Abstract: Provided is a Fe—Mn—C-based twinning-induced plasticity (TWIP) steel which includes 13 wt % to 24 wt % of manganese (Mn), 0.4 wt % to 1.2 wt % of carbon (C), and iron (Fe) as well as other unavoidable impurities as a remainder, is manufactured by caliber rolling, has a microstructure including elongated grains that are elongated in a rolling direction, and has an average grain size of the elongated grains in a direction perpendicular to the rolling direction of 1 ?m or less.

Abstract: Provided is a Fe—Mn—C-based twinning-induced plasticity (TWIP) steel which includes 13 wt % to 24 wt % of manganese (Mn), 0.4 wt % to 1.2 wt % of carbon (C), and iron (Fe) as well as other unavoidable impurities as a remainder, is manufactured by caliber rolling, has a microstructure including elongated grains that are elongated in a rolling direction, and has an average grain size of the elongated grains in a direction perpendicular to the rolling direction of 1 ?m or less.

Abstract: A tough high strength heavy wall steel material having superior weldability is provided, said steel material has a diameter or a side 5 mm or more in length, and comprises oxides 1 ?m or less in particle diameter homogeneously dispersed at a dispersion density in a range of from 10,000 to 100,000 particles/mm2 and uniform ferrite grains 2 ?m or less in grain diameter formed over the entire plane making a right angle with respect to the rolling direction.

Abstract: There are provided a steel for warm working, to be subjected to warm working as various structures, components of cars, and the like, a warm working method thereof, and a steel material and a steel component obtainable from the warm working method. [Solving Means] A steel is to have a particle dispersion type fiber structure formed in the matrix by warm working. The steel is characterized in that the total amount of the dispersed second-phase particles at room temperature is 7×10?3 or more in terms of volume fraction, and the Vickers hardness (HV) is equal to or larger than the hardness H of the following equation (2): H=(5.2?1.2×10?4?)×102??(2) when the steel is subjected to any of annealing, tempering, and aging treatments in the as-unworked state under conditions such that a parameter ? expressed by the following equation (1): ?=T(log t+20)(T; temperature(K), t; time(hr))??(1) is 1.4×104 or more in a prescribed temperature range of 350° C. or more and Ac1 point or less.

Abstract: A tough high strength heavy wall steel material having superior weldability is provided, said steel material has a diameter or a side 5 mm or more in length, and comprises oxides 1 ?m or less in particle diameter homogeneously dispersed at a dispersion density in a range of from 10,000 to 100,000 particles/mm2 and uniform ferrite grains 2 ?m or less in grain diameter formed over the entire plane making a right angle with respect to the rolling direction.

Abstract: A tough high strength heavy wall steel material having superior weldability is provided, said steel material has a diameter or a side 5 mm or more in length, and comprises oxides 1 ?m or less in particle diameter homogeneously dispersed at a dispersion density in a range of from 10,000 to 100,000 particles/mm2 and uniform ferrite grains 2 ?m or less in grain diameter formed over the entire plane making a right angle with respect to the rolling direction.

Abstract: A tough high strength heavy wall steel material having superior weldability is provided, said steel material has a diameter or a side 5 mm or more in length, and comprises oxides 1 &mgr;m or less in particle diameter homogeneously dispersed at a dispersion density in a range of from 10,000 to 100,000 particles/mm2 and uniform ferrite grains 2 &mgr;m or less in grain diameter formed over the entire plane making a right angle with respect to the rolling direction.

Abstract: A fine ferrite-based steel is disclosed, wherein at least 60% of the ferrite grain boundary is a large angle grain boundary of at least 15°, and the mean ferrite grain size is not more than 2.5 &mgr;m. The fine ferrite-based steel of the invention has a high strength and a long fatigue life.

Abstract: The present invention provides a method of designing a high fatigue strength in high tensile strength steel, comprising: obtaining values of tensile strength &sgr;B (unit thereof is MPa) and Vickers hardness Hv of the steel; measuring a flaw area of an inclusion, when a fracture origin is located only at a surface of the steel; and estimating, in designing the high fatigue strength steel, that a fatigue limit &sgr;w (unit thereof is MPa) of the steel satisfies either &sgr;w≧0.5 &sgr;B or &sgr;w≧1.6 Hv, when a square root of the flaw area, (area)½ (unit thereof is m), contained in the steel is no larger than 45.8/&sgr;B2 or 4.47/Hv2. According to the present invention, a method of evaluating high fatigue strength in high tensile strength steel, in which method a relationship between a flaw dimension (area) of ODA and the fatigue strength is considered, and a high fatigue strength material can be provided.

Abstract: A delayed fracture is effectively prevented by appropriately setting a shape and working stress, and working environment of a high strength member having more than 1,000 MPa of tensile strength. For this end, the relationship between a maximum value of diffusible hydrogen contents of unfailed specimens (Hc) and Weibull stress are found and the content of diffusible hydrogen entered into steel from the environment due to corrosion during the use of the steel member (He) is also found. Then, Weibull stress in the Hc that is equal to the He is found, thus determining the shape and working stress of the steel member so as to provide stress below the Weibull stress.

Abstract: A method of producing a ultra fine grain steel made of ferrite having a mean grain size of not larger that 3 &mgr;m as the base phase, after ingoting raw materials, by austenitizing the ingot by heating it to a temperature of at least an Ac 3 point, then, applying compression working of a reduction ratio of at least 50% at a temperature of from an Ae 3 point or lower to an Ar 3 point −150° C., or at a temperature of at least 550° C., and thereafter, cooling, wherein the strain rate as compression working is in the range of from 0.001 to 10/second.

Abstract: A delayed fracture is effectively prevented by appropriately setting a shape and working stress, and working environment of a high strength member having more than 1,000 MPa of tensile strength. For this end, the relationship between a maximum value of diffusible hydrogen contents of unfailed specimens (Hc) and Weibull stress are found and the content of diffusible hydrogen entered into steel from the environment due to corrosion during the use of the steel member (He) is also found. Then, Weibull stress in the Hc that is equal to the He is found, thus determining the shape and working stress of the steel member so as to provide stress below the Weibull stress.

Abstract: The present invention provides a method of designing a high fatigue strength material in high tensile strength steel, comprising the steps of: obtaining the values of tensile strength &sgr;B (unit thereof is MPa) and Vickers hardness Hv of the steel; measuring the flaw area of inclusion, when the fracture origin is located only at the surface of the steel; and estimating, in designing the high fatigue strength material, that the fatigue limit &sgr;w (unit thereof is MPa) of the steel satisfies either &sgr;w≧0.5&sgr;B or &sgr;w≧1.6 Hv, when a square root of the flaw area, (area)½ (unit thereof is m), contained in the steel is no larger than 45.8/&sgr;B2 or 4.47/Hv2. According to the present invention, a method of evaluating high fatigue strength material in high tensile strength steel, in which method the relationship between the flaw dimension (area) of ODA and the fatigue strength is considered, as well as a high fatigue strength material, can be Provided.

Abstract: A tough high strength heavy wall steel material having superior weldability is provided, said steel material has a diameter or a side 5 mm or more in length, and comprises oxides 1 &mgr;m or less in particle diameter homogeneously dispersed at a dispersion density in a range of from 10,000 to 100,000 particles/mm2 and uniform ferrite grains 2 &mgr;m or less in grain diameter formed over the entire plane making a right angle with respect to the rolling direction.

Abstract: Ups and downs having a cycle of not more than 5 &mgr;m and an amplitude of not less than 200 nm are formed in a part of not less than 70% per unit length of a prior-austenite grain boundary which is observed in a linear form when seen from a vertical plane by a series of steps of subjecting a steel in a state of austenite to a deformation in not less than 30% of a total area reduction rate at a temperature region which is lower than recrystallization temperature of austenite, and subsequently cooling a deformed steel without causing neither recrystallization nor phase transformation of a diffusion type.

Abstract: A fine ferrite-based steel is disclosed, wherein at least 60% of the ferrite grain boundary is a large angle grain boundary of at least 15°, and the mean ferrite grain size is not more than 2.5 &mgr;m. The fine ferrite-based steel of the invention has a high strength and a long fatigue life.

Abstract: The invention according to the present application provides a high strength solidification body by solidifying a starting metallic powder of iron and the like by means of plastic working using hydrostatic pressing, which is, for instance, a high strength high toughness steel material and the like having a superfine texture comprising a crystalline texture consisting of grains 5 &mgr;m or less in average diameter, or preferably, 3 &mgr;m or less in average diameter. Furthermore, the present invention provides a steel material included in the high strength solidification body, which contains oxide grain 0.2 &mgr;m or less in diameter at a volume ratio of from 0.5 to 60%.

Abstract: A high strength phosphorus-containing steel having fine texture free from the problem of embrittlement by positively using the presence of P, which is a carbon steel having an average ferritic grain diameter of 3 &mgr;m or less and containing from 0.04 to 0.1% by mass of P, wherein the volume fraction of P segregated in the grain boundary accounts for 0.3 or less in case the grain boundary is covered by a P layer 1 nm in thickness.

Abstract: The invention provides an ultra-fine grain steel comprising fine ferrite grains as oriented at random and surrounded by large angle grain boundaries. The steel comprises fine ferrite grains having a mean grain size of not larger and 3.0 &mgr;m and surrounded by large angle grain boundaries having an misorientation not smaller than 15°.

Abstract: In carbon steel, oxides with grain diameter of 1 .mu.m or less and with grain spacing of 6 .mu.m or less are dispersed to suppress growth of .gamma. grains by heating at .gamma. region temperature.