SE2250518A1 - Method for spheroidizing annealing case-hardening steel - Google Patents

Abstract

Provided is a spheroidizing annealing method that is capable to skip a gradual cooling process, to shorten spheroidizing annealing time, and that is capable to obtain a microstructure with uniformly dispersed carbides. This method includes annealing a steel material having a temperature A1 of 750°C or more and consisting of, by mass%, 0.15 to 0.26% of C, 0.05 to 1.00% of Si, 0.1 to 0.9% of Mn, 0.030% or less of P, 0.030% or less of S, 1.30 to 2.50% of Cr, 0.020 to 0.050% of Al, 0.0040 to 0.0300% of N, optionally at least one or more of 0 to 2.00% of Ni and 0 to 2.00% of Mo, optionally at least one or more of 0 to 0.10% of Nb, 0 to 0.200% of Ti, 0 to 0.0050% of B and 0 to 0.500% of V, and the balance being Fe and unavoidable impurities, so as to satisfy the conditions of a holding temperature T(°C) of the spheroidizing annealing: (A1 - 30) ≤ T ≤ (A1 - 5), and a holding time t(h) of the spheroidizing annealing: t ≥ 120 / (T - A1 + 50).

Description

METHOD FOR SPHEROIDIZING ANNEALING CASE-HARDENING STEEL TECHNICAL FIELD[0001]The present invention relates to a method for spheroidizing annealing case-hardening steel.

BACKGROUND ART 2. 2. id="p-2" id="p-2" id="p-2" id="p-2" id="p-2" id="p-2" id="p-2" id="p-2" id="p-2" id="p-2" id="p-2"

[0002] A variety of methods have been proposed as spheroidizing annealing methods. PatentLiterature 1 (JP2009-242917A), for example, with respect to bearing steel, proposes amethod for conducting spheroidizing annealing efficiently in case that greater softeningthan usual is required. The literature discloses a heat treatment method for the bearingsteel by means of controlling a ratio of a high temperature holding time in the hightemperature holding process to a controlled cooling time in the controlled cooling process,when conducting the spheroidizing annealing including each process of heating, pre-transformation cooling, controlled cooling, and post-transformation cooling. 3. 3. id="p-3" id="p-3" id="p-3" id="p-3" id="p-3" id="p-3" id="p-3" id="p-3" id="p-3" id="p-3" id="p-3"

[0003] Patent Literature 2 (JP2001-131631A) proposes spheroidizing annealing of a steelmaterial for mechanical structures containing 0.15 to 1.10% of C. The literaturediscloses a method of spheroidizing annealing the steel material in a short time by meansof (1) heating at a rate of 0.01°C/s or less in a temperature range between (a temperatureat which austenite appears - 50°C) and (a temperature at which austenite appears - 5°C)in a process of heating; (2) heating to a temperature range between (a temperature atwhich a single phase of austenite is obtained - 30°C) and (a temperature at which a singlephase of austenite is obtained - 5°C), which corresponds to a maximum heatingtemperature, followed by immediate cooling; and (3) cooling at a rate of 0.005°C/s or less in a temperature range between (a temperature at which ferrite appears + 10°C) and (a temperature at which ferrite appears - 40°C) in a process of cooling, followed by air cooling. This method intends to shorten the high temperature holding time by controllinga temperature increasing rate. 4. 4. id="p-4" id="p-4" id="p-4" id="p-4" id="p-4" id="p-4" id="p-4" id="p-4" id="p-4" id="p-4" id="p-4"

[0004] Patent Literature 3 (JPH11-12684A), discloses controlling an area fraction of (ferrite +pearlite) in a microstructure before spheroidizing annealing to 75% or more, and anaverage grain size of ferrite to 40 um or less and an average grain size of pearlite to 30um or less, with respect to case-hardening steel for cold forging containing 0.3% or less ofC. This technology provides uniform distribution of carbides after spheroidizingannealing in order to secure cold forgeability by controlling the microstructure beforespheroidizing annealing. . . id="p-5" id="p-5" id="p-5" id="p-5" id="p-5" id="p-5" id="p-5" id="p-5" id="p-5" id="p-5" id="p-5"

[0005] However, in these Patent Literatures, since the holding temperature of spheroidizingannealing reaches a two-phase region of austenite + ferrite, a long period of gradual cooling had to be conducted in the subsequent cooling process.

CITATION LISTPATENT LITERATURE 6. 6. id="p-6" id="p-6" id="p-6" id="p-6" id="p-6" id="p-6" id="p-6" id="p-6" id="p-6" id="p-6" id="p-6"

[0006] Patent Literature 1: JP2009-242917A Patent Literature 2: JP2001-131631A Patent Literature 3: JPH11-12684A SUMMARY OF INVENTION 7. 7. id="p-7" id="p-7" id="p-7" id="p-7" id="p-7" id="p-7" id="p-7" id="p-7" id="p-7" id="p-7" id="p-7"

[0007] ln case-hardening steel using low-carbon steel, cold forging is sometimes selectedconsidering yield and processing efficiency in the process of shaping machinery parts.Then, in the cold forging, spheroidizing annealing is generally carried out for a total ofabout 20 hours in order to reduce deformation resistance. 8. 8. id="p-8" id="p-8" id="p-8" id="p-8" id="p-8" id="p-8" id="p-8" id="p-8" id="p-8" id="p-8" id="p-8"

[0008] That is, the conventional spheroidizing annealing of the case-hardening steel involves thefollowing processes: (1) increasing the temperature to A1 or higher to transform the previous microstructurebeing pearlite or bainite into austenite; (2) holding the temperature at A1 or higher to form a tvvo-phase structure of ferrite +austenite, wherein carbide nuclei are left in austenite grains; and (3) precipitating spherical carbides on the carbide nuclei in the austenite grains and onaustenite/ferrite grain boundaries as precipitation sites thereafter, by means of a gradualcooling process from just above A1 to just below A1. 9. 9. id="p-9" id="p-9" id="p-9" id="p-9" id="p-9" id="p-9" id="p-9" id="p-9" id="p-9" id="p-9" id="p-9"

[0009] At this time, if a gradual cooling rate across A1 is too fast, formation of hard pearlite cannotbe suppressed. To prevent this, the gradual cooling rate is controlled at about 10°C/h,which takes a long time, resulting in the extension of the total time. . . id="p-10" id="p-10" id="p-10" id="p-10" id="p-10" id="p-10" id="p-10" id="p-10" id="p-10" id="p-10" id="p-10"

[0010] Further, in the microstructure of the spheroidize-annealed case-hardening steel, due tothe process of holding in the two-phase region of ferrite + austenite in (2), the sphericalcarbides cannot be precipitated within the ferrite grains of the two-phase region. Thisinevitably causes non-uniformity of the distribution of the spherical carbides after gradualcooling. Since such non-uniformity of the distribution of the carbides causes crackingduring the cold forging and coarsening of crystal grains in the subsequent carburizingprocess, uniformizing the distribution of carbides is preferred. 11. 11. id="p-11" id="p-11" id="p-11" id="p-11" id="p-11" id="p-11" id="p-11" id="p-11" id="p-11" id="p-11" id="p-11"

[0011] To address these issues, in the aforementioned Patent Literature 1, the total treatmenttime is attempted to be reduced by devising time allocation between the high temperatureholding time and the controlled cooling time with respect to spheroidizing annealingconditions for the bearing steel. Further, in Patent Literature 2, it is attempted to reducethe high temperature holding time by controlling the temperature increasing rate.Furthermore, in Patent Literature 3, it is attempted to uniformize the distribution ofcarbides after spheroidizing annealing by controlling the previous microstructure.

However, the methods disclosed in these Literatures unfortunately cannot skip the gradual cooling process accounting for most of the spheroidizing annealing treatment time andcannot completely uniformize the distribution of carbides. 12. 12. id="p-12" id="p-12" id="p-12" id="p-12" id="p-12" id="p-12" id="p-12" id="p-12" id="p-12" id="p-12" id="p-12"

[0012] Therefore, an object of the present invention is to provide a spheroidizing annealingmethod that can shorten a spheroidizing annealing time by skipping a gradual coolingprocess and can obtain a microstructure in which the carbides are uniformly dispersed.[0013] The inventors have recently devised a spheroidizing annealing method for case-hardeningsteel that is capable to skip the gradual cooling process to shorten the spheroidizingannealing time, and that is capable to obtain the microstructure With uniformly dispersedcarbides, by adjusting sample material composition. 14. 14. id="p-14" id="p-14" id="p-14" id="p-14" id="p-14" id="p-14" id="p-14" id="p-14" id="p-14" id="p-14" id="p-14"

[0014] That is, in order to achieve the spheroidizing annealing in Which the carbides are uniformlydispersed while the annealing time is shortened, by adjusting sample material composition,(a) temperature A1 is made higher than that of normal case-hardening steel, and (b) nano-order fine precipitates (Al nitrides, Nb carbonitrides, Fe carbides, Cr carbides, etc.) aredispersed in ferrite in the microstructure before spheroidizing annealing. . . id="p-15" id="p-15" id="p-15" id="p-15" id="p-15" id="p-15" id="p-15" id="p-15" id="p-15" id="p-15" id="p-15"

[0015] The temperature A1 can be obtained by calculating as in [Formula 1].

[Formula 1]: A1 = 723°C -14Mn [%] +22Si [%] -14.4Ni [%] +23.3Cr [%]. 16. 16. id="p-16" id="p-16" id="p-16" id="p-16" id="p-16" id="p-16" id="p-16" id="p-16" id="p-16" id="p-16" id="p-16"

[0016] The present invention provides the following items: [ltem 1] A method for spheroidizing annealing a steel material, comprising annealing a steelmaterial having a temperature A1 of 750°C or more and consisting of, by mass%, 0.15 to 0.26% of C,0.05 to 1.00% of Si,0.1 to 0.9% of Mn,0.030% or less of P,0.030% or less of S, 1.30 to 2.50% of Cr, 0.020 to 0.050% of Al, 0.0040 to 0.0300% of N, optionally at least one or more of 0 to 2.00% of Ni and 0 to 2.00% of Mo, optionally at least one or more of 0 to 0.10% of Nb, 0 to 0.200% of Ti, 0 to 0.0050%of B and 0 to 0.500% of V, and the balance being Fe and unavoidable impurities,so as to satisfy the following conditions: a holding temperature T(°C) of the spheroidizing annealing: (A1 - 30) S T s (A1 - 5),and a holding time t(h) of the spheroidizing annealing: t 2 120 / (T - A1 + 50).[ltem 2]The method according to item 1, wherein the steel material comprises, by mass%, at leastone or more of 0.02 to 2.00% of Ni and 0.05 to 2.00% of Mo.[ltem 3]The method according to ltem 1 or 2, wherein the steel material comprises, by mass%, atleast one or more of 0.02 to 0.10% of Nb, 0.020 to 0.200% of Ti, 0.0010 to 0.0050% of Band 0.010 to 0.500% of V.[0017]The steel composed of chemical composition of the present invention has a mixed-phasestructure of ferrite, pearlite and bainite, in the microstructure before spheroidizingannealing, and by holding the holding temperature at A1 or less during the spheroidizingannealing treatment, (a) spherical carbides are directly precipitated in the ferrite grainswith fine precipitates as a nuclei, and (b) the spheroidization of carbides constitutingpearlite and bainite is promoted, resulting in a provision of a microstructure With uniformspherical-carbide dispersion Within the ferrite grains after cooling.[0018]Further, when the spheroidizing annealing is conducted by the heat treatment held at A1 or lower, a long gradual cooling process as usually conducted is not necessary because austenite is not involved in formation of the spherical carbides. Therefore, the total timerequired for the spheroidizing annealing treatment can be greatly reduced. 19. 19. id="p-19" id="p-19" id="p-19" id="p-19" id="p-19" id="p-19" id="p-19" id="p-19" id="p-19" id="p-19" id="p-19"

[0019] Further, in the steel material spheroidize-annealed by means of the present invention, themicrostructure is as follows: an area fraction of the ferrite grains in which sphericalcarbides are not precipitated is 3% or less; an area fraction of pearlite or bainite is 5% or less; and no formation of martensite is observed. Also, material hardness of the steel is 83 HRB or less.

BRIEF DESCRIPTION OF DRAWINGS . . id="p-20" id="p-20" id="p-20" id="p-20" id="p-20" id="p-20" id="p-20" id="p-20" id="p-20" id="p-20" id="p-20"

[0020] Figure 1 is a schematic diagram to illustrate a holding temperature T (°C) and a holdingtime t (h) of spheroidizing annealing.

Figure 2 shows photographs of a microstructure observed by an optical microscope of anexample of measuring an area fraction of ferrite in which spheroidized carbides are notprecipitated. The photograph (left) shows an inventive example with 0% of the areafraction of ferrite; and the photograph (right) shows a comparative example with 78.1% ofthe area fraction of ferrite.

Figure 3 shows photographs of a microstructure observed by an optical microscope of anexample of evaluating area fractions of pearlite and bainite. The photograph (left) showsan inventive example with 0% of the area fractions of pearlite and bainite; and thephotograph (right) shows a comparative example with 21.1% of the area fractions ofpearlite and bainite.

Figure 4 shows a photograph of a microstructure observed by an optical microscope of formation of martensite.

DESCRIPTION OF EMBODIMENTS[0021]Prior to description of embodiments for carrying out the present invention, reasons for limiting chemical composition of inventive steel to which a method according to the present invention is applied, and reasons for limiting characteristics in case that a materialof the inventive steel is carburized, is described. ln addition, % in the chemicalcomponents is mass %. 22. 22. id="p-22" id="p-22" id="p-22" id="p-22" id="p-22" id="p-22" id="p-22" id="p-22" id="p-22" id="p-22" id="p-22"

[0022] C: 0.15 to 0.26% C is an element that increases material hardness. However, When C content is less than0.15%, strength becomes insufficient due to reduction of core hardness after carburizing.On the other hand, when the C content exceeds 0.26%, workability is reduced due toincrease in material hardness, resulting in inferior machinability and cold workability.Thus, the C content is 0.15 to 0.26%, preferably 0.17 to 0.26%, and more preferably 0.18to 0.26%. 23. 23. id="p-23" id="p-23" id="p-23" id="p-23" id="p-23" id="p-23" id="p-23" id="p-23" id="p-23" id="p-23" id="p-23"

[0023] Si: 0.05 to 1.00% Si is a deoxidizer and an element that increases material hardness. However, when Sicontent is less than 0.05%, it is insufficient as a deoxidizer and deoxidation is notsufficient. On the other hand, when the Si content exceeds 1.00%, workability is reduceddue to increase in material hardness, and carburizing inhibition also occurs. Thus, the Sicontent is 0.05 to 1.00%, preferably 0.20 to 0.80%, and more preferably 0.30 to 0.70%.[0024] Mn: 0.1 to 0.9% Mn is an element that contributes to hardening. When Mn content is less than 0.1%,sufficient hardenability is not accompanied, i.e., insufficient in hardening. When the Mncontent exceeds 0.9%, workability is reduced. Thus, the Mn content is 0.1 to 0.9%,preferably 0.15 to 0.60%, and more preferably 0.20 to 0.50%. . . id="p-25" id="p-25" id="p-25" id="p-25" id="p-25" id="p-25" id="p-25" id="p-25" id="p-25" id="p-25" id="p-25"

[0025] P: 0.030% or less P is an unavoidable impurity, but when P content exceeds 0.030%, toughness is reduceddue to grain boundary segregation. Thus, the P content is 0.030% or less. The P content is typically more than 0% and 0.030% or less, and more typically 0 to 0.025%.[0026] S: 0.030% or less S is an unavoidable impurity, but when S content exceeds 0.030%, toughness is reduceddue to formation of MnS, and fatigue strength is also reduced. Thus, the S content is0.030% or less. The S content is typically more than 0% and 0.030% or less, and moretypically 0 to 0.025%. 27. 27. id="p-27" id="p-27" id="p-27" id="p-27" id="p-27" id="p-27" id="p-27" id="p-27" id="p-27" id="p-27" id="p-27"

[0027] Cr: 1.30 to 2.50% Cr is an element that contributes to increase of A1 and hardenability. When Cr content isless than 1.30%, the increase of the A1 is insufficient. Further, the hardenability is alsoinsufficient. However, when the Cr content exceeds 2.50%, workability is reduced due tothe increase in material hardness. Thus, the Cr content is 1.30 to 2.50%, preferably 1.40to 2.30%, and more preferably 1.60 to 2.20%. 28. 28. id="p-28" id="p-28" id="p-28" id="p-28" id="p-28" id="p-28" id="p-28" id="p-28" id="p-28" id="p-28" id="p-28"

[0028] Al: 0.020 to 0.050% Al is a deoxidizer and an element that forms fine nitrides and suppresses the coarseningof crystal grain. When Al content is less than 0.020%, it is insufficient as the deoxidizer.Further, this results in short of fine nitrides, coarsening crystal grains, thereby reducing the toughness and fatigue properties. On the other hand, when the Al content exceeds 0.050%, coarse nitrides are formed, resulting in reduced fatigue properties and workability.

Thus, the Al content is 0.020 to 0.050%, preferably 0.020 to 0.045%, and more preferably0.025 to 0.040%. 29. 29. id="p-29" id="p-29" id="p-29" id="p-29" id="p-29" id="p-29" id="p-29" id="p-29" id="p-29" id="p-29" id="p-29"

[0029] N: 0.0040 to 0.0300% When N exceeds 0.0300%, it forms coarse carbonitrides, reducing fatigue properties andworkability. But, when the N content is less than 0.0040%, crystal grains tend to becomecoarse because of a shortage of fine carbonitrides, thereby reducing toughness andfatigue properties. Thus, the N content is 0.0040 to 0.0300%, preferably 0.0050 to0.0250%, and more preferably 0.0100 to 0.0200%. . . id="p-30" id="p-30" id="p-30" id="p-30" id="p-30" id="p-30" id="p-30" id="p-30" id="p-30" id="p-30" id="p-30"

[0030] Next, components that can be added optionally to the chemical composition of thesesteels will be described. That is, the elements described together with their preferredcontent ranges are optional elements, and therefore, the content of each optional elementcan be 0%. 31. 31. id="p-31" id="p-31" id="p-31" id="p-31" id="p-31" id="p-31" id="p-31" id="p-31" id="p-31" id="p-31" id="p-31"

[0031] Nk0to200% Ni is an element that improves hardenability and toughness. When Ni content is 0.02%or more, a hardenability-improving effect becomes greater, and a toughness-improvingeffect also becomes greater. On the other hand, when the Ni content exceeds 2.00%,cost increases, and workability is reduced due to the increase in material hardness.Thus, when Ni is added, the Ni content is 0 to 2.00%, preferably 0.02 to 2.00%, morepreferably 0.05 to 1.50%, and even more preferably 0.15 to 1.00%. 32. 32. id="p-32" id="p-32" id="p-32" id="p-32" id="p-32" id="p-32" id="p-32" id="p-32" id="p-32" id="p-32" id="p-32"

[0032] Mm0to200% Mo is an element that improves hardenability. When Mo content is 0.05% or more, ahardenability-improving effect becomes greater. On the other hand, When the Mocontent exceeds 2.00%, cost increases, and workability is also reduced due to theincrease in material hardness. Thus, when Mo is added, the Mo content is 0 to 2.00%,preferably 0.05 to 2.00%, more preferably 0.05 to 1.50%, and even more preferably 0.15to100%. 33. 33. id="p-33" id="p-33" id="p-33" id="p-33" id="p-33" id="p-33" id="p-33" id="p-33" id="p-33" id="p-33" id="p-33"

[0033] Nb: 0 to 0.10% Nb is an element that suppresses the coarsening of crystal grains through formation offine carbonitrides. When Nb content is 0.02% or more, the fine carbonitrides aresufficiently formed, and a suppressing effect of the coarsening of crystal grains becomesgreater, to improve toughness and fatigue strength. When the Nb content exceeds0.10%, the amount of carbonitrides is excessive, to reduce workability. Thus, when Nb isadded, the Nb content is 0 to 0.10%, preferably 0.02 to 0.10%, more preferably 0.02 to0.09%, and even more preferably 0.03 to 0.08%. 34. 34. id="p-34" id="p-34" id="p-34" id="p-34" id="p-34" id="p-34" id="p-34" id="p-34" id="p-34" id="p-34" id="p-34"

[0034] Ti: 0 to 0.200% Ti is an element that suppresses the coarsening of crystal grains through formation of finecarbonündes. VVhen Ticontentis 0.0209ß orrnore,the arnourfi offine carbonündesissufficient, and N is fixed, to prevent formation of BN, improving hardenability, and also asuppressing effect of the coarsening of crystal grains becomes greater. When the Ticontent exceeds 0.200%, the amount of carbonitrides is excessive, to reduce workability.Thus, when Ti is added, the Ti content is 0 to 0.200%, preferably 0.020 to 0.200%, morepreferably 0.020 to 0.150%, and even more preferably 0.020 to 0.100%. . . id="p-35" id="p-35" id="p-35" id="p-35" id="p-35" id="p-35" id="p-35" id="p-35" id="p-35" id="p-35" id="p-35"

[0035] B: 0 to 0.0050% B is an element that improves hardenability and increases material hardness. When Bcontent is 0.0010% or more, a hardenability-improving effect becomes greater. On theother hand, when the B content exceeds 0.0050%, material hardness is increased, toreduce workability. Thus, when B is added, the B content is 0 to 0.0050%, preferably0.0010 to 0.0050%, more preferably 0.0010 to 0.0040%, and even more preferably 0.0010to 0.0030%. 36. 36. id="p-36" id="p-36" id="p-36" id="p-36" id="p-36" id="p-36" id="p-36" id="p-36" id="p-36" id="p-36" id="p-36"

[0036] V: 0 to 0.500% V is an element that suppresses the coarsening of crystal grains through formation of finecarbonitrides. When V content is 0.010% or more, the fine carbonitrides are sufficientlyformed, and a suppressing effect of coarsening of crystal grains becomes greater, toimprove toughness and fatigue strength. On the other hand, When the V contentexceeds 0.500%, the amount of carbonitrides is excessive, to reduce workability. Thus,when V is added, the V content is 0 to 0.500%, preferably 0.010 to 0.500%, morepreferably 0.050 to 0.400%, and even more preferably 0.100 to 0.350%. 37. 37. id="p-37" id="p-37" id="p-37" id="p-37" id="p-37" id="p-37" id="p-37" id="p-37" id="p-37" id="p-37" id="p-37"

[0037] Next, reasons for specifying temperature A1, and also specifying a holding temperature T(°C) and a holding time t (hr) Which are conditions of spheroidizing annealing Will be described. 38. 38. id="p-38" id="p-38" id="p-38" id="p-38" id="p-38" id="p-38" id="p-38" id="p-38" id="p-38" id="p-38" id="p-38"

[0038] 11 A1 temperature: 750°C or more When temperature A1 is lower than 750°C, the holding temperature of spheroidizingannealing becomes low, and spheroidization of carbides constituting pear|ite and bainitebecomes insufficient, resulting in insufficient softening effect. Thus, temperature A1 is750°C or more. Preferably, temperature A1 is 750°C to 800°C. 39. 39. id="p-39" id="p-39" id="p-39" id="p-39" id="p-39" id="p-39" id="p-39" id="p-39" id="p-39" id="p-39" id="p-39"

[0039] Holding Temperature T (°C) of Spheroidization Annealing: (A1 - 30°C) s T s (A1 - 5°C)Holding Time t (h) of Spheroidization Annealing: t 2 120 / (T - A1 + 50) Firstly, with respect to the holding temperature T and the holding time t in the heattreatment of spheroidization annealing, as schematically shown in Figure 1, the holdingtemperature is a temperature T that is held after a temperature of steel material is raisedfor annealing, and the holding time is time t for which the steel material is held at thetemperature T. 40. 40. id="p-40" id="p-40" id="p-40" id="p-40" id="p-40" id="p-40" id="p-40" id="p-40" id="p-40" id="p-40" id="p-40"

[0040] Next, why the holding temperature T (°C) of spheroidizing annealing is set between (A1 -30°C) and (A1 - 5°C) is as follows: when the holding temperature is lower than (A1 - 30°C),the spheroidization of the carbides constituting pear|ite and bainite becomes insufficient,resulting in insufficient softening effect; and on the other hand, when the holdingtemperature T (°C) of spheroidizing annealing is higher than (A1 - 5°C), the materialtemperature may exceed A1 due to temperature variation in an annealing furnace, causingaustenite to be formed, and therefore pear|ite or martensite is formed at a time ofsubsequent air cooling or water cooling, resulting in increased hardness, to reduceworkability. 41. 41. id="p-41" id="p-41" id="p-41" id="p-41" id="p-41" id="p-41" id="p-41" id="p-41" id="p-41" id="p-41" id="p-41"

[0041] Why the holding time t (h) held at the holding temperature T of spheroidizing annealing isset to 120 / (T - A1 + 50) or more is as follows: when the time held at the holdingtemperature is short, the spheroidization of the carbides constituting pear|ite and bainitebecomes insufficient, resulting in insufficient softening effect. 42. 42. id="p-42" id="p-42" id="p-42" id="p-42" id="p-42" id="p-42" id="p-42" id="p-42" id="p-42" id="p-42" id="p-42"

[0042] 12 When the steel having temperature A1 of 750°C or higher composed of the chemicalcomposition described in the means of the present invention is annealed by apredetermined means, a microstructure that spherical carbides are uniformly dispersed inferrite is obtained, and a steel material having a material hardness of 83 HRB or less isobtained. 43. 43. id="p-43" id="p-43" id="p-43" id="p-43" id="p-43" id="p-43" id="p-43" id="p-43" id="p-43" id="p-43" id="p-43"

[0043] That is, the steel material spheroidize-annealed by the means of the present invention hasa microstructure as follows: (1) an area fraction of the ferrite grains in which sphericalcarbides are not precipitated is 3% or less; (2) an area fraction of pearlite or bainite is 5%or less; and (3) no martensite is formed.

Further, the material hardness of the steel is 83 HRB or less. 44. 44. id="p-44" id="p-44" id="p-44" id="p-44" id="p-44" id="p-44" id="p-44" id="p-44" id="p-44" id="p-44" id="p-44"

[0044] Now, chemical component values of inventive steel composition and comparative steelcomposition used for the sample materials, and A1 obtained by calculation are shown inTable 1. Using the values as references, steel having the temperature A1 of 750°C orhigher can be selected.

Also, the calculated A1 can be obtained by substituting [%] value of each chemicalcomponent into the following formula: A1 = 723°C - 14Mn [%] + 22Si [%] - 14.4Ni [%] +23.3Cr [%]. 45. 45. id="p-45" id="p-45" id="p-45" id="p-45" id="p-45" id="p-45" id="p-45" id="p-45" id="p-45" id="p-45" id="p-45"

[0045][Table1](Unit is mass %)c si Mn P s cr Ai N Ni M6 Ti B Nb v Camäiatedö så* 0.15 0.11 0.65 0.007 0.017 2.20 0.047 0.0242 0.12 0.02 - - - - 766ä B2 stee| 0.22 0.97 0.31 0.021 0.024 1.69 0.037 0.0155 1.15 0.29 - - - - 763É5 så* 0.20 0.53 0.26 0.007 0.009 1.36 0.023 0.0160 0.03 0.03 0.0015 - 0.04 - 773Cl.).E11,_9,31 D 0.20 0.25 0.73 0.009 0.010 1.05 0.310 0.0145 0.10 0.02 - - - - 741gu, steelOO 1) P and S are unavoidable impurities.2) Shaded area shows outside the inventive range.3) A1 = 723°C - 14Mn [%] + 22Si [%] - 14.4Ni [%] + 23.3Cr [%]. 46. 46. id="p-46" id="p-46" id="p-46" id="p-46" id="p-46" id="p-46" id="p-46" id="p-46" id="p-46" id="p-46" id="p-46"

[0046] 13 Next, using each of the prepared sample materials, heat treatment conditions (holdingtemperature, holding time, and cooling method) for spheroidizing annealing treatment, thepost-annealed microstructure (presence or absence of martensite, area fractions of pear|ite and bainite), and hardness of the material are shown in Table 2. 47. 47. id="p-47" id="p-47" id="p-47" id="p-47" id="p-47" id="p-47" id="p-47" id="p-47" id="p-47" id="p-47" id="p-47"

[0047][Table 2]Spheroidizing annealing conditions EffectsNO Holding . ln- Area ffactpn (%) of Area fraction Presence or- Holding . oi grams in which . Hardnesstempeorature T time t (h) Cooling furnace spherica' Carbides (%) of pear|ite Absence of (HRB)( C) time (h) are not precipitated and bainite martensite 1 770 1 water cooling 1.5 0.0 7.2 Presence 82.52 770 4 water cooling 4.5 8.5 0.0 Presence 88.03 770 6 water cooling 6.5 4.5 0.0 Presence 90.04 750 1 air cooling 1.5 0.0 12.9 Absence 84.0stêe' 5 750 4 air cooling 4.5 0.0 2.1 Absence 82.06 750 6 air cooling 6.5 0.0 0.0 Absence 81.07 740 1 air cooling 1.5 1.2 18.6 Absence 85.68 740 4 air cooling 4.5 0.0 10.6 Absence 84.19 740 6 water cooling 6.5 0.0 0.0 Absence 81.8C 10 770 1 water cooling 1.5 0.0 6.2 Presence 82.2På 11 770 4 water cooling 4.5 6.2 0.0 Presence 92.3å. 12 770 6 water cooling 6.5 10.2 0.0 Presence 96.8ä B 13 750 1 air cooling 1.5 0.0 12.0 Absence 84.1ä stee' 14 750 4 air cooling 4.5 0.0 3.3 Absence 82.2ï 15 750 6 air cooling 6.5 0.0 0.0 Absence 81.3å 16 740 1 air cooling 1.5 3.5 20.8 Absence 85.5ä 17 740 4 air cooling 4.5 0.0 10.0 Absence 83.8E 18 740 6 water cooling 6.5 0.0 0.0 Absence 82.119 780 1 water cooling 1.5 0.0 5.6 Presence 81.320 780 4 water cooling 4.5 0.0 0.0 Presence 92.521 780 6 water cooling 6.5 4.2 0.0 Presence 95.622 760 1 air cooling 1.5 0.0 11.2 Absence 83.48:e' 23 760 4 air cooling 4.5 0.0 2.1 Absence 81.824 760 6 air cooling 6.5 0.0 0.0 Absence 79.525 730 1 air cooling 1.5 3.2 21.1 Absence 84.126 730 4 air cooling 4.5 0.0 10.5 Absence 83.427 730 6 air cooling 6.5 0.0 8.5 Absence 82.028 740 2 water cooling 2.5 81.1 16.8 Absence 83.1_ 29 740 4 water cooling 4.5 81.3 13.1 Absence 82.1å: c 30 740 6 water cooling 6.5 82.1 3.7 Absence 81.6å .å D 31 730 2 air cooling 2.5 80.0 18.9 Absence 83.6"å g stee' 32 730 4 air cooling 4.5 82.3 14.5 Absence 82.8å ä 33 730 6 air cooling 6.5 80.8 13.2 Absence 82.08 34 720 1 air cooling 1.5 81.4 18.6 Absence 84.535 720 2 air cooling 2.5 78.1 16.5 Absence 83.536 720 6 air cooling 6.5 82.4 12.1 Absence 82.4 * Shaded portions indicate that T or t is out of condition, or effect is not accompanied. 48. 48. id="p-48" id="p-48" id="p-48" id="p-48" id="p-48" id="p-48" id="p-48" id="p-48" id="p-48" id="p-48" id="p-48"

[0048]After 100 kg of steel composition of A to D composed of chemical components listed in Table 1 and the balance Fe with unavoidable impurities were melted in a vacuum melting 14 furnace and casted into steel ingots, and were subjected to extended forging to a diameterof 32 mm at 1250°C, and then normalized at 925°C for 1 hr. These were cut into 100mm pieces and prepared as the sample materials for spheroidizing annealing. 49. 49. id="p-49" id="p-49" id="p-49" id="p-49" id="p-49" id="p-49" id="p-49" id="p-49" id="p-49" id="p-49" id="p-49"

[0049] With respect to the prepared sample materials, the spheroidizing annealing wasconducted using a Kanthal furnace according to the following procedure. The preparedsample material was put into the furnace set at the holding temperature of spheroidizingannealing specified in Table 2, kept for 30 minutes for heating, and held in the furnace forthe holding time specified in Table 2. Subsequently, air or water cooling was carried outas specified in Table 2. ln this way, the spheroidizing annealing treatment wasconducted. Also, the total time that the sample material was present in the furnace isshown in Table 2. 50. 50. id="p-50" id="p-50" id="p-50" id="p-50" id="p-50" id="p-50" id="p-50" id="p-50" id="p-50" id="p-50" id="p-50"

[0050] As evaluation of the characteristics of each sample material which was subjected to thespheroidizing annealing treatment, (1) microstructure observation (a. an area fraction ofthe ferrite grains in which spherical carbides were not precipitated, b. area fractions ofpearlite and bainite, and c. presence or absence of martensite) was conducted and (2) thematerial hardness was measured. ln Table 2, results of each of sample materials, i.e.,the presence or the absence of martensite, the area fractions of pearlite and bainite, thearea fraction of the ferrite grains (oi grains) in which spherical carbides were not formed,and the hardness, are shown. 51. 51. id="p-51" id="p-51" id="p-51" id="p-51" id="p-51" id="p-51" id="p-51" id="p-51" id="p-51" id="p-51" id="p-51"

[0051] (1) Microstructure To observe the microstructure of the sample material, the sample material was first cutthrough its center and parallel to a rolling direction, the cut surface was polished, and thepolished surface was corroded with a Nital solution.

After that, a D/4 position of the cut surface was observed by using an optical microscope, and evaluated as follows. a. Evaluation of the area fraction of the ferrite grains in which spherical carbides were notprecipitated At the D/4 position, an image was taken with a field of view of ><400, and the area fractionof ferrite in which spherical carbides were not precipitated in the target area of the imagewas measured. b. Evaluation of the area fractions of pearlite and bainite At the D/4 position, an image was taken with the field of view of ><400, and the areafractions of pearlite and bainite in the target area of the image was measured. c. Evaluation of the presence or the absence of the generation of martensite At the D/4 position, an image was taken with the field of view of ><400, and the presence orthe absence of the generation of martensite structure in the target area of the image wasobserved. 52. 52. id="p-52" id="p-52" id="p-52" id="p-52" id="p-52" id="p-52" id="p-52" id="p-52" id="p-52" id="p-52" id="p-52"

[0052] (2) Material hardness The sample material was cut in a direction perpendicular to the rolling direction, and aftersurface grinding of the cut surface, Rockwell hardness testing was conducted at the D/4position, and the hardness obtained was shown in Table 2 as the material hardness.[0053] ln Figures 2 to 4, the images taken by ><400 at the D/4 position are shown.

Figure 2 shows photographs of the microstructure observed by the optical microscope ofan example of measuring the area fraction of ferrite in which spheroidized carbides werenot precipitated. The photograph (left) shows a sectional microstructure of the samplematerial of an inventive example, that is obtained by spheroidizing annealing the C steelhaving an inventive steel composition, under the conditions of the holding temperature of760°C, the holding time of 6 h, and air cooling; and the photograph (right) shows asectional microstructure of the sample material of the comparative example in which the Dsteel having the comparative steel composition was air cooled under the conditions of theholding temperature of 720°C, the holding time of 2 h. The photograph (left) correspondsto the inventive example, and shows that the area fraction of ferrite is 0%; and the photograph (right) shows that the area fraction of ferrite is 78.1%. 16 54. 54. id="p-54" id="p-54" id="p-54" id="p-54" id="p-54" id="p-54" id="p-54" id="p-54" id="p-54" id="p-54" id="p-54"

[0054] Figure 3 shows photographs of the microstructure observed by the optical microscope ofan example of evaluating the area fractions of pearlite and bainite. The photograph (left)shows a sectional microstructure of the sample material of an inventive example, that isobtained by spheroidizing annealing the C steel having an inventive steel composition,under the conditions of the holding temperature of 760°C, the holding time of 6 h, and aircooling; and the photograph (right) shows a sectional microstructure of the samplematerial of the comparative example in which the C steel having an inventive steelcomposition was air cooled under the conditions of the holding temperature of 730°C, theholding time of 1 h. The photograph (left) corresponds to the inventive example, andshows that the area fractions of pearlite and bainite are 0%; and the photograph (right) isthe comparative example and shows that the area fractions of pearlite and bainite are21.1%. 55. 55. id="p-55" id="p-55" id="p-55" id="p-55" id="p-55" id="p-55" id="p-55" id="p-55" id="p-55" id="p-55" id="p-55"

[0055] Figure 4 shows an example of formation of martensite. The photograph shows asectional microstructure of the sample material of the comparative example in which the Csteel having an inventive steel composition was spheroidize-annealed under theconditions of the holding temperature of 780°C, the holding time of 6 h, and water cooling,and generation of the martensite structure was observed. 56. 56. id="p-56" id="p-56" id="p-56" id="p-56" id="p-56" id="p-56" id="p-56" id="p-56" id="p-56" id="p-56" id="p-56"

[0056] Table 2 shows inventive examples (Nos. 5, 6, 9, 14, 15, 18, 23, and 24 in Table 2) inwhich the steel having the inventive steel composition was spheroidize-annealed in a waythat satisfies the spheroidizing annealing conditions of the present invention, comparativeexamples (Nos. 1 to 4, 7, 8, 10 to 13, 16, 17, 19 to 22, and 25 to 27 in Table 2) in whichthe steel having the inventive steel composition was heat-treated in a way that does notsatisfy the spheroidizing annealing conditions of the present invention, and alsocomparative examples (Nos. 28 to 36 in Table 2) in which the steel having the comparative steel composition was spheroidize-annealed. 57. 57. id="p-57" id="p-57" id="p-57" id="p-57" id="p-57" id="p-57" id="p-57" id="p-57" id="p-57" id="p-57" id="p-57"

[0057] 17 The steel having an inventive steel composition and satisfying the spheroidizing annealingconditions of the present invention (Nos. 5, 6, 9, 14, 15, 18, 23 and 24 in Table 2, i.e., theinventive examples) had 3% or less of the area fraction of the ferrite grains in whichspherical carbides were not precipitated and 5% or less of the area fraction of pearlite orbainite, and no generation of martensite was observed, such that the material hardnesswas 83 HRB or less. 58. 58. id="p-58" id="p-58" id="p-58" id="p-58" id="p-58" id="p-58" id="p-58" id="p-58" id="p-58" id="p-58" id="p-58"

[0058] On the other hand, even if the steel had the inventive steel composition, the comparativeexamples treated in the different heat-treatment conditions did not exhibit thecharacteristics of the inventive examples, e.g., in the comparative example in Which theholding temperature was high, the martensite structure was generated, and also in a partof those comparative examples, there was ones that the material hardness became toohard, ones that the area fraction of pearlite or bainite Was high, ones that the area fractionof the ferrite grains in Which spherical carbides Were not precipitated Was high, etc..Further, in a comparative example in Which the C steel having the inventive steelcomposition Was normalized at the low holding temperature, the area fractions of pearliteand bainite became high.

Further, in a comparative example in Which the holding time Was insufficient, the areafractions of pearlite and bainite became high, or in some cases the material hardness became too hard.

Claims (3)

1. 1. A method for spheroidizing annealing a steel material, comprising annealing a steelmaterial having a temperature A1 of 750°C or more and consisting of, by mass%, 0.15 to 0.26% of C, 0.05 to 1.00% of Si, 0.1 to 0.9% of Mn, 0.030% or less of P, 0.030% or less of S, 1.30 to 2.50% of Cr, 0.020 to 0.050% of Al, 0.0040 to 0.0300% of N, optionally at least one or more of 0 to 2.00% of Ni and 0 to 2.00% of Mo, optionally at least one or more of 0 to 0.10% of Nb, 0 to 0.200% of Ti, 0 to 0.0050%of B and 0 to 0.500% of V, and the balance being Fe and unavoidable impurities,so as to satisfy the following conditions: a holding temperature T(°C) of the spheroidizing annealing: (A1 - 30) S T s (A1 - 5),and a holding time t(h) of the spheroidizing annealing: t 2 120 / (T - A1 + 50).

2. The method according to claim 1, wherein the steel material comprises, by mass%, at least one or more of 0.02 to 2.00% of Ni and 0.05 to 2.00% of Mo.

3. The method according to claim 1 or 2, wherein the steel material comprises, bymass%, at least one or more of 0.02 to 0.10% of Nb, 0.020 to 0.200% of Ti, 0.0010 to0.0050% of B and 0.010 to 0.500% of V.