The decarburization sample was taken in flat steel by wire, pre-polished, polished, and 4% nitric acid was etched, and the depth of the carbon layer was measured under a ZEIS microscope. The above physical property test results are seen, and it can be seen that the physical properties meet the requirements of the national standard. In the macroscopic test, the bubbling of the sample was repeatedly flattened and heated to randomly take five samples of the flat steel with bubbling, and the number 48 was No. 1, No. 2, No. 3, No. 4, No. 5.
At the normal temperature of Jiangsu metallurgy, the air bubble is used to level the bubbling lightly, and then 5 samples are placed in the physical property project.) The measured value is changed to the standard value: [321 in IBS26q decarburization > 岌妄 2% box type resistance furnace After heating to 120 ° C, the furnace door was opened and it was found that none of the No. 1 and No. 3 samples showed bubbling, and the No. 2, No. 4 and No. 5 samples showed bubbling during heating. The samples No. 1 and No. 3 were naturally cooled in the air, and no bubbling occurred. The samples No. 2, No. 4 and No. 5 were taken out and leveled with an air hammer, and then placed in a furnace for heating, and the bubbling phenomenon reappeared.
After bubbling and drilling, the 2, 4, and 5 samples of the bubbling are gently leveled with an air hammer at a normal temperature. The drill bit of the middle smm is drilled in a hole of the No. 2 sample, and the hole depth L is: /2d guest L [d (d is the thickness of the flat steel). The samples Nos. 2, 4, and 5 were placed in a box-type resistance furnace for heating, and as a result, no bubbling occurred in the No. 2 sample, and bubbling occurred in the No. 4 and No. 5 samples. Similarly, after the bubbling of No. 4 and No. 5 was leveled at room temperature, one hole was drilled and heated in the furnace, and no bubbling occurred on No. 4 and No. 5.
At room temperature, the scales of No. 1 and No. 3 were peeled off at room temperature, and the cracks in the cross section of No. 1 sample were found as indicated by the arrows. Since the scale of the No. 3 cross-section is firm and not easy to remove, it is taken out in the waste diluted HCI for 48 hours, and it is found that there is also a crack in the cross section. Macroscopic test analysis 5 bubbling samples were flattened at room temperature and heated in the furnace. Forms 2, 4, and 5 formed bubbling during heating, instead of forming during the cooling process, and samples No.1 and No.3 No bubbling was formed even after heating and discharging. It is indicated that the formation of bubbling is independent of the cooling stress and phase transformation stress of the steel.
The cross-sections of No.1 and No.3 have cracks and bubbling, which can not form bubbling during heating; the bubbles of No.2, No.4 and No.5 are leveled and heated, and bubbling and bubbling reappear. The fact that the bubbling no longer occurs after being drilled, fully demonstrates the presence of gas in the rear bubbling. The matrix structure of the steel at room temperature is ferrite ten pearlite, and its strength is much greater than the austenite structure at high temperature. At normal temperature, the bubble is flattened and the gas is compressed in the center of the flat steel. Since the strength of the ferrite + pearlite is high, bubbling cannot be formed.
At this time, the microstructure is a single-phase austenite region, and since the austenite structure has good plasticity, it is easily plastically deformed. The gas molecules in the high temperature state are intensified, and the internal energy is high, so that the plastic deformation of the austenite is easily caused, so that bubbling is formed at a high temperature. Metallographic analysis of the sample observation The sample was cut transversely in the bubble and a metallographic sample was taken. Pre-grinding, polishing, and 4% nitric acid corrosion were observed under a zEIs microscope. The 100-fold observation is as shown: the crack is attached to one side of the negative segregation zone, and the width of the negative segregation zone is about 0.7 mm.
No inclusions were found in or around the crack. The 500-fold observation is as follows: the inner wall of the crack has a pure ferrite layer, the ferrite is a columnar vertical crack inner wall, and the height is 0.03 mm. This pure ferrite is a product of redox reaction, that is, a fully decarburized layer. Mechanism of formation of metallographic structure in cracks During continuous casting and casting, the liquid crystallizes in the crystallizer. There is a certain amount of residual oxidizing gas in the center of the slab during the solidification of the molten steel. The temperature of the crystal is very high, just below the liquidus temperature.
Analysis and discussion of the mechanism of gas enrichment in the center of the slab Before the solidification of the molten steel in the mold, some oxidizing gases are distributed in the molten steel due to accidental factors. These oxidizing gases will react with the carbon atoms in the molten steel at all times. The lower carbon content of the molten steel after decarburization will be driven to the center of the slab as part of the negative segregation zone due to the molten superheat and temperature gradient of the molten steel.
In addition, the residual oxidizing gas and the reacted product CO gas which have not yet been reacted are also rushed to the center of the slab due to the crystallization selected during the crystallization of the continuous casting slab, and finally, with the end of the crystallization of the slab center, These gases will be enriched in the gaps of the crystal bridge, ie at the center of the slab.
Conclusion The bubbling of spring flat steel is caused by the enrichment in the continuous casting billet and the public gas.
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