High temperature flexural strength of magnesia carbon brick

There are many factors that affect the high-temperature flexural strength of magnesia carbon brick, the most important of which is the purity of raw material, carbon content, binder, matrix composition and organizational structure of magnesia carbon brick. The purity of raw material is relatively simple, the purity of magnesia is high, the crystal scale is large, the low melting point phase content distributed in the periclase grain boundary is low, the direct bonding degree is high, the high-temperature flexural strength is better; the purity of graphite The research on matrix composition, microstructure and other aspects is relatively complex, and it is also the most concentrated research field to improve the high-temperature flexural strength of magnesia carbon brick, which is roughly divided into the following three directions.

1、Add metal powder

In the aspect of improving the flexural strength at high temperature, the metal powders mainly include A1, Si, etc

① Al4C3 and SiC are formed by the reaction of metal Al and Si with graphite and resin carbon in magnesia carbon brick, strengthening the combination of carbon and carbon, and improving the strength;

② whiskers and fibers are formed in MgO-C brick by metal Al and Si, which strengthen the material matrix;

③ The formation of magnesia alumina spinel and the improvement of ceramic bonding

2、In situ formation of carbide, nitride and other whiskers

The improvement of high temperature flexural strength of MgO-C brick is usually achieved by in-situ formation of carbide and nitride whiskers.

Whiskers are generally one-dimensional crystalline materials of nanometer or submicron scale, with few internal defects, and the strength and modulus are close to the theoretical value of crystal materials. At the same time, the net distribution of whiskers in the brick or the pinning and locking effect in the microstructure of MgO-C brick also endow the material with a better mechanical properties Good strength. For example, yijingguang et al. Found that with the increase of heat treatment temperature, the high-temperature flexural strength and residual flexural strength of MgO-C brick added with metal Si powder and Al powder increased, while the flexural strength of the sample after heat treatment at 1400 ℃ was larger.

Through the analysis of microstructure, it was found that there was not only needle like AlN formation in the brick, but also inlaid on the surface of magnesia particles at 1400 ℃, At the same time, there are a large number of SiC whiskers and acicular β – Sin whiskers. In such a microstructure, when the material is subjected to external force, the stress can be transferred from the matrix to the whisker through the interface layer, and the whisker disperses the stress on the matrix and reduces the damage effect.

When the crack size of the sample under the action of thermal stress is small, the whisker plays a bridging role, When the crack increases, the whisker at the crack tip will be further destroyed, and the whisker will be pulled out of the matrix to consume energy. At this time, the pull-out effect will give the magnesia carbon brick high temperature mechanical properties

3、Generation or addition of nano carbon in magnesia carbon brick

Carbon nanotubes are new materials which appear in recent years, and their mechanical properties are very outstanding. Therefore, in the aspect of improving the high-temperature flexural strength and microstructure of MgO-C brick, some scholars have formed carbon nanotubes in the material by introducing catalysts and other ways, and achieved good results.

For example, we ì and other scholars modified phenolic resin and prepared low-carbon MgO-C brick by introducing Fe nano sheets. It is found that doping 0 A large number of carbon nanotubes with a diameter of 50-100nm and a length of micrometer are produced by the MgO-C brick with a mass fraction of. 5%. Compared with the sample without doping Fe nano sheet, the high-temperature flexural strength increases from 8.29mpa to 10.29mpa with a amplitude of about 24%, reaching the highest value. The presence of a large number of carbon nanotubes firmly locks the MgO particles.

When the stress is applied to the blank sample, the For the 0.5% doped sample, when the crack passes through the MgO particles, the carbon nanotubes can absorb and release the stress at the crack tip through the bridging and crack deflection mechanism due to its high strength and toughness.

In addition to the formation of carbon nanotubes in MgO-C brick, nano carbon is also introduced to improve the microstructure of MgO-C brick and improve the high-temperature flexural strength of materials. For example, on the basis of 3% mass fraction of graphite, by introducing different proportions of nano carbon and graphite, the high-temperature flexural strength (hMOR) increases with the increase of nano carbon content, and its value increases from 2.5MPa When it reaches 4.5MPa (0.9% mass fraction) and keeps constant when the nano carbon is further increased, as shown in Figure 21. Further analysis shows that with the increase of the content of nano carbon, the filling and compaction effect is better. At the same time, the nano carbon has very high reactivity and can form carbides at a higher rate when contacting with the gold additive, with stronger binding force and higher strength.

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The influence of mill rolls on product quality in the production of plate steel and strip steel

In the rolling process, the quality of the rolls is very important. Often due to the poor quality of the roll itself or improper operation, many rolling accidents occur, which bring losses to the steel mill. The following is a detailed analysis of the phenomena and causes of several rolling accidents that may occur.

What is the cause of mill rolls sticking?

The sticking mill roll is caused by excessive local pressure, broken strip fragments and folded strip steel entering the rolling mill. Generally, the slightly sticky mill roll can continue to be used after manual grinding with grindstone. When the strip surface require is high, the mill roll must be changed.

What are the material requirements for the cold mill rolls?

During the cold rolling process, the surface of the roll bears great squeezing and intense wear. Steel jamming and over-burning during high-speed rolling will cause cracks on the roll surface. Therefore, cold-rolling work rolls should have extremely high and uniform hardness, a certain depth of hardened layer, and good wear resistance and crack resistance. The roll has good resistance to over burning and crack resistance is the main factor to extend the life of the roll.

What are the effects of insufficient mill rolls hardness on the rolling process?

The roll hardness is not enough, the elastic flattening is large during rolling, and the contact area between the roll and the strip surface is increased. To obtain the same thickness of the product, the rolling pressure must be increased, but the large rolling pressure is not good for the adjustment of the plate shape. The roughness of the roll is large, and the friction coefficient between the roll and the strip steel surface increases during rolling, which causes the friction force and the rolling pressure to increase.

How to use bending rolls to eliminate “both sides waves” and “middle waves”?

When waves appear on both sides of the strip during rolling, it means that the rolling pressure on both sides of the strip is too large. At this time, positive bending rolls are used to eliminate the waves on both sides. When there is an middle waves, it means that the original crown or thermal crown of the roll is too large. At this time, use a negative bending roll to eliminate.

How to prevent the skewing when the strip passes through the gap of the roll

First of all, before the strip passes through the gap of the roll, it must be minimize the clearance between upper and lower rollers, given a standard roll gap, it looks like the quality of the material, whether there are edge waves, and observe the strip running condition. In the rolling process, the deviation of the strip generally occurs during the strip passes through the gap of the roll or tail flicking. The main reasons for the skewing of the strip are as follows:

1）Due to the shape of the incoming material is not good, and there are serious edge waves, so that the strip edge control device cannot be accurately and timely adjusted effectively, causing the strip to deviate in the first pass. The measure to be taken is that control the rolling speed don’t be too high! Control the clearance between upper and lower roller, swing adjustment in time or stop in time.

2) Operational reasons: The unreasonable adjustment of the swing due to the the clearance between upper and lower roller causes the strip to run out of direction.

3) For electrical reasons, the coiler tension suddenly decreases or disappears during the rolling process, causing the strip to run off and break.

4) Due to the serious taper of the roll after the roll is ground, the clearance between upper and lower roller is inaccurate. During the rolling, the reduction and swing of the operation increase the difficulty. The lighter one will produce serious side waves and cause the plate shape defect, and the heavy one will cause deviation and broken strip.

5) The strip edge control device failure, the deviation device, the lamp tube or the pollution of the receiving device, etc., make the deviation device failure and cause the first deviation.

What quality defects can occur in rolls grinding? What are the undesirable consequences in the rolling process?

1) The roll body has a taper: the reduction cannot be corrected, and it is easy to cause a side wave, and the plate shape is difficult to control. The roll body taper is required to be less than 0.1mm.

2) Grinding is not round: the roll body has ellipticity. Vibration occurs during the rolling process, the rolling pressure fluctuates, and the strip thickness varies along the length direction.

3) Crown: The roll is required to be a flat roll in the process of the unit. If the roll has a crown,In the rolling process, it is easy to produce the middle wave and the second rib wave of the strip steel.

4) The surface roughness of the roll is too large. During the rolling process, the rolling pressure increases, the strip shape is difficult to control, and the surface quality of the strip is affected.

5) The roll body cracks, because the cracks have not been ditched and ground. The rolls were not inspected for flaw detection, and protruding rolling marks with crack shapes were generated on the surface of the strip during the rolling process.

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