JFE Steel Patente

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Objects are to provide a high strength steel sheet having a tensile strength of 980 MPa or greater and having excellent ductility, fatigue strength, and blankability and to provide a method for manufacturing the same. A high strength steel sheet has a predetermined chemical composition. In the high strength steel sheet, a primary phase is a bainite phase that includes upper bainite and granular bainite and is present in an area fraction of 85% or greater, and a hard secondary phase includes fresh martensite and/or retained austenite phases and is present in an area fraction of less than 15%; the area fraction of the granular bainite with respect to the area fraction of the upper bainite in the bainite phase is 5% or greater and 40% or less; a difference |Hv1 - Hv2| between a hardness Hv1 and a hardness Hv2 is 30% or less with respect to 0.3TS, where Hv1 is a hardness at a 1/2 thickness position, and Hv2 is a hardness at a 1/4 thickness position; and the upper bainite phase has an average grain size of 7 µm or less.

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Provided is a blast furnace operating method with which CO<2> emissions can be reduced. In the blast furnace operating method, molten pig iron is produced by charging agglomerated ore into the blast furnace from the top and supplying a gaseous reducing agent from a lower portion of the blast furnace. The amount of the gaseous reducing agent supplied is adjusted to 100 Nm<3>/t or more, and the concentration of FeO in the agglomerated ore is adjusted to 10% or less.

It is an object of the present invention to evaluate delayed fracture characteristics of a sheared end surface considering a load loading condition in press forming. A delayed fracture characteristic evaluation method for evaluating the delayed fracture characteristics of a sheared end surface of a metal sheet for press forming to be used for press processing includes: producing a plurality of test pieces having the sheared end surface by applying shearing processing under the same shearing condition to a metal sheet as a target; applying a test with a plurality of load stresses of loading stress under load loading conditions in which the stress distributions in the sheet thickness direction are the same or similar to the sheared end surfaces of the test pieces; carrying out the test under a plurality of load loading conditions in which the stress distributions in the sheet thickness direction are different from each other; and determining a limit stress, the limit stress being a limit value of a load stress at which a delayed fracture does not occur, according to the stress distribution in the sheet thickness direction based on the test for each load loading condition.

Provided is a gaseous reduction degree measurement method capable of appropriately measuring the gaseous reduction degree of an iron raw material used in an ironmaking process with consideration given to the influence of stagnation of the reduction reaction. Also provided are a blast furnace operating method, a direct reduction shaft furnace operating method, and an iron raw material.The gaseous reduction degree measurement method is used to measure the gaseous reduction degree of an iron raw material reduced using a gaseous reducing agent and used in an ironmaking process. The measurement time t (min) for measuring the gaseous reduction degree of the iron raw material is longer than or equal to t_ret × 0.9, where t_ret is a time calculated from the following formula (1): t_ret = 5 × d/(Xco - 2 × X_CO2 + 2 × X_H2 - 4 × X_H2O) × exp(6000/T)Here, T is reduction temperature in K, and d is the average particle size mm of the iron raw material. X_CO is the concentration % of CO in the gaseous reducing agent, and X_CO2 is the concentration % of CO₂ in the gaseous reducing agent. X_H2 is the concentration % of H₂ in the gaseous reducing agent, and X_H2O is the concentration % of H₂O in the gaseous reducing agent.

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To provide a method for dephosphorizing molten iron that can effectively improve the phosphorus distribution and thus promote a dephosphorization reaction of molten iron by providing electric energy using an electrochemical method. Specifically, provided is a method for dephosphorizing molten iron, applying a current between molten slag and molten iron via two electrodes, with one in contact with the molten iron serving as an anode and the other in contact with only the molten slag serving as a cathode, characterized in that Relational Expression (1) below is satisfied among the density I of the applied current, the molten steel temperature T of the molten iron, the phosphorus distribution Lp in the molten slag, and the required phosphorus distribution Lp' for the molten slag: I ≥ α × ln Lp ′ / Lp × T <img class="EMIRef" id="20af17b2-0fab-4cfc-897c-8302b69eeef7-ia01" /> where in Relational Expression (1), I represents the density (A/m<2>) of the applied current, α represents the constant, Lp represents the phosphorus distribution (-) in the molten slag, Lp' represents the required phosphorus distribution (-) for the molten slag, and T represents the molten steel temperature (K) of the molten iron.

An object of the present invention is to provide a duplex stainless steel and a stainless steel seamless pipe that have high strength, particularly, a yield strength of 448 MPa (65 ksi) or greater, and have excellent corrosion resistance and excellent low-temperature toughness. A duplex stainless steel having a chemical composition containing, in mass%, C: 0.002 to 0.03%, Si: 0.05 to 1.0%, Mn: 0.1 to 1.7%, P: 0.040% or less, S: 0.020% or less, Cr: 20.0 to 28.0%, Ni: 4.0 to 10.0%, Mo: 2.0 to 5.0%, Al: 0.001 to 0.05%, N: 0.06 to 0.35%, Sb: 0.001 to 1.000%, and O: 0.010% or less, with the balance being Fe and incidental impurities, wherein the duplex stainless steel includes microstructures containing an austenite phase in a volume fraction of 20 to 70% and a ferrite phase in a volume fraction of 30 to 80%, and the duplex stainless steel has a yield strength YS of 448 MPa or greater.

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The present invention provides, in the form of a welded joint for high-Mn steel to be used in cryogenic environments, a gas tungsten arc welding technology that can achieve further improvement of the strength of weld metal and enhanced inhibition of formation of hot cracks. Provided is a gas tungsten arc-welded joint of pieces of a steel material produced using a shielding gas and a filler metal. The steel material is austenitic steel with a high Mn content. Chemical composition of the weld material includes, in mass%, C: 0.15 to 0.80%, Si: 0.10 to 1.00%, Mn: 17.0 to 30.0%, P: 0.030% or less, S: 0.030% or less, Al: 0.005 to 0.100%, Cr: 0.4 to 5.5%, Mo: 0.01 to 3.50%, Ni: 0.01 to 10.00%, and N: 0.0400 to 0.4000%, with the balance being composed of Fe and unavoidable impurities. Further provided is a production method of a welded joint in which a nitrogen gas is contained in a shielding gas, or a content of N in a filler metal is increased, to form the above-described weld metal.