US20180245172A1 - Age-hardenable steel, and method for manufacturing components using age-hardenable steel - Google Patents

Age-hardenable steel, and method for manufacturing components using age-hardenable steel Download PDF

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US20180245172A1
US20180245172A1 US15/558,963 US201615558963A US2018245172A1 US 20180245172 A1 US20180245172 A1 US 20180245172A1 US 201615558963 A US201615558963 A US 201615558963A US 2018245172 A1 US2018245172 A1 US 2018245172A1
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steel
forging
hardenable steel
age hardenable
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Masato Yuya
Motoki TAKASUGA
Taizo Makino
Masashi Higashida
Tatsuya Hasegawa
Kouji Morita
Toshimasa Ito
Tomomitsu FUKUOKA
Tadashi Nishiwaki
Yoshihiro Tanimura
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Denso Corp
Nippon Steel Corp
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Denso Corp
Nippon Steel and Sumitomo Metal Corp
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Assigned to DENSO CORPORATION, NIPPON STEEL & SUMITOMO METAL CORPORATION reassignment DENSO CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: FUKUOKA, TOMOMITSU, ITO, TOSHIMASA, MORITA, KOUJI, NISHIWAKI, TADASHI, TANIMURA, YOSHIHIRO, HASEGAWA, TATSUYA, MAKINO, TAIZO, TAKASUGA, MOTOKI, YUYA, MASATO, HIGASHIDA, MASASHI
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite

Definitions

  • the present invention relates to age hardenable steel. More specifically, it relates to steel for production of a machine part for automobiles, industrial machinery, and construction machinery which is worked into a predetermined shape by hot forging and machining, is treated for age hardening (below, simply referred to as “aging treatment”), and has the desired strength and toughness secured by this aging treatment. Further, the present invention relates to such a method of production of a part using age hardenable steel.
  • machine parts for automobiles, industrial machinery, construction machinery, etc. are required to be high in fatigue strength. If just providing steel with a high fatigue strength, it is possible to easily achieve this by utilizing alloy elements and/or heat treatment to raise the hardness of the steel. However, in general, machine parts are formed by hot forging, then are machined to finish them to predetermined product shapes. For this reason, the steel used as a material for machine parts must be provided with high fatigue strength together with sufficient machinability simultaneously.
  • the fatigue strength becomes better the higher the hardness of the material.
  • the machining resistance and the tool life tend to become more inferior the higher the hardness of the material.
  • precision shaped machine parts are required to remain unchanged in dimensions during use. Depending on the environment of use, these precision shaped machine parts can be instantaneously subjected to higher loads compared with the loads of the extents of usual use. Yield strength is also required so that the dimensions do not change in the face of such loads.
  • Japanese Patent Publication No. 2006-37177A discloses “age hardening steel” obtained by rolling, forging, or solutionization of steel in which the precipitation strengthening elements of Mo and V are contained in amounts limited by specific formulas, and cooling between a temperature of 800° C. to 300° C. by an average cooling rate of 0.05 to 10° C./sec, having an area rate of bainite structures of 50% or more and a hardness of 40 HRC or less before aging treatment, and, after aging treatment, having a hardness of 7 HRC or more higher than the hardness before aging treatment.
  • Japanese Patent Publication No. 2011-236452A discloses, as steel excellent in hot forgeability and machinability after hot forging and able to be raised in strength by age hardening after machining, bainite steel containing specific amounts of Mo and V as precipitation hardening elements.
  • Japanese Patent Publication No. 2000-17374A proposes, as age hardening type high strength bainite steel for hot forging use, age hardening type high strength bainite steel characterized by having a yield point or 0.2% yield strength of 900 MPa or more obtained by hot rolling or hot forging steel containing Mo and V, then cooling it according to the steel components, making the hardness 400 HV or less, making the structure a bainite rate of 70% or more, making the old austenite grain size 80 ⁇ m or less, then machining or plastic forming the steel according to need and applying aging treatment.
  • age hardening type high strength bainite steel characterized by having a yield point or 0.2% yield strength of 900 MPa or more obtained by hot rolling or hot forging steel containing Mo and V, then cooling it according to the steel components, making the hardness 400 HV or less, making the structure a bainite rate of 70% or more, making the old austenite grain size 80 ⁇ m or less, then machining or plastic forming the steel according to need and applying
  • Japanese Patent Publication No. 2013-245363A (PLT 4) describes steel promising both a high machinability and a high fatigue strength which is adjusted in the contents of the alloy elements to satisfy specific parameter formulas and thereby relatively reduce the content of Mo while making the hardness before aging treatment after hot forging 290 HV or less and making the hardness after aging treatment 325 HV or more.
  • WO2012/161323A discloses a steel part for machine structure use using cooling and heat treatment after hot forging to optimize the shapes of V carbonitrides and shapes of bainite structures having a precipitation strengthening ability and provide machinability, fatigue strength, and toughness all together.
  • Japanese Patent Publication No. 2013-213254A discloses steel for cold forging and nitriding use excellent in cold forgeability and chip removal ability after cold forging and able to provide a cold forged nitrided part with a high core hardness, high surface hardness, and deep effective hardened layer depth.
  • steel low in toughness suffers from faster progression of fractures and larger scale fractures once fatigue cracks occur.
  • the steel disclosed in PLT 1 can be adjusted in the contents of the alloy elements so as to satisfy specific parameter formulas to obtain a high age hardening ability, but the toughness is not considered at all.
  • the steel disclosed in PLT 2 adjusts the contents of the alloy elements to satisfy specific parameter formulas so as to relatively reduce the content of Mo while making the hardness before aging treatment 300 HV or less after hot forging and making the hardness after aging treatment 300 HV or more.
  • the steel is not sufficiently designed to be raised in toughness after aging treatment.
  • the steel disclosed in PLT 3 has a C content kept low at 0.06 to 0.20%, but the V content is an extremely high 0.51 to 1.00%, so the steel is remarkably strengthened by age hardening but is not excellent in toughness.
  • the steel disclosed in PLT 4 is not sufficiently designed to raise the toughness and the yield strength after aging treatment.
  • the steel disclosed in PLT 5 is not sufficiently designed to raise the yield strength after aging treatment.
  • the steel disclosed in PLT 6 is low in N content, so nitrides are insufficiently produced and as a result an excellent yield strength is not obtained.
  • an object of the present invention is to provide age hardenable steel satisfying the following ⁇ 1> to ⁇ 3>:
  • ⁇ 1> A low hardness after hot forging related to the machining resistance and tool life. Note that, in the following explanation, the hardness after the above hot forging will be referred to as the “hardness before aging treatment”.
  • ⁇ 2> The ability to provide a machine part with a desired fatigue strength and yield strength by aging treatment.
  • ⁇ 3> A high toughness after aging treatment.
  • an object of the present invention is to provide age hardenable steel having a hardness before aging treatment of 340 HV or less, a fatigue strength explained later after aging treatment of 480 MPa or more, a 0.2% yield strength, found by the offset method using a prescribed plastic strain amount of 0.2% in a tensile test conducted using a tensile test piece of 14A of the JIS having a ⁇ 6 parallel part, of 800 MPa or more, and further having an absorption energy at 20° C. after aging treatment, evaluated by a Charpy impact test conducted using a U-notched standard test piece having a notch depth of 2 mm and a notch bottom radius of 1 mm described in JIS Z 2242, of 25 J or more.
  • the inventors engaged in surveys and studies relating to the chemical composition, structure, and effective V ratio (amount of dissolved V/total amount of V) and the values calculated by formulas using contents of specific elements so as to solve the above problem. Specifically, they investigated the conditions for obtaining good toughness even with steel giving a high fatigue strength and yield strength by causing the precipitation of fine secondary phase particles in the steel due to aging. As a result, they obtained the following findings (a) to (d).
  • the elements for causing deterioration of toughness after aging treatment are C, V, Mo, and Ti.
  • Ti bond with N and/or C to form TiN and/or TiC. If TiN and/or TiC precipitates, the fatigue strength sometimes becomes higher, but the toughness is made to greatly fall.
  • the intensity of the action of Ti in lowering the toughness is extremely large compared with those of V and Mo which are the elements contributing to precipitation strengthening as V. For this reason, Ti must be limited as much as possible.
  • C forms cementite in steel and can become the starting point for cleavage fracture. Even if treating steel containing an amount of V or Mo excessive with respect to the amount of C by aging, part of the cementite remains. Both V and Mo cause precipitation of carbides at the same crystal planes of the matrix along with aging treatment and thereby assist the progression of cleavage fractures and cause deterioration of the toughness. Therefore, to raise the toughness, it is necessary to reduce the contents of C, V, and Mo.
  • C and Mo have the effect of refining the structure and raising the toughness and the action of precipitating as cementite or carbides and lowering the toughness. Overall, C greatly lowers the toughness and Mo slightly lowers the toughness.
  • the effective V ratio defined as the amount of dissolved V to the total amount of V.
  • the effective V ratio being small means the ratio of the amount of V contributing to precipitation strengthening is small and the strengthening ability is small and is not preferable. There is no upper limit of the effective V ratio. The closer to 1, the better.
  • the contents of C, Mn, Cr, V, and Mo have to be controlled so that the value expressed by (2) or (2′) showing an indicator of toughness after aging treatment explained later becomes a specific value or more. Furthermore, the content of Ti has to be made a specific value or less so that inclusions and precipitates harmful to toughness are not contained in the steel.
  • the inventors engaged in further surveys and studies relating to the values calculated by the formulas using the chemical composition and contents of specific elements. Specifically, they adjusted the components of steel able to secure toughness after aging and investigated the conditions relating to the hardness before aging and the hardness after aging and the age hardening ability expressed by the difference of the same. As a result, they obtained the findings of the following (e) to (g).
  • the inventors engaged in further surveys and studies relating to the chemical composition. Specifically, they focused on the fact that even if raising the toughness after aging by reducing the contents of C, V, and Mo and increasing the contents of Mn and Cr, to cause sufficient amounts of precipitation strengthening particles to precipitate and obtain a sufficient age hardening ability and high yield strength, it is necessary to increase the precipitation strengthening ability per unit V amount. Further, the inventors engaged in various studies on techniques for increasing the precipitation strengthening ability of V and obtained the following findings (h) to (j).
  • the bonding force between N and V is larger than the bonding force between C and V, so the effect of promoting the precipitation of V carbonitrides is larger with N than with C.
  • the present invention was made based on the above findings (a) to (j) and has as its gist the following:
  • Age hardenable steel comprising, by mass %, C: 0.09 to 0.20%, Si: 0.01 to 0.40%, Mn: 1.5 to 2.5%, S: 0.001 to 0.045%, Cr: over 1.00% to 2.00%, Al: 0.001 to 0.060%, V: 0.22 to 0.55%, N: over 0.0080 to 0.0170%, and a balance of Fe and impurities, the P and Ti in this impurities being P: 0.03% or less and Ti: less than 0.005%, wherein an area rate of bainite structures is 80% or more, an effective V ratio (amount of dissolved V/total amount of V) is 0.9 or more, and the chemical composition is one where the F1 expressed by the following formula (1) is 1.00 or less and the F2 expressed by the following formula (2) is 0.30 or more:
  • Age hardenable steel comprising, by mass %, C: 0.09 to 0.20%, Si: 0.01 to 0.40%, Mn: 1.5 to 2.5%, S: 0.001 to 0.045%, Cr: over 1.00% to 2.00%, Al: 0.001 to 0.060%, V: 0.22 to 0.55%, Mo: 0.9% or less, N: over 0.0080 to 0.0170%, and a balance of Fe and impurities, the P and Ti in this impurities being P: 0.03% or less and Ti: less than 0.005%, wherein an area rate of bainite structures is 80% or more, an effective V ratio (amount of dissolved V/total amount of V) is 0.9 or more, and the chemical composition is one where the F1′ expressed by the following formula (1′) is 1.00 or less and the F2′ expressed by the following formula (2′) is 0.30 or more:
  • the age hardenable steel according to [1] or [2] further comprising one or more of Cu: 0.3% or less and Ni: 0.3% or less.
  • a method of production of a part using age hardenable steel comprising a forging step of heating age hardenable steel according to any one of [1] to [4] at 1100 to 1350° C. for 0.1 to 300 minutes, then forging it so that a surface temperature after finish forging becomes 900° C. or more, then cooling it down to room temperature while making the average cooling speed in a temperature region from 800 to 400° C. a speed of 10 to 90° C./min, a machining step machining the steel after forging, and an aging treatment step holding the steel after machining in the temperature region from 540 to 700° C. for 30 to 1000 minutes.
  • the age hardenable steel of the present invention has a hardness before aging treatment of 340 HV or less. Further, a machine part using the age hardenable steel of the present invention has a fatigue strength of 490 MPa or more due to aging treatment performed after machining. Further, the machine part has a toughness (absorption energy at 20° C. after aging treatment evaluated by a Charpy impact test performed using a standard test piece with a U-notch of a notch depth of 2 mm and a notch bottom radius of 1 mm) of 25 J or more. Furthermore, the machine part has a yield strength of 800 MPa or more. For this reason, the age hardenable steel of the present invention can be extremely suitably used as a material for a machine part of automobiles, industrial machinery, construction machinery, etc.
  • FIG. 1 A view showing the correlation between a steel material hardness before aging and an F1 value
  • FIG. 2 A view showing the relationship between a Charpy impact value of a steel material after aging and an F2 value.
  • C is an important element in the present invention. C bonds with V to form carbides and strengthen the steel. However, if the content of C is under 0.09%, carbides of V become harder to precipitate, so the desired strengthening effect cannot be obtained. On the other hand, if the content of C becomes too great, the amount of C not bonding with V or Mo forming carbides with Fe (cementite) increases, so the toughness ends up being degraded. Therefore, the content of C was made 0.09 to 0.20%.
  • the content of C preferably is made 0.10% or more, more preferably made 0.11% or more. Further, the content of C preferably is made 0.18% or less, more preferably is made 0.16% or less.
  • Si is useful as a deoxidizing element at the time of steelmaking and simultaneously has the action of improving the strength of the steel by dissolving in the matrix. To sufficiently obtain these effects, Si has to be made 0.01% or more in content. However, in steel containing Mn and Cr in large amounts, if the content of Si becomes excessive, sometimes the amount of residual austenite of the structure after hot forging becomes too great and deformation becomes greater during aging treatment. Therefore, the content of Si was made 0.01 to 0.40%.
  • the content of Si preferably is made 0.05% or more. Further, the content of Si preferably is made 0.35% or less, more preferably 0.30% or less.
  • Mn has the effect of improving the hardenability and making the structure bainite. Furthermore, it has the effect of lowering the bainite transformation temperature and thereby refining the bainite structure to improve the toughness of the matrix. Further, Mn has the action of forming MnS in steel to improve the chip removal ability at the time of machining. To sufficiently obtain these effects, Mn has to be made at least 1.5% in content. However, Mn is an element which easily segregates at the time of solidification of the steel, so if the content becomes too great, the fluctuation in hardness in a part after hot forging unavoidably becomes larger. Therefore, the content of Mn was made 1.5 to 2.5%. The content of Mn preferably is made 1.6% or more, more preferably is made 1.7% or more. Further, the content of Mn preferably is made 2.3% or less, more preferably is made 2.1% or less.
  • the content of S preferably is made 0.005% or more, more preferably is made 0.010% or more. Further, the content of S preferably is made 0.040% or less, more preferably is made 0.035% or less.
  • Cr like Mn, has the effect of raising the hardenability and making the structure bainite. Furthermore, it lowers the bainite transformation temperature to refine the bainite structure. Furthermore, it has the effect of lowering the ease of movement of grain boundaries to refine the austenite grain size at the time of hot forging and as a result refine the bainite structure after transformation. Cr has the effect of raising the toughness of the matrix through the effects of these in refining the bainite structure. To sufficiently obtain these effects, it must be included in over 1.00%. However, if the content of Cr is over 2.0%, the hardenability becomes larger and the hardness before aging treatment sometimes exceeds 340 HV. Therefore, the content of Cr was made over 1.00% to 2.00%.
  • the content of Cr preferably is made 1.10% or more. Further, the content of Cr preferably is made 1.80% or less, more preferably is made 1.60% or less.
  • Al is an element having a deoxidizing action. To obtain this effect, 0.001% or more in content is required. However, if Al is excessively contained, coarse oxides are formed and the toughness falls. Therefore, the content of Al was made 0.001 to 0.060%. The content of Al preferably is made 0.050% or less.
  • V is the most important element in the steel of the present invention. At the time of aging treatment, V bonds with C to form fine carbides and thereby has the action of raising the fatigue strength. Further, when the steel contains Mo, V has the effect of combining with Mo and precipitating due to aging treatment and further raising the age hardening ability. To obtain these effects, V has to be made 0.22% or more in content. However, if the content of V becomes excessive, even in the heating at the time of hot forging, undissolved carbonitrides easily remain inviting a drop in toughness. Further, if the content of V becomes excessive, sometimes the hardness before aging treatment ends up becoming higher. Therefore, the content of V was made 0.22 to 0.55%. The content of V preferably is under 0.45%, more preferably is made 0.40% or less. Further, the content of V preferably is made 0.25% or more, more preferably is made 0.27% or more.
  • N has the effect of promoting the precipitation of V carbonitrides at the time of aging and raising the yield strength. To sufficiently obtain this effect, the content of N has to be made over 0.0080%. However, if the content of N exceeds 0.0170%, at the time of hot forging, the V carbonitrides fail to enter a solution and at the next time of aging and precipitation of a sufficient amount of fine V carbonitrides becomes difficult, so the yield strength falls. Therefore, the content of N was made over 0.0080 to 0.0170%.
  • the content of N preferably is made 0.0090% or more, more preferably is made 0.0100% or more. Further, the content of N preferably is made 0.0160% or less, more preferably is made 0.0150% or less.
  • the age hardenable steel of the present invention is comprised of the above elements from C to N and a balance of Fe and impurities, the P and Ti in the impurities are P: 0.03% or less and Ti: less than 0.005%, the area rate of the bainite structure is 80% or more, and the effective V ratio (amount of dissolved V/total amount of V) is 0.9 or more.
  • the “impurities” indicate elements which enter from the starting materials of the ore and scraps and the manufacturing environment etc. when industrially producing ferrous metal materials.
  • P is an element contained as an impurity and not preferable in the present invention. That is, P segregates at the grain boundaries to thereby cause a drop in toughness. Therefore, the content of P was made 0.03% or less. The content of P preferably is made 0.025% or less.
  • Ti is an element contained as an impurity and is particularly not preferable in the present invention. That is, Ti bonds with N and/or C to form TiN and/or TiC to invite a drop in toughness. In particular, if the content becomes 0.005% or more, the toughness greatly deteriorates. Therefore, the content of Ti was made less than 0.005%. To secure a good toughness, the content of Ti preferably is made 0.0035% or less.
  • the area rate of the bainite structure is 80% or more.
  • the area rate of the bainite structure means the area rate in the case of observing a metal structure at a position from 1 ⁇ 3 depth to 1 ⁇ 2 depth of thickness from the surface of the steel material with an optical microscope. If making the area rate of the bainite structure 80% or more, the precipitation of V is suppressed, the effective V ratio becomes larger, and a high fatigue strength and 0.2% yield strength can be obtained.
  • the effective V ratio (amount of dissolved V/total amount of V) is 0.9 or more.
  • the “effective V ratio” means the amount of dissolved V in the total amount contained in the steel. If the effective V ratio is 0.9 or more, the amount of V carbonitrides precipitating during the aging treatment becomes greater and a high fatigue strength and 0.2% yield strength can be obtained.
  • Mo like V, is an element with a relatively low precipitation temperature of carbides and suitable for age hardening. Mo has the action of raising the hardenability and making the structure after hot forging bainite and of increasing the area rate. Mo has the action of forming carbides together with V to increase the age hardening ability in steel containing 0.22% or more of V. For this reason, Mo may be included in accordance with need. However, Mo is an extremely expensive element, so if the content becomes greater, the cost of manufacture of the steel increases and the toughness also falls. Therefore, the amount of Mo when included was made 0.9% or less. The amount of Mo when included preferably is made 0.75% or less, more preferably is made 0.60% or less, and still more preferable is less than 0.50%. On the other hand, to stably obtain the above effect of Mo, the amount of Mo when included desirably is made 0.05% or more, more desirably is made 0.10% or more.
  • Cu has the action of improving the fatigue strength. For this reason, Cu may be included according to need. However, if the content of Cu becomes greater, the hot workability falls. Therefore, the amount of Cu when included was made 0.3% or less. The amount of Cu when included preferably is made 0.25% or less. On the other hand, to stably obtain the effect of raising the fatigue strength of Cu, the amount of Cu when included is desirably made 0.1% or more.
  • Ni has the action of improving the fatigue strength. Furthermore, Ni also has the action of suppressing the drop in hot workability due to Cu. For this reason, Ni may be included in accordance with need. However, if the content of Ni becomes greater, the cost swells and, in addition, the above effect is also saturated. Therefore, the amount of Ni when included was made 0.3% or less. The amount of Ni when included preferably is made 0.25% or less. On the other hand, to stably obtain the above effects of Ni, the amount of Ni when contained desirably is made 0.1% or more.
  • the above Cu and Ni may be included as just one type of either of the same or as two types combined.
  • the total content of the elements when included can be made 0.6% of the case where the contents of Cu and Ni are at their respective upper limit values.
  • Ca has the action of lengthening the tool life. For this reason, Ca may be included in accordance with need. However, if the content of Ca becomes larger, coarse oxides are formed and the toughness is lowered. Therefore, the amount of Ca when included was made 0.005% or less. The content of Ca when included is preferably made 0.0035% or less. On the other hand, to stably obtain the effect of Ca on increasing tool life, the amount of Ca when included is desirably made 0.0005% or more.
  • Bi has the action of lowering the machining resistance and increasing the tool life. For this reason, Bi may be included in accordance with need. However, if the content of Bi becomes greater, it causes a drop in the hot workability. Therefore, the amount of Bi when included was made 0.4% or less. The amount of Bi when included is preferably made 0.3% or less. On the other hand, to obtain the effect of Bi in prolonging the tool life, the amount of Bi when included is desirably made 0.03% or more.
  • the above Ca and Bi may be included as just one type of either of the same or as two types combined.
  • the total content of the elements when included can be made 0.405% of the case where the contents of Ca and Bi are at their respective upper limit values, but is preferably made 0.3% or less.
  • the age hardenable steel of the present invention satisfies the conditions of the above-mentioned chemical composition (essential components and optional components), structure, and effective V ratio. Further the values F1 (F1′) and F2 (F2′) calculated by formulas using contents of specific elements has to be 1.00 or less and 0.30 or more respectively.
  • F1 and F1′ are indicators showing the hardness before aging treatment. If the age hardenable steel of the present invention satisfies the conditions relating to the above F1 or F1′, the hardness before aging treatment does not become too high, the machining resistance at the time of machining does not become large, and longer tool life is realized.
  • F1 and F1′ are preferably 0.97 or less, more preferably 0.95 or less. Further, F1 and F1′ are preferably 0.60 or more, more preferably 0.65 or more.
  • FIG. 1 is a graph showing the relationship between the hardness before aging (ordinate; HV) and the F1 values of various types of steel (abscissa). As clear from the graph of FIG. 1 , a strong primary positive correlation is found between the two. If F11.00 or less, it is judged that the hardness before aging 340 HV.
  • F2 and F2′ are indicators showing the toughness after aging treatment. That is, by just satisfying the condition of F1 or F1′, sometimes the toughness after aging treatment falls and the targeted toughness cannot be secured, so it is necessary to separately prescribe F2 and F2′.
  • FIG. 2 is a view showing a relationship between a Charpy impact value of a steel material after aging and an F2 value. As shown in this figure, a positive correlative relationship is observed between the Charpy impact value (J) after aging treatment and the F2 value (abscissa). When F2 or F2′ is less than 0.30, toughness after aging treatment is not sufficiently obtained. To obtain a yield strength of 800 MPa or more while securing the targeted toughness, it is necessary to make the contents of the above alloy elements within the prescribed ranges, satisfy the conditions of F1 or F1′, and satisfy the conditions of F2 or F2′.
  • F2 and F2′ preferably are 0.45 or more, more preferably are 0.60 or more.
  • the method of production of the age hardenable steel of the present invention is not particularly limited.
  • a general method may be used to smelt the steel and adjust the chemical composition.
  • a material used for hot forging (below, referred to as a “material for hot forging use”) is prepared from steel with a chemical composition adjusted to the above-mentioned range.
  • a material for hot forging use a billet obtained by blooming from an ingot, a billet obtained by blooming from a continuously cast material, or steel rods obtained by hot rolling or hot forging these billets etc. can be used.
  • the material for hot forging use is hot forged and further is machined to finish the worked material to a predetermined part shape.
  • the hot forging is for example performed by heating the material for hot forging use to 1100 to 1350° C. for 0.1 to 300 minutes, then allowing the surface temperature after the finish forging to fall to 900° C. or more, then cooling down to room temperature by an average cooling rate of 10 to 90° C./min in the temperature region from 800 to 400° C.
  • the worked material was supplied to aging treatment to obtain a machine part for an automobile, industrial machinery, construction machinery, etc. provided with the desired characteristics.
  • the aging treatment is, for example, performed in the temperature region from 540 to 700° C., preferably from 560 to 680° C.
  • the holding time of the aging treatment is adjusted by the size (mass) of the machine part for soaking, but can be made 30 to 1000 minutes.
  • the Steels 1 to 27 of the chemical compositions shown in Table 1 were smelted with a 50 kg vacuum melting furnace.
  • the Steels 1 to 17 in Table 1 are steels with chemical compositions within the ranges prescribed in the present invention.
  • the Steels 18 to 27 in Table 1 are steels with chemical compositions outside the conditions prescribed by the present invention.
  • F1 P2 type mass % (balance: Fe and impurities) structures V or or name V N Mo Ti Others (%) ratio F1′ P2′ 1 0.31 0.0130 0.10 ⁇ 0.001 +0.005Nb 100 0.99 0.76 0.99 2 0.35 0.0161 0.05 0.001 100 0.98 0.87 1.58 3 0.45 0.0090 ⁇ 0.01 ⁇ 0.001 100 0.99 0.87 1.18 4 0.39 0.0099 ⁇ 0.01 ⁇ 0.001 100 0.99 0.87 1.25 5 0.32 0.0115 0.39 ⁇ 0.001 100 0.99 0.91 1.17 6 0.29 0.0089 0.35 ⁇ 0.001 100 0.99 0.88 1.06 7 0.30 0.0126 0.05 0.001 100 0.98 0.78 0.97 8 0.31 0.0126 0.30 0.001 +0.001Ca 100 0.98 0.83 1.05 9 0.30 0.0146 0.30 0.001 +0.015Bi 100 0.98 0.83 1.10 10 0.32 0.0146 0.30 0.001 100 0.98 0.85 1.01 11 0.40 0.0155 0.29 ⁇ 0.001 +
  • the ingots of the various steel were heated at 1250° C., then were hot forged to steel rods of diameters of 60 mm.
  • the hot forged steel rods were cooled to room temperature in the atmosphere. After that, these were heated at 1250° C. for 30 minutes, then, envisioning forging to part shapes, were hot forged to steel rods with a diameter of 35 mm while surface temperatures of the forging rods at the time of finishing was kept from 950 to 1100° C. After the hot forging, all of the rods were cooled to room temperature in the atmosphere.
  • the cooling rate at the time of allowing the rods to cool in the atmosphere was measured after by burying a thermocouple near R/2 of the steel rods hot forged under the above conditions (“R” indicates the radius of the steel rods), again raising the temperature to near the finish temperature in hot forging, then allowing the rods to cool in the atmosphere.
  • the average cooling rate in the temperature region from 800 to 400° C. after forging measured in this way was about 40° C./min.
  • test pieces were cut out from the steel rods and were investigated for absorption energy in a Charpy impact test, fatigue strength, and yield strength after aging treatment.
  • the hardness was measured in the following way. First, a steel rod was cross-cut, was buried in resin so that the cut surface became the measured surface, then was polished to a mirror finish to prepare a test piece. Next, based on “Vickers Hardness Test—Test Method” in JIS Z 2244 (2009), 10 points near the R/2 Part of the measured surface (“R” indicating the radius) were measured for hardness with a test force of 9.8N. The values of the above 10 points were arithmetically averaged to obtain the Vickers hardness. When the hardness before the aging treatment was 340 HV or less, it was judged that mass production was industrially possible even with parts machined under various conditions. This was made the target. The test piece after measurement of the hardness was corroded with Nital and observed for structure, whereupon the structure of the test piece of each steel was also mainly bainite with some MA structures mixed in.
  • the toughness after aging treatment was evaluated by a Charpy impact test conducted using a U-notched standard test piece with a depth of notch of 2 mm and notch bottom radius of 1 mm.
  • the absorption energy at a test temperature of 20° C. was 25 J or more, it was judged sufficiently high. This was made the target.
  • the fatigue strength was investigated by fabricating an Ono type rotating bending fatigue test piece with a diameter of the parallel part of 8 mm and length of 106 mm. That is, the above test piece was taken so that the center of the fatigue test piece becomes the R/2 part of a steel rod.
  • An Ono type rotating bending fatigue test was conducted eight times under conditions of room temperature, the atmosphere, and a stress ratio of ⁇ 1. The maximum value of the stress amplitude up to 1.0 ⁇ 10 7 repetitions while not fracturing was made the fatigue strength. If the fatigue strength was 490 MPa or more, it was judged that the fatigue strength was sufficiently high and this was made the target.
  • the age hardenable steel of the present invention can secure a suitable hardness before aging treatment (340 HV or less) and promises a drop in machining resistance and longer life of tools. Further, if using the age hardenable steel of the present invention, due to the aging treatment performed after machining, a suitable fatigue strength (490 MPa or more), yield strength (800 MPa or more), and impact value (25 J or more) can be secured together. For this reason, the age hardenable steel of the present invention can be extremely suitably used as a material for a machine part in automobiles, industrial machinery, construction machinery, etc.

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US12000028B2 (en) * 2015-08-24 2024-06-04 Nippon Steel Corporation Rail vehicle axle

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