A hot-rolled steel sheet has a specified chemical composition and has such a steel structure that the area ratio of a ferrite is 5 to 50%, the area ratio of a bainite that is composed of an aggregate of a bainitic ferrite having an average crystal orientation difference of 0.4 to 3° is 50 to 95%, and the total area ratio of a martensite, a pearlite and a retained austenite is 5% or less.

Hot-rolled steel sheet

PROBLEM TO BE SOLVED: To provide a hot-rolled steel sheet having high strength and high toughness satisfying the yield strength YS: ≥960 MPa, and suitable as the one for the structural member of a large-sized industrial construction machine, and to provide a method for producing the hot-rolled steel sheet. SOLUTION: A steel stock comprising C, Si, Mn, P, S, Al, Nb, Ti, Mo, Cr and B in proper ranges is heated at 1,100 to 1,250°C, and is subjected to finish rolling where the inlet side temperature in the finish rolling lies in the range of 900 to 1,100°C, the outlet side temperature in the finish rolling lies in the range of 800 to 900°C, and a cumulative draft in a recrystallized γ region is 60 to 90%. Thereafter, immediately, cooling is started, and is performed within 30 s from the start of the cooling to a cooling stop temperature of (an Ms point+50°C) or below at a cooling velocity more than a martensite formation critical cooling velocity, and next, the steel is held for 10 to 60s in the temperature range of (a cooling stop temperature±100°C) and is thereafter coiled so as to obtain a steel sheet having a structure consisting of an M phase or a tempered M phase as the main phase, and in which the aspect ratio of old γgrains in the cross-section in the rolling direction is 3 to 18 and combining high strength and high toughness. COPYRIGHT: (C)2011,JPO&INPIT

High-strength hot-rolled steel sheet having excellent low temperature toughness, and method for producing the same

A hot-rolled steel sheet according to the present invention is a steel sheet containing a predetermined components, and satisfying O < S/Ca < 0.8, N - 14/ 48 x Ti ≧ "0" (zero)%. It is a high-strength hot-rolled steel sheet for a spiral pipe excellent in low-temperature toughness in which a pro-eutectoid ferrite fraction is 3% or more and 20% or less, and the other is a low-temperature transformation phase in a microstructure at a depth of a half thickness of a sheet thickness from a steel sheet surface, a number average crystal grain size of a whole of the microstructure is 2.5 µm or less, an area average grain size is 9 µm or less, a standard deviation of the area average grain size is 2.3 µm or less, and a reflected X-ray intensity ratio {211}/ {111} in a {211} direction and in a {111} direction relative to a plane in parallel to the steel sheet surface at the depth of the half thickness of the sheet thickness from the steel sheet surface is 1.1 or more. The steel sheet has both high-toughness and strength of API5L-X80 standard or more.

Hot-rolled steel sheet and method for producing same

There are provided a high-strength hot-rolled steel sheet securing low-temperature toughness and having excellent stretch flangeability by controlling a structural fraction and a hardness difference among structures, and a manufacturing method thereof. A hot-rolled steel sheet contains: C: 0.01 to 0.2%; Si: 0.001 to 2.5% or less; Mn: 0.10 to 4.0% or less; P: 0.10% or less; S: less than 0.03%; Al: 0.001 to 2.0%; N: less than 0.01%; Ti: (0.005+48/14[N]+48/32[S]) % or more and 0.3% or less; Nb: 0 to 0.06%; Cu: 0 to 1.2%; Ni: 0 to 0.6%; Mo: 0 to 1%; V: 0 to 0.2%; Cr: 0 to 2%; Mg: 0 to 0.01%; Ca: 0 to 0.01%; REM: 0 to 0.1%; and B: 0 to 0.002%, and has: an texture in which, at a central portion of a sheet thickness located between ⅜ to ⅝ thickness positions of the sheet thickness from a surface of the steel sheet, an average value of X-ray random intensity ratios of a group of {100} to {223} orientations of a sheet plane is 6.5 or less and an X-ray random intensity ratio of a {332} crystal orientation is 5.0 or less; and a microstructure in which a total area ratio of tempered martensite, martensite and lower bainite is more than 85%, and an average crystal grain diameter is 12.0 m or less.

Hot-rolled steel sheet and manufacturing method thereof

Disclosed is a hot-rolled steel sheet in which at least the sheet-thickness central part which ranges from 5/8 to 3/8 the sheet thickness in terms of distance from the steel sheet surface has an average X-ray random intensity ratio for the {100} to {223} orientations of 1.0-6.0 and has an X-ray random intensity ratio for the {332} crystal orientation of 1.0-5.0. The steel sheet further has an rC, which is the value of r in a direction perpendicular to the rolling direction, of 0.70-1.10 and an r30, which is the value of r in a direction having an angle of 30o with respect to the rolling direction, of 0.70-1.10.

Hot-rolled steel sheet, cold-rolled steel sheet, galvanized steel sheet, and processes for producing these

PROBLEM TO BE SOLVED: To provide a high-strength hot-rolled steel plate which has a small in-plane anisotropy and superior hole expandability, and to provide a manufacturing method therefor. SOLUTION: The hot-rolled steel plate superior in hole expandability comprises, by mass%, 0.005 to 0.150% C, 2.50% or less Si, 0.10 to 3.00% Mn, 0.150% or less P, 0.0150% or less S, 0.150% or less Al, 0.0100% or less N, 0.005 to 0.07% Nb, and the balance Fe with unavoidable impurities; has a structure formed of ferrite, or ferrite and bainite, while the ferrite has a grain diameter of 30 μm or smaller; has a mean value of random X-ray intensity ratios of crystal grains having orientation groups of {100} to {223} in an amount of 4.0 or less; and has a mean value of random X-ray intensity ratios of crystal grains having orientations of {554} , {111} and {111} in an amount of 4.5 or less, in a plate plane at 1/2 of the plate thickness. COPYRIGHT: (C)2010,JPO&INPIT

Hot-rolled steel plate superior in hole expandability and manufacturing method therefor

PROBLEM TO BE SOLVED: To provide a thick high-strength hot-rolled plate sheet having especially ≥5 mm thickness, and excellent strength-ductility balance and blanking property. SOLUTION: The high-strength hot-rolled steel plate is composed, by mass, of 0.025-0.08% C, 0.005-0.09% Si, 1.0-2.0% Mn, ≤0.02% P, ≤0.01% S, ≤0.05% Al, ≤0.01% N, 0.05-0.15% Ti, and Ce of Si/16≤Ce≤0.0060% and the balance Fe with inevitable impurities and the metallic structure has a bainitic-ferritic phase as the main phase. The steel slab having the above chemical components, is heated to 1,150-1,300°C and is subjected to hot-rolling of 60-90% rolling ratio and at 850-950°C finish temperature so as to become ≥5 mm plate thickness, and is subjected to cooling at ≥20°C/s average cooling speed, and produced by winding at 450-650°C. COPYRIGHT: (C)2009,JPO&INPIT

High-strength hot-rolled steel plate having excellent strength-ductility balance and blanking property, and producing method therefor

To establish the method for determining the amount of carbon in the ferrite phase in ferrite + martensite dual-phase low-alloy steels, mechanical loss measurements have been performed on a series of Fe–C alloys with varying constitution. The observed mechanical loss spectra of two-phase alloys turned out to be simple superposition of those of single phase alloys, of ferrite and of martensite. The concentrations of carbon in solution evaluated from the magnitude of the Snoek relaxation in the two-phase alloys agree well with those expected from the Fe–C phase diagram. It is thus possible to selectively analyse the carbon dissolved in the ferrite phase in the complex structure, at least in simple binary alloys.

Daisuke Maeda

Papers

Hot-rolled steel plate superior in hole expandability and manufacturing method therefor

High-strength hot-rolled steel plate having excellent strength-ductility balance and blanking property, and producing method therefor

Quantitative Analysis of Carbon in Ferrite in Dual-Phase Steels by Mechanical Loss Measurements