AU2006241390B2 - Free-cutting steel having excellent high temperature ductility - Google Patents

Description

FREE-CUTTING STEEL HAVING EXCELLENT HIGH TEMPERATURE DUCTILITY 0 z FIELD OF THE INVENTION The invention relates to steel used in automobiles and general machines or the like. Specifically, the invention relates to steel having excellent machinability such as C excellent properties in tool life, surface roughness, and chip control at the time of cutting.

Moreover, the invention relates to a free-cutting steel having excellent high temperature ductility and showing good ductility at the time of hot rolling.

BACKGROUND ART General machineries and automobiles are manufactured by assembly of various parts. In many cases, the production process of those parts includes a cutting operation in terms of prescribed precision and production efficiency. In those cases, because of the requirement for cost reduction and improvement of production efficiency, improvement in machinability of steel is demanded. For example, machinability has been emphasized in the developments of SUM23 as low-carbon sulfur-bearing free cutting steel and SUM24L as a low-carbon sulfur-lead composite free cutting steel.

Conventionally, it is known that addition of machinability-improving elements such as S, Pb is effective in improving the machinability of steel. However, since some customers avoid the use of Pb due to its environmental load, usage of Pb is tending to decline.

In the prior art, as an alternative to addition of Pb to steel, machinability of the steel has been improved, by a method of forming inclusions, for example sulfides mainly composed of MnS, showing soft properties under cutting conditions. However, since the low-carbon sulfur-lead composite free-cutting steel SUM24L contains S in the same O amount as the low-carbon sulfur-bearing free-cutting steel SUM23, an additional amount of sulfur more than in the conventional case is required. However, addition of an 0 Z excess of sulfur merely coarsens the grain size of sulfides mainly composed of MnS and C is not effective in improving the machinability of the steel. In addition, since it is impossible to achieve a sufficient brittleness of a matrix of the steel, various problems such as deterioration of roughness of the finished surface (hereafter referred to as surface CI roughness) which accompanies drop-out of the constituent cutting edge of a cutting tool and isolation of chips, and defective chip control in parting with insufficient chips are caused to occur. Moreover, in the production process including rolling and forging of the steel, sulfides mainly composed of coarse MnS act as starting points of breakdown and cause many problems concerning the manufacture, for example, rolling flaws.

Therefore, there is a limit for the improvement of macinability by only increasing sulfur.

Although the machinability may be improved to a certain extent by adding machinability-improving elements other than sulfur, for example, Te, Bi, P, and N, it is preferred to limit the addition of those elements to the lowest amount since their addition causes deterioration of surface properties, for example, by generation of cracks and flaws during rolling or hot-forging of the steel. As explained above, in the conventional art, it has been difficult to obtain good machinability and productivity compatibly.

Japanese Unexamined Patent Application, First Publication No. H 11-222646 (patent document 1) proposed a method for improving chip controllability by forming sulfides such that single sulfide grains of not smaller than 20 pLm or sulfide aggregate of not smaller than 20 gtm formed of approximately serially aggregated sulfides exist in the section along the rolling direction to a density of not less than 30 particles per 1 mm 2 However, dispersion of sulfides of submicron meter in size and production method of such occurrence of sulfides are neither described in patent document 1 nor can be O expected from the compositional range disclosed in patent document 1. In the consideration of the inventor of the present invention, the dispersion of sulfides of 0 Z submicron meter in size is a most effective way to improve the machinability of steel.

C-I There have been attempts to utilize non-sulfide inclusions for improving the machinability of steel. For example, Japanese Unexamined Patent Application, First Publication No. H9-17840 (patent document Japanese Unexamined Patent CI Application, First Publication No.2001-329335 (patent document Japanese Unexamined Patent Application, First Publication No.2002-3991 (patent document 4), and Japanese Unexamined Patent Application, First Publication No.2000-178683 (patent document 5) disclosed techniques for improving machinability by utilizing BN.

However, the techniques described in those documents are not intended to improve the surface roughness. The techniques of patent documents 2, 3, and 5 are intended to improve tool life (tool life of a tool used for cutting the steel, and the technique of patent document 4 is intended to improve chip-controllability. On the other hand, a sufficient effect for improving the surface roughness cannot be obtained by applying the chemical composition in ranges of embodiments described in the above-described documents.

Specifically, based on the consideration of the inventor of the present invention, it is impossible to obtain a sufficient effect for improving the surface roughness without homogenizing the matrix by fine dispersion of BN in steel, which is not described in any of the above-described patent documents.

Japanese Unexamined Patent Application, First Publication No.2004-176176 (patent document 6) also disclosed a method for utilizing BN for the improvement of machinability. High temperature ductility for inhibiting generation of flaws during the production process is also considered in patent document 6. However, in the technique described in patent document 6 it is impossible to find a compositional balance by which O generation of flaws is completely inhibited and machinability as a reciprocal property is compatibly ensured.

0 Z Patent document 3 proposed a technique for improving high temperature C ductility by inhibiting grain boundary embrittlement caused by grain-boundary precipitation of BN, and a technique for controlling the amount of N addition in order to utilize the effect of dissolved B (B forming a solid-solution) for inhibiting grain C boundary embrittlement. However, patent document 3 only reduces the amount of N and does not pay sufficient attention to control of the amount of dissolved N at a temperature range of BT heating for working, and therefore, fails to sufficiently reduce the amount of dissolved N in order to prevent the generation of flaws. In addition, since the presence of a large amount of dissolved B highly enhances hardenability of the steel, the surface layer of the steel cooled by cooling water or the like is hardened during rolling and numerous fine flaws occur. In addition, since the amount of N is limited to a value remarkably lower than the stoichiometric composition, a necessary amount of BN for improving the surface roughness cannot be stably provided. In addition, since the amount of S for compensating the machinability lowered by deficient N, excellent machinability, especially excellent surface roughness cannot be obtained.

Japanese Unexamined Patent Application, First Publication No.2004-27297 (patent document 7) proposed a technique for limiting the oxygen content in the steel in order to reduce the surface flaws. However, patent document 7 does not describe a method for controlling the oxygen content in steel. In a low-carbon free-cutting steel before deoxidation, it is impossible to limit the oxygen content in the steel and prevent the generation of flaws without a specific control.

There has been a technique for adding Ca to low-carbon free cutting steel in order to improve machinability. For example, in Japanese Unexamined Patent

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O Application, First Publication No.2000-160284 (patent reference 8) no practical effect for improving the machinability is described, a wide range is described for the amount of Ca 0 Z addition, and the effective amount for improving the machinability is not described.

CI The present invention relates to steel used in automobiles and general machines or the like. Specifically, the invention relates to steel having excellent machinability represented by excellent tool life, surface roughness, and chip-controllability at the time CI of cutting. Moreover, the invention provides a free-cutting steel having excellent high temperature ductility during hot rolling and capable of preventing deterioration of surface properties by the hot rolling, and provides a method for producing such a free-cutting steel.

SUMMARY OF THE INVENTION The inventor of the present invention, considered that cutting was a deformation phenomena for separating chips from steel, and enhancement of the chipping could be a point of the invention. However, as explained above, there is a limit in the effect of increasing the sulfur content. In addition, in terms of productivity, while improving machinability, it is necessary to improve high-temperature ductility, which is a property conflicting the machinability. In addition, amounts of machinability-improving elements should be considered.

Based on various studies, the inventor invented a method for reducing dissolved N at a temperature range of the rolling temperature for improving high temperature ductility and effectively adding B as a BN forming element for ensuring the machinability. Moreover, the inventor found that control of the oxygen content in steel enabled a high temperature ductility and machinability compatibly to be provided.

00 Here, the amount of dissolved N (N forming a solid-solution) is a nitrogen content obtained by subtracting the amount of nitrogen contained in compounds from the total Samount of nitrogen. The amount of nitrogen in compounds is substantially the same as the amount of nitrogen forming BN. Large amount of dissolved N is generated at a temperature range of rolling from 800 to 1100 0 C by dissolution of BN. The inventor found that reduction of the dissolved N at that temperature range is necessary for Sexcellent rolling without causing surface flaws except to a low degree.

The inventor found that it is effective to combine controlling of amount of dissolved at a high temperature to controlling of ratio of amounts of B and N required to form BN 1o which is necessary to achieve machinability at a room temperature at which cutting is operated. In a molten steel, B is easily consumed by formation of oxide. The inventor found an amount of Ca addition for effective control of oxygen content in steel as a technique for improving machinability of the steel by improving yield of B as BN and simultaneously reducing hard oxide for preventing flaw generation.

Is The present invention is based on the above-described finding and is summarized in the following: A first aspect of the invention is a free-cutting steel having excellent hightemperature ductility containing, in mass C: 0.005 to Si:0.001 to Mn: 0.3 to P: 0.001 to 0.2%, S: 0.40 to 0.50%, B: 0.005 to 0.015%, 0: 0.005 to 0.012%, Ca: 0.0001 to 0.001%, Al: not more than 0.01%, a predetermined amount of N, and a balance: Fe and unavoidable impurities, wherein amounts of S and B satisfy Seq.=S+14xB>0.52 amounts of B and N satisfy 1.3xB-0.0022 N 1.3xB+0.0034, and an amount of dissolved nitrogen contained in the steel is substantially same as a value determined by subtracting an amount of nitrogen forming BN from the total amount of nitrogen.

1338010-1HJG O A second aspect of the invention is a free-cutting steel having excellent high-temperature ductility according to the first aspect, wherein the steel includes sulfide Z particles mainly composed of MnS, and in a section perpendicular to a rolling direction of the steel, a density of the sulfide particles of 0.1 to 0.5 jtm in a circle-equivalent diameter is not less than 10000 particles/mm 2 c A third aspect of the invention is a free-cutting steel having excellent i high-temperature ductility according to the first or second aspect, further containing at the expense of the balance, in mass one or more selected from V: 0.05 to Nb: 0.005 to Cr: 0.01 to Mo: 0.05 to W: 0.05 to Ni: 0.05 to Cu: 0.01 to A fourth aspect of the invention is a free-cutting steel having excellent high-temperature ductility according to any one of the first to third aspect, further containing at the expense of the balance, in mass one or two selected from Sn: 0.005 to and Zn: 0.0005 to A fifth aspect of the invention is a free-cutting steel having excellent high-temperature ductility according to any one of the first to fourth aspect, further containing at the expense of the balance, in mass one or more selected from Ti: 0.0005 to Zr: 0.0005 to Mg: 0.0003 to 0.005%.

A sixth aspect of the invention is a free-cutting steel having excellent high-temperature ductility according to any one of first to fifth aspect, further containing at the expense of the balance, in mass one or more selected from Te: 0.0003 to Bi: 0.005 to and Pb: 0.005 to According to the invention, it is possible to provide a free-cutting steel having excellent machinability such as excellent tool-life, surface roughness, and

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O chip-controllability, as well as having excellent ductility during the hot rolling and therefore having excellent surface properties,

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BRIEF EXPLANATION FOR DRAWINGS FIGS. 1A and 1B schematically illustrate a procedure of a plunge cutting test.

Cc A relative relation between a round bar of steel subjected to the plunge cutting test is C, shown in strabismic view in FIG.IA and plan view in FIG. 1B.

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FIG. 2A is a schematic drawing for explaining a method of longitudinal cutting and FIG. 2B is a drawing for explaining an influence of tear on the surface roughness in accordance with the testing method.

FIGS. 3A and 3B show a distribution condition of sulfides mainly composed of MnS in accordance with the invention. FIG. 3A is a replica photograph using TEM.

FIG. 3B is an optical microphotograph.

FIGS. 4A and 4B show a distribution condition of sulfides mainly composed of MnS in accordance with a Comparative Example. FIG. 4A is a replica photograph using TEM. FIG. 4B is an optical microphotograph.

FIG. 5 is a drawing showing a difference in machinability dependent on Seq., where the machinability is represented by surface roughness after 800 times cutting.

FIG. 6 is a drawing showing a balance between a surface roughness and high temperature ductility in cases of longitudinal turning of Examples according to the invention and Comparative Examples.

FIG. 7 is a drawing explaining a portion used for determination of solidification cooling rate.

DETAILED DESCRIPTION OF THE INVENTION 9 In the following, the present invention is explained based on the best mode.

However, it should be noted that the present invention is not limited to the individual 0 Z embodiments described in the following.

IC The invention was completed as a result of finding effective amounts of B and S addition in order to obtain sufficient machinability, especially excellent surface Cc roughness, without adding lead. In addition, the inventor found that adding N with a predetermined ratio to B and controlling of oxygen in steel are effective in order to control an amount of dissolved N and to reduce hard oxides at a temperature range for hot rolling (800 to 1100°C) for preventing generation of surface flaws caused by reduction of high temperature ductility of steel.

In the following, the reason for regulating the component elements in the free-cutting steel of the invention is explained.

Carbon 0.005 to 0.2% by mass.

C is related to fundamental strength of steel and oxygen content in the steel, and therefore has a strong influence on machinability. Addition of excess C enhances the strength of the steel but reduces the machinability. Therefore, the upper limit of C was regulated to On the other hand, if a C content is reduced to too low a level by a simple blow smelting, besides high cost, deoxidization by C does not occur and high residual oxygen content in the steel causes deficiencies such as pin holes. Therefore, as a C content sufficient to inhibit the deficiencies such as pin holes, the lower limit of C content was regulated to 0.005%.

Silicon 0.001 to 0.5% by mass.

Addition of excessive Si generates hard oxides and reduces machinability.

However, a proper amount of Si softens oxides but does not reduce the machinability.

The upper limit of the proper Si content is If the Si content exceeds C generation of hard oxide cannot be avoided. If the Si content is lower than 0.001%, it is difficult to soften the oxides and, in terms of industrial production of steel, production 0 Z cost is expensive.

C Manganese 0.3 to 3.0% by mass.

Mn is necessary for fixing sulfur in steel as MnS and dispersing the MnS. In Cc addition, Mn is necessary for softening and reducing the harmful effects of oxides in the i steel. While the effect of Mn also depends on the content of added S, Mn of less than 0.3% cannot sufficiently fix the added S as MnS and cannot inhibit generation of surface flaws sufficiently. In addition, residual S forming FeS embrittles the steel. On the other hand, a high Mn content enhances the hardness of the matrix and deteriorates the machinability and reduces workability of the steel during cold working, Therefore, the upper limit of the Mn content was set to be Phosphorus 0.001 to 0.2% by mass.

Since P hardens the matrix of steel and deteriorates not only cold workability but also hot workability and casting property, upper limit of P must be On the other hand, since P is an effective element for improving the machinability, the lower limit of P was set to be 0.001%.

Sulfur 0.40 to 0.50% by mass.

S is coupled with Mn and exists as sulfides mainly composed of MnS.

Although the sulfides mainly composed of MnS improve the machinability, elongated sulfides mainly composed of MnS act as a cause of anisotropy at a time of forging.

While sulfides mainly composed of large MnS should be avoided, in terms of improvement of machinability, addition of excess S is preferable. Therefore, it is preferable to disperse fine sulfides mainly composed of MnS. Where Pb is not added, S of not more than 0.4% should be added for improving the machinability. On the other I11 O hand, if too much S is added, it is impossible to avoid the generation of sulfides mainly composed of coarse MnS. In addition, deterioration of casting properties and hot Z deformation properties caused by FeS or the like causes cracking of the steel during I production of the steel. Therefore, the upper limit of S was set to be 0.50 Boron 0.005 to 0.015% by mass.

c B is effective for improvement of machinability when the B precipitates as BN.

I The improvement of machinability is especially remarkable, when the boron nitrides form complex precipitates with sulfides mainly composed of MnS and occur as fine grains dispersed in the matrix. Those effects are not obvious when B content is less than 0.005. On the other hand, if the B content exceeds 0.015%, the excessive B in molten steel is highly reactive with refractory material, and enhances the melting damage of the refractory material during casting, thereby remarkably deteriorating the productivity. Therefore, the range of B content was set to be 0.005% to 0.015%.

Since B easily forms oxides, there is a case in which the amount of BN effective for improving the machinability is reduced by consumption of B by forming oxides when a molten steel has high free oxygen content. For the improvement of machinability, it is effective to improve the yield of B substantially forming BN by adding Ca to reduce the free-oxygen concentration, and subsequently adding the B.

Oxygen 0.005 to 0.012% by mass.

When O exists in the steel as single elements without forming oxides, the O form bubbles during cooling of the steel and causes generation of pin holes. On the other hand, oxygen forming hard oxides causes deterioration of machinability or flaws.

Moreover, when fine MnS grains are dispersed in order to improve the machinability, oxides are utilized as nuclei for precipitation of the sulfides. In some cases, through formation of oxide in the molten steel, oxygen has an influence on the machinability by I12 o consuming B added as a machinability-improving element. Therefore, it is required to control oxygen content. Oxygen of less than 0.005% is not sufficient to disperse fine 0 Z grained sulfides mainly composed of MnS, causes generation of coarse MnS, and has an adverse effect on the machinability and mechanical properties of the steel. In addition, machinability is deteriorated by the generation of sulfides mainly composed of MnS having a morphology of type II sulfide according to Sims's classification. Moreover, I desulfurization is easily caused in the molten steel preventing stable addition of S.

Therefore, the lower limit of the oxygen content was set to be 0.005%. An oxygen content exceeding 0.012% easily causes a generation of boron oxides in the molten steel, thereby decreasing the amount of B substantially forming B, deteriorating machinability, and increasing generation of flaws by forming a large amount of hard oxide. Therefore, the upper limit of O content was set to be 0.012%. In order to control oxygen, it is inevitable to add calcium.

Calcium 0.0001 to 0.0010% by mass.

Ca is a deoxidizing element, capable of controlling the oxygen content in the steel, stabilizing the yield of B tending to form oxides, and inhibiting the generation of hard oxides. In addition, when a small amount of Ca is added, the Ca has an effect of improving machinability of the steel by forming soft oxides. Such effects cannot be observed if the Ca content is less than 0.0001%. If the Ca content exceeds 0.0010%, a large amount of soft oxide is generated, and the soft oxides are adhered to cutting edges of tools forming an irregular surface, and remarkably worsening the surface roughness.

In addition, a large amount of hard oxides is also generated and reduces machinability and high temperature ductility. Therefore, the compositional range of Ca was set to be 0.0001 to 0.0010%.

Aluminum Al 0.01% by mass.

13 O Al is a deoxidizing element and generates Al 2 0 3 and AIN in the steel.

However, the presence of A1 2 0 3 as hard oxide causes tool-damage in the cutting

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Z operation, and enhances wearing of the tool. In addition, formation of AIN decreases N CN for forming BN, thereby reducing machinability. Therefore, the Al content was limited to not more than 0.01% so as not to generate large amounts ofA1 2 0 3 and AIN.

c Sulfur and boron contents satisfying Seq.=S+ 14xB>0.52.

NI B forms BN in the steel and is effective for improving the machinability. B O can be used by replacing a part of S. The amount of B is regulated by the equation Seq.=S+ 14xB>0.52 showing an equivalent weight (in mass for ensuring surface roughness.

A small Seq. value deteriorates the surface roughness. Especially, where Seq.

S0.52, during cutting large amount of steel by longitudinal cutting, as shown in FIG. for the below-mentioned example, by the advancing tool friction, surface roughness depending on the reflection of irregularity of tools is remarkably deteriorated. Therefore, Seq. exceeding 0.52 is required.

Content of N satisfying 1.3xB-0.022 5 N 1.3xB+0.0034.

N is coupled with B to form BN and improves the machinability. BN is an inclusion for improving the machinability. By dispersing the boron nitrides with a high density, it is possible to remarkably improve the machinability. At a mass ratio of B:N =10.8 1.14, B and N form compounds without excess or deficiency, and form boron nitrides In addition, BN has a solubility in steel, and the solubility increases with increasing temperature of the steel. As a result, dissolved N is increased in the steel. Since a high dissolved nitrogen content in a range of rolling temperature (800 to 1100 0 C) causes rolling flaws, It is necessary to limit the dissolved nitrogen

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O content to a value not higher than a predetermined content. Therefore, the amount of N added to the steel must be controlled in accordance with the amount of B added to the Z steel. However, too low a N content reduces the amount of BN generation, and is C1 insufficient for providing BN required for improving the machinability. In addition, the steel tends to be hardened by the presence of excessive dissolved B, and hot ductility is reduced. Therefore the N content should be limited based on the B content and satisfy C1, 1.3xB-0.022<N<.3xB+0.0034 so as to satisfy productivity and machinability

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Scompatibly.

[Dispersion of sulfides mainly composed of MnS] The density of the sulfide particles of 0.1 to 0.5 tm in a circle-equivalent diameter is not less than 10000 particles/mm 2 Sulfides mainly composed of MnS are inclusions for improving machinability.

By dispersing the sulfides having a fine grain size with high density, the machinability of the steel is remarkably improved.

Especially, in a cutting method like longitudinal turning in which the cutting advances while forming a crest called a feed mark, the presence or absence of tear caused by tear type chipping has a large influence on the height of the crest, that is, on the surface roughness. If the sulfide grains mainly composed of MnS occur as fine grained sulfides dispersively distributed in the steel with high density, because of the homogeneous texture, it is possible to improve the rupture property of the steel, thereby reducing tear of the steel and improving the surface roughness. The above-described occurrence of fine grained sulfides is particularly effective for improving surface roughness of a part subjected to cutting by longitudinal turning, for example, a shaft of office automation equipment. In order to exert the effect, the steel should have the fine grained sulfides with a density of 10000 particles/mm 2 A dimension of each grain of the sulfides is preferably, as a circle equivalent diameter, 0.1 to 0.5 gm.

O In general, the distribution of sulfides mainly composed of MnS is observed with an optical microscope. Dimensions and distribution density of the sulfides are also 0 Z measured using the optical microscope. However, the optical microscope is ineffective for observation of sulfides mainly composed of MnS having the above-described dimension. In such a case, a transmission electron microscope (TEM) is used for the observation of sulfides. That is, the difference in dimension and distribution density of the fine grained sulfides mainly composed of MnS cannot be observed with an optical microscope, but can be clearly observed with a TEM. By controlling the fine grained sulfides, and by showing a numerical limitation for the occurrence of the fine grained sulfides, the present invention is apparently differentiated from the conventional art.

If the dimensions of sulfides mainly composed of MnS exceed the above-described upper limit, in order to achieve a distribution of the sulfide with a density of not lower than 10000 particles/mm 2 it is required to add S in a large amount exceeding the above-described range of the present invention. In such a case, addition of a large amount of S increases the probability to occur numerous sulfides mainly composed of coarse MnS, and increases generation of flaws at the time of rolling of the steel. When the dimension of sulfides exceeds the above-described dimension while regulating the amount of S added to be within the above-described range of the present invention, because of the deficiency in the amount of sulfides mainly composed of MnS, it is impossible to maintain a sufficient distribution density of the sulfides required for the improvement of surface roughness at the time of longitudinal cutting. On the other hand, sulfides having a surface-equivalent diameter smaller than the lower limit of 0.1 im do not have an influence on the machinability. Therefore, the dimension and distribution density of the sulfides mainly composed of MnS were regulated such that O distribution density of the sulfide particles of 0.1 to 0.5 pm in a circle-equivalent diameter was not less than 10000 particles/mm 2 0 Z Conventionally, it has been difficult to homogeneously disperse fine BN in steel.

Since the sulfides mainly composed of MnS act as precipitation nuclei of BN, it is possible to homogeneously disperse the fine BN, remarkably enhancing the improvement Cc of surface roughness, especially in the case of longitudinal cutting.

CI In order to achieve the above-described dimension and distribution density of sulfides mainly composed of MnS, it is effective to control the ratio of Mn and S content given by Mn/S to be 1.2 to 2.8.

In order to generate sulfide mainly composed of fine MnS more effectively, it is preferable to control a range of the cooling rate in the solidification process. If the cooling rate is slower than 10C/min, because of too slow a solidification rate, sulfides mainly composed of MnS crystallize as coarse grains causing difficulty in forming a dispersive distribution of fine sulfides. If the cooling rate exceeds 100C/min, because of the saturated distribution density of sulfides mainly composed of fine MnS, hardness of the billet is increased and a possibility of generation of cracking increases. Therefore, at the time of casting the steel, the preferable cooling rate is 10 to 100 0 C/min. The cooling rate in the above-described range can be easily achieved by controlling the cross sectional area of the casting mold, casting speed or the like to be preferable value. Such a control of the cooling rate can be applied to both a continuous casting process and a steel ingot making process.

The solidification cooling rate of the above description is a cooling rate from a liquidus temperature to a solidus temperature of a portion at a center in the width direction of a section of the bloom and one fourth of the thickness (see FIG.7).

In accordance with the following equation, the cooling rate can be calculated 17 from the spacing of a secondary arm of a dendrite formed as a solidification texture in the thickness direction of the bloom.

2 0.41 R1 7R Where, Rc denotes the cooling rate and X2 denotes an arm spacing 'C 5 (gm) of a secondary dendrite.

That is, since the arm spacing of the secondary dendrite varies with the cooling condition, the controlled cooling rate can be evaluated by a measurement of the arm spacing.

The above-described sulfide mainly composed of MnS is a generic of Mn sulfide-based inclusions and expresses, as well as pure MnS, inclusions in which sulfides of Fe, Ca, Ti, Zr, Mg, REM (Rare Earth Metal) or the like are coexistent with MnS in a form of a solid solution or an aggregate, or inclusions in which a compound of Mn and a non-sulfur element, for example MnTe, is coexistent with MnS in a form of a solid solution or an aggregate, or the above-described inclusions precipitated around an oxide as a nucleus of the precipitation. That is, the chemical formula of the above-described Mn sulfide-based inclusions can be described as where X denotes a sulfide-forming element other than Mn, and Y denotes a non-sulfur element bonded with Mn.

[Elements to strengthen steel] Vanadium 0.05 to 1.0% by mass V forms a carbonitride and strengthens steel by secondary precipitation-hardening. V of less than 0.05% has no effect on strengthening of the

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O steel. On the other hand, V exceeding 1.0% results in formation of numerous carbonitrides and reduces a mechanical property of the steel. Therefore, it is preferable

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Z to control the V content to be lower than the above-described upper limit.

(N Niobium (Nb) 0.005 to 0.2% by mass.

Nb also forms a carbonitride and strengthens steel by the effect of secondary precipitation hardening. Nb of less than 0.005% has no effect on strengthening of the C steel. On the other hand, Nb exceeding 0.2% results in formation of numerous

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carbonitrides and reduces a mechanical property of the steel. Therefore, it is preferable to control the Nb content to be lower than the above-described upper limit.

Chromium (Cr) 0.01 to 2.0% by mass.

Cr is an element for improving hardenability and providing resistance to temper softening, and therefore is added to steel requiring high-strength. In that case, at least 0.01% of Cr must be added. However, excess addition of Cr embrittles steel by forming Cr carbide. Therefore, the upper limit of Cr is preferably set to be Molybdenum 0.05 to 1.0% by mass.

Mo is an element which is capable of providing steel with resistance to temper softening and improving hardenability. However, such effects cannot be observed with a Mo content of less than 0.05%. Where the Mo content exceeds the effects of Mo addition are saturated. Therefore, it is preferable to control the range of Mo content to be 0.05 to Tungsten 0.05 to 1.0% by mass.

W forms a carbonitride and is capable of strengthening steel by secondary precipitation hardening. W of less than 0.05% has no effect on strengthening of the steel.

On the other hand, W exceeding 1.0% results in formation of numerous carbonitride and 19 0 reduces a mechanical property of the steel. Therefore, it is preferable to set the upper

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limit of the W content to be 0 Z Nickel 0.05 to CNI Ni is an element capable of strengthening ferrite, and improving ductility, hardenability, and corrosion resistance. Such effects cannot be observed where the Ni c content is less than 0.05%. On the other hand, the effect of Ni on improving a CI mechanical property is saturated where the Ni content exceeds Therefore, it is

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0preferable to set the upper limit of the Ni content to be Copper (Cu) 0.01 to 2.0% by mass.

Cu is an element capable of strengthening ferrite and improving hardenability and corrosion resistance. Such effects are not apparent where the Cu content is less than 0.01%. In terms of mechanical property, the effect of Cu is reduced where the Cu content exceeds Therefore, the upper limit of the Cu content was set to be In addition, since Cu tends to reduce high-temperature ductility and cause flaws at the time of rolling, it is preferable to add Cu and Ni simultaneously.

[Elements for improving machinability of steel by embrittlement] Sn: 0.005 to Sn embrittles ferrites, prolongs tool-life, and improves surface roughness.

Such effects are not apparent where the Sn content is less than 0.005%. The effects of Sn are saturated where the Sn content exceeds Therefore, it is preferable to set the upper limit of the Sn content to be Zinc 0.0005 to Zn embrittles ferrites, prolongs tool life, and has an effect of improving surface roughness. Such effects are not apparent where the Zn content is less than 0.0005%.

O The effects of Zn are saturated where the Zn content exceeds Therefore, it is preferable to set the upper limit of the Zn content to be 0 Z [Elements for improving machinability by controlling deoxidization] C, Titanium 0.0005 to 0.1% by mass.

Ti is a deoxidizing element and is capable of controlling the oxygen content in steel and stabilizing the yield of B, which easily forms oxides. In addition, a small ri amount of Ti has an effect of improving machinability by generating soft oxides. These effects are not observed where the Ti content is less than 0.0005%. Where the Ti content is not less than a large amount of numerous hard oxides are generated, and Ti which is dissolved without forming oxides is combined with N to form TiN and reduces machinability. Therefore, the range of the Ti content was set to be 0.0005 to In addition, since Ti forms TiN and consumes N, which is necessary for forming BN, it is preferable to control the amount of Ti addition to be not higher than 0.01%.

Zircon 0.0005 to 0. 1% by mass.

Zr is a deoxidizing element and is capable of controlling the oxygen content in the steel and stabilizing the yield of B which easily forms oxides. In addition, a small amount of Zr has an effect of improving machinability by generating soft oxides. These effects are not observed where the Zr content is less than 0.0005%. On the other hand, where the Zr content is not less than a large amount of soft oxides are generated. Such soft oxides adhere to the cutting edge of a tool forming irregularities, thereby extremely deteriorating surface roughness. In addition, where the Zr content is not less than large amount of hard oxides are also generated which further reduce the machinability. Therefore, it is preferable to set the compositional range of Zr to be 0.0005 to 0.1 Magnesium 0.0003 to 0.005% by mass.

21 O Mg is a deoxidizing element and is capable of controlling the oxygen content in the steel and stabilizing the yield of B, which easily forms oxides. In addition, a small 0 Z amount of Mg has an effect of improving machinability by generating soft oxides.

CI These effects are not observed where the Mg content is less than 0.0003 On the other hand, where Mg content is not less than 0.005%, a large amount of soft oxides are c generated. Such soft oxides adhere to the cutting edge of a tool forming irregularities C thereby extremely deteriorating surface roughness. In addition, where the Mg content is not less than a large amount of hard oxides are also generated which further reduce the machinability. Therefore, it is preferable to set the compositional range of Mg to be 0.0003 to 0.005 [Elements for improving machinability by controlling morphology of sulfides and by lubrication between a tool and steel] Tellurium 0.0003 to 0.2% by mass.

Te is a machinability-improving element. In addition, Te has an effect of reducing deformability of MnS and inhibiting elongation of MnS morphology by generating MnTe or by coexisting with MnS. Therefore, Te is effective for reducing anisotropy. Such effects are not apparent where the Te content is less than 0.0003%.

Where the Te content exceeds the above-described effects are saturated and flaws are easily generated because of a reduction of high-temperature ductility.

Bismuth (Bi) 0.005 to 0.5% by mass.

Bi is a machinability-improving element. The effects of Bi are not apparent where the Bi content is less than 0.005%. Where the Bi content exceeds the above-described effects are saturated and flaws are easily generated because of a reduction of high-temperature ductility.

Lead 0.005 to 0.5% by mass.

22 O Pb is a machinability improving element. The effects of Pb are not apparent where the Pb content is less than 0.005%. Where the Pb content exceeds the 0 Z above-described effects are saturated and flaws are easily generated because of a C1 reduction of high-temperature ductility.

Examples Cc Effects of the present invention are explained with reference to Examples.

C Tables 1, 2 and 3 show data for Examples 1 to 30 according to the invention. Samples of steel according to the invention was made molten in a 270 t converter and was cast such that the solidification cooling rate was in a range of 4 to 1 8 0 C/min. The other Examples were made molten in a 180kg vacuum melting furnace and cast such that the solidification cooling rate was in a range of 1 to 85 0 C/min. The solidification cooling rates of Examples 1 to 6 of steel samples according to the first aspect of the invention were 1 to 7°C/min. The solidification cooling rate of Examples 7 to 30 of steel samples according to the second to sixth aspects of the invention were 12 to 85°C/min. Tables 4, 5, and 6 show data for steels of Comparative Examples 31-53. Samples of the steel were made molten in a 270 t converter and cast such that the solidification cooling rate was in a range of 4 to 7C/min. The other Examples were made molten in a 180 kg vacuum melting furnace and cast such that the solidification cooling rate was in a range of 1 to 4 0 C/min. Both in the Examples and in the Comparative Examples, blooms obtained from the 270 t converter were worked into billets using a billet mill. Those billets were subsequently rolled into steel bars of (I9.5 to 50 mm. On the other hand, ingots of 180kg vacuum furnace were forged to square bars having a side length of 180 mm, subsequently welded with dummy billets and were rolled into steel bars of09.5 to mm. The 0 9.5 mm bars formed by the rolling were further drawn into the wire rods of 8 mm. In order to evaluate high-temperature ductility of the steel samples, test 23

ID

0 specimens for tensile test were sampled from the billets and the 180 mm forged square

(N

bars before rolling of the billets and the 180mm forged square bars. The solidification 0 Z cooling rates were controlled by controlling the sectional area of the casting mold or by C(N controlling external cooling.

Table I INo. CHEMAL COMPOSITION I C ISi IMn I P ISI B I 0= CalI Al VI Nb ICr IMoI W 0.069 10.003 1.37 10.0401 0.44 10.006910.008910.00051 0.003 2 0.070 0.010 1.48 0.091 0.48 0.0101 0.0092 0,0003 0.002 3 0.060 0.010 1.37 0.073 0.44 0.0069 0,0120 0.0005 0.003 4 0.120 0.002 1.39 0.054 0.45 0.0101 0.0082 0.0007 0.003 0.092 10.006 1.32 10.0611 0.42 0.0148 0,0101 0.0003 0.004 6 0.061 0.005 1.48 0.112 0.41 0.0130 0.0093.0.0005 0.001__ 7 0.053 0.005 0.95 0.029 0.41 0.0128 0.0106 10.0006 0.002 8 0.084 0.004 1.29 0.056 0.50 0.0147 0.0109 0.0007 0.002 9 0.06310.007 0.98 10.0881 0.41 0.0114 0.0111 0.0009 0.003 0.131 0.003 1.22 0.053 0.50 0.0107 0.0108 0.0009 0.003 11 0.093 0.009 1.07 0.083 0.46 0.0116 10.0080 0.0005 0.002 12 0.064 0.008 1.15 0.131 0.48 0.0109 0.0084 0.0004 0.002 13 0.05610.001 0.88 10.047 0.44 0.0091 0.0087 0.0002 0.001 14 0.070 0.005 1.04 0.092 0.49 0.0080 0.0094 0.0008 0.002 0.055 0.009 1.07 0.069 0.46 0.0144 0.0081 0.0008 0.001 10.11 16 0.134 0.008 1.10 0.050 0.46 0.0111 0.0079 0.0008 0.002 0.021 17 0.066 0.008 0.70 0.089 0.42 0.0101 0,0088 0.0002 0.002 0.77 18 0.070 10.009 0.83 10.054 0.46 0.0071 10,0115 0.0008 0.001 0.10 19 0.068 0.006 0.54 0.072 0.47 0.0103 0.0083 0.0007 0.002 -0.14 0.076 0.004 0.97 0.087 0.50 0.0125 0.0085 0.0008 0.003 21 0.069 0.005 0.99 0.109 0.45 0.0086 0.0076 0.0007 0.001 22 0.051 0.008 1.15 0.097 0.44 0.0122 0.0097 0.0006 0.001 23 0.072 0.005 1.06 0.067 0.43 0.0139 0.0110 0.0004, 0.002 24 0.090 0.002 1.01 0.089 0.41 0.0177 0.0076 0.0007 0.002 0.055 0.001 0.88 0.080 0.45 0.0098 0.0074 0.0008 0.001 26 0.080 0.003 1.20 0.095 0.43 0.0105 0.0103 0.0006 0.004 27 0.061 0.006 0.63 0.094 0.45 0.0132 0.0114 0.0007 0.002 28 0.092 0.002 1.25 0.070 0.46 0.0061 0.0106 0.0004 0.002 29 0.089 0.003. 1.11 0.062. 0.43 0.0103 0.0114 0.0003 0.002 130 0.099 0.002 1.20 0.061 0. 44 0.0096 10.0102 0.000510. 001 Table 2 CHEMICAL COMPOSITION (wt. ______RESULTS OF CALCULATION RANGE OF N CONTENT No. Ni Cu Sn Zn Ti Zr Mg Te Bi Pb N ALLOWED BY EQ. Seq.

0.0089. 0.0068 to 0.124 0.54 0.0123 0.0109 toO.0165 0.62 0.0070 0.0068 to 0.0124 0.54 0.0122 0.0109 toO.0165 0.59 .0.0180 0.0l70OtoO.02261 0.63 10.01551 0.0147 to 0.0203 0.59 0.0153 0.0144 to 0.0200 0.59 0.0169 0.0169 to 0.0225 0.70 0.0167 0.0126 to 0.0182 0.57 0.0139 0.0118 toO.0174 0.65 0.0139 0.0129 to 0.0185 0.63 0.0161 0.0120OtoO.0176 0.63 0.0109 0.0096 to 0.0152 0.57 0.0102 0.0082 to 0.0138 0.60 10.0175, 0.0166 to 0.0222 0.66 10.0140 0.0122 to 0,0178 0.62 E 0.0146 0.0109 toO.0165 0.56 18 0.01000.00 7to.127 0.56 0.0151 0.0112 to 0.0168 0.61 u200.35 0.0181 10.0141 to 0.0197 0.67 C21 0.26 0.15-- 0.0130 0.0089 to 0.0145 0.57 Lu 22 0.22 0.0160 0.0136 to 0.0192 0.61 23 0.13 I- 0.0167 0.0159 to 0.0215 0.62 24 -0.003 0.0158 0.0130 to 0.0186 0.57 0.008 0.0141 0.0105 to 0.0161 0.59 0.0149 0.0115 to 0.0171 0.58 0.0009 0.0170 0.0150 to 0.0206 0.63 -10.02 0.0101 10.0057 to 0.0113 0.55 0.08 0.0137 0.0112 to 0.0168 0.57 0.18 0.0125 0.0103 to 0.0159 0.57 Table 3

EVALUATION

SURFACE ROUGHNESS HOT DUCTILITY RE MnS VL LONGI- REMARK DENSITY 1000 PLUNGE TUDINAL CHIP- REDUCTIONACCEP- No. CUTTING TURNING CONTROL OF AREA TANCE 4125 92.2 EXAMPLE OF FIRST ASPECT.

2 1468 120 8.0 6.4 A 93.9 A EXAMPLE OF FIRST ASPECT.

3 563 129 8.0 7.9 A 90.6 A EXAMPLE OF FIRST ASPECT.

4 967 129 7.7 7.4 A 88.5 A EXAMPLE OF FIRST ASPECT.

6133 125 8.0 6.4 A 94.9 A EXAMPLE OF FIRST ASPECT.

6 1328 129 5.1 4.1 A 95.8 A EXAMPLE OF FIRST ASPECT.

7 26334 134 4.4 3.5 A 87.6 A EXAMPLE OF SECOND ASPECT.

8 17001 126 5.3 4.8 A 91.4 A EXAMPLE OF SECOND ASPECT.

9 24000 130 5.6 4.5 A 90.3 A EXAMPLE OF SECOND ASPECT.

21334 131 5.7 5.4 A 90.5 A EXAMPLE OF SECOND ASPECT.

11 26667 145 5.0 4.7 A 94.2 A EXAMPLE OF SECOND ASPECT.

12 23334 127 5.1 4.1 A 89.0 A EXAMPLE OF SECOND ASPECT.

13 36667 139 6.1 5.8 A 90.7 A EXAMPLE OF SECOND ASPECT.

14 32585 128 5.1 5.8 A 93.1 A EXAMPLE OF SECOND ASPECT.

26027 155 6.2 5.0 A 90.2 A EXAMPLE OF THIRD ASPECT, V ADDITION.

16 24065 138 7.1 5.7 A 90.4 A EXAMPLE OF THIRD ASPECT. Nb ADDITION.

17 47778 135 8.3 8.2 A 91.4 A EXAMPLE OF THIRD ASPECT, Cr ADDITION.

18 43525 141 7.2 5.8 A 90.8 A EXAMPLE OF THIRD ASPECT, Mo ADDITION.

19 65036 144 7.8 6.2 A 92.7 A EXAMPLE OF THIRD ASPECT, W ADDITION.

37990 139 7.0 5.6 A 90.0 A EXAMPLE OF THIRD ASPECT, Ni ADDITION.

EXAMPLE OF THIRD ASPECT, 21 29206 136 7.9 6.3 A 87.1 A Ni AND Cu ADDITION.

22 16150 141 7.0 5.6 A 92.6 A EXAMPLE OF THIRD ASPECT, Cu ADDITION.

23 21163 144 8.1 6.5 A 93.7 A EXAMPLE OF FOURTH ASPECT, Sn ADDITION.

24 21220 134 7.1 5.7 A 94.7 A EXAMPLE OF FOURTH ASPECT, Zn ADDITION.

38148 136 8.0 6.4 A 94.4 A EXAMPLE OF FIFTH ASPECT, Ti ADDITION.

26 10311 145 8.1 7.6 A 90.0 A EXAMPLE OF FIFTH ASPECT, Zr ADDITION.

27 56667 144 7.0 5.6 A 92.4 A EXAMPLE OF FIFTH ASPECT, Mg ADDITION.

28 12754 139 9.6 9.2 A 87.9 A EXAMPLE OF SIXTH ASPECT, Te ADDITION.

29 17287 139 6.8 5.4 A 84.3 A EXAMPLE OF SIXTH ASPECT, Bi ADDITION.

12425 85.9 EXAMPLE OF SIXTH ASPECT, Pb ADDITION.

MnS density is determined by TEM, VL1000: tool life of a drill, A: Excellent 2006241390 27 Nov 2006 CHEMICAL COMPOSITION (wt No. C Si Mn P S B 0 Ca Al IV Nb Cr Mo W Ni Cu Sn Zn Ti Zr 31 0.088 0.009 1.34 0.056 0.60 0.0058 0.0074 0.0004 0.003 32 0.078 0.006 1.72 0.081 0.41 0.0176 0.0092 0,0006 0.003 33 0.088 0.009 1.05 0.060 0.42 0.0078 0,0055 0.0050 0.003 34 0.028 0.013 0.70 0.078 0.48 0.0091 .0.0216 .0.004 0.048 0.013 2.68 0.077 0.61 0.0055 0.0169 0.002 36 0.069 0.013 1.06 0.074 0.51 0.0112 0.0221 0.002 37 0.033 0.010 1.83 0.082 0.52 0.0028 0.0156 0.0018 0.001 38 0.050 0.009 1.94 0.089 0.52 0.0060 0.0052 0.003 0.003 S39 0.026 0.003 1.33 0.072 0.41 .0.0091 0.0197 0.004. 0.063 0.012 0.91 0.084 0.49 0.0026 0.0196 0.003 uj41 0.065 0.006 1.48 0.109 0.21 0.0106 0.0109 0.0006 0.002 ~420.061 0.007 0.71 .0.077 0.46 0.0169 0.002 ~430.049 0.010 0.51 0.085 0.33 0.0206 0.003 ci44 0.070 0.010 1.05 0.070 0.33 10.0104 0.0158 0.002 0.098 0.009 1.34 0.056 0.41 10.0059 0.0074 0.0004 0.003 46 0.078 0.006 1.72 10.081 0.43 0.0051 0.0092 0.0006 0.003 47 0.028 0.013 0.70 10.078 0.43 0.0038 0,006810.0006 0.004 48 0.048 0.013 2.68 0.077 0.43 0.0081 0,0078 0.0004 0.002 49 0.069 0.013 1.35 0.074 0.46 0.0064 0.0081 0.0003 0.002 0.048 0.011 2.77 0.076 0.42 0.0073 0.0055 0.0006 0.002 51 0.064 0.013 1.10 0.074 0.43 0.0068 0.0057 0.0005 0.002 52 0.098 0.011 11.36 10.058 0.10 0.0010 10.008810.000510.003 1 53 0.110 0.021 11.52 10.060 0.15 10.0110 10.000210.004 1-

H

Table CHEMICAL COMPOSITION (wt RESULTS OF CALCULATION RANGE OF N CONTENT Seq.

No. Mg Te Bi Pb N ALLOWED BY EQ. (2) 31 0.0077 0.0053 to 0.0109 0.68 32 0.0216 0.0207 to 0.0263 0.66 33 0.0088 0.0079 to 0.0135 0.53 34 0.0118 0.0096 to 0.0152 0.60 0.0092 0.0050 to 0.0106 0.69 36 0.0203 0.0124 to 0.0180 0.66 37 0.0160 0.0014 to 0.0070 0.56 n 38 0.0073 0.0056 to 0.0112 0.60 39 0.0038 0.0096 to 0.0152 0.54 x 40 0.0167 0.0012 to 0.0068 0.53 S41 0.0119 0.0116 to 0.0172 0.36 42 0.0124 0.000 to 0.0034 0.46 M 43 0.0088 0.000 to 0.0034 0.33 8 44 0.0081 0.0113 to 0.0169 0.47 0.0077 0.0055 to 0.0111 0.49 46 0.0059 0.0044 to 0.0100 0.50 47 0.0074 0.0027 to 0.0083 0.48 48 0.0038 0.0083 to 0.0139 0.54 49 0.0025 0.0061 to 0.0117 0.55 0.0176 0.0073 to 0.0129 0.52 51 0.0290 0.0066 to 0.0122 0.53 52 0.0044 0.0000 to 0.0047 0.11 53 0.0020 0.0000 to 0.0034 0.15 Table 6

EVALUATION

SURFACE ROUGHNESS HOT DUCTILITY REMARKS MnS VL LONGI- No. DENSITY 1000 PLUNGE TUDINAL CHIP- REDUCTION ACCEP- CUTTING TURNING CONTROL OF AREA TANCE 4520 1120 S EXCEEDS UPPER LIMIT.

32 3401 125 9.1 11.1 A 59.3 B B EXCEEDS UPPER LIMIT.

33 1060 106 11.0 12.4 A 75.3 B Ca EXCEEDS UPPER LIMIT.

34 3190 121 9.6 11.6 A 68.3 B Ca ABS. 0 EXCEEDS UPPER LIMIT.

35 120 9.8 9.7 A 65.3 B Ca ABS. S, 0 EXCEED UPPER LIMITS.

36 489 119 9.8 8.6 A 44.3 B Ca ABS. S. 0. AND EQ. (2) 6 49 EXCEED UPPER LIMITS.

37 192 105 9.7 11.6 A 5.S, O, AND E. EXCEED UPPER LIMITS.

I I_ B IS LESS THAN LOWER LIMIT.

S AND Ca EXCEED UPPER LIMIT S. 38 2750 129 9.3 10.9 A 59.3 B EQ(2ISLSTHNOWRIM.

E IS LESS THAN LOWER LIMIT.

39 756 10 9 106 A 71.3 B Ca ABS. 0 EXCEED S UPPER LIMITS.

E Q. IS LESS THAN LOWER LIMIT.

3654 120 9.5 11.0 A 44.7 B Ca ABS. B IS LESS THAN LOWER LIMIT.

0 AND E 0. EXCEED UPPER LIMITS.

41 2 80 14.6 18.7 B 85.3 A S AND E ARE LESS THAN LOWER LIMITS.

42 579 109 11.9 17.9 A 45.6 B BCa ABS. 0 EXCEEDS UPPER LIMIT.

E 0. IS LESS THAN LOWER LIMIT.

43 126 100 1108 109 A 63 B B, Ca ABS. 0 EXCEEDS UPPER LIMIT.

I IS AND EO. ARE LESS THAN LOWER LIMITS.

44 6 100 16 109 B 70.1 B Ca ABS. 0 EXCEEDS UPPER LIMIT.

S AND EO. ARE LESS THAN LOWER LIMITS.

39 99 12.3 18.1 A 51.3 B EQ.(1) IS LESS THAN LOWER LIMIT.

46 11 110 12.1 17.9 A 59.3 B EQ.(1) IS LESS THAN LOWER LIMIT.

47 1985 95 12.1 17.7 A 68.3 B B AND EO.(1) ARE LESS THAN LOWER LIMITS.

48 31 125 12.0 13.0 A 73.0 B IS LESS THAN LOWER LIMIT.

49 2692 120 9.8 11.9 A 63.6 B IS LESS THAN LOWER LIMIT.

9 121 12.1 13.0 A 60.5 B EQ.(2) EXCEEDS UPPER LIMIT.

51 1003 118 10.3 12.0 A 49.7 B E. EXCEEDS UPPER LIMIT.

52 2 55 15.9 23.6 B 81.9 A S, B. AND EO.(1) ARE LESS THAN LOWER LIMITS.

80.5 B ABS. S AND EO.(1): LESS THAN LOWER LIMITS.

MnS density is determined by TEM, VL1000: tool life of a drill, A: Excellent, B: Not good, abs.: not added.

Table 7 CUTTING CONDITIONS DRILL REMARK Cutting speed 10-200m/min 5mm Depth of hole 15 mm Feed 0.33mm/rev NACHI NORMAL Tool life until breakage Lubricant Water-immiscible DRILL cutting fluid Extrusion 45 mm Machinabilities of the steel samples were evaluated based on cutting tests by three representative cutting methods; namely drilling test, plunge cutting test, and longitudinal cutting test. Tables 7, 8 and 9 show conditions for the drilling test, plunge cutting test, and longitudinal cutting test respectively.

The drilling test is a method for evaluating the machinability of steel based on the maximum cutting speed that allows drilling of he steel to the accumulated depth of 1000 mm (so called VL1000 expressed by unit of m/min).

Table 8 CUTTING CONDITIONS DRILL REMARK Cutting speed 80 m/min SKH51 equivalent Evaluation after 200 times Feed 0.05 mm/rev Rake angle 150 Until drill life.

Lubricant Water-immiscible Clearance angle 6° cutting fluid Table 9 CUTTING CONDITIONS DRILL REMARK Cutting speed 80 m/min Hard metal tool Evaluation after cutting of Feed 0.05 mm/rev corresponding to P10. 800 times.

Lubricant Water-immiscible Rake angle 100 cutting fluid Clearance angle The plunge cutting test is a method for evaluating surface roughness by transferring the tool morphology (morphology of constituent cutting edge) using a cut-off tool made of high-speed tool steel. The method of the plunge cutting test is schematically shown in FIG. 1. In the experiment, the surface roughness of the steel after 200 times of grooving was measured using a stylus surface roughness tester. Ten point surface roughness Rz (unit: m) was used as an indicator of the surface roughness of the finished-surface.

The longitudinal turning test is a method for cutting a periphery of the steel while feeding a hard metal tool in the longitudinal direction. As with the plunge cutting method, the surface roughness of the steel is evaluated by transferring the tool morphology.

The method of the longitudinal turning test is schematically shown in FIG. 2A.

In this method, the cutting operation advances while forming a crest called a feed mark on the surface of the steel. As shown in the explanatory drawing of FIG. 2B, the presence of tear greatly affects the height of the crest, that is, the surface roughness.

Without tear, the surface roughness is analogous to an ideal value. The surface roughness is reduced (deteriorated) under the presence of tear. Since densely o distributed fine sulfides mainly composed of MnS can homogenize the steel and thereby reduce tear and improve the surface roughness, the effect of the densely distributed Z sulfides mainly composed of MnS can be remarkably represented by the result of the IC longitudinal turning method. The wear of a tool after massive cutting causes irregularities of the tool morphology. The condition of the surface roughness caused by Cc a transfer of such irregularites can also be remarkably represented by the result of IC longitudinal turning. In the experimental conditions, the periphery of a test specimen 4 was cut to a depth of 1 mm using a hard metal tool 3. In order to evaluate a difference in machinability of steel samples under the progress of tool wear, the evaluation was made based on the surface roughness after cutting of 800 pieces. Surface roughness was measured using a using a stylus surface roughness tester, ten point surface roughness Rz (unit: pLm) was used as an indicator of the surface roughness of the finished-surface.

With regard to chip-controllability, it is preferable that chips show curling with a small radius or are parted from the steel. Where a sample shows such occurrence of chips, the case was evaluated as positive. Although a chip showed a large amount of curling, the sample was evaluated as positive if the curling had a small curvature radius. In addition, although a chip showed curling having a large curvature radius, the sample was evaluated as positive if the length of the chip was shorter than 100 mm. Where a chip had a curvature radius longer than 20 mm and had an amount of curvature not less than three, the sample was evaluated as negative.

In a section perpendicular to a rolling direction of the steel, the density of the sulfide particles of 0.1 to 0.5 gtm in a circle-equivalent diameter was not less than 10000 particles/mm 2 Measurement of the distribution density of fine sulfides mainly composed of MnS and having a dimension within a minimum diameter of O. 1 to a maximum diameter O of 0.5 in a circle equivalent diameter was performed as follows. After rolling of the samples into steel bars of (50mm, the wire rods were sectioned along the plane 0 Z perpendicular to the rolling direction. Specimens for evaluation were sampled from a C 1/4 diameter portion of each section by an extracting replica method and subjected to observation using a transmission electron microscope. At a magnification of 10000, not less than 40 fields respectively having a field-of-view of 80 pm 2 were subjected to I observation. From the results of observation, the number of sulfides mainly composed 8 of MnS per 1mm area was calculated.

High temperature ductility of steel was evaluated based on the reduction of area after the high temperature tensile test at 1000 0 C. Where a reduction of area was not less than 50%, rolling can be performed satisfactorily. However, where the reduction of area was less than 80%, numerous surface flaws were generated. In such a case, after the rolling, large area must be subjected to flaw depletion treatment. Therefore, the reduction of area of less than 80% cannot be allowed in a high grade steel requiring strict control of surface property. Where, the reduction of area was not less than generation of surface flaws was remarkably reduced and the steel could be used without surface treatment, and therefore classified as high grade. In addition, the cost of treatment can also be reduced. Therefore, a reduction of area of not less than was regarded to represent a high temperature ductility of positive grade, and a reduction of area of less than 80% was regarded to represent a high temperature ductility of negative grade.

Compared with Comparative Examples 31 to 53 shown in Tables 4, 5 and 6, Examples 1 to 30 shown in Tables 1, 2 and 3 had excellent tool life of a drill, and excellent surface roughness in cases of plunge cutting and longitudinal turning, and excellent high temperature ductility represented by a reduction of area of not less than For example, as shown in Examples 1 to 6 according to the first aspect of the 0 Z invention, by adding a well-balanced amount of B and N so as to control the N content C and by adding Ca so as to control the oxygen content, it was possible to obtain excellent machinability. As shown in Examples 7 to 14 according to the second aspect of the invention, where the distribution density of fine sulfides mainly composed of MnS was I satisfied, it was possible to further improve the surface roughness, especially the surface

\O

roughness in the case of longitudinal turning. Examples 15 to 30 with optional elements added in accordance with third to sixth aspects of the invention could also provide excellent surface roughness.

On the other hand, since Comparative Examples were respectively cast at a low level solidification cooling rate, fine sulfides mainly composed of MnS showed a small distribution density and inferior values of machinability and surface roughness.

Because the chemical compositions of Comparative Examples deviated from the range of the present invention, surface roughness of Comparative Examples were inferior compared with that of Examples 1 to 6 according to the invention. For example, where Ca was not added as in Comparative Example 34, it was impossible to control the oxygen content, and generation of numerous hard oxide grains resulted in a low value of high temperature ductility smaller than 80%. Comparative Example 36 shows a relatively satisfactory surface roughness since sulfur and boron content satisfy Seq. >0.52 in accordance with equation However, high temperature ductility of Comparative Example 36 shows extremely low value since Ca is not added, sulfur and oxygen content exceeds the upper limit in accordance with the invention, and N content relative to B content exceeds the upper limit of the above-described equation In Comparative Example 44 where Ca is not added and equation is not satisfied, high temperature O ductility and machinability showed inferior values compared with the invention. In Comparative Examples 48 and 49, where the N contents relative to the B contents were 0 Z lower than the lower limit of equation high temperature ductility showed low values C since an increase of dissolved B caused an increase of hardness. In Comparative Example 50 and 51, where the N contents relative to the B contents exceeded the upper limit of equation high temperatue ductility showed undesirable value because of ,I increase of an dissolved nitrogen.

FIG. 3A and FIG. 3B respectively show a photograph of sulfide mainly composed of MnS in accordance with the invention taken by transmission electron microscopy using the replica method (FIG. 3A) and by optical microscopy (FIG. 3B).

FIG. 4A and FIG. 4B respectively show a photograph of sulfide mainly composed of MnS in accordance with Comparative Example taken by transmission electron microscopy using the replica method (FIG. 4A) and by optical microscopy (FIG. 4B).

As shown in FIGS. 3B and 4B, in a field-of-view of an optical microscope, the dimensions and distribution density of sulfides mainly composed of MnS do not show a significant difference. However, as shown in FIGS. 3A and 4A, a remarkable difference in dimensions and distribution density of sulfides mainly composed of MnS can be observed in the field-of view by transmission electrom microscopy using the replica method.

FIG. 5 shows the difference of machinability in accordance with Seq. In FIG.

machinability was exemplified by the surface roughness after 800 times of cutting by longitudinal cutting. Since the progress in wear of a tool at the time of massive cutting is remarkable where Seq.<0.52, Seq.=0.52 provides a boundary condition for distinguishing inferiority and superiority of the surface roughness affected by transfer of irregularities caused by tool wear.

36 O FIG. 6 shows the balance between the surface roughness and high temperature ductility in the case of longitudinal cutting of Examples and Comparative Examples.

0 Z Examples of the invention show a satisfactory surface roughness and high-temperature ductility of not less than 80%. Comparative Examples belong to a grade in which both the surface roughness and high temperature ductility are within an unsatisfactory range or a grade having satisfactory high temperature ductility but having an unsatisfactory C surface roughness. Examples of the invention having well-balanced sulfur, boron, and nitrogen contents and further having a controlled oxygen content show satisfactory productivity and machinability.

While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, omissions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description, and is only limited by the scope of the appended claims.

Claims (4)

1. A free-cutting steel having excellent high-temperature ductility containing, in 0 mass SC: 0.005 to Si:0.001 to Mn: 0.3 to P: 0.001 to 0.2%, S: 0.40 to 0.50%, B: 0.005 to 0.015%, 0: 0.005 to 0.012%, Ca: 0.0001 to 0.001%, Al: not more than 0.01%, a predetermined amount of N, and a balance: Fe and unavoidable impurities, wherein amounts of S and B satisfy Seq.=S+14xB>0.52 0o amounts of B and N satisfy C I 1.3xB-0.0022 N 1.3xB+0.0034, and an amount of dissolved nitrogen contained in the steel is substantially same as a value determined by subtracting an amount of nitrogen forming BN from the total amount of nitrogen.

2. A free-cutting steel having excellent high-temperature ductility according to claim 1, wherein the steel includes sulfide particles mainly composed of MnS, and in a section perpendicular to a rolling direction of the steel, a density of the sulfide particles of 0.1 to 0.5 jtm in a circle-equivalent diameter is not less than 10000 particle/mm 2

3. A free-cutting steel having excellent high-temperature ductility according to claim 1 or 2, further containing at the expense of the balance, in mass one or more selected from V: 0.05 to Nb: 0.005 to Cr: 0.01 to Mo: 0.05 to

1338010-1HJG IO W: 0.05 to Ni: 0.05 to Cu 0.01 to

4. A free-cutting steel having excellent high-temperature ductility according to 0 any one of claims 1 to 3, further containing at the expense of the balance, in mass one or two selected from Sn: 0.005 to and Zn: 0.0005 to A free-cutting steel having excellent high-temperature ductility according to 0any one of claims 1 to 4, further containing at the expense of the balance, in mass one Sor more selected from CN Ti: 0.0005 to Zr: 0.0005 to Mg: 0.0003 to 0.005%. 0 o6. A free-cutting steel having excellent high-temperature ductility according to CN any one of claims 1 to 5, further containing the expense of the balance, in mass one or more selected from Te: 0.0003 to Bi: 0.005 to and Pb: 0.005 to 7. A free-cutting steel as claimed in claim 1 and substantially as hereinbefore is described with reference to any one of the examples and/or any one of the accompanying drawings. 8. A free-cutting steel according to any one of claims 1 to 7 when used in general machinery or automobiles. Dated 27 November, 2006 Nippon Steel Corporation Patent Attorneys for the Applicant/Nominated Person SPRUSON FERGUSON 585460-IHJG