A primary object of the present invention is to provide a continuous-cast slab where central porosity is reduced by surly crushing the slab, and a method and apparatus of manufacturing the same.

The continuous-cast slab 1 having horizontally symmetrical granular equiaxed crystals at least in the center in the thickness direction includes a first reduction dent 2 and a second reduction dent 3 that further dents from the bottom surface of the first reduction dent 2 and is narrower than the first reduction dent 2 at least on one long side surface. Such a configuration can makes it possible to achieve the continuous-cast slab 1 of no more than 2.5×10.sup.−4 cm.sup.3/g in a maximum porosity volume while segregation is reduced.

Free-cutting steel includes a specific composition, with an A value defined by a following formula (1) satisfying 4.0 or more and 20.0 or less, and remainder having Fe and an unavoidable impurity; and texture with 1000 or more sulfide particles with an equivalent circle diameter of less than 1 μm per mm.sup.2, 500 or more sulfide particles with an equivalent circle diameter of 1 μm or more and 5 μm or less per mm.sup.2, and 1000 or more Pb particles with an equivalent circle diameter of 1 μm or less per mm.sup.2, wherein

A value=(Mn+5Cr)/S   (1)

here, symbols of elements in the formula indicate contents (% by mass) of the elements.

Free-cutting steel includes a specific composition, with an A value defined by a following formula (1) satisfying 4.0 or more and 20.0 or less, and remainder having Fe and an unavoidable impurity; and texture with 1000 or more sulfide particles with an equivalent circle diameter of less than 1 μm per mm.sup.2, 500 or more sulfide particles with an equivalent circle diameter of 1 μm or more and 5 μm or less per mm.sup.2, and 1000 or more Pb particles with an equivalent circle diameter of 1 μm or less per mm.sup.2, wherein

A value=(Mn+5Cr)/S   (1)

here, symbols of elements in the formula indicate contents (% by mass) of the elements.

A titanium product includes an inner layer portion and a surface layer portion joined to the inner layer portion. The surface layer portion has a composition consisting of, by mass %, O: 0.4% or less, Fe: 0.5% or less, Cl: 0.020% or less, the balance: Ti and impurities. The inner layer portion 3 has pores and a composition consisting of, by mass %, O: 0.4% or less, Fe: 0.5% or less, Cl: more than 0.020% and 0.60%, the balance: Ti and impurities. The area fraction of the pores in the inner layer portion in a cross-section perpendicular to the longitudinal direction of the titanium product is more than 0% and not more than 30%. The Cl content (Cl.sub.I) of the inner layer portion, a thickness (t.sub.S) of the surface layer portion, and a thickness (t.sub.I) of the inner layer portion satisfy the expression [Cl.sub.I≤0.03+0.02×t.sub.S/t.sub.I].

A titanium product includes an inner layer portion and a surface layer portion joined to the inner layer portion. The surface layer portion has a composition consisting of, by mass %, O: 0.4% or less, Fe: 0.5% or less, Cl: 0.020% or less, the balance: Ti and impurities. The inner layer portion 3 has pores and a composition consisting of, by mass %, O: 0.4% or less, Fe: 0.5% or less, Cl: more than 0.020% and 0.60%, the balance: Ti and impurities. The area fraction of the pores in the inner layer portion in a cross-section perpendicular to the longitudinal direction of the titanium product is more than 0% and not more than 30%. The Cl content (Cl.sub.I) of the inner layer portion, a thickness (t.sub.S) of the surface layer portion, and a thickness (t.sub.I) of the inner layer portion satisfy the expression [Cl.sub.I≤0.03+0.02×t.sub.S/t.sub.I].

A hot rolling line control system includes a rolling condition setting unit, an operation data collection unit that collects rolling conditions and operation data of a line during rolling, an operation data storage unit, a material measurement data storage unit that stores material actual measurement data obtained by measuring a material of a rolled steel sheet, a material prediction unit that predicts material of rolled steel sheet, and a material prediction data storage unit that stores material prediction data in the material prediction unit, and the material prediction unit includes a classification criteria creation and material model regression unit that creates classification criteria using operation data and the material actual measurement data, classifies the operation data and the material actual measurement data according to the created classification criteria, and regresses the classified operation data and material actual measurement data to create a material model for each classification.

A hot rolling line control system includes a rolling condition setting unit, an operation data collection unit that collects rolling conditions and operation data of a line during rolling, an operation data storage unit, a material measurement data storage unit that stores material actual measurement data obtained by measuring a material of a rolled steel sheet, a material prediction unit that predicts material of rolled steel sheet, and a material prediction data storage unit that stores material prediction data in the material prediction unit, and the material prediction unit includes a classification criteria creation and material model regression unit that creates classification criteria using operation data and the material actual measurement data, classifies the operation data and the material actual measurement data according to the created classification criteria, and regresses the classified operation data and material actual measurement data to create a material model for each classification.

Provided is a titanium cast product for hot rolling made of commercially pure titanium, the titanium cast product including a melted and resolidified layer in a range of more than or equal to 1 mm in depth on a surface serving as a rolling surface, the melted and resolidified layer being obtained by adding one or more elements out of any one of or both of at least one α stabilizer element and at least one neutral element to the surface, and melting and resolidifying the surface. An average value of a total concentration of the at least one α stabilizer element and the at least one neutral element in the range of more than or equal to 1 mm in depth is higher than a total concentration of the at least one α stabilizer element and the at least one neutral element in a base metal by, in mass %, more than or equal to 0.1% and less than 2.0%.

Provided is a titanium cast product for hot rolling made of commercially pure titanium, the titanium cast product including a melted and resolidified layer in a range of more than or equal to 1 mm in depth on a surface serving as a rolling surface, the melted and resolidified layer being obtained by adding one or more elements out of any one of or both of at least one α stabilizer element and at least one neutral element to the surface, and melting and resolidifying the surface. An average value of a total concentration of the at least one α stabilizer element and the at least one neutral element in the range of more than or equal to 1 mm in depth is higher than a total concentration of the at least one α stabilizer element and the at least one neutral element in a base metal by, in mass %, more than or equal to 0.1% and less than 2.0%.

A method for producing a hot-rolled titanium plate includes, [1] melting at least one part of the side surface of the titanium slab by radiating a beam or plasma toward the side surface, not toward the surface to be rolled, and thereafter causing re-solidification to form, in the side surface, a layer having grain diameter of 1.5 mm or less and a depth of 3.0 mm or more from the side surface; [2] performing a finishing process on the surface to be rolled of the titanium slab in which the layer is formed, to thereby bring a slab flatness index X to 3.0 or less; and [3] subjecting the titanium slab after the finishing process to hot rolling under a condition in which a length of an arc of contact of a roll L in a first pass of rough rolling is 230 mm or more.