Medium and thick plate Basic Knowledge Questions and Answers (1)
Section 1 Quality control and heating of medium thick slab material
one What are the advantages and disadvantages of using raw materials for blooming?
Advantages: The fledgling billet is made of large steel ingot rolled by the blooming mill, so its structure is relatively dense, there is no as-cast structure stress, and it can be heated at a larger heating speed than the ingot; Because the bloom has been compressed, there is no need to be limited by the compression ratio, so the thickness of the raw material can be as small as possible when the bloom is used as the raw material. Therefore, the medium and thick plate mill using blooming billet as raw material has high yield and good quality.
Disadvantages: blooming mill investment is large and energy consumption is high, and the comprehensive yield from ingot to steel plate is low.
2. What are the advantages and limitations of using continuous casting billet as raw material?
Advantages: The internal structure of continuous casting billet is denser than that of flat ingot, and the thickness size is smaller than that of flat ingot, which can increase the yield and improve the yield.
Restriction: Under the influence of continuous casting equipment, the width size and thickness size of continuous casting billet can not be too much, thus reducing the production efficiency of rolling mill and heating furnace. Due to the limitation of compression ratio, the thickness of thick plate using continuous casting billet as raw material cannot be too large.
3. What is the principle of raw material selection for plate mill?
(1) The thickness of the raw material is as small as possible, and the small thickness is conducive to the improvement of the efficiency of the rolling mill and the heating furnace. In order to ensure the performance of the steel plate, the thickness of the raw material should meet the requirements of the compression ratio of the steel plate, and the compression ratio of the raw material is different.
(2) The width and size of raw materials should be as large as possible. The large width of the raw material is conducive to the operation of the mill. In order to meet the requirements of the billet feed operation during cross rolling, each mill has a minimum feedstock width limit, less than this width of feedstock cannot be fed into the mill during cross rolling.
(3) The length of the raw material is as close as possible to the maximum allowable length of the raw material. The heating capacity of the heating furnace is affected by the raw materials. When the length of raw material is within the allowable loading length of the heating furnace, the longer the raw material is, the larger the production capacity of the heating furnace is and the smaller the unit fuel consumption is.
(4) The length of the raw material must also meet the various restrictions of the rolling mill equipment and the heating furnace, such as the allowable thickness of the heating furnace, the maximum allowable length of the rolling mill, and the convenience of production operation.
4. What are the common defects of medium and thick plate raw materials?
The surface of a flat ingot, bloom billet or continuous casting billet used in the production of medium thick steel plate may have some defects due to the influence of the casting process operation or the operating conditions of the billet process. The common defects of flat steel ingot are transverse crack, longitudinal crack, scar, surface porosity, subcutaneous bubble, shrinkage hole, heavy skin, etc. The common defects in the bloom are ear, folding, crack, pull, scar, hairline, bubble exposure, overburning, surface inclusion, scratches, etc. The common defects of continuous casting billet include surface longitudinal crack, surface transverse crack, star crack, subskin porosity and inclusion, bulging belly, internal crack, center segregation and center porosity, nonmetallic inclusion, vibration ripple mark, scratch and so on.
5. How to clean up the defects of medium and thick plate raw materials?
The defects existing on the surface of the raw material, in addition to some relatively minor defects because they are oxidized and burned during the heating process will not affect the quality of the steel plate and do not need to be cleaned up, the defects exceeding a certain size need to be removed by some cleaning method, so as not to affect the surface quality of the steel plate or cause waste. Therefore, the surface cleaning of raw materials can not only improve the surface quality of steel plates, but also reduce the waste caused by surface defects of steel plates, which has a certain significance for saving raw materials and reducing costs, and should pay attention to this process. There are many methods of surface cleaning, common flame cleaning, wind shovel cleaning, grinding wheel grinding, machine tool processing, arc cleaning and so on. When selecting the cleaning method, the steel grade of the raw material, the specific situation of heating and rolling, and the requirements for the surface quality of the finished steel plate should be considered.
6. What problems should be paid attention to when selecting medium thick slab material?
(1) The content of harmful elements phosphorus and sulfur is as low as possible;
(2) The upper and lower limits of the element content should be as small as possible, especially the content of carbon;
(3) The content of non-metallic inclusions should be as little as possible.
7. What should be considered in the design of medium and thick plate materials?
(1) It must be ensured that the size and performance of the finished steel plate can meet the requirements of use;
(2) can give full play to the process conditions and equipment capacity of the steelmaking workshop and the thick plate car;
(3) The steel plate produced has the lowest cost.
8. What are the common defects of continuous casting slab?
Surface longitudinal crack, surface transverse crack, star crack, subcutaneous porosity and inclusion, bulging belly, internal crack, central segregation and central porosity, nonmetallic inclusion.
9. What are the surface cleaning methods, and what factors should be paid attention to when choosing?
Flame cleaning, wind shovel cleaning, grinding wheel grinding, machine tool processing, arc cleaning, etc. When selecting, we should pay attention to the steel grade of raw materials, the specific conditions of heating and rolling, and the requirements of the surface quality of the finished steel plate.
10. When cutting raw materials, what problems should be paid attention to in length?
It may be double, triple, or more times the required length. Cut to the required length before loading the furnace.
11. What is the purpose of heating medium thick slab?
Heating has the following purposes: (1) to improve the plasticity of steel; (2) Make the temperature inside and outside the billet uniform; (3) Change the crystalline structure of the metal.
The quality of billet heating directly affects the quality, output, energy consumption and rolling life of medium and thick steel plate.
12. Which kinds of heating furnace can be divided into according to its structure?
According to its structure, the heating furnace for medium and thick plate can be divided into three kinds: continuous heating furnace, chamber heating furnace and soaking furnace.
Soaking furnace is mostly used for rolling extra thick plate from ingot. Chamber furnace is suitable for extra-heavy, extra-light, extra-thick and extra-short slab, or multi-variety and small batch and alloy steel billet or ingot, the production is more flexible; Continuous heating furnace is suitable for mass production of few varieties, it is not easy to do special heating for a few slabs, so in the workshop of multi-variety mass production, in addition to continuous heating furnace, often with a chamber furnace. In order to overcome the cold mark of the slide, in recent years, the continuous heating furnace of the medium and thick plate has been changed to the hot slide or stepping type.
13. What are the effects of heating temperature and heating time on the material?
The heating temperature and the length of time in the furnace directly affect the size of the original austenite grain. The longer the heating time, the higher the heating temperature and the larger the original grain size. More importantly, the heating temperature directly affects the level of the rolling temperature, the higher the rolling temperature, whether it is disadvantageous to temperature control rolling or reduction control rolling.
14. What should we pay attention to when determining the heating temperature of steel?
The heating temperature of heated carbon and low-alloy structural steels can be determined by iron-carbon balance diagram. The heating temperature of low alloy steel is mainly
According to the carbon content to determine, for high alloy steel should not only refer to the phase diagram, but also consider the plastic diagram, deformation resistance curve and metallographic structure. The minimum heating temperature of steel should be determined by the final rolling temperature and taking into account the heat loss during billet release and rolling.
15. Why control the heating rate of steel?
Due to the thermal resistance of the metal, the surface temperature of the blank always rises faster than the center temperature during the heating process, resulting in internal temperature stress. It depends on the temperature gradient. The faster the heating rate and the larger the temperature gradient, the greater the thermal stress. When the thermal stress exceeds the breaking strength limit of the steel, cracks or fractures will occur inside the billet.
16. What are the requirements for heating temperature of steel plate varieties?
Different heating temperatures are determined according to the thickness of the production plate. Due to the few rolling passes of the thick steel plate and the high final rolling temperature, the heating temperature of the thick steel plate is required to be 20-30 ℃ lower than that of the thin steel plate, and the micro-alloyed steel also reduces the heating temperature accordingly. This is because reducing the heating temperature can not only shorten the waiting time of the rolling process, but also refine the size of the initial grain size of austenite and improve the strength and toughness of the steel plate.
17. What is the effect of heating temperature on austenite grain size?
With the increase of heating temperature, the austenite grains of all kinds of steels grow, and there is a plateau at 1150℃, where the austenite grain size is more uniform. When the temperature exceeds 1180℃, the austenite grain grows rapidly. According to this characteristic, we will control the heating temperature between 1130 ~ 1160℃, by controlling the austenite grain size, to achieve the purpose of refining the ferrite grain.
18. What is the role of slab weighing machine. What are the three parts?
Function: It is mainly to weigh the billet ready for heating and determine the flow direction of the slab.
The weighing operation of the weighing machine consists of three parts: to confirm the uniqueness and correctness of the plate rolling number of the weighing machine; Weighing operation; Confirm the quality control of the medium thick slab in section 1 and the flow direction of the slab after heating and weighing.
19. What is the function and composition of the furnace pushing machine?
The steel pushing machine of heating furnace is a device for pushing billet to continuous heating furnace, which is composed of motor, reducer, push rod, roller, press roller and push head.
20. What are the characteristics of steel reducer?
(1) The output direction of the output axis is changed, that is, the output axis is orthogonal to the output axis.
(2) The structure is relatively compact, in the case of the transverse position is relatively tight, the motor is arranged in the longitudinal reducer, so that the transverse position can be greatly saved, for the installation position requires special circumstances, the reducer can solve certain problems.
21. What device is the slab flame cutting machine?
The slab automatic cutting machine is installed on the slab cleaning line. It is an automatic cutting device to obtain the fixed size slab by gas flame cutting. The device is made of welded steel plates and is composed of a trolley, a frame and a transverse automatic running device.
22. What are the characteristics of the reducer of the billet turning machine?
(1) Without sliding, accurate rotation can be obtained;
(2) The roller chain rotates smoothly, even at the end of the life of the roller chain, the motion of the rotating mechanism is almost unchanged;
(3) Because the roller chain is elastic, it can absorb the impact load;
(4) Large transmission power, long roller chain life;
(5) The assembly accuracy is not high, easy to maintain and repair, is a more economical transmission device.
23. What are the causes of stuck furnace wall and steel pipe?
Billet loading deviation, its end is too close to the furnace wall or water pipe; Loading blank is too long; The size head of the flat ingot is not paired with inverted discharge and parallel feeding; The billet bending is too large, and the uneven thrust of the two pusher heads of the steel pusher will push it sideways. Uneven bottom of the furnace or the height of the two roller is inconsistent, so that the billet runs off.
24. What are the causes of tipping and arch steel?
Billet side is not straight, there is obvious bending or warping; The billet is thin and the push-steel ratio is too large. The thickness of adjacent billets is very different, and the small section billets are installed after the large section. Uneven pit surface of the guide rail or soaking section.
25. How to observe the flame color to determine whether the ratio of air and gas is appropriate?
Whether the flame is normal or not can be observed and identified from the combustion channel in the burner brick. The yellow flame indicates that the ratio of air and gas is appropriate, and the black flame indicates that the air volume is too small, and the air volume or gas should be adjusted in time.
26. What are the attention points of ignition operation?
When the torch is extinguished, the gas opener and close should be immediately closed, the flue gate should be opened for furnace release, and the ignition should be re-ignited after 15min, that is, the gas accumulated in the furnace can be completely eliminated before ignition again, so as to avoid the explosion caused by excessive concentration of combustible gas in the furnace. When the temperature of the furnace or burner brick is above 701 ~ 800 ° C, the temperature of the furnace itself is reliable for direct ignition.
27. How to do the outbreak test?
Take out the gas sample from the end of the gas pipe with the explosion test cylinder. When the gas in the sampling cylinder is contacted with fire, there is no burning phenomenon, indicating that the gas pipe has no gas and the steam can be stopped.
28. What should be done when tempering occurs?
To close the burner, check the treatment. If the tempering time is long, the burner mixing tube has been burned red, should be waiting for the mixing tube to cool before ignition, in actual work, should grasp the principle that the gas pressure should not be too low.
29. How to prevent explosions?
A power cut off valve should be installed on the gas pipeline to automatically cut off the gas when the power is cut off; Install explosion-proof holes on the air pipeline, and relieve pressure to protect the air pipeline once the gas enters the air pipeline; Before the combustion air blower stops, the gas valve of the burner should be closed immediately; The use of gas and air mutual lead belt safety burner, no matter which kind of air or gas supply interruption is not easy to invade each other's pipelines.
30. What are the causes of scorching of nozzle heads and nozzle tiles?
Poor atomization of heavy oil; Nozzle installation is not correct or nozzle brick aperture or Angle is too small; When the flow collides with the nozzle brick, the hot cracking of oil droplets leads to carbon coking. When the nozzle is closed, the residual oil in the head is hot cracked and carbon coking.
31. What are the main reasons for poor atomization?
The oil temperature is too low is the main reason, so it is necessary to increase the heating temperature of the oil, reduce the viscosity of the heavy oil, and improve the atomization quality. The failure of the air regulating mechanism in the low-pressure nozzle will also cause poor atomization, and the nozzle needs to be cleaned or repaired regularly.
32. What are the scope of application of primary heating rolling method and secondary heating rolling method?
One time rolling method is the direct rolling of steel ingot into steel plate after one time heating, which is suitable for rolling heavy or thick steel plate; The secondary heating method is that the ingot is heated once and then the billet is rolled into steel plate by secondary heating, which is suitable for some special steel plates with relatively thin thickness.
33. What is the overheating of steel, how to eliminate the adverse effects of overheating?
Overheating is the phenomenon that when the heating temperature exceeds A and continues to heat to a certain temperature, the steel grains grow excessively, resulting in the weakening of the intergranular binding force and the deterioration of the mechanical properties of the steel.
The overheated steel can be normalized annealed, that is, the steel is slowly heated to A temperature slightly below A, and then slowly cooled to allow the tissue to recrystallize to improve its mechanical properties. However, severely overheated steel is difficult to restore its mechanical properties by annealing treatment. In order to prevent the overheating of the steel, the heating temperature and the residence time in the high temperature section should be mastered during the heating process of the billet, and the heating time should be appropriately shortened. Especially when the rolling mill is stopped for a long time due to faults, the fire watch should reduce the fuel consumption and properly reduce the furnace temperature.
34. What is the overburning of steel, how to prevent overburning?
When the steel is heated to a higher temperature than overheating and for a long time, not only the grain of the steel grows, but also the boundary between the grains begins to melt, oxygen enters the grain gap, oxidizing the metal and promoting its melting, resulting in the destruction of the intergranular binding force and making the steel lose its own strength and plasticity, which is the overburning of steel. The burnt steel cannot be saved, it has to be scrapped and smelted again.
In order to prevent overburning, the heating temperature should be avoided and its long-term stay in the high temperature section. When the mill fails or rolls are changed, efforts should be made to reduce the furnace temperature and reduce the amount of air entering the furnace.
35. What is the decarbonization of steel. How to prevent it?
During the heating process of steel, the reduction of carbon content in its surface layer is called decarbonization. Decarbonization is closely related to the chemical composition of steel, heating temperature, heating time and the atmosphere in the furnace. The atmosphere in the furnace refers to the content of oxygen, hydrogen, carbon dioxide and water vapor in the furnace gas. They react with cementites as follows:
2FeC+O=6Fe+2CO
FeC+2H=3Fe+CH
FeC+CO=3Fe+2CO
FeC+HO=3Fe+H+CO
Under normal circumstances, the higher the heating temperature, the longer the time, the more serious the decarbonization. Steel at high temperature for a long time than at low temperature insulation decarburization layer is much thicker. The higher the carbon content of carbon steel, the easier it is to decarbonize. When heating, the more oxidizing the atmosphere in the furnace, the easier it is to decarbonize.
Therefore, in order to prevent decarbonization, rapid heating should be done to shorten the residence time of steel in the high temperature section; Select the heating temperature correctly to avoid the decarbonization peak range of easily decarbonized steel; Proper regulation and control of the furnace atmosphere; It is best heated in a walking beam heating furnace.
15358968703