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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • 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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium

Definitions

• cold forging (including form rolling) can ameliorate surface conditions, dimensional accuracy, and the like of a product and realizes a favorable yield. Therefore, cold forging has been widely applied as a method of manufacturing relatively small-sized machine parts such as bolts.
• a machine part is manufactured by performing cold forging, as a material, for example, a carbon steel (medium carbon) for machine structural use or an alloy steel regulated in JIS G 4051, JIS G 4052, JIS G 4104, JIS G 4105, JIS G 4106, and the like are used.
• the manufacturing cost increases due to significant wear on a die at the time of cold forging. Cracking is likely to occur at the time of cold forging due to insufficient ductility of a material in a state of being hot-rolled with no additional treatment, so that the yield is degraded.
• a boron steel for bolts realized by adding a slight amount of B to a steel has been developed (for example, Patent Document 1 and Patent Document 3).
• a boron steel is characterized by the factors as follows. Hardness of a wire rod in a state of being hot-rolled with no additional treatment is decreased and ductility is improved by reducing the carbon content in the steel and addition amounts of alloying elements such as Cr and Mo, so that annealing is no longer necessary. Degradation in hardenability due to reduction of the addition amounts of alloying elements is supplemented with an effect of improving hardenability by adding a slight amount of B, which does not increase the hardness of a rolled material.
• Patent Document 4 discloses that precipitation of BN is suppressed by setting Ti/N (mass% ratio) to 4 or more. In principle, if Ti/N is 3.42 or more, precipitation of BN can be suppressed.
• the foregoing technology has a disadvantage. That is, in a case where a large amount of fine TiC or Ti(CN) is dispersed in a structure after hot rolling, there is an adverse reaction in which hardness of ferrite is increased due to precipitation strengthening by fine precipitate particles, thereby resulting in a problem that the effect of a boron steel softening a hot rolled material is decreased. That is, in a case where the amount of fine TiC or Ti(CN) is increased, generation of coarse grains can be suppressed, but the service life of a die for cold forging is degraded by increasing the hardness of a rolled material due to precipitation strengthening.
• Patent Document 7 also discloses a technical idea similar to that in the foregoing technology of preventing generation of coarse grains in a boron steel. That is, in the technology, the relationship among the amounts of Ti, Nb, Al, and N is set to be within certain ranges, so that carbonitride of these elements is dispersed in a steel, and coarsening of grains is prevented. Moreover, Patent Document 7 discloses an effect of enhancing machinability by adding 0.01 % or more of Bi. However, Patent Document 7 discloses only the effect of enhancing machinability, as the effect of Bi. There is no disclosure related to a relationship between Bi and the properties of coarsening of grains at all. Since Bi is added for the purpose of the effect of improving machinability, only a case of adding a relatively large amount of Bi is examined in Patent Document 7. In this case, as disclosed in Patent Document 7, there is concern that hot workability will be degraded due to added Bi.
• Patent Document 8 discloses a case hardening steel. An object thereof is to provide a case hardening steel which exhibits excellent grain coarsening resistance even in a case where carburizing is performed at a higher temperature than an example in the related art, and has excellent cold workability even if softening annealing is not performed.
• Patent Document 8 only utilization of fine Ti carbide, Ti-containing composite carbide, and the like is proposed as a way of ensuring grain coarsening resistance.
• the hot rolling temperature is set to be extremely low in order to ensure cold workability. Therefore, productivity of a case hardening steel is impaired.
• One of problems of steels for cold forging is softly maintaining a steel without performing annealing after hot rolling and before cold forging and without adopting manufacturing conditions impairing productivity in order to improve cold forgeability of the steel and productivity of the steel.
• Another problem of steels for cold forging is exhibiting high hardenability after cold forging in order to apply high strength to machine parts.
• further another problem of steels for cold forging is suppressing generation of coarse grains during quenching after cold forging in order to prevent deterioration in dimensional accuracy of machine parts, impact values, fatigue strength, delayed fracture properties, and the like.
• an object of the present invention is to provide a steel in which generation of coarse grains at the time of quenching is suppressed without using Ti carbide and Ti carbonitride, such as TiC and Ti(CN), so that all of manufacturability, cold forgeability, and mechanical properties after quenching are excellent.
• the gist of the present invention is as follows.
• the steel according to the present invention it is possible to provide a steel in which both softening before cold forging and suppressing of generation of coarse grains at the time of quenching after cold forging can be achieved.
• the steel according to the present invention can be manufactured under conditions within a range in which cracking is not generated at the time of casting, at the time of rolling, and the like and no load is applied to manufacturing equipment, thereby being excellent in manufacturability.
• wear on a die at the time of cold forging is suppressed and the service life of a die can be improved.
• the steel according to the present invention is applied to cold-forged components, the cost of expensive dies can be reduced. Therefore, it is possible to particularly contribute to reduction of the manufacturing cost of high strength bolts of which tensile strength is 800 MPa or more.
• the steel according to the present invention also has excellent machinability. Therefore, industrial contribution of the present invention is extremely significant.
• a steel according to an embodiment of the present invention will be described.
• the steel according to the present embodiment has the following features.
• the inventors have examined a technology of suppressing generation of coarse grains, separately from a technology in the related art, in which hardness of ferrite is noticeably increased due to precipitation strengthening such that hardness of a steel is increased, and TiC, Ti(CN), and the like which are particles impairing cold workability of a steel are finely dispersed.
• the foregoing features are based on the following knowledge obtained by the inventors who have intensely investigated a technology of suppressing abnormal grain growth of austenite grains at the time of heating for quenching of a steel.
• C is an element which is necessary to enhance strength of a steel having a tempered martensite structure.
• the C content is required to be set to 0.15% or more.
• a preferable lower limit for the C content is 0.17%, 0.19%, or 0.23%.
• the upper limit for the C content is set to 0.40%.
• a preferable upper limit for the C content is 0.35%, 0.34%, 0.33%, or 0.30%.
• Mn is an element which is effective in improving hardenability of a steel.
• the Mn content is required to be set to 0.10% or more.
• a preferable lower limit for the Mn content is 0.20%, 0.35%, or 0.40%.
• the upper limit for the Mn content is set to 1.50%.
• a preferable upper limit for the Mn content is 1.30%, 1.00%, or 0.80%.
• the S content is required to be set to 0.002% or more.
• a preferable lower limit for the S content is 0.003%.
• the S content may be less than that in the technology in the related art. Moreover, if the S content exceeds 0.020%, S embrittles prior austenite grain boundaries of a steel after quenching, so that delayed fracture resistance (hydrogen embrittlement resistance) are degraded. Furthermore, since Ti 2 C 2 S described above is particles impairing machinability of a steel, there is concern that if the S content exceeds 0.020%, machinability of a steel will be deteriorated. Therefore, the S content is required to be limited to 0.020% or less. Preferably, the upper limit value for the S content is 0.015%, 0.010%, or 0.005%.
• Ti forms compounds with C, N, and S in a steel and is present in the steel as Ti-based inclusions such as TiN, Ti(CN), TiC, TiS, and Ti 2 C 2 S.
• Ti has the effect of suppressing abnormal grain growth of austenite grains by serving as pinning particles at the time of heating for quenching.
• Ti has strong chemical attraction to a solute N in a steel. Therefore, Ti an element extremely effective in fixing the solute N in a steel in advance as TiN and suppressing generation of BN.
• the Ti content is required to be set to 0.005% or more.
• a preferable lower limit for the Ti content is 0.010%, 0.015%, or 0.020%.
• the Ti content may be less than that in the technology in the related art. Moreover, if the Ti content exceeds 0.050%, Ti-based inclusion particles cause precipitation strengthening, and hardness of a rolled material after hot rolling excessively is increased, so that the service life of a die for cold forging is significantly degraded. In order to suppress hardness of a rolled material after hot rolling while increasing the amount of Ti-based inclusion particles, the hot rolling temperature is required to be lowered. However, it is not preferable in regard to productivity, the service life of equipment, and the like.
• the upper limit for the Ti content is set to 0.050%.
• a preferable Ti content is 0.040% or less, 0.030% or less, less than 0.030%, or 0.025% or less.
• B is an element which contributes to improvement of hardenability of a steel in a case where a slight amount thereof is contained. B achieves an effect of improving hardenability and can increase hardness after cold forging and quenching, without increasing hardness of a rolled material after hot rolling and before cold forging.
• B is an essential element particularly for a boron steel for bolts.
• B has an effect of suppressing fractures at grain boundaries by being segregated in prior austenite grain boundaries and strengthening the prior austenite grain boundaries.
• the B content is required to be set to 0.0005% or more.
• the lower limit value for the B content is 0.0010%, 0.0012%, or 0.0015%.
• the B content is set to 0.0050% or less.
• the upper limit value for the B content is 0.0030%, 0.0025%, 0.0020%, or 0.0018%.
• Bi content is required to be set to 0.0010% or more.
• a preferable lower limit value for the Bi content is 0.0020%, 0.0025%, or 0.0030%.
• the Bi content is set to 0.0100% or less.
• the Bi content is preferably less than 0.0100%, 0.0080% or less, or 0.0060% or less.
• the P content is required to be limited to 0.020% or less.
• the upper limit value for the P content is 0.015%, 0.013%, or 0.010%.
• the lower limit value for the P content is 0%.
• the lower limit value for the P content may be set to 0.001%.
• the lower limit value for the N content is 0%.
• the lower limit value for the N content may be set to 0.0001%, 0.0005%, or 0.0010%.
• the N content is required to be limited to 0.0100% or less.
• the upper limit value for the N content is 0.0070%, 0.0050%, or 0.0040%.
• a steel for springs according to the present embodiment may further include one or more selected from the group consisting of Si, Cr, and Al, within the range described below.
• Si, Cr, and Al are not essential, the lower limit for each of the Si content, the Cr content, and the Al content is 0%.
• the lower limit value for the Si content is 0%.
• Si is an element which is effective in improving hardenability of a steel and improving resistance to temper softening of martensite.
• it is preferable that the Si content is set to be more than 0% or 0.01% or more.
• the lower limit value for the Si content may be set to 0.05% or 0.15%.
• the Si content is set to less than 0.30%.
• a preferable upper limit for the Si content is 0.27%, 0.25%, or 0.20%.
• the lower limit value for the Cr content is 0%.
• Cr is an element which is effective in improving hardenability of a steel and improving resistance to temper softening of martensite.
• it is preferable that the Cr content is set to be more than 0% or 0.01% or more.
• the lower limit value for the Cr content may be set to 0.10%, 0.20%, or 0.30%.
• the upper limit for the Cr content is set to 1.50%.
• a preferable upper limit for the Cr content is 1.20%, 1.00%, or 0.80%.
• Al is an element which is effective in deoxidation of a steel.
• the lower limit value for the Al content is 0%.
• the Al content is 0.001% or more, 0.005% or more, or 0.010% or more.
• the upper limit for the Al content is set to 0.050%.
• the upper limit for the Al content is preferably 0.040%, 0.030%, or 0.025%.
• the steel for springs according to the present embodiment may further include one or more selected from the group consisting of Mo, Cu, Ni, and Nb, within the range described below.
• Mo, Cu, Ni, and Nb are not essential, the lower limit for each of the Mo content, the Cu content, the Ni content, and the Nb content is 0%.
• the lower limit value for the Mo content is 0%.
• Mo is an element which contributes to improvement of hardenability of a steel even if the content thereof is small.
• the Mo content is set to 0.02% or more. More preferably, the lower limit value for the Mo content is 0.03%, 0.04%, or 0.05%.
• the Mo content is set to 0.20% or less.
• the upper limit value for the Mo content is 0.16%, 0.13%, or 0.10%.
• the lower limit value for the Cu content is 0%.
• Cu is an element which improves corrosion resistance of a steel.
• the Cu content is set to 0.20% or less.
• the upper limit value for the Cu content is 0.15%, 0.10%, or 0.08%.
• the lower limit value for the Ni content is 0%.
• Ni is an element which improves corrosion resistance of a steel and is an element which is also effective in improvement of toughness of a steel.
• the Ni content is set to 0.02% or more. More preferably, the lower limit value for the Ni content is 0.03%, 0.04%, or 0.05%.
• Ni is an expensive alloying element
• the Ni content if the Ni content exceeds 0.20%, it is disadvantageous to the manufacturing cost. Therefore, even in a case where Ni is contained, the Ni content is set to 0.20% or less.
• the upper limit value for the Ni content is 0.15%, 0.12%, 0.10%, or 0.08%.
• the lower limit value for the Nb content is 0%.
• Nb forms compounds with C in a steel and is present in the steel as Nb-based inclusions such as NbC or TiNb(CN).
• Nb has the effect of suppressing abnormal grain growth of austenite grains, as pinning particles at the time of heating for quenching.
• the Nb content is set to 0.002% or more. More preferably, the lower limit value for the Nb content is 0.003%, 0.005%, or 0.006%.
• the Nb content exceeds 0.030%, not only the effect thereof is saturated, but also Nb-based inclusions cause precipitation strengthening, so that manufacturability at the time of continuous casting is impaired. Otherwise, in this case, since Nb-based inclusions cause precipitation strengthening, hardness of a rolled material after hot rolling is excessively increased. Therefore, if the Nb content exceeds 0.030%, problems such as degradation in manufacturability and significant degradation in the service life of a die for cold forging become noticeable. Therefore, even in a case where Nb is contained, the Nb content is set to 0.030% or less. Preferably, the upper limit value for the Nb content is 0.015%, 0.013%, or 0.010%.
• the steel according to the present embodiment includes the foregoing alloy composition, and the remainder of the chemical composition includes Fe and impurities.
• impurities are compositions which are incorporated due to raw materials such as ores and scraps, and other factors when a steel is industrially manufactured. Impurities mean elements of an amount at a level at which the operational effects of the steel according to the present embodiment are not impaired.
• the N fixation index I FN defined by the following Expression 1 is set to 0 or more.
• the lower limit value for the N fixation index I FN may be set to 0.0005, 0.0010, 0.0014, or 0.0050.
• the N fixation index I FN is not particularly limited, as long as the Ti content and the N content are controlled to be within the range described above, the steel according to the present embodiment is softened before cold forging, and generation of coarse grains at the time of quenching can be suppressed.
• I FN Ti ⁇ 3.5 ⁇ N [Ti] and [N] in the foregoing Expression 1 indicate the Ti content and the N content in a steel by unit mass%, and in a case where these elements are not contained, the contents thereof are set to 0%.
• N is fixed as TiN by using Ti, and the amount of the solute N is reduced.
• the inventors have found that Nb also has a similar operation as Ti.
• fine Ti-Nb-based precipitates such as TiC, Ti(CN), NbC, TiNb(CN), and Ti 2 C 2 S which are precipitates present in a steel, have the effect of suppressing generation of coarse grains by suppressing abnormal grain growth of austenite grains at the time of heating for quenching as pinning particles.
• these Ti-Nb-based precipitate particles are dispersed in a structure after hot rolling, there is adverse reaction such as an increase in hardness of ferrite caused by precipitation strengthening due to fine precipitate particles.
• the Ti-Nb-based precipitate generation index I P calculated by using the following Expression 2 is set to 0.0100 or less.
• the Ti-Nb-based precipitate generation index I P may be set to 0.0075 or less, less than 0.0050, 0.0045 or less, 0.0040 or less, or 0.0035 or less.
• the Ti-Nb-based precipitate generation index I P is not particularly limited, as long as the Ti content, the Nb content, and the N content are controlled to be within the range described above, the steel according to the present embodiment is softened before cold forging, and generation of coarse grains at the time of quenching can be suppressed.
• I P 0.3 ⁇ Ti + 0.15 ⁇ Nb ⁇ N [Ti], [N], and [Nb] in the foregoing Expression 2 indicate the Ti content, the N content, and the Nb content in a steel by unit mass%, and in a case where these elements are not contained, the contents thereof are set to 0%.
• a steel having the chemical composition described above is smelted in a converter.
• the steel passes through a secondary refining process and is formed into a cast slab by performing continuous casting.
• This cast slab is reheated and is subjected to blooming, thereby obtaining a material for wire-rod rolling (steel piece) having a cross section of 162 square mm (the length of 162 mm and the width of 162 mm), for example.
• the steel piece is heated at a temperature of approximately 1,000°C to 1,280°C.
• wire-rod rolling is performed to have a wire rod shape with a diameter of 6 mm to 20 mm.
• the wire rod is wound into a coil shape through hot rolling by a winding device, and then is cooled to room temperature. In this manner, the steel of the present embodiment is obtained.
• the amount of Ti-based precipitate particles causing precipitation strengthening is suppressed. Therefore, in the method of manufacturing the steel according to the present embodiment, there is no need to apply a load to hot rolling equipment by lowering the hot rolling temperature in order to suppress hardness of a steel. In addition, imperfections such as cracking and defects due to an increase in hardness are unlikely to be generated in a steel. Moreover, in the steel according to the present embodiment, hardness thereof is suppressed without performing annealing after hot rolling. Therefore, the steel according to the present embodiment is also excellent for having high productivity.
• the steel of the present embodiment softening before cold forging and suppression of generation of coarse grains at the time of quenching can be compatible.
• the steel of the present embodiment is excellent in manufacturability.
• the hardness of the steel according to the present embodiment can be suitably adjusted in accordance with its usage, it is not particularly limited. However, in a case where there is a need to ensure cold forgeability, it is suitable for the hardness of the steel according to the present embodiment to be set to Hv 180 or less, and it is more suitable to be set to Hv 170 or less, or Hv 160 or less.
• the lower limit value for the hardness of the steel according to the present embodiment is not particularly limited. However, in consideration of the chemical composition thereof, it is assumed that the lower limit value therefor is substantially set to approximately Hv 130 or approximately Hv 140.
• the hardness of the steel according to the present embodiment can be within the suitable range described above even if annealing is not performed after hot rolling. In addition, the steel according to the present embodiment is also excellent in machinability.
• the steel according to the present embodiment in a case where the steel according to the present embodiment is heated to a temperature of 840°C to 1,100°C and is retained for 30 minutes, and then quenching is performed under a condition of performing water cooling or oil cooling and tempering treatment in which the steel according to the present embodiment is heated and retained at a temperature within a range of 150°C to 450°C is further performed, tensile strength thereof can be set to 800 MPa or more. Therefore, the steel according to the present embodiment becomes suitable for a material of components which require high strength.
• heat treatment conditions are not particularly limited and can be suitably selected in accordance with its usage.
• the usage of the steel according to the present embodiment is not particularly limited. It is suitable to be applied to high strength machine parts manufactured by performing cold forging and quenching, and is particularly suitable to be applied to high strength bolts.
• the steel according to the present embodiment having high cold forgeability is used as a material of high strength machine parts, wear on a die at the time of cold forging is suppressed and the service life of a die can be improved.
• the cost of expensive dies can be reduced, it is possible to particularly contribute to reduction of the manufacturing cost of high strength bolts of which tensile strength is 800 MPa or more.
• a test piece for measuring Vickers hardness was cut out from the wire rod after rolling. Specifically, the test piece having a cross section including the central axis of the wire rod was cut out in a direction parallel to the rolling direction. After the cut-out cross section was polished, Vickers hardness of a region of the wire rod at the depth of 1/4 (1/4 portion) in a diameter from the surface of the wire rod was measured. The test load was set to 10 kgf, and the average value of four measurements was recorded in Table 2-1 and Table 2-2 as "Hardness after rolling", and this was regarded as an index for estimating the service life of a die for cold forging.
• prior austenite grain boundaries were manifested through corrosion, and the grain size of prior austenite after quenching and tempering was measured through observation using an optical microscope.
• the grain size of prior austenite was measured based on JIS G 0551.
• 10 or more visual fields were adopted at magnification of 400 times.
• any of cold forgeability, properties of preventing coarse grains, and manufacturability was poor. That is, in B 1 to B4, hot ductility was degraded due to the excessive addition amount of Bi, and manufacturability was poor. In B5 to B7, Bi was not added or the addition amount was excessively small, so that properties of preventing coarse grains were poor. In B8 and B9, the Ti-Nb-based precipitate generation index I P exceeded due to the excessive addition amount of Ti or the small amount of the N content with respect to the addition amount of Ti, so that hardness of the wire rod after rolling was high and cold forgeability was poor.
• the steel according to the present invention it is possible to provide a steel in which both softening at the time of cold forging and suppressing of generation of coarse grains at the time of quenching after cold forging can be achieved.
• the steel according to the present invention can be manufactured under conditions within a range in which cracking is not generated at the time of casting, at the time of rolling, and the like and no load is applied to manufacturing equipment, thereby being excellent in manufacturability.
• the steel according to the present invention is applied to cold-forged components, wear on a die at the time of cold forging is suppressed and the service life of a die can be improved.
• the steel according to the present invention is applied to cold-forged components, the cost of expensive dies can be reduced. Therefore, it is possible to particularly contribute to reduction of the manufacturing cost of high strength bolts of which tensile strength is 800 MPa or more.
• the steel according to the present invention also has excellent machinability. Therefore, industrial contribution of the present invention is extremely significant.

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