During dendritic solidification of castings and ingots, a number of processes take place simultaneously within the semisolid region. These include crystallization, solute redistribution, ripening, interdendritic fluid flow, and solid movement. The dendritic structure which forms is greatly affected by convection during the early stages of solidification. In the limit of vigorous convection and slow cooling, grains become spheroidal. Alloys with this microstructure possess rheological properties in the semisolid state which are quite different from those of dendritic alloys. They behave thixotropically, and viscosity can be varied over a wide range, depending on processing conditions. The metal structure and its rheological properties are retained after solidification and partial remelting. The semisolid alloys can be formed in new ways, broadly termed «semisolid metal (SSM) forming processes». Some of these are now employed commercially to produce metal components and are also used to produce metal-matrix composites

Behavior of metal alloys in the semisolid state

The influence of composition and cooling rate on the hot ductility of steels has been reviewed. Models to predict hot ductility behaviour have been discussed and the parts of the trough which can be used to predict the likelihood of cracking occurring are highlighted. On tensile testing both deformation induced ferrite in sufficient quantity to improve ductility and dynamic recrystallisation occur but not when straightening during continuous casting; the strain being too low. This limits the use of the hot ductility curve in predicting cracking behaviour. The temperature range in which straightening of the continuously cast strand should be carried out is either 30°C below the Ar 3 when there is a large amount of ferrite (∼40%) present before deformation or above the T d, the temperature at which dynamic recrystallisation starts to take place in a tensile test; this being when the ferrite film no longer forms and precipitates are sufficiently coarse and few in number to influence the ducti...

Hot ductility of steels and its relationship to the problem of transverse cracking in continuous casting

A simple analytical model of microsegregation for the solidification of multicomponent steel alloys is presented. This model is based on the Clyne-Kurz model and is extended to take into account the effects of multiple components, a columnar dendrite microstructure, coarsening, and the δ/γ transformation. A new empirical equation to predict secondary dendrite arm spacing as a function of cooling rate and carbon content is presented, based on experimental data measured by several different researchers. The simple microsegregation model is applied to predict phase fractions during solidification, microsegregation of solute elements, and the solidus temperature. The predictions agree well with a range of measured data and the results of a complete finite-difference model. The solidus temperature decreases with either increasing cooling rate or increasing secondary dendrite arm spacing. However, the secondary dendrite arm spacing during solidification decreases with increasing cooling rate. These two opposite effects partly cancel each other, so the solidus temperature does not change much during solidification of a real casting.

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Simple Model of Microsegregation during Solidification of Steels

In order to elucidate the decomposition mechanism of Na2CO3 and Li2CO3 in mold-powder systems employed in the continuous casting of steel, decompositions of Na2CO3 and Li2CO3 were investigated using thermogravimetric (TG) and differential scanning calorimetric (DSC) methods at temperatures up to 1200 °C, under a flow of argon gas. For the case of pure Na2CO3, the thermal decomposition started from its melting point and continued as the temperature was increased, but at a very slow rate. For Li2CO3, however, the decomposition occurred at much faster rates than that for Na2CO3. When carbon black was added to the carbonate particles, the decomposition rates of both Na2CO3 and Li2CO3 were significantly enhanced. From mass-balanced calulations and X-ray diffraction (XRD) analyses of the reaction products, it is concluded that decompositions of Na2CO3 and Li2CO3 with carbon black take place according to the respective reactions of Na2CO3 (1) + 2C (s) = 2Na (g) + 3CO (g) and Li2CO3 (l) + C (s) = Li2O (s) + 2CO (g). It was found that liquid droplets of Na2CO3 were initially isolated due to carbon particles surrounding them, but, as the carbon particles were consumed, the liquid droplets were gradually agglomerated. This effected a reduction of the total surface area of the carbonate, resulting in a dependence of the decomposition rate on the amount of carbon black. For the case of Li2CO3, on the other hand, hardly any agglomeration occurred up to the completion of decomposition, and, hence, the rate was almost independent of the amount of carbon black mixed. The apparent activation energies for the decomposition of Na2CO3 and Li2CO3 with carbon black were found to be similar and were estimated to be 180 to 223 kJ mole−1.

Thermal and carbothermic decomposition of Na 2 CO 3 and Li 2 CO 3

To estimate the cracking condition in continuously cast steels, a new model for critical fracture stress given from the measured critical strain has been proposed, which can take into account the brittle temperature range and strain rate. The effects of brittle temperature range and strain rate on critical strain for internal crack formation were analyzed. When the brittle temperature range and strain rate were increased, the possibility of internal crack formation increased due to the decreasing critical strain. To describe the thermomechanical property model of the mushy zone between zero strength temperature (ZST) and zero ductility temperature (ZDT), the yield criterion for porous metals, which can take into account δ/γ transformation, was used. Using the fitting equation for the measured critical strain and the microsegregation analysis, the thermomechanical behavior of the mushy zone could be successfully described by the proposed model, which incorporates the effects of microsegregation of solute elements and δ/γ transformation on hot tear during solidification at the given range of steel compositions and strain rates. A cracking criterion based on the difference of deformation energy in the brittle temperature range is proposed to explain the cracking phenomenon of whole carbon range.

A new criterion for internal crack formation in continuously cast steels

Characteristic features of the embrittlement occurring in the temperature range from 1 000 to 600°C in various kinds of steels were examined by hot stage tensile test as well as fractographic analysis, and the mechanism of this embrittlement is discussed.The embrittlement is caused by the slower strain rate of the tensile test and fracture occurs along the austenite grain boundary by either grain boundary sliding or by localization of strain in the film-like proeutectoid ferrite produced by the γ-α transformation.Steels such as ferritic steel and electrolytic iron do not have this embrittlement, while carbon steels containing 0.05 to 0.4% carbon and fully austenitic steels reveal this embrittlement. In the case of carbon steels, sulfur, nitrogen and niobium are detrimental elements while aluminum and phosphorus suppress it depending on their content and state of existence in the steels.The stress-strain analysis by an Instron type machine shows that the restoration process either by dynamic recovery or by dynamic recrystallization plays an important role in this embrittlement. Another finding is that proeutectoid ferrite deforms preferentially in the austenite plus ferrite region because of very low levels of flow stress and work hardening rate in this ferrite.Thus, factors governing this embrittlement are the degree of ease of the recrystallization in austenitic steel relating to the grain boundary sliding, and the formation of film-like proeutectoid ferrite produced by the γ-α transformation in the case of carbon steels. Grain boundary precipitates such as sulfides and carbonitrides act as nuclei of voids and thus promote this embrittlement.

Embrittlement of Steels Occurring in the Temperature Range from 1 000 to 600°C

To improve the quality of continuously cast carbon steels, the characteristics of the hot ductility were studied via in-situ melted and solidified tensile testing method. Special emphases were placed on the effect of C and P on the hot ductility in the temperature range between melting temperature and 700°C.It was found that zero strength temperature (ZSTc) and zero ductility temperature (ZDTc) have a linear relationship with the equilibrium solidus temperature in Fe-C binary alloys ranging from 0.003 to 1.6% C and does not have the singularity between 0.1 to 0.2% C content. The embrittlement due to P exists only in the carbon steels having C content higher than 0.25%, which are subjected to the melting thermal history. This embrittlement is caused by the segregation of P along the austenite grain boundary.Mild cooling in the secondary cooling stage is very effective to diminish the embrittlement resulting in producing defect free cast slabs.

Improvement of Hot Ductility of Continuously Cast Carbon Steels

An in-situ melt-bending test method has been developed, which permits the simple evaluation of the critical strain for the internal crack initiation in continuously cast slabs. In this testing method, the center of the top surface of a specimen in the form of rectangular parallelepiped is melted by high-frequency induction heating and the heating current is then lowered to form a solidified layer about 20mm in thickness. The specimen under this condition is deformed by bending. The amount of strain applied to the solid-liquid interface is changed by changing the amount of bending. This test is conducted in an argon gas atmosphere. After the test, the internal cracking in the specimen is investigated and the critical strain for the internal crack initiation is determined by comparing the cracking condition with the amount of the strain applied. The critical strain was measured in six carbon steels by the above-mentioned method. It has been found that the difference in the critical strain among the steel grades is correlated to the difference between the calculated liquidus temperature and the calculated solidus temperature and that the greater the difference, the smaller the critical strain will be. It is also revealed that the critical strain values obtained by the other researchers show the same tendency.

An Evaluation of Critical Strain for Internal Crack Formation in Continuously Cast Slabs

Hot Ductility in Steels in the Temperature Range between 900 and 600°C

A laboratory-scale test was made in which iron oxide contained in a water-cooled crucible was melted and reduced by using a 10-50% H/sub 2/-Ar transferred arc plasma. The degree of reduction was found to be proportional to the amount of hydrogen fed. The efficiency of hydrogen utilization for the reduction was 50-70%, which is much higher than equilibrium values below 3000 K. This high efficiency was attributable partially to the reactivity of the hydrogen atom in a plasma and partially to the continuous contact of the hydrogen plasma with the molten iron oxide layer floating over the liquid iron formed. During the plasma reduction, evaporative loss of phosphorus was observed. The degree of phosphorus removal depended on the weight ratio, CaO/(SiO/sub 2/+Al/sub 2/O/sub 3/). H/sub 2/-Ar plasma was shown to be far superior for the phosphorus removal, compared with Ar and Ar-N/sub 2/ plasma.

Yasushi Nakamura

Papers

Embrittlement of Steels Occurring in the Temperature Range from 1 000 to 600°C

Improvement of Hot Ductility of Continuously Cast Carbon Steels

An Evaluation of Critical Strain for Internal Crack Formation in Continuously Cast Slabs

Hot Ductility in Steels in the Temperature Range between 900 and 600°C

Reduction and dephosphorization of molten iron oxide with hydrogen-argon plasma