With the development and progress of medium thick steel plate technology, people pay more attention to the product quality, and the production enterprises pay more attention to the requirements of product quality and energy saving. From the current development situation in the world, the heat treatment thick plate products in the treatment, Japan's technology is the most advanced.
Compared with the advanced technology in China and the world, there are many quality defects in production, which have a serious negative impact on the quality of medium thick steel plates, which leads to the repair and treatment in a large range in the operation, which greatly affects the whole production order, and also causes the steel plate to be scrapped due to the repair and grinding, resulting in a certain amount of economic loss of the enterprise and adverse impact on the development of the enterprise.
With the improvement of social productivity, people have gradually improved the quality of steel. The mechanical properties of steel can be improved by adding a certain amount of alloy elements to the steel production process and then heat treatment. The heat treatment of medium thick steel plates is mainly in four aspects: normalizing, high temperature, tempering, spheroidization.
1.1 normalizing treatment
Normalizing treatment is also called normalization treatment. The strength of medium thick steel plates is usually large, but the toughness is very low. Therefore, normalizing treatment should be carried out, and then placed in air for cooling. Thus, fine gold particles will exist after cooling, and high-tech plate materials can be obtained after normalization treatment.
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 inside the steel plate. There is a significant difference between the treatment and the normalizing treatment, which requires that the temperature control is within AC3.
1.3 conditioning and quality control
The quenching and tempering process is a combination of quenching and tempering. The requirements of this process for heat treatment are relatively high. Heat treatment furnace, low temperature tempering furnace and pressure quenching machine are the necessary equipment. Martensite structure can be formed in the production of medium thick steel plates. It can be heated and tempered at low temperature, which is the whole process of tempering treatment.
1.4 spheroidization treatment
Spheroidization is to form spheroidized structure after heating treatment of medium thick steel plates. After the treatment, the plate can be used as tool steel plate, and its comprehensive performance is relatively high.
In addition to the above mentioned heat treatment processes, in the specific production practice, technicians can optimize and integrate many aspects, and then form new heat treatment processes, such as the new normalizing cooling control process and the normalizing tempering new process. The former is to avoid the insufficient strength of steel plate after normalizing treatment. After the heat treatment of steel plate, the treatment with water-cooled equipment can realize cold control treatment, and can prevent the steel plate strength from falling. The heat treatment of boiler pressure vessel steel plate is the best by normalizing tempering heat treatment. While the normalizing tempering treatment technology can be used for CrMo steel plate.
2. defect analysis
2.1 shot press in
When the roller brush of shot blasting machine enters the end of a cycle, it will lead to the existence of wear. In the treatment of smaller steel plate, the shot material will follow the steel plate directly out of the shot blasting machine body. If there is no effective cleaning treatment, the shot material of the sheet material will be pressed after the stacking of the steel plate.
In addition, due to the lack of shot blasting machine design, shot material is easy to appear in the position between steel plate and roller table in the process of processing. Because of the effect of self weight, the steel plate presses the shot material into the lower part, and enters into the steel plate after several times of rolling.
2.2 indentation of bottom roller
The formation of the defects of the bottom roller indentation is that the bottom rolling surface is nodular, and the larger thickness of the steel plate appears indentation due to the high temperature on the lower surface. Causes of the surface nodulation of the bottom roller: there is a certain amount of oxide iron on some positions of the roller surface, and the increasing of the charging quantity causes the increase of the oxide scale, and then the accumulation in this position. At the same time, there is high temperature oxidation reaction under the soft condition of hot melt, and the roll surface nodule will increase.
Under quenching or normalizing conditions, the layer oxide skin bonded on the roller surface is rolled by steel plate, and the layer by layer increases, and the density increases. In this case, the surface of the steel plate with larger thickness appears indentation under the influence of high temperature. In addition, the defects of drum and deformation at the bottom of the furnace will also have the problem of nodule.
Source of iron oxide: one is outside the production link, the other is the abnormal situation in the furnace. The oxide skin outside the production link is the unclean shot blasting and the residual of shot material, because the design of shot blasting machine is unreasonable, the mechanical cleaning is not complete, the surface of steel plate has shot material, and there is oxide skin on the steel plate itself.
The reason for the formation of iron oxide sheet in furnace is that the internal environment of heat treatment is abnormal and the residual oxygen is too large. The reason for the abnormal environment in the furnace is the quality problem of radiation tube. It can be checked by shutdown to determine whether it is caused by this reason.
The furnace heating is carried out after mixing gas and combustion supporting air. The combustion in the radiation tube is realized by radiation. The sealing effect of the whole furnace is good. Therefore, the existence of internal oxygen is that air enters the inside when the furnace door is open.
Because the furnace is designed as positive pressure and the furnace door is opened, nitrogen is used for automatic cleaning, so the probability of air entering into the furnace door is low; after the radiation pipe is treated for a long time with high temperature, the quality problems of the ceramic inner pipe, or the problems such as breakage and fracture due to the long service time, are caused, The reason for the metal appearance to be broken due to the lack of uniformity of heating may be due to the quality problems of the metal appearance itself. The burning through situation occurs in the work, which leads to the combustion air entering the inside from the damaged part.
2.3 extrusion deformation of side
The deformation of the side extrusion is mainly due to the defects of the drum deformation in the roller table at the bottom of the heat treatment furnace. In the high temperature production process, the lower part of the steel plate contacts the drum at the bottom of the furnace and its gravity action will produce extrusion deformation under the movement in the furnace. The reason for the formation of drum drum is that the temperature difference between bottom roller and steel plate exceeds the standard, and the thickness of bottom roller wall is very small and the strength is relatively low. It is deformed when it contacts the steel plate due to the high temperature.
3. control measures
3.1 control of shot material pressing
In order to effectively prevent the residual shot material on the surface of steel plate after shot blasting and prevent the shot material entering the furnace, generally, the scraper structure should be set at the outlet position of the shot blasting machine. At this time, the blowing device can be added at the outlet position to better clean the surface shot material;
The following two points should be taken for pressing the shot material on the lower surface. One is that adding scraper in the cleaning room can prevent the existence of shot material on the lower surface of the steel plate; the second is to set up a supporting ring device on the surface of the conveying roller in the cleaning room of the shot blasting machine, which can effectively reduce the contact area between the steel plate and the roller, and prevent the pellet from entering the furnace.
The above-mentioned shot blasting machine transformation can effectively prevent the pressing of shot material, eliminate these problems, and realize that the steel plate with thickness less than 50mm will not have any problem of shot pressing; the pressing of steel plate with thickness of more than 50mm generally appears on both sides of the steel plate. After treatment, the probability of shot pressing is reduced to less than 1%, which can improve the quality of steel plate, The productivity of shot blasting is promoted and the economic benefit is obviously improved.
3.2 control of the indentation of furnace bottom roller
Strengthen the quality management of shot blasting to ensure that there is no residual and pressing of shot material on the surface of steel plate. The shot blasting effect reaches Sa2.5. According to the requirements of production process, the furnace cleaning can be carried out. According to the influence of the surface weight and friction force of the steel plate, the oxide skin can be crushed, and then the oxide skin will be removed before entering the furnace, so as to ensure the steel plate quality is qualified, and the service life of the bottom roller can be prolonged, and the indentation of the bottom roller can be eliminated from the root.
After the above treatment, the treatment practice analysis of the indentation of the bottom roller is carried out for a long time. Under the condition that the temperature in the production environment is controlled below 900 ℃, there is no bottom roller indentation. When the production temperature is above 900 ℃, and the thickness is more than 30mm, the indentation of the bottom roller will be found, and the depth has been reduced from 0.3 to 1.0mm to 0.1-0.3mm, and the decrease is obvious.
After the above treatment, the workload of manual grinding can be effectively reduced, the work efficiency is improved, and the surface nodular defects of bottom roller can be eliminated completely, which has high quality requirements.
3.3 control of extrusion deformation at the side
To make the temperature of furnace bottom roll reach the uniformity standard, one is to raise the roller speed properly. The longer the steel plate contacts the roller surface, the lower the temperature will be, the temperature difference will increase, which will lead to the appearance of drum. If the low speed operation time is longer, the problem of roller table bending deformation will appear, and the speed can be improved to lower the temperature better, thus avoiding the problem of drum drum.
Second, increase the swing amplitude appropriately. When the bottom roller rotates for one week, it can be determined as the best swing amplitude. At this time, the contact between the roller and the steel plate can be more uniform, and the heating of the roller table will become more uniform. Through installing the furnace offset control device, in the production of high temperature steel plate, it should be classified according to the thickness and time: the plate with thickness no more than 100mm is normally installed in the furnace; the quenched (positive) fire plate with thickness greater than 100m shall be padded on one side of wood block when aligning, so that the steel plate will be offset 200-300mm to the side.
Through the implementation of the control measures of offset charging, the working surface of the bottom roller can be used alternately, and the temperature difference on the bottom roller position can be reduced better, and the heating is more uniform, and then the problem of drum drum of bottom roller can be prevented and the whole production level can be promoted.
After the above treatment measures, the quenching furnace produces 145 pieces of 2725t steel plates with thickness of 150 mm, and has been put into the furnace 335 times. After the production of steel plates in the enterprise, the quality level of the side is relatively high, and there is no defect of side extrusion deformation.
In conclusion, the causes of surface quality defects of medium thick steel plate heat treatment are analyzed, and necessary treatment measures are taken. After some time of test, it is found that the quality improvement of steel plate is obvious, such as the pressing rate of shot material is controlled below 1% for a long time, the depth of the bottom roller indentation is kept at 0.1-0.3mm, the manual grinding quantity is small, and the edge of steel plate has no extrusion deformation defects, The overall quality level is very high, which brings high economic benefits to the enterprise.
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