Through low-magnification inspection of the cast slab and electron microscope scanning of the triangular crack area, the types and causes of the triangular cracks in the continuous casting slab were analyzed. Improvement measures were proposed in terms of molten steel composition, secondary cooling water distribution, driving pressure, light reduction, and support sections. The results showed that sulfur in the steel and the support section of the cast slab were the main factors affecting the cracks in the triangle zone. The implementation of specific measures has significantly reduced the incidence and level of cracks in the triangle area.
Keywords: continuous casting slab; triangular zone crack; support section; sulfur; low magnification inspection
The No. 1 casting machine of the Third Steelmaking Plant of Jinan Iron and Steel Co., Ltd. is a straight arc-shaped slab continuous caster with multi-point bending and multi-point straightening of the straight mold. The metallurgical length is 34.2 m. The support section consists of 1 curved section and 14 sector sections. Composition; Sector sections 7 to 14 are equipped with intelligent light reduction technology; its production section is (200, 270) mm × (1200 ~ 2100) mm, and the main production sections are 200 mm × 2100 mm and 270 mm × 2 100 mm. The maximum pulling speed for the 200 mm section is 1.60 m/min, which is generally controlled between 1.25 and 1.40 m/min; the maximum pulling speed for the 270 mm section is 1.25 m/min, which is generally controlled between 1.0 and 1.15 m/min. The fixed length is 2 000 ~ 3 200 mm, providing blanks for medium and thick plate factories. Since it was put into production, with the continuous increase in its output, the internal quality of the cast slabs has fluctuated greatly. In some furnaces, especially the cast slabs of ordinary carbon steel, serious triangular crack defects have occurred, and delamination defects have also appeared on the edges of the steel plates. Serious At that time, the proportion was as high as 1.12%, which increased the number of modifications and scrapping of steel plates. To this end, a comprehensive investigation and analysis of the causes of cracks in the triangle area of the cast slab were carried out, and effective measures were taken to effectively control them, providing raw material guarantee for rolling thick-sized plates.
1 Investigation and analysis of cracks in the triangle zone
Shortly after the slab comes out of the mold, the triangular area on the narrow surface is subjected to thermal stress caused by strong cooling of the narrow surface and cooling of the wide surface. As well as the mechanical stress caused by the improper position of the narrow-surface foot roller and poor wide-surface support, the strength and plasticity of the steel in this part are reduced due to the accumulation of enriched solute elements at the solid-liquid interface and nearby areas at the solidification front. When the combined stress exceeds the critical strength near the solidus temperature of the steel bell, the shell at the solid-liquid interface cannot resist the combined stress and strain and cracks occur.
1.1 Electron microscope analysis of triangular zone cracks
A low-magnification sample of the Q 23 5B slab with serious internal cracks (w(S)=0.034%, the crack in the triangular zone is 2.0) was cut into small samples, and the crack locations were analyzed by scanning electron microscopy. The main elements oxygen and iron at the crack of the sample are on average 31.59% and 53.60% respectively, followed by manganese and sulfur at 5.33% and 4.49% respectively. There are also very small amounts of silicon, calcium and other elements, mainly oxides and sulfide inclusions. The sulfur segregation at the crack is as serious as 24.63%, which affects the strength and toughness of the cast slab at this location, and triangular zone cracks occur under the action of less than the ultimate stress. To facilitate analysis, oxygen and iron elements were removed and Figure 1 was produced.
It can be seen from the figure that sulfur and manganese, and some calcium and sulfur go hand in hand, with high and low levels corresponding to each other. Therefore, it can be inferred that the inclusions at the cracks are mainly manganese sulfide, calcium sulfide and other small amounts of oxide inclusions. It can be inferred from the presence of sodium that the inclusions are caused by the entangled mold slag remaining in the molten steel and enriched at the cracks.
Analysis shows that the cracks in the triangle area of the cast slab mainly enrich the sulfide and oxide inclusions in the steel. Therefore, reducing the sulfur in the steel, reducing the oxide inclusions and controlling the fluctuation of the crystallizer liquid level are effective ways to reduce the cracks in the triangle area of the cast slab.
Figure 1 Electron microscope scanning results
1.2 Low-magnification analysis of cracks in the triangular zone
In order to more clearly understand the main influencing factors of the cracks in the triangle area of the casting slab, low-magnification samples were taken to analyze the cracks in the triangle area of the casting slab according to different production process conditions. Take 136 pieces of various Q235B low magnification samples, the pulling speed is 1.2~1.3 m/min, and the crack level is the average value. The statistical comparison of low-magnification inspection of cast slabs is shown in Table 1.
Table 1 Statistical comparison of low-magnification inspection of cast slabs
|w(S)/% in steel
|Press down lightly
|Triangular area crack/level
It can be seen from Table 1 that sulfur in steel has the greatest impact on the crack level in the triangle zone. It is obvious that the crack levels of the samples with w(S)>0.020% are all higher. Except for the intermediate cooling and dynamic roll gap, which is level 0.8, the others are all above level 1.0. Secondly, the influencing factor is secondary cooling. Using intermediate cooling, the cracks in the triangle area of the slab are improved to a certain extent. The crack level in the triangle area is reduced by about 0.32 levels on average compared with weak cooling. With the dynamic roll gap, the cracks in the triangle area are reduced by an average of 0.28 levels compared with the static roll gap.
2 Types and influencing factors of triangular zone cracks
2.1 Triangular zone crack types
In order to facilitate analysis and problem solving, triangular zone cracks are divided into subcutaneous triangular zone cracks and deep triangular zone cracks based on the differences in crack occurrence locations and influencing factors. The former refers to cracks in the triangular area with a depth of 10 to 40 mm on the narrow surface (side) of the slab (Figure 2 (I)). The length is generally less than 10 mm and is grade 0.5. The cracks account for a small proportion and have no significant impact on the performance of the plate. Influence. The latter refers to triangular zone cracks less than 40mm on the narrow surface (side) of the cast slab (Figure 2 (II)). The length is generally 10 to 30 mm and is grade 0.5 to 2.5. The cracks account for the largest proportion and have an impact on the performance of the plate. It is also the largest, so the cracks in the deep triangle area are the focus of research.
Ⅰ-Triangular zone cracks on the narrow surface (side) of the slab with a depth of 10 to 40 mm;
Ⅱ – Cracks in the triangular area below 40 mm on the narrow surface (side) of the slab
Figure 2 Triangular zone crack types
2.2 Factors affecting cracks in the subcutaneous triangle zone and control measures
According to the solidification theory calculation of the cast slab, the subcutaneous triangular zone cracks mainly occur in the bending section area.
2.2.1 Crystallizer narrow surface foot roller
The gap between the narrow-surface foot roller at the lower part of the crystallizer and the sample plate is small. The foot roller accumulates the cast slab and causes depressions in the narrow surface. Subcutaneous triangular cracks occur in the narrow surface of the cast slab under excessive mechanical stress. According to the shrinkage of the slab and the shape of the slab combined with the narrow surface of the slab, the gap between the foot roller and the template was readjusted. After the adjustment, the cracks in the subcutaneous triangle area were significantly reduced, and the overall shape of the slab was also very good (Table 2).
Table 2 Comparison of adjustment of gap between narrow foot rollers
|Distance between foot roller and crystallizer bottom/mm
2.2.2 The cooling intensity in the narrow surface area is too large
Supercooling of the narrow surface causes the narrow surface of the billet to shrink and appear narrow surface dents, and the billet in the triangular area is subject to tensile stress. When the strain of the billet shell in the local area of the solidification front exceeds the limit deformation value there, intergranular cracking occurs along the columnar crystals, resulting in triangular cracks. The water flow rate in the narrow foot roller area was reduced from 98L/min to 70L/min, and the cracks in the subcutaneous triangular area were significantly reduced.
2.2.3 Support rollers in the upper area of the casting machine
The support roller opening in the upper area of the sector section has a large deviation and poor arc alignment, which causes the billet to have a bulge on the wide surface and a depression on the narrow surface, resulting in subcutaneous triangular cracks. Through the roll gap test combined with the low-magnification inspection of the cast slab (Table 3), it was found that the opening and arc deviation values of the No. 1 machine were controlled within 0.5 mm and 1 mm respectively, which can effectively reduce cracks in the subcutaneous triangle area.
Table 3 Effect of support roller deviation on cracks in the subcutaneous triangle zone
|Subcutaneous triangle crack
Table 4 Effect of sulfur in steel on subcutaneous triangular zone cracks
|One maintenance interval
|Another maintenance interval
|Cracks in subcutaneous triangular zone
|Cracks in subcutaneous triangular zone
2.2.5 Measures and effects of controlling cracks in the subcutaneous triangle zone
By adjusting the gap between the narrow surface foot roller of the mold and the template, strengthening the assembly accuracy of the bending section, and reducing the cooling intensity of the narrow surface of the casting slab, the cracks in the subcutaneous triangular area were effectively controlled, with an average level of 0.02, which almost eliminated the cracks in the subcutaneous triangular area.
2.3 Deep triangle cracks
2.3.1 The influence of the lower area support section on the cracks in the deep triangle area
It mainly occurs in the support section area of the lower part of the casting machine. In this area, due to poor centering of the support roller, inappropriate opening, and poor roller dynamics, the cast slab will bulge and cracks will occur. The deformation of the slab belly increases with the thickness of the slab, and the cracks in the lower area are all long cracks, which are very harmful. It can be seen from Table 5 and Table 3 that the cracks in the deep triangle area are generated when the deviation value of the support roller is large, and the cracks are long and high-level. This is confirmed by the disassembly measurement of the lower line sector section and the low-magnification inspection of the slab.
Table 5 Effect of support roller deviation on cracks in deep triangular zone
|deep triangle crack
2.3.2 Effect of driving roller pressure on cracks in deep triangle zone
When excessive tension and straightening roller pressure acts on a cast slab that is not fully solidified, the solidification front is subjected to tension strain. The mechanism is the same as the strain when the bulging cast slab enters the next pair of normal rollers. At this time, the shell of the slab is thin. Therefore, the strain produced is greater and the harm is more serious. Comparing the effects of different tension-straightening pressures on the cracks in the deep triangle area of the cast slab, from the comparison of low-magnification samples of the cast slab before and after the tension-straightening pressure adjustment, it can be seen that the tension-straightening pressure was adjusted from 4.6 MPa to 4.2 MPa. Cracks in the deep triangular area are alleviated to a certain extent, and the average crack level can be reduced by about 0.5 levels. This shows that lowering the tension and straightening pressure is effective in reducing cracks in the internal triangle zone.
2.3.3 Impact of secondary cooling water distribution
Poor wide-surface cooling in this area or excessive uneven cooling between the rollers causes uneven cooling and reheating of the slab, causing thermal stress.
Overcooling of the billet will lead to the development of columnar crystals, which will reduce the high-temperature strength of the steel; insufficient cooling will make the billet shell too thin and prone to bulges, affecting the uniformity of the billet temperature and leading to the occurrence of internal cracks. By analyzing the low-magnification cracks of Q235B weakly cooled and medium-cooled cast slabs (Table 1), it was found that the cracks in the triangular area of the cast slabs were improved to a certain extent by Q235B medium-cooled. The average crack level in the deep triangular zone is reduced by about 0.5 levels, and the average crack level in the triangular zone is reduced by 0.32 levels. Stronger cooling or weak cooling is not very helpful in reducing cracks in the triangular zone.
2.3.4 Influence of dynamic roll gap
By comparing the effects of dynamic roll gaps and static roll gaps on internal cracks in the cast slab (Table 1), it is found that dynamic light reduction is more conducive to improving the deep triangular zone cracks and internal center segregation of the billet than static light reduction. The cracks in the triangular area are reduced by an average of 0.28 levels, and the cracks in the deep triangular area are reduced by an average of 0.5 levels. Especially when the condition of the sector section deteriorates, the effect of the dynamic roll gap is more obvious.
2.3.5 Superheat of molten steel
The temperature of the molten steel is high, and the columnar crystals of the cast slab are developed, which reduces the crack resistance of the cast slab, and the gas inclusions in the steel are also high. The shrinkage of the cast slab is large, the billet shell with the same cooling intensity is thinner, and the high-temperature mechanical strength of the billet shell is relatively low. . Statistics found that the difference in casting superheat was 12°C, and the corresponding crack levels in the deep triangle area differed by 0.5 on average. The casting temperature is too high and the casting speed is low, resulting in serious deterioration of the accuracy of the casting machine (serious damage to the bearings and bending of the pinch rollers), which indirectly leads to the formation of internal cracks in the slab.
2.3.6 Pulling speed
The pulling speed and change rate have a great influence on the shell thickness of the cast slab, the position of the solidification end, and the composition of the solidification structure. Changes in the pulling speed will cause changes in the secondary cooling water, and will also cause changes in the solidification end position, which will lead to bridging during solidification of the slab, and it is easy to generate deep triangular cracks. A pulling speed that is too fast will also increase the deformation rate of the billet shell at the abnormal roller. These factors can easily lead to the formation of internal cracks; a pulling speed that is too low will cause the billet temperature to be too low and damage the accuracy of the casting machine.
2.3.7 Effect of sulfur in steel
Judging from the corresponding relationship between the low-magnification sample analysis results and the sulfur in the steel (Figure 3), as the sulfur in the steel increases, the crack level in the triangle area of the cast slab shows a significant increasing trend. This is consistent with the fact that the inclusions at the cracks found in the energy spectrum analysis of the cracks in the triangle area of the cast slab are mainly sulfide. Statistical analysis results show that sulfur in steel has the greatest impact on the internal cracks of the cast slab. When w (S) in the steel is ≤ 0.020%, the internal quality of the cast slab during low-magnification inspection is better, and the cracks in the deep triangular area are generally not larger than 0.5.
2.3.8 Measures and effects of controlling cracks in deep triangular areas
After taking measures, the cracks in the deep triangular area of Q235B billet were effectively controlled. The specific measures are: ① Use a combination of low-magnification sulfur prints and roll gap meters to guide the maintenance of the casting machine in a timely and accurate manner to ensure that the sector opening, arc and dynamic conditions of the support rollers meet the requirements; ② The driving roller pressure is controlled to no more than 4.2 MPa; ③ Strengthen production scheduling, optimize the temperature system, and control the superheat of molten steel at 20±5°C to create prerequisites for stable casting machine speed operation; ④ When conditions permit, all molten iron is pretreated to control w in the steel ( S)<0.020%; ⑤ Carry out operations such as changing the middle bag and water port that affect the stability of the liquid level in strict accordance with the standards to reduce slag entrainment on the liquid level.
Figure 3 Correspondence between cracks in the deep triangular area of the cast slab and sulfur in the steel type
The results of crack research in the triangle zone are shown in Table 6. A total of 53 low-magnification samples were taken, the grade was the average, and there were a total of 2 samples with w(S)>0.020% in the steel. Comparing with Table 1, it can be seen that these two types of triangular zone cracks have been effectively controlled, and the research has achieved satisfactory results.
Table 6 Statistical comparison of low-magnification inspection of cast slabs
|in steel Subcutaneous triangular zone crack level/level
|Deep triangular zone crack level/level
(1) The gap between the narrow surface of the mold and the sample plate and the cooling intensity of the narrow surface of the slab have an important impact on the occurrence of cracks in the subcutaneous triangular area. It is necessary to ensure that the narrow surface of the slab is a slightly concave billet shape.
(2) Reducing sulfur in steel is an effective measure to reduce cracks in the deep triangle zone of the cast slab. When conditions permit, molten iron should be desulfurized as much as possible, converter smelting should be strengthened, and top slag treatment should be added during the tapping process to reduce sulfur in steel.
(3) Use low-magnification sulfur printing technology to promptly guide the maintenance of the casting machine to ensure that the curvature and opening of the online support section of the casting machine are within the required range. During each maintenance, the roll gap measuring instrument must be used to adjust the measured data to the sector section so that the opening and arc of the casting machine reach the set range. This is the equipment guarantee to reduce cracks in the triangle area.
(4) The tension and straightening pressure of the casting machine has a certain influence on the cracks in the deep triangle zone. The tension and straightening pressure should be reduced as much as possible while ensuring normal steel drawing conditions.
(5) Through production organization coordination and related operations, ensure the temperature of the molten steel to the casting machine platform, so that the target temperature of the molten steel tundish meets the requirements of each steel type. Provide pre-process conditions for constant speed casting, which is of great help in reducing internal cracks in the slab.
(6) The dynamic roll gap is more helpful than the static roll gap in improving the deep triangular zone cracks and internal center segregation of the slab. Especially when the condition of the sector section deteriorates, the effect of the dynamic roll gap is more obvious.