Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium

Definitions

• the present invention relates to steel products which exhibit excellent machinability, as well as to machined steel parts. More particularly, the invention relates to steel products which exhibit excellent machinability and are suitable for steel stocks of structural steel parts for a variety of machinery such as transportation machinery including automobiles, machinery for industrial use, construction machinery, and the like, and to a variety of machined structural steel parts for machinery, such as crankshafts, connecting rods, gears, and the like.
• such steel parts are generally either (a) formed roughly into predetermined shapes through hot working, then formed into desired shapes through machining, followed by thermal refining through quenching and tempering, or (b) subjected to hot working, and then quenching and tempering, followed by machining.
• the life of the drill to a steel having the following composition based on % by weight, C: 0.45%; Si: 0.29%; Mn: 0.78%; P: 0.017%; S: 0.041%; Al: 0.006%; N: 0.0087%; Ti: 0.228%; O: 0.004%; and Ca: 0.001%, is adversely short, and therefore, machinability of above-mentioned steel is poor. Consequently, it is concluded that machinability of steel is not improved through simple addition of Ti.
• Document JP-A-59 17 3250 discloses that the machinability of steel is improved by the formation of MnS dispersed in the steel matrix and by the addition of Te which enhances the spheroidisation of MnS in order to avoid anisotropy of strength in the steel body.
• US-patent 5,102,619 teaches that mechanical properties such as fracture toughness and strength of steel can be improved by replacing the MnS inclusions therein by titanium carbosulfide inclusions.
• This document recommends a very low content of S and Ti, especially of Ti which according to examples given in the specification is comprised between 0.012 and 0.021% by weight.
• GB-patent 1 514 093 relates to a steel sheet having excellent workability, and in particular formability, and excellent resistance to the occurrence of "fish-scale" in an enamel layer fired thereon.
• the titanium content is comprised between 0.03% and 0.10% and the sulfur content is between 0.025% to 0.035% whereas the carbon content is not greater than 0.015%. Due to this low carbon content only titanium sulfide and no titanium carbosulfide is formed.
• an object of the present invention is to provide steel products which not only have remarkable mechanical properties, especially toughness and tensile strength but show also excellent machinability and thus are suitable for steel stocks of structural steel parts for a variety of machinery such as transportation machinery including automobiles, machinery for industrial use, construction machinery, and the like, and to provide a variety of machined structural steel parts for machinery, such as crankshafts, connecting rods, gears, and the like.
• the present invention relates to a steel product which exhibits excellent machinability and which has the following chemical composition based on % by weight : C 0.05% to 0.6% S 0.002% to 0.2% Ti 0.04% to 1.0% N 0,008% or less Nd 0.0% to 0.1% Se 0.0% to 0.5% Te 0.0% to 0.5% Ca 0.0% to 0.01% Pb 0.0% to 0.5% Bi 0.0% to 0.4% Si 0.0% to 1.61% Mn 0.0% to 3.5% P 0.07% or less Al 0.0% to 0.05% Cu 0.0% to 1.10% Ni 0.0% to 2.0% Cr 0.0% to 3.0% Mo 0.0% to 0.54% V 0.0% to 0.31% Nb 0.0% to 0.1% B 0.0% to 0.02% The balance being Fe and unavoidable impurities, and wherein the maximum diameter of titanium carbosulfide contained in the steel is not greater than 10 ⁇ m, and its amount expressed in the index of cleanliness of the steel is equal to or more than 0.05%.
• the present invention further refers to a non-heat-treated type steel product, according to (I) above, whose microstructure is constituted by at least 90% of ferrite and pearlite and which has the following chemical composition based on % by weight : C 0.2% to 0.6% Si 0.05% to 1.5% Mn 0.1% to 2.0% P 0.07% or less S 0.01% to 0.2% Al 0.002% to 0.05% Cu 0% to 1.0% Ni 0% to 2.0% Cr 0% to 2.0% Mo 0% to 0.5% V 0% to 0.3% Nb 0% to 0.1% and wherein the balance are Fe and unavoidable impurities.
• the present invention aims also at a heat-treated type steel product, according to (I) above, whose microstructure is constituted by at least 50% of martensite and which has the following chemical composition based on % by weight : C 0.1% to 0.6% Si 0.05% to 1.5% M
• titanium carbosulfide as used herein encompasses titanium sulfide.
• maximum diameter (of titanium carbosulfide) refers to "the longest diameter among the diameters of respective titanium carbosulfide entities.”
• the index of cleanliness of the steel is determined by "the microscopic testing method for the non-metallic inclusions in steel" prescribed in JIS G 0555, and performed by means of an optical microscope at x400 magnification and 60 visual fields.
• non-heat-treated type steel product refers to a steel product manufactured without “quenching and tempering" which are so-called “thermal refining,” and includes “steel which may be used under the as-cooled condition after hot working” as well as “steel obtained through aging corresponding to tempering after hot working and cooling.”
• heat-treated type steel product refers to steel products obtained through "quenching and tempering”.
• Ratios referred to in terms of microstructure denote those observed under a microscope, i.e. area percentage.
• At least 90% of the microstructure of the steel is constituted by ferrite and pearlite means that the total of the respective contents of ferrite and pearlite in the microstructure where ferrite and pearlite coexist is at least 90%.
• At least 90% of the microstructure of the steel is constituted by bainite means that the bainite content in the microstructure where no ferrite exists is at least 90%
• at least 90% of the microstructure of the steel is constituted by ferrite and bainite means that the total of the respective contents of ferrite and bainite in the microstructure where ferrite and bainite coexist is at least 90%.
• "at least 50% of the microstructure of the steel is constituted by martensite” means that the martensite content in the microstructure is at least 50%.
• the above (IV) is directed to a "heat-treated type steel product" which has undergone quenching and tempering.
• the above mentioned martensite refers to martensite which has undergone tempering, i.e. "tempered martensite,” and will hereinafter be referred to simply as "martensite".
• the present inventors conducted various experiments to investigate the effects of the chemical composition and the microstructure of steel products on machinability and mechanical properties.
• machinability of a steel product is improved by (a) addition of a proper amount of Ti to the steel, (b) transformation of sulfides to titanium carbosulfides for controlling inclusions in the steel, and (c) minute dispersion of the titanium carbosulfides in the steel.
• the present invention has been accomplished based on the above findings.
• C binds to Ti together with S to form titanium carbosulfide and to have an effect of improving machinability. Also, C is an element effective for securing strength. However, if the carbon content is less than 0.05%, these effects cannot be obtained. On the other hand, if the carbon content is in excess of 0.6%, toughness will be impaired. Therefore, the carbon content shall be from 0.05% to 0.6%.
• the carbon content shall be, desirably, from 0.2% to 0.6%, more desirably, from 0.25% to 0.5%.
• the carbon content shall be, desirably, from 0.05% to 0.3%, more desirably, from 0.1% to 0.24%.
• the carbon content shall be, desirably, from 0.1% to 0.6%.
• S binds to Ti together with C to form titanium carbosulfide and to have an effect of improving machinability. However, if the sulfur content is less than 0.002%, the effect cannot be obtained.
• MnS machinability-improving effect of MnS relies on the effect of improving lubrication between the chips and the face of a tool during machining.
• MnS may become large and cause a large macro-streak-flaw for steel products, resulting in a defect.
• the machinability-improving effect of S is obtained by forming titanium carbosulfide through incorporation of adequate amounts of C and Ti. Therefore, as mentioned above, the sulfur content is required to be not less than 0.002%. By contrast, if the sulfur content is in excess of 0.2%, although no effect is provided for machinability, coarse MnS is produced in the steel again, which leads to problems such as a macro-streak-flaw. In addition, since hot workability is considerably impaired, plastic working becomes difficult and toughness may be impaired. Therefore, the sulfur content shall be from 0.002% to 0.2%.
• the sulfur content shall be, desirably, from 0.01% to 0.2%, more desirably, from 0.02% to 0.17%.
• the sulfur content shall be, desirably, from 0.005% to 0.17%.
• Ti is an important alloy element to control inclusion. If the titanium content is less than 0.04%, S is not fully incorporated into the titanium carbosulfide and thus improved machinability is not obtained. By contrast, if the titanium content is in excess of 1.0%, not only the cost increases as machinability-improving effect saturates, but also the toughness and hot-workability decrease excessively. Therefore, the titanium content shall be from 0.04% to 1.0%.
• titanium content shall be, desirably, from 0.08% to 0.8%.
• titanium content shall be, desirably, from 0.06% to 0.8%.
• titanium content shall be, desirably, from 0.06% to 0.8%.
• the nitrogen content shall be 0.008% or less.
• the upper limit of the nitrogen content shall be, desirably, 0.006%.
• Nd may be omitted.
• Nd if added, becomes Nd 2 S 3 serving as a chip breaker to have an effect of improving machinability.
• Nd 2 S 3 is finely produced in molten steel in a dispersing manner at relatively high temperatures, the growth of austenite grains, due to heat, is restricted during hot working or quenching in the subsequent process and thus the microstructure becomes fine, resulting in high strength and toughness of steel.
• the neodymium content shall be, desirably, not less than 0.005%.
• the neodymium content shall be from 0% to 0.1%. Desirably, the upper limit of the neodymium content shall be 0.08%.
• Se may be omitted.
• Se if added, has an effect of further improving the machinability of steel.
• the selenium content shall be, desirably, not less than 0.1%.
• the selenium content shall be from 0% to 0.5%.
• Te may be omitted. Te, if added, has an effect of further improving machinability of steel. To reliably obtain this effect, the tellurium content shall be, desirably, not less than 0.005%. However, when the tellurium content is in excess of 0.05%, not only the above-mentioned effect saturates, but also fatigue strength and/or toughness of the steel decrease as coarse inclusions are produced. Further, addition of a great amount of Te leads to decreased hot-workability. Specifically, if the tellurium content is in excess of 0.05%, scratches are formed in the surfaces of steel products which have undergone hot working. Therefore, the tellurium content shall be from 0% to 0.05%.
• Ca may be omitted.
• Ca if added, has an effect of remarkably improving machinability of steel.
• the calcium content shall be, desirably not less than 0.001%.
• the calcium content shall be from 0% to 0.01%.
• Pb may be omitted.
• Pb if added, has an effect of further improving the machinability of steel.
• the lead content shall be, desirably, not less than 0.05%.
• the lead content when the lead content is in excess of 0.5%, not only the above-mentioned effect saturates, but also fatigue strength and/or toughness decrease as coarse inclusions are produced.
• addition of a great amount of Pb leads to decreased hot-workability. Specifically, if the lead content is in excess of 0.5%, scratches are formed in the surfaces of steel products which have undergone hot working. Therefore, the lead content shall be from 0% to 0.5%.
• Bi may be omitted.
• Bi if added, has an effect of further improving the machinability of steel.
• the bismuth content shall be, desirably, not less than 0.05%.
• the bismuth content when the bismuth content is in excess of 0.4%, not only the above-mentioned effect saturates, but also fatigue strength and/or toughness decrease as coarse inclusions are produced. Further, addition of a great amount of Bi leads to decreased hot-workability, resulting in scratches which are formed in the surfaces of steel products which have undergone hot working. Therefore, the bismuth content shall be from 0% to 0.4%.
• Si is an element effective for deoxidizing a steel and for strengthening the ferrite phase. Further, the increased silicon content improves lubrication on the surface of the chips during machining and thus the service life of the tool is extended, resulting in improved machinability. However, if the silicon content is less than 0.05%, the effect of the addition is insignificant, whereas if the silicon content is in excess of 1.5%, not only the above-mentioned effect saturates, but also toughness is impaired. Therefore, the silicon content shall be, desirably, from 0.05% to 1.5%, more desirably, from 0.3% to 1.3%, most desirably, from 0.5% to 1.3%.
• Mn is an element effective for improving fatigue strength through solid-solution strengthening.
• the manganese content shall be, desirably, from 0.1% to 2.0%, more desirably, from 0.4% to 2.0%, and most desirably, from 0.5% to 1.7%.
• P may be intentionally added. This is because P has an effect of improving tensile strength and fatigue strength in "steel products under Condition X". In order to reliably obtain this effect, the phosphorus content shall be, desirably, not less than 0.01%. However, if the phosphorus content is in excess of 0.07%, toughness decreases remarkably and hot-workability is impaired. Therefore, the phosphorus content shall be, desirably, not greater than 0.07%. If P is added intentionally, the phosphorus content shall be, desirably, from 0.015% to 0.05%.
• Al is an element effective for deoxidizing a steel.
• the aluminum content shall be, desirably, from 0.002% to 0.05%, more desirably, from 0.005% to 0.03%.
• Cu may be omitted.
• Cu if added, has an effect of improving strength, especially fatigue strength of a steel, through precipitation strengthening.
• the copper content shall be, desirably, not less than 0.2%.
• the copper content shall be, desirably, from 0% to 1.0%.
• Ni may be omitted.
• Ni if added, has an effect of improving strength.
• the nickel content shall be, desirably, not less than 0.02%.
• the nickel content shall be, desirably, from 0% to 2.0%.
• Cr may be omitted. Cr, if added, has an effect of improving fatigue strength through solid-solution strengthening. To reliably obtain this effect, the chromium content shall be, desirably, not less than 0.02%. However, if the chromium content is in excess of 2.0%, in "steel products under Condition X", endurance ratio and yield ratio may be impaired. Therefore, the chromium content shall be, desirably, from 0% to 2.0%. In the case where Cr is added, the chromium content shall be, desirably, from 0.05% to 1.5%.
• Mo may be omitted.
• Mo if added, has an effect of improving strength, especially fatigue strength of a steel, since the microstructure composed of ferrite and pearlite becomes fine.
• the molybdenum content shall be, desirably, not less than 0.05%.
• the molybdenum content shall be, desirably, from 0% to 0.5%.
• V may be omitted.
• V if added, has an effect of improving strength, especially fatigue strength of a steel, since V precipitates as fine nitride or carbonitride.
• the vanadium content shall be, desirably, not less than 0.05%.
• the vanadium content shall be, desirably, from 0% to 0.3%.
• Nb may be omitted.
• Nb if added, has an effect of preventing coarsening of austenite grains, to thereby enhance strength, especially fatigue strength of a steel, since Nb precipitates as fine nitride or carbonitride.
• the niobium content shall be, desirably, not less than 0.005%.
• the niobium content shall be, desirably, from 0% to 0.1%. More desirably, the upper limit of niobium content shall be 0.05%.
• the value of fn1 expressed by the equation (1) is greater than 0%, and/or the value of fn2 expressed by the equation (2) is greater than 2, the machinability-improving effect of titanium carbosulfides is enhanced.
• the value of fn2, expressed by the equation (2) is greater than 2, the pinning effect of titanium carbosulfides is enhanced, to thereby improve tensile strength and fatigue strength. Therefore, it is desired that the value of fn1 shall be greater than 0%, or alternatively, the value of fn2 shall be greater than 2. No particular limitation is imposed on the upper limits of the values of fn1 and fn2, and they may be determined so as to comply with compositional requirements.
• O (oxygen) as an impurity element forms hard oxide-type inclusions, by which the machine tool may be damaged, resulting in lowered machinability.
• the oxygen content in excess of 0.015% may considerably impair machinability. Consequently, in order to maintain excellent machinability, the amount of O as an impurity element shall be, desirably, 0.015% or less. More desirably, the oxygen content shall be 0.01% or less.
• Si has an effect of deoxidizing a steel and improving hardenability. Furthermore, in “steel products under Condition Y", the increased silicon content improves lubrication on the surface of the chips during machining and thus the service life of the tool is extended, resulting in improved machinability.
• the silicon content shall be, desirably, from 0.05% to 1.5%. More desirably, the silicon content shall be from 0.5% to 1.3%.
• Al is an element having powerful deoxidizing effect on a steel.
• the aluminum content shall be, desirably, not less than 0.002%.
• the aluminum content shall be, desirably, from 0.002% to 0.05%, more desirably, from 0.005% to 0.04%.
• Cu may be omitted.
• Cu if added, has an effect of improving machinability as well as strength of the steel without lowering toughness.
• the copper content shall be, desirably, not less than 0.2%.
• the copper content shall be, desirably, from 0% to 1.0%.
• Mo may be omitted.
• Mo if added, has an effect of improving hardenability and strength of a steel by rendering the microstructure of the steel very fine.
• the molybdenum content shall be, desirably, not less than 0.05%.
• the molybdenum content shall be, desirably, from 0% to 0.5%.
• V may be omitted.
• V if added, has an effect of improving strength, since V precipitates as fine nitride or carbonitride, and moreover, has an effect of improving lubrication on the surface of the chips during machining.
• the vanadium content shall be, desirably, not less than 0.05%.
• the vanadium content shall be, desirably, from 0% to 0.30%.
• Nb may be omitted.
• Nb if added, has an effect of preventing coarsening of austenite grains and improving strength and toughness of the steel, since Nb precipitates as fine nitride or carbonitride.
• the niobium content shall be, desirably, not less than 0.005%.
• the niobium content shall be, desirably, from 0% to 0.1%.
• B may be omitted.
• B if added, has an effect of improving strength and toughness of a steel due to increased hardenability.
• the boron content shall be, desirably, not less than 0.0003%.
• the boron content shall be, desirably, from 0% to 0.02%.
• the value of fn3 is correlated to the microstructure and toughness of a non-heat-treated type steel product having a certain chemical composition.
• the primary microstructure of the non-heat-treated type steel product comes to be bainite, or a combination of ferrite and bainite, thus achieving well-balanced strength and toughness.
• Si, Mn, Cr and Ni which form the terms of the equation for fn3, have the effect of enhancing hardenability of the steel.
• the value of fn3 is less than 2.5%, intended improvement in hardenability cannot be obtained, with toughness being sometimes degraded.
• the values of fn3 in excess of 4.5% result in excessive hardenability, which may in turn degrade toughness. Therefore, it is desired that the value of fn3 expressed by the equation (3) shall be from 2.5% to 4.5%.
• the contents of the respective elements other than Si are not particularly limited, so long as the above-mentioned fn3 falls within the range of 2.5-4.5%.
• Mn, Cr and Ni shall be contained in amounts of 0.4-3.5%, 3.0% or less, and 2.0% or less, respectively.
• the machinability-improving effect of titanium carbosulfides is enhanced when the value of fn1 expressed by the equation (1) is greater than 0%, and/or the value of fn2 expressed by the equation (2) is greater than 2. Furthermore, when the value of fn2, expressed by the equation (2), is greater than 2, the pinning effect of titanium carbosulfides increases as well, to thereby improve tensile strength and fatigue strength. Therefore, it is desired that the value of fn1 shall be greater than 0%, or alternatively, the value of fn2 shall be greater than 2.
• the upper limits of the values of fn1 and fn2 are not particularly limited, and they may be determined based on compositional requirements.
• O (oxygen) as an impurity element forms hard oxide-type inclusions, by which the machine tool may be damaged, resulting in lowered machinability.
• the oxygen content in excess of 0.015% may invite significant degradation in machinability. Therefore, even in the case of "steel products under Condition Y", in order to maintain excellent machinability, the amount of O as an impurity element shall be, desirably, 0.015% or less. More desirably, the oxygen content shall be 0.01% or less.
• phosphorus (P) as an impurity element shall be, desirably, suppressed to 0.05% or less.
• Si has an effect of deoxidizing a steel and improving hardenability. Furthermore, in the case of "steel products under Condition Z", increased silicon content improves lubrication on the surface of the chips during machining and thus the service life of the tool is extended, resulting in improved machinability.
• the silicon content shall be, desirably, from 0.05% to 1.5%.
• Mn improves hardenability of a steel and improves fatigue strength through solid-solution strengthening.
• the manganese content shall be, desirably, from 0.4% to 2.0%.
• Al is an element having strong deoxidizing effect on a steel.
• the aluminum content shall be, desirably, not less than 0.002%.
• the aluminum content shall be, desirably, from 0.002% to 0.05%, more desirably, from 0.005% to 0.04%.
• Cu may be omitted.
• Cu if added, has an effect of improving strength without lowering toughness, and in addition, enhances machinability.
• the copper content shall be, desirably, not less than 0.2%.
• the copper content shall be, desirably, from 0% to 1.0%.
• Ni may be omitted.
• Ni if added, has an effect of improving hardenability of a steel.
• the nickel content shall be, desirably, not less than 0.02%.
• the nickel content shall be, desirably, from 0% to 2.0%.
• the chromium content shall be, desirably, not less than 0.03%.
• the chromium content shall be, desirably, from 0% to 2.0%.
• Mo may be omitted.
• Mo if added, has an effect of improving hardenability of a steel.
• the molybdenum content shall be, desirably, not less than 0.05%.
• the molybdenum content shall be, desirably, from 0% to 0.5%.
• V may be omitted.
• V if added, has an effect of improving strength, especially fatigue strength of a steel, since V precipitates as fine nitride or carbonitride.
• the vanadium content shall be, desirably, not less than 0.05%.
• the vanadium content shall be, desirably, from 0% to 0.3%.
• Nb may be omitted.
• Nb if added, has an effect of preventing coarsening of austenite grains, to thereby enhance strength, especially fatigue strength and toughness of a steel, since Nb precipitates as fine nitride or carbonitride.
• the niobium content shall be, desirably, not less than 0.005%.
• the niobium content shall be, desirably, from 0% to 0.1%. More desirably, the upper limit of niobium content shall be 0.05%.
• B may be omitted.
• B if added, has an effect of improving strength and toughness of a steel due to increased hardenability.
• the boron content shall be, desirably, not less than 0.0003%.
• the boron content shall be, desirably, from 0% to 0.02%.
• O (oxygen) as an impurity element forms hard oxide-type inclusions, by which the machine tool may be damaged, resulting in lowered machinability.
• the oxygen content in excess of 0.015% may considerably impair machinability. Consequently, also in "steel products under Condition Z", in order to maintain excellent machinability, the amount of O as an impurity element shall be, desirably, 0.015% or less. More desirably, the oxygen content shall be 0.01% or less.
• P (phosphorus) as an impurity element shall be, desirably, suppressed to 0.05% or less.
• titanium carbosulfides In order to improve machinability of steel products having chemical compositions described in (A) above through use of titanium carbosulfides, it is important that the size and the index of cleanliness in terms of titanium carbosulfides be optimized. As described herein above, the expression “titanium carbosulfides” encompasses titanium sulfides.
• the amount expressed by the index of cleanliness in terms of titanium carbosulfide having a maximum diameter of not more than 10 ⁇ m is less than 0.05%, titanium carbosulfides cannot exhibit their machinability-improving effect.
• the above-mentioned index of cleanliness shall be, desirably, not less than 0.08%.
• the upper limit of the above-mentioned index of cleanliness in terms of titanium carbosulfides shall be, desirably, approximately 2.0%.
• the maximum diameter of titanium carbosulfide is set to 10 ⁇ m ⁇ is that sizes in excess of 10 ⁇ m reduce fatigue strength and/or toughness.
• the maximum diameter of titanium carbosulfide shall be 7 ⁇ m.
• the lower limit of the maximum diameter of titanium carbosulfide shall be, desirably, about 0.5 ⁇ m.
• titanium carbosulfide is basically determined by the amounts of Ti, S and N contained in the steel. In order to bring the size and the index of cleanliness in terms of titanium carbosulfides within the predetermined ranges, it is important to prevent overproduction of titanium oxides. To this end, according to a preferred steelmaking process, steel is first sufficiently deoxidized with Si and Al, then Ti is added, since, in some cases, satisfaction of the compositional requirements for the steel mentioned in (A) is not sufficient by itself.
• Titanium carbosulfides can be discerned from other inclusions based on their color and shape through mirror-like polishing of test pieces cut from steel products and through observation of the polished surface under an optical microscope at x400 or higher multiplication. That is, titanium carbosulfides have a very pale gray color and a granular (spherical) shape corresponding to B-type inclusions according to JIS (Japanese Industrial Standards). Detailed determination of titanium carbosulfides may also be performed through observation of the aforementioned mirror-like-polished surface under an electron microscope equipped with an analytical device such as EDX (energy dispersive X-ray spectrometer).
• EDX energy dispersive X-ray spectrometer
• the index of cleanliness in terms of titanium carbosulfides is determined as described hereinabove; i.e., in accordance with "the microscopic testing method for the non-metallic inclusions in steel” prescribed in JIS G 0555, and performed by means of an optical microscope at x400 magnification and 60 visual fields.
• steel products excellent in machinability of the present invention can be obtained by simply prescribing the amounts of C, S, Ti, N, Nd, Se, Te, Ca, Pb and Bi as described in (A) above and also prescribing the size and the index of cleanliness in terms of titanium carbosulfide as described in (B) above.
• the microstructure of steel products may be additionally prescribed as well.
• a semi-finished product having a chemical composition described in (II) above may first be heated to 1050-1300°C, then subjected to hot working such as hot forging to finish at a temperature not lower than 900°C, and subsequently subjected to air cooling or atmospheric cooling at a cooling rate of not more than 60°C/min for at least a period until the temperature reaches 500°C.
• cooling rate refers to the cooling rate as measured on the surface of the steel product.
• ferrite accounts for 20-70% in terms of the area percentage; ferrite grain size is 5 or more as expressed by the JIS grain size number; the average lamellar spacing of pearlite is 0.2 ⁇ m or less.
• a semi-finished product having a chemical composition described in (III) above may first be heated to 1050-1300°C, then subjected to hot working such as hot forging to finish at a temperature not lower than 900°C, and subsequently subjected to air cooling or atmospheric cooling at a cooling rate of not more than 60°C/min for at least a period until the temperature reaches 300°C.
• working ratio is used to refer to the ratio A 0 /A where A 0 represents a sectional area before working and A represents a sectional area after working.
• a non-heat-treated type steel product in which not less than 90% of the microstructure is constituted by bainite or a combination of ferrite and bainite i.e., a "steel product under Condition Y"
• the expression "prior austenite grains” in a non-heat-treated type steel product refers to austenite grains right before bainite or ferrite is generated therefrom as a result of transformation under heat and hot working.
• Prior austenite grains in a non-heat-treated type steel product in which not less than 90% of the microstructure is constituted by bainite or a combination of ferrite and bainite can be readily determined through corrosion with nital and observation under an optical microscope.
• a semi-finished product having a chemical composition described in (IV) above may be treated as follows. Briefly, the semi-finished product is first heated to 1050-1300°C, then subjected to hot working such as hot forging at a working ratio of 1.5 or more and to finishing at a temperature not lower than 900°C. Subsequently the finished steel material is subjected to air cooling or atmospheric cooling at a cooling rate of not more than 60°C/min for at least a period until the temperature reaches 300°C.
• the steel product is heated to a temperature range of 800-950°C, maintained for 20-150 minutes, then quenched by use of a cooling medium such as water or oil, followed by heating to 400-700°C, maintained for 20-150 minutes, and then subjected to air cooling, atmospheric cooling, or alternatively, depending on cases, water cooling or oil cooling followed by tempering.
• a cooling medium such as water or oil
• the quenching treatment may be performed by way of so-called "direct quenching," in which steel products are quenched directly from the austenite region or austenite-ferrite dual phase region after hot working.
• the microstructure be made martensite.
• the remaining portion of the microstructure other than martensite is constituted by microstructure resulting from tempering of ferrite, pearlite or bainite in the case in which an austenite region undergoes quenching, microstructure resulting from tempering of ferrite in the case in which an austenite-ferrite dual-phase region undergoes quenching, or microstructure resulting from temperering of austenite which has remained untransformed even when quenching was performed (so-called retained austenite).
• Substantially 100% of the microstructure may represent martensite.
• a heat-treated type steel product in which not less than 50% of the microstructure is constituted by martensite i.e., a "steel product under Condition Z"
• a heat-treated type steel product in which not less than 50% of the microstructure is constituted by martensite can be consistently imparted with extremely well-balanced strength and toughness.
• the expression "prior austenite grains" in a heat-treated type steel product refers to austenite grains right before being subjected to quenching.
• Prior austenite grains in a heat-treated type steel product in which not less than 50% of the microstructure is constituted by martensite can be readily identified as follows, for example. A steel product is quenched or is quenched and then tempered, and a sample steel piece is cut out. The test piece is etched with aqueous solution of picric acid to which a surfactant has been added. The etched surface of the test piece is observed under an optical microscope.
• Steels having chemical compositions shown in Tables 1 to 4 were manufactured through a melting process in a 150 kg vacuum melting furnace or a 3-ton vacuum melting furnace.
• Steels 1, 6, and 36 to 40 were manufactured through a melting process in the 3-ton vacuum melting furnace, and other steels were manufactured through a melting process in the 150 kg vacuum melting furnace.
• all steels other than steels 36 and 38 underwent adjustment of the size and the index of cleanliness of titanium carbosulfide. This adjustment was carried out by adding Ti, after various elements had been added, subsequent to sufficient deoxidization with Si and Al.
• Ti was added to a molten steel during deoxidation with Si and Al.
• Steels 1 to 36 in Tables 1 to 3 are examples of the present invention, and contain each component element in an amount falling in a range specified by the present invention.
• steels 37 to 46 in Table 4 are comparative examples, in which any of component elements falls outside a range specified by the present invention.
• each of the steels was hot forged, such that the steel was heated to a temperature of 1250°C and then finished at a temperature of 1000°C, to obtain a round bar having a diameter of 60 mm.
• the hot-forged round bars were cooled to a temperature of 300°C, at a cooling rate of 5°C/min to 35°C/min by air cooling or atmospheric cooling, thereby adjusting their microstructures, so as to obtain a tensile strength of about 845 MPa to 870 MPa.
• the hot-forged round bars were cooled as above and then heated at a temperature of 770°C to 900°C for 1 hour, followed by water quenching.
• the water-quenched round bars were tempered at a temperature of 550°C to 560°C (followed by air cooling), so as to adjust their microstructures and strengths.
• Test pieces were obtained from each of the round bars at a position 15 mm deep from the surface (at a position described as a R/2 site, where R denotes the radius of the round bar).
• the obtained test pieces were JIS No. 14A tensile test pieces, Ono-type rotating bending fatigue test pieces (diameter of straight portion: 8 mm; length of straight portion: 18.4 mm), and JIS No. 3 impact test pieces (2 mm U-notch Charpy test pieces), which were used for testing tensile strength, fatigue strength (fatigue limit), and toughness (impact value), respectively, at room temperature.
• a test piece was obtained from each of the round bars at a position described as a R/2 site in accordance with Fig. 3 of JIS G 0555.
• the mirror-like polished surface to be observed measured 15 mm (width) by 20 mm (height).
• the polished surface was observed through an optical microscope at 400 magnifications over a range of 60 visual fields.
• the index of cleanliness in terms of titanium carbosulfides in the steel was measured such that titanium carbosulfides were distinguished from other inclusions, and also the maximum diameter of titanium carbosulfides was obtained.
• the mirror-like polished surface of each test piece was etched with nital. The etched surface was observed through the optical microscope at 100 magnifications so as to observe the state of microstructure, i.e. to obtain the occupancy rate (area percentage) of individual constituent phases of microstructure, at the R/2 site.
• a drilling test was conducted for evaluation of machinability. Specifically, each of the round bars having a diameter of 60 mm was cut to obtain round bar blocks, each having a length of 55 mm. The blocks were drilled 50 mm deep in the length direction. The number of bores were counted and drilled until the drilling tool became disabled due to failure of the top cutting edge. The number of the drilled bores was defined as a machinability index indicative of machinability of steel. The drilling test was conducted through use of a 6 mm-diameter straight shank drill of high speed tool steel, JIS SKH59, and a watersoluble lubricant, at a feed of 0.20 mm/rev and a revolution of 980 rpm.
• Tables 5 to 8 show the results of the above tests. Tables 5 to 8 also contain quenching and tempering conditions for steels 6, 7, 9, 11, 29 to 36, 40, 45 and 46.
• the machinability indices are in excess of 200.
• the tested steels 1 to 35 contain C, S, Ti and N in amounts falling within respective ranges, as specified in the present invention and have a maximum diameter of titanium carbosulfides, not greater than 10 ⁇ m and a index of cleanliness in terms of titanium carbosulfide not lower than 0.05%.
• the machinability index is as low as 51, since the tested steel 36 has a index of cleanliness in terms of titanium carbosulfide lower than 0.05% despite its C, S, Ti and N contents, falling within respective ranges as specified in the present invention.
• the machinability indices are as low as 58, 40 and 45, respectively, since some of the C, Ti and N contents of the tested steels 37, 39 and 40 fall outside the corresponding range as specified in the present invention.
• the machinability index is as low as 31, since the S content of the tested steel 38 falls outside the corresponding range, as specified in the present invention, and also the tested steel 38 has a index of cleanliness in terms of titanium carbosulfide lower than 0.05%.
• the steels, according to the present invention show excellent machinability.
• test Nos. 41 to 46 in which the Nd, Se, Te, Ca, Pb and Bi contents of the tested steels 41 to 46, respectively, fall outside respective ranges as specified in the present invention, machinability is favorable, but fatigue strength and/or toughness is inferior to that of test Nos. 2 to 7, in which the tested steels 2 to 7 contain these elements in amounts falling within respective ranges, as specified in the present invention.
• Steels 47 to 54 having chemical compositions shown in Table 9 were manufactured through a melting process in a 150 kg vacuum melting furnace or a 3-ton vacuum melting furnace. Steels 47 to 49 were manufactured through a melting process in the 3-ton vacuum melting furnace, and other steels were manufactured through a melting process in the 150 kg vacuum melting furnace. In order to prevent the generation of titanium oxides, the steels underwent adjustment of the size and the index of cleanliness of titanium carbosulfide. This adjustment was carried out by adding Ti, after various elements had been added, subsequent to sufficient deoxidization with Si and Al. Steels 47 to 54 in Table 9 are examples of the present invention, and contain each component element in an amount falling in a range specified by the present invention.
• each of the steels was hot forged, such that the steel was heated to a temperature of 1250°C and then finished at a temperature of 1000°C, to obtain a round bar having a diameter of 60 mm.
• the hot-forged round bars were cooled to a temperature of 400°C, at a cooling rate of 5°C/min to 35°C/min by air cooling or atmospheric cooling, thereby adjusting tensile strength through attainment of a microstructure which is primarily composed of ferrite and pearlite.
• Test pieces for use in various tests were obtained from each of the round bars at a position as deep as R/2 from the surface of the round bar in a manner similar to that of Example 1.
• the obtained test pieces were JIS No. 14A tensile test pieces, Ono-type rotating bending fatigue test pieces (diameter of straight portion: 8 mm; length of straight portion: 18.4 mm), and JIS No. 3 impact test pieces (2 mm U-notch Charpy test pieces), which were used for testing tensile strength, fatigue strength (fatigue limit), and toughness (impact value), respectively, at room temperature.
• a test piece was obtained from each of the round bars at a position described as a R/2 site in accordance with Fig. 3 of JIS G 0555.
• the mirror-like polished surface to be observed measured 15 mm (width) by 20 mm (height).
• the polished surface was observed through an optical microscope at 400 magnifications over a range of 60 visual fields.
• the index of cleanliness in terms of titanium carbosulfides in the steel was measured such that titanium carbosulfides were distinguished from other inclusions, and also the maximum diameter of titanium carbosulfides was obtained.
• the mirror-like polished surface of each test piece was etched with nital.
• the etched surface was observed through the optical microscope at 100 magnifications so as to observe the state of microstructure, i.e. to obtain the occupancy rate (area percentage) of individual constituent phases of microstructure, at the R/2 site.
• the occupancy rate (area percentage) of individual constituent phases of microstructure at the R/2 site.
• the ferrite grain size number as specified in JIS was measured, and the average lamellar spacing of pearlite was obtained from photographs taken through a scanning electron microscope.
• Table 10 shows the results of the above tests.
• the area percentage of ferrite is 20% to 70%; the grain size of ferrite in terms of JIS grain size number is not smaller than 5; and the average lamellar spacing of pearlite is 0.2 ⁇ m or less.
• the machinability index assumes a relatively large value when the value of fn1 represented by the aforementioned equation (1) is greater than 0%, and/or the value of fn2 represented by the aforementioned equation (2) is greater than 2.
• the value of fn2 represented by the equation (2) is greater than 2
• fatigue strength is also relatively high.
• Steels 55 to 59 having chemical compositions shown in Table 11 were manufactured through a melting process in a 150 kg vacuum melting furnace or a 3-ton vacuum melting furnace. Steels 55 and 56 were manufactured through a melting process in the 3-ton vacuum melting furnace, and other steels were manufactured through a melting process in the 150 kg vacuum melting furnace. In order to prevent the generation of titanium oxides, the steels underwent adjustment of the size and the index of cleanliness of titanium carbosulfide, in this example too. This adjustment was carried out by adding Ti, after various elements had been added, subsequent to sufficient deoxidization with Si and Al. Steels 55 to 59 in Table 11 are examples of the present invention, and contain each component element in an amount falling in a range specified by the present invention.
• each of the steels was hot forged, such that the steel was heated to a temperature of 1250°C and then finished at a temperature of 1000°C, to obtain a round bar having a diameter of 60 mm.
• the hot-forged round bars were cooled to a temperature of 300°C, at a cooling rate of 5°C/min to 35°C/min by air cooling or atmospheric cooling, thereby adjusting tensile strength through attainment of a microstructure which is primarily composed of bainite, or ferrite and bainite.
• aged steel was also tested (test Nos. 60 and 61). Specifically, the hot-forged round bars of steels 57 and 58 were cooled as above and then aged, i.e. heated at a temperature of 560°C for 1 hour, followed by air cooling.
• Test pieces for use in various tests were obtained from each of the round bars at a position as deep as R/2 from the surface of the round bar in a manner similar to that of Example 1.
• the obtained test pieces were JIS No. 14A tensile test pieces, Ono-type rotating bending fatigue test pieces (diameter of straight portion: 8 mm; length of straight portion: 18.4 mm), and JIS No. 3 impact test pieces (2 mm U-notch Charpy test pieces), which were used for testing tensile strength, fatigue strength (fatigue limit), and toughness (impact value), respectively, at room temperature.
• a test piece was obtained from each of the round bars at a position described as a R/2 site in accordance with Fig. 3 of JIS G 0555.
• the mirror-like polished surface to be observed measured 15 mm (width) by 20 mm (height).
• the polished surface was observed through an optical microscope at 400 magnifications over a range of 60 visual fields.
• the index of cleanliness in terms of titanium carbosulfides in the steel was measured such that titanium carbosulfides were distinguished from other inclusions, and also the maximum diameter of titanium carbosulfides was obtained.
• the mirror-like polished surface of each test piece was etched with nital. The etched surface was observed through the optical microscope at 100 magnifications so as to observe the state of microstructure, i.e. to obtain the occupancy rate (area percentage) of individual constituent phases of microstructure, at the R/2 site.
• Table 12 shows the results of the above tests. Table 12 also contains the conditions of aging treatment conducted on steels 57 and 58 in test Nos. 60 and 61.
• Steels 60 to 64 having chemical compositions shown in Table 13 were manufactured through a melting process in a 150 kg vacuum melting furnace or a 3-ton vacuum melting furnace. Steels 60 and 61 were manufactured through a melting process in the 3-ton vacuum melting furnace, and other steels were manufactured through a melting process in the 150 kg vacuum melting furnace. In order to prevent the generation of titanium oxides, the steels underwent adjustment of the size and the index of cleanliness of titanium carbosulfide, in this example too. This adjustment was carried out by adding Ti, after various elements had been added, subsequent to sufficient deoxidization with Si and Al. Steels 60 to 64 in Table 13 are examples of the present invention, and contain each component element in an amount falling in a range specified by the present invention.
• each of the steels was hot forged, such that the steel was heated to a temperature of 1250°C and then finished at a temperature of 1000°C, to obtain a round bar having a diameter of 60 mm.
• the hot-forged round bars were cooled to a temperature of 300°C, at a cooling rate of 5°C/min to 35°C/min by air cooling or atmospheric cooling.
• the hot-forged round bars were heated at a temperature of 850°C to 900°C for 1 hour, followed by water quenching.
• the water-quenched round bars were tempered at a temperature of 550°C (followed by air cooling) so as to adjust their microstructures and strengths.
• Test pieces for use in various tests were obtained from each of the round bars at a position as deep as R/2 from the surface of the round bar in a manner similar to that of Example 1.
• the obtained test pieces were JIS No. 14A tensile test pieces, Ono-type rotating bending fatigue test pieces (diameter of straight portion: 8 mm; length of straight portion: 18.4 mm), and JIS No. 3 impact test pieces (2 mm U-notch Charpy test pieces), which were used for testing tensile strength, fatigue strength (fatigue limit), and toughness (impact value), respectively, at room temperature.
• a test piece was obtained from each of the round bars at a position described as a R/2 site in accordance with Fig. 3 of JIS G 0555.
• the mirror-like polished surface to be observed measured 15 mm (width) by 20 mm (height).
• the polished surface was observed through an optical microscope at 400 magnifications over a range of 60 visual fields.
• the index of cleanliness in terms of titanium carbosulfides in the steel was measured such that titanium carbosulfides were distinguished from other inclusions, and also the maximum diameter of titanium carbosulfides was obtained.
• the mirror-like polished surface of each test piece was etched with nital. The etched surface was observed through the optical microscope at 100 magnifications so as to observe the state of microstructure, i.e. to obtain the occupancy rate (area percentage) of individual constituent phases of microstructure, at the R/2 site.
• Table 14 shows the results of the above tests. Table 14 also contains quenching and tempering conditions for steels 60 to 64.
• the machinability index assumes a relatively large value when the value of fn1 represented by the aforementioned equation (1) is greater than 0%, and/or the value of fn2 represented by the aforementioned equation (2) is greater than 2.
• the value of fn2, expressed by the equation (2) is greater than 2
• fatigue strength is also relatively high.
• each of the square bars was hot die forged, such that the square bar was heated to a temperature of 1250°C and then finished at a temperature not less than 1000°C.
• the hot-die-forged square bars were cooled to a temperature of 300°C, at a cooling rate of 5°C/min to 35°C/min by air cooling or atmospheric cooling, in order to obtain near net shape products of crankshafts.
• the thus-obtained near net shape products were machined to obtain finished crankshafts.
• the hot-die-forged square bars were cooled as above and then heated at a temperature of 890°C to 900°C for 1 hour, followed by water quenching.
• the water-quenched square bars were tempered at a temperature of 550°C (followed by air cooling) to obtain near net shape products of crankshafts.
• the thus-obtained near net shape products were machined to obtain finished crankshafts.
• the coated carbide insert having the shape as defined by the designation code CNMG12041N-UX in JIS.
• the machining was of dry type and carried out at a cutting speed of 100 m/min, a depth of cut of 1.5 mm, and a feed of 0.25 mm/rev.
• an oil hole was drilled in each of the crankshafts through use of a 6 mm-diameter straight shank drill of high speed tool steel, JIS SKH59, and a watersoluble lubricant, at a feed of 0.20 mm/rev and a revolution of 980 rpm.
• the number of the drilled crankshafts was defined as a machinability index indicative of machinability of steel.
• a test piece was obtained from each of the crankpins (70 mm diameter) of the above-mentioned near net shape products of crankshafts in accordance with Fig. 3 of JIS G 0555 and with respect to the reference line which passes a position as deep as 15 mm from the surface of the crankpin.
• the mirror-like polished surface to be observed measured 15 mm (width) by 20 mm (height).
• the polished surface was observed through an optical microscope at 400 magnifications over a range of 60 visual fields.
• the index of cleanliness in terms of titanium carbosulfides in the steel was measured such that titanium carbosulfides were distinguished from other inclusions, and also the maximum diameter of titanium carbosulfides was obtained.
• test pieces were obtained from each of the crankshafts, in parallel with the axial direction of the crankshaft.
• the obtained test pieces were JIS No. 14A tensile test pieces, Ono-type rotating bending fatigue test pieces (diameter of straight portion: 8 mm; length of straight portion: 18.4 mm), and JIS No. 3 impact test pieces (2 mm U-notch Charpy test pieces), which were used for testing tensile strength, fatigue strength (fatigue limit), and toughness (impact value), respectively, at room temperature.
• Table 15 shows the results of the above tests. Table 15 also contains quenching and tempering conditions for test Nos. 68, 69, 73, 79, and 80.
• crankshafts manufactured from the steel products according to the present invention show excellent machinability. Moreover, the crankshafts manufactured from the steel products according to the present invention are superior, in balance between strength and toughness, to the crankshafts manufactured from the steel products of the comparative examples.
• the steel products of the present invention have excellent machinability and excellent balance between strength and toughness, they can be used as steel stocks of structural steel parts for a variety of machinery such as transportation machinery including automobiles, machinery for industrial use, construction machinery, and the like.
• Various kinds of structural steel parts for machinery can relatively readily be manufactured from the steel products of the present invention through machining.

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• 1997
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