A steel sheet of the present invention is a steel sheet having a predetermined chemical composition and containing at least ferrite, residual austenite, and/or martensite in a microstructure, and furthermore, is a steel sheet in which, in a plane parallel to a rolled surface, an average distance between centers of high Mn regions adjacent to each other is 1.00 mm or less, a density D.sub.A of the high Mn regions at a sheet width center portion and a density D.sub.B of the high Mn regions at a ¼ position from a sheet width end portion satisfy a relationship of 0.77≤D.sub.A/D.sub.B≤1.30, a ratio of an average hardness of the high Mn regions to an average hardness of the low Mn regions is 1.1 to 2.0, and a difference between an average of a top 5% and an average of a bottom 5% of Mn contents in the low Mn regions is 0.3% or more.

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.

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 manufacturing a roll mantle or roll body for a roll line of a continuous casting apparatus that has a shaft includes casting a metal to form the roll mantle or roll body such that the roll mantle or roll body includes at least one internal channel. The roll mantle or roll body has a first end region, a second end region and a central region between the first end region and the second end region, the central region extending along at least 50% of a length of the roll mantle or roll body, and the internal channel may be formed in the central region. The internal channel may also include a pattern or projection.

A method for manufacturing a roll mantle or roll body for a roll line of a continuous casting apparatus that has a shaft includes casting a metal to form the roll mantle or roll body such that the roll mantle or roll body includes at least one internal channel. The roll mantle or roll body has a first end region, a second end region and a central region between the first end region and the second end region, the central region extending along at least 50% of a length of the roll mantle or roll body, and the internal channel may be formed in the central region. The internal channel may also include a pattern or projection.

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.

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.

Device (1) for coupling a coolant supply and/or coolant drainage to a roll (2), especially in a continuous casting installation, wherein the roll (2) comprises a journal (3), which is mounted rotatably in a bearing block (4), and a lateral recess, extending in the axial direction, and at least one roll cooling channel, through which a coolant can pass, wherein the device (1) comprises: at least one coolant channel (11, 12), which can be brought into fluidic connection with the roll cooling channel on the journal (3) side; and a sealing unit for sealing the fluidic connection between the coolant channel and the roll cooling channel, comprising an inner module and an outer module; wherein the inner module comprises a hollow cylindrical element (7), which can be inserted into the lateral recess of the journal (3), characterized in that the hollow cylindrical element (7) has no flange on the outer side of the journal, looking in the axial direction.

Device (1) for coupling a coolant supply and/or coolant drainage to a roll (2), especially in a continuous casting installation, wherein the roll (2) comprises a journal (3), which is mounted rotatably in a bearing block (4), and a lateral recess, extending in the axial direction, and at least one roll cooling channel, through which a coolant can pass, wherein the device (1) comprises: at least one coolant channel (11, 12), which can be brought into fluidic connection with the roll cooling channel on the journal (3) side; and a sealing unit for sealing the fluidic connection between the coolant channel and the roll cooling channel, comprising an inner module and an outer module; wherein the inner module comprises a hollow cylindrical element (7), which can be inserted into the lateral recess of the journal (3), characterized in that the hollow cylindrical element (7) has no flange on the outer side of the journal, looking in the axial direction.

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.