Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/0081—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for slabs; for billets
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
  • C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/0075—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rods of limited length
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/22—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for drills; for milling cutters; for machine cutting tools
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

• the present disclosure relates to a free-cutting steel, in particular a steel that is a substitute for a free-cutting steel containing sulfur and a small amount of lead as elements for improving machinability by cutting.
• the present disclosure relates to a free-cutting steel having machinability by cutting higher than or equal to that of a low carbon sulfur-lead composite free-cutting steel, and a method of producing the same.
• Low carbon sulfur-lead free-cutting steel as represented by JIS SUM24L contains a large amount of lead (Pb) and sulfur (S) as free-cutting elements and thus has excellent machinability by cutting.
• lead In steel materials, lead is useful for reducing tool wear and improving chip treatability in cutting work. Hence, lead is regarded as an important element that significantly improves the machinability by cutting of materials, and is used in many steel products produced by cutting work. With the rise of environmental awareness in recent years, however, there is a growing movement to abolish or restrict the use of environmentally hazardous substances worldwide. Lead is one of such environmentally hazardous substances, and restriction on the use of lead is required.
• JP H9-25539 A discloses a non-Pb-containing free-cutting non-heat-treated steel.
• J P 2000-160284 A discloses a non-Pb-containing free-cutting steel.
• JP H2-6824 B discloses a free-cutting steel containing Cr which can form a compound with S more easily than Mn to thereby cause a Mn—Cr—S-based inclusion to be present and ensure machinability by cutting.
• the technique described in PTL 1 is intended for a non-heat-treated steel that contains 0.2% or more of C and thus is hard, and the use of Nd which is a special element requires high production costs.
• adding a large amount of S causes low hot ductility and induces cracking during continuous casting or hot rolling, which is problematic in terms of surface characteristics.
• Cr and S are added while reducing the amount of Mn.
• due to high Cr content of 3.5% or more not only cost reduction is difficult but also a large amount of CrS forms, causing a production problem in that material smelting treatment in the steelmaking process is difficult.
• a free-cutting steel comprising: a chemical composition that contains (consists of), in mass %, C: less than 0.09%, Mn: 0.50% to 1.50%, S: 0.250% to 0.600%, O: more than 0.0100% and 0.0500% or less, and Cr: 0.50% to 1.50%, with a balance consisting of Fe and inevitable impurities, and in which a A value defined by the following formula (1) is 6.0 to 18.0,
• [Mn], [Cr], and [S] respectively denote contents in mass % of elements Mn, Cr, and S; and a steel microstructure in which at least 500 particles/mm 2 of sulfide of less than 1 ⁇ m in equivalent circle diameter and at least 2000 particles/mm 2 of sulfide of 1 ⁇ m to 5 ⁇ m in equivalent circle diameter are distributed.
• the free-cutting steel according to 1. or 2. wherein the chemical composition further contains, in mass %, one or more selected from the group consisting of Ca: 0.0010% or less, Se: 0.30% or less, Te: 0.15% or less, Bi: 0.20% or less, Sn: 0.020% or less, Sb: 0.025% or less, B: 0.010% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ti: 0.100% or less, V: 0.20% or less, Zr: 0.050% or less, and Mg: 0.0050% or less.
• the chemical composition further contains, in mass %, one or more selected from the group consisting of Ca: 0.0010% or less, Se: 0.30% or less, Te: 0.15% or less, Bi: 0.20% or less, Sn: 0.020% or less, Sb: 0.025% or less, B: 0.010% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ti: 0.100% or less, V:
• a method of producing a free-cutting steel comprising: rolling a rectangular cast steel at a heating temperature of 1120° C. or more and an area reduction rate of 60% or more to obtain a billet, the rectangular cast steel having a chemical composition that contains, in mass %, C: less than 0.09%, Mn: 0.50% to 1.50%, S: 0.250% to 0.600%, O: more than 0.0100% and 0.0500% or less, and Cr: 0.50% to 1.50% with a balance consisting of Fe and inevitable impurities, and in which a A value defined by the following formula (1) is 6.0 to 18.0,
• [Mn], [Cr], and [S] respectively denote contents in mass % of elements Mn, Cr, and S, and a side length of a cross section of the rectangular cast steel perpendicular to a longitudinal direction being 250 mm or more; and hot working the billet at a heating temperature of 1050° C. or more and an area reduction rate of 75% or more.
• the method of producing a free-cutting steel according to 4. or 5. wherein the chemical composition further contains, in mass %, one or more selected from the group consisting of Ca: 0.0010% or less, Se: 0.30% or less, Te: 0.15% or less, Bi: 0.20% or less, Sn: 0.020% or less, Sb: 0.025% or less, B: 0.010% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ti: 0.100% or less, V: 0.20% or less, Zr: 0.050% or less, and Mg: 0.0050% or less.
• the chemical composition further contains, in mass %, one or more selected from the group consisting of Ca: 0.0010% or less, Se: 0.30% or less, Te: 0.15% or less, Bi: 0.20% or less, Sn: 0.020% or less, Sb: 0.025% or less, B: 0.010% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ti: 0.100% or
• a free-cutting steel according to the present disclosure will be described in detail below. First, the reasons for limiting the content of each component in the chemical composition of the free-cutting steel will be described below.
• “%” with regard to components is mass % unless otherwise stated.
• C is an important element that greatly influences the strength and the machinability by cutting of the steel. If the C content is 0.09% or more, the steel hardens and the strength increases excessively, and as a result the machinability by cutting degrades. The C content is therefore less than 0.09%.
• the C content is preferably 0.07% or less. From the viewpoint of ensuring the strength, the C content is preferably 0.01% or more, and more preferably 0.03% or more.
• Mn is a sulfide forming element important for improvement in machinability by cutting. If the Mn content is less than 0.50%, the amount of sulfide is small, and sufficient machinability by cutting cannot be obtained. The lower limit is therefore 0.50%.
• the Mn content is preferably 0.70% or more. If the Mn content is more than 1.50%, sulfides not only coarsen but also extend long, causing a decrease in machinability by cutting. In addition, the mechanical properties decrease. The upper limit of the Mn content is therefore 1.50%.
• the Mn content is preferably 1.20% or less.
• S is a sulfide forming element effective in improving the machinability by cutting. If the S content is less than 0.250%, fine sulfides are few, so that the machinability by cutting cannot be improved. If the S content is more than 0.600%, sulfides coarsen excessively and the number of fine sulfides decreases, as a result of which the machinability by cutting decreases. Moreover, the hot workability and the ductility which is an important mechanical property decrease.
• the S content is therefore in a range of 0.250% to 0.600%.
• the S content is preferably 0.300% or more.
• the S content is preferably 0.450% or less.
• O is an element that forms oxide and serves as a sulfide precipitation nucleus and also is effective in suppressing extension of sulfides during hot working such as rolling. This action can improve the machinability by cutting. If the O content is 0.0100% or less, the sulfide extension suppressing effect is insufficient and extended sulfides remain, so that the foregoing effect cannot be expected. The O content is therefore more than 0.0100%. If the O content is more than 0.0500%, not only the sulfide extension suppressing effect is saturated but also the amount of hard oxide-based inclusions increases. Adding an excessive amount of O is also economically disadvantageous. The upper limit of the O content is therefore 0.0500%.
• Cr has an effect of forming sulfides and improving the machinability by cutting through lubricating action during cutting. Cr also suppresses extension of sulfides during hot working such as rolling, and thus can improve the machinability by cutting. If the Cr content is less than 0.50%, the formation of sulfides is insufficient and extended sulfides tend to remain, so that the foregoing effect cannot be expected. If the Cr content is more than 1.50%, not only the steel hardens but also sulfides coarsen. Moreover, the extension suppressing effect is saturated, and the machinability by cutting decreases. Besides, adding an excessive amount causes an increase in alloy costs, which is economically disadvantageous.
• the Cr content is therefore 0.50% to 1.50%.
• the Cr content is preferably 0.70% or more.
• the Cr content is preferably 1.30% or less.
• the free-cutting steel contains the above-described components with the balance consisting of Fe and inevitable impurities, or contains the above-described components and further contains the below-described optional components.
• the free-cutting steel preferably contains the above-described components or preferably contains the above-described components and further the below-described optional components, with the balance consisting of Fe and inevitable impurities.
• the A value is an important index that influences refinement of Mn—Cr—S-based sulfide during hot working such as rolling, and limiting the A value can improve the machinability by cutting. If the A value is less than 6.0, sulfide of Mn—S alone forms, which tends to be coarse. Consequently, the machinability by cutting degrades. If the A value is more than 18.0, not only the sulfide refining effect is saturated but also the amount of the sulfide forming elements is excessively large relative to sulfur, causing sulfides to coarsen. The A value is therefore 6.0 to 18.0. The A value is preferably 6.5 or more. The A value is preferably 17.0 or less.
• the free-cutting steel according to the present disclosure may optionally contain one or more selected from the group consisting of
• N 0.0150% or less.
• Si is a deoxidizing element. Moreover, Si oxide acts as a sulfide formation nucleus to promote the formation of sulfides and refine the sulfides and thus improve the cutting tool life. Accordingly, Si may be contained in the steel in order to further extend the tool life. If the Si content is more than 0.50%, the oxide increases in size and decreases in number. Such oxide is ineffective as a sulfide formation nucleus, and also hard oxide induces abrasive wear and leads to degradation in tool life. The Si content is therefore 0.50% or less. The Si content is preferably 0.03% or less. To achieve the foregoing action by Si, the Si content is preferably 0.001% or more.
• the P content is an element effective in suppressing the formation of built-up edges during cutting work to thus reduce finishing surface roughness.
• the P content is preferably 0.01% or more. If the P content is more than 0.10%, the material hardens, so that the machinability by cutting decreases and the hot workability and the ductility decrease significantly.
• the P content is therefore preferably 0.10% or less.
• the P content is more preferably 0.08% or less.
• Al is a deoxidizing element as with Si, and may be contained in the steel. Al forms Al 2 O 3 in the steel. This oxide is hard and causes degradation in cutting tool life due to abrasive wear. Hence, adding an excessive amount of Al needs to be avoided. From this viewpoint, the Al content is preferably 0.010% or less. The Al content is more preferably 0.005% or less. From the viewpoint of achieving the deoxidizing effect by Al, the Al content is preferably 0.001% or more.
• N forms nitride with Cr and the like.
• an oxide film called belag forms on the tool surface.
• Belag has an action of protecting the tool surface and thereby improving the tool life.
• N may be contained in the steel.
• the N content is preferably 0.0050% or more. If the N content is more than 0.0150%, not only the effect of belag is saturated but also the material hardens, as a result of which the tool life shortens.
• the N content is therefore preferably 0.0150% or less.
• the N content is more preferably 0.0060% or more.
• the N content is more preferably 0.0120% or less.
• the free-cutting steel according to the present disclosure may optionally further contain one or more selected from the group consisting of
• V 0.20% or less
• Mg 0.0050% or less.
• Ca, Se, Te, Bi, Sn, Sb, B, Cu, Ni, Ti, V, Zr, and Mg each have an action of improving the machinability by cutting, and accordingly may be added in the case where the machinability by cutting is considered important.
• these elements in order to improve the machinability by cutting, if their respective contents are Ca: less than 0.0001%, Se: less than 0.02%, Te: less than 0.10%, Bi: less than 0.02%, Sn: less than 0.003%, Sb: less than 0.003%, B: less than 0.003%, Cu: less than 0.05%, Ni: less than 0.50%, Ti: less than 0.003%, V: less than 0.005%, Zr: less than 0.005%, and Mg: less than 0.0005%, sufficient effect cannot be achieved.
• their respective contents are preferably Ca: 0.0001% or more, Se: 0.02% or more, Te: 0.10% or more, Bi: 0.02% or more, Sn: 0.003% or more, Sb: 0.003% or more, B: 0.003% or more, Cu: 0.05% or more, Ni: 0.05% or more, Ti: 0.003 or more, V: 0.005% or more, Zr: 0.005% or more, and Mg: 0.0005% or more.
• their respective contents are preferably Ca: 0.0010% or less, Se: 0.30% or less, Te: 0.15% or less, Bi: 0.20% or less, Sn: 0.020% or less, Sb: 0.025% or less, B: 0.010% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ti: 0.100% or less, V: 0.20% or less, Zr: 0.050% or less, and Mg: 0.0050% or less.
• Fine dispersion of sulfides in the microstructure of the free-cutting steel is advantageous in promoting the lubricating action between the tool and the work material during cutting work.
• at least a predetermined amount of sulfides of less than 1 ⁇ m in equivalent circle diameter and at least a predetermined amount of sulfides of 1 ⁇ m to 5 ⁇ m in equivalent circle diameter need to be dispersed in the steel microstructure.
• Sulfides of less than 1 ⁇ m in equivalent circle diameter are mainly effective for lubrication between the tool and the work material.
• Sulfides of 1 ⁇ m to 5 ⁇ m in equivalent circle diameter not only have the foregoing lubrication effect but also are effective for chip partibility.
• the number of sulfides of less than 1 ⁇ m in equivalent circle diameter is at least 500 particles/mm 2
• the number of sulfides of 1 ⁇ m to 5 ⁇ m in equivalent circle diameter is at least 2000 particles/mm 2 .
• a rectangular cast steel that has the above-described chemical composition and whose side length of a cross section perpendicular to the longitudinal direction is 250 mm or more is rolled at a heating temperature of 1120° C. or more and an area reduction rate of 60% or more to obtain a billet, and the billet is hot worked at a heating temperature of 1050° C. or more and an area reduction rate of 75% or more.
• a molten steel adjusted to the chemical composition is cast to obtain a cast steel.
• a cast steel a rectangular cast steel whose side length of a cross section perpendicular to the longitudinal direction is 250 mm or more is used.
• the cast steel is produced as a cast steel having a rectangular cross section by continuous casting or ingot casting. If the side length of the rectangular cross section is less than 250 mm, sulfide particles increase in size in the solidification of the cast steel. Consequently, coarse sulfides remain even after the cast steel is subsequently rolled to obtain a billet, which is disadvantageous in terms of sulfide refinement after final hot working.
• the side length of the cast steel in the cross section is therefore 250 mm or more.
• the side length of the cast steel in the cross section is more preferably 300 mm or more. Although no upper limit is placed on the side length of the cast steel in the cross section, the side length is preferably 600 mm or less from the viewpoint of the rollability in the hot rolling following the casting.
• the cast steel is hot rolled into a billet.
• the heating temperature in the hot rolling needs to be 1120° C. or more. If the heating temperature is less than 1120° C., coarse sulfides crystallized during cooling-solidification in the casting stage do not dissolve, and remain even in the billet. Consequently, the sulfides remain coarse even after the hot working, and the desired fine sulfide distribution state cannot be achieved. Accordingly, the heating temperature when hot rolling the cast steel into the billet is 1120° C. or more, and is preferably 1150° C. or more. Although no upper limit is placed on the heating temperature of the cast steel, the heating temperature is preferably 1300° C. or less and more preferably 1250° C. or less from the viewpoint of preventing scale loss.
• the sulfide particles crystallized during the solidification are large in size, the sulfide particles need to be reduced in size to some extent in bloom rolling. If the area reduction rate in the hot rolling is low, the sulfide particles remain large in the billet. In such a case, it is difficult to refine the sulfide particles in heating/rolling when subsequently hot working the billet into a steel bar or a wire rod. In view of this, the area reduction rate in the hot rolling of the cast steel into the billet is 60% or more.
• the area reduction rate (%) in the hot rolling can be calculated according to the following formula:
• S0 is the cross-sectional area of a cross section perpendicular to the hot rolling direction of the cast steel before the hot rolling
• S1 is the cross-sectional area of a cross section perpendicular to the hot rolling direction of the billet produced as a result of the hot rolling.
• the heating temperature when hot working the billet into a steel bar or a wire rod is an important factor. If the heating temperature is less than 1050° C., the sulfides do not disperse finely, so that the lubricating action during cutting work is poor. This facilitates tool wear, and shortens the tool life.
• the heating temperature of the billet is therefore 1050° C. or more.
• the heating temperature of the billet is more preferably 1080° C. or more. Although no upper limit is placed on the heating temperature of the billet, the heating temperature is preferably 1250° C. or less from the viewpoint of suppressing a yield rate decrease caused by scale loss.
• the area reduction rate when hot working the billet into a steel bar or a wire rod is also an important factor for sulfide refinement. If the area reduction rate is less than 75%, sulfide refinement is insufficient. Accordingly, the lower limit of the area reduction rate is 75%.
• the area reduction rate is more preferably 80% or more.
• the area reduction rate in the hot working can be calculated according to the following formula:
• S1 is the cross-sectional area of a cross section perpendicular to the hot working direction of the billet before the hot working
• S2 is the cross-sectional area of a cross section perpendicular to the hot working direction (stretching direction) of the steel bar or wire rod produced as a result of the hot working.
• the sulfides can be refined and the machinability by cutting can be improved.
• the obtained billet was heated at the corresponding heating temperature in Table 2-1 and Table 2-2, and hot rolled into a steel bar having the corresponding diameter in Table 2-1 and Table 2-2.
• Each of the obtained steel bars (disclosed steels and comparative steels) was subjected to the following test.
• a test piece was collected from a cross section parallel to the rolling direction of the obtained steel bar, and the 1 ⁇ 4 position in the radial direction from the peripheral surface of the cross section was observed with a scanning electron microscope (SEM) to investigate the equivalent circle diameter and number density of sulfide in the steel.
• SEM scanning electron microscope
• precipitate composition analysis was conducted by energy dispersive X-ray spectrometry (EDX).
• EDX energy dispersive X-ray spectrometry
• the machinability by cutting was evaluated by an outer periphery turning test.
• BNC-34C5 produced by Citizen Machinery Co., Ltd. was used as a cutting machine
• Carbide EX35 Tool TNGG160404R-N produced by Hitachi Tool Engineering, Ltd. was used as a turning tip
• DTGNR2020 produced by KYOCERA Corporation was used as a holder.
• a 15-fold diluted emulsion of YUSHIROKEN FGE1010 produced by Yushiro Chemical Industry Co., Ltd. was used.
• the cutting conditions were cutting rate: 120 m/min, feed rate: 0.05 mm/rev, cut depth: 2.0 mm, and machining length: 10 m.
• the machinability by cutting was evaluated based on the flank wear Vb of the tool after the end of the cutting test over a length of 10 m. In the case where the flank wear Vb after the end of the cutting test was 200 ⁇ m or less, the machinability by cutting was evaluated as “good”. In the case where the flank wear was more than 200 ⁇ m, the machinability by cutting was evaluated as “poor”.
• Table 2-1 and Table 2-2 The test results of the disclosed steels and the comparative steels are shown in Table 2-1 and Table 2-2. As is clear from Table 2-1 and Table 2-2, the disclosed steels had favorable machinability by cutting as compared with the comparative steels.

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