US20180044757A1 - Age-hardening steel and method of manufacturing parts using age-hardening steel - Google Patents

Age-hardening steel and method of manufacturing parts using age-hardening steel Download PDF

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US20180044757A1
US20180044757A1 US15/556,473 US201615556473A US2018044757A1 US 20180044757 A1 US20180044757 A1 US 20180044757A1 US 201615556473 A US201615556473 A US 201615556473A US 2018044757 A1 US2018044757 A1 US 2018044757A1
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steel
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Yutaka Neishi
Masato Yuya
Tatsuya Hasegawa
Motoki TAKASUGA
Masashi Higashida
Hitoshi Matsumoto
Taizo Makino
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite

Definitions

  • the present invention relates to age hardening steel, more particularly relates to age hardening steel which is cut etc. to work it to a predetermined shape, then is treated for age hardening (below, referred to simply as “aging treatment”). Further, the present invention relates to a method of production of a part using such age hardening steel.
  • age hardening steel which can be kept low in hardness at the shaping stage and, after that, can be treated to age it to raise the hardness in the final product stage has been proposed (for example, see WO2010/090238A (PLT 1), Japanese Patent Publication No. 2012-246527A (PLT 2), Japanese Patent Publication No. 2011-241441A (PLT 3), Japanese Patent Publication No. 2012-193416A (PLT 4), and Japanese Patent No. 5343923B2 (PLT 5)).
  • PLTs 1 and 2 disclose a method of production controlling the cooling rate after shaping by hot forging, suppressing the formation of structures other than bainite, keeping down the amount of precipitation of VC during cooling, and securing the amount of solute V so as to be able to obtain a sufficient age hardenability.
  • the method of production described in PLTs 1 and 2 at the time of the cooling step after hot forging, it is necessary to control the cooling rate at each specific temperature region, there are restrictions in facilities, apparatuses, etc., and also sometimes quenching is not possible on an actual production line, so it was difficult to stably produce age hardening steel.
  • PLTs 3 to 5 propose age hardening steel for use as a material of a machine part not requiring strict conditions to be set at the cooling step after hot forging and enabling cooling by air cooling and fans in production.
  • the steel used as a material for a machine part is required to be excellent in machinability at the stage of production of the machine part and to be excellent in fatigue strength after the machine part is completed.
  • the above demands can be met by steel provided with the characteristics of being low in hardness before the aging treatment and being raised in hardness after aging treatment.
  • a large difference between the hardness before the aging treatment and the hardness after aging treatment (that is, a high age hardenability) is preferable for obtaining a machine part which is excellent in both productivity and fatigue strength.
  • the methods of production for obtaining age hardening steel according to the prior art require inclusion of a step of quenching the steel. This quenching step causes an increase in the cost of manufacturing age hardening steel.
  • the present invention was made in consideration of such an actual situation and has as its object the provision of age hardening steel in which the production conditions are not particularly limited, the machinability before aging treatment is excellent, hardening by aging treatment can be used to stably improve the fatigue strength, and a drop in toughness due to aging treatment can be suppressed.
  • V is present forming a solid solution in steel during the hot forging performed at a general temperature.
  • the reason is that the starting temperature for formation of V carbides or V carbonitrides (precipitation temperature) is low.
  • V is strong in ability to form precipitates (V carbides or V carbonitrides) at the aging treatment, so is an element effective for hardening by aging treatment.
  • the content of N is large, V nitrides are formed at the time of cooling after hot forging and before aging treatment, the hardness rises before aging treatment, and the machinability becomes impaired. Based on these discoveries, the inventors experimented with the promotion of formation of V carbides or V carbonitrides after aging treatment and suppression of formation of V nitrides before aging treatment.
  • Nb precipitates in steel as carbides or carbonitrides in the process of heating and being worked at the time of hot forging and has the effect of refining the austenite crystal grain size due to the pinning effect, and, after that, refining the bainite structure in the bainite transformation. Furthermore, part of the Nb in steel does not precipitate as carbides or carbonitrides at the time of hot forging but is present as solute Nb. This solute Nb precipitates as Nb carbides or Nb carbonitrides at the time of aging treatment after hot forging and has the effect of raising the hardness without inviting a drop in toughness and thereby achieving an improvement in the low cycle fatigue strength and fatigue strength. Based on these discoveries, the inventors experimented with utilizing Nb to suppress the drop in toughness due to aging treatment.
  • REMs are elements which form sulfide-based inclusions or oxide-based inclusions and have the effect of causing the inclusions to finely disperse and making the inclusions spherical in shape.
  • the content of REMs is too great, a drop in the hot ductility of the steel material at the time of hot rolling or hot forging will end up being caused.
  • the inventors tried to find the suitable REM content and searched for and determined conditions enabling stable improvement of the fatigue strength by hardening by aging treatment and stably preventing a drop in toughness after aging treatment.
  • the present invention was made based on such a discovery and has as its gist the following:
  • An age hardening steel containing, by mass %, C: 0.05 to 0.20%, Si: 0.01 to 0.50%, Mn: 1.50 to 2.50%, S: 0.005 to 0.080%, Cr: 0.03 to 1.60%, Al: 0.005 to 0.050%, V: 0.25 to 0.50%, Nb: 0.010 to 0.100%, Ca: 0.0005 to 0.0050%, and REM: 0.001 to 0.05%, limiting P to 0.030% or less, Ti to less than 0.005%, and N to less than 0.0080%, having a balance of Fe and impurities, having an area ratio of a bainite structure of 70% or more and, furthermore, having a chemical composition where the following F1 expressed by formula (1) is 0.68 or more, F2 expressed by formula (2) is 0.85 or less, F3 expressed by formula (3) is 0.00 or more, and F4 expressed by formula (4) is 0.012 to 0.08:.
  • An age hardening steel containing, by mass %, C: 0.05 to 0.20%, Si: 0.01 to 0.50%, Mn: 1.50 to 2.50%, S: 0.005 to 0.080%, Cr: 0.03 to 1.60%, Al: 0.005 to 0.050%, V: 0.25 to 0.50%, Nb: 0.010 to 0.100%, Ca: 0.0005 to 0.0050%, and REM: 0.001 to 0.05%, having an area ratio of bainite structures of 70% or more, furthermore, satisfying any one or more of the conditions of the composition shown by the following ⁇ a> to ⁇ c>, limiting P to 0.030% or less, Ti to less than 0.005%, and N to less than 0.0080%, having a balance of Fe and impurities, and, furthermore, having a chemical composition where the following F1′ expressed by formula (1′) is 0.68 or more, F2′ expressed by formula (2′) is 0.85 or less, and F3′ expressed by formula (3′) is 0.00 or more,
  • a method of production of a part using age hardening steel comprising a forging step of heating age hardening steel according to [1] or [2] at 1200 to 1250° C. for 5 to 60 minutes, then forging it so that a surface temperature after final forging becomes 1100° C. or more, then, after that, cooling it by an average cooling rate in a 800 to 400° C. temperature region of 15 to 60° C./min down to room temperature, a cutting step of cutting the forged steel, and an aging treatment step of holding the cut steel in a 540 to 700° C. temperature region for 30 to 1000 minutes.
  • age hardening steel which is not particularly limited in production conditions, is excellent in machinability before aging treatment, can use the hardening by the aging treatment to stably improve the fatigue strength, and can keep down a drop in toughness due to aging treatment. Further, by using the age hardening steel of the present invention as a material, it is possible to provide a machine part which is excellent in productivity, excellent in fatigue strength, and is sufficient in toughness.
  • the age hardening steel of the present invention has, as an indicator of the cutting resistance, a Vicker's hardness before aging treatment of 290 Hv or less.
  • the amount of rise of the Vicker's hardness (age hardenability, ⁇ Hv) due to the aging treatment when making the age hardening steel of the present invention a substantially columnar shape with a diameter of 35 mm and holding this steel at a temperature of 620° C. for 120 minutes is 30 Hv or more.
  • the age hardening steel of the present invention after the aging treatment has a fatigue strength of 425 MPa or more.
  • the age hardening steel of the present invention after aging treatment has an absorption energy at 20° C. of 50 J or more in a Charpy impact test performed using a standard test piece with a U-notch with a notch depth of 2 mm and a notch bottom radius of 1 mm and has a low cycle fatigue strength of 520 MPa or more.
  • the age hardening steel of the present invention can be extremely suitably used as a material for a machine part of automobiles, industrial machinery, construction machinery, etc. Its contribution to industry is extremely remarkable.
  • FIG. 1 is a view showing the shape of a monoaxial tension and compression type of fatigue test piece used in the examples.
  • the numerical values in the figure show the dimensions (units: mm).
  • the chemical composition of the age hardening steel according to the present embodiment will be explained focusing on the elements important from the viewpoint of age hardening.
  • the main application for the age hardening steel according to the present embodiment (below, sometimes abbreviated as the “steel according to the present embodiment”) is as the material of a machine part produced by a method of production including hot forging, machining, aging treatment, etc. Therefore, to explain the features of the steel according to the present embodiment, sometimes reference will be made to characteristics of steel after hot forging, machining, and aging treatment.
  • the steel according to the present embodiment does not necessarily require such treatment. That is, the applications of the steel according to the present embodiment are not limited to hot forging, machining, etc.
  • the fact that the V content has to be made 0.25 mass % or more was discovered by the present inventors.
  • the V content 0.25 mass % or more it is possible to make the amount of carbides of V or carbonitrides of V formed due to the aging treatment increase, raise the hardness after aging treatment, and secure fatigue strength.
  • V once forming a solid solution in steel, will not precipitate until the steel is cooled down to near 850° C. and is strong in ability to form carbides or carbonitrides at the age hardening treatment temperature. Furthermore, in the steel according to the present embodiment, in the same way as V, it is also possible to add Mo which is relatively low in temperature of precipitation of carbides and can be easily used for age hardening. If making steel containing V in 0.25 mass % or more further contain Mo, the aging treatment causes composite carbides of V and Mo or composite carbonitrides of V and Mo to be formed, so the hardness after aging treatment rises more.
  • V has the property of not precipitating once forming a solid solution in the steel until cooling the steel down to near 850° C., so is an element which can remain stably present in the steel in the solute state.
  • V carbides easily precipitate at the phase boundaries when austenite transformed to ferrite. If the amount of precipitation of V carbides increases, the amount of solute V decreases. That is, if a large amount of proeutectoid ferrite is formed during the cooling after hot forging, V carbides will precipitate at the phase boundaries, so it becomes no longer possible to secure the amount of solute V necessary for precipitation hardening due to aging treatment after that.
  • phase accounting for an area ratio of a 70% or more (below, sometimes referred to as “main phase”) becomes bainite. Further, to prevent skyrocketing costs of manufacturing a machine part, such control of the structure has to be performed not by controlling the hot forging conditions, but by controlling the chemical composition of the steel.
  • the structure after hot forging is closely correlated with the C, Mn, and Cr improving the hardenability and, furthermore, the content of Mo.
  • the present inventors discovered that if making the values of the indicators F1 and F1′ of hardenability expressed by the following formula (1) and formula (1′) specific numerical values or more by controlling the contents of these elements in these formulas, precipitation of a large amount of proeutectoid ferrite harmful to securing the solute V is suppressed in the usual cooling process after hot forging (cooling rate 15° C./min to 60° C./min).
  • the present inventors discovered that by controlling F1 and F1′, the steel structure easily becomes a structure having bainite as its main phase, that is, becomes a structure including bainite by an area ratio of 70% or more, so a sufficient amount of solute V can be secured:
  • the present inventors discovered that if controlling the chemical composition of the steel according to the present embodiment so that the values of indicators F2 and F2′ of hardness before aging treatment expressing the contents of C, Si, Mn, Cr, V, and Mo by the following formula (2) and formula (2′) become specific numerical values or less, it is possible to keep the hardness before aging treatment low and possible to suppress a rise in the cutting resistance:
  • the present inventors produced steel containing 0.25 mass % or more of V and adjusted in ingredients so that F1 and F2 or F1′ and F2′ in which the contents of C, Si, Mn, Cr, Mo, and V are found by the above formula (1) and formula (2) or formula (1′) and formula (2′) satisfy specific ranges of numerical values, hot forged, then treated this steel to age to prepare samples, and investigated the toughness of the samples. Specifically, they hot forged and treated the above steel to age, prepared standard test samples with U-notches with a notch depth of 2 mm and notch bottom radius of 1 mm, ran the test pieces through a Charpy impact test, and investigated the effects of the ingredients on the toughness after aging treatment.
  • F3 and F3′ are large, the toughness of the steel after aging treatment will be sufficient. Further, increasing the contents of C, V, and Mo decreases F3 and
  • formula (3) or formula (3′) mean that the contents of C, V, and Mo, which are required for improving the hardness and the fatigue strength after aging treatment, have to be reduced to suppress a drop in toughness due to aging treatment.
  • Nb is effective as a means for sufficiently raising the machinability before aging treatment and low cycle fatigue strength after aging treatment and suppressing a drop in toughness due to the aging treatment.
  • Nb has the effect of refining the austenite grain size before bainite transformation in the same way as Ti.
  • Nb is an element having the effect of refining the austenite grain size and has the ability to form a compound at the aging treatment temperature (secondary phase). This is because Nb has a higher precipitation temperature than V and Mo. That is, since Nb has a relatively high precipitation temperature, part of the contained Nb precipitates as carbides or carbonitrides at the time of hot forging. The carbides and other Nb precipitates contribute to the refining of the austenite grain size.
  • the present inventors obtained the discovery relating to age hardening steel which is not particularly limited in the production conditions of the steel material, is excellent in the machinability before aging treatment, is improved in the fatigue strength by hardening by aging treatment, and can suppress a drop in toughness after aging treatment.
  • the fatigue strength and toughness after aging treatment become somewhat low levels though still within the ranges of the desired values.
  • the present inventors carefully investigated the mechanism by which the fatigue strength or toughness becomes lower in level after aging treatment and as a result discovered that the coarse inclusions contained inside the steel are the cause. That is, they clarified that by suppressing the formation of such coarse inclusions, it is possible to stably improve the fatigue strength after aging treatment and suppress a drop in toughness after aging treatment.
  • a REM has the effect of forming sulfide-based inclusions or oxide-based inclusions and making both the sulfide-based inclusions and oxide-based inclusions finely disperse.
  • the REM content is too large, the hot ductility of the steel material at the time of hot rolling or hot forging ends up being made to fall.
  • both the sulfide-based inclusions and the oxide-based inclusions cannot stably finely disperse.
  • the present inventors learned that it is necessary to control the contents of Ca and a REM so that the value of the control indicator F4 of the forms of inclusions with a REM and Ca expressed by the following formula (4) becomes a specific range. The inventors discovered that by doing this, the sulfide-based inclusions disperse finely in spherical shapes and the oxide-based inclusions finely disperse:
  • both the sulfide-based inclusions and the oxide-based inclusions stably finely disperse. Further, if F4 is over 0.08, the effect becomes saturated and the hot ductility of the steel is liable to be lowered, so it is necessary to make the contents of Ca and REM suitable.
  • the present invention relates to age hardening steel obtained based on the results of the studies of the present inventors and the findings they obtained explained above. Below, the requirements of age hardening steel according to an embodiment of the present invention will be explained in detail.
  • the composition of the age hardening steel according to the present embodiment will be explained. Note that, the “%” of the content of the elements mean “mass %”.
  • C is an important element in the present embodiment. C bonds with V due to the aging treatment to form carbides and strengthen the steel. However, if the content of C is less than 0.05%, the force driving the precipitation of V carbides becomes smaller and it becomes difficult for V carbides to precipitate, so the desired strengthening effect cannot be obtained. On the other hand, if the content of C is over 0.20%, the C not bonded with V bonds with Fe to form carbides (cementite) and cause a remarkable degradation of toughness of the steel. Further, if the content of C exceeds 0.20%, the concentration of C concentrating in the austenite also becomes higher in the middle of the transformation from austenite to bainite and the structure after bainite transformation becomes partially contaminated by martensite.
  • the content of C is made 0.05 to 0.20%.
  • the content of C is preferably made 0.08% or more, more preferably 0.10% or more. Further, the content of C is preferably made 0.18% or less, more preferably 0.16% or less.
  • Si is used as a deoxidizing element at the time of steelmaking and simultaneously has the action of forming a solid solution in the matrix and improving the strength of the steel.
  • Si has to be made 0.01% or more in content.
  • the content of Si becomes excessive, a rise occurs in the hot workability and cutting resistance of the steel and a drop in machinability is invited.
  • the content of Si is over 0.50%, a drop in the hot workability and a rise in the cutting resistance of the steel become remarkable.
  • Si is liable to promote the formation of proeutectoid ferrite and decrease the amount of bainite, so to stably obtain bainite, excessive inclusion of Si is not preferable.
  • the content of Si is made 0.01 to 0.50%.
  • the content of Si is preferably made 0.06% or more. Further, the content of Si is preferably made 0.45% or less, more preferably is made less than 0.35%.
  • Mn has the effect of improving the hardenability and making the main phase of the structure bainite. Furthermore, Mn has the action of causing a reduction in the bainite transformation temperature and by doing so has the effect of refining the structure to raise the toughness of the matrix. Note that, the structure which accounts for the majority of the volume of the steel will be called the “matrix”.
  • the matrix of the steel according to the present embodiment is bainite. Further, Mn has the action forming MnS in the steel to cause a drop in the cutting resistance and thereby improve the machinability.
  • Mn has to be made at least 1.50% in content.
  • Mn is an element which easily segregates at the time of solidification of the steel, so the content increases.
  • the content of Mn is made 1.50 to 2.50%.
  • the content of Mn is preferably made 1.60% or more, more preferably is made 1.70% or more. Further, the content of Mn is preferably made 2.30% or less, more preferably is made 2.10% or less.
  • the content of S bonds with the Mn in the steel to form MnS which causes a drop in the cutting resistance and improvement of the machinability.
  • 0.005% or more of S has to be included.
  • the content of S is made 0.005 to 0.080%.
  • the content of S is preferably made 0.010% or more. Further, the content of S is preferably made 0.050% or less, more preferably is made 0.030% or less.
  • Cr has the effect of raising the hardenability and making the main phase of the structure bainite. Furthermore, Cr has the action of causing a drop in the bainite transformation temperature and has the effect of refining the structure by doing so and thereby raising the toughness of the matrix.
  • the content of Cr is made 0.03 to 1.60%.
  • the lower limit of the content of Cr is preferably made 0.05% or more, more preferably is made 0.10% or more.
  • the upper limit of the content of Cr is preferably 1.00% or less, more preferably is made 0.50% or less.
  • Al is an element having a deoxidizing action. To obtain such an action, the content has to be made 0.005% or more. However, if the content of Al is over 0.050%, coarse oxides are formed and the steel falls in toughness and fatigue strength. Therefore, the content of Al is made 0.005 to 0.050%. The content of Al is preferably 0.040% or less.
  • V 0.25 to 0.50%
  • V is the most important element in the steel according to the present embodiment.
  • V has the action at the time of aging treatment of bonding with C to form fine V carbides or of bonding with C and N to form fine V carbonitrides and thereby raise the strength of the steel after the aging treatment.
  • V also has the effect of precipitating compositely with Mo due to the aging treatment and further raising the age hardenability of the steel.
  • V has to be made 0.25% or more in content.
  • the content of V becomes excessive, even in heating at the time of hot forging, undissolved carbonitrides will easily remain and a drop in toughness will be invited. In particular, if the content exceeds 0.50%, the drop in toughness will become remarkable.
  • the content of V is made 0.25 to 0.50%.
  • the content of V is preferably made less than 0.45%, more preferably is made 0.40% or less. Further, the content of V is preferably made 0.27% or more.
  • Nb is one of the important elements in the steel according to the present embodiment.
  • Part of the Nb contained in the steel precipitates in steel as Nb carbides or Nb carbonitrides in the process of heating and being worked at the time of hot forging and causes refinement of the austenite crystal grains due to the pinning effect.
  • the refinement of the austenite crystal grains at the time of hot working has the effect of refining the bainite structure in the bainite transformation after the end of hot working.
  • part of the Nb in the steel at the time of hot forging remains present as solute Nb.
  • Nb precipitates as Nb carbides or Nb carbonitrides at the time of aging treatment after hot forging so has the effect of raising the hardness and achieving improvement of the low cycle fatigue strength and improvement of the fatigue strength without inviting a drop in toughness.
  • Nb has to be made 0.010% or more in content.
  • the content of Nb becomes excessive, undissolved carbonitrides easily remain even at the time of heating at the time of hot forging and the effect of improvement of the hardness after aging treatment and/or the effect of improvement of the fatigue strength after aging treatment becomes saturated.
  • the content of Nb is made 0.010 to 0.100%.
  • the content of Nb is preferably made less than 0.080%, more preferably is made 0.050% or less. Further, the content of Nb is preferably made 0.020% or more.
  • Ca is one of the important elements in the steel according to the present embodiment.
  • the Ca contained in the steel finely disperses and precipitates in steel as sulfide-based inclusions or oxide-based inclusions and therefore has the effect of suppressing increase of the fatigue strength after aging treatment and drop of the toughness after aging treatment.
  • Ca has to be made 0.0005% or more in content.
  • the content of Ca is made 0.0005 to 0.0050%.
  • the content of Ca is preferably made 0.0010% or more, more preferably is made 0.0015% or more.
  • a REM is one of the important elements in the steel according to the present embodiment.
  • a REM contained in the steel finely disperses and precipitates in the steel as sulfide-based inclusions or oxide-based inclusions and thereby has the effect of improving the fatigue strength after aging treatment and, further, suppressing a drop in toughness after aging treatment.
  • the REM content has to be made 0.001% or more.
  • the content of a REM exceeds 0.05%, a drop in the hot ductility is invited. Therefore, the content of a REM is made 0.001 to 0.05%.
  • the content of a REM is preferably made 0.003% or more, more preferably is made 0.005% or more.
  • the age hardening steel of the present embodiment is steel comprised of the above-mentioned C, Si, Mn, S, Cr, Al, V, Nb, Ca, and REM and a balance of Fe and impurities, restricting P, Ti, and N in the later mentioned impurities to P: 0.030% or less, Ti: less than 0.005%, and N: less than 0.0080%, and, furthermore, having a chemical composition where the F1 expressed by the above formula (1) is 0.68 or more, the F2 expressed by the formula (2) is 0.85 or less, the F3 expressed by the formula (3) is 0.00 or more, and the F4 expressed by the formula (4) is 0.012 to 0.08.
  • impurities indicates elements entering from the starting material ore, scraps, the manufacturing environment, etc. when producing a ferrous material industrially.
  • P is an element contained as an impurity and not preferable in the steel according to the present embodiment. That is, P segregates at the grain boundaries and causes a drop in the toughness. In particular, if its content is over 0.030%, the drop in toughness becomes extremely remarkable. Therefore, the content of P is limited to 0.030% or less. The content of P is preferably limited to 0.025% or less. Note that, the effect of the steel according to the present embodiment is exhibited without particularly determining the lower limit of the content of P. The lower limit value of the P content may also be made 0%. However, if excessively decreasing the P, an extreme rise in the cost of dephosphorization is invited and the result becomes disadvantageous economically, so the lower limit of the amount of P is preferably made 0.005%.
  • Ti is an element contained as an impurity and not preferable in the steel according to the present embodiment. That is, Ti bonds with the N and C to form coarse Ti carbonitrides which invite a drop in toughness. In particular, if the content becomes 0.005% or more, the toughness is made to greatly degrade. Therefore, the content of Ti is limited to less than 0.005%. To suppress a drop in toughness due to the aging treatment, the content of Ti is preferably limited to 0.0035% or less, more preferably is limited to 0.0015% or less. The lower limit value of the Ti content may also be made 0%.
  • N is an element contained as an impurity and ends up fixing V as VN so is not preferable in the steel according to the present embodiment. That is, the V precipitating as VN no longer contributes to age hardening, so to suppress precipitation of VN, the content of N must be lowered. To suppress the precipitation of VN and secure a sufficient amount of solute V at the stage before the aging treatment, it is necessary to limit the content of N to less than 0.0080%.
  • the upper limit value of the N content is preferably 0.0070%, 0.0060%, or 0.0050%.
  • the lower limit value of the N content is 0%.
  • Another embodiment of the age hardening steel according to the present embodiment is steel comprising elements from the above C to REMs, a composition satisfying one or more of any of the above ⁇ a> to ⁇ c>, and a balance of Fe and impurities, having P, Ti, and N in the impurities limited to P: 0.03% or less, Ti: less than 0.005%, and N: 0.020% or less, and, furthermore, having a chemical composition where the F1′ expressed by formula (1′) is 0.68 or more, F2′ expressed by formula (2′) is 0.85 or less, F3′ expressed by formula (3′) is 0.00 or more, and F4 expressed by formula (4) is 0.012 to 0.08.
  • Mo is not essential, so the lower limit value of the Mo content is 0%.
  • Mo has the action of raising the hardenability, making the main phase of the structure of the steel after hot forging bainite, and increasing the area ratio of the bainite.
  • Mo also has the action of forming carbides together with V to increase the age hardenability in steel containing 0.25% or more of V. For this reason, if necessary, Mo may also be included.
  • Mo is an extremely expensive element, so if the content increases, the cost of manufacturing the steel increases and furthermore the toughness also falls. Therefore, the amount of Mo when included is made 1.0% or less.
  • the amount of Mo when included is preferably made 0.50% or less, more preferably is made 0.40% or less, still more preferably is made less than 0.30%.
  • the amount of Mo when included is preferably made 0.01% or more, more preferably is made 0.05% or more, still more preferably is made 0.10% or more.
  • Inclusion of Cu is not essential, so the lower limit value of the Cu content is 0%.
  • Cu has the action of increasing the fatigue strength of the steel after aging treatment. For this reason, Cu may also be included in accordance with need. However, if the content of Cu exceeds 0.30%, the hot workability falls. Therefore, the amount of Cu when included is made 0.30% or less.
  • the amount of Cu when included is preferably made 0.25% or less.
  • the amount of Cu when included is preferably made 0.01% or more, more preferably is made 0.05% or more, still more preferably is made 0.10% or more.
  • Ni is not essential, so the lower limit value of the Ni content is 0%.
  • Ni has the action of improving the fatigue strength of steel after aging treatment. Furthermore, Ni has the action of suppressing a drop in the hot workability due to Cu. For this reason, Ni may also be included in accordance with need. However, if the content of Ni exceeds 0.30%, the cost swells and in addition the above effect becomes saturated. Therefore, the amount of Ni when included is made 0.30% or less. The amount of Ni when included is preferably made 0.25% or less. On the other hand, to stably obtain the above effect of Ni, the amount of Ni when included is preferably made 0.01% or more, more preferably is made 0.05% or more, still more preferably is made 0.10% or more.
  • the above Cu and Ni may be included as single types among these or as combination of the two types.
  • the total content of the elements when included may be 0.6% when the contents of Cu and Ni are respectively the upper limit values.
  • Bi has the action of lowering the cutting resistance and increasing the machinability of the steel before the aging treatment. For this reason, when desiring to obtain a better machinability, Bi may be included in the range explained below.
  • the lower limit value of the Bi content is 0%.
  • Bi has the action of lowering the cutting resistance of the steel before the age hardening and the action of improving the scrap disposability. For this reason, if necessary, Bi may also be included. However, if the content of Bi is over 0.400%, a drop in the hot workability is caused. Therefore, the amount of Bi when included is made 0.400% or less. The amount of Bi when included is preferably made 0.300% or less. On the other hand, to stably obtain the above-mentioned effect of reducing the cutting resistance of Bi and the effect of improving the scrap disposability, the amount of Bi when included is preferably made 0.010% or more, more preferably is made 0.030% or more.
  • the balance besides the above elements is substantially Fe and unavoidable impurities, but other elements may be added in trace amounts to an extent not impairing the action and effect of the present invention.
  • F1 and F1′ are indicators of the hardenability. If the amounts of the alloy elements contained in the steel satisfy the above ranges and F1 and F1′ satisfy the above conditions, even if water cooling or other accelerated cooling was not performed after hot forging, the structure after hot forging becomes one having bainite as its main phase.
  • F1 and F1′ are preferably 0.70 or more, more preferably are 0.72 or more.
  • F1 and F1′ are preferably 1.00 or less, more preferably are 0.98 or less.
  • age hardening steel according to the present embodiment does not contain Mo, F2 expressed by
  • F2 and F2′ are indicators showing the hardness before aging treatment. Even if steel satisfies the condition of the above F1 or F1′, if F2 or F2′ becomes within a suitable range, sometimes the hardness before aging treatment becomes too high, the cutting resistance rises, and a good machinability can no longer be secured. That is, if F2 or F2′ exceeds 0.85, the hardness of the bainite structure becomes too high. For this reason, sometimes a rise in the cutting resistance becomes unavoidable and a good machinability can no longer be secured.
  • F2 and F2′ are preferably 0.82 or less, more preferably are 0.80 or less.
  • F2 and F2′ are preferably 0.55 or more, more preferably are 0.60 or more.
  • F3 or F3′ 0.00 or more
  • age hardening steel according to the present embodiment does not contain Mo, F3 expressed by
  • F3 and F3′ are indicators of the toughness after aging treatment. That is, even if the conditions of F1 or F1′ and F2 or F2′ were satisfied, if F3 or F3′ is in a suitable range, sometimes the toughness of the steel after aging treatment falls and the targeted toughness can no longer be secured. That is, when F3 or F3′ is less than 0.00 (negative number), the toughness after aging treatment falls.
  • F3 and F3′ are preferably 0.01 or more.
  • F1 is 0.68 or more and F2 is 0.85 or less, it is not particularly necessary to provide a limit for the upper limit of F3.
  • F1′ is 0.68 or more and F2′ is 0.85 or less, it is not particularly necessary to provide a limit for the upper limit of F3′.
  • F4 is an indicator showing a control indicator of the form of inclusions. That is, even if the steel satisfies the conditions of F1 or F1′ and F2 or F2′ and F3 or F3′, if F4 is in a suitable range, sometimes the fatigue strength of the steel after aging treatment will not stably rise or the drop in toughness after aging treatment cannot be stably suppressed. That is, when F4 is less than 0.012, sometimes sulfide-based inclusions and oxide-based inclusions cannot be finely dispersed, the fatigue strength of the steel after aging treatment will not stably rise, and the drop in toughness after aging treatment cannot be stably suppressed. Therefore, F4 was made 0.012 or more.
  • F4 is preferably 0.014 or more. Further, 0.016 or more is more preferable. Further, the more F4 increases, the more the effect of refining the inclusions can be exhibited, but if F4 is over 0.08, conversely sometimes coarsening of the oxide-based inclusions is invited or a drop in the hot ductility is invited, so the upper limit of F4 was made 0.08 or less. F4 is preferably 0.07 or less, more preferably is 0.06 or less.
  • the main phase before the aging treatment of the age hardening steel according to the present embodiment be made bainite. That is, to secure a sufficient machinability and solute V, the structure before the aging treatment must have an area ratio of bainite of 70% or more.
  • the area ratio of bainite is preferably 80% or more, while the bainite single phase, that is, the area ratio of bainite of 100%, is most preferable. If the area ratio of bainite is less than 100%, the phases other than the main phase of bainite includes a ferrite phase, pearlite structure, martensite structure, etc., but the smaller these phase and structures, the better.
  • the lower limit value of the age hardenability of the steel according to the present embodiment is preferably 30 Hv, more preferably is 33 Hv, 35 Hv, or 40 Hv.
  • “treatment holding the steel at a temperature of 620° C. for 120 minutes” means general aging treatment conditions when treating the steel according to the present embodiment for age hardening to produce a machine part. If the age hardenability is 30 Hv or more, the steel according to the present embodiment has an excellent machinability before the aging treatment and has an excellent fatigue strength after the aging treatment.
  • the method of production of age hardening steel of the present embodiment is not particularly limited.
  • a general method may be used to smelt the steel and adjust the chemical composition.
  • Below, one example of the method of production of a machine part in automobiles, industrial machinery, construction machinery, etc. using as a material the age hardening steel according to the present embodiment produced in the above way will be shown.
  • intermediate material a billet obtained by blooming an ingot, a billet obtained by blooming a continuously cast material, a steel rod obtained by hot rolling or hot forging these billets, or other such material may be used.
  • the time of fabrication of the intermediate material if holding the material for a certain time in a temperature region where V carbides easily precipitate, the age hardenability is liable to be lost. For example, when maintaining the temperature of the intermediate materials within the range of 540 to 700° C.
  • the intermediate material is allowed to stand in a room temperature environment after blooming or after hot rolling or hot forging, such a situation will not arise.
  • the above intermediate material was hot forged and furthermore was cut to finish it into a predetermined part shape.
  • the above hot forging for example, heats the intermediate material at 1200 to 1250° C. for 5 to 60 minutes, then forges it so that the surface temperature after the final forging becomes 1100° C. or more, then, after that, makes the average cooling rate in the 800 to 400° C. temperature region 15 to 60° C./min to cool the material down to room temperature.
  • Such an average cooling rate is easily obtained by allowing the forged steel to stand in a room temperature environment.
  • the cooling rate is less than 15° C./min, the V carbides precipitate during cooling and the age hardenability is liable to become 30Hv or less. After the material is cooled in this way, it is further cut to finish it to the desired part shape.
  • the roughly shaped material formed to a predetermined part shape is treated to age it to obtain a machine part of automobiles, industrial machinery, construction machinery, etc. provided with the desired characteristics.
  • the above aging treatment is, for example, performed in a 540 to 700° C. temperature region, preferably a 560 to 680° C. temperature region, more preferably a 580 to 660° C. temperature region.
  • the holding time of this aging treatment is suitably adjusted to, for example, 30 to 1000 minutes etc. by the size (mass) of the machine part. If the aging treatment temperature is less than 540° C., V carbides or V carbonitrides cannot be sufficiently formed and the desired age hardenability of 30Hv cannot be obtained.
  • the aging treatment temperature exceeds 700° C.
  • the formed V carbides or V carbonitrides become coarser, so no longer contribute to hardening and the desired age hardenability of 30 Hv cannot be obtained.
  • the holding time is less than 30 minutes, V carbides or V carbonitrides cannot be sufficiently formed, so the desired age hardenability of 30Hv cannot be obtained.
  • the holding time exceeds 1000 minutes, the formed V carbides or V carbonitrides become coarser, so no longer contribute to hardening and the desired age hardenability of 30Hv cannot be obtained.
  • Examples 1 and 2 will be used to explain the present invention in further detail.
  • the conditions in Examples 1 and 2 shown below are examples of conditions adopted for confirming the workability and advantageous effects of the present invention.
  • the present invention is not limited to these examples of conditions. Further, the present invention can adopt various conditions so long as not departing from the gist of the present invention and achieving the object of the present invention.
  • Each of the Steels 1 to 26 of the chemical compositions shown in Table 1 was smelted by a 50 kg vacuum melting furnace.
  • the Steels 1 to 13 in Table 1 are steels with chemical compositions within the ranges prescribed by the present invention.
  • the Steels 14 to 26 in Table 1 are steels with chemical compositions outside the conditions prescribed by the present invention. Note that, in the section on Ti, “ ⁇ 0.001” indicates the content of Ti as an impurity is below the lower limit value of detection in emission spectroscopy of 0.001%.
  • each steel was heated at 1250° C., then hot forged to a diameter 60 mm steel rod.
  • Each hot forged steel rod was allowed to cool once in the atmosphere to cool it down to room temperature. After that, furthermore, the steel rod cooled down to room temperature was heated as an intermediate material to 1250° C. and was again hot forged to a diameter 35 mm steel rod while making the finishing temperature 950° C. or more.
  • This second hot forging was performed for simulating forging to a part shape.
  • the second hot forged steel rod was allowed to cool in the atmosphere to cool it down to room temperature.
  • the cooling rate at the time of the second hot forging was measured using a radiant thermometer.
  • the average cooling rate after hot forging in the 800 to 400° C. temperature region (in Table 2, indicated as “cooling rate”) was 50° C./min in each case.
  • the Vicker's hardness was measured in the following way.
  • the amount of hardening ⁇ HV the difference between the Vicker's hardness after aging treatment and the Vicker's hardness before aging treatment (below, referred to as the “amount of hardening ⁇ HV”) was 30 Hv or more, the age hardenability was judged sufficiently high and was used as the target.
  • the area ratio of the bainite in the structure was measured in the following way. First, a test piece used for measurement of hardness and obtained by burial in a resin and polishing to a mirror finish was etched by Nital. The etched test piece was photographed for structure at a power of 200 ⁇ using an optical microscope. From the obtained photograph, image analysis was used to measure the area ratio of bainite. If the area ratio of the bainite was 70% or more, the structure was judged to be sufficiently converted to bainite and was used as the target.
  • the toughness was measured using a standard test piece with a U-notch with a notch depth of 2 mm and a notch bottom radius of 1 mm. If the absorption energy at 20° C. after aging treatment evaluated in this Charpy impact test was 50 J or more, it was judged sufficiently high and was used as the target.
  • the fatigue strength was investigated by taking a monoaxial tension and compression type fatigue test piece. That is, a smooth fatigue test piece shaped with a diameter of the parallel part shown in FIG. 1 of 3 . 4 mm and a length of the parallel part of 12.7 mm was taken in parallel to the forging direction from the R/2 part of the steel rod (longitudinal direction of steel rod) and was tested for fatigue room temperature, in the atmosphere, under conditions of a stress ratio of 0.05 and a test rate of 10 Hz. The maximum stress where the test piece does not break at 10 7 repeated application of stress under the above conditions was made the fatigue strength. If the fatigue strength was 425 MPa or more, the fatigue strength was judged sufficiently high and was used as the target.
  • the low cycle fatigue strength was found by the following method: First, a parallelopiped member of vertical and horizontal spans in the longitudinal direction cross-section of respectively 13 mm and a length of 100 mm was taken from a steel rod in parallel with the forging direction (longitudinal direction of steel rod) so that the sampled portion become the R/2 part of the steel rod. After that, furthermore, a four-point bending test piece provided with a radius 2 mm semicircular notch at a part at the center in the longitudinal direction of one face of that parallelopiped member (that is, the face having the part for evaluation of fatigue) was obtained. The low cycle fatigue test was conducted at room temperature in the atmosphere.
  • Table 2 shows the results of the above investigations. Note that, in the column on “Bainitization”, test pieces with a bainite area ratio of 70% or more or meeting the target were indicated as “GOOD”, while test pieces of less than 70% or not reaching the target were indicated as “BAD”. Further, in Table 2, an “absorption energy in a Charpy impact test” was indicated as the “Charpy absorption energy”.
  • the Steel 1 and Steels 27, 30, 33, and 36 to 39 in Table 3 are steels with chemical compositions within the scope prescribed by the present invention.
  • Steels 28, 29, 31, 32, 34, and 45 in Table 3 are steels with chemical compositions off from the conditions prescribed in the present invention. Note that, in the section on Ti, “ ⁇ 0.001” indicates the content of Ti as an impurity is below the lower limit value of detection in emission spectroscopy of 0.001%.
  • each steel was heated at 1250° C., then hot forged to a diameter 60 mm steel rod.
  • Each hot forged steel rod was allowed to cool once in the atmosphere to cool it down to room temperature. After that, furthermore, the steel rod cooled down to room temperature was heated as an intermediate material to 1250° C. and was again hot forged to a diameter 35 mm steel rod while making the finishing temperature 950° C. or more.
  • This second hot forging was performed for simulating forging to a part shape.
  • the second hot forged steel rod was allowed to cool in the atmosphere to cool it down to room temperature.
  • the cooling rate at the time of the second hot forging was measured using a radiant thermometer.
  • the average cooling rate after hot forging in the 800 to 400° C. temperature region (in Table 4, indicated as “cooling rate”) was 50° C./min in each case.
  • the hardness before aging treatment is 290 Hv or less so a good machinability can be expected.
  • aging treatment performed after cutting causes the steel to harden to a Vicker's hardness of 30 Hv or more, so a 425 MPa or more fatigue strength is obtained.
  • the absorption energy at 20° C. after aging treatment is 50J or more and a drop in toughness due to the aging treatment can be sufficiently suppressed.
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