S&F Ref: 868991D1 AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT Name and Address Nippon Steel Corporation, of 6-3, Otemachi 2-chome, of Applicant : Chiyoda-ku, Tokyo, 1008071, Japan Actual Inventor(s): Masayuki Hashimura Kenichiro Miyamoto Kei Miyanishi Atsushi Mizuno Address for Service: Spruson & Ferguson St Martins Tower Level 35 31 Market Street Sydney NSW 2000 (CCN 3710000177) Invention Title: Machine structural steel excellent in machinability and strength properties The following statement is a full description of this invention, including the best method of performing it known to me/us: 5845c(2547203_1):LMP DESCRIPTION MACHINE STRUCTURAL STEEL EXCELLENT IN MACHINABILITY AND STRENGTH PROPERTIES 5 FIELD OF THE INVENTION This invention relates to a machine structural steel that is to be machined and particularly to a machine structural steel excellent in machinability and strength 10 properties that is amenable to machining over a broad spectrum of machining speeds ranging from relatively low speed machining with a high-speed steel drill to relatively high speed machining such as longitudinal turning with a super-steel coated tool. 15 DESCRIPTION OF THE RELATED ART Although recent years have seen the development of steels of higher strength, there has concurrently emerged a problem of declining machinability. An increasing need is therefore felt for the development of steels that 20 maintain excellent strength without experiencing a decline in machining performance. Addition of machinability-enhancing elements such as S, Pb and Bi is known to be effective for improving steel machinability. However, while Pb and Bi are known to improve 25 machinability and to have relatively little effect on forgeability, they are also known to degrade strength properties. Moreover, Pb is being used in smaller quantities these days owing to the tendency to avoid use because of 30 concern about the load Pb puts on the natural environment. S improves machinability by forming inclusions, such as MnS, that soften in a machining environment, but MnS grains are larger than the those of Pb and the like, so that it readily becomes a stress 35 concentration raiser. Of particular note is that at the time of elongation by forging or rolling, MnS produces anisotropy, which makes the steel extremely weak in a - 2 particular direction. It also becomes necessary to take such anisotropy into account during steel design. When S is added, therefore, it becomes necessary to utilize a technique for reducing the anisotropy. 5 As pointed out in the foregoing, it has been difficult to achieve good strength properties and good machinability simultaneously because addition of machinability-enhancing elements degrades the strength properties. Further technological innovation is therefore 10 needed to enable simultaneous realization of satisfactory steel machinability and strength properties. This situation has led to efforts to provide a machine structural steel enabling prolongation of machine tool life by, for example, incorporating a total of 0.005 15 mass% or greater of at least one member selected from among solute V, solute Nb and solute Al, and further incorporating 0.001% or greater of solute N, thereby enabling nitrides formed by machining heat during machining to adhere to the tool to function as a tool 20 protective coating (see Japanese Patent Publication (A) No. 2004-107787). In addition, there has been proposed a machine structural steel that achieve improved shavings disposal and mechanical properties by defining C, Si, Mn, S and Mg contents, defining the ratio of Mg content to S 25 content, and optimizing the aspect ratio and number of sulfide inclusions in the steel (see Japanese Patent No. 3706560). The machine structural steel taught by Patent No. 3706560 defines the content of Mg as 0.02% or less (not including 0%) and the content of Al, when included, 30 as 0.1% or less. SUMMARY OF THE INVENTION However, the foregoing existing technologies have the following drawbacks. The steel taught by Japanese 35 Patent Publication (A) No. 2004-107787 is liable not to give rise to the aforesaid phenomenon unless the amount of heat produced by the machining exceeds a certain 3 level. The machining speed must thereof be somewhat high to realize the desired effect, so the invention has a problem in the point that the effect cannot be anticipated in the low speed range. Japanese Patent No. 3706560 is totally silent regarding the strength properties of the steel it teaches. Moreover, the steel of this patent is incapable of 5 achieving adequate strength properties because it gives no consideration to machine tool life of yield ratio. The present invention was achieved in light of the foregoing problems and has as its object to provide a machine structural steel that has good machinability over a broad range of machining speeds and also has high impact properties and high yield ratio. 10 The machine structural steel excellent in machinability and strength properties according to the present invention consists essentially of, in mass%, C: 0.1 to 0.85%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P: 0.005 to 0.2%, S: 0.001 to 0.15%, total Al: 0.080% to 0.3%, total N: 0.0035 to 0.020%, solute N: 0.0009 to 0.0020%, and a balance of Fe and unavoidable impurities. is The machine structural steel can further consist essentially of, in mass%, ca: 0.0003 to 0.0015%. The machine structural steel can further consist essentially of, in mass%, one or more elements selected from the group consisting of Ti: 0.001 to 0.1%, Nb: 0.005 to 0.2%, W: 0.01 to 1.0%, and V: 0.01 to 1.0%. 20 The machine structural steel can further consist essentially of, in mass%, one or more elements selected from the group consisting of Mg: 0.0001 to 0.0040%, Zr: 0.0003 to 0.01%, and REMs: 0.0001 to 0.015%. The machine structural steel can further consist essentially of, in mass%, one or more elements selected from the group consisting of Sn: 0.005 to 2.0%, 25 Zn: 0.0005 to 0.05%, B: 0.0005 to 0.015%, Te: 0.0003 to 0.2%, Bi: 0.005 to 0.5%, 4 and Pb: 0.005 to 0.5%. The machine structural steel can further consist essentially of, in mass%, one or two elements selected from the group consisting of Cr: 0.01 to 2.0% and Mo: 0.01 to 1.0%. 5 The machine structural steel can further consist essentially of, in mass%, one or two elements selected from the group consisting of Ni: 0.05 to 2.0% and Cu: 0.01 to 2.0%. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a region from which a Charpy impact test piece 10 was cut. DETAILED DESCRIPTION OF THE INVENTION Preferred embodiments of the present invention are explained in detail in the following. The machine structural steel excellent in machinability and strength properties according to the present invention achieves the foregoing object by providing a machine is structural steel wherein solute N acting to degrade machinability and impact properties is minimized by adjusting the amounts of added N and nitride-forming elements such as Al, wherein effective cutting performance is established with respect to a broad cutting speed range extending from low to high speed by ensuring presence of suitable amounts of solute Al serving to improve high-temperature embrittlement property and machinability, 20 and Sb serving to produce a matrix embrittlement effect, and forming a crystal structure exhibiting high-temperature embrittlement effect and cleavage, thereby ensuring an appropriate amount of AIN serving to improve machinability, and wherein high impact properties are also realized by increasing Al addition so that at the slab stage segregation is made smaller and MnS of highly uniform dispersibility (type III MnS by SIMS 25 analysis) is made more abundant than in conventional Al -killed steel. Moreover, the steel further achieves a high yield ratio owing to fine precipitation of AIN and presence of solute -5 Al. Specifically, the machine structural steel according to the present invention comprises, in mass%, C: 0.1 to 0.85%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P: 0.005 to 5 0.2%, S: 0.001 to 0.15%, total Al: greater than 0.05% and not greater than 0.3%, Sb: less than 0.0150% (including 0%), and total N: 0.0035 to 0.020%, solute N being limited to 0.0020% or less, and a balance of Fe and unavoidable impurities. 10 The individual elements constituting the machine structural steel of the present invention and the contents thereof of will first be explained. In the ensuing explanation, percentage composition by mass of the steel components is denoted simply by the symbol %. 15 C: 0.1 to 0.85% C has a major effect on the fundamental strength of the steel. When the C content is less than 0.1%, adequate strength cannot be achieved, so that large amounts of other alloying elements must be incorporated. When C 20 content exceeds 0.85%, machinability declines markedly because carbon concentration becomes nearly hypereutectoid to produce heavy precipitation of hard carbides. In order to achieve sufficient strength, the present invention therefore defines C content as 0.1 to 25 0.85%. The preferred lower limit of C content is 0.2%. Si: 0.01 to 1.5% Si is generally added as a deoxidizing element but also contributes to ferrite strengthening and temper softening resistance. When Si content is less than 0.01%, 30 the deoxidizing effect is insufficient. On the other hand, an Si content in excess of 1.5% degrades the steel's embrittlement and other properties and also impairs machinability. Si content is therefore defined as 0.01 to 1.5%. The preferred upper limit of Si content is 35 1.0%. Mn: 0.05 to 2.0% Mn is required for its ability to fix and disperse - 6 sulfur (S) in the steel in the form of MnS and also, by dissolving into the matrix, to improve hardenability and ensure good strength after quenching. When Mn content is less than 0.05%, the steel is embrittled because S 5 therein combines with Fe to form FeS. When Mn content is high, specifically when it exceeds 2.0%, base metal hardness increases to degrade cold workability, while its strength and hardenability improving effects saturates. Mn content is therefore defined as 0.05 to 2.0%. 10 P: 0.005 to 0.2% P has a favorable effect on machinability but the effect is not obtained at a P content of less than 0.005%. When P content is high, specifically when it exceeds 0.2%, base metal hardness increases to degrade 15 not only cold workability but also hot workability and casting properties. P content is therefore defined as 0.005 to 0.2%, S: 0.001 to 0.15% S combines with Mn to produce MnS that is present in 20 the steel in the form of inclusions. MnS improves machinability but S must be added to a content of 0.001% or greater for achieving this effect to a substantial degree. When S content exceeds 0.15%, the impact value of the steel declines markedly. In the case of adding S to 25 improve machinability, therefore, the S content is made 0.001 to 0.15%. Total Al: greater than 0.05% and not greater than 0.3% Al not only forms oxides but also promotes precipitation of AlN, which contributes to grain size 30 control and machinability, and further improves machinability by passing into solid solution. Al must be added to a content of greater than 0.05% in order to form solute Al in an amount sufficient to enhance machinability. Al also affects the form of MnS 35 grains/precipitation. Moreover, when Al is added in an amount exceeding 0.05%, segregation at the slab stage can be made smaller and MnS of highly uniform dispersibility (type III MnS by SIMS analysis) be made more abundant than in a conventional Al-killed steel. This makes it possible to obtain a machine structural steel also having high impact properties and further to achieve a high 5 yield ratio owing to fine precipitation of AlN and the presence of solute Al. However, machinability starts to decline when total Al content exceeds 0.3%. Total Al content is therefore defined as greater than 0.05% and not greater than 0.3%. The lower limit of total Al 10 content is preferably 0.08% and more preferably 0.1%. Sb: less than 0.0150% (including 0%) Sb improves machinability by suitably embrittling ferrite. This effect of Sb is pronounced particularly when solute Al content is high but is not observed when 15 Sb content is less than 0.0005%. When Sb content is high, specifically when it reaches 0.0150% or greater, Sb macro-segregation becomes excessive, so that the impact value of the steel declines markedly. Sb content is therefore defined as 0.0005% or greater and less than 20 0.0150%. When high machinability is not required or total Al is greater than 0.1%, addition of Sb can be omitted (Sb content of 0%). Total N: 0.0035 to 0.020% N, which is present not only as solute N but also in 25 nitrides of Ti, Al V and the like, suppresses austenite grain growth. However, no substantial effect is obtained at a total N content of less than 0.0035%. When total N content exceeds 0.020%, it leads to the occurrence of roll marks during rolling. Total N content is therefore 30 defined as 0.0035 to 0.020%. Solute N: 0.0020% or less Solute N hardens the steel. Of particular concern is that it shortens cutting tool life by causing steel near the cutting edge to harden under dynamic strain aging. It 35 also causes occurrence of roll marks during rolling. High solute N content, specifically a content in excess of 0.0020%, aggravates tool wear during cutting because -8 cutting resistance rises due to increased local hardness. Solute N content is therefore held to 0.0020% or less. This helps to reduce tool wear. Moreover, high solute N content also degrades impact properties by causing matrix 5 embrittlement, but such matrix embrittlement can also be mitigated by holding solute N content to 0.0020% or less. Solute N content as termed here means the value obtained by subtracting the N content of AlN, NbN, TiN, VN and other such nitrides from total N content. It can be 10 calculated, for example, in accordance with Equation (1) shown below, using the total N content determined by the inert gas fusion thermal conductivity method and the N content of nitrides determined by SPEED (Selective Potentiostatic Etching by Electrolytic Dissolution) 15 analysis and indophenol absorbency analysis of residue electrolytically extracted using a 0.1 pm filter. (Solute N content) = (Total N content) - (Nitride N content) ... (1) Solute N content can be lowered by the methods 20 explained below: 1) Hold total N content to a low level within the range defined by the present invention. Although total N is defined as 0.020% or less, it is preferably held to 0.01% or less and more preferably to 0.006% or less. 25 2) When total N content is high, it is helpful to increase the amount of N compounds by adding suitable amounts of Al, a nitride-forming element, as well as other nitride-forming elements. 3) Solute N reduction by fine precipitation of nitrides 30 is preferable in a machine structural steel from the viewpoint of inhibiting grain coarsening. Taking into account that reduction of solute N content by fine precipitation of nitrides requires holding at a high temperature enabling more complete solution treatment 35 into N and nitride-forming element content, solution heat treatment is conducted at a temperature of 1100 *C or -9 greater, preferably 1200 *C or greater, and more preferably 1250 *C or greater, whereafter precipitation is performed by conducting a heat treatment such as normalizing or carburization. Of particular note is that 5 in the case of AlN, solute N can be reduced by utilizing prolonged retention near 850 *C to increase precipitation. By "prolonged" here is meant 0.8 hr or greater, preferably 1 hr or greater and more preferably 1.2 hr or greater. 10 The machine structural steel of the present invention can contain Ca in addition to the foregoing components. Ca: 0.0003 to 0.0015% Ca is a deoxidizing element that forms oxides in the 15 steel. In the machine structural steel of the present invention, which has a total Al content of greater than 0.05% and not greater than 0.3%, Ca forms calcium aluminate (CaOAl 2 0 3 ) . As CaOAl 2 0 3 is an oxide having a lower melting point than A1 2 0 3 , it improves machinability 20 by constituting a tool protective film during high-speed cutting However, this machinability-improving effect is not observed when the Ca content is less than 0.0003%. When Ca content exceeds 0.0015%, CaS forms in the steel, so that machinability is instead degraded. Therefore, 25 when Ca is added, its content is defined as 0.0003 to 0.0015%. When the machine structural steel of the present invention needs to be given high strength by forming carbides, it can include in addition to the foregoing 30 components one or more elements selected from the group consisting of Ti: 0.001 to 0.1%, Nb: 0.005 to 0.2%, W: 0.01 to 1.0%, and V: 0.01 to 1.0%. Ti: 0.001 to 0.1% Ti forms. carbonitrides that inhibit..aus-tenite grain_ 35 growth and contribute to strengthening. It is used as a grain size control element for preventing grain - 10 coarsening in steels requiring high strength and steels requiring low distortion. Ti is also a deoxidizing element that improves machinability by forming soft oxides. However, these effects of Ti are not observed at 5 a content of less than 0.001%, and when the content exceeds 0.1%, Ti has the contrary effect of degrading mechanical properties by causing precipitation of insoluble coarse carbonitrides that cause hot cracking. Therefore, when Ti is added, its content is defined as 10 0.001 to 0.1%. Nb: 0.005 to 0.2% Nb also forms carbonitrides. As such, it is an element that contributes to steel strength through secondary precipitation hardening and to austenite grain 15 growth inhibition and strengthening. Ti is therefore used as a grain size control element for preventing grain coarsening in steels requiring high strength and steels requiring low distortion. However, no high strength imparting effect is observed at an Nb content of less 20 than 0.005%, and when Nb is added to a content exceeding 0.2%, it has the contrary effect of degrading mechanical properties by causing precipitation of insoluble coarse carbonitrides that cause hot cracking. Therefore, when Nb is added, its content is defined as 0.005 to 0.2%. 25 W: 0.01 to 1.0% W is also an element that forms carbonitrides and can strengthen the steel through secondary precipitation hardening. However, no high strength imparting effect is observed when W content is less than 0.01%, Addition of W 30 in excess of 1.0% has the contrary effect of degrading mechanical properties by causing precipitation of insoluble coarse carbonitrides that cause hot cracking. Therefore, when W is added, its content is defined as 0.01 to 1.0%. 35 V: 0.01 to 1.0%. V is also an element that forms carbonitrides and can strengthen the steel through secondary precipitation - 11 hardening. It is suitably added to steels requiring high strength. However, no high strength imparting effect is observed when V content is less than 0.01%, Addition of V in excess of 1.0% has the contrary effect of degrading 5 mechanical properties by causing precipitation of insoluble coarse carbonitrides that cause hot cracking. Therefore, when V is added, its content is defined as 0.01 to 1.0%. When the machine structural steel of the present 10 invention is subjected to deoxidization control for controlling sulfide morphology, it can comprise in addition to the foregoing components one or more elements selected from the group consisting of Mg: 0.0001 to 0.0040%, Zr: 0.0003 to 0.01%, and REMs: 0.0001 to 0.015%. 15 Mg: 0.0001 to 0.0040% Mg is a deoxidizing element that forms oxides in the steel. When Al deoxidization is adopted, Mg reforms A1 2 0 3 , which impairs machinability, into relatively soft and finely dispersed MgO and A1 2 0 3 -Mg. Moreover, its oxide 20 readily acts as a precipitation nucleus of MnS and thus works to finely disperse MnS. However, these effects are not observed at an Mg content of less than 0.0001%. Moreover, while Mg acts to make MnS spherical by forming a metal-sulfide complex therewith, excessive Mg addition, 25 specifically addition to a content of greater than .0.0040%, degrades machinability by promoting simple MgS formation. Therefore, when Mg is added, its content is defined as to 0.0001 to 0.0040%. Zr: 0.0003 to 0.01%. 30 Zr is a deoxidizing element that forms an oxide in the steel. The oxide is thought to be ZrO 2 , which acts as a precipitation nucleus for MnS. Since addition of Zr therefore increases the number of MnS precipitation sites, it has the effect of uniformly dispersing MnS. 35 Moreover, Zr dissolves into MnS to form a metal-sulfide complex therewith, thus decreasing MnS deformation, and therefore also works to inhibit MnS grain elongation - 12 during rolling and hot forging. In this manner, Zr effectively reduces anisotropy. But no substantial effect in these respects is observed at a Zr content of less than 0.0003%. On the other hand, addition of Zr in excess 5 of 0.01% radically degrades yield. Moreover, by causing formation of large quantities of ZrO 2 , ZrS and other hard compounds, it has the contrary effect of degrading mechanical properties such as machinability, impact value, fatigue properties and the like. Therefore, when 10 Zr is added, its content is defined as to 0.0003 to 0.01%.. REMs: 0.0001 to 0.015% REMs (rare earth metals) are deoxidizing elements that form low-melting-point oxides that help to prevent 15 nozzle clogging during casting and also dissolve into or combine with MnS to decrease MnS deformation, thereby acting to inhibit MnS shape elongation during rolling and hot forging. REMs thus serve to reduce anisotropy. However, this effect does not appear at an REM content of 20 less than 0.0001%. When the content exceeds 0.015%, machinability is degraded owing to the formation of large amounts of REM sulfides. Therefore, when REMs are added, their content is defined as 0.0001 to 0.015%. When the machine structural steel of the present 25 invention is to be improved in machinability, it can include in addition to the foregoing components one or more elements selected from the group consisting of Sn: 0.005 to 2.0%, Zn: 0.0005 to 0.5%, B: 0.0005 to 0.015%, Te: 0.0003 to 0.2%, Bi: 0.005 to 0.5%, and Pb: 0.005 to 30 0.5%. Sn: 0.005 to 2.0% Sn extends tool life by embrittling ferrite and also improves surface roughness. These effects are not observed when the Sn content is less than 0.005%, and the 35 effects saturate when Sn is added in excess of 2.0%. Therefore, when Sn is added, its content is defined as 0.005 to 2.0%.
- 13 Zn: 0.0005 to 0.5% Zn extends tool life by embrittling ferrite and also improves surface roughness. These effects are not observed when the Zn content is less than 0.0005%, and 5 the effects saturate when Zn is added in excess of 0.5%. Therefore, when Zn is added, its content is defined as 0.0005 to 0.5%. B: 0.0005 to 0.015% B, when in solid solution, has a favorable effect on 10 grain boundary strength and hardenability. When it precipitates, it precipitates as BN and therefore helps to improve machinability. These effects are not notable at a B content of less than 0.0005%. When B is added to a content of greater than 0.015%, the effects saturate and 15 mechanical properties are to the contrary degraded owing to excessive precipitation of BN. Therefore, when B is added, its content is defined as 0.0005 to 0.015%. Te: 0.0003 to 0.2% Te improves machinability. It also forms MnTe and, 20 when co-present with MnS, decreases MnS deformation, thereby acting to inhibit MnS shape elongation. Te is thus an element effective for reducing anisotropy. These effects are not observed when Te content is less than 0.0003%, and when the content thereof exceeds 0.2%, the 25 effects saturate and hot rolling ductility declines, increasing the likelihood of flaws. Therefore, when Te is added, its content is defined as: 0.0003 to 0.2%. Bi: 0.005 to 0.5% Bi improves machinability. This effect is not 30 observed when Bi content is less than 0.005%. When it exceeds 0.5%, machinability improvement saturates and hot rolling ductility declines, increasing the likelihood of flaws. Therefore, when Bi is added, its content is defined as 0.005 to 0.5%. 35 Pb: 0.005 to 0.5% Pb improves machinability. This effect is not observed when Pb content is less than 0.005%. When it - 14 exceeds 0.5%, machinability improvement saturates and hot rolling ductility declines, increasing the likelihood of flaws. Therefore, when Pb is added, its content is defined as 0.005 to 0.5%. 5 When the machine structural steel of the present invention is to be imparted with strength by improving its hardenability and/or temper-softening resistance, it can include in addition to the foregoing components one or two elements selected from the group consisting of Cr: 10 0.01 to 2.0% and Mo: 0.01 to 1.0%. Cr: 0.01 to 2.0% Cr improves hardenability and also imparts temper softening resistance. It is therefore added to a steel requiring high strength. These effects are not obtained 15 at a Cr content of less than 0.01%. When Cr content is high, specifically when it exceeds 2.0%, the steel is embrittled owing to formation of Cr carbides. Therefore, when Cr is added, its content is defined as 0.01 to 2.0%. Mo: 0.01 to 1.0% 20 Mo imparts temper-softening resistance and also improves hardenability. It is therefore added to a steel requiring high strength. These effects are not obtained at an Mo content of less than 0.01%. When Mo is added in excess of 1.0%, its effects saturate. Therefore, when Mo 25 is added, its content is defined as 0.01 to 1.0%. When the- machine structural steel of the present invention is to be subjected to ferrite strengthening, it can include in addition to the foregoing components one or two elements selected from the group consisting of Ni: 30 0.05 to 2.0% and Cu: 0.01 to 2.0%. Ni: 0.05 to 2.0% Ni strengthens ferrite, thereby improving ductility, and is also effective for hardenability improvement and anticorrosion improvement. These effects are not observed 35 an Ni content of less than 0.05%. When Ni is added in excess of 2.0%, mechanical property improving effect saturates and machinability is degraded. Therefore, when - 15 Ni is added, its content is defined as 0.05 to 2.0%. Cu: 0.01 to 2.0% Cu strengthens ferrite and is also effective for hardenability improvement and anticorrosion improvement. 5 These effects are not observed a Cui content of less than 0.01%. When Cu is added in excess of 2.0%, mechanical property improving effect saturates. Therefore, when Cu is added, its content is defined as 0.01 to 2.0%. A particular concern regarding Cu is that its effect of 10 lowering hot rollability may lead to occurrence of flaws during rolling. Cu is therefore preferably added simultaneously with Ni. As explained in the foregoing, the machine structural steel of the present invention is minimized in 15 solute N content and therefore achieves better machinability and impact properties than conventional machine structural steels. Moreover, total Al content and Sb content are controlled to suitable levels to ensure presence of proper amounts of solute Al, Sb and AlN 20 serving to improve machinability, thereby establishing effective cutting performance with respect to a broad cutting speed range extending from low to high speed. The steel also achieves a high yield ratio owing to fine precipitation of the AlN and presence of solute Al. In 25 addition, excellent impact properties are realized by appropriately regulating the contents of elements affecting MnS precipitation so as to obtain an abundance of MnS of highly uniform dispersibility. The machine structural steel excellent in 30 machinability and strength properties according to the present invention can be produced by hot-forging a billet having the aforesaid steel composition into a bar at a temperature of 1200 *C or greater, subjecting the bar to solution heat treatment at a temperature of 1100 'C or 35 greater, and then to a heat treatment such as normalizing or carburization. Of particular note is that in the case of a steel containing the carbide AlN, a machine - 16 structural steel markedly reduced in solute N can be obtained by prolonged retention following the solution heat treatment at 1100 *C or greater for 0.8 hr or greater, preferably 1 hr or greater, and more preferably 5 1.2 hr or greater. EXAMPLES First Set of Examples The effects of the present invention will now be 10 specifically explained giving Examples and Comparative Examples. In this set of Examples, steels of the compositions shown in Table 1 and Table 2, 150 kg each, were produced in a vacuum furnace, hot-forged under a temperature condition of 1250 *C, and elongation-forged 15 into 65-mm diameter bars. The properties of the Example and Comparative Example steels were evaluated by subjecting them to machinability testing, Charpy impact testing and tensile testing by the methods set out below. In Table 2, underlining indicates a value outside the 20 invention range.
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" 0D 0 0) 0 0 3 0 L LA 0 0 l In "I I- n m c 0 c: -I0 %D -4 'm 0 LA Hl 'D r-0 CD co (N c4 M r 0 LA LA o 0 n kD C vk H H4 0 .- 4 0D -I 0) HI 0D 0 H 0 0 0 0 0D 0: 0 M~ .-4 -1 0) H (N 0D ~-W 5 , 5 D 5000000000 0 0 0 0c; 0 0 0 0* 0 0 0) 0 00 M CA I- No I- (N 00 % D 0 N CD OD No (N LA Nn No n LA H I CD CA CD N- CDLA CD %D'0 r- MD' C ID 0D L ' 0 L N L A N- (N .00 0 0 0 0 0 0 0 0 0c0 0 0Z 0 H4 H, (i (N (2 0000000000000 0 0> 0 0) 0 0 0 ) 0 5 c 0 000000000000 0 0 0)0C)C 0 0 00 0 0 0) 0.c >c))C U H H H H Ln A CD w' LA) CN LA 0) co N LA (NI to H LA (N v4 o4 H N- CD %o -K LA LA (N4 m~ c,) LA "N (N c,4 -w r ri -y (4 -; (N LA LA (N4 m~ Pn (1 " 0 0000a(D0 0 0000)D0 0 0cD 0 0 0 0D 0 0 0> 0 0 -I-4---4 4 -4---4-4 H-4 H H H -4 H- H H H H 1wQ - C 000000 0000 0 0 0 0 0T 04 0n 0 0 0 ,' 000000.0000000 0 03 0 0 0 0C 00 N; C I N; ID c N; c; C D CD- N c; '0 P2 CD 8NN ( ' C 0 P r- N NW N r N - N N- N N- CD N No N NNNNN 0) H .4 LAL CD C) CD H C 0 LA C, LA C) 0 C) C C CD 00 00 0 00 0 H 0- nV 7 ,C r 4O 0 0W 0 04 0 0r 0 0 0 .1 LA 04 (qN '4 H _4 (N I - 2~ L 2 - 4 ( A~ L (DC)( 0>( H( P~ A0C) C O ( P C) LA a0 N) C ) )C D 0 0- (NP2 C>C V4 UZ _ n -W C14 LA (nAfnr a) Ex0pe Coprtv Examples D -4CN(n1 - 19 Machinability test Machinability testing was conducted with respect to Example and Comparative Example steels that had been elongation-forged under heating at 1250 *C by first 5 subjecting them to heat treatment consisting of normalization under temperature condition of 850 *C for 1 hr, 0.5 hr in the case of Comparative Examples No. 49 and No. 50, followed by air-cooling. A machinability evaluation test piece was then cut from each heat-treated 10 steel and the machinabilities of the Example and Comparative Example steels were evaluated by conducting drill boring testing under the cutting conditions shown in Table 3 and to longitudinal turning testing under the conditions shown in Table 4. The maximum cutting speed 15 VL1000 enabling cutting up to a cumulative hole depth of 1000 mm was used as the evaluation index in the drill boring test, and the maximum width VBmax of wear of the relief flank after 10 min was used as the evaluation index in the longitudinal turning test. 20 Table 3 Speed: 10 - 120 m/min Cutting Feed: 0.25 mm/rev conditions Cutting fluid: Water-soluble cutting oil Drill diameter: 3 mm Drill NACHI ordinary drill Overhang: 45 mm Other Hole depth: 9 mm Tool life: Until breakage - 20 Table 4 Cutting speed: 250 m/min Cutting Feed: 0.3 mm/rev conditions Depth of cut: 1.5 mm Dry cutting Holder: PTGNR2525M16 Tool Tool shape: TNMG160408N UZ Material: AC2000 Charpy impact test FIG. 1 is a diagram showing a region from which a 5 Charpy impact test piece was cut. In the Charpy impact test, first, as shown in FIG. 1, a cylinder 2 measuring 25 mm in diameter was cut from a steel 1 heat-treated by the same method and under the same conditions as the aforesaid machinability test piece so that its axis was 10 perpendicular to the elongation-forging direction of the steel 1. Next, the cylinder 2 was held under temperature condition of 850 *C for 1 hr, 0.5 hr in the case of Comparative Examples No. 49 and No. 50, then oil-quenched by cooling to 60 *C, and further subjected to tempering 15 with water cooling in which it was held under temperature condition of 550 *C for 30 min. Next, the cylinder 2 was machined to fabricate a Charpy test piece 3 in conformance with JIS Z 2202, which was subjected to a Charpy impact test at room temperature in accordance with 20 the method prescribed by JIS Z 2242. Absorbed energy per unit area (J/cm 2 ) was adopted as the evaluation index. Tensile test A cylinder 2 sampled parallel to the elongation forging direction was oil-quenched and tempered by the 25 same methods and under the same conditions as in the aforesaid Charpy impact test, whereafter it was processed into a tensile test piece measuring 8 mm in parallel section diameter and 30 mm in parallel section length, and then tensile tested at room temperature in accordance 30 with the method prescribed by JIS Z 2241. Yield ratio - 21 (= (0.2% proof stress YP) / (tensile strength TS) was adopted as the evaluation index. The results of the foregoing tests are shown in Tables 5 and 6.
- 22 Table 5 No. VL1000 VBmax Impact YP / TS (m/min) (P.m) value (J/cm 2 _ 1 87 118 27 0.85 2 89 119 22 0.83 3 85 124 20 0.89 4 88 126 25 0.82 5 85 125 20 0.84 6 89 123 21 0.86 7 89 128 22 0.83 8 92 129 22 0.86 9 89 126 20 0.86 10 92 122 21 0.82 11 91 121 21 0.83 12 87 130 22 0.84 13 90 127 22 0.82 14 90 125 21 0.84 15 90 125 24 0.86 16 85 121 26 0.87 17 93 128 24 0.86 18 86 124 22 0.85 19 90 128 25 0.86 x 20 89 126 20 0.82 21 89 121 24 0.84 ' 22 96 125 21 0.82 23 93 129 22 0.83 24 94 126 23 0.82 25 82 126 26 0.82 26 86 120 25 0.84 27 89 130 26 0.85 28 86 127 20 0.86 29 83 129 20 0.84 30 86 122 24 0.86 31 95 129 26 0.86 32 89 130 22 0.83 33 89 123 21 0.87 34 90 126 24 0.85 35 94 121 22 0.82 36 83 127 20 0.85 37 83 121 20 0.85 38 82 127 30 0.83 39 93 127 21 0.83 40 90 124 27 0.86 41 89 127 23 0.84 42 91 126 21 0.87 - 23 Table 6 No. VL1000 VB max Impact YP / TS (m/min) (Pm) value (J / cm 2 ) 43 54 149 21 0.68 n 44 62 141 24 0.66 1 45 60 142 25 0.67 46 53 158 22 0.68 47 64 178 9 0.65 48 48 178 22 0.82 D49 62 149 16 0.83 50 69 150 15 0.85 51 99 122 14 0.85 2, 52 98 124 11 0.87 3 53 104 128 12 0.84 54 100 128 13 0.85 The Steels No.1 to No. 42 shown in Tables 1, 2 and 5 are Examples of the present invention, and the steels No. 5 43 to No. 51 shown in Tables 2 and 6 are Comparative Example steels. As shown in Tables 5 and 6, the steels of Examples No 1 to No. 42 exhibited good values for all of the evaluation indexes, namely VL1000, VB max, impact value (absorbed energy), and YP / TS (yield ratio), but 10 the steels of the Comparative Examples were each inferior to the Example steels in at least one of the properties. Specifically, the steels of Comparative Examples No. 43 to No. 46 had total Al contents below the range of the present invention and were therefore inferior to the 15 Example steels in machinability evaluation index VL1000 and yield ratio (YP / TS). Moreover, the steel of Comparative Example No.47 had a total Al content far below the range of the present invention, so that its solute N content was above the range of the present 20 invention, and the steel was therefore inferior to the steels of the Examples in machinability (VL1000, VBmax), impact value, and yield ratio (YP / TS). The steel of Comparative Example No. 48 had a total Al content above the range of the present invention, so 25 that its hardness increased, and the steel was therefore - 24 inferior in machinability (VL1000, VBmax). The steels of Comparative Examples No. 49 and No. 50 were maintained at 850 *C, the temperature at which AlN readily precipitates, for a shorter holding time than the steels of the 5 Examples, so that their solute N contents were above the range of the present invention, and the steels were therefore inferior to the steels of the Examples in machinability (VL1000, VBmax) and impact value. The steels of Comparative Examples No. 51 to No. 54 had Sb 10 contents above the range of the present invention and were therefore inferior to the steels of the Examples in impact value. Second Set of Examples In this set of Examples, steels of the compositions 15 shown in Table 7 and Table 8, 150 kg each, were produced in a vacuum furnace, hot-forged under a temperature condition of 1250 *C, and elongation-forged into 65 mm diameter bars. The properties of the Example and Comparative Example steels were evaluated by subjecting 20 them to machinability testing, Charpy impact testing and tensile testing by the methods set out below. In Tables 7 and 8, underlining indicates a value outside the invention range.
- 25 Table 7 No ______com position massss) ______________ C Si IMn P IS ICr Ca ITt1 Al Ttl N SoliN Other 1 0.44 0.25 0.76 0.01510.017 - 0.0000 0.121 0.0052 0.0012 2 0.44 0.26 0.76 0.0150.012 - 0.0006 0.101 0. 00520.0012 3 0.44 0.25 0.75 0.016 0.010 - 0.0008 0.250 0.0060 0.0013 4 0.44 0.25 0.76 0.015 0.008 - 0.0010 0.075 0.0045 0.0011 5 0.46 0.26 0.76 0.015 0.013 - 0.0006 0.099 0.0049 0.002Mg: 0.0020 6 0.44 0.24 0.74 0.015 0.011 - 0.0008 0.193 0.0065 .0013Ti:0.04 7 0.45 0.25 0.74 0.015 0.013 - 0.0008 0.178 0.0075 0.006 Nb:0.02 8 10.44 0.24 0.74 0.015 0.011 - 0.0006 0.169 0.0058 0.0014 W:0.2 9 0.45 0.24 0.74 0.016 0.010 - 0.0012 0.175 0.0055 0.0013Ni:0.2 10 0.46 0.26 0.76 0.01510.014 - 0.0005 0.142 0.0045 0.7Cu:0.5 11 0.44 0.26 0.75 0.01510.015 - 0.0007 0.127 0.0052 0.0010 Sn: 0.05 12 0.44 0.25 0.76 0.015 0.011 0.0004 0.147 0.0051 0.0014 Zn:0.007 130.45 0.24 0.76 0.01410.011 - 0.0012 0.144 0.00530.0019B:0.002 14 0.45 0.26 0.75 0.01510.011 - 0.0012 0.187 0.0046 0.O005Te:0.002 15.0.41 0.24 0.78 0.015 0.014 1.0 0.0010 0.108 0.0047 .0016 16 0.44 0.25 0.76 0.01510.013 0.1 0.0012 0.112 0.0048 0.0013 17 0.44 0.24 0.74 0.015 0.010 0.0006 0.131 0.0071 0.0lOTi:.03, Mg:0.0025 18 0.45 0.26 0.75 0.016)0.010 0.0009 0.109 0.0075 0.1680Ti:0.04, Zn:0-004 19 0.41 0.24 0.75 0.016 0.010 1.0 0.0008 0.168 0.0071 0.O002Ti:0.03 20 0.44 0.26 0.74 0.016 0.010 - 0.0011 0.113 0.0077 0.0015Ti:0.03 Cu:0.3 2104 .40.75 0.016 0.014 - 0.000 014 .0054 0.O0010Ti: 0. 02, Mgj:0.0025, Sn:0.04 x 31 .45 0.2 0.7 ). 0 100 22 0.41 0.25 0.75 0.015 0.010 1.1 0.0005 0.192 0.0074 0.On9sTi:y0. 03, Mg:0.0025 230.45 0.24 0.75 0.015 0.013 - 0.0009 0.119 0.00620.Ol9Ti:0.03, Mg:0.0025, Cu:0.4 24 0.40 0..26 0.75 0.015 0.012 1.0 0.0005 0.198 0.0061 0.0015 i:0.02, Mg:0.00 2 5, Sn:0.04 250.44 0.26 0.75 0.014 0.015 - 0.0008 0.169 0.0060 .0012Ti:0.03, Mg:0.0025, Sn:0.04, 05 .Cu0.3 26 0.42 0.24 0.75 0.01410.013 1.0 0.0011 0.116 0.0074 0.010Ti: 0.03, Mg:0.0025, Cu:0.4 27 0.41 0.24 0.74 0.01510.014 1.0 0.00041 0.198 0.0055 0.0012 T:0.03, Sn:0.04 28 0.46 0.25 0.75 0.01510.010 - 0.0010 0.179 0.0061 0.006Ti: 0.03, Sn:0.04, Cu:0.3 29 0.41 0.25 0.75 0.0160.011 0.9 0.00081 0.156 0.0062 0.0017 Ti:0.04, Sn:0.04, Cu:0.3 30 41 0.26 0.75 0.0140.012 1.0 0.0009 0.137 0.005610. 2 Ti:0.03, Cu:0.3 31.0.45 0.24 0.75 0.015 0.013 - 0.0013 0.109 0.0047 0.0010 g:0.00 2 5, Zn:0.003 320.41 0.25 0.76 0.0160.015 1.0 0.0011 0.104 0.00490.0013Mg0.0019, Zn:0.003 33 0.45 0.24 0.75 0.015 0.011 - 0.0013 0.109 0.00480. 0014 ,,2, Cu::0.3 34 0.40 0.25 0.75 0.016 0.015 1.0 0.0008 0.105 0.00460.0014 g:0.0020, Sn:0.04 35 0.45 0.24 0.74 0.015 0.014 - 0.00091 0.110 0.0050 0.008 n:0.04, Cu:0.1 36 0.42 0.25 0.75 0.014 0.012 1.0 0.0012 0.107 0.0047 0019Mg:0.0021, Sn-0.02, Cu:0.1 37 0.41 0.24 0.75 0.015 0.014 1.1 0.0005 0.104 0.0049 0. l9Mg:0.0029, Cu::. 38 0.42 0.25 0.76 0.015 0.011 1.0 0.0009 0.102 0.0049 0.0010 n:0.04 39 0.44 0.24 0.76 0.015 0.012 - 0.0001 0.110 0.0046 0.0016Sn:0.03, Cu:0.1 40 0.41 0.25 0.75 0.015 0.011 1.0 0.0009 0.108 0.0046 0.00091Sn:0.04, Cu:0.1 41 0.41 0.25 0.75 0.015 0.011 0.9 0.0003 0.102 0.0046 0.0014 Cu:0.2 42 0.46 0.25 0.76 0.015 0.011 - 10.00031 0.102 0.0055 0.0014 14b:0.01, Mg:0.0026, Sn:0.04, Cu: 0.3 ) 43 0.44 0.24 0.76 0.014,0.011 - 0.0006 0.025 0.00520.0018 g 40.45 0.25 0.76 0.015 0.014 - 0.0006 0.035 0.0051 0.0019 a 45 0.45 0.24 0.75 0.015 0.014 - 0.0008 0.040 0.0053 0.0017: cu~ 46 0.45 0.25 0.76 0.014 0.011 - 1 0.0010 0.030 0.0034 0.0019 .47 0.46 0.25 0.74 0.016 0.011 - 10.008 0.003 0.0043 0.00341 mt 48 0.44 0.24 .0.75 0.014 0.009, - j0.000 7 0.103 0.0058 0.00251_____________ - 26 Table 8 No ___ __ __ composition (mass%) _______________ C Si Mn p S Cr Ca Tt1AlTtlN SolN Other 52 0.45 0.26 0.75 0.016 0.025 - 0.0002 0.101 0.005210.0 53 0.44 0.25 0.76 0.015 0.030 - 0.0000 0.250 0.0060 0.0013 54 0.45 0.25 0.74 0.015 0.042 - 0.0001 0.123 0. 004810.0012 55 0.45 0.24 0.75 0.015 0.090 - 0.0002- 0.106 0.00490.0013 56 0.45 0.24 0.75 0.014 0.042 - 0.0001 0.102 0.00520.O0llMg:0.0020 57 0.45 0.24 0.74 0.015.0.042 - 0.000 0 00650.0016Ti:0.04 58 0.46.0.25 0.76 0.01610.047 - 0.0001 0.154 0.007 5.0012Nb:0.02 59 0.4510.25 0.74 0. 0150.044 - 0.0001 0.129 0.080.0017W:0.2 60 0.4410.25 0.76 0.01510.044 0.0001 0.109 0.0055.0015Ni:0.2 61 0.4510.26 0.74 0.0160.041 - 0.0001 0.148 .. 00450.0015Cu:0.5 62 0.4610.25 0.75 0.01610.047 - 0.0000 0.111 0.0052 0.0013 Sn:0.03 63 0.4610.25 0.75 0.01510.051 - 0.0001 0.188 0.0051 0.0012 Zn.0.007 64 0.4510.24 0.76 0.01510.073 - 0.0002 0.197 0.00530.00119B0.002 65 0.4410.25 0.75 0.01510. - -2 0.109 0.00460.0010Te:0.002 66 0.4510.25 0.74 0.015 0.062 - 0.0000 0.200 0.00460.O011Cr:0.1 67 0.45 0.26 0.76 0.01410.049 - 0.0001 0.109 0.00700.0012 68 0.450.26 0.76 0.0160.040 - 0.0000 0.172 0.0072001(Ti:0.03, Mg:0.0025 69 0.45 0.25 0.75 0.014 0.040 0.0001 0.110 0.0068 0.001(Ti:0.04, Zn:0.004 70 0.41 0.25 0.75 0. 01510. 04 3 0.910.0000 0.125 0.0075 0.0009 Ti -0. 03 71 0.45 0.25 0.76 0.01510.043 -0.0002 0.110 0.0069 0.0009 Ti: 0. 03, Cu:0.3 72 0.45 0.24 0.76 0. 0150.047 0.0000 0.125 0.06 l i:0.03, Mg:0.0015, Sn:0.04 73 0.40 0.26 0.75 0.014 0.049 1.0 0.0001 0.142 0.00650.O007Ti:0.03, mg:0.0025 74 0.45 0.24 0.75 0.015 0.044 - 0.0001 0.149 0.0062 0.017Ti:0.03, Mg:0.0025, Cu:0.4 75 0.41 0.26 0.76 0.0160.041 1.0 0.0001 0.129 0.00590.001 i:0.05, Mg:0.0025, Sn:0.04 76 0.44 0.24 0.76 0.015 0.043 - 0.0001 0.188 0.0061 0.0014 Ti:0.03, Mg:0.0025, 77:004 0.2u:010.04 77 10.40 0.26 0.75 0.016 0.046 0.910.0001 0.172 0.0064 0 0OIl3Ti:0 01, Mg:0.0025, Cu:0.4 78 0.41 0.25 0.75 0. 0160.042 1.00.0000 0.111 0.00630.O003Ti:0.03, Sn:0.04 79 0.46 0.25 0.76 0. 0150.047 - 0.0001 0.151 0.00670.0012Ti:0.03, Sn:0.04, Cu:0.3 80 0.40 0.26 0.76 0.0160.G43 0.910.0001 0.120 0.00720.O007Ti:0.02, Sn:0.04, Cu:0.3 81 0.41 0.26 0.74 0.01510.046 1.1 0.0001 0.144 0.0069 0,010Ti:0.03, Cu:0.3 82 0.46 0.24 0.76 0.014 0.040 - 0.0001 0.105 0.00510.001OMg:0.0028, Zn:0.003 83 0.41 0.24 0.76 0.01410.047 0.9 0.0000 0.102 0.0052 0.0013 :0.0019, Zn:0.003 84 0.45 0.24 0.76 0.01510.041 - 0.0001 0.102 0.0069 0.0011 M.002 2 , Cu::0.3 85 0.41 0.26 0.75 0.016 .041 1.010.0000 0.109 0.00550.0012Mq: 0.0 2 0 Sn:0.04 86 0.44 0.25 0.76 0.016 0.047 - 0.0001 0.103 0.0062 0.0010Mg0.0025, Sn:0.04, Cu:0.1 87 0.42 0.26 0.75 0.015 0.042 1.0 0.0001 0.101 0.0057 0.0011Mg:0.001 7 , Sn:0.04, Cu:0.1 88 0.41 0.25 0.75 0.0150.046 1.1 0.0001 0.106 0.0067 0.003Mg:0.0025, Cu::. 89 0.41 0.25 0.74 0.014 0.046 1.0 0.0000 0.109 0.00590.O006Sn:0.02 90 0.45 0.26 0.75 0.0150.042 - 0.0001 0.100 0.00660.013Sn:0.04, Cu:0.1 91 0.41 0.24 0.74 0.015 0.046 1.1 0.00011 0.105 0.00650.O02Sn:0.04, Cu:0. 92 0.41 0.26 0.75 0.015 .040 1.1 0.0000 0.109 0.00580.O0l4Cu:0.1 93 0.44 0.24 0.75 0.015 0.057 - 0.0001 0.101 0.00510.0l7Nb:0.01, Mg:0.0025, 3.4~~n0.4 3.4 .7 )01 )04 94 0.45 0.25 0.74 0.014 0.026 - 0.0001 0.025 0.0051 0.0017 0 95 0.45 0.24 0.75 0.0140.043 - 0.0001 0.024 0.0052 0.0018 96 0.46 0.24 0.75 0. 01610. 046 - 0.0002 0.032 10.0051 0.0019 ______________ 9b 97 0.46 0.24 0.76 0.015 0.046 - 0.0002 0.104 0.0078 0.0034 _____________ cm t98 0.45 0.25 0.74 0.016 0.043 - 0.0001 0.103 0.0058 0.0025 99 0.44 0.26 0.76 0.016 0.051 - 0.0000 0 .002 m 100 0.45 0.24 0.75 0.014 73 - 0.0001 0.111 0.0067 0.0031 101 0.46 5 0.75 0.014 0.0001 0.142 0.007710.00351 - 27 Machinability test Machinability testing was conducted with respect to Example and Comparative Example steels that had been elongation-forged under heating at 1250 'C by first 5 subjecting them to heat treatment consisting of normalization under temperature condition of 850 *C for 1 hr, 0.5 hr in the case of Comparative Examples No. 48, No. 49 and No. 97 to No. 101, followed by air-cooling. A machinability evaluation test piece was then cut from 10 each heat-treated steel and the machinabilities of the Example and Comparative Example steels were evaluated by conducting drill boring testing under the cutting conditions shown in Table 9 and to longitudinal turning testing under the conditions shown in Table 10. The 15 maximum cutting speed VL1000 enabling cutting up to a cumulative hole depth of 1000 mm was used as the evaluation index in the drill boring test, and the maximum width VBmax of wear of the relief flank after 10 min was used as the evaluation index in the longitudinal 20 turning test. Table 9 Speed 10 - 120 m/min Cutting Feed 0.25 mm/rev conditions Cutting Water-soluble fluid cutting oil Drill 3 mm Drill diameter ( NACHI Ordinary drill ,Overhang 45 mm Other Hole depth 9 mm _Tool life Until breakage - 28 Table 10 Cutting 250 m/min Cutting speed conditions Feed 0.3 mm/rev Mode Dry cutting Holder PTGNR2525M16 Tool Shape TNMG160408N-UZ Material AC2000 Charpy impact test FIG. 1 is a diagram showing a region from which a 5 Charpy impact test piece was cut. In the Charpy impact test, first, as shown in FIG. 1, a cylinder 2 measuring 25 mm in diameter was cut from a steel 1 heat-treated by the same method and under the same conditions as the aforesaid machinability test piece so that its axis was 10 normal to the elongation-forging direction of the steel 1. Next, the cylinder 2 was held under temperature condition of 850 0 C for 1 hr, 0.5 hr in the case of Comparative Examples No. 48, No. 49 and No. 97 to No. 101, then oil-quenched by cooling to 60 "C, and further 15 subjected to tempering with water cooling in which it was held under temperature condition of 550 *C for 30 min. Next, the cylinder 2 was machined to fabricate a Charpy test piece 3 in conformance with JIS Z 2202, which was subjected to a Charpy impact test at room temperature in 20 accordance with the method prescribed by JIS Z 2242. Absorbed energy per unit area (J/cm 2 ) was adopted as the evaluation index. Tensile test A cylinder 2 oil-quenched and tempered by the same 25 methods and under the same conditions as in the aforesaid Charpy impact test was processed into a tensile test piece measuring 8 mm in parallel section diameter and 30 mm in parallel section length, and then tensile tested at room temperature in accordance with the method 30 prescribed by JIS Z 2241. Yield ratio (= (0.2% proof stress YP) / (tensile strength TS) was adopted as the - 29 evaluation index. The results of the foregoing tests are shown in Tables 11 and 12.
- 30 Table 11 No. VL1000 VBmax Impact YP / TS (m/min) (pm) value (J/cM 2 1 70 121 39 0.87 2 65 121 40 0.82 3 65 123 41 0.84 4 65 125 42 0.80 5 70 115 39 0.83 6 65 121 42 0.83 7 65 123 41 0.86 8 65 120 40 0.85 9 65 116 42 0.85 10 65 122 43 0.86 11 70 123 44 0.85 12 70 120 38 0.83 13 70 119 39 0.84 14 70 120 40 0.85 15 55 132 37 0.89 16 65 124 40 0.86 17 65 125 40 0.82 18 70 124 39 0.85 19 55 131 39 0.84 X 20 65 126 38 0.82 1 21 70 118 39 0.83 ,. 22 55 133 39 0.84 (D 23 70 128 38 0.83 24 60 130 39 0.85 25 70 119 39 0.83 26 55 131 40 0.83 27 65 132 40 0.83 28 70 121 40 0.84 29 60 131 39 0.86 30 55 131 38 0.85 31 70 120 41 0.83 32 55 133 37 0.86 33 70 125 40 0.87 34 60 134 39 0.86 35 70 120 39 0.87 36 60 133 41 0.87 37 55 131 41 0.84 38 60 130 38 0.86 39 70 119 39 0.86 40 60 131 38 0.84 41 55 132 37 0.84 42 65 124 41 0.83 43 45 122 41 0.66 44 45 116 40 0.67 10 45 45 117 41 0.67 46 50 110 42 0.67 (D 47 35 156 22 0.68 48 50 149 30 0.87 49 50 140 29 0.85 31 Table 12 No. VL1000 VB max Impact YP / TS (m/min) (PM) value (J/cm2 52 85 121 25 0.85 53 85 123 24 0.85 54 95 121 23 0.82 55 105 112 21 0.85 56 95 120 22 0.85 57 95 123 22 0.85 58 90 123 22 0.85 59 95 121 22 0.83 60 90 124 22 0.83 61 95 120 22 0.87 62 95 115 22 0.83 63 95 125 22 0.85 64 100 117 21 0.84 65 105 113 21 0.84 66 95 121 20 0.86 67 80 131 20 0.83 68 95 122 25 0.87 69 100 120 24 0.84 70 80 131 20 0.84 M 71 95 122 23 0.85 1 72 100 118 26 0.83 2 73 80 130 21 0.84 - 74 95 122 25 0.85 75 85 128 20 0.85 76 100 119 25 0.86 77 80 132 22 0.83 78 85 128 20 0.82 79 100 120 24 0.82 80 85 126 21 0.84 81 80 133 21 0.83 82 105 120 23 0.84 83 80 130 21 0.83 84 95 124 24 0.86 85 85 129 22 0.87 86 95 117 23 0.87 87 85 128 21 0.84 88 80 129 21 0.86 89 85 126 20 0.83 90 95 119 21 0.86 91 85 125 20 0.87 92 80 133 20 0.83 93 100 112 22 0.86 94 60 180 22 0.68 0 95 65 179 20 0.69 5 9 96 65 174 19 0.68 g 97 70 157 18 0.84 a 98 75 149 18 0.82 E - 99 70 143 18 0.86 ( 100 75 152 15 0.79 101 75 163 12 0.86 - 32 The steels No.1 in Tables 7 and 11 are embodiments of claim 1 and the steels No.2 to No. 42 in the same tables are embodiments of claim 2. The steels No. 52 to No. 93 in Table 8 and Table 12 are embodiments of claim 5 1. The comparative steels No. 43 to No. 49 satisfy the S content and Ca content requirements of claim 2, and the comparative steels No. 94 to No. 101 satisfy the S content and Ca content requirements of claim 1. As shown in Tables 11 and 12, the steels of Examples 10 No 1 to No. 42 and No. 52 to No. 93 exhibited good values for all of the evaluation indexes, namely VL1000, VBmax, impact value (absorbed energy), and YP / TS (yield ratio), but the steels of the Comparative Examples were each inferior to the Example steels in at least one of 15 the properties. Specifically, the steels of Comparative Examples No. 43 to No. 46 had total Al contents below the range of the present invention and were therefore inferior to the Example steels in machinability (VL1000) and yield ratio (YP / TS). Moreover, the steel of 20 Comparative Example No.47 had a total Al content below the range of the present invention, so that its solute N content was above the range of the present invention, and the steel was therefore inferior to the steels of the Examples in machinability (VL1000, VBmax), impact value, 25 and yield ratio (YP / TS). The steels of Comparative Examples No. 48 and No. 49 were maintained at 850 'C, the temperature at which AlN readily precipitates, for a shorter holding time than the steels of the Examples, so that their solute N contents 30 were above the range of the present invention, and the steels were therefore inferior to the steels of the Examples in machinability (VL1000, VBmax) and impact value. Moreover, the steels of Comparative Examples No.94 to No. 96 had a total Al content below the range of the 35 present invention and were therefore inferior to the steels of the Examples in machinability (VL1000, VBmax) and yield ratio (YP / TS). Further, the steels of - 33 Comparative Examples No. 97 to No. 101 were maintained at 850 *C, the temperature at which AlN readily precipitates, for a shorter holding time than the steels of the Examples, so that their solute N contents were above the 5 range of the present invention, and the steels were therefore inferior to the steels of the Examples in machinability (VL1000, VBmax) and impact value. INDUSTRIAL APPLICABILITY 10 The present invention provides a machine structural steel that has good machinability over a broad range of machining speeds and also has high impact properties and high yield ratio.