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During the production of Al-killed titanium-alloyed interstitial free steel, to reduce defects in cold rolled sheets and decrease the influence of inclusions on the properties of the steel, it is important to clarify the distribution of inclusions in the thickness direction of IF (interstitial free) steel along the slab. In this study, standard metallographic techniques were employed to analyze the total oxygen and nitrogen by performing scanning electron microscopy, energy spectroscopy, automatic scanning electron microscopy, and original morphology analysis. The results show that the average mass fractions of T.O and N are 1.6 × 10−5 and 1.7 × 10−5, respectively, and the T.O for the 1/8 thickness from the inner arc is 2.0 × 10−5, while the content of N for between the 1/4 and 3/8 thickness from the inner arc is 1.8 × 10−5. A total of 1177 inclusions were counted. More than 70% of inclusions are within 5 μm in size, and the average size of inclusions in the thickness direction is 2.8 μm. The sizes of inclusions for the 3/8 thickness from both the inner and outer arcs are larger at 4.0 μm and 4.4 μm, respectively. The amount of precipitation of TiN is large in the slab center, and there are mainly Al2O3 and Al2O3–TiOx near the inner and outer arc surfaces with sizes between 5 and 10 μm. Al2O3–TiN distributes irregularly in the 1/4 thickness from the inner and outer arcs, and the size fluctuates between 3 and 5 μm. The size of TiN during solidification fluctuates between 3 and 6 μm. TiN precipitates in the liquid and δ phase of the solidification front when the solidification rate is between 0.646 and 0.680, and the size fluctuates between 3 and 6 μm.

Hook characteristics in continuous casting slabs of low-carbon steel under different casting conditions were investigated. According to hook morphology, hooks were classified into different types. Distributions of bubbles and inclusions near hooks were analyzed, and the influence of casting parameters on hook depth was discussed. Bikerman's equations and theory of meniscus solidification were used to explain the various hook morphology. Results show that on the basis of morphology hooks can be classified into four types, including whole-leaf type, double-hook type, truncated type and re-solidified type. The percentage of re-solidified type is the highest (46.8%), and the percentages of whole-leaf type, truncated type and double-hook type are respectively 25.3%, 7.6% and 6.3%. Studies demonstrate that the inclusion number near hooks is obviously higher than that in other zone, indicating floating inclusions in the mold can be captured by hooks. Comparing different casting parameters, the use of FC-Mold, the decrease of the submerged depth of the submerged entry nozzle (SEN) and the increase of casting speed can reduce the hook depth. Calculated results of Bikerman's equation and theory of meniscus solidification can explain the formation mechanisms of different hook morphologies.

Aimed at the frequently occurred problem of slab camber in rough rolling of hot strip mills, the main factors causing slab camber, including the difference in longitudinal stiffness of the mill on both sides, incoming wedge and running deviation of rolled pieces, were analyzed by combining with the spring equations and analytical method in this paper, and the corresponding adjustment values were respectively calculated. Based on rolling force difference on both sides, the tilting control model of slab camber was established. This model could reflect the quantitative relationship between the main factors and camber, such as the difference in longitudinal stiffness of the mill on both sides, incoming wedge and running deviation of rolled pieces, etc., and then the tilting values of roll gap for controlling camber were calculated in each pass of the rough rolling mill. Comparing with the real values by off-line verification, the averaged ratio of measured and calculated values is 0.977. The results show that the tilting values of roll gap can be estimated by the tilting control model of slab camber in each pass. Since putting the model into a 2250 mm hot strip mill, the camber values not complying with tolerance declined from 24.88% to 6.62%. By improving the control effect of camber, the slab camber problem has been greatly eased.

To solve the detection problems of slab surface defects by conventional image recognition algorithms, this article introduces an improved multi-block local binary pattern algorithm which considers the image's pixels. In this algorithm, the original image is divided into several small regions, each small region is equally divided, and the average gray value is calculated. Then the local binary pattern algorithm is used. Five different kinds of 1697 samples gathered from a production line of slabs were examined, including cracks, scratches, indentations, dents, and no defect. The recognition rate reaches 94.9%, while the recognition rate of the traditional local binary pattern method is 89.1%. The results show that the proposed algorithm has the characteristics of high precision, better robustness and noise immunity.

The crack morphology and element segregation in continuous casting slabs were studied to solve the problem of internal cracks. The stress of the solidification front was analyzed by a finite element model at a series of reduction parameters and compared with the critical value. It is found that C,P and S enrich in dendrites and exist in grain boundaries,these are just the internal causes,but the true cause for internal cracks is the tensile stress of the solidification front. When the slab passes through the straightening segment,the reduction effect of rollers results in the stress of the solidification front exceeding the critical value,however,the corresponding stresses in the arc and horizontal segments do not exceed the critical value,so the solidification front in the straightening segment would be much easy to crack. Statistics analysis shows that when soft reduction and straightening are separately carried out,the occurrence rate of internal cracks reduces by about 41.3%.

Inclusions in the surface of low carbon aluminum killed steel slabs cast at high casting speed were analyzed by Aspex automated scanning electron microscopy in a large area. The changes in number and size of inclusions were tested at different casting speeds,and the influence of flow fields and sulfur content on the distribution of inclusions was explained in detail. It is found that when the casting speed increases,the depth and length of hooks decrease. Hooks in the slab subsurface is not obvious when the casting speed larger than 2 m·min-1,so inclusions larger than 200 μm cannot be found in the slab surface. Inclusions with the size of 50-200 μm in the slab surface are mainly entrapped by the solidified shell,and the entrapment of inclusions is influenced by forces acting on inclusions at the solidifying front. The flow velocity of molten steel increases due to increasing casting speed,then the washing force becomes large and the entrapping force becomes small,and therefore the number of entrapped inclusions decreases. If the sulfur content in the steel is high at high casting speed,inclusions will be easily entrapped by the solidified shell due to the thermal Marangoni force acting on inclusions obviously.

The cleanness evolution and inclusion distribution of IF slabs in the thickness direction were studied by original morphology analysis, scanning electron microscopy and energy spectroscopy analysis, and total oxygen (T. O) and nitrogen analysis. The average values of mass fraction of total oxygen and N in the slabs in the thickness direction are within 17 × 10-6. The mass fractions of T. O and N in the area within 1/16 thickness from the slab surface are 5% to 10% higher than the average values, in the area between 1/4 thickness from the inner are and 1/4 thickness from the outer arc are 5% to 10% lower than the average values and in other area are close to the average levels. The average size of inclusions is 5.7 txm based on a total of 963 statistical inclusions. Inclusions with the particle size smaller than 5μm account for 60% and with the particle size smaller than 10 txm account for 90%. Al2O3 mainly exits within 5 mm from the slab surface with the size of 2 to 10μm; TiN-Al2O3 and TiN exist in the area from 5 to 80 mm in the thickness direction, and their particle size grows up as the thickness increasing; TiN-TiS and TiS inclusions mainly exist in the area from 80 to 130 mm in the thickness direction with the size of 1 to 5μm. Inclusions from the slab surface to center are mainly Al2O3, Al2O3-TiN, TiN, TiN-TiS, TiS, and MnS in order.

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