A heat treatment process for eliminating banded structure in steel

Abstract

A heat treatment process for eliminating banded structure in steel belongs to the field of metal heat treatment technology. The process is as follows: solution quenching: heating the steel from room temperature to a certain temperature and keeping it warm for a period of time before rapidly cooling; the certain temperature is the carbide dissolution temperature at the banded structure + 100-150°C, and the period of time is the time it takes for the carbides at the banded structure to completely dissolve back at the solution temperature; cyclic quenching: the first quenching temperature is 10-20°C below the solution quenching temperature, and the steel core is rapidly cooled after reaching the temperature; this quenching process is repeated until the number of cyclic quenching times is set, and the quenching temperature is reduced by 10-20°C compared with the previous quenching temperature for each additional quenching time; the number of cyclic quenching times is 2-4 times; the tempering temperature and time are set according to the service performance index. The present invention regulates the organization of the banded structure based on the phase change principle, with a low process temperature, a short time, and greatly improved impact properties of the steel.

Description

Technical Field

[0001] The invention belongs to the technical field of metal heat treatment, and in particular relates to a heat treatment process for eliminating banded structure in steel. Background Art

[0002] Due to the inherent dendritic segregation of the ingot, the ingot will have a banded structure after rolling or forging. For medium and high alloy steels, the banded structure is enriched with a large amount of alloy elements, and a large amount of carbides are easily generated in the banded structure after the final heat treatment, such as Figure 1 As shown. In addition, the grain size of the banded structure is much larger than that of the non-banded structure. The presence of the banded structure will not only significantly reduce the impact performance of the steel, but also reduce the isotropy of the steel performance. The root cause is that during service, the banded structure becomes a crack source under the action of impact loads, causing the component to fail prematurely, such as Figure 2 Therefore, eliminating the influence of banded structure on the mechanical properties of steel is of great significance for improving steel quality and achieving long service life of components.

[0003] As mentioned above, the banded structure is enriched with a large amount of alloying elements. To achieve uniform diffusion with the matrix, a long diffusion annealing process is usually performed at a temperature slightly below the solidus, typically between 1100-1200°C. However, this method is high in temperature, time, and cost. Summary of the Invention

[0004] The purpose of the present invention is to solve the problem of banded structure in medium and high alloy steels and provide a heat treatment process for eliminating banded structure in steel. The process refines the grains and carbides in the banded structure through solid solution quenching + cyclic quenching + tempering phase transformation, eliminates the banded structure in the steel, and greatly improves its impact energy.

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

[0006] A heat treatment process to eliminate banded structure in steel, such as Figure 7 As shown, the process is specifically as follows:

[0007] Step 1: Solution quenching: The steel is heated from room temperature to a certain temperature, kept at this temperature for a period of time, and then rapidly cooled; the certain temperature is the carbide dissolution temperature of the banded structure + 100-150°C, and the period of time is the time it takes for the carbides in the banded structure to completely dissolve at the solution temperature;

[0008] The microstructure of the steel after quenching at different temperatures is characterized using a scanning electron microscope. If no carbides are observed in the banded structure, the corresponding quenching temperature is the carbide dissolution temperature. After the solution temperature is determined, the solution quenching holding time is determined based on experiments. The microstructure of the steel after quenching at this temperature for different holding times is characterized using a scanning electron microscope. The time when no carbides are observed is set as the holding time. The purpose of solution quenching is mainly to dissolve the coarse carbides in the banded structure. Therefore, the dissolution temperature of the carbides in the banded structure can be determined based on thermodynamic calculations or experiments, and the solution quenching temperature is set to the carbide dissolution temperature + 100-150°C.

[0009] Step 2: Cycle quenching: The first quenching temperature is 10-20°C below the solution quenching temperature. The steel core is cooled rapidly after reaching the temperature. This quenching process is repeated repeatedly until the set number of quenching cycles is reached. Each additional quenching time reduces the quenching temperature by 10-20°C compared to the previous quenching temperature. The number of quenching cycles is 2-4 times. The purpose of cyclic quenching is to refine the grain size and provide more nucleation sites for carbide precipitation during tempering to refine the carbides. To prevent grain growth, the core is cooled as soon as it reaches the temperature, and the cooling rate must be fast. While ensuring hardenability, prevent carbide precipitation during the cooling process.

[0010] Step 3: The tempering temperature and time are set according to the service performance indicators.

[0011] Furthermore, the steel is a medium-high alloy steel. Low alloy steels have an alloying element content of less than 5wt%, medium alloy steels have an alloying element content of 5-10wt%, and high alloy steels have an alloying element content of more than 10wt%. This method is suitable for medium and high alloy steels because the alloying element content is so low that coarse carbides do not form in the banded structure.

[0012] Furthermore, in step one, the rapid cooling is one of air cooling, oil cooling or water cooling.

[0013] Furthermore, in step 1, the carbide dissolution temperature is obtained by the following method: using a spectrometer to determine the chemical composition of the banded structure in the steel, and calculating the equilibrium phase composition-temperature diagram based on the chemical composition using commercial thermodynamic calculation software such as Thermo-calc or Jmatpro, thereby obtaining the carbide dissolution temperature.

[0014] Furthermore, in step three, the setting is based on the service performance index, that is, the tempering temperature and time are selected according to the required hardness and impact energy index.

[0015] Take 1Cr11Ni2W2MoV steel as an example. Its service performance requirements are a Brinell hardness of 269-321 HBW after quenching and tempering. After quenching, its hardness is 441 HBW. After tempering at 680°C for 1 hour, the hardness is 287 HBW, after tempering for 2 hours, the hardness is 273 HBW, and after tempering for 4 hours, the hardness is 259 HBW. Therefore, a tempering temperature of 680°C and a tempering time of 1 or 2 hours are recommended. After tempering at 700°C for 1 hour, the hardness is 276 HBW, after tempering for 2 hours, the hardness is 270 HBW, and after tempering for 4 hours, the hardness is 261 HBW. Therefore, a tempering temperature of 700°C and a tempering time of 1 or 2 hours are also recommended. For 1Cr11Ni2W2MoV steel, the tempering temperature is 660-710°C and the tempering time is 1-4 hours.

[0016] The advantages of this invention over existing technologies are as follows: The diffusion annealing process, based on the principle of alloying element diffusion, requires high temperatures (~1200°C) and long periods (tens to dozens of hours) to achieve uniform composition throughout the steel. This invention, based on the principle of phase transformation, regulates the structure of the banded structure, achieving low temperatures and short processing times while significantly improving the steel's impact resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a scanning electron microscope image of the banded structure in steel;

[0018] Figure 2 This is a scanning electron microscope image of the impact fracture of the steel containing banded structure after heat treatment;

[0019] Figure 3 is the metallographic diagram of Comparative Example 1;

[0020] Figure 4 is the metallographic diagram of Example 1;

[0021] Figure 5 This is a scanning electron microscope image of the banded structure of Comparative Example 1;

[0022] Figure 6 This is a scanning electron microscope image of the banded structure of Example 1;

[0023] Figure 7 It is a three-stage heat treatment process curve;

[0024] Figure 8 This is the metallographic diagram of Comparative Example 2;

[0025] Figure 9 This is the metallographic diagram of Example 2;

[0026] Figure 10 This is a scanning electron microscope image of the banded structure of Comparative Example 2;

[0027] Figure 11This is a scanning electron microscope image of the banded structure of Example 2;

[0028] Figure 12 Schematic diagram of the temperature field and curve during the heating process of metal components. DETAILED DESCRIPTION

[0029] The technical solution of the present invention is further described below with reference to the accompanying drawings and embodiments, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention that does not depart from the spirit and scope of the technical solution of the present invention should be included in the protection scope of the present invention.

[0030] Example 1:

[0031] 1. The carbide dissolution temperature at the banded structure of 1Cr11Ni2W2MoV steel was experimentally determined to be 950°C. The composition of the 1Cr11Ni2MoV steel is as follows: C: 0.10-0.16 wt%, Cr: 10.5-12.0 wt%, Ni: 1.40-1.80 wt%, W: 1.50-2.00 wt%, Mo: 0.35-0.50 wt%, V: 0.18-0.30 wt%, Si≤0.60 wt%, Mn≤0.50 wt%, and P≤0.03 wt%.

[0032] 2. Solution quenching: Heat 1Cr11Ni2W2MoV steel from room temperature to 1050℃ and keep it warm for 30 minutes. After the insulation is completed, air cool (or oil cool) to room temperature.

[0033] 3. First quenching: Heat the 1Cr11Ni2W2MoV steel after the first quenching from room temperature to 1030℃, and then air cool (or oil cool) to room temperature after the core reaches temperature.

[0034] 4. Second quenching: Heat the 1Cr11Ni2W2MoV steel after secondary quenching from room temperature to 1010℃, and air cool (or oil cool) to room temperature after the core reaches temperature.

[0035] 5. Tempering: Heat the 1Cr11Ni2W2MoV steel after three-cycle quenching from room temperature to 700℃, keep it at this temperature for 2h, and then take it out of the furnace and air cool it.

[0036] 6. Characterize the microstructure and mechanical properties of 1Cr11Ni2W2MoV steel after cyclic quenching and tempering.

[0037] In this embodiment, the explanation of the heart temperature is as follows:

[0038] Since the actual components have dimensions, Figure 12Take the infinitely large flat plate shown in the figure as an example. It can be seen that there is a temperature difference between the surface and core of the component during the heating process. The heating time of a metal is defined as the heating time + the equalization time + the holding time, where the heating time is the time it takes for the component surface temperature to reach the set temperature, the equalization time is the time it takes for the component core temperature to reach the set temperature minus the time it takes for the component surface temperature to reach the set temperature, and the holding time is the heat treatment process time.

[0039] (1) The length of the heating time mainly depends on the heating method. For example, the heating time is short when using an electric furnace (mainly through radiation heat transfer), while the heating time is long when using a fuel furnace (mainly through convection heat transfer).

[0040] (2) The length of the temperature averaging time depends mainly on the load and size of the workpiece and the material composition. The larger the component size and the larger the load, the longer the temperature averaging time. The temperature averaging time of alloy steel is longer than that of carbon steel. (The higher the alloying element content in the steel, the lower the thermal conductivity)

[0041] (3) The holding time mainly depends on the heat treatment process requirements and the initial structure of the material. For the quenching process, the holding time of pearlite structure should be longer than that of ferrite structure. (The austenitization kinetics of steel consists of four steps: nucleation, growth, cementite dissolution, and composition homogenization)

[0042] Therefore, theoretically, for a certain component, the heating time should be determined by comprehensively considering the heating method, component composition, size, furnace load, process requirements and initial structure, and accurately calculated. However, in actual production, for convenience, the following semi-empirical formula is usually used:

[0043] τ=α×KD

[0044] Wherein, τ is the heating time of the metal, min; α is the heating coefficient, min / mm, usually selected in the range of 0.7-0.8; K is the correction coefficient of the reaction furnace charge, usually selected in the range of 1.0-1.3; D is the effective thickness of the workpiece, mm.

[0045] Therefore, the core temperature refers to the core of the steel part (the steel part has dimensions) reaching the set quenching temperature, which can be calculated by an empirical formula, or calculated by simulation, or measured in real time by inserting a thermocouple into the core of the simulated steel part.

[0046] Comparative Example 1:

[0047] 1. Quenching: Heat 1Cr11Ni2W2MoV steel from room temperature to 1010℃ and keep it warm for 30 minutes. After the end of the heat preservation, air cool (or oil cool) to room temperature.

[0048] 2. Tempering: Heat the quenched 1Cr11Ni2W2MoV steel from room temperature to 700℃, keep it warm for 2h, then take it out of the furnace and air cool it.

[0049] 3. Characterize the microstructure and mechanical properties of 1Cr11Ni2W2MoV steel after quenching and tempering.

[0050] Organization comparison:

[0051] from Figure 3 and 4 It can be seen that the grain size of 1Cr11Ni2W2MoV steel after quenching and tempering is 21μm, and the grain size of some of the steel after cyclic quenching and tempering is refined to 3μm. Figure 5 and 6 It can be seen that coarse carbides still exist in the banded structure of 1Cr11Ni2W2MoV steel after quenching and tempering. After cyclic quenching and tempering, the grains are refined and the grain boundaries become more numerous, and the carbides are also refined.

[0052] Comparison of mechanical properties:

[0053] The cyclic quenching and tempering process significantly improves its impact energy KU2, from 74J of the original quenching and tempering process to 154J.

[0054] Example 2:

[0055] 1. The carbide dissolution temperature in the banded structure of 1Cr11Ni2W2MoV steel is determined to be 950℃ through experiments.

[0056] 2. Solution quenching: Heat 1Cr11Ni2W2MoV steel from room temperature to 1050℃ and keep it warm for 30 minutes. After the insulation is completed, air cool (or oil cool) to room temperature.

[0057] 3. First quenching: Heat the 1Cr11Ni2W2MoV steel after the first quenching from room temperature to 1030℃, and then air cool (or oil cool) to room temperature after the core reaches temperature.

[0058] 4. Second quenching: Heat the 1Cr11Ni2W2MoV steel after secondary quenching from room temperature to 1020℃, and air cool (or oil cool) to room temperature after the core reaches temperature.

[0059] 5. Third quenching: Heat the 1Cr11Ni2W2MoV steel after the second quenching from room temperature to 1010℃, and air cool (or oil cool) to room temperature after the core reaches temperature.

[0060] 6. Tempering: Heat the 1Cr11Ni2W2MoV steel after four cycles of quenching from room temperature to 680℃, keep it at this temperature for 2h, and then take it out of the furnace and air cool it.

[0061] 7. Characterize the microstructure and mechanical properties of 1Cr11Ni2W2MoV steel after cyclic quenching and tempering.

[0062] Comparative Example 2:

[0063] 1. Quenching: Heat 1Cr11Ni2W2MoV steel from room temperature to 1010℃ and keep it warm for 30 minutes. After the end of the heat preservation, air cool (or oil cool) to room temperature.

[0064] 2. Tempering: Heat the quenched 1Cr11Ni2W2MoV steel from room temperature to 680℃, keep it at this temperature for 2 hours, then take it out of the furnace and air cool it.

[0065] 3. Characterize the microstructure and mechanical properties of 1Cr11Ni2W2MoV steel after quenching and tempering.

[0066] Organization comparison:

[0067] from Figure 8 and 9 It can be seen that the grain size of 1Cr11Ni2W2MoV steel after quenching and tempering is 25μm, and the grain size of some of the steel after cyclic quenching and tempering is refined to 4μm. Figure 10 and 11 It can be seen that coarse carbides still exist in the banded structure of 1Cr11Ni2W2MoV steel after quenching and tempering. After cyclic quenching and tempering, the grains are refined and the grain boundaries become more numerous, and the carbides are also refined.

[0068] Comparison of mechanical properties:

[0069] The cyclic quenching and tempering process significantly improves its impact energy KU2, from 72J of the original quenching and tempering process to 122J.

Claims

1. A heat treatment process for eliminating banded structure in steel, characterized by: The process is specifically as follows: Step 1: Solution quenching: The steel is heated from room temperature to a certain temperature, held at that temperature for a period of time, and then rapidly cooled; the certain temperature is the carbide dissolution temperature of the banded structure + (100-150)°C, and the period of time is the time it takes for the carbides in the banded structure to completely dissolve at the solution temperature; the steel is 1Cr11Ni2W2MoV steel; the carbide dissolution temperature is obtained by determining the chemical composition of the banded structure in the steel using a spectrometer, calculating the equilibrium phase composition-temperature diagram based on the chemical composition, and obtaining the carbide dissolution temperature from the obtained diagram; Step 2: Cycle quenching: The first quenching temperature is 10-20℃ below the solution quenching temperature. The core of the steel piece is cooled quickly after it reaches the temperature. Repeat this quenching process until the set number of cycle quenching times is reached. Each time the quenching times are increased, the quenching temperature is reduced by 10-20℃ compared with the previous quenching temperature. The number of cycle quenching times is 2-4 times. Step 3: Tempering: Heat the steel parts after cyclic quenching to 680-700℃, keep them at this temperature for 1-4 hours, and then take them out of the furnace and air cool them.

2. The heat treatment process for eliminating banded structure in steel according to claim 1, characterized in that: In step 1, the rapid cooling is one of air cooling, oil cooling or water cooling.

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