Abstract: A hot-dip galvanized cold-rolled steel sheet has a tensile strength of 750 MPa or higher, a composition consisting, in mass percent, C: more than 0.10% and less than 0.25%, Si: more than 0.50% and less than 2.0%, Mn: more than 1.50% and 3.0% or less, and optionally containing one or more types of Ti, Nb, V, Cr, Mo, B, Ca, Mg, REM, and Bi, P: less than 0.050%, S: 0.010% or less, sol. Al: 0.50% or less, and N: 0.010% or less, and a main phase as a low-temperature transformation product and a second phase as retained austenite. The retained austenite volume fraction is more than 4.0% and less than 25.0% of the whole structure, and has an average grain size of less than 0.80 ?m. A number density of retained austenite grains having a grain size of 1.2 ?m or more is 3.0×10?2/?m2 or less.

Abstract: Provided is a manufacturing apparatus and manufacturing method of hot-rolled steel sheet which enables uniform cooling of a rolled material and improvement of the surface properties thereof. The manufacturing apparatus comprises: a rolling stand; a supplying device capable of supplying lubricant to work rolls and/or backup rolls; an online roll grinding device; and a removing device capable of removing at least part of the lubricant before the surface of the work rolls is ground by the grinding device. The manufacturing method comprises the steps of: removing at least part of the lubricant adhered to the work rolls, or to the work rolls and backup rolls using the lubricant removing device after completing rolling of a preceding material; grinding the work rolls using the online roll grinding device after the removing step; and supplying the lubricant to the work rolls and/or backup rolls from the lubricant supplying device.

Abstract: A method for manufacturing a hot-rolled sheet attains grain refinement of the steel sheet whose grain size is extremely fine. In particular, a ferrite grain size of less than average 2 ?m is obtained, which is not laminar but has ferrite grains with equiaxed morphology and exhibits high formability in forming. The method comprises the steps of rolling and cooling, wherein the rolling reductions, cooling steps, and temperature are closely regulated. A hot rolled sheet made from the method of manufacturing has a controlled ferrite grain in different regions of sheet thickness.

Abstract: Provided is a method of controlling operation of a tandem rolling mill which enables large reduction rolling in the latter-stage stand of the tandem rolling mill necessary for manufacturing fine-grained steel, and so on. The method comprises: a step of determining a first exit-side sheet thickness in rolling a constant portion of a material to be rolled; and a step of determining a second exit-side sheet thickness in rolling a front end portion of the material, such that a pre-tightening load becomes a set value or less; the material is rolled into the second exit-side sheet thickness, until the front end portion is fed into the stands; the constant portion is rolled by the stand given a pre-tightening load into the first exit-side sheet thickness; and the second exit-side sheet thicknesses of the stands given the pre-tightening load are made larger than the first exit-side sheet thickness.

Abstract: A hot-dip galvanized cold-rolled steel sheet has a tensile strength of 750 MPa or higher, a composition consisting, in mass percent, of C: more than 0.10% and less than 0.25%, Si: more than 0.50% and less than 2.0%, Mn: more than 1.50% and 3.0% or less, and optionally containing one or more types of Ti, Nb, V, Cr, Mo, B, Ca, Mg, REM, and Bi, P: less than 0.050%, S: 0.010% or less, sol. Al: 0.50% or less, and N: 0.010% or less, and a main phase as a low-temperature transformation product and a second phase as retained austenite. The retained austenite volume fraction is more than 4.0% and less than 25.0% of the whole structure, and has an average grain size of less than 0.80 ?m. A number density of retained austenite grains having a grain size of 1.2 ?m or more is 3.0?10?2/?m2 or less.

Abstract: A method for manufacturing a hot-rolled sheet attains grain refinement of the steel sheet whose grain size is extremely fine. In particular, a ferrite grain size of less than average 2 ?m is obtained, which is not laminar but has ferrite grains with equiaxed morphology and exhibits high formability in forming. The method comprises the steps of rolling and cooling, wherein the rolling reductions, cooling steps, and temperature are closely regulated. A hot rolled sheet made from the method of manufacturing has a controlled ferrite grain in different regions of sheet thickness.

Abstract: A method for manufacturing a hot-rolled sheet is provided, wherein the method attains grain refinement of the steel sheet containing C, Si, and Mn, wherein the grain size thereof is set to particularly below average of 2 ?m, which has ferrite grain with equiaxed morphology, which has high formability in forming, and the ferrite grain-size deviation in the thickness direction is uniformed down to the level not higher than a predetermined amount whereby uniform formability in forming is high. The method includes a first rolling for rolling the sheet such that the total rolling reduction is 80% or more or the average grain size is 30 ?m or less in a form of single phase of austenite, a second rolling of a single-pass, a third rolling being conducted thereafter, and a following cooling.

Abstract: Provided is a method of controlling operation of a tandem rolling mill which enables large reduction rolling in the latter-stage stand of the tandem rolling mill necessary for manufacturing fine-grained steel, and so on. The method comprises: a step of determining a first exit-side sheet thickness in rolling a constant portion of a material to be rolled; and a step of determining a second exit-side sheet thickness in rolling a front end portion of the material, such that a pre-tightening load becomes a set value or less; the material is rolled into the second exit-side sheet thickness, until the front end portion is fed into the stands; the constant portion is rolled by the stand given a pre-tightening load into the first exit-side sheet thickness; and the second exit-side sheet thicknesses of the stands given the pre-tightening load are made larger than the first exit-side sheet thickness.

Abstract: Provided is a manufacturing apparatus and manufacturing method of hot-rolled steel sheet which enables uniform cooling of a rolled material and improvement of the surface properties thereof. The manufacturing apparatus comprises: a rolling stand; a supplying device capable of supplying lubricant to work rolls and/or backup rolls; an online roll grinding device; and a removing device capable of removing at least part of the lubricant before the surface of the work rolls is ground by the grinding device. The manufacturing method comprises the steps of: removing at least part of the lubricant adhered to the work rolls, or to the work rolls and backup rolls using the lubricant removing device after completing rolling of a preceding material; grinding the work rolls using the online roll grinding device after the removing step; and supplying the lubricant to the work rolls and/or backup rolls from the lubricant supplying device.

Abstract: The present invention provides a method for manufacturing the hot-rolled sheet, which includes: a step A including a first rolling in which a steel sheet containing 0.04-0.20% C, 0.01-2.0% Si, 0.5-3.0% Mn by mass, and the reminder being Fe and inevitable impurities, is rolled by successive multi-pass rolling at a total rolling reduction of 80% or more while keeping the steel sheet at temperatures not lower than the para-equilibrium transformation temperature Ae3; a step B including a second rolling in which a single-pass rolling is carried out at a rolling reduction of 30-55% when an entry side temperature is not lower than the para-equilibrium transformation temperature Ae3; a step C including a third rolling in which a single-pass rolling is carried out at a rolling reduction of 35-70% when an entry side temperature is set within a predetermined range; and a step D in which within 0.2 seconds after the third rolling, the rolled sheet is cooled at a cooling rate of 600 degree C.

Abstract: There is provided a rolled plate joining apparatus equipped with a truck that can travel reversibly in the rolling direction, tailing end pinch rolls that are mounted on the truck and can be moved vertically with the tailing end of a preceding rolled plate pinched horizontally, leading end pinch rolls that are mounted on the truck and can pinch horizontally the leading end of a succeeding rolled plate, machining apparatuses for cutting one surface of the tailing end of the preceding rolled plate and the other surface of the leading end of the succeeding rolled plate, and a pressure welding apparatus for compressing the preceding rolled plate and the succeeding rolled plate with the machined surfaces of the preceding rolled plate and the succeeding rolled plate overlapped to reduce them approximately to the thickness of the rolled plates, including a tailing end centering apparatus placed between the tailing end pinch rolls and the pressure welding apparatus for pressing the opposite width ends of the tailing

Abstract: There is provided a rolled plate joining apparatus equipped with a truck that can travel reversibly in the rolling direction, tailing end pinch rolls that are mounted on the truck and can be moved vertically with the tailing end of a preceding rolled plate pinched horizontally, leading end pinch rolls that are mounted on the truck and can pinch horizontally the leading end of a succeeding rolled plate, machining apparatuses for cutting one surface of the tailing end of the preceding rolled plate and the other surface of the leading end of the succeeding rolled plate, and a pressure welding apparatus for compressing the preceding rolled plate and the succeeding rolled plate with the machined surfaces of the preceding rolled plate and the succeeding rolled plate overlapped to reduce them approximately to the thickness of the rolled plates, including a tailing end centering apparatus placed between the tailing end pinch rolls and the pressure welding apparatus for pressing the opposite width ends of the tailing