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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 74 (1988), No. 7

  • Consideration on Direct Connection between Continuous Casting and Hot Strip Mill

    pp. 1199-1202

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  • Metallurgical Researches and Developments Forwarding Hot-Direct Rolling and Hot-Charge Rolling Processes

    pp. 1203-1209

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    x

    Readers Who Read This Article Also Read

    1. Scheduling Hot Slabs in Parallel Hot Rolling Production Lines by Column Generation ISIJ International Vol.54(2014), No.12
  • Thermodynamics on the Equilibrium Distribution Coefficients of Solute Elements between Solid and Liquid Phases in Iron Alloys

    pp. 1210-1218

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  • Embrittlement and Mechanical Behavior of Steels at High Temperatures

    pp. 1219-1226

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  • Production Techniques of High Temperature CC slabs for Remote Direct Rolling

    pp. 1227-1234

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    The high temperature slab production techniques to realize the new type of CC-HDR (the remote HDR), in which the caster is located some distance from the mill, are discussed.
    In continuous casting; Firstly, it is required to adopt the optimum slab thickness. Next, the following techniques are indispensable in order to raise the slab edge temperature high enough for remote HDR process; secondary cooling pattern leaving slab edge not sprayed with water, slab surface reheating by utilization of latent heat of solidification, slab edge insulation in caster, slab edge heating system around cutter equipment.
    Slab transportation; High speed, insulated slab transfer car is advantageous for remote HDR process.
    Prior to hot rolling mill; Jet gas heating system is more advantageous for remote HDR process than induction heating system due to the effective heating of slab cutting section. In addition, longitudinal turning of slab is effective to save energy for slab edge heating.
    It is required to efficiently arrange these techniques between caster and hot rolling mill.
    Based on the concept mentioned above, a new remote HDR process at Yawata Works in Nippon Steel Corporation has been built and put into operation successfully in June 1987.
  • The Technology of CC-Hot Direct Rolling in NKK

    pp. 1235-1239

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  • Hot Charge Rolling Process at SMI Kashima No. 3 Caster

    pp. 1240-1247

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    The establishment of "Hot charge rolling process (HCR)" at No. 3 caster in Kashima Steel Works has led to much higher productivity, energy saving and improvement of material flow. In HCR, cast slabs are directly put into the reheating furnace and after a short time, they are rolled by the hot strip mill. The following technological developments and new systems were implemented; high speed casting technology, high temperature slab discharging technology, technology for width change during casting, slab quality assurance system and production control system.
    Since it has started up in June, 1983, HCR has been running satisfactorily. The latest production amount reaches 270 kt/M and the ratio of hot charge rolling, exceeds 80%.
  • Direct Hot Charging Process at Kure No. 2 Continuous Caster, Nisshin Steel Co., Ltd.

    pp. 1248-1255

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    Kure No. 2 Continuous Casting Plant, which was designed to connect steelmaking plant and hot rolling mill directly, has started its operation in July 1982.
    Since the start up, the hot charging ratio has been increased under several technical developments. Since 1986, the hot charging ratio has been kept over 70%. The present monthly production is 210000t.
    In this paper, the operation of No. 2 CC and techniqes developed for increasing the hot charging ratio are introduced. Main items of the developments are as follows,
    1) Technical development for defect free slabs
    2) Technical development for increasing slab temperature
  • Development of Continuous Casting Technology for Special Steel Wire Rod and Bar

    pp. 1256-1265

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  • Physical and Chemical Properties of Casting Powders Affecting Mold Lubrication during Continuous Casting of Slabs

    pp. 1266-1273

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    Physical and chemical properties of casting powder, which affect mold lubrication during continuous casting of slabs, are evaluated. Experiments of insulating characteristics of unmelted layer of casting powder are made to study the effect of the size and shape of granule of casting powder on melting rate. The effect of chemical composition of casting powder on crystallization and freezing temperature of molten slag is examined in laboratory and practice. Special additives such as BaO, MgO, and B2O3 are effective in lowering freezing temperature and also in prevention of crystallizing of the slag. The rate of slag flow from meniscus and the resulting slag film thickness between solidified shell and mold wall are analyzed theoretically by taking into account the viscosity change in slit.
    The above research work is successfully applied to the development of casting powder for high speed continuous casting over 1.7m/min.
  • Reduction of Friction Force in Mold and Prevention of Sticking Type Breakout for High Speed Continuous Casting of Slabs

    pp. 1274-1281

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    Friction force between mold and slab was measured by load cell in order to optimize mold powder and oscillation conditions for No.3 continuous caster at Chiba Works. Furthermore, a method for prediction and prevention of the breakout was also studied to minimize reduction of productivity when a countermeasure was taken for avoiding the breakout. Results obtained are as follows:
    1) Adoption of mold powder with low viscosity and reduction of negative stripping ratio made it possible to cast low carbon aluminum killed steel stably at 2.0m/min by decreasing friction force less than 200gf/cm2.
    2) Effect of the viscosity of mold powder and negative stripping ratio on the friction force can be reasonably explained by considering viscous friction.
    3) The breakout can be successfully predicted by thermocouples set in parallel to meniscus in mold copper plate.
    4) When the breakout sensor alarms, the casting speed is instantly lowered down to the predetermined allowable level to avoid the breakout.
  • The Secondary Cooling Technology of Continuous Casting for Production of High Temperature and High Quality Slab

    pp. 1282-1289

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    Delivery of sound slabs at high temperature is one of the most important conditions for the caster to realize the direct rolling (HDR) practice. Secondary cooling system was to be modified to make plan of Fukuyama No. 5 machine for HDR practice. To improve the uniformity and the controllability of cooling intensity, an air-mist cooling nozzle was newly developed. The developed nozzle has several features such as small amount of air consumption, stable spray pattern, easy maintenance, and others. Empirical equation to describe the cooling intensity of the air-mist cooling nozzle was obtained as a function of its operational factors. Evaluation of the nozzle performance in the secondary cooling zone was carried out through plant test with existing casters prior to the installation of No. 5 machine. Improvement of surface qualities of slabs was confirmed as the results of the test. Average heat transfer coefficient of the secondary cooling zone was also determined. Based on the knowledge, the secondary cooling zone and the reheating zone of No. 5 machine was designed and operated. Sound and high temperature slabs are delivered by the caster to realize the smooth HDR operation.
    x

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    1. Speed Disturbance Compenation in the Secondary Cooling Zone in Continuous Casting ISIJ International Vol.40(2000), No.5
    2. Simulator Based Design of Secondary Cooling Zone Control in Continuous Casting Tetsu-to-Hagané Vol.96(2010), No.7
  • Influence of Carbo-Nitride and Proeutectoid Ferrite on Hot Ductility of Nb, V Containing Steel

    pp. 1290-1297

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    Surface crack which occurs in unbending process during continuous casting, was investigated by hot tensile test mainly at 750°C in relation to Nb, V carbo-nitride and proeutectoid ferrite in Nb, V containing steels.
    Amount of dynamically precipitating carbo-nitride has close relation to the value of load integration which was newly introduced parameter.
    Surface crack sensitivity grows higher as the parameter becomes larger because more Nb, V carbo-nitrides precipitate at grain boundary and intercrystalline, which leads to the increase of total amount of precipitates.
    Thin Proeutectoid Ferrite around austenite grain boundaries was observed when deformation was introduced at 750°C, and the ferrite also increased crack sensitivity.
    To avoid surface cracks in continuous casting, reducing the number and zone length of unbending points is effective because load integration value decreases. Furthermore, surface crack sensitivity can also be improved when the temperature at unbending is higher than 950°C because the amount of Nb, V carbo-nitride becomes less and ferrite precipitation does not occur.
  • Analysis of Unbending Strains of Strands in Multi-Points Unbending Low Head Continuous Casters for High Temperature Slabs

    pp. 1298-1305

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    Unbending strains of strands in multi-points unbending profiles of low head continuous casters, which will serve to direct rolling of high temperature cast slabs, were studied through the newly developed mathematical model and the accurate simulation experiments giving the following results.
    (1) Agreement between calculated strains with this mathematical model and experiments is excellent, while the conventional equation needs to be corrected.
    (2) The distribution of unbending strain is decided mainly by the arrangement of rolls rather than by stress-strain characteristics or rigidity distribution of a strand.
    (3) The effects of roll arrangements on the characteristics of strain distribution are as follows.
    In a one-point unbending profile, the maximum strain increment is about a half of the value estimated by the conventional equation.
    In a continuous multi-points unbending profile the maximum strain increment agrees with the conventional equation, while it is about half as large as that of the conventional equation in an intermittent multi-points unbending profile.
  • Fundamental Investigation of Inline Prereduction Shearing Method of Continuous Casting Steel

    pp. 1306-1313

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    The prereduction shearing method of continuous casting steel has been fundamentally investigated in which reduction of thickness and sequential shearing are applied to the steel for the purpose of decreasing the capacity of a shearing press as well as saving energy.
    The prereduction shearing model tests of hot carbon steel were carried out, and a prereduction shearing apparatus was made according to the results.
    The prereducing tests of unsolidified ingots were also carried out to show that no internal cracks are found when the ratio of solid shell thickness to total thickness is more than about 30%. In the prereduction shearing tests of unsolidified ingots, a few internal defects appeared, but they were very fine and thought to be harmless. Some cracks and leaks of liquid steel through the cut section were observed too, and they caused to the surface defects at the end of rolled sheets. However, they were not so serious because they would be cut off with crop loss.
    The press capacity is minimum when a prereducing load equals to a shearing load, and for a large section continuous casting steel it will be remarkably reduced by angular tools after solidification, and by circular tools before solidification.
    It is concluded that the prereduction shearing method is very useful for promoting hot direct rolling process of continuous casting steel.
  • System Technology Supporting Operation of Direct Rolling

    pp. 1314-1322

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  • Production Control System for Direct-linked Process between Continuous Casters and Hot Strip Mill or Plate Mill at Oita Works, Nippon Steel Corp

    pp. 1323-1330

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    In the development of the direct-linked process are required not only technical innovation in the manufacturing processes involved but also the renovation of the wide spectrum of production control from the control technology through the computer system to the production organization according to new ideas. "Dynamic production control system" (named as DIAS) for dynamically controlling the material flow and schedule matching of the direct-linked process of ironmaking, steelmaking and rolling was developed at Oita Works in 1980. "Dynamic energy demand/supply control system" (named as DIAS II) linking with "DIAS" for dynamically controlling the energy balance and cost saving was additionally developed in 1985.
  • The Integrated Control System of the Directly Connected Production Process between Continuous Casting Machines and Hot Strip Mill

    pp. 1331-1337

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    In Kashima Steel Works hot slabs supplied from two continuous casting machines are rolled at a hot strip mill in one rolling cycle.
    Hence, in order to realize efficient casting and rolling operations while syncronizing these two processes under many equipmental and operational constraints, we have developed a consistent production control system, which includes the integrated cast and roll scheduling system and the instruction system for proper actions in case of abnormal occurrences.
    In this paper we describe the concept of the mixed rolling schedule system(its aims, functions and features), and also its operational problems, improved points and a view for the future.
  • Synchronized Production Control System between Steel Making and Hot Rolling at Mizushima Works, Kawasaki Steel Corp

    pp. 1338-1345

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    A synchronized production control system between steel making and hot rolling was developed at Mizushima Works, (which is named "P2 system"). This system is composed of integrated planning system and operation control system.
    The former includes 5-day scheduling, daily scheduling and schedule adjustment system. And the latter covers operations of hot metal, BOF, CCM, slab yard, hot strip mill and coil yard.
    In order to synchronize the production rate, these were the key technologies to be developed; decreasing the number of slab sizes in continuous casting, quick quality check of slabs before hot rolling and schedule free rolling in hot strip mill (HSM). HSM-in-line sizing press was developed and installed for the first time in the world to decrease the number of slab sizes. Hot slab samplers were also developed and equipped in slab finishing line for the quick check of chemical composition, and all the finishing stands of HSM were equipped with work-roll-shift devices for the schedule free rolling.
    This system was started in March 1987, and now demonstrating noticeable results such as improvement of material flow, hot charge rolling, production yield, quality of products, energy saving, man power saving and so on.
  • The Optimum Process on Continuous Casting and Hot Strip Mill

    pp. 1346-1353

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    The effect of Hot Direct Rolling(HDR) process and Direct Hot Charged Rolling (D-HCR) process between continuous casting and hot strip mill was studied by the simulate calculation and the real operation.
    1) HDR process is superior in heat consumption to D-HCR process under the same ratio of these productions.
    2) In D-HCR process, high-efficient furnace operation, for example using only one furnace is profitable.
    3) HDR process in which the heating can be adopted in a short time and partially is superior in scale loss.
    4) The temperature deviation of the strip in the rolling direction can be decreased by these process, and the effect of the quality's improvement such as the improvement of the accuracy about the dimension can be obtained.
    5) Though the temperature profile in the width direction is a little different by the process, the fixed temperature for the quality is maintained in each process.
  • Synchronized Operation between Continuous Casting and Rolling Processes in the New Billet Mill of Mizushima Works, Kawasaki Steel Corp.

    pp. 1354-1361

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    A modernization program for billet, bloom and shape products was completed at Mizushima Works generally aiming at streamlining the complicated crisscross of products from the two continuous bloom casters to the succeeding rolling mills. A new billet mill, which was a significant part of this modernization, was brought into operation in February, 1984. The billet rolling facility has the 'roll chance free" function in order to realize continuous and syncronized production between the continuous casters and the new billet mill.
    This new factory has remarkably realized labor-saving and rationalization of handling and transporting by several new techniques, such as the highly computer controlled vehicle system and quick roll changing system.
    The control of operation and information has been fully computerized over the entire processes from steelmaking to billet shipping.
  • Establishment of HCR Technology at Heavy Plate Mill in Kimitsu Works, Nippon Steel Corp.

    pp. 1362-1369

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    At a Heavy Plate Mill in Kimitsu Works, HCR operation has been introduced since July 1982.
    Since start-up, the production management between caster and mill was made more suitable for the HCR operation and several measures were taken to raise HCR ratio and the slab temperature, such as decrease of slab surface cracks, building of slab heat insulator, optimized scheduling system and so on.
    As a result, the HCR ratio and temperature is now more than 75% and 680°C.
    In this paper, we describe the concepts, contents and effects of these measures.
  • Thermal Study on Possibility of Direct Rolling between the Continuous Casting and Hot Rolling Processes

    pp. 1370-1377

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    The possibility of unification between the continuous casting and hot rolling processes has been investigated from a point of view of a slab temperature. Then, we have made a numerical model to estimate the change in slab temperature from a meniscus to a crater-end as well as the growing process of shell thickness, and calculated the temperature process of slab and the position of crater-end under various conditions. Also, the effect of the thermal insulator, the casting speed and the width as well as length of the secondary cooling zone on the slab temperature has been studied to find out the optimal condition for producing slabs with a possibly high temperature. Next, we have numerically examined whether or not the slab transported directly from the continuous casting machine is permissible to be hot-rolled under such a temperature condition. The results so obtained show the possibility of direct rolling for producing hot coils thicker than 2mm.
  • A Universal Continuous Caster-Hot Strip Mill Direct Rolling Process by Effective Utilization of the Unique Schedule Free Rolling Operation System

    pp. 1378-1385

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    A new 66-in. Yawata hot strip mill put into operation in Apr. 1982 and the total production amounted to 17 million tons at the end of June 1987. In this mill, up-to-date equipments, such as the 6Hi mill, work-roll shift device, etc. and large scale computer control system were introduced. This SFR operation has been established by a lot of developments about the unique softwares, such as the work-roll shift system, automatic crown and flatness control system, the resistance to deformation model employing the dislocation theory, forward slip measuring system, specialized operation of the reheating furnaces, etc. As a result, the coupled and synchronized operation between continuous caster and hot strip mill has been carried out successfully, so-called hot charged rolling (HCR), which has brought not only to the cost reduction but also to the market need without production shortage. This epoch-making SFR operation has surely changed the image of hot strip mill operation. And, an advanced and lay-out free direct rolling (CC-HDR) operation started-up in June 1987.
  • Technology for the Development of Hot Direct Rolling in Hot Strip Mill

    pp. 1386-1393

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    In NKK Fukuyama Works, Hot Direct Rolling (HDR) process on No. 5 continuous casting machine and No. 2 hot strip mill is operated for the purpose of the saving of the energy and the materials, and the manpower. In the hot strip mill, the process is maintained by the technology of the prevention of temperature going down such as the heat insulating cover, the induction heater, and the technology of the schedule free rolling and the equalize of the product ability between the continuous casting and the hot strip mill, and the new computer system which controls these. By the achievement about each technology, the large quantities of production by HDR is operated stably, and the effect of the improvement about the quality is obtained.
  • Schedule-free Rolling at Existing Hot Strip Mill

    pp. 1394-1401

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    In order to achieve schedule matching between the continuous casting and hot rolling operations, thus ensuring the maximum benefits of "Hot charge rolling, "we have established the schedule-free hot rolling technology that frees the continuous casting operation from various restrictions of the downstream hot rolling operation. Starting from the Kimitsu-developed crown control process ("NBCM rolling method"), we have developed the rolling-mill arrangement, which combines the NBCM (New Back up roll Crowning Mill), the high-strength work roll bender, and the work roll shifting device and has functions nearly equal to those of the 6-high rolling mill. In addition, we have also developed and successfully put into practice the high precision preset control model, the hot-rolling dynamic control model and the automatic scheduling function, aimed at the maximum utilization of the above rolling-mill arrangement, resulting in substantial achievements in energy conservation and yield improvement.
  • Profile and Shape Control in Hot Strip Mill

    pp. 1402-1409

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    A new profile and shape control system has been developed for use in hot strip mills at Wakayama Steel Works and Kashima Steel Works.
    The control system consists of variable crown(VC) rolls, work roll benders and work roll shifting devices on the last three stands of the finishing train. These devices can be preset so that the desired strip profile can be obtained. To achieve this, new mathematical models were developed which could be used to calculate the thermal roll crown, the roll wear, the VC roll profile and the strip profile. These models are considered to be suitable for use in online calculations.
  • Slab Sizing Mill and Its Tension Control System

    pp. 1410-1417

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    In 1980, slab sizing mill was constructed at Oita Works, Nippon Steel Corporation, which is V-H-V stands. (V: Vertical, H: Horizontal).
    This mill is 3 stand tandem mill, and rolls the width of continuous casting slabs.
    The products of this mill is sent to hot strip mill and plate mill.
    This paper describes the mill, and interstand tension control. It utilized the rolling torque and the rolling force, in particular, the rolling torque was separated from the acceleration/deceleration torque. And the interstand tension was controlled within ±0.2 kgf/mm2 by free tension control. Thus high accuracy width deviation was attained.
  • Effect of Cabon Content on Deformation Resistance of Cabon Steels in Austenite Temperature Range

    pp. 1418-1425

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    Deformation resistance of carbon steels with carbon content from 0.05 to 0.80wt% is examined in the austenite temperature range and at the strain rates ranging from 10-3 to 200/s with special respect to its dependence of carbon content. Hot deformation resistance during work hardnening stage increases with increasing carbon content at lower temperature of austenite range and at higher strain rate. On the contrary, it decreases with increasing carbon content at higher temperature range and at lower strain rate. In early work hardening stage, work hardening is promoted with increase of carbon content. Work hardening rate decreases with increase of carbon content at all conditions of strain, strain rate and temperature. Such phenomena are discussed with respect to the dependence of the stacking fault energy on carbon content. The deformation resistance in second deformation stage after interruption of deformation comes close to the deformation resistance without interruption of deformation as strain increases. The precise mathematical model of resistance is obtained by a quadratic formula of regression with strain, strain rate, temperature and carbon content.
  • Metallargical Problems on Hot Direct Rolling from Continuous Casting

    pp. 1426-1429

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  • α→γ Transformation Behavior during Heating from(α+γ) Region in 0.18% Carbon Steel

    pp. 1430-1437

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    α→γ transformation behavior during continuous heating at a rate of 3K/s from the (α+γ) region in 0.18% carbon steel has been investigated by dilatometric measurement and metallography.
    The transformation was delayed with increasing an initial amount of proeutectoid α before the heating. When the amount of proeutectoid α before the heating was same, the transformation kinetics was delayed with increases in initial γ grain size and in the γ→α transformation temperature in the (α+γ) region. Microscopic observation showed that the α→γ transformation proceeded only by the growth of pre-existing γ, namely the dissolution of proeutectoid α, without nucleation of new γ grains in the proeutectoid α particles. When the temperature (T0.9) for 90% γ in the α→γ transformation was plotted against the size of α particles before the heating, the experimental data lay on a straight line and T0.9 increased with increasing the α particle size, regardless of various factors such as the initial γ grain size, the γ→α transformation temperature, and the amount of proeutectoid α. The effect of the various factors on the α→γ transformation behavior could be well explained by considering the influence of those factors on the size of proeutectoid α particles.
  • Effect of Hot Charge Rolling Conditions from Austenite Region on Microstructure and Mechanical Properties of Nb and Ti Bearing Steel Plates

    pp. 1438-1445

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    The effect of γ-HCR conditions on microstructures and mechanical prorerties of Si-Mn, Nb and Ti bearing steels were investigated. Here, γ-HCR is defined as hot charge rolling in which casted slab is charged into furnace at austenite(γ) state. The results are summarized as follows.
    (1) Si-Mn steel produced by γ-HCR and cold charge rolling has almost the same mechanical properties. On the contrary, Nb and Ti bearing steels produced by γ-HCR has significantly mixed grain size structure and low toughness.
    (2) The increase in charging temperature, increase in heating temperature and reduction of heating time in γ-HCR process lead to increase in strength and microstructural refinement in Nb and Ti bearing steels.
    (3) Increase in rolling start temperature in γ-HCR process leads to microstructural refinement and improvement of toughness in Nb and Ti bearing steels.
    (4) The amount of precipitates prior to rolling decreases in accordance with the change on γ-HCR conditions as mentioned in (2). Precipitates prior to rolling retard the recrystallization of as-casted coarse grain. From these results, (2) was explained.
    (5) In the case of applying γ-HCR process to micro-alloyed steel plates, it is important to charge the slab at high temperature, beet for short time and start rolling from the temperature above 1100°C with high one pass reduction such as 20%/pass.
  • Effect of Hot Charge Direct Rolling Condition on Mechanical Properties of Nb Bearing Steel Plate

    pp. 1446-1453

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    A large amount of steel plates is currently produced by HCR (Hot Charge Direct Rolling) process. In this study, the effect of HCR condition on the mechanical properties of Nb bearing steel is investigated.
    If the charging temperature is above Ar1, the toughness of Nb bearing steel through HCR process is inferior to conventionally rolled one. This phenomenon comes from the larger amount of Nb in solution in the HCR process than in the CCR process. This Nb in solution markedly retards the recrystallization of austenite and then it precipitates as Nb(CN) finely during and after rolling in the HCR process. To improve the balance between the tensile strength and brittle-ductile transition temperature in Nb bearing steel at a charging temperature above Ar1, 1100°C is the most favorable for the rolling start temperature, because the finest austenite grain is obtained at this temperature range due to recrystallization.
  • Effects of Hot Charge Rolling on the Mechanical Properties and Microstructure of Controlled-rolled High Strength Low Alloy Steel

    pp. 1454-1461

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    A study was made on effects of hot charge rolling(HCR) on the mechanical properties and microstructures of the plate, in relation to the behaviors of austenite (γ) grains and precipitates.
    In this process, it becomes very difficult to refine γ grains by conventional controlled-rolling, because of the coarse as-cast γ grains before rolling.
    Therefore, low-temperature toughness remarkably deteriorates. In order to improve low-temperature toughness of HCR plate, the optimum rolling method of γ grains was examined in the recrystallization and non-recrystallization regions of γ.
    On the other hand, it is possible to get higher strength in HCR process, compared with that in conventional process, because precipitation-hardening elements such as Nb are fully dissolved. The strength of HCR plate, however, strongly depends on the processing conditions.
    Furthermore, it was found that a microalloyed Ti of about 0.01 % remarkably raises the strength by the precipitation-hardening in HCR process.
  • Mathematical Model for Microstructural Changes and Mechanical Properties in Microalloyed Steels in Continuous Casting-Direct Rolling Process

    pp. 1462-1469

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    A microstructural control model for conventional reheating and rolling process has been applied to computer simulation of microstructural changes in direct rolling process. Results are summarized as follows:
    (1) Effects of secondary cooling conditions in continuous casting on carbonitride precipitation behaviours predicted by the model are in good agreement with those observed by hot ductility simulation tests.
    (2) It is known that effect of charging temperature on kinetics of phase transformation and grain growth and low temperature toughness is distinctive: The computer simulation results show that low temperature toughness of steel plate deteriorates if hot charging temperature is higher than Ar1. This result is consistent with that of laboratory rolling experiment.
    (3) The mathematical models for carbonitride precipitation, phase transformation and grain growth for the conventional process can be applicable to microstructural control in continuous casting-direct rolling process.
  • Precipitation Kinetics in Cast Steels Prior to Hot Rolling and Mechanical Properties of Rolled Products

    pp. 1470-1477

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    Lowering the slab reheating temperature for subsequent rolling is effective for energy saving. However, the temperature has rarely been decreased to lower than 1150°C because of the necessity to desolve precipitates in steels with Nb, V, Al etc. To produce steels with good mechanical properties by a process of hot charging or direct rolling, the understanding of precipitation kinetics is the most important because the thermal history of the new process is quite different from that of a conventional process.
    Experiments were carried out on Nb-microalloyed steels and Al-killed steels for batch annealing. Small ingots were hot charged into a furnace at 10501100°C and held for various times. Mechanical properties of rolled (and annealed) steels and precipitates were closely examined. With hot charging, the possibility arises that the temperature is free from any restriction; as long as the slab is maintained above Ar3, precipitation of the alloying carbide or nitride will hardly occur in γ prior to deformation.
  • Mechanical Properties of Continuous-Cast and Hot Direct-Rolled Ti-added High Strength Steel Sheets

    pp. 1478-1485

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    Continuous-cast and hot direct-rolling processes(CC-HDR) have been under operation in Japan and contributed to saving energy and time.
    From a metallurgical point of view, some differences of mechanical properties of low alloyed steels may take place between CC-HDR and conventional reheat-rolling process (Reheat), because of its change in dissolution and precipitation behavior of the alloying elements.
    Mechanical properties of Ti-added high strength hot rolled steel sheets manufactured by CC-HDR in Sakai Works, Nippon Steel Corp., have been examined in comparison with those of conventional Reheat steels.
    Both of the tensile and yield strength of the HDR steel sheets, having 55 and 60 kgf/mm2 tensile strengths level, were 45 kgf/mm2 higher than those of Reheat steels. The amount of H3PO4(2+1) soluble Ti of HDR steels were larger than that of Reheat steels and the number of relatively coarse Ti-precipitates measured by TEM were few in comparison with that of Reheat steels.
    Therefore it is concluded that the amount of very small size Ti-precipitates in HDR steels which contribute to the strength of steel may probably be larger than that of Reheat steels and so the strength of HDR steels are higher than that of Reheat steels. This difference of precipitation behavior between these processes was explained by their different thermal histories.
  • Effect of the Small Addition of Ti on the Mechanical Properties of Hot Direct Rolled Steel Sheets

    pp. 1486-1492

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    The effect of a small addition of titanium on the mechanical properties of hot direct rolled steels was examined in the laboratory simulation. The small addition of titanium increases the strength of hot direct rolled steels, however the strength of conventional reheating hot rolled steels does not change with the small addition of titanium. The ferrite grain refinement of hot direct rolled steels is also observed with the small addition of titanium.
    In hot direct rolling process, titanium seems to be mostly solute during the cooling stage from solidification to the start of hot rolling, and to precipitate as fine carbides/nitrides during or after hot rolling. The dependence of the strength and low temperature toughness on the nitrogen content in the titanium bearing hot direct rolled steels suggests that the strength in low nitrogen steels is due to the fine TiC precipitates and TiN precipitates for high nitrogen steels.
  • Effects of Cooling Rate during Solidification and Charging Temperature Prior to Hot-rolling on Mechanical Properties of Multi-phase Hot-rolled Sheet Steels

    pp. 1493-1500

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    A study has been made of the effects of the cooling rate during solidification and the charging temperature prior to hot-rolling on the mechanical properties of multi-phase high strength hot-rolled sheet steels.The strength of B-bearing multi-phase sheet is independent on the cooling rate during solidification and decreases with dropping the charging temperature prior to hot-rolling. That may be due to the decrease in solute boron content segregated to austenite grain boundary to improve hardenability, because of the precipitation of BN during cooling the ingots below 1000°C.
    The strength of Nb-bearing multi-phase sheet steel decreases with dropping hot-charging temperature.The decrement occurrs at lower temperature in the hot-rolled sheet produced from the ingot solidified in cast-iron mold to simulate the solidification condition of thin-slab casting than in sand mold sample to simulate conventional continuous casting, an effect which is explained in terms of effective suppression of NbC precipitation during cooling in the cast-iron mold ingot.
  • Effects of Cooling Rate during Solidification and Charging Temperature Prior to Hot-rolling on Mechanical Properties of Cold-rolled Sheet Steels with Deep Drawability

    pp. 1501-1508

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    A study has been made of the effects of the cooling rate during solidification and the charging temperature prior to hot-rolling on the mechanical properties of cold-rolled sheet steels with deep drawability. The r-value deteriorates abruptly as the charging temperature decreases below 900°C in the batch-annealed low carbon Al-killed steel sheet hot-rolled from the ingot solidified in cast-iron mold to simulate the solidification condition of thin-slab casting, while r-value remains high even at the charging temperature of 600°C in sand mold sample to simulate the solidification condition of conventional continuous casting.
    Ther-value of continuously annealed extra-low carbon Ti-bearing steel sheet hot-rolled directly from the ingot solidified in the cast-iron mold is about 2.0 and is same as that of sand mold sample. However, with decreasing the charging temperature, the r-value of sand mold sample decreases, whereas that of castiron mold sample remains almost constant.
  • AlN Precipitation of Al-killed Steel for Cold-rolled Sheets in Continuous Casting and Direct Rolling Process

    pp. 1509-1516

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    A continuous casting and direct rolling process for Al-killed cold-rolled steel sheets was examined. AlN precipitation in steels which were melted before precipitation treatment was studied in the laboratory and AlN precipitation diagrams (TTP) were obtained. AlN precipitation in melting-treated steels is delayed and a larger amount of aluminum and nitrogen are kept in solution in comparison with that in AlN solution-treated steels. When the temperature of steels after casting and solidification is kept above Ar3 transformation temperature, AlN hardly precipitates. Production-scale tests of the process were performed by using the continuous casting machine and the hot strip mill. AlN precipitation in hot-rolled steel sheets was also observed. Enough amount of solute aluminum and nitrogen for obtaining high r-value after cold-rolling and box-annealing were maintained in hot-rolled steel sheets. Even in case of high content of aluminum and nitrogen, most of them are in solution in hot-rolled sheets and cold-rolled steel sheets which have high r-value are obtained by using the process.
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  • 埋もれた知的資産の行方

    pp. 1517-1517

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