Steel casting by diffusion solidification

A new process for casting and welding carbon steels is described in which carbon diffuses isothermally or adiabatically within an intimate mixture of solid low carbon steel and high carbon liquid iron to effect solidification and subsequent homogenization with respect to carbon. Advantages over conventional casting processes and products result from 1) 150 to 200°C lower casting temperature; 2) reduced solidification shrinkage, obviating the need for risers in most cases; and 3) more rapid solidification, especially for castings with large ratios of volume to area. In its most versatile form the process involves low pressure forced infiltration of a mold filled with preheated spherical low carbon steel particles by a higher-carbon liquid. The process can reliably produce castings with greater than 99 pct of theoretical density; solidification time typically range from a few seconds to several minutes; and tensile strengths as high as 185 ksi with 15 pct reduction of area to break have been attained. The ductility of such castings is approximately one order of magnitude more sensitive to total oxygen content than the ductility of wrought steels, probably because of cavitation nucleated by oxides during solidification of the pools of liquid trapped between the shot particles. An analysis of the kinetics of the infiltration and solidification is performed for steel casting by diffusion of carbon, manganese or heat in iron. The iron-carbon system is most tractable; steel casting by thermal diffusion has also been demonstrated but no attempt was made to test the iron-manganese system. GEORGE LANGFORD, formerly with the Monsanto Triangle Park Development Center, Inc.     

Interrupted and Isothermal Solidification Studies of Low and Medium Carbon Steels

Low and medium carbon steels experience multiple phase transformations during solidification and subsequent cooling. The sequence, extent, and nature of the different transformations have a significant bearing on the microstructural evolution that occurs in the steel. The change in microstructure with temperature is very important, since it may influence the hot ductility of the steel during casting and/or rolling and the subsequent response of the material to thermoprocessing. The aim of this investigation was to gain a better understanding of the development of the as-cast structure in low and medium carbon steels. Of particular interest is the origin of the large austenite grains frequently associated with poor hot ductility. Interrupted and isothermal solidification experiments were therefore conducted to study the nonequilibrium and near-equilibrium structures which form at different stages of the freezing process. The results of the investigation established delta-ferrite as the primary solidifying phase in low carbon steels. Austenite forms as the secondary phase by nucleation at the solidification (delta-ferrite) boundaries. While excessive austenite grain coarsening is suppressed by the coexistence of the second phases delta-ferrite or liquid, this suppression occurs over only a limited temperature range, just below the peritectic temperature. Subsequent cooling leads to very large austenite grains, ranging up to 5 mm in diameter, in steels of low carbon content. N.S. POTTORE, formerly with the Basic Metals Processing Research Institute, Department of Materials Science and Engineering, University of Pittsburgh     

特殊钢的连铸

特殊钢一般含有一定的合金元素,冷却凝固状态不同于普通碳素钢,在连铸坯中更易产生成分 和组织偏析,和表面裂纹等缺陷。所以冶炼特殊钢时应提高钢水的纯洁度,减少夹杂物含量和改善夹杂物形 态,连铸时采用钢液二次氧化和污染,以减少铸坯低过热度(10～15℃)浇注和保护浇铸技术,防止成分和组 织偏析,提高铸坯表面质量和内部致密度。叙述了连铸保护浇铸技术,中间包冶金,电磁搅拌、凝固末端轻压 下特殊钢连铸专用覆盖剂、保护渣和自动化技术。讨论了特殊钢连铸工艺参数,存在的问题和对策。     

Observations of the columnar-to-equiaxed transition in stainless steels

Observations are reported for the columnar-to-equiaxed transition (CET) in stainless steel bars which have been solidified slowly and progressively in a horizontal configuration. For ferritic, austenitic, and ferritic/austenitic stainless steels containing more than 0.085 wt pct carbon, CETs occur at about the same distance from the start of solidification at a given growth rate. With increasing growth rates, the transition occurs closer to the start of solidification. At low carbon levels, near 0.02 wt pct carbon, the ferritic/austenitic steel is entirely columnar, in most cases. Adding nickel to the ferritic/austenitic steel, which makes the leading phase austenitic, produces a CET with small equiaxed grains. This suggests that different particles which are effective with austenitic growth become operative as nucleants. The transition from a columnar to an equiaxed structure occurs abruptly across the diameter of the sample. There is extensive fluid flow in the bulk melt, which produces shallow temperature gradients in the melt prior to the onset of solidification. The bulk melt flow does not appear to interact significantly with the melt in the interdendritic region or the melt immediately ahead of this region. The width of the solid/liquid region in the present experiments is observed to be between 10 and 20 mm, depending on the growth velocity and the distance from the start of solidification.     

A metallographic study of porosity and fracture behavior in relation to the tensile properties in 319.2 end chill castings

A metallographic study of the porosity and fracture behavior in unidirectionally solidified end chill castings of 319.2 aluminum alloy (Al-6.2 pct Si-3.8 pct Cu-0.5 pct Fe-0.14 pct Mn-0.06 pct Mg-0.073 pct Ti) was carried out using optical microscopy and scanning electron microscopy (SEM) to determine their relationship with the tensile properties. The parameters varied in the production of these castings were the hydrogen (∼0.1 and ∼0.37 mL/100 g Al), modifier (0 and 300 ppm Sr), and grain refiner (0 and 0.02 wt pct Ti) concentrations, as well as the solidification time, which increased with increasing distance from the end chill bottom of the casting, giving dendrite arm spacings (DASs) ranging from ∼15 to ∼95 /im. Image analysis and energy dispersive X-ray (EDX) analysis were employed for quantification of porosity/microstructural constituents and fracture surface analysis (phase identification), respectively. The results showed that the local solidification time(viz. DAS) significantly influences the ductility at low hydrogen levels; at higher levels, however, hydro-gen has a more pronounced effect (porosity related) on the drop in ductility. Porosity is mainly observed in the form of elongated pores along the grain boundaries, with Sr increasing the porosity volume percent and grain refining increasing the probability for pore branching. The beneficial effect of Sr modification, however, improves the alloy ductility. Fracture of the Si, β-Al₅FeSi, α- Al₁₅(Fe,Mn)₃Si₂, and Al₂Cu phases takes place within the phase particles rather than at the particle/Al matrix interface. Sensitivity of tensile properties to DAS allows for the use of the latter as an indicator of the expected properties of the alloy.     

The effect of carbon content on solidification of steel in the continuous casting mold

This investigation examines the effect of steel carbon content on microsegregation and strand deformation during the first stage of solidification in the continuous casting mold. Calculation of microsegregation for phosphorus indicates a minimum at 0.10 wt pct C, and a maximum around 0.25 wt pct C of the decrease in solidus temperature. This leads to very different effective shell thicknesses and determines whether or not the strand shell can contract. As a result, mainly steels around 0.10 wt pct C can produce a finite gap in the early stages of strand formation, explaining the pronounced waviness of the surface of such steels. On the other hand, steels with more than 0.20 wt pct C are forced by ferrostatie pressure to remain in contact with the mold wall leading to uniform shell growth and smooth strand surfaces but also undergoing enhanced mold friction.     

Feeding and risering of high-alloy steel castings

A more accurate, less conservative set of feeding distance (FD) and riser sizing rules is developed for high-alloy steel castings produced from alloy grades CF-8M, CA-15, HH, HK, and HP. These rules are designed to produce radiographically sound castings at 2 pct sensitivity. By comparing results between plate casting trials and the corresponding simulations of those trials, a relationship is shown to exist between a local thermal parameter known as the Niyama criterion and ASTM shrinkage X-ray level. This relationship is then used in an extensive set of casting simulations to numerically determine FDs for a wide range of casting conditions. It is shown that the FD rule developed in an analogous earlier study for carbon and low-alloy (C&LA) steels can also be used for these high-alloy grades, provided that the FD is modified by a multiplier that accounts for the high-alloy steel grade. In addition, it is shown that multipliers for superheat, sand mold materials, and the use of chills developed in the earlier work are also valid with these high-alloy steel grades. In comparison with previously published high-alloy FD rules, the present rules are shown to provide longer FDs (and hence higher casting yields) in most casting situations. This study also investigates riser sizing rules. It is determined that for open top risers, the previously published C&LA riser sizing rule is also valid for high-alloy steels. This rule is less conservative than existing high-alloy riser sizing rules, specifying smaller risers that produce higher casting yields. In addition, for vented blind top risers, it is shown that the previously published rules are also overly conservative.     

Influence of nitrogen on hot ductility of steels and its relationship to problem of transverse cracking

Abstract

The present review examines the influence of nitrogen on the hot ductility of steels, with particular relevance to the problem of transverse cracking during continuous casting. Nitrogen itself is not detrimental to hot ductility, but when it is present with aluminium or microalloying additions, ductility can be adversely affected through the formation of nitrides or carbonitrides. The addition of aluminium to low nitrogen C–Mn steels (0·005%N)impairs ductility during casting at an acid soluble level as low as 0·02%Al. This arises because segregation of aluminium to the grain boundaries occurs on solidification, and the temperature cycling that takes place when the strand is cooled encourages AlN precipitation. However, for low nitrogen, high strength low alloy (HSLA) steels with carbon levels in the peritectic range 0·08–0·17%C, transverse cracking is not generally encountered until the aluminium level is >0·04%. Higher nitrogen levels are likely to cause problems even at very low aluminium levels, as precipitation of AlN is controlled by the product of the aluminium and nitrogen contents. The microalloying additions vanadium and niobium are detrimental to ductility but, of the two elements, niobium is more damaging, as it gives finer precipitation. Increasing the nitrogen level has a more pronounced influence on ductility in vanadium containing steels, since vanadium forms a nitride while niobium forms Nb (CN), which is mainly carbon based. Nevertheless, the product of vanadium and nitrogen contents has to approach 1·2 × 10^-3, for example 0·1%V and 0·012%N, before ductility deteriorates to that normally given by a niobium containing steel with 0·03%Nb and 0·005%N. When small titanium additions are made to low nitrogen C–Mn–Al steels (0·005%N), the best ductility is likely to be given by a high Ti/N ratio of 4–5 : 1; the excess titanium in solution encourages growth of the TiN particles. For high nitrogen steels (0·01%N), a low titanium level (0·01%)is recommended to limit the volume fraction of TiN particles. A low soluble aluminium level is also needed to prevent the excess nitrogen from combining to form AlN. For C–Mn–Nb–Al steels, similar recommendations can be made with regard to adding titanium. However, the presence of niobium and aluminium appears to have little influence on ductility, since these elements coarsen the titanium containing precipitates.     

The ductility of continuously-cast steel near the melting point—hot tearing

At temperatures near the melting point steels fail in a brittle manner. This brittle failure can lead to the formation of surface and internal cracks in continuously cast steel, during casting, if the steel is subject to a tensile strain. In this investigation the nature of the brittle failure has been considered for a wide range of continuously-cast steels, by examining the strength and ductility of the steels as a function of temperature, composition, and cast structure. The results show that for steels containing 0.05 to 0.12 pct C, brittle failure is due to incipient melting at grain boundaries at temperatures between approximately 40°C below the solidus and the solidus. The incipient melting is ascribed to solute or residual segregation, at the grain boundaries following extensive boundary migration. For steel containing approximately 0.16 pct C, with increasing test temperature brittle failure starts 70°C below the solidus. For steels containing 0.25 to 1.0 pct C brittle failure starts 40°C below the solidus over the entire carbon range. Failure due to melting alone occurs interdendritically at temperatures above the solidus. In general the melting or ductile-brittle transition temperatures are independent of the initial cast structure, or large increases in the solute or residual levels, other than carbon.     

Surface Solidification Behaviour of Micro‐Alloyed Steels under Continuous Casting Conditions

Micro‐alloyed steels are important in steel industry with regard to their unique mechanical properties. Their characterisitcs are mainly caused by the refinement of ferrite grain size by controlled precipitation of nitrides during thermomechanical treatment in hot rolling. Uncontrolled precipitations of titanium nitrides in the surface region during casting and solidification can negatively influence the surface quality of continuously cast steel, particularly when casting, thermal soaking and hot rolling are carried out in a combined process chain. Focus of this work are experimental simulations and mathematical investigations of early solidification in a CC mould, primary precipitation of nitrides, and effects of different influences such as mould contact through casting flux or direct mould contact. A laboratory rig to lead solidification on liquid casting flux was developed. The carbon content of the steel melts was varied.     

The critical amount of nitrogen on the formation of nitrogen gas pores during solidification of 25Cr-7Ni duplex stainless steels

The effects of casting thickness, nitrogen contents, cooling rate, and Mn contents on the formation of nitrogen gas pores during solidification of 25Cr-7Ni-1.5Mo-3W duplex stainless steels (DSS) were quantitatively investigated. In the case of a sand mold, the formation of nitrogen gas pore was not affected by the thickness of castings, which ranged from 13 to 52 mm, and the critical initial nitrogen content for the formation of gas pore was 0.30 wt pct. In the case of the molds made of a stainless steel (STS) and water-cooled Cu, the critical initial nitrogen content did not change much compared to the sand mold. The amount of nitrogen gas pores increased with initial nitrogen contents of castings. The segregation of nitrogen and alloying elements was calculated with Thermo-Calc. The calculated data and the experimental results were compared to estimate the critical nitrogen partial pressure in the residual melt for the nucleation of gas pores. The effect of Mn content on the formation of gas pores was also investigated. The increase of Mn content from 1 wt pct to 2.6 wt pct changed the critical initial nitrogen content 0.30 wt pct to 0.40 wt pct.     

锰质量分数对Fe-Mn-C钢热物性参数的影响

 为促进Fe-Mn-C钢连铸技术的发展,对不同锰质量分数的Fe-Mn-C钢铸锭的热物性参数进行了研究。结果表明,0Mn钢的导热系数高于3Mn钢。低于750 ℃时,6Mn钢的导热系数最低;高于900 ℃时,6Mn钢的导热系数最高。3个钢种的平均线膨胀系数为1.0×10－5～1.6×10－5 ℃－1。以Z＜60%作为判据,6Mn钢的第III脆性区为600～800 ℃,3Mn钢和0Mn钢的第III脆性区分别为600～850 ℃和600～900 ℃。在6Mn钢和3Mn钢中,大量生成的形变诱导铁素体（DIF）导致低温区热塑性的恢复。然而,由于连铸矫直过程的应变速率较低,不能生成大量的DIF。因此在连铸过程中,低温区6Mn钢和3Mn钢的热塑性不能恢复。     

Influence of microstructure on tensile properties of spheroidized ultrahigh-carbon (1.8 Pct C steel

Ultrahigh-carbon steel (UHCS) containing 1.8 pct carbon was processed to create microstructures consisting of fine-spheroidized carbide particles (0.2- to 1.5-μm size range) within a fine-grained ferrite matrix (0.3- to 5-μm range) through a variety of thermomechanical processing and heat-treatment combinations. Tensile ductility, yield, and fracture strengths, and strain-hardening behavior were evaluated at room temperature. Yield strengths ranged from 640 to 1450 MPa, and uniform tensile elongation ranged from 3 to 23 pct. Quantitative analyses revealed that a Hall-Petch type relationship exists between the yield strength and the ferrite grain size and carbide particle size within grain interiors. The fracture strength, on the other hand, was found to be uniquely dependent on the coarse carbide particle size typically found at grain boundaries. Data from other investigators on spheroidized carbon steels were shown to correlate well with the data for the UHCS (1.8 pct C) material. It was shown that the tensile ductility will increase when the difference between the fracture strength and the yield strength is increased and when the strain-hardening rate is decreased. The basis for the trends observed is that the tensile ductility is limited by the fracture process that appears to be dictated by the nucleation of cracks at large carbide particles. The results obtained indicate that UHCSs have significant potential for sheet applications where high strength and good ductility are primary requirements.     

Feeding and Distribution of Porosity in Cast Al-Si Alloys as Function of Alloy Composition and Modification

Unmodified, Na-modified, and Sr-modified castings of Al-7?pct Si and Al-12.5?pct Si alloys were cast in molds in which it was possible to create different cooling conditions. It is shown how solidification influences the distribution of porosity at the surface and the center of the castings as a function of modification and Si content in sand- and chill-cast samples. Eutectic modification, Si content, and cooling conditions have a great impact on the distribution of porosity. Unmodified and Na-modified castings are more easily fed with porosity tending to congregate near the centerline of the casting, while Sr-modified castings solidify in a mushy manner that creates a more homogeneous distribution of porosity in the casting. The amount of porosity was highest in the Sr-modified alloys, lower in the Na-modified alloys, and lowest in the unmodified alloys. The size of the porosity-free layer and the effectiveness of the feeders were greater in the castings made with the steel chills due to the increased thermal gradients and consequent increase in the directionality of solidification.     

Structure and Properties of Cast Near-Congruent Copper-Manganese Alloys

Microstructure development in the casting of copper-manganese alloys based on the congruent point at 34.6 wt pct Mn and 1146 K (873 °C) has been studied. The alloys were prepared by induction melting of electrolytic Cu and Mn in clay-graphite crucibles in open air. Under conventional casting conditions, the alloys exhibit fine cellular (non-dendritic) solidification morphology with a distinct absence of solidification shrinkage microporosity, and they maintain these attributes over a composition range of approximately 3 wt pct Mn about the congruent point. The high Mn concentration in the alloy admits carbon into solution in the melt, resulting in formation of manganese carbide Mn₇C₃ particles having two different forms (globular and angular) in the cast microstructure. The Mn carbide was eliminated or controlled to low levels by melting in an alumina or a silicon carbide crucible, or in a clay-graphite crucible at lower temperatures. Microstructure development in casting the alloy was analyzed in terms of the available phase diagrams and thermochemical data. Hardness and tensile testing indicated a potent solid solution strengthening effect of Mn and high ductility in the as-cast condition, with additional hardness (strength) when the alloy contains the Mn carbide phase.     

Strain-aging of vanadium, niobium or titanium-strengthened high-strength low-alloy steels

Four commercially available high-strength low-alloy (HSLA) steels were evaluated in this study. It was determined that all four steels were susceptible to strain-aging by interstitial solutes. The increase in strength due to strain-aging was similar to that observed in a low carbon steel studied for comparison. At high levels of prestrain, the percent loss in ductility in the HSLA steels was comparable to that observed in the low-carbon steel in specimens prestrained to the same fraction of the total elongation of the as-received metal. However, when considered on an absolute basis, the residual ductility in the HSLA steels was 25 to 50 pct of that observed in the low-carbon steel. The kinetics of strain-aging were briefly examined. Indications are that the kinetics are slower in the HSLA steels than they are in the low-carbon steel.     

Delta-Ferrite Distribution in a Continuous Casting Slab of Fe-Cr-Mn Austenitic Stainless Steel

The delta-ferrite distribution in a continuous casting slab of Fe-Cr-Mn stainless steel grade (200 series J4) was analyzed. The results showed that the ferrite fraction was less than 3 pct. The “M” type distribution was observed in the thickness direction. For the distribution at the centerline, the maximum ferrite content was found in the triangular zone of the macrostructure. In addition, in this zone, the carbon and sulfur were severely segregated. Furthermore, an equilibrium solidification calculation by Thermo-Calc® software indicates that the solidification mode of the composition in this triangular zone is the same as the solidification mode of the averaged composition, i.e., the FA (ferrite-austenite) mode. None of the nickel-chromium equivalent formulas combined with the Schaeffler-type diagram could predict the ferrite fraction of the Cr-Mn stainless steel grade in a reasonable manner. The authors propose that more attention should be paid to the development of prediction models for the ferrite fraction of stainless steels under continuous casting conditions.     

Strain-aging of vanadium,niobium or titanium-strengthened high-strength low-alloy steels

Four commercially available high-strength low-alloy (HSLA) steels were evaluated in this study. It was determined that all four steels were susceptible to strain-aging by interstitial solutes. The increase in strength due to strain-aging was similar to that observed in a low carbon steel studied for comparison. At high levels of prestrain, the percent loss in ductility in the HSLA steels was comparable to that observed in the low-carbon steel in specimens prestrained to the same fraction of the total elongation of the as-received metal. However, when considered on an absolute basis, the residual ductility in the HSLA steels was 25 to 50 pct of that observed in the low-carbon steel. The kinetics of strain-aging were briefly examined. Indications are that the kinetics are slower in the HSLA steels than they are in the low-carbon steel.     

Effect of Titanium Addition on As Cast Structure and Macrosegregation of High‐Carbon High‐Chromium Steel

In casting heavy ingots of high‐chromium high‐carbon cold work steels, macrosegregation develops in the center of the ingot, causing difficulties during subsequent hot working. Heat transfer and solidification of an industrial scale high‐carbon high‐chromium steel ingot was simulated and thereafter a laboratory scale representative ingot was designed to model the solidification of the industrial scale ingot. Titanium in the range of 0.3–1% was added to the high‐chromium high‐carbon (12%Cr–2%C) steel during melting process. Microstructures, macrosegregation and phase formations were studied using optical microscopy, scanning electron microscopy, energy dispersive X‐ray spectrometry, wave dispersive X‐ray spectrometry, optical emission spectroscopy, and X‐ray diffraction. Addition of 0.3% titanium was sufficient to diminish the macrosegregation; however it did not have a significant effect on the grain size. Addition of 0.7 and 1% titanium had a substantial effect on grain size in the longitudinal direction and refined the primary carbides structure. The formation of small TiC carbides that precipitated before solidification of liquid iron acted as nuclei for primary pro‐eutectic austenite grains.     

Delta ferritic heat-resistant chromium-molybdenum steels with improved rupture strength

Nine experimental delta-ferritic steels have been examined as potential low expansion heat-resistant steels for use in fossil fuel power generation, nuclear power generation, nuclear process heat plants and coal gasification plants. The steels contain 10 to 14 pct Cr and 2 to 6 pct Mo, with additions of columbium, titanium, vanadium, aluminum and boron. Room-temperature tensile properties and oxidation resistance of all steels were determined. Selected steels were aged for 1000 h at 760 °C (1400 °F) and subjected to elevated temperature tensile tests at the aging temperature. Creep-rupture properties of selected steels were determined at 760 and 815 °C (1400 and 1500 °F). Extensive metallographic and phase identification studies were conducted. Of the two steels tested for creep-rupture strength, the 10Cr-6Mo-0.5Cb steel, with good room-temperature ductility, has rupture strength exceeding that of martensitic 12Cr-1Mo-V steel. The 14Cr-3Mo-0.5Cb-lTi-2Al steel exhibits an even higher rupture strength, but has only marginal ductility at room temperature.