Title of invention: Hot forged steel
Technical field
[0001]
　The present disclosure relates to steel products, and more particularly to hot forged steel products that are hot forged steel products.
Background technology
[0002]
　Large steel parts are used as frames for mechanical products such as plunger pumps.
[0003]
　Such a large steel part is usually manufactured by the following manufacturing method. Prepare a thick steel plate made of machine structural steel. A cutting process is performed on the prepared thick steel plate to manufacture a plurality of intermediate steel materials. Support ribs are sandwiched between a plurality of intermediate steel materials produced by cutting a thick steel plate, and the support ribs and the intermediate steel materials are welded together to bond the plurality of intermediate steel materials. Steel parts are manufactured through the above steps.
[0004]
　As described above, when a thick steel plate is cut to manufacture an intermediate steel material, the intermediate steel material and the supporting ribs are welded to manufacture a steel part. In this case, the number of welding steps increases.
[0005]
　On the other hand, when a hot forged steel material obtained by hot forging a steel material is used as the intermediate steel material, it is possible to manufacture a hot forged steel material in which the supporting ribs are integrally formed with the intermediate steel material. If the hot forged steel material is used, the step of welding the supporting rib and the intermediate steel material can be reduced, and the number of welding steps can be reduced in the production of steel parts. Furthermore, since the support ribs are formed integrally with the intermediate steel material, the strength of the joint between the support ribs and the intermediate steel material is increased. Therefore, it is preferable to manufacture a steel part using a hot forged steel material manufactured by hot forging.
[0006]
　As described above, when manufacturing a steel part using a hot forged steel material, the hot forged steel material is required to have high tensile strength and toughness. By the way, mechanical products such as plunger pumps may be used in cold regions. Therefore, in the hot forged steel material for steel parts, in particular, high tensile strength and excellent low temperature toughness are required.
[0007]
　Steel for steel parts is disclosed in, for example, Japanese Patent Application Laid-Open No. 11-256267 (Patent Document 1) and Japanese Patent Application Laid-Open No. 60-262941 (Patent Document 2).
[0008]
　The structural steel material described in Patent Document 1 is C: 0.04 to 0.18%, Si: 0.60% or less, Mn: 0.80 to 1.80%, P: 0. 030% or less, S: 0.015% or less, V: 0.04 to 0.15%, N: 0.0050 to 0.0150%, and further, Al: 0.005 to 0.050% and Ti: contains 0.005 to 0.050% of one or two kinds, the balance being Fe and impurities, and has a chemical composition satisfying the following formula. 0.34≦C+Si/24+Mn/6+V/14+Ni/40+Cr/5+Mo/4≦0.48%. This structural steel material further contains 0.02 to 0.07% of VN precipitates and has a structure in which 10 6 to 10 10 VN precipitates having a particle diameter of 5 to 200 nm are precipitated /mm 3 . The crystal grain size of ferrite of this structural steel material is 5 or more in the grain size number defined by JIS G 0552, and the area ratio of ferrite grains is 50 to 100%. It is described in Patent Document 1 that the structural steel material having the above structure can have excellent fracture toughness under high-speed deformation.
[0009]
　The steel for warm forging described in Patent Document 2 is, by weight %, C: 0.1 to 0.5%, Si: 0.03 to 1.0%, Mn: 0.2 to 2.0%. , Al: 0.015 to 0.07%, N: 0.009 to 0.03%, the balance Fe and impurities are hot-worked steel, after warm forging at 300 to 950 ℃ The crystal grains at the time of reheating such as normalizing, carburizing or carbonitriding are fine grains having a grain size number of 6 or more. It is described in Patent Document 2 that the warm forging steel can increase the strength of parts by the above-mentioned configuration.
Prior art documents
Patent literature
[0010]
Patent Document 1: Japanese Patent Laid-Open No. 11-256267
Patent Document 2: Japanese Patent Laid-Open No. 60-262941
Summary of the invention
Problems to be Solved by the Invention
[0011]
　However, as long as the examples (see Table 2-1 and Table 2-2) in Patent Document 1 are referred to, the grain size number of ferrite of the structural steel material of Patent Document 1 is as low as 7.5 or less. Therefore, the low temperature toughness may be low. Further, in Patent Document 1, when the deformation rate in the tensile test is about 0.2 mm/s, which is a normal value, high tensile strength may not be obtained.
[0012]
　In the warm forging steel of Patent Document 2, the forging temperature is as low as 950°C or lower. Therefore, high tensile strength and high low temperature toughness may not be obtained in some cases.
[0013]
　It is known that high temperature low-temperature toughness can be obtained by containing Ni and a rare earth element. However, these elements are expensive and increase manufacturing costs. Therefore, there is a demand for a hot forged steel material having high strength and excellent low temperature toughness even if the content of these elements is omitted or the content of these elements is suppressed to a low level.
[0014]
　An object of the present disclosure is to provide a hot forged steel material having high strength and excellent low temperature toughness.
Means for solving the problem
[0015]
　The hot forged steel material according to the present disclosure has C: 0.14 to 0.20%, Si: 0.20 to 1.00%, Mn: 1.00 to 1.90%, P: 0. 030% or less, S: 0.030% or less, V: 0.16 to 0.30%, Al: 0.015 to 0.050%, N: 0.0050 to 0.0250%, Cr: 0.10. .About.0.30%, Cu:0.about.0.10%, Nb:0.about.0.10%, and the balance Fe and impurities, and has a chemical composition satisfying the formulas (1) and (2). The grain size number of the ferrite in the hot forged steel material is 9.0 or more, and the absorbed energy at −30° C. is 100 J or more in the Charpy impact test using the V-notch test piece.
　0.36≦C+(Si+Mn)/6+(Cr+V)/5+Cu/15<0.68 (1)
　51/12×C−V≦0.52 (2)
　Here, the formula (1) and the formula (2) The content (mass %) of the corresponding element is substituted for each element symbol therein.
Effect of the invention
[0016]
　The hot forged steel product according to the present disclosure has high strength and high low temperature toughness.
MODE FOR CARRYING OUT THE INVENTION
[0017]
　The present inventors have conducted investigations and studies in order to enhance the strength and low temperature toughness of hot forged steel materials used for large steel parts. As a result, the present inventors initially thought that if the C content was lowered, the weldability of steel materials would be improved. As a result of further study, the present inventors have found that, in mass %, C: 0.14 to 0.20%, Si: 0.20 to 1.00%, Mn: 1.00 to 1.90%, P: 0.030% or less, S: 0.030% or less, V: 0.16 to 0.30%, Al: 0.015 to 0.050%, N: 0.0050 to 0.0250%, Cr : 0.10 to 0.30%, Cu: 0 to 0.10%, Nb: 0 to 0.10%, and the strength if it is a hot forged steel material having a chemical composition with the balance being Fe and impurities , And low temperature toughness.
[0018]
　However, the strength may not be sufficiently obtained by simply adjusting the chemical composition of the hot forged steel material to the above-described chemical composition having a low C content. Therefore, the present inventors further studied. As a result, it was found that if the above chemical composition further satisfies the following formula (1), the strength is increased.
　0.36≦C+(Si+Mn)/6+(Cr+V)/5+Cu/15<0.68 (1)
　Here, each element symbol in the formula (1) indicates the content (% by mass) of the corresponding element. Substituted.
[0019]
　It is defined as F1=C+(Si+Mn)/6+(Cr+V)/5+Cu/15. F1 is an index of weldability and strength and corresponds to carbon equivalent. If F1 is 0.36 or more, sufficient strength can be obtained even in the above chemical composition. It is generally known that the lower the carbon equivalent, the better the weldability. Therefore, in the steel material of the present embodiment having the above chemical composition, F1 is set to less than 0.68. In this case, it is considered that better weldability can be obtained than when F1 is 0.68 or more. Further, if F1 is less than 0.68, bainite is unlikely to be generated in the microstructure, so the low temperature toughness is enhanced.
[0020]
　Further, in the present embodiment, in the hot forged steel material, as shown in the above chemical composition, 0.16 to 0.30% of V is contained, and fine V carbonitrides (VC, VN, and V( C, N), or a composite precipitate of these and other elements) is deposited. As shown in the above chemical composition and satisfying the formula (1), the V content is set to 0.16 to 0.30% to precipitate fine V carbonitrides, etc. The present inventors considered that the value was 600 MPa or more, and high strength was obtained.
[0021]
　However, in the above chemical composition, when the formula (1) is satisfied and the V content is 0.16 to 0.30%, high strength can be obtained, but the low temperature toughness of the hot forged steel material becomes low. Turned out to be. Therefore, the present inventors have further studied a hot forged steel material that can obtain not only strength but also sufficient low temperature toughness. As a result, the inventors have found that not only the strength but also the low temperature toughness can be enhanced by satisfying the formula (1) and further satisfying the following formula (2) in the above chemical composition.
　51/12×CV≦0.52 (2)
　Here, the content (mass %) of the corresponding element is substituted for each element symbol in the formula (2).
[0022]
　It is defined as F2=51/12×CV. F2 is an index of the amount of C remaining in a solid solution state (hereinafter referred to as the amount of solid solution C) in the hot forged steel material after the precipitation of V carbonitride. When F2 exceeds 0.52, the amount of solid solution C in the steel material is too large even after V carbonitrides and the like are deposited. In this case, the low temperature toughness of the hot forged steel material decreases. In the above chemical composition, if the formula (1) is satisfied and F2 is 0.52 or less, the amount of solid solution C after V carbonitride and the like is sufficiently suppressed, and as a result, hot The low temperature toughness of the forged steel becomes high. Specifically, in a Charpy impact test using a V-notch test piece at −30° C., assuming that the crystal grain size number of ferrite grains according to JIS G 0551 (2013) described later is 9.0 or more. Absorbed energy of 100 J or more.
[0023]
　In the above hot forged steel material, if the crystal grains of ferrite (pro-eutectoid ferrite) are refined, the low temperature toughness is further enhanced. Specifically, if the crystal grain size number of ferrite grains according to JIS G 0551 (2013) is 9.0 or more, excellent low temperature toughness is obtained.
[0024]
　When carrying out hot forging, the ferrite grains after hot forging are coarse grains. Therefore, the hot forged steel material of the present invention contains 0.015 to 0.050% Al and 0.0050 to 0.0250% N as shown in the above chemical composition, and is burned at 875 to 950° C., for example. Perform semi-processing. In this case, not only the ferrite grains become finer by the normalizing treatment, but also the ferrite grains become finer by the pinning effect of AlN formed during the normalizing treatment. Since TiN, V carbonitride, and the like are extremely fine, the pinning effect is not exhibited. The pinning effect of AlN is effective for making the ferrite grains fine during normalizing treatment.
[0025]
　In the present invention, Ti and Mo are impurities. Ti forms TiN and reduces the low temperature toughness of the hot forged steel material. Mo forms bainite in the steel and reduces the low temperature toughness of the hot forged steel material. Therefore, Ti and Mo are impurities.
[0026]
　The hot forged steel material according to the present embodiment completed based on the above findings is, in mass %, C: 0.14 to 0.20%, Si: 0.20 to 1.00%, Mn: 1.00 to 1.90%, P: 0.030% or less, S: 0.030% or less, V: 0.16 to 0.30%, Al: 0.015 to 0.050%, N: 0.0050 to 0 0.0250%, Cr: 0.10 to 0.30%, Cu: 0 to 0.10%, Nb: 0 to 0.10%, and the balance Fe and impurities, and the formula (1) and the formula ( It has a chemical composition satisfying 2), the grain size number of ferrite in the hot forged steel is 9.0 or more, and the absorbed energy at −30° C. is 100 J in the Charpy impact test using the V-notch test piece. That is all.
　0.36≦C+(Si+Mn)/6+(Cr+V)/5+Cu/15<0.68 (1)
　51/12×C−V≦0.52 (2)
　Here, the formula (1) and the formula (2) The content (mass %) of the corresponding element is substituted for each element symbol therein.
[0027]
　The above chemical composition may contain one or more selected from the group consisting of Cu: 0.01 to 0.10% and Nb: 0.01 to 0.10%.
[0028]
　In the hot forged steel material according to the present embodiment, the tensile strength TS may be 600 MPa or more.
[0029]
　Hereinafter, the hot forged steel material according to the present invention will be described in detail. "%" regarding an element means mass% unless otherwise specified.
[0030]
　[Chemical composition]
　The chemical composition of the hot forged steel material according to the present embodiment contains the following elements.
[0031]
　C: 0.14 to 0.20%
　Carbon (C) enhances the tensile strength of the steel material. If the C content is less than 0.14%, this effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the C content exceeds 0.20%, the weldability and low temperature toughness of the steel material deteriorate even if the content of other elements is within the range of this embodiment. Therefore, the C content is 0.14 to 0.20%. The preferable lower limit of the C content is more than 0.14%, more preferably 0.15%, and further preferably 0.16%. The preferable upper limit of the C content is 0.19%, more preferably 0.18%, further preferably 0.17%.
[0032]
　Si: 0.20-1.00%
　Silicon (Si) deoxidizes steel. Si further forms a solid solution with ferrite in the steel material to strengthen the ferrite and enhance the strength of the steel material. If the Si content is less than 0.20%, these effects cannot be sufficiently obtained even if the content of other elements is within the range of this embodiment. On the other hand, if the Si content exceeds 1.00%, the scale tends to remain on the surface of the hot forged steel material, and the appearance of the hot forged steel material deteriorates. Therefore, the Si content is 0.20 to 1.00%. The preferable lower limit of the Si content is 0.30%, more preferably 0.40%, further preferably 0.50%. The preferable upper limit of the Si content is 0.90%, more preferably 0.80%, further preferably 0.70%.
[0033]
　Mn: 1.00 to 1.90%
　Manganese (Mn) deoxidizes steel. Mn further forms a solid solution in the steel material to enhance the strength of the steel material. If the Mn content is less than 1.00%, these effects cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mn content exceeds 1.90%, bainite is generated in the steel material and the low temperature toughness of the hot forged steel material is reduced even if other elements are within the range of this embodiment. Therefore, the Mn content is 1.00 to 1.90%. The preferable lower limit of the Mn content is 1.20%, more preferably 1.30%, and further preferably 1.40%. The preferable upper limit of the Mn content is less than 1.90%, more preferably 1.80%, further preferably 1.70%, and further preferably 1.60%.
[0034]
　P: 0.030% or less
　Phosphorus (P) is an unavoidable impurity. That is, the P content is more than 0%. If the P content exceeds 0.030%, even if the content of other elements is within the range of the present embodiment, P segregates at the grain boundaries of the steel material and embrittles the steel material. Therefore, the P content is 0.030% or less. The preferable upper limit of the P content is 0.020%, more preferably 0.015%, and further preferably 0.010%. The P content is preferably as low as possible. However, if the P content is extremely reduced in the steelmaking process, the manufacturing cost will increase and the productivity will also decrease. Therefore, the lower limit of the P content is preferably 0.001%, and more preferably 0.002%.
[0035]
　S: 0.030% or less
　Sulfur (S) is an unavoidable impurity. That is, the S content is more than 0%. If the S content exceeds 0.030%, even if the content of other elements is within the range of this embodiment, S deteriorates the hot workability of the steel material. Therefore, the S content is 0.030% or less. The preferable upper limit of the S content is 0.020%, more preferably 0.015%, and further preferably 0.013%. It is preferable that the S content is as low as possible. However, if the S content is extremely reduced in the steelmaking process, the manufacturing cost will increase and the productivity will also decrease. Therefore, the lower limit of the S content is preferably 0.001%, and more preferably 0.002%.
[0036]
　V: 0.16 to 0.30%
　Vanadium (V) is a fine V carbonitride or the like (VC, VN, and V(C, N), or these and other substances combined with carbon and/or nitrogen. It forms a complex precipitate with the element) and enhances the strength of the hot forged steel material. If the V content is less than 0.16%, this effect cannot be sufficiently obtained even if the content of other elements is within the range of this embodiment. On the other hand, if the V content exceeds 0.30%, coarse V carbonitrides and the like are generated even if the content of other elements is within the range of this embodiment. Coarse V carbonitrides reduce the low temperature toughness of hot forged steel. Therefore, the V content is 0.16 to 0.30%. The preferable lower limit of the V content is 0.17%, more preferably 0.18%, further preferably 0.19%, further preferably 0.20%. The preferable upper limit of the V content is 0.29%, more preferably 0.28%, further preferably 0.27%, further preferably 0.26%.
[0037]
　Al: 0.015 to 0.050%
　Aluminum (Al) deoxidizes steel. Further, Al forms AlN to refine the ferrite grains of the hot forged steel material by the pinning effect. This enhances the low temperature toughness of the hot forged steel material. If the Al content is less than 0.015%, these effects cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Al content exceeds 0.050%, coarse Al 2 O 3 -based inclusions and coarse AlN are likely to be generated even if the content of other elements is within the range of the present embodiment . Coarse Al 2 O 3 -based inclusions and coarse AlN reduce the low temperature toughness of the hot forged steel material. Therefore, the Al content is 0.015 to 0.050%. The preferable lower limit of the Al content is 0.016%, more preferably 0.018%, and further preferably 0.020%. The preferable upper limit of the Al content is 0.040%, more preferably 0.035%, further preferably 0.030%. The “Al” content as used herein means the content of “acid-soluble Al”, that is, “sol.Al”.
[0038]
　N: 0.0050 to 0.0250%
　Nitrogen (N) combines with Al and V to form AlN and V carbonitride and the like. AlN refines the ferrite grains of the hot forged steel material by the pinning effect to enhance the low temperature toughness of the hot forged steel material. V carbonitride and the like enhance the strength of the hot forged steel material by precipitation strengthening. If the N content is less than 0.0050%, these effects cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the N content exceeds 0.0250%, coarse AlN and coarse V carbonitrides are produced, and the low temperature toughness of the hot forged steel material deteriorates. Therefore, the N content is 0.0050 to 0.0250%. The preferable lower limit of the N content is 0.0060%, more preferably 0.0070%, further preferably 0.0080%, further preferably 0.0090%. The preferable upper limit of the N content is 0.0220%, more preferably 0.0210%, further preferably 0.0200%, further preferably 0.0190%, further preferably 0.0180. %.
[0039]
　Cr: 0.10 to 0.30%
　Chromium (Cr) enhances the strength of the steel material. If the Cr content is less than 0.10%, this effect cannot be sufficiently obtained even if the content of other elements is within the range of this embodiment. On the other hand, when the Cr content exceeds 0.30%, the low temperature toughness and weldability of the steel material deteriorate even if the content of other elements is within the range of this embodiment. Therefore, the Cr content is 0.10 to 0.30%. The preferable lower limit of the Cr content is 0.12%, more preferably 0.15%, and further preferably 0.16%. The preferable upper limit of the Cr content is 0.25%, more preferably 0.22%, and further preferably 0.20%.
[0040]
　The balance of the chemical composition of the hot forged steel material according to the present embodiment is Fe and impurities. Here, the impurities, when industrially manufacturing the hot forged steel, ore as a raw material, scrap, or those that are mixed from the manufacturing environment, etc., adversely affect the hot forged steel of the present invention. It means what is allowed in the range not given.
[0041]
　In the hot forged steel material of the present embodiment, the impurities are, for example, Ti and Mo. Ti forms TiN. TiN significantly reduces the low temperature toughness of hot forged steel. When Mo is contained, bainite is easily generated in the steel material after the normalizing treatment. As a result, the low temperature toughness of the steel material decreases. In the hot forged steel material of the present embodiment, Ti and Mo reduce the low temperature toughness of the hot forged steel material. Therefore, the lower the Ti and Mo contents, the better, and the Ti and Mo contents may be 0%. In this embodiment, the Ti content is 0.010% or less. Mo content is 0.10% or less. The Ti content and the Mo content can be adjusted within the above ranges if they are manufactured by a person having ordinary technical common knowledge in this field in the manufacturing process described below. The preferable upper limit of the Ti content is 0.008%, more preferably 0.005%, and further preferably less than 0.003%. The preferable upper limit of the Mo content is 0.09%, more preferably 0.08%.
[0042]
　[Regarding Optional Elements (Optional Elements)]
　The chemical composition of the hot forged steel material described above may further include one or more selected from the group consisting of Cu and Nb, instead of part of Fe. All of these elements are optional elements, and all increase the strength of the hot forged steel material.
[0043]
　Cu: 0 to 0.10%
　Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When contained, Cu enhances the strength of the hot forged steel material. If Cu is contained even a little, the above effect can be obtained to some extent. However, if the Cu content exceeds 0.10%, the hot workability of the hot forged steel material deteriorates even if the content of other elements is within the range of this embodiment. Therefore, the Cu content is 0 to 0.10%. The preferable lower limit of the Cu content is more than 0%, more preferably 0.01%, and further preferably 0.02%. The preferable upper limit of the Cu content is 0.08%, more preferably 0.07%, and further preferably 0.05%.
[0044]
　Nb: 0 to 0.10%
　Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, Nb combines with carbon and/or nitrogen in the crystal grains to form fine Nb carbonitrides (NbC, NbN, and Nb(C, N), or a composite of these with other elements. Precipitates) and strengthen the strength of the hot forged steel material by precipitation strengthening. If Nb is contained even a little, the above effect can be obtained to some extent. However, in the chemical composition of the hot forged steel material of the present embodiment, the above-mentioned Nb carbonitride or the like is unlikely to contribute to the refinement of ferrite grains. On the other hand, if the Nb content exceeds 0.10%, coarse Nb carbonitrides and the like are generated even if the content of other elements is within the range of the present embodiment, and the low temperature toughness of the hot forged steel material is generated. To lower. Therefore, the Nb content is 0 to 0.10%. The preferable lower limit of the Nb content is more than 0%, more preferably 0.01%, and further preferably 0.02%. The preferable upper limit of the Nb content is 0.08%, more preferably 0.05%.
[0045]
　[Regarding Formula (1)]
　The chemical composition of the hot forged steel material of the present embodiment further satisfies Formula (1).
　0.36≦C+(Si+Mn)/6+(Cr+V)/5+Cu/15<0.68 (1)
　Here, each element symbol in the formula (1) indicates the content (% by mass) of the corresponding element. Substituted.
[0046]
　It is defined as F1=C+(Si+Mn)/6+(Cr+V)/5+Cu/15. F1 is an index of the strength of the hot forged steel material and corresponds to the carbon equivalent. When F1 is less than 0.36, the strength of the hot forged steel material is insufficient. Specifically, even if the content of each element in the chemical composition is within the above range and the formula (2) is satisfied, the tensile strength of the hot forged steel material becomes less than 600 MPa. It is generally known that the lower the carbon equivalent, the better the weldability. The upper limit of F1 is set to less than 0.68 so that the weldability does not deteriorate excessively. When F1 is 0.68 or more, bainite is further generated in the microstructure, and the hot forged steel material becomes too hard. As a result, the low temperature toughness decreases. When F1 is 0.36 to less than 0.68, assuming that the content of each element in the chemical composition is within the range of the present embodiment and the formula (2) is satisfied, the tensile strength of 600 MPa or more is It is possible to obtain excellent low temperature toughness. The preferable lower limit of F1 is 0.40, more preferably 0.45, and further preferably 0.50. The preferable upper limit of F1 is 0.65, more preferably 0.63, and further preferably 0.61. In addition, F1 is obtained by rounding off to the second decimal place.
[0047]
　[Regarding Formula (2)]
　The chemical composition of the hot forged steel material of the present embodiment further satisfies Formula (2).
　51/12×CV≦0.52 (2)
　Here, the content (mass %) of the corresponding element is substituted for each element symbol in the formula (2).
[0048]
　It is defined as F2=51/12×CV. F2 is an index relating to the amount of solid solution C remaining in the steel after precipitation of V carbonitride in the hot forged steel. "51" in F2 means the atomic weight of V, and "12" means the atomic weight of C. If F2 exceeds 0.52, the amount of solid solution C remaining in the steel is too large even after V carbonitrides and the like are precipitated. In this case, the low temperature toughness of the hot forged steel material decreases. In the above chemical composition, if the formula (1) is satisfied and F2 is 0.52 or less, the amount of solid solution C in the steel material after V carbonitrides and the like are precipitated is sufficiently low, The low temperature toughness of the forged steel becomes high. As a result, the content of each element in the chemical composition is within the range of this embodiment, the chemical composition satisfies the formula (1), and the ferrite grain size number in the microstructure is 9.0 or more. Assuming that, in the Charpy impact test using the V-notch test piece, the absorbed energy at −30° C. becomes 100 J or more.
[0049]
　The preferable upper limit of F2 is 0.50, more preferably 0.49, and further preferably 0.48. The lower limit of F2 is not particularly limited. However, considering the lower limit value of C content and the upper limit value of V content in the above-mentioned chemical composition, the preferable lower limit of F2 is 0.30, and more preferably 0.32.
[0050]
　[Microstructure]
　The microstructure (matrix structure) of the hot forged steel material of the present invention is composed of ferrite and pearlite. The term “ferrite” as used herein means proeutectoid ferrite unless otherwise specified. If the microstructure is composed of ferrite and pearlite, it is assumed that the content of each element in the chemical composition is within the range of this embodiment and the chemical composition satisfies the formulas (1) and (2). Excellent low temperature toughness can be obtained in hot forged steel. In the hot forged steel material of the present embodiment, if the content of each element in the chemical composition is within the range of the present embodiment and the chemical composition satisfies the formula (1) and the formula (2), the manufacturing method described later. Assuming that the above is performed, a microstructure composed of ferrite and pearlite can be obtained. The microstructure in this specification means a so-called matrix (base material) structure excluding precipitates and inclusions. That the microstructure is composed of ferrite and pearlite means that the total area ratio of ferrite and pearlite obtained by the method for measuring each phase of the microstructure described below is 95.0% or more.
[0051]
　[Method of Measuring Each Phase in
　Microstructure ] Each phase (ferrite, pearlite, etc.) in the microstructure can be specified by the following method.
[0052]
　A sample is taken from an arbitrary portion inside the hot forged steel material at a depth of 5 mm or more from the surface. The size of the sample is not particularly limited as long as the observation visual field described below can be secured. After the surface (observation surface) of the sample is mirror-polished, the sample is etched with an ethanol solution (nital etchant) containing 2% by volume of nitric acid. The structure is observed on the etched observation surface. A 100× optical microscope is used for the tissue observation, and the observation visual field is 200 μm×200 μm. Observe any one visual field in the observation plane. In the observation visual field, the contrast of each phase (ferrite, pearlite, bainite, etc.) is different. Therefore, the phase is specified based on the contrast. Among the identified phases, the total area of ​​ferrite and the total area of ​​pearlite are calculated. The ratio of the total area of ​​the total of ferrite and pearlite to the total area of ​​the observation visual field (hereinafter referred to as the total area ratio of ferrite and pearlite) is obtained. If the total area ratio of ferrite and pearlite is 95.0% or more, it is determined that the microstructure is a microstructure composed of ferrite and pearlite.
[0053]
　[Regarding Grain Size]
　Further, in the hot forged steel material of the present embodiment, in the ferrite in the microstructure, the grain size number specified by JIS G 0551 (2013) is 9.0 or more. In the hot forged steel material of this embodiment, since the grain size number of ferrite is as fine as 9.0 or more, it is excellent in low temperature toughness. Specifically, in a Charpy impact test using a V-notch test piece, the absorbed energy at −30° C. is 100 J or more.
[0054]
　In the hot forged steel material of the present embodiment, the preferred lower limit of the grain size number of ferrite in the microstructure according to JIS G 0551 (2013) is 9.5, and more preferably 10.0. The upper limit of the crystal grain size number of ferrite in the microstructure according to JIS G 0551 (2013) is not particularly limited, but in the case of the above chemical composition satisfying the formulas (1) and (2), the upper limit of the crystal grain size number is For example, it is 15.0 and may be 14.5. The grain size number of ferrite defined in the present embodiment means the grain size number of proeutectoid ferrite, as described above, and is not the grain size number of ferrite in pearlite.
[0055]
　[Measurement method of grain size number] The grain size number of
　ferrite in the microstructure is determined by the following method. A sample is taken from the area of ​​the depth of 3.0 mm to the depth of 20.0 mm from the surface of the hot forged steel material. The size of the sample is not particularly limited as long as the visual field described below can be secured. One of the surfaces of the sample is specified as the observation surface, the observation surface is mirror-polished, and then etched with an ethanol solution (nital corrosive liquid) containing 2% by volume of nitric acid to form ferrite grain crystals on the observation surface. Bring out grain boundaries. From the etched observation surface, the grain size number of the ferrite grains in each visual field is determined in any 10 visual fields including ferrite (the area of ​​each visual field is 40 mm 2 ). Specifically, the grain size number of the ferrite grains in each field of view is determined by comparison with the grain size standard diagram specified in 7.2 of JIS G 0551 (2013). The average of the grain size numbers of each field of view is defined as the grain size number of the hot forged steel material of this embodiment. The grain size number is a value obtained by rounding off the second decimal place (that is, the value of the grain size number of the ferrite grains is the first decimal place).
[0056]
　[Regarding Low Temperature Toughness] In
　the hot forged steel material of the present embodiment, the absorbed energy at −30° C. is 100 J or more in the Charpy impact test using the V-notch test piece according to JIS Z 2242 (2005). The hot forged steel material of the present embodiment has a microstructure composed of ferrite and pearlite, and since the crystal grain size number of ferrite in the microstructure according to JIS G 0551 (2013) is 9.0 or more, In the above Charpy impact test, the absorbed energy at −30° C. is 100 J or more, and excellent low temperature toughness is obtained. In the hot forged steel material of the present embodiment, in a Charpy impact test using a V notch test piece conforming to JIS Z 2242 (2005), a preferable lower limit of absorbed energy at −30° C. is 105 J or more, and more preferably It is 115 J or more.
[0057]
　[Measurement Method of Low Temperature Toughness]
　The low temperature toughness of the hot forged steel material of the present embodiment can be measured by the following method. In the hot forged steel material, a V-notch test piece specified in JIS Z 2242 (2005) is taken from within a region having a depth of 3.0 mm to a depth of 20.0 mm from the surface. The cross section of the V-notch test piece is a 10 mm×10 mm square, and the length of the V-notch test piece in the longitudinal direction is 55 mm. That is, the V-notch test piece is a so-called full-size test piece. That is, the full-size test piece is sampled from the depth of 3.0 mm to the depth of 20.0 mm from the surface of the hot forged steel material. The longitudinal direction of the V-notch test piece is parallel to the axial direction (longitudinal direction) of the hot forged steel material. The V notch is formed at the center position of the length of the V notch test piece (that is, the center position of the length 55 mm). The V notch angle is 45°, the notch depth is 2 mm, and the notch bottom radius is 0.25 mm. A Charpy impact test according to JIS Z 2242 (2005) is carried out using the V-notch test piece to determine the absorbed energy at -30°C. Specifically, the Charpy impact test according to JIS Z 2242 (2005) was carried out in the air on three V-notch test pieces cooled to -30°C, and the average of the absorbed energy obtained. Is defined as the absorbed energy (J) at −30° C. The absorbed energy (J) is an integer value rounded off to one decimal place.
[0058]
　[Regarding Tensile Strength]
　The tensile strength of the hot forged steel material of this embodiment is 600 MPa or more. In the hot forged steel material of this embodiment, a large number of fine V carbonitrides and the like are precipitated in the ferrite due to the phase interface precipitation. Therefore, the hot forged steel material of this embodiment has high tensile strength. Since the size of the fine V carbonitrides in the ferrite is at the nano level, it is necessary to quantitatively measure the areal density (pieces/μm 2 ) of the fine V carbonitrides in the ferrite. Is extremely difficult. Therefore, in the hot forged steel material of the present embodiment, the degree of precipitation of fine V carbonitrides, etc. is substituted by the regulation of tensile strength.
[0059]
　The preferable lower limit of the tensile strength of the hot forged steel material of the present embodiment is 605 MPa, more preferably 610 MPa. The upper limit of the tensile strength of the hot forged steel material of the present embodiment is not particularly limited, but in the case of the above chemical composition, the upper limit of the tensile strength is, for example, 750 MPa.
[0060]
　[Measurement Method of Tensile Strength]
　The tensile strength of the hot forged steel material of the present embodiment can be measured by the following method. A round bar tensile test piece having a diameter of 6.35 mm and a parallel portion length of 35 mm is produced from a region of a depth of 3.0 mm to a depth of 20.0 mm from the surface of the hot forged steel material. The parallel portion of the round bar tensile test piece is parallel to the axial direction (longitudinal direction) of the hot forged steel material. Using a round bar tensile test piece, a tensile test is carried out in the atmosphere at room temperature (10 to 35° C.) according to JIS Z 2241 (2011) to obtain the tensile strength (MPa). The deformation rate in the tensile test is 0.2 mm/s.
[0061]
　[Use of hot forged steel material]
　The hot forged steel material of the present embodiment is widely applicable to applications requiring strength and low temperature toughness. The hot forged steel material is applied, for example, as a steel part manufactured by welding. The steel part is, for example, a frame member of an industrial machine represented by a plunger pump. When a hot forged steel material is applied as a frame member of an industrial machine, for example, a plurality of hot forged steel materials are combined and adjacent hot forged steel materials are fixed by welding or the like, so that a frame (a housing ) Can be manufactured.
[0062]
　[Manufacturing Method]
　An example of a method for manufacturing the hot forged steel material of the present embodiment will be described. The hot forged steel material of the present embodiment is not limited to the following manufacturing method as long as it has the above configuration. However, the manufacturing method described below is a suitable example for manufacturing the hot forged steel material of the present embodiment.
[0063]
　The method of manufacturing a hot forged steel material includes a step of preparing a material (preparation step), a step of hot forging the material (hot forging step), and a normalizing treatment for the hot forged material. And a step of manufacturing a hot forged steel material (normalizing treatment step). Hereinafter, each step will be described in detail.
[0064]
　[Preparation Step]
　A molten steel having a chemical composition in which the content of each element satisfies the range of the present embodiment described above and has the chemical formulas (1) and (2) is manufactured. The material is manufactured using molten steel. Specifically, slabs or blooms are produced by continuous casting using molten steel. You may manufacture an ingot by the ingot making method using molten steel. If necessary, a slab, a bloom, or an ingot may be slab-rolled into a billet. The material (slab, bloom, ingot, or billet) is manufactured by the above steps. When performing slabbing, the heating temperature of the slab, bloom, and ingot before slabbing may be within a known temperature range (for example, 1050 to 1300° C.).
[0065]
　[Hot Forging Step] The
　prepared material is hot forged to produce a rough intermediate product. The heating temperature during hot forging is 1200 to 1300°C. The material is heated in, for example, a heating furnace. Here, the heating temperature during hot forging corresponds to the surface temperature of the raw material at the start of hot forging. The heating temperature during hot forging can be measured, for example, by a thermometer installed at the extraction port of the heating furnace.
[0066]
　By setting the heating temperature during hot forging to 1200 to 1300° C., V carbonitride and the like in the raw material can be sufficiently dissolved. If V carbonitrides in the material can be sufficiently dissolved by heating during hot forging, fine V carbonitrides, etc. can be dissolved in ferrite (pro-eutectoid ferrite) due to phase interface precipitation in the cooling process after hot forging. Can be dispersed and precipitated. When the heating temperature during hot forging is less than 1200° C., V carbonitrides and the like remain in the steel material after the heating during hot forging without being sufficiently dissolved. In this case, V carbonitrides and the like remaining in the raw material become coarse in the cooling process after the hot forging and the normalizing process in the post process of the hot forging process. As a result, the low temperature toughness of the hot forged steel material is lowered, and the absorbed energy at −30° C. becomes less than 100 J in the Charpy impact test using the V-notch test piece. On the other hand, if the hot forging temperature is too high, the manufacturing cost will be high. Therefore, the hot forging temperature is 1200 to 1300°C. The hot forging may be performed multiple times. When performing hot forging a plurality of times, it is sufficient if the final hot forging temperature is 1200 to 1300°C. The intermediate product after hot forging is allowed to cool. The rate during cooling is, for example, 3 to 50° C./minute. In this case, it is possible to suppress the coarsening of V carbonitride and the like during cooling, and to suppress the generation of a hard structure such as bainite in the microstructure.
[0067]
　[Normalizing Treatment Step] In the
　normalizing treatment step, the normalizing treatment is performed on the intermediate product after hot forging. By normalizing, the grain size number of ferrite in the steel material is set to 9.0 or more. The temperature in the normalizing treatment (normalizing temperature) is not lower than the Ac3 transformation point, specifically, 875 to 950°C. By setting the normalizing temperature within the above range, a part of V carbonitride or the like is re-dissolved during normalizing treatment, and the phase boundary is precipitated again during cooling. In this case, fine V carbonitrides or the like are generated, and the growth of coarse V carbonitrides or the like is suppressed. As a result, the tensile strength TS of the hot forged steel material becomes 600 MPa or more. The holding time at the normalizing temperature is not particularly limited, but is, for example, 40 to 150 minutes.
[0068]
　Further, in the normalizing treatment, the ferrite grains become finer. Further, in the hot forged steel material having the chemical composition of the present embodiment, fine AlN is produced in the above normalizing temperature range. Therefore, not only the normalizing treatment but also the pinning effect of AlN generated during the normalizing treatment further reduces the size of ferrite grains. Specifically, by the above-mentioned normalizing treatment, the grain size number of ferrite grains becomes 9.0 or more, which is excellent in the hot forged steel material having the chemical composition satisfying the above formulas (1) and (2). Further, low temperature toughness is obtained, and specifically, in the Charpy impact test using the V-notch test piece, the absorbed energy at −30° C. is 100 J or more.
[0069]
　Through the above steps, the hot forged steel material of the present embodiment is manufactured. The above-described manufacturing method is an example of the method for manufacturing the hot forged steel material of the present embodiment, and the hot forged steel material of the present embodiment is not limited to the above manufacturing method. The hot forged steel material of the present embodiment having the above-described configuration may be manufactured by another method different from the above manufacturing method.
[0070]
　In addition, you may implement machining etc. with respect to the hot forged steel material manufactured. Using the manufactured hot forged steel as a frame member, welding is performed on multiple hot forged steels to manufacture steel parts such as the frame (housing) of industrial machinery such as plunger pumps. May be.
Example
[0071]
　Materials having the chemical composition shown in Table 1 (a round bar having a diameter of 80 to 100 mm) were prepared.
[0072]
[table 1]

[0073]
　"-" in Table 1 means that the content of the corresponding element was below the detection limit. The "F1" column in Table 1 shows the F1 value for each test number. When the F1 value was less than 0.68, it was judged that the weldability was excellent, and "○" was entered in the "Weldability" column of Table 1. When the F1 value was other than that, it was judged that the weldability was low, and “X” was described in the “weldability” column. The "F2" column in Table 1 shows the F2 value of each test number.
[0074]
　Hot forging (hot forging) was performed on the round bar which is the above material to manufacture an intermediate product (a round bar having a diameter of 60 mm). The heating temperature (corresponding to the temperature at the start of hot forging) of the material (round bar) during hot forging was as shown in Table 1. The intermediate product after hot forging was cooled to room temperature at 3 to 50° C./min. Normalizing treatment was performed on the intermediate product after cooling. The temperature in the normalizing treatment (normalizing temperature) was 875 to 950° C., and the holding time was 60 to 120 minutes. The hot forged steel material was manufactured by the above steps.
[0075]
　[Evaluation Test]
　[Microstructure Observation Test]
　Samples were taken from the region of the depth of 3.0 mm to the depth of 20.0 mm from the surface of the hot forged steel material of each test number. The surface (observation surface) of the sample was mirror-polished, and then etched with an ethanol solution (nital etchant) containing 2% by volume of nitric acid. The structure was observed on the etched observation surface. A 100× optical microscope was used for tissue observation, and the visual field was 200 μm×200 μm. Any one visual field in the observation plane was observed. In the observation visual field, the contrast of each phase (ferrite, pearlite, bainite, etc.) is different. Therefore, the phase was identified based on the contrast. Among the identified phases, the total area of ​​ferrite and the total area of ​​pearlite were obtained. The ratio (total area ratio of ferrite and pearlite) of the total area of ​​ferrite and pearlite to the total area of ​​the observation visual field was determined. If the total area ratio of ferrite and pearlite is 95.0% or more, the microstructure was determined to be a microstructure composed of ferrite and pearlite. "F+P" in the "Microstructure" column in Table 1 indicates that the microstructure was a structure composed of ferrite and pearlite. On the other hand, when the total area ratio of ferrite and pearlite was less than 95.0% and bainite was observed in addition to ferrite and pearlite, it was determined that the microstructure was not a structure composed of ferrite and pearlite. “F+P+B” in the “Microstructure” column in Table 1 indicates that the total area ratio of ferrite and pearlite is less than 95.0% in the microstructure, and the microstructure was a structure containing ferrite, pearlite, and bainite. Show.
[0076]
　[Crystal Grain No. Measurement Test] A
　sample was taken from a region within a depth range of 3.0 mm to a depth of 20.0 mm from the surface of the hot forged steel material of each test number. The observation surface of the sample was mirror-polished and then etched with an ethanol solution (nital corrosive solution) containing 2% by volume of nitric acid to reveal ferrite crystal grain boundaries on the observation surface. The grain size number of ferrite in each visual field was determined in 10 arbitrary visual fields (each area of ​​the visual field was 40 mm 2 ) containing ferrite in the etched observation plane . Specifically, the grain size number of ferrite in each visual field was determined by comparison with the grain size standard diagram specified in 7.2 of JIS G 0551 (2013). The average of the grain size numbers in each field of view was defined as the grain size number of the hot forged steel product of the present embodiment. The grain size number was a value obtained by rounding off the second decimal place.
[0077]
　[Tensile Strength Test]
　A round bar tensile test piece having a diameter of 6.35 mm and a parallel portion length of 35 mm was selected from within a region of a depth of 3.0 mm to a depth of 20.0 mm from the surface of the hot forged steel material of each test number. It was made. The parallel part of the round bar tensile test piece was parallel to the axial direction of the hot forged steel material. Using a round bar tensile test piece, a tensile test was carried out in the atmosphere at room temperature (10 to 35° C.) according to JIS Z 2241 (2011) to obtain a tensile strength TS (MPa). The deformation rate in the tensile test was 0.2 mm/s. When the tensile strength TS was 600 MPa or more, it was evaluated as having high tensile strength.
[0078]
　[Charpy impact test]
　A V-notch test piece specified in JIS Z 2242 (2005) is produced from the area of ​​the depth of 3.0 mm to the depth of 20.0 mm from the surface of the hot forged steel material of each test number. did. The cross section of the V-notch test piece was a square of 10 mm×10 mm, and the length of the V-notch test piece in the longitudinal direction was 55 mm. The longitudinal direction of the V-notch test piece was parallel to the axial direction (longitudinal direction) of the hot forged steel material. The V notch was formed at the center position of the length of the V notch test piece (that is, the center position of 55 mm in length). The V notch angle was 45°, the notch depth was 2 mm, and the notch bottom radius was 0.25 mm. Using a V-notch test piece, a Charpy impact test according to JIS Z 2242 (2005) was performed to determine the absorbed energy at -30°C. Specifically, a Charpy impact test according to JIS Z 2242 (2005) was carried out in the atmosphere on three V-notch test pieces cooled to -30°C, and the average of the obtained absorbed energy Was defined as the absorbed energy (J) at −30° C. The absorbed energy (J) was an integer value rounded off to one decimal place.
[0079]
　[Test Results]
　Table 1 shows the test results.
[0080]
　With reference to Table 1, the chemical compositions of the hot forged steel products of Test Nos. 1 to 6 were appropriate. Further, F1 was 0.36 to less than 0.68. Furthermore, F2 was 0.52 or less, and the grain size number of ferrite in the steel material was 9.0 or more. Therefore, the tensile strength TS was as high as 600 MPa or more, and the absorbed energy at −30° C. was 100 J or more, showing excellent low temperature toughness.
[0081]
　On the other hand, in the hot forged steel material of test number 7, the C content was low. As a result, the tensile strength TS was less than 600 MPa and the strength was low.
[0082]
　In the hot forged steel material of test number 8, since the Mn content was high and the V content was low, bainite was generated in the microstructure. As a result, the absorbed energy at −30° C. was less than 100 J, and the low temperature toughness was low.
[0083]
　In the hot forged steel material of test number 9, the V content was low. Therefore, the tensile strength TS was less than 600 MPa and the strength was low.
[0084]
　In the hot forged steel material of test number 10, the V content was low and Mo was contained. Therefore, bainite was generated in the microstructure. As a result, the absorbed energy at −30° C. was less than 100 J, and the low temperature toughness was low.
[0085]
　In the hot forged steel material of test number 11, the N content was low. Therefore, the grain size number of the ferrite grains was less than 9.0. As a result, the absorbed energy at −30° C. was less than 100 J, and the low temperature toughness was low.
[0086]
　In the hot forged steel material of test number 12, the Al content was low. Furthermore, F2 did not satisfy the formula (2). Therefore, the grain size number was less than 9.0. As a result, the absorbed energy at −30° C. was less than 100 J, and the low temperature toughness was low.
[0087]
　In the hot forged steel material of test number 13, F1 was 0.68 or more. Therefore, the weldability was considered to be low. In addition, bainite was formed in the microstructure. As a result, the absorbed energy at −30° C. was less than 100 J, and the low temperature toughness was low.
[0088]
　In the hot forged steel material of Test No. 14, the heating temperature during hot forging was less than 1200°C. As a result, the absorbed energy at −30° C. was less than 100 J. Since the heating temperature during hot forging was low, it is considered that the V carbonitrides and the like remaining after heating during hot forging coarsened in the normalizing process step, and as a result, the low temperature toughness decreased.
[0089]
　In the hot forged steel material of test number 15, F2 did not satisfy the formula (2). As a result, the absorbed energy at −30° C. was less than 100 J. It is considered that the low temperature toughness was lowered because the amount of solid solution C in the steel material after the annealing treatment step was high.
[0090]
　The embodiments of the present invention have been described above. However, the embodiments described above are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments without departing from the spirit thereof.
The scope of the claims
[Claim 1]
　Hot forged steel,
　mass%
　C: 0.14 to 0.20%,
　Si: 0.20 to 1.00%,
　Mn: 1.00 to 1.90%,
　P: 0.030 % 　Or less,
　S: 0.030% or less,
　V: 0.16 to 0.30%,
　Al: 0.015 to 0.050%,
N: 0.0050 to 0.0250%,
　Cr: 0.10 to 0.30%,
　Cu:0-0.10%,
　Nb:0-0.10%, and the
　balance Fe and impurities, having a chemical composition satisfying the formulas (1) and (2),
　The hot forged steel material, wherein the grain size number of ferrite in the hot forged steel material is 9.0 or more, and
　the absorbed energy at −30° C. is 100 J or more in a Charpy impact test using a V-notch test piece
　.
　0.36≦C+(Si+Mn)/6+(Cr+V)/5+Cu/15<0.68 (1)
　51/12×C−V≦0.52 (2)
　Here, the formula (1) and the formula (2) The content (mass %) of the corresponding element is substituted for each element symbol therein.
[Claim 2]
　The hot forged steel material according to claim 1,
　wherein the chemical composition is 　selected from the group consisting of
　Cu: 0.01 to 0.10% and
Nb: 0.01 to 0.10%.
　Hot forged steel material containing more than one kind .
[Claim 3]
　The hot forged steel material according to claim 1 or 2, wherein the
　tensile strength TS is 600 MPa or more
　.