A steel sheet has a composition containing, by mass %, C: 0.04% to 0.20%, Si: 0.6% to 1.5%, Mn: 1.0% to 3.0%, P: 0.10% or less, S: 0.030% or less, Al: 0.10% or less, N: 0.010% or less, one, two, or all of Ti, Nb, and V in an amount of 0.01% to 1.0% each, and the balance being Fe and inevitable impurities, a microstructure including, in terms of area ratio, 50% or more of ferrite, in which an average grain diameter at a position located 50 μm from a surface of the steel sheet in a thickness direction is 3000×(tensile strength TS (MPa)).sup.−0.85 μm or less, C precipitates having a grain diameter of less than 20 nm formed in steel is 0.010 mass % or more, and a amount of precipitated Fe is 0.03 mass % to 1.0 mass %, and a roughness Ra of 3.0 μm or less.

A batch furnace for annealing material, in particular a single chamber furnace or single coil furnace, with a furnace housing. The batch furnace has a closable charging opening, a receiving chamber for receiving furnace material, and a device for convective heat transfer onto the furnace material by a heat transfer medium. The batch furnace includes at least one fan, which is arranged in the furnace housing, at least one heating device for the heat transfer medium and/or at least one inlet for an externally heated heat transfer medium, wherein the heating device and/or the inlet is arranged directly in front of the intake side or directly behind the pressure side of the fan or circumferentially in an annular gap between the fan and the furnace housing, and a receiving chamber for the furnace material, which is arranged on the pressure side of the fan.

A batch furnace for annealing material, in particular a single chamber furnace or single coil furnace, with a furnace housing. The batch furnace has a closable charging opening, a receiving chamber for receiving furnace material, and a device for convective heat transfer onto the furnace material by a heat transfer medium. The batch furnace includes at least one fan, which is arranged in the furnace housing, at least one heating device for the heat transfer medium and/or at least one inlet for an externally heated heat transfer medium, wherein the heating device and/or the inlet is arranged directly in front of the intake side or directly behind the pressure side of the fan or circumferentially in an annular gap between the fan and the furnace housing, and a receiving chamber for the furnace material, which is arranged on the pressure side of the fan.

The present invention provides a method for manufacturing a cold-rolled or zinc-plated dual-phase steel plate over 980 MPa. After being subjected to hot rolling, coiling, bundling, and online heat preservation, a slab is directly sent to a cold rolling and continuous annealing process, or a cold rolling, continuous annealing, and zinc plating process, so as to obtain a cold-rolled or zinc-plated dual-phase steel plate, wherein the coiling temperature is controlled to be over 450° C. The online thermal preservation means that after uncoiling of each hot-rolled coil, an independent and airtight thermal preservation cover is closed, and the hot-rolled coil with the closed thermal preservation cover is transferred to coil rolling by means of a steel coil conveying chain or a traveling car; the thermal preservation temperature for the hot-rolled coil in the thermal preservation cover is over 450° C., and the thermal preservation duration is less than 20 hours. According to the present disclosure, by means of the design of a thermal preservation process with or without a heat source after hot rolling and coiling, the manufacturing problems such as edge cracks and sharp fluctuation in thickness after cold rolling are solved, and good cold rolling manufacturability is achieved.

The present invention provides a method for manufacturing a cold-rolled or zinc-plated dual-phase steel plate over 980 MPa. After being subjected to hot rolling, coiling, bundling, and online heat preservation, a slab is directly sent to a cold rolling and continuous annealing process, or a cold rolling, continuous annealing, and zinc plating process, so as to obtain a cold-rolled or zinc-plated dual-phase steel plate, wherein the coiling temperature is controlled to be over 450° C. The online thermal preservation means that after uncoiling of each hot-rolled coil, an independent and airtight thermal preservation cover is closed, and the hot-rolled coil with the closed thermal preservation cover is transferred to coil rolling by means of a steel coil conveying chain or a traveling car; the thermal preservation temperature for the hot-rolled coil in the thermal preservation cover is over 450° C., and the thermal preservation duration is less than 20 hours. According to the present disclosure, by means of the design of a thermal preservation process with or without a heat source after hot rolling and coiling, the manufacturing problems such as edge cracks and sharp fluctuation in thickness after cold rolling are solved, and good cold rolling manufacturability is achieved.

A continuous annealing apparatus includes: a pre-treatment device to prepare a strip unwound from a coil; a heating device to heat the strip prepared by the pre-treatment device; a heat holding device to isothermally maintain the strip heated by the heating device; a first cooling device to cool the strip heat-maintained by the heat holding device; an annealing device including a first annealing device for annealing the strip, which, is cooled by the first cooling device, for a first time, and a second annealing device for winding the strip, which is cooled by the first cooling device, into a coil and then unwinding the coil into the strip again after annealing for a second time; a second cooling device to cool the strip annealed by the first annealing device or the second annealing device; and a post-treatment device to wind the strip cooled by the second cooling device into the coil.

A continuous annealing apparatus includes: a pre-treatment device to prepare a strip unwound from a coil; a heating device to heat the strip prepared by the pre-treatment device; a heat holding device to isothermally maintain the strip heated by the heating device; a first cooling device to cool the strip heat-maintained by the heat holding device; an annealing device including a first annealing device for annealing the strip, which, is cooled by the first cooling device, for a first time, and a second annealing device for winding the strip, which is cooled by the first cooling device, into a coil and then unwinding the coil into the strip again after annealing for a second time; a second cooling device to cool the strip annealed by the first annealing device or the second annealing device; and a post-treatment device to wind the strip cooled by the second cooling device into the coil.

Provided is a method of easily producing a non-oriented electrical steel sheet that contains substantially no Al and contains large amounts of Si and Mn and has low iron loss, comprising hot rolling a slab having a specified chemical composition to obtain a hot-rolled sheet; coiling the hot-rolled sheet; cold rolling the hot-rolled sheet once or twice with intermediate annealing being performed therebetween, to obtain a cold-rolled sheet; and subjecting the cold-rolled sheet to final annealing, wherein the hot-rolled sheet after the hot rolling is cooled at an average cooling rate from 800? C. to 650? C. of 30? C./s or more, and thereafter the coiling is performed at 650? C. or less.

Provided is a method of easily producing a non-oriented electrical steel sheet that contains substantially no Al and contains large amounts of Si and Mn and has low iron loss, comprising hot rolling a slab having a specified chemical composition to obtain a hot-rolled sheet; coiling the hot-rolled sheet; cold rolling the hot-rolled sheet once or twice with intermediate annealing being performed therebetween, to obtain a cold-rolled sheet; and subjecting the cold-rolled sheet to final annealing, wherein the hot-rolled sheet after the hot rolling is cooled at an average cooling rate from 800? C. to 650? C. of 30? C./s or more, and thereafter the coiling is performed at 650? C. or less.

A steel sheet has a composition containing, by mass %, C: 0.04% to 0.20%, Si: 0.6% to 1.5%, Mn: 1.0% to 3.0%, P: 0.10% or less, S: 0.030% or less, Al: 0.10% or less, N: 0.010% or less, one, two, or all of Ti, Nb, and V in an amount of 0.01% to 1.0% each, and the balance being Fe and inevitable impurities, a microstructure including, in terms of area ratio, 50% or more of ferrite, in which an average grain diameter at a position located 50 m from a surface of the steel sheet in a thickness direction is 3000(tensile strength TS (MPa)).sup.0.85 m or less, C precipitates having a grain diameter of less than 20 nm formed in steel is 0.010 mass % or more, and a amount of precipitated Fe is 0.03 mass % to 1.0 mass %, and a roughness Ra of 3.0 m or less.