High strength steel sheet having a tensile strength of 800 MPa or more comprising a middle part in sheet thickness and a soft surface layer arranged at one side or both sides of the middle part in sheet thickness, wherein each soft surface layer has a thickness of more than 10 μm and 30% or less of the sheet thickness, the soft surface layer has an average Vickers hardness of 0.60 time or less the average Vickers hardness of the sheet thickness ½ position, and the soft surface layer has a nano-hardness standard deviation of 0.8 or less is provided.

One embodiment of the present invention provides an ultra-thick structural steel having excellent brittle crack initiation resistance, and a manufacturing method therefor, the ultra-thick structural steel comprising, by wt %, 0.03-0.08% of C, 1.6-2.2% of Mn, 0.6-1.3% of Ni, 0.005-0.03% of Nb, 0.005-0.02% of Ti, 0.1-0.4% of Cu, 100 ppm or less of P, 40 ppm or less of S, 1.5 ppm or less of H, and the balance of Fe and other inevitable impurities, wherein the sum of acicular ferrite and granular bainite in the microstructure is 80% or more by area fraction, the sum of the total length of cracks having a size of 30 μm or more per unit area of 1 mm2 in a ±1 mm region on the basis of the thickness center of the steel is 130 μm or less, and the yield strength is 500 MPa or more.

A method and apparatus for improving the fatigue and stress corrosion cracking performance of irregular surfaces, such as welds assemblies of components, using a positioning system, such as a robotic or CNC machine, to position a tool head for inducing compression along and into the surface of a workpiece to automatically follow the surface irregularities. The method and apparatus operates to follow a virtual control surface located below the actual surface of the workpiece thereby allowing the irregular topography surface to be uniformly processed with closed loop process control.

A method and apparatus for improving the fatigue and stress corrosion cracking performance of irregular surfaces, such as welds assemblies of components, using a positioning system, such as a robotic or CNC machine, to position a tool head for inducing compression along and into the surface of a workpiece to automatically follow the surface irregularities. The method and apparatus operates to follow a virtual control surface located below the actual surface of the workpiece thereby allowing the irregular topography surface to be uniformly processed with closed loop process control.

An object of the invention is to provide: a two-phase alloy as a metal material that can be preferably utilized under circumstances of a temperature range and a high corrosion as in an oil well, the two-phase alloy having a high corrosion resistance and good mechanical properties that are equivalent or more than those of conventional ones, and saving a cost; a product of the two-phase alloy; and a method for producing the product. There is provided a two-phase alloy containing Cr as a major component and including two phases of an austenite phase and a ferrite phase in a mixed state. The alloy has a chemical composition containing: 34-70 mass % of Cr; 17-45 mass % of Ni; 10-35 mass % of Fe; 0.1-2 mass % of Mn; 0.1-1 mass % of Si; and impurities. The total content of the Ni and the Fe is 30-65 mass %.

A forging process is conducted in a temperature range of 350-600° C. on at least a portion that is required to have a fatigue strength in an intermediate forged product having a ferrite and pearlite texture obtained by conducting a hot forging on a steel in which N is not greater than an amount at which N is unavoidably dissolved as a solid, thereby improving strength of the portion that is required to have a fatigue strength. With this, there is provided a forged product having a good strength and a low price.

A forging process is conducted in a temperature range of 350-600° C. on at least a portion that is required to have a fatigue strength in an intermediate forged product having a ferrite and pearlite texture obtained by conducting a hot forging on a steel in which N is not greater than an amount at which N is unavoidably dissolved as a solid, thereby improving strength of the portion that is required to have a fatigue strength. With this, there is provided a forged product having a good strength and a low price.

A rail comprises a predetermined chemical composition. In a hardness distribution in a region from a rail head surface to a depth of 16.0 mm, a part having higher hardness than V1 that is minimum hardness in a first internal region is present in a second internal region, and hardness of the rail head surface is HBW 400 to 520 and average hardness in the region from the rail head surface to the depth of 16.0 mm is HBW 350 or more.

An ultrahigh-strength ultrahigh-toughness and low-density dual-phase lamellar steel plate is disclosed. The steel plate comprises the following alloy components in percentage by mass: 0.200-0.320% of C, 0.600-2.000% of Mn, 0.200-0.600% of Si, 2.000-4.000% of Al, 0.300-1.200% of Ni, 0.001-0.005% of B, P not greater than 0.012%, S not greater than 0.005%, and the balance of Fe and inevitable impurities. The steel plate consists of dual phases of ferrites and martensites, the ferrites are high-temperature delta ferrites, the martensites are lath martensites, the delta ferrites are distributed in the lath martensites in a lamellar mode. The steel plate has excellent mechanical properties, for example, the yield strength in the rolling direction is not less than 1000 MPa, the tensile strength is not less than 1600 MPa, the elongation is not less than 8.0%, and the average value of Charpy V-Notch impact energy at −40° C. is not less than 350J.

An ultrahigh-strength ultrahigh-toughness and low-density dual-phase lamellar steel plate is disclosed. The steel plate comprises the following alloy components in percentage by mass: 0.200-0.320% of C, 0.600-2.000% of Mn, 0.200-0.600% of Si, 2.000-4.000% of Al, 0.300-1.200% of Ni, 0.001-0.005% of B, P not greater than 0.012%, S not greater than 0.005%, and the balance of Fe and inevitable impurities. The steel plate consists of dual phases of ferrites and martensites, the ferrites are high-temperature delta ferrites, the martensites are lath martensites, the delta ferrites are distributed in the lath martensites in a lamellar mode. The steel plate has excellent mechanical properties, for example, the yield strength in the rolling direction is not less than 1000 MPa, the tensile strength is not less than 1600 MPa, the elongation is not less than 8.0%, and the average value of Charpy V-Notch impact energy at −40° C. is not less than 350J.