Heat treatment of medium and thick steel plate
Heat treatment of medium and thick steel plate
Problems of Heat Treatment Defects of Medium and Thick Steel Plates and Treatment Methods
With the all-round development and progress of plate technology, people pay more attention to product quality, and manufacturers pay more attention to the requirements of product quality and energy saving. From the analysis of the current development situation in the world, Japan's technology is the most advanced in the processing of heat-treated thick plate products.
Comparing my country with international advanced technology, there are many quality defects in production, which will have a serious negative impact on the quality of medium and heavy plates, leading to a large-scale repair process in the operation, which greatly affects the entire production order , It will also cause the steel plate to be scrapped due to grinding, which will cause a certain amount of economic loss to the enterprise, and will have an adverse impact on the development of the enterprise.
1 Heat treatment process of medium and heavy plate
With the improvement of social productivity, people's requirements for steel quality are gradually increasing. Adding a certain amount of alloying elements in the steel production process, and then implementing heat treatment, can promote the improvement of the mechanical properties of the steel. The heat treatment of medium and heavy plates mainly includes four aspects: normalizing, high temperature, quenching and tempering, and spheroidization.
1.1 Normalizing
Normalizing treatment is also called normalization treatment. The strength of medium and thick plates is usually relatively large, but the toughness is very low, so it should be normalized and then placed in the air to cool, so that fine gold particles will exist after cooling. , After normalizing treatment, high-tech medium and thick plate materials can be obtained.
1.2 High temperature treatment
High temperature treatment is also called tempering treatment. The main function of this treatment is to remove the stress existing in the steel plate. There is a significant difference with normalizing treatment, and the treatment in this respect needs to ensure that the temperature is controlled within the Ac3 point.
1.3 Quenching and tempering treatment
The quenching and tempering treatment process is a combination of quenching and tempering. This treatment process has relatively high requirements for heat treatment. Heat treatment furnace, low temperature tempering furnace and pressure quenching machine are necessary equipment. The martensite structure is formed in the production of medium and heavy plates, which can be heated and tempered at low temperature. This is the entire process of quenching and tempering.
1.4 Spheroidization
The spheroidizing treatment is to form the spheroidized structure after the heating treatment of the medium and thick plate. After the treatment is completed, the medium and thick plate can be used as a tool steel plate, and the overall performance is relatively high.
In addition to the above-mentioned heat treatment processes, in specific production practices, technicians can optimize and integrate multiple aspects, and then form new heat treatment processes, such as normalizing-controlled cooling new process and normalizing-tempering new process. The function of the former is to avoid insufficient strength of the steel plate after the normalizing treatment. After the steel plate is heat treated, the water cooling equipment can be used to realize the cold control treatment, which can prevent the decrease of the strength of the steel plate. In the heat treatment process of boiler pressure vessel steel plate, the normalizing-tempering heat treatment method has the best effect, and when the CrMo steel plate is heat-treated, the normalizing-tempering treatment technology can be used.
2 Defect analysis
2.1 Pill press-in
When the roller brush of the shot blasting machine enters the end of a cycle, it will cause the existence of wear problems. In the processing of steel plates with a small thickness, the shot material will follow the steel plate and will be directly taken out of the shot blasting machine body. If there is no effective cleaning Processing will cause the shots of the plates to be pressed in after the steel plates are stacked.
In addition, due to the shortcomings in the design of the shot blasting machine, the shot material is easy to appear in the position between the steel plate and the roller table during processing. The steel plate presses the shot material to the lower part due to its own weight, and will enter after multiple rolling. To the inside of the steel plate.
2.2 Hearth roll impression
The formation of indentation defects of the furnace bottom roll is due to nodules on the surface of the furnace bottom rolling, and indentations appear on the lower surface of the steel plate with a larger thickness due to the effect of high temperature. The cause of nodulation on the surface of the furnace bottom roller: a certain amount of oxide scale adheres to some positions on the roller surface, and the gradual increase in the furnace load causes the number of oxide scales to increase, and then accumulates at this position, and at the same time, under the soft condition of hot melt There is a high-temperature oxidation reaction under the temperature, and the roll surface nodules will increase.
Under quenching or normalizing conditions, the layered oxide skin bonded on the roller surface is processed by the steel plate rolling, and the layer is increased layer by layer, and the density increases.In this case, the surface of the steel plate with a larger thickness will be compressed under the influence of high temperature. mark. In addition, the furnace bottom has defects such as bulging and deformation, which will also cause nodules.
The source of oxide scale: First, it exists outside the production process, and the other is caused by abnormal conditions in the furnace. The oxide scale that exists outside the production process is not clean shot blasting, shot material residue, etc., because the shot blasting machine design lacks rationality, mechanical cleaning is not thorough, there is shot material on the surface of the steel plate, and there is oxide scale on the steel plate itself.
The reason for the formation of iron oxide scale in the furnace is that the internal environment of the heat treatment is abnormal and the residual oxygen is too large. The reason for the abnormality of the furnace environment is the quality problem of the radiant tube. The furnace can be shut down and checked to determine whether this is the cause.
The heating of the quenching furnace is carried out by mixing gas and combustion air. The combustion in the radiant tube is realized by radiation. The sealing effect of the entire quenching furnace is better, so the existence of internal oxygen is that air enters when the furnace door is opened. To the inside.
Because the furnace is designed with positive pressure, and nitrogen is used for automatic cleaning when the furnace door is opened, the probability of air entering the interior from the furnace door is low; the radiant tube has been processed for a long time at high temperature, and the ceramic The inner tube will have quality problems, or problems such as broken or broken due to excessive use time, resulting in insufficient heat uniformity of the metal appearance and fracture, or it may be because the metal appearance itself has quality problems and burns during work. In the case of wear, the combustion-supporting air will enter the interior from the damaged part.
2.3 Squeeze deformation of edges
The side extrusion deformation is mainly due to the defect of bulging deformation of the bottom roller table of the heat treatment furnace. In the high temperature production process, the lower part of the steel plate contacts the bottom bulge and its own gravity will produce extrusion deformation under the movement of the furnace. The reason for the formation of the furnace bottom roller bulge is that the temperature difference between the furnace bottom roller and the steel plate exceeds the standard, and the thickness of the furnace bottom roller wall is very small, and the strength is relatively low. The high temperature causes it to contact with the steel plate and deform.
3 Control measures
3.1 Shot press-in control
In order to effectively prevent residual shots on the surface of the steel plate after shot blasting and prevent shots from entering the furnace, it is generally necessary to set a scraper structure at the exit position of the shot blasting machine. At this time, a purge can be added at the exit position. The device can better clean up the pellets on the surface;
The injection of shot materials on the lower surface should be based on the following two points. One is that the addition of a scraper in the cleaning room can prevent the presence of shots on the lower surface of the steel plate; the other is that the surface of the conveyor roller in the cleaning room of the shot blasting machine should be equipped with a support ring. The device can effectively reduce the contact area between the steel plate and the roller table, and can prevent the shot material from entering the furnace.
The above-mentioned shot blasting machine modification treatment can effectively prevent the shot intrusion situation, eliminate these existing problems, and realize that the steel plate with a thickness of less than 50mm will not have any shot intrusion problem; the steel plate with a thickness of 50mm or more will be pressed. In general, the injection occurs on both sides of the steel plate. After treatment, the probability of shot intrusion is reduced to less than 1%, which can improve the quality of the steel plate, promote the improvement of shot blasting productivity, and increase economic benefits significantly.
3.2 The bottom roller indentation control
Strengthen the quality management of shot blasting to ensure that there will be no shot residue or press-in on the surface of the steel plate, and the shot blasting effect can reach Sa2.5 level. The furnace is washed and cleaned according to the requirements of the production process. According to the influence of the weight and friction of the surface of the steel plate, the oxide scale can be crushed, and then the oxide scale will be eliminated before entering the furnace to ensure that the quality of the steel plate is qualified, and it can also extend the bottom of the furnace. The service life of the rollers can fundamentally eliminate the indentation of the furnace bottom roller on the lower surface of the steel plate.
After the above treatment, a relatively long-term analysis of the treatment practice of the bottom roller indentation was carried out. When the temperature in the production environment is controlled below 900 ℃, there is basically no bottom roller indentation; the production temperature is above 900 ℃ When the thickness is more than 30mm, the bottom roller indentation will appear, and its depth has been reduced from 0.3~1.0mm to 0.1~0.3mm, and the decline is more obvious.
Through the above treatment, the workload of manual grinding can be effectively reduced, the work efficiency can be improved, and the surface nodules of the furnace bottom roller can be completely eliminated, and it has relatively high quality requirements.
3.3 Edge extrusion control
In order to make the temperature of the furnace bottom roller reach the uniformity standard, one is to appropriately increase the roller table speed. The longer the steel plate contacts the roller surface, the lower its temperature will be, and the temperature difference will increase, which will lead to the appearance of bulging. If the low-speed running time is relatively long, the problem of bending deformation of the roller table will occur, and the increase of the speed can better reduce the temperature, which can avoid the problem of bulging.
The second is to appropriately increase the swing amplitude. When the hearth roll is swinging, one revolution can be determined as the best swing amplitude. At this time, the contact between the roller table and the steel plate can be more even, and the heating of the roller table will become more uniform. By installing the furnace offset control device, in the production of high-temperature steel plates, it should be classified according to the thickness and time: the plate with a thickness of not more than 100mm is normally centered in the furnace; the quenched (normal) fired plate with a thickness of more than 100m is only in the centering time. The side chocks are used to offset the steel plate by 200~300mm to the side.
After the implementation of offset furnace loading control measures, the working surface of the furnace bottom roller can be used alternately, which can better reduce the temperature difference at the position of the furnace bottom roller, so that the heating is more uniform, and thus it can prevent the furnace bottom from appearing. The problem of bulging has promoted the improvement of the entire production level.
After the above treatment measures, the quenching furnace produced 145 sheets of steel sheets with a thickness of 150mm, 2725t, which were put into the furnace 335 times. After the production of steel sheets in the enterprise, the edge quality level is relatively high, and there is no defect of edge extrusion deformation.
4 summary
To sum up, analyze the causes of surface quality defects in the heat treatment of medium and heavy plates, and take necessary treatment measures. After a certain period of test treatment, it is found that the quality of the steel plate has improved significantly, such as the long-term control of the shot intrusion rate at 1% Below, the indentation depth of the furnace bottom roller is maintained at 0.1~0.3mm, the manual grinding amount is small, and the steel plate edge does not have extrusion deformation defects. The overall quality level is very high, which brings higher economic benefits to the enterprise.
Problems of Heat Treatment Defects of Medium and Thick Steel Plates and Treatment Methods
With the all-round development and progress of plate technology, people pay more attention to product quality, and manufacturers pay more attention to the requirements of product quality and energy saving. From the analysis of the current development situation in the world, Japan's technology is the most advanced in the processing of heat-treated thick plate products.
Comparing my country with international advanced technology, there are many quality defects in production, which will have a serious negative impact on the quality of medium and heavy plates, leading to a large-scale repair process in the operation, which greatly affects the entire production order , It will also cause the steel plate to be scrapped due to grinding, which will cause a certain amount of economic loss to the enterprise, and will have an adverse impact on the development of the enterprise.
1 Heat treatment process of medium and heavy plate
With the improvement of social productivity, people's requirements for steel quality are gradually increasing. Adding a certain amount of alloying elements in the steel production process, and then implementing heat treatment, can promote the improvement of the mechanical properties of the steel. The heat treatment of medium and heavy plates mainly includes four aspects: normalizing, high temperature, quenching and tempering, and spheroidization.
1.1 Normalizing
Normalizing treatment is also called normalization treatment. The strength of medium and thick plates is usually relatively large, but the toughness is very low, so it should be normalized and then placed in the air to cool, so that fine gold particles will exist after cooling. , After normalizing treatment, high-tech medium and thick plate materials can be obtained.
1.2 High temperature treatment
High temperature treatment is also called tempering treatment. The main function of this treatment is to remove the stress existing in the steel plate. There is a significant difference with normalizing treatment, and the treatment in this respect needs to ensure that the temperature is controlled within the Ac3 point.
1.3 Quenching and tempering treatment
The quenching and tempering treatment process is a combination of quenching and tempering. This treatment process has relatively high requirements for heat treatment. Heat treatment furnace, low temperature tempering furnace and pressure quenching machine are necessary equipment. The martensite structure is formed in the production of medium and heavy plates, which can be heated and tempered at low temperature. This is the entire process of quenching and tempering.
1.4 Spheroidization
The spheroidizing treatment is to form the spheroidized structure after the heating treatment of the medium and thick plate. After the treatment is completed, the medium and thick plate can be used as a tool steel plate, and the overall performance is relatively high.
In addition to the above-mentioned heat treatment processes, in specific production practices, technicians can optimize and integrate multiple aspects, and then form new heat treatment processes, such as normalizing-controlled cooling new process and normalizing-tempering new process. The function of the former is to avoid insufficient strength of the steel plate after the normalizing treatment. After the steel plate is heat treated, the water cooling equipment can be used to realize the cold control treatment, which can prevent the decrease of the strength of the steel plate. In the heat treatment process of boiler pressure vessel steel plate, the normalizing-tempering heat treatment method has the best effect, and when the CrMo steel plate is heat-treated, the normalizing-tempering treatment technology can be used.
2 Defect analysis
2.1 Pill press-in
When the roller brush of the shot blasting machine enters the end of a cycle, it will cause the existence of wear problems. In the processing of steel plates with a small thickness, the shot material will follow the steel plate and will be directly taken out of the shot blasting machine body. If there is no effective cleaning Processing will cause the shots of the plates to be pressed in after the steel plates are stacked.
In addition, due to the shortcomings in the design of the shot blasting machine, the shot material is easy to appear in the position between the steel plate and the roller table during processing. The steel plate presses the shot material to the lower part due to its own weight, and will enter after multiple rolling. To the inside of the steel plate.
2.2 Hearth roll impression
The formation of indentation defects of the furnace bottom roll is due to nodules on the surface of the furnace bottom rolling, and indentations appear on the lower surface of the steel plate with a larger thickness due to the effect of high temperature. The cause of nodulation on the surface of the furnace bottom roller: a certain amount of oxide scale adheres to some positions on the roller surface, and the gradual increase in the furnace load causes the number of oxide scales to increase, and then accumulates at this position, and at the same time, under the soft condition of hot melt There is a high-temperature oxidation reaction under the temperature, and the roll surface nodules will increase.
Under quenching or normalizing conditions, the layered oxide skin bonded on the roller surface is processed by the steel plate rolling, and the layer is increased layer by layer, and the density increases.In this case, the surface of the steel plate with a larger thickness will be compressed under the influence of high temperature. mark. In addition, the furnace bottom has defects such as bulging and deformation, which will also cause nodules.
The source of oxide scale: First, it exists outside the production process, and the other is caused by abnormal conditions in the furnace. The oxide scale that exists outside the production process is not clean shot blasting, shot material residue, etc., because the shot blasting machine design lacks rationality, mechanical cleaning is not thorough, there is shot material on the surface of the steel plate, and there is oxide scale on the steel plate itself.
The reason for the formation of iron oxide scale in the furnace is that the internal environment of the heat treatment is abnormal and the residual oxygen is too large. The reason for the abnormality of the furnace environment is the quality problem of the radiant tube. The furnace can be shut down and checked to determine whether this is the cause.
The heating of the quenching furnace is carried out by mixing gas and combustion air. The combustion in the radiant tube is realized by radiation. The sealing effect of the entire quenching furnace is better, so the existence of internal oxygen is that air enters when the furnace door is opened. To the inside.
Because the furnace is designed with positive pressure, and nitrogen is used for automatic cleaning when the furnace door is opened, the probability of air entering the interior from the furnace door is low; the radiant tube has been processed for a long time at high temperature, and the ceramic The inner tube will have quality problems, or problems such as broken or broken due to excessive use time, resulting in insufficient heat uniformity of the metal appearance and fracture, or it may be because the metal appearance itself has quality problems and burns during work. In the case of wear, the combustion-supporting air will enter the interior from the damaged part.
2.3 Squeeze deformation of edges
The side extrusion deformation is mainly due to the defect of bulging deformation of the bottom roller table of the heat treatment furnace. In the high temperature production process, the lower part of the steel plate contacts the bottom bulge and its own gravity will produce extrusion deformation under the movement of the furnace. The reason for the formation of the furnace bottom roller bulge is that the temperature difference between the furnace bottom roller and the steel plate exceeds the standard, and the thickness of the furnace bottom roller wall is very small, and the strength is relatively low. The high temperature causes it to contact with the steel plate and deform.
3 Control measures
3.1 Shot press-in control
In order to effectively prevent residual shots on the surface of the steel plate after shot blasting and prevent shots from entering the furnace, it is generally necessary to set a scraper structure at the exit position of the shot blasting machine. At this time, a purge can be added at the exit position. The device can better clean up the pellets on the surface;
The injection of shot materials on the lower surface should be based on the following two points. One is that the addition of a scraper in the cleaning room can prevent the presence of shots on the lower surface of the steel plate; the other is that the surface of the conveyor roller in the cleaning room of the shot blasting machine should be equipped with a support ring. The device can effectively reduce the contact area between the steel plate and the roller table, and can prevent the shot material from entering the furnace.
The above-mentioned shot blasting machine modification treatment can effectively prevent the shot intrusion situation, eliminate these existing problems, and realize that the steel plate with a thickness of less than 50mm will not have any shot intrusion problem; the steel plate with a thickness of 50mm or more will be pressed. In general, the injection occurs on both sides of the steel plate. After treatment, the probability of shot intrusion is reduced to less than 1%, which can improve the quality of the steel plate, promote the improvement of shot blasting productivity, and increase economic benefits significantly.
3.2 The bottom roller indentation control
Strengthen the quality management of shot blasting to ensure that there will be no shot residue or press-in on the surface of the steel plate, and the shot blasting effect can reach Sa2.5 level. The furnace is washed and cleaned according to the requirements of the production process. According to the influence of the weight and friction of the surface of the steel plate, the oxide scale can be crushed, and then the oxide scale will be eliminated before entering the furnace to ensure that the quality of the steel plate is qualified, and it can also extend the bottom of the furnace. The service life of the rollers can fundamentally eliminate the indentation of the furnace bottom roller on the lower surface of the steel plate.
After the above treatment, a relatively long-term analysis of the treatment practice of the bottom roller indentation was carried out. When the temperature in the production environment is controlled below 900 ℃, there is basically no bottom roller indentation; the production temperature is above 900 ℃ When the thickness is more than 30mm, the bottom roller indentation will appear, and its depth has been reduced from 0.3~1.0mm to 0.1~0.3mm, and the decline is more obvious.
Through the above treatment, the workload of manual grinding can be effectively reduced, the work efficiency can be improved, and the surface nodules of the furnace bottom roller can be completely eliminated, and it has relatively high quality requirements.
3.3 Edge extrusion control
In order to make the temperature of the furnace bottom roller reach the uniformity standard, one is to appropriately increase the roller table speed. The longer the steel plate contacts the roller surface, the lower its temperature will be, and the temperature difference will increase, which will lead to the appearance of bulging. If the low-speed running time is relatively long, the problem of bending deformation of the roller table will occur, and the increase of the speed can better reduce the temperature, which can avoid the problem of bulging.
The second is to appropriately increase the swing amplitude. When the hearth roll is swinging, one revolution can be determined as the best swing amplitude. At this time, the contact between the roller table and the steel plate can be more even, and the heating of the roller table will become more uniform. By installing the furnace offset control device, in the production of high-temperature steel plates, it should be classified according to the thickness and time: the plate with a thickness of not more than 100mm is normally centered in the furnace; the quenched (normal) fired plate with a thickness of more than 100m is only in the centering time. The side chocks are used to offset the steel plate by 200~300mm to the side.
After the implementation of offset furnace loading control measures, the working surface of the furnace bottom roller can be used alternately, which can better reduce the temperature difference at the position of the furnace bottom roller, so that the heating is more uniform, and thus it can prevent the furnace bottom from appearing. The problem of bulging has promoted the improvement of the entire production level.
After the above treatment measures, the quenching furnace produced 145 sheets of steel sheets with a thickness of 150mm, 2725t, which were put into the furnace 335 times. After the production of steel sheets in the enterprise, the edge quality level is relatively high, and there is no defect of edge extrusion deformation.
4 summary
To sum up, analyze the causes of surface quality defects in the heat treatment of medium and heavy plates, and take necessary treatment measures. After a certain period of test treatment, it is found that the quality of the steel plate has improved significantly, such as the long-term control of the shot intrusion rate at 1% Below, the indentation depth of the furnace bottom roller is maintained at 0.1~0.3mm, the manual grinding amount is small, and the steel plate edge does not have extrusion deformation defects. The overall quality level is very high, which brings higher economic benefits to the enterprise.
