structures (such as ferrite and / or pearlite) may be precipitated during the cooling process, thereby reducing the bainite hardenability of the steel. Therefore, their contents and lower limits need to be limited. However, Mn and Cr increase the tendency to crack during quenching, so the upper limit of their sum needs to be limited. By adding a certain amount of V and Ti elements, dispersed carbide particles are formed, which helps to refine the grains of the steel and can also act as hydrogen traps to avoid the risk of hydrogen embrittlement.

[0050] The two-stage temperature-variable composite heat treatment method for low-carbon low-alloy bainite steel parts provided by the present invention mainly includes three stages: conventional normalizing treatment in S1, composite heat treatment in S2, and medium-low temperature tempering treatment in S3.

[0051] Conventional normalizing treatment of S1

[0052] Normalizing is a heat treatment process in which steel is heated to austenitize and kept warm for a period of time. After the heat preservation is completed, the steel is removed from the furnace and naturally air-cooled to room temperature to obtain finer ferrite and pearlite structures.

[0053] In one embodiment, the conventional normalizing process includes: placing the steel piece in a state where the steel piece has been heated to T 11 (℃) in a heating furnace and keep warm 11 (min), then heated to T 12 And keep warm 12 (min), and then naturally air-cool to room temperature after heat preservation, where T 11 500~600℃, t 11 100~200min, T 12 880~1050℃, t 12 30 to 240 minutes.

[0054] In a preferred embodiment, the process of the conventional normalizing treatment comprises the following steps:

[0055] S1-1: Place the steel piece in the first heating furnace and heat it at 80-150℃ / h to T 11 (500~600℃) and keep warm 11 (100-200 min), then heated at 150-200 °C / h to T 12 (880~1050℃), and keep warm 12 (30~240min).

[0056] S1-2: The steel piece that has completed S1-1 is removed from the first heating furnace and cooled naturally in air to room temperature. In a preferred embodiment, the microstructure of the steel piece after the S1 treatment is ferrite and pearlite.

[0057] It should be noted that, in S1-1, heating to T 11 The speed may be, but is not limited to, 80°C / h, 90°C / h, 100°C / h, 110°C / h, 120°C / h, 130°C / h, 140°C / h or 150°C / h; T 11 The temperature may be, but is not limited to, 500°C, 520°C, 540°C, 560°C, 580°C or 600°C; t 11 It can be, but is not limited to, 100 min, 120 min, 140 min, 160 min, 180 min or 200 min; heating to T 12 The speed may be, but is not limited to, 150°C / h, 160°C / h, 170°C / h, 180°C / h, 190°C / h or 200°C / h; T 12The temperature may be, but is not limited to, 880°C, 900°C, 920°C, 940°C, 960°C, 980°C, 1000°C, 1020°C or 1050°C; t 12 It can be, but is not limited to, 30 min, 50 min, 100 min, 150 min, 200 min or 240 min.

[0058] Composite quenching treatment of S2

[0059] Quenching is a heat treatment method that includes continuous cooling and quenching and austempering. Continuous cooling and quenching of steel involves heating the steel to a temperature above the critical temperature Ac3 (for hypoeutectoid steel) or Ac1 (for hypereutectoid steel), holding it for a period of time to fully or partially austenitize it, then rapidly cooling it at a cooling rate greater than the critical cooling rate to a temperature below Ms (or isothermally holding it near Ms) to undergo a martensite (or bainite) transformation. Austempering of steel involves heating the steel to a temperature above the critical temperature Ac3 (for hypoeutectoid steel) or Ac1 (for hypereutectoid steel), holding it for a period of time to fully or partially austenitize it, then rapidly cooling it at a cooling rate greater than the critical cooling rate to a temperature between the bainite start and bainite end temperatures, and isothermally holding it for a period of time to undergo a bainite transformation.

[0060] In the present invention, the composite quenching treatment adopts a first-level controlled cooling treatment-a first-level low-temperature heat preservation-a second-level heating treatment-a second-level high-temperature heat preservation. During the cooling stage, the steel is subjected to a primary controlled cooling treatment, which is carried out for an appropriate time to a temperature between [martensite start transformation temperature] and [martensite start transformation temperature + 1 / 2 (bainite start transformation temperature - martensite start transformation temperature)]. During this process, the structure of the steel does not change and exists in the form of supercooled austenite structure. Then, a primary low-temperature holding treatment is carried out for an appropriate time to transform the structure of the steel from supercooled austenite to a lath bainite structure with a fine structure. Then, a secondary heating treatment is carried out for an appropriate time to heat the steel to a temperature between [martensite start transformation temperature + 1 / 2 (bainite start transformation temperature - martensite start transformation temperature)] and [bainite start transformation temperature]. Then, a secondary high-temperature holding treatment is carried out for an appropriate time to homogenize the carbon atoms in the untransformed austenite. Finally, a cooling treatment is carried out, i.e., the steel is cooled to room temperature. At this time, part of the homogenized untransformed austenite is transformed into martensite, forming martensite / austenite islands with a lath structure.

[0061] In order to realize the above-mentioned tissue phase change process, S2 specifically includes the following steps:

[0062] S2-1: Heating and heat preservation treatment. At this time, the steel structure is completely transformed from ferrite and pearlite structure after normalizing treatment to austenite structure.

[0063] S2-2: First-stage controlled cooling treatment, after S2-1, the steel piece is cooled to the first low-temperature holding temperature T iso1 (℃), at this time no structural transformation occurs in the steel.

[0064] S2-3: First-level low-temperature insulation, after S2-2, the steel piece is kept at the first low-temperature insulation temperature T iso1 (℃) Keep warm for a period of time t iso1 (min), so that the steel part forms a fine structure of bainitic ferrite.

[0065] S2-4: Secondary heating treatment, after S2-3, the steel piece is heated to the second high temperature holding temperature T iso2 (℃), at this time no structural transformation occurs in the steel.

[0066] S2-5: Secondary high temperature insulation, after S2-4, the steel piece is kept at the second high temperature insulation temperature T iso2 (℃) Keep warm for a period of time t iso2 (min), so that the carbon atoms in the steel are evenly distributed in the untransformed austenite structure.

[0067] S2-6: Slow cooling treatment: The steel is slowly cooled after S2-5, and the austenite that has not been transformed in the S2-5 stage is partially transformed into martensite.

[0068] In a preferred embodiment, the specific method of S2 is preferably as follows:

[0069] S2-1: Place the steel parts that have completed the S1 conventional normalizing treatment into a heating furnace that has been heated to T2 (850-950°C) and keep it warm for t2 (30-120 minutes).

[0070] T2 can be, but is not limited to, 850°C, 880°C, 900°C, 920°C, 940°C, or 950°C; t2 can be, but is not limited to, 30 min, 50 min, 70 min, 90 min, 110 min, or 120 min.

[0071] S2-2: Take the steel piece that has completed S2-1 out of the heating furnace and cool it to the first low temperature holding temperature T iso1 (℃).

[0072] In a preferred embodiment, the first low temperature holding temperature T iso1 (°C) is a temperature between Ms and [Ms + 1 / 2(Bs - Ms)], where Ms is the martensite start temperature and Bs is the bainite start temperature.

[0073] Ms=539-423C-30.4Mn-17.7Ni-12.1Cr-7.5Mo (℃) Formula 1.

[0074] Bs=830-270C-90Mn-37Ni-70Cr-83Mo(℃)Formula 2

[0075] In the above formula, C, Mn, Ni, Cr, and Mo are the mass percentages of carbon, manganese, nickel, chromium, and molybdenum in the steel, respectively. Temperature T iso1 (℃) preferably 150~360℃; T iso1 The temperature may be, but is not limited to, 150°C, 160°C, 180°C, 200°C, 220°C, 240°C, 260°C, 280°C, 300°C, 320°C, 340°C or 360°C.

[0076] In a preferred embodiment, in S2-2, the cooling method is strong wind cooling or spray cooling.

[0077] S2-3: Place the steel parts that have been completed in S2-2 into the heated steel parts. iso1 (℃) in a heating furnace iso1 (min) a period of time.

[0078] In a preferred embodiment, first calculate the iso1 The time t1 (min) corresponding to the completion of bainite transformation during heat preservation, t iso1 (min) is obtained as follows:

[0079] t iso1 =t1+(0.5~0.8)×L(min); where L is the thickness of the steel part in millimeters. iso1 (min) is preferably 30 to 180 min; t iso1 (min) can be, but is not limited to, 30 min, 40 min, 60 min, 80 min, 100 min, 120 min, 140 min, 160 min or 180 min.

[0080] In a preferred embodiment, the time t1 is obtained by material science calculation software such as Jmat-Pro.

[0081] S2-4: Place the steel piece that has been completed in S2-3 into the second high temperature T iso2 (℃) in a heating furnace.

[0082] In a preferred embodiment, the second high temperature is T iso2(℃) is a temperature between the steel component [Ms+1 / 2(Bs-Ms)] and Bs; Ms and Bs are calculated by formula 1 and formula 2 respectively. Temperature T iso2 (℃) preferably 360~560℃; T iso2 The temperature may be, but is not limited to, 360°C, 380°C, 400°C, 420°C, 440°C, 460°C, 480°C, 500°C, 520°C, 540°C, 550°C or 560°C.

[0083] In a preferred embodiment, the heating method is to directly place the steel piece into a iso2 (℃) in a heating furnace and heated in the furnace.

[0084] S2-5: Keep the steel parts completed in S2-4 in a heating furnace for t iso2 (min) a period of time.

[0085] In a preferred embodiment, the holding time t iso2 (min) is determined as follows:

[0086] Z1: Physical and chemical tests to determine the composition of the steel at the holding temperature T iso1 After the heat treatment, the half width l(m) of the untransformed austenite is obtained.

[0087] Z2: Determine the theoretical uniform time t according to formula 3 and formula 4:

[0088]

[0089]

[0090] Where D is the diffusion coefficient of carbon atoms, D0 is a constant, and its value is 6.2×10 -7 m 2 / s, Q is the carbon atom diffusion activation energy, which is 135.7 kJ / mol, and R is the gas constant, 8.314 J / (mol×K).

[0091] Z3:t iso2 (min) is determined by Formula 5

[0092] t iso2 =t / 60+(0.4~0.6)×L(min) Formula 5.

[0093] Where L is the thickness of the steel part in millimeters. iso2 (min) is preferably 10 to 120 min; t iso1(min) can be, but is not limited to, 10 min, 20 min, 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, 100 min, 110 min or 120 min.

[0094] S2-6: The steel piece that has completed S2-5 is taken out of the heating furnace and cooled to room temperature.

[0095] In a preferred embodiment, the cooling method is weak wind cooling or natural air cooling.

[0096] S3 medium and low temperature tempering treatment

[0097] Tempering is a heat treatment method in which, after hardening, the workpiece is heated to a temperature below Ac1 (the temperature at which pearlite begins to transform to austenite during heating), held at that temperature for a specified period of time, and then cooled to room temperature. Tempering is categorized into low-temperature tempering (tempering of the workpiece at 150-250°C), medium-temperature tempering (tempering of the workpiece between 360-500°C), and high-temperature tempering (tempering of the workpiece above 500-650°C).

[0098] The medium-low temperature tempering treatment of S3 includes the following steps: placing the steel part that has completed S2 into a heating furnace that has been heated to T3 (50-500°C) and keeping it warm for time t3 (30-360 minutes), then taking it out of the furnace and cooling it to room temperature.

[0099] T3 may be, but is not limited to, 50°C, 100°C, 150°C, 200°C, 250°C, 300°C, 350°C, 400°C, 450°C or 500°C; t3 may be, but is not limited to, 30 min, 50 min, 60 min, 80 min, 100 min, 120 min, 140 min, 160 min, 180 min, 200 min, 220 min, 240 min, 260 min, 280 min, 300 min, 320 min, 340 min or 360 min.

[0100] In a preferred embodiment, the cooling method of S3 can be natural air cooling or strong wind cooling.

[0101] The medium-low carbon low alloy bainitic steel parts obtained by the present invention through the above-mentioned low-temperature phase transformation and high-temperature carbonization two-stage variable temperature composite heat treatment method have a microstructure of lath bainite, lath martensite / austenite islands and film-like residual austenite complex phase structure, which has the advantages of high strength, high toughness and good mechanical property matching, can simultaneously resist high-speed direct wear and large-scale direct impact, and are particularly suitable for manufacturing mining mechanical components such as concrete mixing blades and concrete conveying bends.

[0102] Example 1, preferred ingredients:

[0103] The composition and content of the medium-low carbon low alloy steel part 1 are shown in Table 1:

[0104] Table 1 Chemical composition and content (mass percentage) of low carbon low alloy steel part 1 in Example 1

[0105] element C Si Mn Cr V P S content 0.10 1.8 2.3 0.8 0.07 0.01 0.005

[0106] The continuous cooling transformation curve of the low carbon low alloy steel material 1 in Example 1 was obtained by using the Jmat-Pro software. Figure 1 As shown in ) and isothermal transformation curve (as shown in 2). Figure 1 It can be seen that the austenite start transformation temperature Ac1 and the austenite end transformation temperature Ac3 of the low carbon low alloy steel material 1 in Example 1 are 729°C and 860°C respectively; the bainite start transformation temperature Bs and the martensite start transformation temperature Ms are 487°C and 417°C; and the minimum cooling rate to avoid high temperature structures (ferrite and pearlite) is about 29°C / s. Figure 2 It can be seen that the bainite transformation temperature range of the low-carbon low-alloy steel part 1 material in Example 1 is between 417°C and 487°C; the bainite transformation temperature is relatively narrow; compared with the medium-temperature bainite structure, the high-temperature ferrite and pearlite structures are significantly shifted to the left, indicating that the bainite hardenability of this composition is poor, which is not conducive to obtaining a bainite structure.

[0107] The composition and content of medium-low carbon low alloy steel 2 are shown in Table 2:

[0108] Table 2 Chemical composition and content (mass percentage) of low carbon low alloy steel part 2 in Example 1

[0109] element C Si Mn Cr V P S content 0.55 1.8 2.3 0.8 0.07 0.01 0.005

[0110] The continuous cooling transformation curve of the low carbon low alloy steel material 2 in Example 1 was obtained by using the material science Jmat-Pro software (such as Figure 3 As shown in ) and isothermal transformation curve (as shown in 4). Figure 3 It can be seen that the austenite start transformation temperature Ac1 and the austenite end transformation temperature Ac3 of the low carbon low alloy steel material 2 in Example 1 are 750℃ and 785℃ respectively; the bainite start transformation temperature and the martensite start transformation temperature are 417℃ and 227℃ respectively; and the minimum cooling rate to avoid high temperature structure (ferrite and pearlite) is about 13℃ / s. Figure 4 It can be seen that the bainite transformation temperature range of the low-carbon low-alloy steel material 2 in Example 1 is between 227°C and 417°C; compared with the medium-temperature bainite structure, the high-temperature ferrite and pearlite structures are obviously shifted to the left, indicating that the bainite hardenability of this composition is poor, which is not conducive to obtaining a bainite structure.

[0111] The composition and content of medium-low carbon low alloy steel 3 are shown in Table 3:

[0112] Table 3 Chemical composition and content (mass percentage) of low carbon low alloy steel part 3 in Example 1

[0113] element C Si Mn Cr V P S content 0.32 1.8 0.6 0.2 0.07 0.01 0.005

[0114] The continuous cooling transformation curve of the low carbon low alloy steel material 3 in Example 1 was obtained by using the material science Jmat-Pro software (such as Figure 5 As shown in Figure 6) and isothermal transformation curve (as shown in Figure 6). Figure 5 It can be seen that the austenite start transformation temperature Ac1 and the austenite end transformation temperature Ac3 of the low carbon low alloy steel material 3 in Example 1 are 768°C and 845°C respectively; the bainite start transformation temperature and the martensite start transformation temperature are 510°C and 382°C respectively; and the minimum cooling rate to avoid high temperature structure (ferrite and pearlite) is about 500°C / s. Figure 6 It can be seen that the bainite transformation temperature range of the low-carbon low-alloy steel material 3 in Example 1 is between 382°C and 510°C; compared with the medium-temperature bainite structure, the high-temperature ferrite and pearlite structures are obviously shifted to the left, indicating that the bainite hardenability of this composition is poor, which is not conducive to obtaining a bainite structure.

[0115] The composition and content of medium-low carbon low alloy steel 4 are shown in Table 4:

[0116] Table 4 Chemical composition and content (mass percentage) of low carbon low alloy steel part 4 in Example 1

[0117] element C Si Mn Cr V P S content 0.32 1.8 6.5 4.0 0.07 0.01 0.005

[0118] The continuous cooling transformation curve of the low carbon low alloy steel material 4 in Example 1 was obtained by using the material science Jmat-Pro software (such as Figure 7 As shown in ) and isothermal transformation curve (as shown in 8). Figure 7 It can be seen that the austenite start transformation temperature Ac1 and the austenite end transformation temperature Ac3 of the low carbon low alloy steel material 4 in Example 1 are 713°C and 790°C respectively; the bainite start transformation temperature and the martensite start transformation temperature are 298°C and 157°C respectively; and the minimum cooling rate to avoid high temperature structures (ferrite and pearlite) is about 0.4°C / s. Figure 8 It can be seen that the bainite transformation temperature range of the low-carbon low-alloy steel material 4 in Example 1 is between 325°C and 500°C; compared with the medium-temperature bainite structure, the high-temperature ferrite and pearlite structures are significantly shifted to the left, indicating that the bainite hardenability of this composition is poor, which is not conducive to obtaining a bainite structure.

[0119] The components and contents of the medium-low carbon low alloy bainite steel 5 are shown in Table 5 below:

[0120] Table 5 Chemical composition and content (mass percentage) of low carbon low alloy bainite steel part 5 in Example 1

[0121] element C Si Mn Cr V P S content 0.32 1.8 2.3 0.8 0.07 0.01 0.005

[0122] The continuous cooling transformation curve of the low carbon low alloy bainite steel 5 in Example 1 was obtained by using the Jmat-Pro software (see Figure 9 As shown) and isothermal transformation curve (as shown in 10). Figure 9 It can be seen that the austenite start transformation temperature Ac1 and the austenite end transformation temperature Ac3 of the low carbon low alloy bainite steel 5 in Example 1 are 756°C and 854°C respectively; the bainite start transformation temperature and the martensite start transformation temperature are 500°C and 325°C; and the minimum cooling rate to avoid high temperature structures (ferrite and pearlite) is about 15°C / s. Figure 10 As can be seen, the bainite transformation temperature range for Example 1 is between 325°C and 500°C. When the bainite transformation temperature is between 330°C and 480°C, the bainite transformation takes less than 1 hour. Compared with the medium-temperature bainite structure, the high-temperature ferrite and pearlite structures are significantly shifted to the right, indicating that this component has good bainite hardenability and is conducive to obtaining a bainite structure. Therefore, Examples 2 to 6 all adopted the chemical composition and content shown in Table 5.

[0123] Example 2, conventional quenching and tempering test

[0124] According to the chemical composition and content in Table 5, a conventional smelting method was used to prepare a Keel sample with reference to GB / T26651-2011 Wear-resistant Steel Castings, and the Keel sample was subjected to conventional normalizing treatment S1.

[0125] The specific conventional normalizing treatment S1 process is:

[0126] S1-1: The Keel sample was heated in a furnace at 120°C / h to 550°C and kept at this temperature for 2h, then heated at 160°C / h to 1050°C and kept at this temperature for 2h;

[0127] S1-2: Then turn off the power of the heating furnace, take the Keel sample out of the heating furnace, and allow it to cool naturally to room temperature.

[0128] Steel pieces with a length of 70 mm, a width of 50 mm and a thickness of 20 mm were taken from the Keel test bars that had completed the conventional normalizing treatment and were subjected to conventional quenching and tempering heat treatment tests.

[0129] Conventional quenching and tempering heat treatment process system includes S2 quenching and S3 tempering treatment. The process flow is as follows: Figure 11 :

[0130] S2 quenching treatment includes:

[0131] S2-1: Place the steel parts completed in S1 into the first heating furnace and heat to 900℃, then keep at this temperature for 45 minutes;

[0132] S2-2: The steel parts completed in S2-1 are cooled by air jet to room temperature.

[0133] S3 tempering treatment includes:

[0134] S3-1: Place the steel parts prepared in S2 into a second heating furnace heated to 250°C and keep the temperature for 120 minutes;

[0135] S3-2: The steel parts obtained in S3-1 are taken out from the second heating furnace and air-cooled to room temperature.

[0136] The tissue photos corresponding to the sample of Example 2 are as follows: Figure 12 As shown, it is willow-leaf bainite + martensite + a small amount of film-like retained austenite.

[0137] The conventional mechanical properties of the steel prepared in Example 2 were tested according to the national standard GB / T 228-2002, and the impact toughness of the steel prepared in Example 2 was tested according to the national standard GB / T 229-1994, as shown in Table 6.

[0138] Table 6 Conventional mechanical properties and impact toughness of different physical and chemical tests in Example 2

[0139]

[0140] Example 3, single-stage high-temperature constant-temperature heat treatment test

[0141] According to the chemical composition and content in Table 5, a conventional smelting method was used to prepare a Keel sample with reference to GB / T26651-2011 Wear-resistant Steel Castings, and the Keel sample was subjected to conventional normalizing treatment S1.

[0142] The specific conventional normalizing treatment S1 process is:

[0143] S1-1: The Keel sample was heated in a furnace at 120°C / h to 550°C and kept at this temperature for 2h, then heated at 160°C / h to 1050°C and kept at this temperature for 2h;

[0144] S1-2: Then turn off the power of the heating furnace, take the Keel sample out of the heating furnace, and allow it to cool naturally to room temperature.

[0145] A steel piece with a length of 70 mm, a width of 50 mm and a thickness of 20 mm was taken from a Keel test bar that had completed conventional normalizing treatment and subjected to a single-stage high-temperature constant-temperature heat treatment test.

[0146] The single-stage high-temperature constant-temperature heat treatment process system includes single-stage high-temperature isothermal quenching of S2 and tempering treatment of S3. The process flow is as follows: Figure 13 :

[0147] S2's single-stage high-temperature austempering treatment includes:

[0148] S2-1: Place the steel parts completed in S1 into the first heating furnace and heat to 900℃, then keep at this temperature for 45 minutes;

[0149] S2-2: Cool the steel parts prepared in S2-1 by air jet cooling to 450°C;

[0150] S2-3: The steel parts prepared in S2-2 are quickly placed into a second heating furnace heated to 450°C and kept at this temperature for 120 minutes.

[0151] S2-4: The steel parts obtained in S2-3 are taken out from the second heating furnace and naturally cooled to room temperature;

[0152] S3 tempering treatment includes:

[0153] S3-1: Place the steel parts prepared in S2 into a third heating furnace heated to 250°C and keep the temperature for 120 minutes;

[0154] S3-2: The steel parts obtained in S3-1 are taken out from the third heating furnace and air-cooled to room temperature.

[0155] The tissue photos corresponding to the sample of Example 3 are as follows: Figure 14 As shown, there are lath bainite + large martensite / austenite islands (width of about 8μm).

[0156] The conventional mechanical properties of the steel prepared in Example 3 were tested according to the national standard GB / T 228-2002, and the impact toughness of the steel prepared in Example 3 was tested according to the national standard GB / T 229-1994, as shown in Table 7.

[0157] Table 7 Conventional mechanical properties and impact toughness of different physical and chemical tests in Example 3

[0158]

[0159] Example 4, single-stage low-temperature constant-temperature heat treatment test

[0160] According to the chemical composition and content in Table 5, a conventional smelting method was used to prepare a Keel sample with reference to GB / T26651-2011 Wear-resistant Steel Castings, and the Keel sample was subjected to conventional normalizing treatment S1.

[0161] The specific conventional normalizing treatment S1 process is:

[0162] S1-1: The Keel sample was heated in a furnace at 120°C / h to 550°C and kept at this temperature for 2h, then heated at 160°C / h to 1050°C and kept at this temperature for 2h;

[0163] S1-2: Then turn off the power of the heating furnace, take the Keel sample out of the heating furnace, and allow it to cool naturally to room temperature.

[0164] A steel piece with a length of 70 mm, a width of 50 mm and a thickness of 20 mm was taken from a Keel test bar that had completed conventional normalizing treatment and subjected to a single-stage low-temperature constant-temperature heat treatment test.

[0165] The single-stage low-temperature constant-temperature heat treatment process system includes single-stage low-temperature isothermal quenching of S2 and tempering treatment of S3. The process flow is as follows: Figure 15 :

[0166] S2's single-stage low-temperature austempering treatment includes:

[0167] S2-1: Place the steel parts completed in S1 into the first heating furnace and heat to 900℃, then keep at this temperature for 45 minutes;

[0168] S2-2: The steel parts after S2-1 are cooled by air jet to 350°C.

[0169] S2-3: The steel parts prepared in S2-2 are quickly placed into a second heating furnace heated to 350°C and kept at this temperature for 120 minutes;

[0170] S2-4: The steel parts obtained in S2-3 are taken out from the second heating furnace and naturally cooled to room temperature;

[0171] S3 tempering treatment includes:

[0172] S3-1: Place the steel parts prepared in S2 into a third heating furnace heated to 250°C and keep the temperature for 120 minutes;

[0173] S3-2: The steel parts obtained in S3-1 are taken out from the third heating furnace and air-cooled to room temperature.

[0174] The tissue photos corresponding to the sample of Example 2 are as follows: Figure 16 As shown, there are lath bainite + martensite / austenite islands (width is about 3 μm).

[0175] The conventional mechanical properties of the steel prepared in Example 4 were tested according to the national standard GB / T228-2002, and the impact toughness of the steel prepared in Example 4 was tested according to the national standard GB / T229-1994, as shown in Table 8.

[0176] Table 8 Conventional mechanical properties and impact toughness of different physical and chemical tests in Example 4

[0177]

[0178] The homogenization time of carbon atoms in austenite at different temperatures and different austenite widths is obtained using the following formula as shown in Table 9.

[0179]

[0180]

[0181] Where D is the diffusion coefficient of carbon atoms, D0 is a constant, and its value is 6.2×10 -7 m 2 / s, Q is the activation energy of carbon atom diffusion, its value is 135.7kJ / mol, R is the gas constant, 8.314J / (mol×K);

[0182] Table 9 Time required for homogenization of carbon atoms in austenite

[0183]

[0184]

[0185] Depend on Figure 14 It can be seen that after the steel piece of Example 3 was treated with a single-stage high-temperature constant-temperature heat treatment process, the width of the austenite was 8 microns. From Table 9, it can be seen that the time required for the carbon atoms to be homogenized in the austenite is 0.4772 hours, that is, the carbon atoms can be completely homogenized in the austenite after being isothermal at 450°C for 2 hours. However, the austenite / martensite islands formed are too large, which deteriorates the toughness of the steel piece. Figure 16 It can be seen that after the steel piece of Example 4 is treated with a single-stage low-temperature constant-temperature heat treatment process, the width of the austenite is 3 microns. It can be seen from Table 9 that the time required for the homogenization of carbon atoms in the austenite is 2.2189 hours, that is, isothermal treatment at 350°C for 2 hours cannot make the carbon atoms completely homogenized in the austenite, which reduces the stability of the austenite and is not conducive to the toughness of the steel piece.

[0186] Example 5: Two-stage high-to-low temperature composite heat treatment test

[0187] According to the chemical composition and content in Table 5, a conventional smelting method was used to prepare a Keel sample with reference to GB / T26651-2011 Wear-resistant Steel Castings, and the Keel sample was subjected to conventional normalizing treatment S1.

[0188] The specific conventional normalizing treatment S1 process is:

[0189] S1-1: The Keel sample was heated in a furnace at 120°C / h to 550°C and kept at this temperature for 2h, then heated at 160°C / h to 1050°C and kept at this temperature for 2h;

[0190] S1-2: Then turn off the power of the heating furnace, take the Keel sample out of the heating furnace, and allow it to cool naturally to room temperature.

[0191] A steel piece with a length of 70 mm, a width of 50 mm and a thickness of 20 mm was taken from a Keel test bar that had completed conventional normalizing treatment and subjected to a two-stage high-temperature-then-low-temperature variable temperature composite heat treatment test.

[0192] The two-stage high-to-low temperature composite heat treatment process system includes S2 high-to-low temperature composite quenching treatment and S3 tempering treatment. The process flow is as follows: Figure 17 :

[0193] The S2 high-to-low temperature composite quenching treatment includes:

[0194] S2-1: Place the steel parts completed in S1 into the first heating furnace and heat to 900℃, then keep at this temperature for 45 minutes;

[0195] S2-2: Cool the steel parts prepared in S2-1 by air jet cooling to 450°C;

[0196] S2-3: The steel parts prepared in S2-2 are quickly placed in a second heating furnace heated to 450°C and kept at this temperature for 60 minutes;

[0197] S2-4: Blow-cool the steel parts obtained in S2-3 to 350°C;

[0198] S2-5: The steel parts prepared in S2-4 are quickly placed into a third heating furnace heated to 350°C and kept at this temperature for 60 minutes;

[0199] S2-6: The steel parts that have completed S2-5 are taken out from the third heating furnace and naturally cooled to room temperature.

[0200] S3 tempering treatment includes:

[0201] S3-1: Place the steel parts prepared in S2 into a second heating furnace heated to 250°C and keep the temperature for 120 minutes;

[0202] S3-2: The steel parts obtained in S3-1 are taken out from the second heating furnace and air-cooled to room temperature.

[0203] The tissue photos corresponding to the sample of Example 5 are as follows: Figure 18 As shown, the lath bainite + martensite / austenite islands (width of approximately 6 μm) are shown. Combined with Table 9, it can be seen that the time required for carbon atoms to homogenize in the austenite is approximately 10 hours. This means that in Example S2-5, the third heating furnace at 350°C for 60 minutes cannot completely homogenize carbon atoms in the austenite, which is detrimental to the toughness of the steel part.

[0204] The conventional mechanical properties of the steel prepared in Example 5 were tested according to the national standard GB / T 228-2002, and the impact toughness of the steel prepared in Example 5 was tested according to the national standard GB / T 229-1994, as shown in Table 10.

[0205] Table 10 Conventional mechanical properties and impact toughness of different physical and chemical tests in Example 5

[0206]

[0207] Example 6: Two-stage low-to-high temperature composite heat treatment test

[0208] According to the chemical composition and content in Table 5, a conventional smelting method was used to prepare a Keel sample with reference to GB / T26651-2011 Wear-resistant Steel Castings, and the Keel sample was subjected to conventional normalizing treatment S1.

[0209] The specific conventional normalizing treatment S1 process is:

[0210] S1-1: The Keel sample was heated in a furnace at 120°C / h to 550°C and kept at this temperature for 2h, then heated at 160°C / h to 1050°C and kept at this temperature for 2h;

[0211] S1-2: Then turn off the power of the heating furnace, take the Keel sample out of the heating furnace, and allow it to cool naturally to room temperature.

[0212] A steel piece with a length of 70 mm, a width of 50 mm and a thickness of 20 mm was taken from a Keel test bar that had completed conventional normalizing treatment and subjected to a two-stage low-to-high temperature composite heat treatment test.

[0213] The two-stage low-to-high temperature composite heat treatment process system includes S2 low-to-high temperature composite quenching treatment and S3 tempering treatment. The process flow is as follows: Figure 19 :

[0214] The S2 low-to-high temperature composite quenching treatment includes:

[0215] S2-1: Place the steel parts completed in S1 into the first heating furnace and heat to 900℃, then keep at this temperature for 45 minutes;

[0216] S2-2: Cool the steel parts prepared in S2-1 by air jet cooling to 350°C;

[0217] S2-3: The steel parts prepared in S2-2 are quickly placed in a second heating furnace heated to 350°C and kept at this temperature for 60 minutes;

[0218] S2-4: The steel parts prepared in S2-3 are quickly placed in a third heating furnace heated to 450°C and kept at this temperature for 60 minutes;

[0219] S2-5: The steel parts prepared in S2-4 are taken out from the third heating furnace and naturally cooled to room temperature.

[0220] S3 tempering treatment includes:

[0221] S3-1: Place the steel parts prepared in S2 into a second heating furnace heated to 250°C and keep the temperature for 120 minutes;

[0222] S3-2: The steel parts obtained in S3-1 are taken out from the second heating furnace and air-cooled to room temperature.

[0223] The tissue photos corresponding to the completed sample of Example 6 are as follows: Figure 20 As shown, the structure is a composite structure of lath bainite, lath martensite / austenite islands, and film-like retained austenite (approximately 3 microns wide). Combined with Table 9, it can be seen that in the third heating furnace at 450°C in S2-4, the time required for carbon atoms to homogenize in the austenite is approximately 0.067 hours. This means that in this embodiment, the third heating furnace at 450°C in S2-4 takes 60 minutes to complete the homogenization of carbon atoms in the austenite, which is beneficial to the toughness of the steel.

[0224] The conventional mechanical properties of the steel prepared in Example 6 were tested according to the national standard GB / T 228-2002, and the impact toughness of the steel prepared in Example 6 was tested according to the national standard GB / T 229-1994, as shown in Table 11.

[0225] Table 11 Conventional mechanical properties and impact toughness of different physical and chemical tests of Example 6

[0226]

[0227] It can be understood that those skilled in the art can combine the features mentioned in one or more embodiments mentioned throughout the present application with the features of other embodiments in any appropriate manner to implement the present application.

[0228] Note that the above are only preferred embodiments of the present application and the technical principles employed. Those skilled in the art will understand that the present application is not limited to the specific embodiments described herein, and that various obvious changes, readjustments, and substitutions can be made by those skilled in the art without departing from the scope of protection of the present application. Therefore, although the present application has been described in more detail through the above embodiments, the present application is not limited to the above embodiments, and may include many other equivalent embodiments without departing from the technical concept of the present application, all of which fall within the scope of protection of the present application.

Claims

1. A two-stage variable temperature composite heat treatment method for medium-low carbon low alloy bainite steel parts, characterized in that: The following steps are involved: S1: After conventional normalizing treatment of the steel, the steel structure is ferrite and pearlite; S2: After S1, the steel piece is subjected to a two-stage variable temperature composite heat treatment, which includes: S2-1: Heating and holding treatment, at this time the steel structure is completely transformed from ferrite and pearlite structure after normalizing treatment to austenite structure; S2-2: First-stage controlled cooling treatment, after S2-1, the steel piece is cooled to the first low-temperature holding temperature T iso1 (℃), at this time, the steel part does not undergo structural transformation; The first low temperature insulation temperature T iso1 is a temperature between the steel component Ms and (Ms+(Bs-Ms) / 2); Ms = 539-423C-30.4Mn-17.7Ni-12.1Cr-7.5Mo Formula 1; Bs = 830-270C-90Mn-37Ni-70Cr-83Mo Formula 2; In the above formula, C, Mn, Ni, Cr, and Mo are the mass percentages of carbon, manganese, nickel, chromium, and molybdenum in the steel respectively; In S2-2, the cooling method is strong air cooling or spray cooling; S2-3: First-level low-temperature insulation, after S2-2, the steel parts are kept at the first low-temperature insulation temperature T iso1 (℃) Keep warm for a period of time t iso1 (min), so that the steel part forms a fine structure of lath bainite ferrite structure; S2-4: Secondary heating treatment, heating the steel piece to the second high temperature holding temperature T after S2-3 iso2 (℃), at this time, the steel part does not undergo structural transformation; The second high temperature insulation temperature T iso2 is a temperature between (Ms+(Bs-Ms) / 2) and Bs of the steel component; Ms and Bs are calculated by formula 1 and formula 2 respectively; S2-5: Secondary high temperature insulation, after S2-4, the steel parts are kept at the second high temperature insulation temperature T iso2 (℃) Keep warm for a period of time t iso2 (min), so that the carbon atoms in the steel are evenly distributed in the untransformed austenite structure; S2-6: Slow cooling treatment: After S2-5, the steel piece is subjected to a slow cooling treatment, whereby the untransformed austenite portion of the steel piece in S2-5 is transformed into lath martensite; and S3: after S2, the steel is subjected to a medium-low temperature tempering treatment, and the obtained microstructure of the steel is a fine-structured lath bainite, a small amount of lath martensite / austenite islands, and a small amount of film-like retained austenite duplex structure; Calculated by mass percentage, the components of the steel include: 0.18% to 0.42% C, 0.8% to 4% Mn, 0.6% to 3% Si and 0.2% to 3% Cr, and at least one of 0.02% to 0.22% V and 0.02% to 0.15% Ti, with the remainder being Fe and other unavoidable impurities; at the same time, the sum of the contents of Mn and Cr is not less than 2.5% and not more than 4.2%, and the sum of the contents of Mn, Si, Cr, V, and Ti is not more than 6%.

2. The two-stage temperature-variable composite heat treatment method according to claim 1, characterized in that: In S2-3, t iso1 Obtained as follows: t iso1 = t1+(0.5~0.8)×L, where L is the thickness of the steel part in millimeters, and t1 (min) is the time taken at T iso1 The time corresponding to the completion of bainite transformation of steel parts during insulation.

3. The two-stage temperature-variable composite heat treatment method according to claim 1, characterized in that: In S2-4, the heating method is to directly place the steel piece into a iso2 The heating furnace is heated along with the furnace.

4. The two-stage temperature-variable composite heat treatment method according to claim 1, characterized in that: In S2-5, the holding time t iso2 Determined by: Z1: Physical and chemical tests to determine the composition of the steel at the holding temperature T iso1 After heat preservation treatment, the half width l (m) of the untransformed austenite is obtained; Z2: Determine the theoretical uniform time t according to formula 3 and formula 4: Formula 3: Formula 4: Where D is the carbon atom diffusion coefficient, and D0 is a constant, whose value is 6.2×10 -7 m 2 / s, Q is the activation energy of carbon atom diffusion, which is 135.7 kJ / mol, and R is the gas constant, which is 8.314 J / (mol×K); Z3:t iso2 Determined by Formula 5: t iso2 = t / 60 + (0.4~0.6)×L Formula 5; Where L is the thickness of the steel part in millimeters.

5. The two-stage temperature-variable composite heat treatment method according to any one of claims 1 to 4, characterized in that: In S2-6, the cooling method is weak wind cooling or natural air cooling; In S2-1, the heating and heat-insulating treatment includes: placing the steel piece that has completed S1 into a heating furnace that has been heated to T2, and keeping it warm for t2, wherein T2 is 850~950℃ and t2 is 30~120min.

6. The two-stage temperature-variable composite heat treatment method according to any one of claims 1 to 4, characterized in that: In S1, the normalizing treatment includes: placing the steel piece in a 11 in a heating furnace and keep warm 11 , then heated to T 12 And keep warm 12 After the heat preservation is completed, it is naturally cooled to room temperature. 11 500~600℃, t 11 100~200min, T 12 880~1050℃, t 12 30~240min; The structure of the steel after S1 treatment is ferrite and pearlite; In S3, the low-temperature tempering treatment includes: placing the steel piece after S2 into a heating furnace that has been heated to T3, holding the temperature for a time period of t3, and then taking it out of the furnace and cooling it to room temperature; wherein T3 is 50-350°C and t3 is 30-360 minutes; In S3, the cooling method is natural air cooling or strong wind cooling.

7. A medium-low carbon low alloy bainite steel part obtained by the two-stage variable temperature composite heat treatment method according to any one of claims 1 to 4; The microstructure of the medium-low carbon low alloy bainite steel part is a lath bainite with a fine structure, a small amount of lath martensite / austenite islands and a small amount of film-like retained austenite duplex structure.

8. Use of the two-stage variable temperature composite heat treatment method according to any one of claims 1 to 4 or the medium-low carbon low alloy bainite steel according to claim 7 in the manufacture of blades, liners, and elbows in mining machinery.

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