Comparative analysis of the structure of solid and hollow steel cast billets

This paper presents the research results of solid structure and hollow steel billets obtained by continuous casting. To substantiate the feasibility of using a hollow billet as an initial one in the production of seamless hot-rolled pipes, a comparative analysis of the distribution of non-metallic inclusions, macro- and microstructure, as well as segregation by structural zones was carried out. When analyzing the macrostructure of a hollow billet, two distinct zones were revealed: equiaxed small and columnar crystals, which distinguishes it, compared with a solid billet, by the absence of a zone of misoriented crystals. This, in turn, helps to eliminate defects such as axial porosity and segregation. The improved quality of the macrostructure during casting of a hollow billet is explained by more favourable conditions for heat removal and a higher rate of solid-phase advance due to bilateral cooling, and less shrinkage of the melt due to its cross-sectional geometry. The distribution of nonmetallic inclusions, consisting of oxide, sulfide and oxysulfide compounds, and liquidation elements, showed that they are concentrated mainly at the boundaries of crystalline zones, and for a solid billet and in the central part. This fact is caused by the development of a zone of intense heat removal. When research the microstructures of solid and hollow workpieces, a ferrite-pearlite mixture is observed in both cases. The microstructure of the hollow billet is more dispersed, which is confirmed by durometric measurements.

Estimation of minimal slab reduction to eliminate of axial porosity

In the production of the continuous cast slabs using continuous-casting machine, the formation of axial porosity is observed. Method for determination of the minimal reduction of the workpiece during hot rolling in order to eliminate the axial defect is presented. The theoretical dependences for transformation of the axial porosity on drawing and the reduction rate of continuously cast slab during hot rolling are obtained based on assumptions.

Additional shearing impact on the effectiveness of MSR technology in conditions of billet CCM

Aside from electromagnetic stirring, casting with low superheat and intensive cooling of the strand in the upper range of secondary cooling zone, Mechanical Soft Reduction (MSR) has proved, above all, to be very effective in reducing segregation and axial porosity in continuously cast billet. Implementation of MSR technology in the production of continuously cast billets has a number of features that are due to their square shape. In this case, particularly promising is the use of blocks of segment design, so called pinch-roll segment. The presence in CCM line of MSR block of such design allows to implement a two-stage deformation scheme. The paper proposes a new two-stage scheme of MSR technology realizing the combined deformation on the basis of cobbing in vertical plane and shearing relative displacement of the faces at the first stage, and at the second stage – deformation on the basis of cobbing in vertical plane. This approach additionally helps to correct deformations of the profile cross section, namely the rhomboidity defect. We present the results of a comparative study using physical modeling methods to assess the contribution of additional shear relative displacement of faces in the horizontal plane to the overall efficiency of MSR technology of continuous casting. The use of a flat model in conjunction with the proposed form of deforming rolls and a combination of modeling materials allowed to achieve a good similarity in geometric criterion, as well as in the criterion of stress ratio equivalence arising at the interface of crystallization front. The obtained experimental data helps to develop ideas about the mechanisms of additional positive effect from the application of shear action. In particular, the deformation of metal surface and adjacent layers of the billet in the rolls with a special above-described profiling will improve their quality due to the occurrence of shear deformations intensifying the process of collapse of subcortical bubbles, “healing” of microcracks, etc. In turn, the artificial creation of torque effect in cross section of the billet will contribute to the occurrence of shear deformations in the crystallized “bridges” of axial liquid-solid region of the ingot, thereby intensifying the process of their destruction and improving the quality of the billet’s macrostructure.

Mathematical Model Approach to the Solidification of Different Geometry Ingots and the Development of Shrinkage Defects in them

A mathematical modeling approach as well as experimental data analysis have made it possible to establish significant factors affecting the relative diameter of the axial porosity zone. The minimal values of this parameter determine if the ingot can be used for the fabrication of rolled steel rods over 300 mm in diameter, because chill extensive axial defects prevent from producing high quality bars of a large diameter. Commercial information analysis and experimental results have enabled to develop a model relating the axial porosity zone dimension, ingot geometry and process parameters of teeming 6.61 ton and 7.0 ton ingots. The improvement of the model obtained has enabled to establish that the axial porosity zone is primarily affected by the following factors: hot top size, slenderness ratio, the H/D ratio and insulation heat capacity. When these parameters are controlled to reduce the relative diameter of the axial porosity zone, the number of shrinkage defects decreases and the quality of large diameter rolled steel becomes better. The proposed ingot geometry improves the direction of the advance of the metal solidification front to the ingot thermal center, located in the hot top. Besides, the solidifying metal is better fed with the hot top melt.

Effect of Continuous Casting Parameters on Quality of Billets Manufactured by UMMC Steel LLC

The analysis of the production records on casting 150õ150mm billets at UMMC Steel LLC has made it possible to define the basic factors worsening the billet quality and increasing the rejection of the rolled materials: steel casting in case the sulfur and phosphorus content exceeds 0.015 % and overheat is more than 30°Ñ. In order to minimize the formation of local nonuniformity in the billets manufactured from low carbon and medium carbon steel, open cast by CCM, it is necessary to maintain a maximum [Mn]:[S] ratio with [Mn]:[Si] ratio at the level less than 2-3, which ensures the formation of fusible deoxidation products and prevents the formation of solid conglomerates in steel. In order to reduce the formation of axial porosity it is advisable to retain the carbon content at the lower limit for the steel grade and avoid overheating of steel above the specified values. In order to minimize the consequences of the overheated steel casting it is necessary to adjust the billet casting rate in accordance with the operating procedures.

The Influence of the Shape of Grooves on the Intensity of Closing Axial Material Discontinuities During Rolling

The article discusses the influence of rolling in the newly designed slitting-bending grooves on closing internal material discontinuities in continuous ingots. The defects located in the axial area of a strand, i.e. axial porosity and contraction cavity, were analysed. Numerical and experimental studies of the rolling process of flat bars for feedstock with marked materials discontinuities simulating actual defects occurring in an ingot were conducted. For comparison purposes, rolling of feedstock with discontinuities in traditional grooves was conducted. The numerical simulations were carried out using the Forge 2008¯software programme. The experimental studies were conducted in a D150 laboratory rolling mill. In the research, the S355J2G3 structural steel and lead were used. Distributions of temperature, intensity of deformation and stress in a cross-section of strands after rolling were analysed. Changes in the surface areas of the discontinuities in feedstock after rolling in shape and traditional grooves were examined. It was concluded that introduction of grooves with complex shapes may contribute to the improvement of internal quality of finished products. The complex shape of the tools has allowed for an almost 100% closure of the discontinuities located in the central part of a strand, already at the initial stages of the process, unlike in case of the traditional production method, where the discontinuities were transferred even until the final product. Applying the newly designed slitting-bending grooves enables utilisation of a smaller number of edge grooves at the initial stage of the rolling process, where reopening of freshly welded defects might occur.

Features of Deformation of Partly Crystallization Blooms at their Two-Stage Soft Reduction

Steady increase of requirements to quality an axial zone continuously cast blooms stimulated in last time development of principles of its deformation at the end of solidifications with the purpose of suppression of axial porosity and segregation: soft reduction or mechanical soft reduction. The technology of soft reduction is one of the most effective ways of improvement quality of internal layers continuously cast blooms. In this case the central part of section of continuously cast blooms is in liquid or liquid-solid condition (mushy zone). At the same time, in practice now there are, at least, some original technical decisions for realization of this method.