A method of casting a metal ingot with a microstructure that facilitates further working, such as hot and cold rolling. The metal is cast in a direct chill casting mold, or the equivalent, that directs a spray of coolant liquid onto the outer surface of the ingot to achieve rapid cooling. The coolant is removed from the surface at a location where the emerging embryonic ingot is still not completely solid, such that the latent heat of solidification and the sensible heat of the molten core raises the temperature of the adjacent solid shell to a convergence temperature that is above a transition temperature for in-situ homogenization of the metal. A further conventional homogenization step is then not required. The invention also relates to the heat-treatment of such ingots prior to hot working.

A steel sheet having a specified chemical composition and a tensile strength of 1,320 MPa or more, and methods for producing the steel sheet. The steel sheet has a specified microstructure including martensite and bainite, the total area fraction of the martensite and the bainite being 95% or more and 100% or less, the balance being one or more selected from ferrite and retained austenite. The forumulae [% Ti]+[% Nb]>0.007 and [% Ti]×[% Nb].sup.2≤7.5×10.sup.−6 are satisfied in the chemical composition.

A steel sheet has a composition comprising, by weight: 0.010%≤C≤0.080%, 0.06%≤Mn≤3%, Si≤1.5%, 0.005%≤Al≤1.5%, S≤0.030%, P≤0.040%, Ti and B such that: 3.2%≤Ti≤7.5% and (0.45×Ti)−1.35≤B≤(0.45×Ti)−0.43, optionally Ni≤1%, Mo≤1%, Cr≤3%, Nb≤0.1%, V≤0.1%, the remainder being iron and unavoidable impurities resulting from the smelting. The steel sheet has a structure consisting of ferrite, at most 10% of austenite, and precipitates comprising eutectic precipitates of TiB.sub.2, the volume fraction of TiB.sub.2 precipitates with respect to the whole structure being of at least 9%, the proportion of TiB.sub.2 precipitates having a surface area lower than 8 μm.sup.2 being of at least 96%.

Disclosed are an electromagnetic stirring device and a method for a secondary cooling zone during slab continuous casting. The device comprises an electromagnetic stirring device main body comprising a protection housing (3), a phase sequence control assembly, an iron core (4) and an electromagnetic coil (5) for carrying out variable-direction electromagnetic stirring on molten steel by means of three-phase current phase sequence transformation; an opening adjustment assembly comprising an air cylinder (7), a fixed base (8), a movable joint shaft (12) and a silicon steel sheet group insert (13) for adjusting online the opening degree of the closed annular iron core by means of a movable joint structure; and the secondary cooling assembly comprising a cooling water inlet (9) and a cooling water nozzle (10) for cooling the electromagnetic coil and spraying cooling water to a surface of a cast slab (1).

A method for semi-continuous casting of a strand (1) of steel in a strand casting machine and a strand casting machine for such casting. The strand has little segregation of the center and porosity. Yet it is castable rapidly. The method steps are: at a casting start of the strand casting machine, pouring liquid steel into an open-ended mold (2). The mold is closed by a cold strand (6). The liquid steel forms, together with the cold strand, a completely solidified strand start (1a) and subsequently forms a semi-solidified strand (1b). Then extracting the semi-solidified strand (1b) from the open-ended mold (2). Supporting and guiding the semi-solidified strand (1b) in a strand guide (3). Cooling the semi-solidified strand (1b) by secondary cooling (4) at the casting end of the strand casting machine, ending the pouring of liquid steel into the open-ended mold (2) and forming a strand end (1c). Extracting the strand end (1c) from the open-ended mold (2). Ending the extraction such that the strand end (1c) lies outside the open-ended mold (2). Ending the secondary cooling (4). Controlling or regulating cooling of the semi-solidified strand (1b) until complete solidification of the strand (1) in a tertiary cooling zone (5) of the strand casting machine. The cooling effect is stronger at the strand start (1a) and decreases towards the strand end (1c). Discharging the strand (1) from the strand casting machine.

A method for improving center segregation and surface crack of continuous casting medium-thick slab of peritectic steel reduces the cooling intensity at the earlier stage of solidification and enhancing the cooling intensity at the final stage of solidification. For example, the cooling water amount of the wide face of the mould is 3400-3600 L/min, and the cooling water amount of the narrow face of the mould is 480-530 L/min. The cooling water amount of the wide face of the foot roller section is 239-298 L/min, and the cooling water amount of the narrow face of the foot roller section is 61-65 L/min. The total cooling water amount of the sector segment is 1517-2166 L/min.

A method for continuously casting steel capable of reducing center segregation that occurs in a slab. In a section in a continuous casting machine in a slab withdrawal direction, a section from a start point at which the average value of solid phase ratios along a thickness direction at a widthwise center of a slab is within a range of 0.4 or more and 0.8 or less to an end point at which the average value of solid phase ratios along the thickness direction at the widthwise center of the slab is greater than the average value of solid phase ratios at the start point and is 1.0 or less is set as a first section. The slab is cooled by water in the first section at a water flow rate per surface area of the slab within a range of 50 L/(m.sup.2×min) or more and 2,000 L/(m.sup.2×min) or less.

This steel sheet has a predetermined chemical composition, a microstructure contains, in terms of a volume fraction, ferrite: 10% to 75%, martensite: 20% to 90%, retained austenite: 0% to 5%, bainite and bainitic ferrite in total: 0% to 50%, and pearlite: 0% to 5%, a proportion of unrecrystallized ferrite in the ferrite is 0% to 25%, cementite that is contained in the martensite satisfies a predetermined relational expression, a density of transition carbide included in the martensite is 1.0×10.sup.13 pieces/m.sup.3 or more, a density of coarse inclusion having an equivalent circle diameter of 10 μm or more is 0.50 pieces/mm.sup.2 or less, in a surface parallel to the surface at a position ¼ of the sheet thickness deep from the surface in the sheet thickness direction, a ratio of a maximum value Hv.sub.max of Vickers hardness to a minimum value Hv.sub.min of the Vickers hardness is 1.40 or less, and an average value of minimum distances between peaks of the Vickers hardness in a distribution map of the Vickers hardness is 1.00 mm or less.

A method for continuously casting steel capable of reducing center segregation that occurs in a slab. In a section in a continuous casting machine in a slab withdrawal direction, a section from a start point at which the average value of solid phase ratios along a thickness direction at a widthwise center of a slab is within a range of 0.4 or more and 0.8 or less to an end point at which the average value of solid phase ratios along the thickness direction at the widthwise center of the slab is greater than the average value of solid phase ratios at the start point and is 1.0 or less is set as a first section. The slab is cooled by water in the first section at a water flow rate per surface area of the slab within a range of 50 L/(m.sup.2×min) or more and 2,000 L/(m.sup.2×min) or less.

An electromagnetic semi-continuous device comprises a crystallizer frame, an internal sleeve, a primary cooling water cavity, a secondary cooling water cavity and a tertiary cooling water cavity. An electromagnetic semi-continuous casting method comprises the steps of (1) adjusting angles of the adjustable spherical nozzles; (2) inserting a dummy bar head in a bottom of the internal sleeve; (3) feeding cooling water to the primary cooling water cavity and the secondary cooling water cavity, then spraying the cooling water to form primary cooling water and secondary cooling water, and exerting a magnetic field on the internal sleeve; (4) pouring the melts into the internal sleeve, starting the dummy bar head, and beginning to perform continuous casting; and (5) spraying tertiary cooling water through the tertiary cooling water cavity, so that casting billets reduce temperature until the continuous casting is completed.