Semikilled Steel Research Articles

METALLOGRAPHIC ANALYSIS OF NON-METALLIC INCLUSIONS IN SEMI-KILLED STEEL (GRADE 25G2S) DEOXIDIZED WITH A COMPLEX ALLOY Fe-Si-Mn-Al

This study presents a metallographic analysis of non-metallic inclusions in steel samples obtained from six different heats of the oxygen converter shop at JSC «Qarmet» and under laboratory conditions at the Karaganda Industrial University. The investigation focused on semi-killed steel samples deoxidized using traditional technology under industrial conditions and with a complex alloy containing Fe-Si-Mn-Al under laboratory conditions, aiming to identify the types and assess the level of contamination by non-metallic inclusions. The results revealed the presence of various types of non-metallic inclusions, such as corundum, chromite spinel, chromium oxides, as well as titanium nitrides and carbonitrides. The degree of contamination by non-metallic inclusions ranged from 2 to 3 points. The analysis showed that the use of a complex Fe-Si-Mn-Al alloy instead of traditional deoxidizers can affect the quantitative and qualitative composition of non-metallic inclusions. It was established that the complex alloy promotes the formation of more stable inclusion phases, which can positively impact the mechanical properties of the steel. It was also found that different production technologies and the use of deoxidizers can significantly influence the quantity and types of non-metallic inclusions, requiring further study and optimization of technological processes. Therefore, the results of this study can be valuable for metallurgical enterprises in selecting optimal steel deoxidation methods and improving steel quality, as well as for further scientific research in the fields of metallurgy and materials science.

Comparative studies on weldability and mechanical characteristics of semi- killed steel using different arc welding technique

The main objective of this study is to examine the weldability and mechanical properties of semi-killed steel of E250BR grade which is mostly used in gear box casing. Three different arc welding methods i) GMAW (Gas Metal Arc Welding) ii) GTAW (Gas Tungsten Arc Welding), and iii) A-GTAW (Activated Gas Tungsten Arc Welding) were employed in this study, and the respective weld joints mechanical properties were evaluated and compared with each other. Visual inspection of the specimens shows that the GTAW and A-GTAW processes offer excellent weld bead geometry. Visual and radiographic inspections showed no crack or discontinuity is in the weld joints. The weldment produced by A-TIG welding has the highest aspect ratio, which is due to the reverse Marangoni convection effect. The tensile and hardness test outcomes exhibited that the mechanical properties of the parent metal were not affected by the welding processes, but impact test results showed a lack of energy absorption by the specimen from the A-GTAW process. A-GTAW welding had the highest Ultimate Tensile strength, which was 2.5 percent to 5 percent higher than the other processes employed in this study. Weldments fabricated using GTAW welding have the highest impact toughness, which is 43.5 percent and 51.85 percent more than the other two processes. Vickers hardness test reveals that joints made with the GTAW process are better than the other counter parts.

Mechanical Properties of Hot Rolled Ribbed and Plain Steel Rods

This study focuses on microstructure and mechanical behaviour of 3PS (Semi-killed mild steel) hot rolled ribbed and plain carbon steel. 3PS billet steel samples and hot rolled ribbed and plain steel rods of different heat numbers and profiles were characterized for its chemical composition, microstructure, and tensile behaviour. The composition analysis of 5 (five) 3PS billet samples shows that there was no appreciable variation in chemical composition of the hot-rolled plain and ribbed steel rods. The microstructures of as-received steel billet (3PS) examination revealed large grains of ferrite and pearlite while those of hot-rolled 3 PS mild steel samples of different heat numbers contain smaller grains of ferrite and some amount of pearlite. The results also indicated that yield and ultimate tensile strength reach maximum values (492 and 361 N/mm2) at 0.31% elongation for heat number 43 while maximum values for heat number 56 (478 and 362 N/mm2) at 0.33 % respectively. The ribbed steel rod of the same diameter as plain steel exhibit slightly better mechanical properties with higher values of yield and ultimate tensile strength. There is consistency in the chemical composition of the as-received billet and the hot rolled products.

Member Ductility and Design Detail of Some Welded Joints

The behaviour of tubular steel members and their connections are studied under ultimate tensile load conditions. Three end connection details are considered. These are practical connections, but one has a deliberately introduced defect of welding fabrication detail. Two types of steel are used: one is a conventional semi-killed mild steel, while the other is a rimmed steel that has been cold formed to produce an enhanced yield stress. The ratio of ultimate stress to yield stress (strain hardening ratio) is widely different for the two steels. The tests highlight the reduction in ultimate-load capacity and the severely reduced ductility that follows when defects of fabrication and low strain hardening ratio steels occur together.

Microstructure-Mechanical Property Relationships for a Fe/Mn/Cr Rock Bolt Reinforcing Steel

The influence of low chromium additions to a 0.25C-1.5Mn semikilled steel on microstructure, and tensile and impact behaviors of high strength rock bolt reinforcing bars has been investigated. Although chromium imparted adequate tensile properties at ambient temperature (yield stress: 624 MPa; ultimate tensile stress: 819 MPa; elongation: 12.5%) by forming transformation products such as tempered martensite, lower and upper bainite, and small amounts of acicular ferrite, it increased the ductile-to-brittle transition temperature due to coarser upper bainite in the core region of bar having larger unit crack paths. The synthesized steel is considered to be effective in realizing the desired tensile properties, and suitable for application in rock bolt, as well as other reinforced concrete structures.

Simulation Test for Crystallization of Inclusions in Semi-killed Steels during Heat Treatment Prior to Hot Rolling

The SHTT (Single Hot Thermocouple Technique) has been used to determine the crystallization behavior of a composition of the CaO–Al2O3–SiO2 system, representative of inclusions in semi-killed steels. XRD and SEM have been employed, respectively, to detect the crystalline phases and to analyze the morphology, quantity and properties of crystals. The same composition has also been studied by quenching the oxides in iron crucibles, a method which had been proven reliable to simulate the actual behavior of inclusions in steel samples during heat treatment prior to hot rolling. The TTT diagrams obtained by using the above two techniques are almost identical. The observed crystalline phases for this composition are gehlenite (2CaO·Al2O3·SiO2) and anorthite (CaO·Al2O3·2SiO2). The TTT diagram from SHTT gives the nose position of gehlenite and anorthite of, respectively, 1200 s at 1200°C and 4800 s at 1300°C. Thus, the Hot Thermocouple Technique (DHTT or SHTT), already largely used to study the crystallization behavior of CC mold slags is shown to be suitable to simulate the crystallization behavior of inclusions with much higher SiO2 contents.

Assessment of the Fe-Sn-Zn phase diagram at 450°C

The Fe-Sn-Zn system is of interest because Sn is one element added to the Zn galvanizing bath to overcome the drawbacks due to the presence of Si in semi-killed steels. This work has been undertaken with the aim to understand the tin effect on the microstructure and the layers growth in batch galvanized coatings on low alloyed steels.

Measurement of sensitivity to dynamic strain aging in C–Mn steels by internal friction experiments

The dynamic strain aging (DSA) phenomenon is present in C–Mn steels and associated welds commonly used in many industrial applications. This phenomenon, which appears in metals containing solute atoms interacting with dislocations, induces an increase in flow stress and ultimate tensile strength (UTS) as well as a decrease in ductility. The DSA phenomenon was studied by tensile, strain rate sensitivity and internal friction tests between 20°C and 350°C in two C–Mn steels with very different sensitivities to DSA (a fully killed A42 steel and a semikilled A48 steel). The internal friction technique, which allows one to evaluate the balance between carbon and nitrogen atoms free in the lattice and carbon and nitrogen atoms interacting with mobile dislocations, appears to give results well correlated with those obtained by tensile (and strain rate sensitivity) tests. The ductility loss or the UTS increase is shown to be proportional to Snoek peak height. It is concluded that, even in industrial materials, a single internal friction test can give a good evaluation of DSA sensitivity.

Prediction of deoxidation and inclusion precipitation in semikilled steel

A generalized thermodynamic calculation procedure for the prediction of oxide inclusion precipitation of desired compositions for Mn-Si-Al deoxidation has been developed. The procedure directly uses established ternary iso-activity diagrams in the MnO-SiO2-Al2O3 system, and does not assume any slag model. The thermodynamic data required for calculation, such as deoxidation constants, interaction coefficients, and activity vs composition relations in the oxide phase, reported in the literature, were examined and the ones considered to be most satisfactory were accepted. On the basis of the computed results for inclusions in the spessartite phase field of the MnO-SiO2-Al2O3 system, discussions have been presented on the applicability of the regular solution model as well as an empirical model reported in the literature. Some predictions for plain carbon steels have also been included.

Kinetic model of non-metallic inclusions' precipitation during steel solidification

A numerical simulation which describes precipitation of non-metallic inclusions (liquid or solid oxides, nitrides) during steel solidification is proposed. Its aim is to predict the compositions and size distributions of these precipitates. The model takes into account microsegregation and includes homogeneous nucleation in the interdendritic liquid, and a mixed-controlled growth stage combining transport of solute elements in the boundary layer around precipitates and interfacial kinetics. We have observed that the calculated compositions of oxide inclusions differ from those of inclusions precipitating at equilibrium and are close to industrial observations on semi-killed steels.

PRODUCTION OF A QUENCHED AND TEMPERED ROLLED PRODUCT FOR THE REINFORCEMENT OF CONCRETE

Until recently, metallurgical plants in the nations of the CIS (Commonwealth of Independent States) ‐ including Russia and Ukraine ‐ produced weldable rolled products for the reinforcement of concrete by using hot-rolled killed low-alloy steels 35GS and 25G2S. These steels belong to strength class A400S (A-III) under GOST 5781 (mimimum yield point 400 MPa). The production of these steels consumed 28‐40 kg of ferroalloys per ton of steel, and the consumption coefficient realized in obtaining the rolled products from the ingots was 1.24‐1.30 kg/ton. The carbon content of steel 25G2S was limited to the standard 0.31%, while the carbon content of steel 35GS was 0.40%. Attempts were made to replace the hot-rolled killed low-alloy steel with hot-rolled semikilled steel 32G2Rps or quenched and tempered semikilled carbon steel St5ps, which also contains up to 0.40% C and correspond to strength class A400S (under GOST 10884, quenched and tempered steel St5ps corresponds to class At-IIIS). At such carbon contents, the reinforcement steel is characterized by poor cold shortness and limited weldability and fails to meet existing international standards. International standard YSO 9655/2, European standard EN 10080, and the national standards of different European nations specify a maximum carbon content of 0.24% for finished rolled products of the given designation. However, these norms stipulate only the carbon equivalent, not the methods used to make or cast the steel. In addition, the construction industry worldwide has instituted the widespread use of a new class of reinforcement steel, now using grade A500S instead of grade A400S. The export-oriented strategy being adopted for the growth of ferrous metallurgy in the CIS nations due to stagnation of the domestic construction industry makes it necessary to improve the quality of their rolled products and bring those products’ chemical composition and properties in line with world standards, to ensure that the products will be competitive in the world market. A reduction in production costs also improves their competitive standing. In 1993, a group of metallurgical researchers based at the West Siberian Metallurgical Combine organized the mass

Edge cracking in hot-rolled coils of semi-killed steels

Various processing parameters leading to edge cracking in hot-rolled coils are described, along with a study of the effect of these parameters on the occurrence of edge cracking during commercial production of semi-killed hot-rolled coils. High-temperature tensile testing revealed that ductility decreased with a decrease in test temperature from 850 to 750 °C and improved with a further drop in temperature. This phenomenon is attributed to the closed-packed structure of single-phase austenite at higher temperatures, which transforms to ferrite + austenite at the intermediate temperature range. Edge cracking was frequently observed in hot-rolled coils that were finish rolled within this intermediate temperature range. Optical microscopy revealed large ferrite grains near edges of the coil as well as at inner surfaces of cracks caused by decarburization at higher temperatures. A minimum manganese/sulfur ratio of 12 to 8 for a sulfur level of 0.03 to 0.04%, proper teeming practice, adequate soaking, and a finish rolling temperature above 875 °C resulted in overall improvement in the edge quality of hot-rolled coils.

Experimental Study and Modeling of the Precipitation of Non-metallic Inclusions during Solidification of Steel.

An original model for the calculation of the precipitation of non-metallic inclusions during solidification has been developed at IRSID. The microsegregation equations and the equilibrium conditions between liquid steel and oxide, sulphide, nitride, carbide inclusions are combined in a general multiphase equilibrium code.Sulphide precipitation in several steel grades: plate grades, medium carbon and bearing steels has been analyzed in laboratory samples quenched from a partially solidified state. For the various steel grades investigated, the computed results of sulphides (Mn, Fe)S or (Mn, Fe, Cr)S compositions are in good agreement with the results of the experimental study.Another example concerns the precipitaion of oxide inclusions in semi-killed high-carbon steels: The calculation represents very precisely the observed heterogeneous population of oxide inclusions formed at different stages of the industrial process.

CO2 shielding gas effects in laser welding mild steel

The most widely used shielding gases for laser welding of steels are helium and argon. Helium produces significantly more penetration than argon in penetration-mode laser beam welding. Another gas that has been proposed as an alternative to these inert gases is carbon dioxide. The benefits of using carbon dioxide as a shielding gas for laser welding are that it costs less than helium and argon and that it provides nearly the same penetration as helium. The major drawback to its use as a shielding gas for mild steel is that it can cause porosity and other weld discontinuities. In this investigation helium and carbon dioxide were investigated as shielding gases in the laser welding of a fully killed 1018 steel, a semikilled 1018 steel, and an unkilled 1018 steel. Helium was generally found to produce discontinuity-free welds, although small quantities of very fine centerline porosity were observed in the fully killed and semikilled steels. Carbon dioxide was found to provide an average of 70% of the penetration obtained with helium. The surface appearance and formation of porosity in welds made using carbon dioxide shielding was found to be dependent on the amount of deoxidizing element in the weld pool. While porosity-free welds were formed in the semikilled steel (0.22% Si), low silicon or other deoxidizer content resulted in poor surface appearance and entrapped porosity in both the fully killed and unkilled steels in this investigation, presumably due to the formation of carbon monoxide in the weld pool. The addition of ferrosilicon powder to the weld pool effectively controlled the formation of carbon monoxide in these welds and resulted in porosity-free welds with carbon dioxide shielding. The results of this investigation indicate that carbon dioxide can be used as a cost-effective shielding gas for penetration-mode welding of carbon steels. It was found that sufficient levels of deoxidizing elements must, however, be available in the molten pool to control porosity.

Material Effects—Statistics of Steel Tube Struts

The test results of 92 tubular steel, pin‐ended struts under different conditions are studied by statistical analysis. The significant influence of material effects, such as strain aging, strain hardening, and the Bauschinger effect on strut load capacity, is investigated by applying a two‐way analysis of variance test. These three material effects are found to exercise a significant influence on strut load capacity for both the rimming steel and the semikilled steel used. The Bauschinger effect plays a dominant role in reducing strut load capacity, while both strain hardening and strain aging have a significant influence on increasing load capacity.

The magnetic properties of plastically deformed steels

The magnetic properties of a semi-killed pearlitic pipe steel have been measured as a function of plastic deformation. Coercivity was found to increase with plastic strain while remanence and maximum permeability decreased. High voltage electron microscopy showed increased pinning at dislocation tangles as deformation proceeded.

Service life of rammed ladle linings obtained from a quartzite-argillaceous body

aceous body (TU 14-87252-77) obtained from the Krasnogorovsk Refractories Plant. The following parameters were used for lining the ladles: thickness at the lower portion of the ladle 190-220 mm, thickness in the central and the upper portions 160-220 mm, and the ramming duration 20-25 min; the surface hardness measured using a 071-model hardness tester amounted to (85  5) conditional units. The rammed lining was subjected to drying using a single-pipe natural-gas burner measuring i00 mm in diameter and 2000 n~n in length according to the following flow regime: 140-150 m3/h for 3 h, 260-270 m3/h for 8 h, and 350-390 m3/h for 8-10 h. The steel-teeming ladles are operated under the following conditions: the teeming temperature 1600-1655~ the residence duration of steel in the ladles 1 h 5 min-2 h 50 min, the basicity of the slags 1.8-3.4, the weight content of FeO in the slags 7.7-22.7%, and the weight of steel in the ladles 200-240 ton; molten steels handled in the ladles - rimming and killed (semikilled) steels in a I:i ratio. The life of the ladle lining is determined by the service conditons of the ladles and the quality of the lining. An analysis of the service life of the rammed ladle linings was carried out by processing the melt-charts of the open-hearth furnaces. We analyzed the data concerning the melting and teeming technology (in particular, the teeming temperature and the grade of the steel, the weight and the residence period of the steel in the ladle, the basicity of the slag and the FeO content in it, and the turnover factor of the ladle, i.e., the periodicity of filling it during teeming). Our analysis showed that the poor service life of the rammed ladle lining (less than I0 teemings) can be attributable mainly to the following factors: filling up the ladles for the first time after making the linings or filling them up in succession (2-3 times or more) with rimming steels (in particular, with the molten steels superheated up to 1635-1655~ low weight of molten steel in the ladles (220-240 ton with an average weight of 236 ton) and an increased weight of slag; and using insufficiently cooled ladles for successive teemings (after 1-3 h intervals). As a rule, under the simultaneous action of all the aforementioned factors, the life of the rammed linings does not exceed 5-7 teemings. High basicity and FeO content of the slags and a prolonged residence period of molten steel in the ladles (exceeding 2 h 30 min) reduce the service life of the ladle lining to a lesser extent. In the absence of proper technological measures (using one flinging port instead of two ports, imbalancing of the rotor of the sand-slinger equipment, using old ladles with different sizes of the abandoned portions, using ladles having a high ellipicity, deviations from the optimum drying regimes for the newly made linings etc.), the life of the rammed linings decreases and, as a rule, does not exceed 12 teemings. Our experience shows that deterioration of the quality of the ramming body (deviations from the desired chemical composition, granulometry, and moisture content) can reduce the life of the linings by 20-30%. When the rammed linings of the steel-teeming ladles of the open-hearth shop are made deviating from the norms of the ramming technology and are used properly, they exhibit a stable service life of not less than 15-17 teemings (in contrast to 8-9 teemings in the case of brick linings).

Member Ductility and Design Detail of Some Welded Joints

The behaviour of tubular steel members and their connections are studied under ultimate tensile load conditions. Three end connection details are considered. These are practical connections, but one has a deliberately introduced defect of welding fabrication detail. Two types of steel are used: one is a conventional semi-killed mild steel, while the other is a rimmed steel that has been cold formed to produce an enhanced yield stress. The ratio of ultimate stress to yield stress (strain hardening ratio) is widely different for the two steels. The tests highlight the reduction in ultimate-load capacity and the severely reduced ductility that follows when defects of fabrication and low strain hardening ratio steels occur together.

Production and use of low-alloy steel for mine supports

The Institute of Ferrous Metallurgy and the Kommunarsk combine developed a new low-alloy semikilled steel 20G2AFps. The steel was made in a two-bath furnace with the following modifications: metal was tapped at a temperature of 15-20/degree/C higher, alloyed mainly in the ladle with blast-furnace ferromanganese and nitrided ferro-vanadium, and deoxidized with ferrosilicon or silicomanganese. The lightweight steel was equal in service to steel St5ps while more durable. There was a 15% decrease in rejections which reduced metal waste. Weight of the new steel was reduced by 19%.

Optimizing the blocking of semikilled steel in the ingot mold

Optimizing the blocking of semikilled steel in the ingot mold