A thermal simulation method for obtaining a set cooling rate from a static CCT experiment

Abstract

The present invention discloses a thermal simulation method for obtaining a set cooling rate in a static CCT test. The method is to conduct a static CCT test on an MMS-200 testing machine. During the heating, holding, and cooling stages of the sample, the instrument's own cooling system and pneumatic system cooperate to manually adjust the gear position of a movable axis cylinder. This method effectively solves the problems of increased expansion of the sample due to bulging, and inconsistency between the actual cooling rate and the set cooling rate due to thermal expansion and contraction. The method effectively avoids the sample from falling during the heating process; avoids the phenomenon of bulging in the middle of the sample due to excessively high heating temperature or excessively long holding time, which leads to increased expansion and misjudgment of the steel type undergoing phase change at this temperature; and avoids the situation of the sample from generating a gap with a fixture due to volume reduction during the cooling process, resulting in an actual cooling rate equivalent to an air-cooled state that is faster or slower than the set cooling rate and misjudging the phase change temperature.

Description

Technical Field

[0001] The present invention belongs to the technical field of physical simulation tests for thermal processing of metal materials, and relates to a method for obtaining consistency between an actual cooling rate of a sample and a set cooling rate during a cooling process when a static CCT test is performed on a thermal simulation MMS-200 testing machine. Background Art

[0002] The continuous cooling transformation curve (CCT) of steel is often referred to as a curve. The CCT curve of the steel being processed is a crucial reference when developing new steel grades, optimizing rolling processes, determining post-rolling cooling schedules, and developing heat treatment processes. CCT curves are obtained through CCT testing.

[0003] CCT tests are categorized into static and dynamic. Static CCT testing involves heating, holding, and cooling a specimen using a thermal simulator, while dynamic CCT testing involves heating, holding, deforming, and cooling the specimen using a thermal simulator. Using static CCT testing to study the phase transformation behavior of austenite during continuous cooling is a fundamental aspect of developing new steel grades. During heating or cooling, the high-temperature microstructure, austenite, and its transformation products—ferrite, pearlite, bainite, and martensite—exhibit volume effects due to their varying specific volumes. Using a highly sensitive radial sensor (C-Srain sensor), the change in specimen diameter (hereinafter referred to as "expansion") is measured to determine the corresponding phase transformation start and end temperatures. This allows for the creation of a static CCT diagram, providing an important basis for developing rolling and heat treatment processes.

[0004] Chinese steel companies currently use the MMS-200 thermal simulation tester, manufactured by Northeastern University, as their testing equipment. In practice, after the cooling phase, the specimen expands along the diameter due to volume effects caused by phase transformation. This expansion-temperature curve allows for the determination of the phase transformation start and end temperatures. However, along the longitudinal direction, due to thermal expansion and contraction, the rapid cooling causes a volume reduction that creates a gap between the specimen and the fixture, resulting in an actual cooling rate equivalent to air cooling. This is inconsistent with the set cooling rate, making the measured phase transformation start and end temperatures inaccurate, making the CCT diagram difficult to draw.

[0005] Therefore, it is urgent to explore a method that can ensure that the sample fits tightly with the fixture during the cooling process, and the actual cooling rate obtained is consistent with the set cooling rate, so as to draw the correct CCT diagram for subsequent research. Summary of the Invention

[0006] The present invention provides a thermal simulation method for obtaining a set cooling rate during a static CCT test. When a specimen undergoes a static CCT test on an MMS-200 thermal simulation testing machine, the actual cooling rate during the cooling phase is consistent with the set cooling rate, enabling accurate determination of the phase transition start and end temperatures and the creation of a CCT diagram.

[0007] The technical solution of the present invention:

[0008] A thermal simulation method for obtaining a set cooling rate in a static CCT test comprises the following steps:

[0009] (1) The sample is processed into a cylindrical sample with a specification of Φ8×15mm, and the surface roughness of the sample is Ra≤3.2um;

[0010] (2) Use sandpaper to polish the middle of the sample to make it metallic, and weld the positive and negative thermocouple wires to the 1 / 2 position of the sample with a spot welder; select the appropriate thermocouple according to the peak temperature. When the peak temperature is ≤1250℃, use nickel-chromium-nickel-aluminum thermocouple, and the thermocouple type is TK1; when the peak temperature is greater than 1250℃, use platinum-rhodium-platinum thermocouple, and the thermocouple type is TK4;

[0011] (3) Start the air compressor, adjust the gear of the cylinder of the fixed shaft to the "compression" gear, press the button to move the fixed shaft to the compressed limit position, and use the movable shaft cylinder to adjust the position of the movable shaft so that the distance between the left and right end clamps is slightly smaller than the length of the sample; adjust the gear of the cylinder of the movable shaft to the "compression" gear, press the button so that both ends of the sample can be firmly clamped by the clamps; after the sample is installed, firmly clamp the C-Srain sensor in the middle of the sample in the diameter direction, so that the quartz rod of the sensor and the thermocouple wire welding position of the sample round rod are in the same cross section, so as to measure the expansion of the sample;

[0012] (4) Setting the test conditions: heat the sample to 850-1000°C at a rate of 10°C / s, keep it at that temperature for 300s, and then cool it to room temperature at a cooling rate of 0.05-20°C / s;

[0013] (5) Start the main system, turn on the circulating water pump and vacuum pump, evacuate the system for one or two minutes, then turn off the vacuum pump, fill with protective gas argon, and start the test;

[0014] (6) During the experiment, the expansion-temperature data of the sample during the heating and cooling process are collected: when the test enters the heating stage, the cylinder gear of the movable axis is adjusted to the "compression" gear; when the test enters the heat preservation stage, the cylinder gear of the movable axis is adjusted from the "compression" gear to the "no force" gear; when the test enters the cooling stage, the cylinder gear of the movable axis is adjusted from the "no force" gear to the "compression" gear;

[0015] (7) The phase transition temperature of the collected expansion-temperature curve is measured, and the metallographic observation of the thermal simulation sample is performed, and the static CCT diagram is drawn by combining the two.

[0016] Beneficial effects of the present invention: The present invention conducts a static CCT test on an MMS-200 testing machine. During the heating stage, insulation stage, and cooling stage of the sample, the instrument's own cooling system and pneumatic system cooperate, and the gear position of the movable axis cylinder is manually adjusted to effectively solve the problem of increased expansion of the sample due to bulging, and the inconsistency between the actual cooling rate and the set cooling rate due to thermal expansion and contraction. This is different from the current stage of relying solely on a computer to input parameters through the system operation interface to collect the expansion-temperature curve. Compared with the existing technology, the present invention can effectively prevent the sample from falling during the heating process; avoid the sample from bulging in the middle due to excessively high heating temperature or too long insulation time, resulting in increased expansion, and misjudging the steel type that has undergone phase change at this temperature; avoid the sample from generating a gap with the fixture due to volume reduction during the cooling process, resulting in an actual cooling rate that is faster or slower than the set cooling rate in the air cooling state, and misjudging the phase change temperature. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is the expansion-temperature diagram of the sample during the heating stage.

[0018] Figure 2 This is the expansion-temperature diagram of the sample during the cooling stage.

[0019] Figure 3 The metallographic structures of the samples at different cooling rates.

[0020] Figure 4 This is the static CCT diagram of the sample. DETAILED DESCRIPTION

[0021] The following is further described with reference to the embodiments. Example 1

[0022] A static CCT experiment was conducted using the MMS-200 thermal simulation testing machine to study the austenite phase transformation behavior of Q550D steel during the continuous cooling process. This experiment used the MMS-200 thermal simulation testing machine produced by Northeastern University. The main structure is mainly composed of two crossbeams, a main hydraulic cylinder, a hydraulic motor, a movable shaft, a fixed shaft, an operating box, a positioning beam and other components. Among them, a movable shaft and a fixed shaft are installed on both sides of the operating box. The movable shaft is connected to a tensile sensor and a fixture. The fixed shaft is connected to another fixture through a screw, and the sample is installed between the two fixtures. After the sample is installed, the C-Srain sensor is firmly clamped in the middle of the sample along the diameter direction. During the test, the expansion-temperature curve of the sample is collected by the sensor. The process steps are as follows:

[0023] (1) The sample is processed into a cylindrical sample with a specification of Φ8×15mm, and the surface roughness of the sample is Ra≤3.2um.

[0024] (2) Use sandpaper to polish the middle of the sample to make it present a metallic luster. Use a spot welder to weld the nickel-chromium-nickel-aluminum thermocouple wire to the 1 / 2 position of the sample, and connect the positive and negative thermocouple wires to the positive and negative poles of the TK1 thermocouple.

[0025] (3) Start the air compressor, adjust the gear of the cylinder of the fixed shaft to the "compression" gear, press the button to move the fixed shaft to the compressed limit, and use the movable shaft cylinder to adjust the position of the movable shaft so that the distance between the left and right end clamps is slightly smaller than the length of the sample; adjust the gear of the cylinder of the movable shaft to the "compression" gear, press the button to allow both ends of the sample to be firmly clamped by the clamps; after the sample is installed, firmly clamp the C-Srain sensor in the middle of the sample along the diameter direction, so that the quartz rod of the sensor and the thermocouple wire welding position of the sample round rod are in the same cross section, so as to measure the expansion of the sample.

[0026] (4) The specific test process is as follows: heat the sample to 930℃ at a rate of 10℃ / s, keep it at that temperature for 300s, and then cool it to room temperature at cooling rates of 0.2℃ / s, 0.5℃ / s, 1℃ / s, 2℃ / s, 5℃ / s, 10℃ / s, 15℃ / s, and 20℃ / s, respectively.

[0027] (5) Start the main system, turn on the circulating water pump and vacuum pump, evacuate the system for one or two minutes, then turn off the vacuum pump, fill in the protective gas argon, and start the test.

[0028] (6) Click the "Heating" button and the "Program Run" button to start the experiment. When the sample is heating, adjust the cylinder gear of the movable axis to the "Compression" gear; when the sample is keeping warm, adjust the cylinder gear of the movable axis from the "Compression" gear to the "No Force" gear; when the sample is cooling, adjust the cylinder gear of the movable axis from the "No Force" gear to the "Compression" gear.

[0029] During the specimen cooling phase, observe the "Actual Temperature" and "Set Temperature" lines on the PLC cabinet. If the two lines decrease at the same rate and overlap, the actual cooling rate is consistent with the set cooling rate. If the actual temperature line decreases faster than the set temperature line, the actual cooling rate is faster than the set cooling rate (slow cooling). If the actual temperature line decreases slower than the set temperature line, the actual cooling rate is slower than the set cooling rate (fast cooling). The latter two conditions are caused by shrinkage, resulting in a loose fit between the specimen and the fixture. Manually adjust the cylinder stop on the movable axis from the "no force" position to the "compression" position to ensure a tight fit between the specimen and the fixture, thus achieving two overlapping temperature lines.

[0030] (7) The thermally simulated specimen was split in the middle, inlaid, ground, and polished, and then corroded with 3% nitric acid. Finally, the thermally simulated specimen after continuous cooling was observed using a LEICA DMi8 metallographic microscope. Combined with the expansion-temperature data collected in the experiment, the tangent method was used to find the starting and ending points of the phase change at different cooling rates, and the temperature-time coordinates of these points were recorded. Based on these coordinates and metallographic structure analysis, the static CCT diagram was drawn using Origin software. Example 1

[0031] The static CCT experiment was carried out using the MMS-200 thermal simulation test machine to study the austenite phase transformation behavior of XG2311 steel during the continuous cooling process. This experiment used the MMS-200 thermal simulation test machine produced by Northeastern University. The main structure is mainly composed of two crossbeams, a main hydraulic cylinder, a hydraulic motor, a movable shaft, a fixed shaft, an operating box, a positioning beam and other components. Among them, a movable shaft and a fixed shaft are installed on both sides of the operating box. The movable shaft is connected to a tensile sensor and a fixture. The fixed shaft is connected to another fixture through a screw, and the sample is installed between the two fixtures. After the sample is installed, the C-Srain sensor is firmly clamped in the middle position of the sample along the diameter direction. During the test, the expansion-temperature curve of the sample is collected by the sensor. The process steps are as follows:

[0032] (1) The sample is processed into a cylindrical sample with a specification of Φ8×15mm, and the surface roughness of the sample is Ra≤3.2um.

[0033] (2) Use sandpaper to polish the middle of the sample to make it present a metallic luster. Use a spot welder to weld the nickel-chromium-nickel-aluminum thermocouple wire to the 1 / 2 position of the sample, and connect the positive and negative thermocouple wires to the positive and negative poles of the TK1 thermocouple.

[0034] (3) Start the air compressor, adjust the gear of the cylinder of the fixed shaft to the "compression" gear, press the button to move the fixed shaft to the compressed limit, and use the movable shaft cylinder to adjust the position of the movable shaft so that the distance between the left and right end clamps is slightly smaller than the length of the sample; adjust the gear of the cylinder of the movable shaft to the "compression" gear, press the button to allow both ends of the sample to be firmly clamped by the clamps; after the sample is installed, firmly clamp the C-Srain sensor in the middle of the sample along the diameter direction, so that the quartz rod of the sensor and the thermocouple wire welding position of the sample round rod are in the same cross section, so as to measure the expansion of the sample.

[0035] (4) The specific test process is as follows: heat the sample to 880℃ at a rate of 10℃ / s, keep it at that temperature for 300s, and then cool it to room temperature at cooling rates of 0.05℃ / s, 0.1℃ / s, 0.2℃ / s, 0.5℃ / s, 1℃ / s, 5℃ / s, 10℃ / s, and 20℃ / s, respectively.

[0036] (5) Start the main system, turn on the circulating water pump and vacuum pump, evacuate the system for one or two minutes, then turn off the vacuum pump, fill in the protective gas argon, and start the test.

[0037] (6) Click the "Heating" button and the "Program Run" button to start the experiment. When the sample is heating, adjust the cylinder gear of the movable axis to the "Compression" gear; when the sample is keeping warm, adjust the cylinder gear of the movable axis from the "Compression" gear to the "No Force" gear; when the sample is cooling, adjust the cylinder gear of the movable axis from the "No Force" gear to the "Compression" gear.

[0038] During the specimen cooling phase, observe the "Actual Temperature" and "Set Temperature" lines on the PLC cabinet. If the two lines decrease at the same rate and overlap, the actual cooling rate is consistent with the set cooling rate. If the actual temperature line decreases faster than the set temperature line, the actual cooling rate is faster than the set cooling rate (slow cooling). If the actual temperature line decreases slower than the set temperature line, the actual cooling rate is slower than the set cooling rate (fast cooling). The latter two conditions are caused by shrinkage, resulting in a loose fit between the specimen and the fixture. Manually adjust the cylinder stop on the movable axis from the "no force" position to the "compression" position to ensure a tight fit between the specimen and the fixture, thus achieving two overlapping temperature lines.

[0039] (7) The thermally simulated specimen was split in the middle, inlaid, ground, and polished, and then corroded with 3% nitric acid. Finally, the thermally simulated specimen after continuous cooling was observed using a LEICA DMi8 metallographic microscope. Combined with the expansion-temperature data collected in the experiment, the tangent method was used to find the starting and ending points of the phase change at different cooling rates, and the temperature-time coordinates of these points were recorded. Based on these coordinates and metallographic structure analysis, the static CCT diagram was drawn using Origin software.

[0040] Figure 1 Display: Critical point temperature of Q550D steel during heating measured by expansion method A c1 =717℃, A c3 =861℃.

[0041] Figure 2 Display: Phase transformation starting temperature of Q550D steel at 0.5℃ / s cooling rate measured by dilatation method T 1=767℃, phase transition end temperature T 2=671℃.

[0042] Figure 3It shows that when the cooling rate of Q550D steel is 0.2-0.5℃ / s, the metallographic structure obtained is ferrite + pearlite; when the cooling rate is 1-2℃ / s, the structure obtained is ferrite + pearlite + bainite; when the cooling rate is 10-20℃ / s, the structure obtained is bainite + ferrite.

[0043] Figure 4 It shows that when the cooling rate of Q550D steel is 0.2-0.5℃ / s, the phase transformation temperature range is 770-671℃; when the cooling rate is 1-2℃ / s, the phase transformation temperature range is 753-635℃; when the cooling rate is 10-20℃ / s, the phase transformation temperature range is 722-535℃.

Claims

1. A thermal simulation method for obtaining a set cooling rate by a static CCT test, characterized in that The following steps are involved: (1) The sample is processed into a cylindrical sample with a specification of Φ8×15mm, and the surface roughness of the sample is Ra≤3.2um; (2) Use sandpaper to polish the middle of the sample to make it metallic, and weld the positive and negative thermocouple wires to the 1 / 2 position of the sample with a spot welder; select the appropriate thermocouple according to the peak temperature. When the peak temperature is ≤1250℃, use nickel-chromium-nickel-aluminum thermocouple, and the thermocouple type is TK1; when the peak temperature is greater than 1250℃, use platinum-rhodium-platinum thermocouple, and the thermocouple type is TK4; (3) Start the air compressor, adjust the gear of the cylinder of the fixed shaft to the "compression" gear, press the button to move the fixed shaft to the compressed limit position, and use the movable shaft cylinder to adjust the position of the movable shaft so that the distance between the left and right end clamps is slightly smaller than the length of the sample; adjust the gear of the cylinder of the movable shaft to the "compression" gear, press the button to ensure that both ends of the sample can be firmly clamped by the clamps; after the sample is installed, firmly clamp the C-Srain sensor in the middle of the sample in the diameter direction, so that the quartz rod of the sensor and the thermocouple wire welding position of the sample round rod are in the same cross section, so as to measure the expansion of the sample; (4) Setting the test conditions: heat the sample to 850-1000°C at a rate of 10°C / s, keep it at that temperature for 300s, and then cool it to room temperature at a cooling rate of 0.05-20°C / s; (5) Start the main system, turn on the circulating water pump and vacuum pump, evacuate the system for one or two minutes, then turn off the vacuum pump, fill with protective gas argon, and start the test; (6) During the experiment, the expansion-temperature data of the sample during the heating and cooling process are collected: when the test enters the heating stage, the gear position of the moving axis cylinder is adjusted to the "compression" gear; When the test enters the heat preservation stage, the gear position of the movable axis cylinder is adjusted from the "compression" gear position to the "no force" gear position; when the test enters the cooling stage, the gear position of the movable axis cylinder is adjusted from the "no force" gear position to the "compression" gear position; (7) The phase transition temperature of the collected expansion-temperature curve is measured, and the metallographic observation of the thermal simulation sample is performed, and the static CCT diagram is drawn by combining the two.

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