SPHC steel hot-rolled coils
Abstract : Aiming at the serious edge breaking phenomenon of hot-rolled SPHC steel in BBNIron and Steel Co., Ltd. , the causes and prevention measures of edge breaking in hot-rolled coils were studied. Starting from the understanding of mechanism, combined with metallographic examination, scanning electron microscope analysis, composition analysis and other methods, the characteristics and causes of broken edge defects were analyzed. The results show that the broken edge of the hot-rolled coil may not be directly related to the apparent quality of the corner of the slab , and the low Mn / S ratio in the steel and the high heating temperature will also lead to serious edge broken defects in the hot-rolled coil. Accordingly, from the improvement of the production process, put forward the corresponding solutions and achieved significant results.
Key words : hot-rolled coil ; broken edge defect ; Mn/S ratio ; heating temperature
Since the production of SPHC steel on the 1780mm hot rolling production line in August 2007 , edge cracks were once the main quality problem encountered in production. In April 2009 , when rolling low-carbon low-silicon steel SPHC with a thickness of 2. 3mm , 6 coils with broken edges were found after rolling. Severe edge cracking was also found on the intermediate billet of this steel type, and the middle of the wide surface was also partially cracked along the trough of the vibration mark. There are serious broken edges in hot continuous rolling SPHC steel , but there are often no obvious defects such as transverse cracks in the corners of continuous casting slabs. Therefore, combined with metallographic analysis , the cause of formation and control measures were studied from the aspects of molten steel composition and heating system of heating furnace.
1 Research progress of hot-rolled coil edge breaking
Broken edges of hot-rolled coils are one of the main defects of hot-rolled products. Its main form is that there are severe transverse cracks on one or both sides of the edge along the length direction, also known as rotten edges. When the SPHC steel coil is broken, the edge appears jagged. It is generally believed that the broken edge defects of hot-rolled coils originate from the surface edge transverse cracks (including deep vibration marks) and small longitudinal cracks on the edge of the continuous casting slab, and continue to expand during the heating and rolling process [ 1 ] . So far, the understanding of the formation mechanism of hot rolling edge broken defects can be attributed to the following aspects.
1) The composition of molten steel.
P , S , N and other residual elements in molten steel and the ratio of Mn/S .
when the casting billet enters the furnace at a low temperature (the temperature at the corner is lower ) , it will lead to the precipitation of N in the steel at the grain boundary of the corner in the form of AlN and MnS . Precipitates at grain boundaries will reduce the plasticity of steel, leading to cracks in the steel rolling process, and in severe cases, broken edges will be formed. When the content of inclusions in steel is high, the severity of broken edges will be aggravated.
P and S are elements with a large segregation tendency , and micro-segregation reduces the solidification temperature of the interdendritic liquid phase, resulting in embrittlement of the steel at the rolling temperature. Studies have shown [ 2 ] that microscopic segregation is the origin of transverse cracks on the slab surface , and the cracks propagate along the segregation area. Segregation is the oscillatory wave in the crystallizer
There are two main reasons : one is that in the meniscus area of the mold, the semi-solidified billet shell at the solidification front is periodically deformed when the mold vibrates, and the concentrated molten steel between the dendrites ( rich P , impurity elements ) are segregated at the troughs of the vibration marks ; the second is that the grains at the troughs of the vibration marks are relatively coarse, and the concentrated molten steel seeps out along the grain boundaries to the surface. Microscopic segregation exists at the trough of vibration marks on the surface of the slab , which is the main cause of transverse cracks . Then it expands along the austenite grain boundary to form larger transverse cracks. In addition , since S is an extremely active surface-active element, it is likely to segregate between grains at a higher heating temperature [ 3 ] .
S can not only cause interdendritic segregation during solidification , but also form low melting point compounds , causing embrittlement of steel. MnS can be formed by adding Mn , To avoid adverse effects of low melting point compounds. But when the solidification cooling rate is too large , Too late to form MnS , Then (Fe,Mn)S® is formed .
Therefore , The content of residual elements such as P , S and N in molten steel should be reduced as much as possible , Properly increase the Mn / S ratio.
- Continuous casting process and slab quality. Defects such as transverse cracks and subcutaneous air bubbles on the edge of the billet are the causes of
The main reason for the broken edge of the steel coil after hot rolling. In particular, the transverse cracks and corner transverse cracks at the vibration marks continue to expand during the heating and rolling process, and when severe, they develop into broken edges. Transverse cracks generally occur at the troughs of the vibration marks, which are related to the vibration parameters of the crystallizer. Corner transverse cracks are formed due to the overcooling of the corners of the slab and the high brittle temperature range during straightening. During the heating process of the heating furnace, when the heating temperature of the slab is higher than 800 °C , decarburization and oxidation occur around the corner transverse crack.
Therefore, appropriate process parameters, such as crystallizer taper, vibration parameters, secondary cooling intensity, etc., should be adopted to ensure the quality of the slab. There are specific measures : ①Using high-frequency and small-amplitude crystallizer vibration mode , To reduce the depth of vibration marks; ②Reduce the force acting on the solidified shell. For example, use mold powder with better lubricity, and use appropriate mold taper to match cooling shrinkage , Avoid the occurrence of bulging as much as possible ; ③Reduce thermal stress in the secondary cooling section and use weak cooling; ④Develop the secondary cooling mode according to the high-temperature thermoplasticity of the steel, and avoid bending and straightening in the low plasticity area. Generally avoid bending and straightening when the surface temperature is between 700-900C .
- hot rolling process.
Billets are subjected to thermal stress, mechanical stress, phase transformation stress, etc. during rolling, laminar cooling, and coiling. If the thermoplasticity of the steel is not enough, it will cause cracks at the edge of the plate, and in severe cases, it will develop into broken edges.
Factors affecting the thermoplasticity of steel mainly include heating temperature, strain rate, chemical composition, grain size, precipitates, non-metallic inclusions, mechanical heat treatment, etc. In some cases, the inherent low thermoplasticity of billets may lead to edge cracks and other defects [ 3-4 ] . In order to prevent edge breaking defects of steel coils, the heating system of the heating furnace should be formulated. If the heating temperature is too high or the time in the furnace is too long, it may cause overheating and overburning, which will easily crack the edge of the slab. When the heating temperature is too low, the thermoplasticity of the steel may not meet the rolling requirements and cause cracks.
The roll shape and roll gap selection of the roll are also very important. Studies have shown [ 5 ] that the roll gap during rolling has a great influence on the crack depth and width. When the front of the crack is in contact with the roll, the crack speed changes rapidly from the horizontal direction to the tangential speed direction of the roll, and the crack will be elongated toward the gap between the rolls; in the gap between the rolls, the expansion of the crack is reduced; when the crack leaves the roll, Cracks also propagate slightly due to friction. In order to reduce the expansion of cracks and prevent edge breaking and oxide layer wrinkling and falling off, measures such as lightly pressing the roll gap, reducing the radius of the roll, increasing friction, and reverse rolling can be taken.
The edge of the rolled piece is subjected to additional tensile stress when it is widened, while the middle part is subjected to additional compressive stress. The greater the spread of the slab, the greater this stress. If the billet is of good quality and the heating and rolling conditions are normal, the presence of these stresses should not affect the integrity of the edge of the rolled piece. Otherwise, the stress on the edge of the rolled piece will exceed the strength of the metal, causing cracking. In addition, the small opening of the coiled side guide plate will also cause broken edges.
Chemical Composition of Process Parameters of SPHC Broken Edge Coil
Low-carbon aluminum-killed steel SPHC is currently one of the most widely used steel sheet varieties for stamping, and it has strict requirements on chemical composition. See Table 1 for the chemical composition, standard composition, and target composition of the heats with chipped edges in the hot continuous rolling of SPHC coils at BBN Iron and Steel Co. , Ltd. It can be seen from Table 1 that the manganese content of the heats with broken edges is low, which is lower than the manganese content of the standard composition.
Table 1 Chemical composition and standard composition requirements ( mass fraction
|
furnace number |
C |
Si |
mn |
P |
S |
al |
|
9M01727 |
0 ． 07 |
0.01 |
0.07 |
0.010 |
0.013 |
0.045 |
|
9M01728 |
0 ． 06 |
0.02 |
0.05 |
0.006 |
0.010 |
0.031 |
|
9M01729 |
0 ． 06 |
0.02 |
0.06 |
0.006 |
0.010 |
0.038 |
|
9M01731 |
0 ． 04 |
0.01 |
0.08 |
0.011 |
0.013 |
0.060 |
|
judgement standard |
C0.07 |
C0.05 |
C0.35 |
C0.025 |
C0.015 |
0.015-0.06 |
|
Original internal control components |
C0.07 |
C0.04 |
C0.25 |
C0.015 |
C0.015 |
0.015-0.045 |
Pouring parameters of heats with chipped edges on steel coils
The pouring process parameters of the heats with broken edges on the steel coil ( 4 for each heat)
The average value of times) is shown in Table 2. It can be seen that the pouring process parameters are not abnormal.
Table 2 The pouring process parameters of the heats with broken edges in steel coils
furnace number |
Average pouring temperature /C |
Average pulling speed/ (m • min — 1 ) |
|
9M01727 |
1555-3 |
1-05 |
|
9M01728 |
1550-0 |
1-05 |
|
9M01729 |
1566-3 |
1-05 |
 |
Blank analysis
-
- low magnification test
5 pieces of 150mm X 150mm low-magnification samples on the same furnace slab with broken edges after rolling , and the sampling positions are shown in Figure 1 . The samples were taken out after being etched in hydrochloric acid for 2 hours , and it was found that the center segregation of Nos . obvious flaws. The above shows that the continuous casting slab has no obvious macro defects.
-
- Metallographic analysis
from the SPHC rolling intermediate billet, and after grinding and polishing, it was corroded with 4% nitric acid alcohol to observe the structure. The metallographic photo is shown in Figure 2 . It can be seen from Figure 2 that the sample structure is mainly ferrite and pearlite , the ferrite grain size near the edge is about 8 grades , and the matrix ferrite grain size is about 6 grades. The grain size of ferrite at the edge is coarse, there are traces of oxidation at the grain boundary , there is no decarburization around the crack , and there is a part of Widmanstatten morphology. This shows that the rolling intermediate billet has local overburning at the corners,
-
- SEM analysis
In order to observe the microscopic morphology of the intermediate billet after SPHC rolling, a 10mm sample was taken from the intermediate billet for scanning electron microscope (SEM ) analysis. Fig . 3 is a scanning electron microscope picture of the intermediate billet sample after 1780mm hot continuous rolling and rough rolling. Within the field of view of the scanning electron microscope, cracks of different degrees can be seen, which are distributed within the surface layer of the intermediate billet and within 5mm of the surface layer . The crack morphology is distributed in a network shape.
4 Causes and prevention measures
4.1 Cause Analysis
- molten steel composition.
Broken edge is mainly related to the Mn/S ratio of molten steel. The biggest hazard of S is that it is easy to cause "hot embrittlement" during rolling and hot working. The segregation tendency of S is large , and the segregation makes the local sulfur content too high . It mainly exists in the steel in the form of FeS , and its melting point is low (1190°C) . When the molten steel solidifies , FeS and Fe form a eutectic with a lower melting point, the melting point of which is 988 C , and a continuous or discontinuous network film is distributed at the grain boundary. This low-melting eutectic melts at furnace heating and rolling temperatures (800-1 200 C) . Since the corners have the highest temperature in the heating furnace, the low-melting point substances in the corners melt more, and the edge of the rolled piece is prone to reticular cracks or even large cracks on the surface under the action of additional tensile stress during rolling, resulting in waste products .
Mn in steel can not only improve the strength of steel, but also a good deoxidizer and desulfurizer. The main reason is that the affinity between manganese and sulfur is much greater than that between sulfur and iron, so Mn can promote sulfur in the steel to preferentially form MnS (1620C) with a higher melting point than FeS , thereby preventing FeS from intergranular precipitation to form a network film, and then "Hot embrittlement" occurs during rolling. Therefore, the key to controlling the composition of molten steel is that there must be a sufficient amount of manganese in the steel, which can form high-melting sulfide (MnS) with sulfur and exist inside the grains to offset the hot embrittlement caused by the low-melting eutectic formed by FeS and Fe . phenomenon, to achieve the purpose of suppressing thermal cracking.
From the analysis of Table 1 after rolling the steel coil edge cracked furnace times Mn/S = 5. 00 ~ 6. 15 . The low Mn/S ratio can appropriately increase the manganese content in molten steel and reduce the sulfur content to reduce cracks.
- Heating regime.
Studies have shown that 6] , for SPHC steel , the grain boundary state of steel with low Mn/S ratio changes at 1210C , and the grain boundary state of steel with high Mn/S ratio changes at 1240C . Therefore, it can be considered that 1210C is the superheating onset temperature of samples with low Mn/S ratio, and 1240C is the superheating onset temperature of samples with high Mn/S ratio. It can be seen that samples with low Mn/S ratio are more than Mn/S The overheating effect of the sample with a higher ratio is serious. Therefore, if the heating temperature is too high, it will lead to intergranular cracking of the slab with low Mn/S ratio, and cracking will occur during the rolling process.
the SPHC hot-rolled coil edge breakage occurs in Angang , the corresponding heating furnace control situation is as follows: the total time in the furnace is not less than 180min ; -The target temperature of adding is 1250-1290C , the target temperature of secondary adding is 1260 ～ 1300C, the target temperature of soaking is 1240-1280C ; the target temperature of billet is 1240-1280C . It can be seen that the heating temperature in the heating furnace is up to 1300C. The preliminary analysis is that the high heating temperature causes the intergranular fracture of the SPHC slab with low Mn/S ratio, and the broken edge occurs after rolling.
4.2 Prevention and control measures
Through the research on the mechanism and process of SPHC hot-rolled coil edge breaking, Angang implemented the following prevention and control measures and achieved good results, and no edge breaking was found.
- increase the mass fraction of Mn in the steel to 0.23%-0.30% .
- Because excessive sulfur content affects the ductility of steel and increases crack sensitivity, therefore, for SPHC steel, the mass fraction of sulfur in molten steel is controlled below 0.010% .
- Increase the Mn/S ratio above 20 .
- Appropriately reduce the heating temperature of the heating furnace, and control the billet discharge temperature of the heating furnace at 1200-1 210 C to prevent edge breaking defects caused by excessive heating temperature. After adjusting the heating system of the heating furnace, the temperature control standard of the heating furnace is: the total time in the furnace is not less than 140min ; - Add target temperature 1120-1150C , add target temperature 1200-1220C , soak target temperature 1190-1210C ; billet target temperature 1200-1210C .
5 Conclusion
- Chipping of hot-rolled coils may not be directly related to the apparent quality of slab corners.
- Angang SPHC hot-rolled steel coil edge cracking is mainly due to the low Mn/S ratio of molten steel and the segregation of low-melting-point sulfides at grain boundaries, resulting in high hot brittleness of the slab. In addition, the heating temperature of the heating furnace is too high, and the weakening effect of the slab edge due to grain boundary segregation is more significant, resulting in intergranular cracking under the action of the additional tensile stress during rolling deformation.
- By increasing the mass fraction of manganese in SPHC molten steel to 0-23 %-0.30% and reducing the sulfur content as much as possible, the Mn/S ratio can be increased , thereby reducing the broken edge defects of hot-rolled coils.
- Appropriately reducing the temperature of the heating furnace can effectively reduce the broken edge defects of hot-rolled coils. The billet temperature of the SPHC steel heating furnace is controlled between 1200-1210C .
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