Coordinating optimisation of F-EMS and soft reduction during bloom continuous casting process for special steel

To control the halfway cracks and shrinkage cavities during bloom continuous casting (CC) process, final electromagnetic stirring (F-EMS) and soft reduction techniques have been coordinately optimised. The halfway crack and shrinkage cavity can be successfully controlled by adopting the CC technique parameters described as follows: (1) casting speed is 0.62 m min-1, (2) secondary cooling water ratio is 0.2?L?kg^?1, (3) reduction amount is 18?mm; (4) reduction region ranges from 19.9?m (location of M3 roller) to 25.4?m (location of M9 roller) distance from meniscus; and (5) corresponding distributions of reduction amount for each roller are 1, 1, 2, 3, 4, 4, and 3?mm along the casting direction. As compared to origin scheme for bearing steel, the difference between the maximum and minimum segregation degrees at the strand centre can be reduced from 0.35 to 0.09 under the optimal case.     

Ternary diffusion in the α and γ phases of the Fe-Ni-P system

Ternary diffusion coefficients have been determined in the α-bcc and γ-fcc phases of the Fe-Ni-P system and at four temperatures 1200, 1100, 1000, and 900°C. At 1100°C the ratio ofD _NiNi ^Fe /D _PP ^Fe in the α phase is 0.3 to 0.4 and the ratio of the cross coefficientD _PNi ^Fe /D _PP ^Fe is 0.03 to 0.05. m the γ phase the corresponding ratios are 0.01 to 0,02 and 0.0015 to 0.0025. The other cross coefficientD _NiP ^Fe could not be evaluated because of experimental uncertainties. Estimates of the ratioD _NiP ^Fe /D _NiNi ^Fe using the interaction parameter ε₁₂ are 0.004 to 0.01 in the α phase and 0.02 to 0.04 in the γ phase. The addition of P in both the α and γ phase increases the major ternary coefficients up to as much as a factor of ten at one temperature. This is consistent with the fact that P lowers the melting point of FeNi in the ternary system, up to 500°C. Isodiffusion coefficient contours obtained at 1100°C plot approximately parallel to the α and γ solidus boundaries and are similar in shape to the α and γ solidus boundaries as a function of temperature. Activation energies andD ₀ values were computed at selected compositions in both α and γ phases forD _PP ^Fe andD _NiNi ^Fe and are given below: Formerly Graduate Student, Department of Metallurgy and Materials Science, Lehigh University, Bethlehem, Pennsylvania     

Determination of the heat-transfer coefficient between the AK7ch (A356) alloy casting and no-bake mold

The heat-transfer coefficient h between a cylindrical cast made of AK7ch (A356) aluminum alloy and a no-bake mold based on a furan binder is determined via minimizing the error function, which reflects the difference between the experimental and calculated temperatures in the mold during pouring, solidification, and cooling. The heat-transfer coefficient is h _L = 900 W/(m² K) above the liquidus temperature (617°C) and h _S = 600 W/(m² K) below the alloy solidus temperature (556°C). The variation in the heat-transfer coefficient in ranges h _L = 900–1200 W/(m² K) (above the alloy liquidus temperature) and h _S = 500–900 W/(m² K) (below the solidus temperature) barely affects the error function, which remains at ~22°C. It is shown that it is admissible to use a simplified approach when constant h = 500 W/(m² K) is specified, which leads to an error of 23.8°C. By the example of cylindrical casting, it is experimentally confirmed that the heat-transfer coefficient varies over the casting height according to the difference in the metallostatic pressure, which affects the casting solid skin during its solidification; this leads to a closer contact of metal and mold at the casting bottom.     

The thermodynamics of melts in the system VO2−V2O5

Using a gravimetric technique, the oxygen isobars in the liquid phase field of the system VO₂−V₂O₅ have been determined in the temperature range 1020° to 1100°C and in the oxygen pressure range 0.01 to 0.4 atm, and the VO₂ liquidus and solidus curves have been determined in the temperature range 953° to 1081°C. From these results have been calculated the activities of the components VO₂ and VO_2.5 in the system and the standard free energy change for the reaction 2VO_{2 (S)}+1/2O_{2 (g)}=V₂O_{5 (1)}. The latter was determined to be ΔG ^o=−17780+7.64T in the temperature range 1020° to 1100°C.     

Influence of Isothermal Treatment on MnS and Hot Ductility in Low Carbon,Low Mn Steels

Hot ductility tests were used to determine the hot-cracking susceptibility of two low-carbon, low Mn/S ratio steels and compared with a higher-carbon plain C-Mn steel and a low C, high Mn/S ratio steel. Specimens were solution treated at 1623 K (1350 °C) or in situ melted before cooling at 100 K/min to various testing temperatures and strained at 7.5 × 10^?4 s^?1, using a Gleeble 3500 Thermomechanical Simulator. The low C, low Mn/S steels showed embrittlement from 1073 K to 1323 K (800 °C to 1050 °C) because of precipitation of MnS at the austenite grain boundaries combined with large grain size. Isothermal holding for 10 minutes at 1273 K (1000 °C) coarsened the MnS leading to significant improvement in hot ductility. The higher-carbon plain C-Mn steel only displayed a narrow trough less than the Ae₃ temperature because of intergranular failure occurring along thin films of ferrite at prior austenite boundaries. The low C, high Mn/S steel had improved ductility for solution treatment conditions over that of in situ melt conditions because of the grain-refining influence of Ti. The higher Mn/S ratio steel yielded significantly better ductility than the low Mn/S ratio steels. The low hot ductility of the two low Mn/S grades was in disagreement with commercial findings where no cracking susceptibility has been reported. This discrepancy was due to the oversimplification of the thermal history of the hot ductility testing in comparison with commercial production leading to a marked difference in precipitation behavior, whereas laboratory conditions promoted fine sulfide precipitation along the austenite grain boundaries and hence, low ductility.     

Structure of powder brazing alloys based on the system Ni-Cr-Si-Fe-B-C(-Mo)

Conclusions A study was made of the sinterability of PKh12N75S8R brazing alloy powder and a mechanical mixture of this powder with 15% of molybdenum (brazing alloy No. 6MA), which are widely used for the brazing of high-alloy steels. Appreciable shrinkage of compacts begins at 1000°C and steadily increases with rise in sintering temperature. The most intense shrinkage is observed above 1100°C, i.e., at temperature close to the solidus. The structure of brazing alloy No. 6MA produced by sintering at 1175–1200°C or melting consists of a solid solution and molybdenum suicides and chromium borides of various compositions.Translated from Poroshkovaya Metallurgiya, No. 10 (118), pp. 77–84, October, 1972.     

Formation and control of macrosegregation for round bloom continuous casting

Based on the developed coupled model of electromagnetism, heat and solute transportation, the macrosegregation formation and effect of secondary cooling water ratio on macrosegregation degree in strand during round bloom continuous casting process have been investigated. The solute segregation degree fluctuates from a positive to a negative value with distance from strand surface in the initial solidified shell region within thickness of 20?mm. A negative segregation region in concave shape and an irregular positive segregation zone are presented in the fixed and loosened side of strand respectively due to the gravity and thermosolutal convection. As the secondary cooling water ratio decreases from 0.25 to 0.15?L?kg^??1, the solidification ratio at final electromagnetic stirring (F-EMS) centre increases from 73.14 to 77.83%. For the steel grade of 50Mn casted by round bloom casting within diameter of 0.35?m, the optimal solidification ratio at F-EMS centre is 75.05%, where the radial centre crack and shrinkage cavity at strand cross-section are removed.     

Optimisation of cutting length of continuously cast strands

Abstract

Weight per metre and strand circumference were measured, using sensitive methods, on 177 mm round strands, with particular attention being paid to phase transformation and thermal shrinkage in the vicinity of the peritectic. The following results were achieved for a broad range of steel grades. With constant mould dimensions, the weight per metre of round continuously cast strand increases as casting speed rises. Soft steels, such as grade S35 containing 0·10%C, have a low weight per metre, whereas harder grades, such as C60 with 0·60%C, have the highest weight per metre. Low alloyed steels and oilfield tubular grades occupy rankings between these extremes. Martensitic and more highly resulphurised round billets have a conspicuously low weight per metre. The following definitive influencing factors on weight per metre became apparent: expansion of the mould tube under exposure to heat; shrinkage as a result of δ - γ transformation; creep processes under exposure to ferrostatic pressure; density of the compact steel; and porosity in the strand centre. These influences lessen in the order in which they are listed; they are, in some cases, contradictory, and balance one another out.     

The solubility of Ca in Mn melt and the third-element interaction effects

Solubility experiments of Ca in Mn melt were carried out in a Mo-wire-heated furnace. Small CaO crucibles, charged with the Mn alloys with small amount of Ca chips on top, were placed in an iron crucible, its lid being tightly sealed under red-hot conditions. Relation of the solubility of Ca in pure Mn with temperature was formulated between 1 250 and 1 380°C. The effects of 3rd elements upon the solubility of Ca in Mn at 1 350°C were studied. Diagram showing the relation of Ig f^j_Ca with [%j] (j = Si, Cr, Ni, Fe, Al, C and Ca) was drawn and e^j_Ca evaluated. With the formula of Wagner, In γ_Ca⁰ and ε_Ca^Ca were found by regression analysis from the experimental data. The standard free energy of solution of Ca in Mn, based on the 1 wt.% standard at 1350°C was calculated, and its relation with T estimated.     

The rate of formation of intermetallic layers between aluminum and steel below 660°C

The rate of formation of intermetallic compounds between aluminum and three ferritic steels, one austenitic steel, and Inconel has been determined by an electrolytic method. The steel was held at zero potential with respect to aluminum in a NaCl-AlCl₃ melt, and the current measured. Comparison of measured thicknesses of intermetallic layers with those calculated from the integrated current gives an average deposition efficiency of 95 pct. For the Type 304 austenitic steel thickness (min), andk is given by logk= −6400/T(⁰K) +4.469. The ferritic steels show a linear rate of growth of Al₅Fe₂, with an initial higher rate such that extrapolation of the linear curve back to zero time gives an intercept of 16±7 μm. The rate constants (mm min⁻¹) may be represented by log (rate)=α/T+β, and the values of α and β are respectively −2650 and−0.788 for a plain carbon steel,−6580 and + 3.469 for a 1.3 pct Cr, 0.4 pct Mo steel, and−5950 and +2.466 for a 2.2 pct Cr, 0.9 pct Mo steel. The more highly alloyed steels are thus attacked, more slowly. Results for Inconel could not be fitted to any simple equation. With the ferritic steels growth is by aluminum diffusing inwards; with Inconel it is by nickel diffusing outward.     

Effect of solidification cooling rate on the morphology and number per unit volume of primary Mg<Subscript>2</Subscript>Si particles in a hypereutectic Al-Mg-Si alloy

Growth morphology and number per unit volume have been determined vs withdrawal velocity V over the range 0.1 to 4 mm/s for primary Mg₂Si in a Bridgman-solidified hypereutectic Al-Mg-Si alloy. Primary Mg₂Si shows a transition from irregular or regular polyhedral to dendritic with increasing V, and increases with solidification cooling rate according to the relationship . These results are compared with corresponding ones for Al-Mg-Si alloy wedge castings and for primary silicon in hypereutectic Al-Si alloys.     

Slurry Erosion Characteristics and Erosive Wear Mechanisms of Co-Based and Ni-Based Coatings Formed by Laser Surface Alloying

In the present work, an attempt has been made to study the slurry erosion properties and operating erosive wear mechanisms of Co-based Stellite 6 and Ni-based Colmonoy 88 coatings, and also to list the conditions at which maximum and minimum erosion rates occur. Laser surface alloying (LSA) has been done on 13Cr-4Ni steels with commercial Co-based Stellite 6 and Ni-based Colmonoy 88 powders. Slurry erosion tests have been conducted on LSA-modified steels for a constant slurry velocity of 12 m/s and for a fixed slurry concentration of 10 kg/m³ of irregular, sharp-edged SiO₂ particles with average sizes of 375 and 100 μm and at impingement angles of 30, 45, 60, and 90 deg. A mixed (neither ductile nor brittle) mode of erosion behavior for Stellite 6 coatings and a brittle mode of erosion behavior for Colmonoy 88 coatings were observed when these materials were impacted with particles with an average size of 375 μm, whereas only a brittle mode of erosion was observed for both Stellite 6 and Colmonoy 88 coatings when impacted with particles with an average size of 100 μm. Mainly, chip formation, chip fracture, microcutting, plowing, and crater lip and platelet formation were observed for Stellite 6 coatings and progressive fracture of carbides, carbide pullout and carbide/boride intact were observed for the case of Colmonoy 88 coatings.     

Effect of carbon content on the plastic flow of plain carbon steels at elevated temperatures

The elevated-temperature plastic-flow behavior of plain carbon steels with a base composition of 0.8 Mn and 0.25 Si was examined as a function of carbon content in the range 0.005 to 1.54 wt pct at strain rates from 6 x 10^-6 to 2 x 10^-2 sec^-1. Beyond 0.05 C the flow stress at a strain of 0.1 decreased with increasing carbon content at the rate of 13 MPa per pct carbon. However, the degree of softening depended on the strain level at which the flow stress was measured, because the increasing carbon content also decreased the rate of work hardening. The inferred increase in recovery processes with increasing carbon content is in agreement with the effects of carbon on diffusivity, elastic modulus, and lattice spacing, as well as the observed increase in grain growth with increasing carbon content. In the range 850 to 1300 °C (1562 to 2372 °F), the temperature dependence of the flow stress can be represented by σ= A exp (-BT) whereA depends on carbon content and strain, andB depends primarily on strain rate. Extrapolation to higher temperatures yields the carbon-content dependence of the flow stress at the austenite solidus.     

Effect of overaging on mechanical properties and stress corrosion cracking of HY-180M steel

The tensile properties, fracture toughness and stress corrosion cracking (SCC) behavior of HY-180 M steel at 22 °C were studied after final 5 h overaging treatments >510 ≤650 °C. SCC tests were conducted for 1000 h with compact tension specimens in aqueous 3.5 pct NaCl solutions at a noble (anodic) potential of −0.28 V_SHE ( −0.48 V_Ag/AgC1) and a cathodic protection potential of −0.80 V_SHE (−1.0 V_Ag/AgC1). The SCC resistance improved at aging temperatures >565 °C, the most significant improvement being at −0.80 V_She, especially after 650 ° aging whereK _ISCC was raised to at least 110 MPa · m^1/2. However, this was at the expense of mechanical properties. Provided low crack propagation rates of ∼3 X 10⁻¹¹ m/s at −0.80V _SHEmay be tolerated, the best compromise between strength, toughness, and SCC resistance was obtained after 594 °C aging. Under these conditions, stress intensities as high as ∼ 110 MPa · m^1/2 can be used, with a yield strength of ∼ 1150 MPa and fracture toughness of ∼ 170 MPa · m^1/2. The retained austenite content after aging increased with aging temperature up to 25 pct by vol at 650 °C. It appeared to correlate with improved SCC resistance, but other microstructural effects associated with aging may be involved. Formerly Research Associate with theDepartment of Metallurgical Engineering , University of BritishColumbia     

Directional solidification and characterization of near eutectic Sm2Co17/Co alloys

The effects of directional solidification processing on the microstructural, compositional, and magnetic properties of near eutectic Co-Sm alloys (∼9 at. pct Sm) have been studied. Because these sytems have high melting temperatures (T _m > 1000 °C) and are quite reactive to oxidizing environments, special containment techniques during solidification were developed. Initial investigations have been performed at modest thermal gradients in the liquid,G _L ≤ 60 °C/cm (1 °C/cm = 10⁻² K/m), and over a range of furnace (solidification) velocities, 0.8 ≤V ≤ 45.4 cm/h (1 cm/h = 2.8 × 10⁻⁶m/s). Since the range ofG _L/V values, a measure of the degree of interfacial morphological stability, was rather low, aligned dendritic morphologies, macrosegregation, and transition to rod eutectic growth were encountered. The primary dendrite spacing for near eutectic Sm₂Co₁₇/Co scaled withV ^−1/2 and varied from ∼50 μm for V ≥20 cm/h to hundreds of microns forV < 10 cm/h while the rod eutectic diameter and interred spacing were an order of magnitude smaller. For both dendritic and cooperative growth, the associated permanent magnet properties were rather poor,e.g., remanence less than 4 kG (1 gauss = 10⁻⁴ Tesla) and coercive force less than 1 kOe (1 Oe = 79.577 A/m) for the smallest dendrite and rod diameter dimensions encountered, although the magnetic hardness for the rod eutectic was larger than for the dendritic microstructure. Magnetization as a function of sample orientation indicated that the easy axis of magnetization was primarily along the direction of solidification for both ferromagnetic phases.     

A Study about the Influence of Carbon Content in the Steel on the Casting Behavior

In the casting process of steels with a C‐content ranging from 0.09 to 0.53 mass%, austenite is formed as secondary crystal phase by peritectic reaction between crystal of δ ferrite and residual melt. For unalloyed or micro‐alloyed steels the C‐content or C‐equivalent influences the casting behavior of steel in the mould, such as strand shell growth, crack formation, heat transfer, temperature fluctuation in the copper plate, mould level fluctuation and oscillation marks formation. The negative casting behavior like the uneven strand shell growth, the deep oscillation mark formation, the high mould level fluctuation, the crack formation on the strand surface were found mostly for steel with C‐content or Cp between 0.10–0.13 mass%. The strand shell structure (strand shell growth, mushy zone, δ + γ phase transformation) and shrinkage of the strand shell were simulated depending on the C‐content by means of mathematical simulation. On the basis of the simulation results and of the measured high temperature strength of steel the dependence of stiffness and the irregularity of the shrinkage of strand shell on the C‐content was investigated. It was found that the stiffness and irregularity of the shrinkage of the strand shell reach the maximum value at a C‐content of about 0.12 mass%.     

Sulfide capacity of CaO-CaF2-SiO2 slags

The sulfide capacityC _S ^2- = (pct S^2-) · (P _{O 2}/P _{S 2})^1/2) of CaO-CaF₂-SiO₂ slags saturated with CaO, 3CaO · SiO₂ or 2CaOSiO₂ was determined at 1200 °C, 1250 °C, 1300 °C, and 1350 °C by equilibrating molten slag, molten silver, and CO-CO₂ gas mixtures. Higher sulfide capacities were obtained for CaO-saturated slags. A drastic decrease was observed in those values when the ratio pct CaO/pct SiO₂ is less than 2. The sulfur partition between carbon-saturated iron melts and presently investigated slags was calculated by using the sulfide capacities obtained and the activity coefficient of sulfur in carbon-saturated iron, which was also experimentally determined. For slags saturated with CaO, partitions of sulfur as high as 10,000 were obtained at 1300 °C and 1350 °C. Correlations between the sulfide capacity and other basicity indexes such as carbonate capacity and theoretical optical basicity were also discussed. Formerly with the Department of Metallurgy, The University of Tokyo.     

The ductile to brittle transition in tin-lead alloys near melting temperatures

Tensile tests were carried out on tin containing up to 2.0 wt pct lead between 160 and 220 °C at strain rates of 8.3 x 10^-3 and 8.3 x 10^-4 s^-1. Both as-cast and heavily deformed samples were examined. The samples exhibited a ductile-brittle transition at a temperatureTDB below the equilibrium solidus temperature of the corresponding alloy.TDB was found to increase with decreasing strain rate and was sensitive to the structure of the test sample. The smooth faceted surface of the brittle failures indicated the presence of a liquid layer at the grain boundaries at failure. Homogenizing the samples prior to testing markedly increasedTDB to near the solidus temperature. Formerly Visiting Scientist, Department of Metallurgical Engineering, University of British Columbia