Roles of titanium-rich precipitates as inoculants during solidification in low carbon steel

The effect of titanium on the as-cast structure and the growth form of titanium precipitates, and the effect of cooling rate on the size and distribution of titanium precipitates were studied. It is shown that Ti-rich precipitates acting as heterogeneous nucleation sites play an important role in refining the grain size and increasing the equiaxed grain ratio. Cooling rate has a great effect on the size and distribution of precipitates. The number of precipitates increases and the size decreases with the increase of cooling rate. Ti-rich particles acting as het- erogeneous nucleation sites at the onset of solidification are observed in the experiment. This result suggests that TiN nucleated on Ti2O3 is an effective inoculant for δ-ferrite during solidification in low carbon steel.

Longitudinal corner crack of chamfered slab and optimization of mould

Chamfered mould has gradually become a new technology equipment to eliminate transverse corner crack.However,longitudinal corner crack ratio of chamfered slab was very high.Solidification microstructure was detected in the area where the longitudinal corner crack occurred.Effect of narrow face shape and taper distribution of mould copper plate on longitudinal corner crack was studied by industrial tests.Water velocity distribution in the narrow copper plate was studied by numerical simulation.On the premise of preventing cooling water from nuclear boiling,improvement measures of mould cooling process and water seam structure were put forward through heat transfer calculation.The results showed that local taper of the meniscus region of mould should be increased to prevent generation of longitudinal corner crack.Chamfering slope length of narrow copper plate should be controlled within 55 mm,and chamfering angle should be controlled at about 30°.Average water velocity should be more than 7.0 m/s.The flat and chamfering regions of narrow copper plate working face should be designed as double taper and funnel structure,respectively.The water seam in the chamfering region should preferentially choose the combined cooling structure with two holes and one slot.In order to prevent the extension of longitudinal corner crack,chamfered narrow face foot roller should be used.

Technology Development for Controlling Slab Transverse Corner Crack of Typical Micro-alloyed Steels

A mathematical model for simulating the fluid flow, heat transfer and solidification in the conventional mold and the chamfer mold, together with a finite element stress-strain model in the straightening process of both molds, were established for the typical niobium, vanadium, and titanium micro-alloyed steels. On the basis of both numerical analysis, the mold copper plate with an optimum chamfered shape was designed and applied in industrial tests. The predicted results from numerical simulation of fluid flow, heat transfer and solidification in the conven tional mold and the chamfer mold show that the increased chamfered angle leads to an approximately linear increase o[ the slab surface temperature, but it also causes strong flow near the slab corner. Very small chamfered length can lead to a significant increase of the temperature near the slab corner. However, with further increasing the chamfered length, the temperature of the slab corner increased slightly. The calculated results from the finite element analysis of stress-strain during the straightening process show that at the same slope width, the tangential strain on the slat） edges and corners is minimum when the chamfered angle is 30° and 45°, which is only 40° to 46° of rectangular slabs with the same cross-section area. At the same chamfered angle of 30°, when the chamfered length is controlled between 65-85 mm, the tangential strain on the part of the slab edges and corners is relatively smaller. Industrial test results show that the slab corner temperature at straightening segment increases about 100 ℃ by using chamfer mold compared to the conventional molds. The slab transverse corner cracks have been reduced more than 95° in comparison with those in the conventional mold.

Mechanism and Improvement of Straight Edge Seam Defect on Hot-rolled Plate Surfaces through Use of Chamfered Slabs

To improve the straight edge seam defect on hot-rolled steel plates,the deformation and temperature distribution of rectangular slabs and chamfered slabs during rolling in a pilot rolling experiment were analyzed in detail using the finite element method.The results showed that the crease formed on the lateral side near the edge of the plate as a result of uneven stress during broadside rolling.The creases rose to the surface with unrestricted spread and evolved into a straight edge seam during the subsequent straight rolling.To eliminate the straight edge seam defect,chamfered slabs were developed and investigated for rolling.The use of the chamfered slabs provided two advantages for rolling：the distribution of the temperature near the edge was ameliorated,and the deformation shape was improved by the chamfered shape.As a result,the risk of forming a straight edge seam defect was reduced by the use of a chamfered slab.

Effect of Solute Elements and Cooling Rate on Strain in Brittle Temperature Range of Continuously Cast Strand

A micro-segregation model of solute elements in mushy zone with δ/γ transformation during solidification was established based on the regular hexagon transverse cross section of dendrite shape proposed by finite difference method under the non-equilibrium solidification condition. The model was used to calculate the non-equilibrium pseudo binary Fe-C phase diagram and the strain of steels induced by variation of temperature in brittle temperature range. On the basis of the phase diagram and the strain, the strain curve in brittle temperature range as a function of carbon content for continuously fast strand was introduced and obtained, Solute elements change the position of the strain curve. And cooling rate changes the position arid the shape of the strain curve. The comprehensive formula of the strain as functions of solute elements and cooling rate in brittle temperature range has been obtained by nonlinear fitting program.