An impeller for stirring a melt pool includes: an impeller body extending in the length direction; a blowing nozzle which is provided in such a way as to pass through one part at the bottom end of the impeller body; and a blade provided on the upper part of the impeller body. As a result, when the impeller is used, a stirring flow produced due to the blade and a stirring flow due to substances blown into the melt-pool via the blowing nozzle correspond to each other, and the two flows are combined such that the overall stirring force is improved. Consequently, it is possible to improve the efficiency of stirring by the impeller as compared with hitherto, and, as a result, refining efficiency in the refining step is improved as the rate of reaction between the melt-pool and additives is increased.

An impeller for stirring a melt pool includes: an impeller body extending in the length direction; a blowing nozzle which is provided in such a way as to pass through one part at the bottom end of the impeller body; and a blade provided on the upper part of the impeller body. As a result, when the impeller is used, a stirring flow produced due to the blade and a stirring flow due to substances blown into the melt-pool via the blowing nozzle correspond to each other, and the two flows are combined such that the overall stirring force is improved. Consequently, it is possible to improve the efficiency of stirring by the impeller as compared with hitherto, and, as a result, refining efficiency in the refining step is improved as the rate of reaction between the melt-pool and additives is increased.

A load current value I of a motor rotationally driving an impeller is read out when the impeller is rotated at a predetermined set rotational speed X.sub.0, and the rotational speed of the impeller is increased so that the load current value I becomes equal to or larger than a threshold value I.sub.0 when it is determined that the load current value I is smaller than the predetermined threshold value I.sub.0.

A load current value I of a motor rotationally driving an impeller is read out when the impeller is rotated at a predetermined set rotational speed X.sub.0, and the rotational speed of the impeller is increased so that the load current value I becomes equal to or larger than a threshold value I.sub.0 when it is determined that the load current value I is smaller than the predetermined threshold value I.sub.0.

A process for producing clean steel products with low nitrogen content, below 35 ppm, in a steelmaking plant comprising a direct reduced iron (DRI) source, which may be a direct reduction plant or a DRI storage facility, an electric arc furnace (EAF), a vacuum degassing system (DS), and a continuous casting system (CC) is disclosed. The process comprises a first stage of melting and refining a metallic iron charge, a second stage of tapping molten steel from the electric arc furnace (EAF) into a ladle, a third stage of exposing molten steel to a pressure below the atmospheric pressure and a fourth stage of casting molten steel to clean steel products. Optionally, the molten steel tapped from the EAF is treated in a ladle furnace (LF) prior to being treated in the degassing system (DS). The metallic iron charge fed to the EAF comprises more than 70% by weight of DRI in the form of pellets or briquettes having a carbon content above 2.5 weight %. Preferably, the metallic iron charge is fed to the EAF at a temperature of 400 C. or higher. The low nitrogen level in the steel products made according to the Application is achieved by forming a first foamy slag in said first process stage and is maintained in a foamy state by controlling the feed of fluxes, oxygen, and carbonaceous materials to the EAF and by forming a second slag, after molten steel is tapped from the EAF, having a predetermined composition capable of continuing the desulfurization and providing a thermal and chemical insulation to prevent nitrogen pickup and promote nitrogen removal of molten steel. The process also comprises carrying out one or more of the following actions: (a) controlling the concentration of nitrogen and sulfur in the raw materials at each process stage, (b) promoting nitrogen removal from steel, (c) decreasing the time spent by the molten steel at each process stage and between each and subsequent process stages, and (d) preventing nitrogen pickup by the molten steel all along said process stages. Steel products made according to the Application comprise the following elements expressed in weight %: C0.05%, Si4.5%, Al2.0%; Mn2.0%; P0.20%; Ni0.200%, Cu0.200%; N0.0030%, Ni0.200%, S0.0035%.

A cold iron source melting ratio estimation device that estimates a melting ratio of a cold iron source charged into a converter-type refining furnace during refining of molten iron in the converter-type refining furnace. The device includes: an input section to which measured values of in-furnace information or estimated values of the in-furnace information is input, the in-furnace information including a molten iron temperature and a carbon concentration in the molten iron during refining; a database section that stores a model equation and parameters related to a refining reaction of the molten iron in the converter-type refining furnace; a computation section that computes the melting ratio of the cold iron source using the measured values or the estimated values input to the input section; and an output section that displays the melting ratio of the cold iron source computed by the computation section.

A top-blowing lance includes a refining oxygen gas blowing nozzle having a plurality of ejection openings through which oxygen gas is blown into an iron bath in a reaction vessel, the ejection openings being disposed along a circular orbit at intervals, and a burner nozzle having an axis coaxial with the central axis of the circular orbit, forming a flame inside the refining oxygen gas blowing nozzle, and having ejection openings for blowing a powder heated by the flame into the iron bath, wherein an indicator A that indicates the positional relationship between the ejection openings of the refining oxygen gas blowing nozzle and the ejection openings of the burner nozzle satisfies the specified conditions.

A top-blowing lance includes a refining oxygen gas blowing nozzle having a plurality of ejection openings through which oxygen gas is blown into an iron bath in a reaction vessel, the ejection openings being disposed along a circular orbit at intervals, and a burner nozzle having an axis coaxial with the central axis of the circular orbit, forming a flame inside the refining oxygen gas blowing nozzle, and having ejection openings for blowing a powder heated by the flame into the iron bath, wherein an indicator A that indicates the positional relationship between the ejection openings of the refining oxygen gas blowing nozzle and the ejection openings of the burner nozzle satisfies the specified conditions.

The present invention relates to a steel rebar comprising the following ingredients: 0.005%-0.030% of C, 0.3%-0.6% of Si, 1.2%-2.5% of Mn, 0.01% or less of P, 0.01% or less of S, 8.0%-10.0% of Cr, 1.0%-3.0% of Mo, 0.2%-0.4% of Sn, 0.01%-0.05% of Rare Earth element, and the remainder being Fe and unavoidable impurities. The present invention also provides a production method of steel rebar. The steel rebar of the present invention has excellent comprehensive mechanical properties and corrosion resistance performance, while meeting the requirements of anti-knock, the service life in sea water of the steel rebar is increased, thus it can be widely used in reinforced concrete structures in ocean environment.

A method capable of efficiently producing grained iron with a low P concentration includes a first step of melting reduced iron to obtain primary molten iron, a second step of separating the primary molten iron from slag, a third step of subjecting the primary molten iron separated from the slag to dephosphorization to obtain secondary molten iron, and a fourth step of solidifying the secondary molten iron into a grained form to obtain grained iron, in which in the third step, the dephosphorization is performed by supplying an oxygen source and a CaO source to the primary molten iron, and a temperature of the secondary molten iron at the end of the dephosphorization is set to a temperature of the primary molten iron at the start of the dephosphorization or lower.