Patent Application
20250092496
The present disclosure relates to a steel material, and more particularly relates to a steel material that can be utilized as a starting material for a bolt.
Bolts are used in industrial machinery, automobiles, architectural structures represented by bridges and the like. In recent years, there is a demand for bolts with higher strength as the performance of industrial machinery and automobiles increases and the sizes of architectural structures and the like increase.
There is a possibility that hydrogen embrittlement susceptibility increases in bolts that have high strength. Hydrogen embrittlement is a factor that causes delayed fracture of bolts. Therefore, bolts that have high strength are required to have low hydrogen embrittlement susceptibility.
International Application Publication No. WO2020/162616 (Patent Literature 1) proposes a steel material that serves as a starting material for a bolt that has high strength and low hydrogen embrittlement susceptibility.
The steel material disclosed in Patent Literature 1 consists of, by mass %, C: 0.35 to 0.45%, Si: 0.02 to 0.10%, Mn: 0.20 to 0.84%, Cr: 0.60 to 1.15%, V: 0.30 to 0.50%, Mo: 0.25 to 0.99%, Al: 0.010 to 0.100%, N: 0.0010 to 0.0150%, P: 0.015% or less, S: 0.015% or less, and the balance: Fe and impurities, and satisfies the following Formula (1) and Formula (2).
0.48≤Mo/1.4+V<1.10 (1)
0.80<Mo/V<3.00 (2)
In this steel material, the content of Mo and the content of V in the chemical composition are adjusted so as to satisfy Formula (1) and Formula (2). By this means, it is easy for MC-type carbides to be dispersed in a bolt produced using this steel material as a starting material. As a result, the hydrogen embrittlement susceptibility of the bolt decreases. The foregoing information is described in Patent Literature 1.
A bolt produced using the steel material disclosed in Patent Literature 1 as a starting material has low susceptibility to hydrogen embrittlement. However, the hydrogen embrittlement susceptibility of a bolt produced using a steel material as a starting material may also be lowered by means other than the means described in Patent Literature 1.
An objective of the present disclosure is to provide a steel material that can lower the hydrogen embrittlement susceptibility of a bolt when used as a starting material for the bolt.
A steel material according to the present disclosure is as follows.
A steel material consisting of, by mass %,
When the steel material according to the present disclosure is used as a starting material for a bolt, the steel material can lower the hydrogen embrittlement susceptibility of the bolt.
The present inventors conducted studies regarding a steel material which can lower the hydrogen embrittlement susceptibility of a bolt when the steel material is used as the starting material for the bolt. As a result, the present inventors obtained the following findings.
First, the present inventors conducted studies from the viewpoint of the chemical composition regarding a steel material which can lower the hydrogen embrittlement susceptibility of a bolt when the steel material is used as the starting material for the bolt. As a result, the present inventors considered that if a steel material has a chemical composition consisting of, by mass %, C: 0.30 to 0.50%, Si: 0.01 to 0.30%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.01 to 0.80%, Al: 0.005 to 0.100%, N: 0.0010 to 0.0300%, Cu: 0 to 0.40%, Ni: 0 to 0.40%, B: 0 to 0.0100%, Zr: 0 to 0.300%, Hf: 0 to 0.100%, Ta: 0 to 0.100%, W: 0 to 0.200%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Bi: 0 to 0.020%, and Te: 0 to 0.010%, with the balance being Fe and impurities, there is a possibility that, in a case where the steel material is used as the starting material for a bolt, the steel material can lower the hydrogen embrittlement susceptibility of the bolt.
The present inventors also investigated means for lowering hydrogen embrittlement susceptibility from the viewpoint of the microstructure of a bolt having the chemical composition described above. As a result, the present inventors found that the hydrogen embrittlement susceptibility of the bolt can be lowered by causing a large number of fine MC-type carbides to be dispersed in the bolt.
Therefore, the present inventors considered that if the chemical composition described above also contains Mo and V, fine MC-type carbides will be formed in the bolt and the hydrogen embrittlement susceptibility of the bolt will be reduced. Therefore, the present inventors conducted further studies regarding the chemical composition of the steel material that took into consideration the formation of MC-type carbides in a process for producing a bolt. As a result, the present inventors discovered that if the contents of the steel material of the present embodiment satisfy the chemical composition described in the following Characteristic 1, when a bolt is produced which uses the steel material as a starting material, the hydrogen embrittlement susceptibility of the bolt will be lowered to a certain extent.
The chemical composition consists of, by mass %, C: 0.30 to 0.50%, Si: 0.01 to 0.30%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.01 to 0.80%, Mo: 0.70 to less than 1.50%, V: 0.01 to 0.50%, Al: 0.005 to 0.100%, N: 0.0010 to 0.0300%, Cu: 0 to 0.40%, Ni: 0 to 0.40%, B: 0 to 0.0100%, Zr: 0 to 0.300%, HE 0 to 0.100%, Ta: 0 to 0.100%, W: 0 to 0,200%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Bi: 0 to 0.020%, and Te: 0 to 0.010%, with the balance being Fe and impurities.
However, even when steel materials had a chemical composition satisfying Characteristic 1, in some cases the hydrogen embrittlement susceptibility of the produced bolts could not be lowered sufficiently. Therefore, the present inventors conducted studies regarding means for further lowering the hydrogen embrittlement susceptibility of the bolts. As a result, the present inventors obtained the following finding.
In the case of a steel material having a chemical composition that satisfies Characteristic 1, cementite particles form in the steel material. In addition, in a process for producing a bolt that uses the aforementioned steel material as a starting material, the steel material is subjected to spheroidizing annealing, quenching, and tempering. As a result, the aforementioned fine MC-type carbides containing Mo and/or V form in the bolt.
In order to form fine MC-type carbides in the bolt, it is preferable that cementite particles in the steel material sufficiently dissolve during quenching. The cementite particles in the steel material before quenching contain Mo and V. If a plurality of cementite particles in the steel material are a lamellar shape (that is, if a plurality of cementite particles are a part of pearlite (lamellar pearlite)), the cementite particles in the steel material will not dissolve sufficiently during quenching in the process for producing the bolt. In this case, even if tempering is performed, a sufficient amount of fine MC-type carbides will not form.
Therefore, in the steel material of the present embodiment, the microstructure is adjusted so as to satisfy the following Characteristic 2.
The area fraction of hard structure in the microstructure is 90% or more and the Vickers hardness of the steel material is 220 to 400 HV. Here, the term “hard structure” refers to a structure which is composed of ferrite that contains carbides, and which does not have a lamellar structure of carbides and ferrite. That is, the hard structure does not include pearlite. This kind of hard structure is also sometimes referred to as “bainite”.
In the microstructure of a steel material having a chemical composition satisfying Characteristic 1, if the area fraction of hard structure is 90% or more and the Vickers hardness of the steel material is 220 to 400 HV, cementite particles in the microstructure will be a substantially granular shape, and not a lamellar shape. In comparison to lamellar cementite particles in pearlite, granular cementite particles in pearlite tend to dissolve sufficiently during quenching. Therefore, the microstructure of the steel material is to be made a substantially hard structure.
Note that, in the microstructure of the steel material of the present embodiment, when a structure other than pro-eutectoid ferrite and pearlite is defined as a “hard structure”, if the area fraction of hard structure in the microstructure is 90% or more and the Vickers hardness of the steel material is 220 to 400 HV, the microstructure of the steel material is substantially a hard structure. Accordingly, for the steel material of the present embodiment, the area fraction of hard structure and the Vickers hardness are defined as described in Characteristic 2.
As described above, the present inventors considered that if a steel material satisfies Characteristic 1 and Characteristic 2, when a bolt is produced using the steel material as a starting material, the hydrogen embrittlement susceptibility of the bolt can be sufficiently lowered. However, even when steel materials satisfied the aforementioned Characteristic 1 and Characteristic 2, there were still some cases where the hydrogen embrittlement susceptibility of bolts produced using the steel materials as a starting material could not be sufficiently lowered. Therefore, the present inventors conducted further studies and, as a result, obtained the following finding.
Even when the microstructure of the steel material is a substantially hard structure and cementite particles in the steel material are formed in a granular shape, the amount of fine MC-type carbides formed in the bolt after tempering is influenced by the number density of cementite particles and the particle size distribution of cementite particles in the steel material. If the number density of cementite particles in the steel material is low, nucleation sites for MC-type carbides will not be sufficiently obtained. Further, even if the average particle diameter of cementite particles in the steel material is small, if the particle size distribution of the cementite particles is broad, the sizes of the cementite particles in the steel material will vary. In such a case, the amount of V and the amount of Mo contained in the cementite particles will be greater in large-sized cementite particles than in small-sized cementite particles. Consequently, even if the cementite particles in the steel material are sufficiently dissolved by quenching. V and Mo will be concentrated in micro-regions in which large-sized cementite particles dissolved in comparison to other micro-regions (regions where small-sized cementite particles dissolved). Therefore, even in a case where the average particle diameter of cementite particles is small, if the particle size distribution of the cementite particles is broad, regions where V and Mo are concentrated will be locally generated during quenching due to large-sized cementite particles. In such a case, it will be difficult for a sufficient amount of fine MC-type carbides to form during tempering. Consequently, it will not be possible to secure sufficient trap sites for hydrogen in a bolt produced using the steel material as a starting material. As a result, the hydrogen embrittlement susceptibility of the bolt will not be sufficiently low.
Therefore, in the steel material of the present embodiment, in order to suppress the occurrence of concentrated regions of V and Mo during quenching, the number density of cementite particles in the steel material is made as high as possible. In addition, among the plurality of cementite particles, the number ratio of fine cementite particles is increased as much as possible. Furthermore, in order to suppress variations in the size of cementite particles, the particle size distribution of the cementite particles is made a sharp distribution. Specifically, a steel material that satisfies Characteristic 1 and Characteristic 2 is also caused to satisfy following Characteristic 3.
A number density ND of cementite particles having an area of 0.0005 μm2 or more in the hard structure is 4.0 pieces/μm2 or more, a number ratio NR of the cementite particles having an area of 0.0005 to 0.0100 μm2 among a plurality of the cementite particles in the hard structure is 50.0% or more, and a sample standard deviation σ of areas of a plurality of the cementite particles having an area of 0.0005 μm2 or more in the hard structure is 0.070 μm2 or less.
In this case, cementite particles are sufficiently present in the steel material. In addition, the cementite particles are sufficiently fine, and the particle size distribution of the cementite particles is sufficiently sharp. That is, variations in the size of the cementite particles are sufficiently suppressed. Therefore, during quenching in a production process for producing a bolt using the steel material as a starting material, the cementite particles sufficiently dissolve and it is difficult for concentrated regions of V and Mo to occur. As a result, the hydrogen embrittlement susceptibility of the bolt is sufficiently lowered.
The mechanism described above is the presumed mechanism. Therefore, there is also a possibility that the hydrogen embrittlement susceptibility of a bolt produced using the steel material of the present embodiment as a starting material is sufficiently lowered by a mechanism that is different to the mechanism described above. However, as is also demonstrated by examples to be described later, when a steel material satisfying Characteristic 1 to Characteristic 3 is used as a starting material to produce a bolt, the hydrogen embrittlement susceptibility of the bolt becomes sufficiently low.
The steel material according to the present embodiment, which has been completed based on the findings described above, is as follows.
[1]
A steel material consisting of, by mass %,
[2]
The steel material according to [1], containing one or more elements selected from a group consisting of:
[3]
The steel material according to [1] or [2], containing one or more elements selected from a group consisting of:
[4]
The steel material according to any one of [1] to [3], containing one or more elements selected from a group consisting of:
[5]
The steel material according to any one of [1] to [4], containing one or more elements selected from a group consisting of:
[6]
The steel material according to any one of [1] to [5], containing one or more elements selected from a group consisting of:
Hereunder, the steel material of the present embodiment is described in detail. Note that, the symbol “%” in relation to an element means mass percent unless otherwise stated.
The steel material of the present embodiment has the following characteristics.
Hereunder, Characteristic 1 to Characteristic 3 are described.
The chemical composition of the steel material of the present embodiment contains the following elements.
Carbon (C) increases the hardenability of the steel material, and thereby increases the strength of a bolt produced using the steel material as a starting material. C also forms cementite particles and thereby sufficiently increases a number density ND of cementite particles that is described later. Consequently, the hydrogen embrittlement susceptibility of the bolt becomes sufficiently low. If the content of C is less than 0.30%, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
On the other hand, if the content of C is more than 0.50%, the hydrogen embrittlement susceptibility of the bolt will increase even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of C is to be 0.30 to 0.50%.
A preferable lower limit of the content of C is 0.32%, and more preferably is 0.35%.
A preferable upper limit of the content of C is 0.48%, and more preferably is 0.45%.
Silicon (Si) increases the hardenability of the steel material, and thereby increases the strength of the bolt. If the content of Si is less than 0.01%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
On the other hand, if the content of Si is more than 0.30%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of Si is to be 0.01 to 0.30%.
A preferable lower limit of the content of Si is 0.02%, and more preferably is 0.03%.
A preferable upper limit of the content of Si is 0.25%, more preferably is 0.20%, and further preferably is 0.15%.
Manganese (Mn) increases the hardenability of the steel material, and thereby increases the strength of the bolt. If the content of Mn is less than 0.10%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
On the other hand, if the content of Mn is more than 1.50%, the hydrogen embrittlement susceptibility of the bolt will increase even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of Mn is to be 0.10 to 1.50%.
A preferable lower limit of the content of Mn is 0.15%, and more preferably is 0.20%.
A preferable upper limit of the content of Mn is 1.30%, more preferably is 1.20%, and further preferably is 1.10%.
Phosphorus (P) is an impurity. That is, the lower limit of the content of P is more than 0%. If the content of P is more than 0.030%, P will segregate to grain boundaries even if the contents of other elements are within the range of the present embodiment. As a result, the hydrogen embrittlement susceptibility of the bolt will increase.
Therefore, the content of P is to be 0.030% or less.
The content of P is preferably as low as possible. However, extremely reducing the content of P will significantly increase the production cost. Therefore, when industrial manufacturing is taken into consideration, a preferable lower limit of the content of P is 0.001%, more preferably is 0.002%, and further preferably is 0.003%.
A preferable upper limit of the content of P is 0.025%, and more preferably is 0.020%.
Sulfur (S) is an impurity: That is, the lower limit of the content of S is more than 0%. If the content of S is more than 0.030%, S will segregate to grain boundaries even if the contents of other elements are within the range of the present embodiment. As a result, the hydrogen embrittlement susceptibility of the bolt will increase.
Therefore, the content of S is to be 0.030% or less.
The content of S is preferably as low as possible. However, extremely reducing the content of S will significantly increase the production cost. Therefore, when industrial manufacturing is taken into consideration, a preferable lower limit of the content of S is 0.001%, more preferably is 0.002%, and further preferably is 0.003%.
A preferable upper limit of the content of S is 0.025%, and more preferably is 0.020%.
Chromium (Cr) increases the hardenability of the steel material, and thereby increases the strength of the bolt. In addition, Cr increases the temper softening resistance of the steel material, and thereby increases the strength of the bolt. If the content of Cr is less than 0.01%, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
On the other hand, if the content of Cr is 0.80% or less, even when the contents of other elements are within the range of the present embodiment, penetration of hydrogen into the steel material will be sufficiently suppressed. As #result, the hydrogen embrittlement susceptibility of the bolt will be sufficiently suppressed.
Therefore, the content of Cr is to be 0.01 to 0.80%.
A preferable lower limit of the content of Cr is 0.03%, and more preferably is 0.05%.
A preferable upper limit of the content of Cr is 0.70%, more preferably is 0.60%, and further preferably is 0.50%.
Mo: 0.70 to Less than 1.50%
Molybdenum (Mo) increases the temper softening resistance of the steel material, and thereby increases the strength of the bolt. In addition, Mo concentrates in MC-type carbides and thereby enhances a hydrogen-trapping function of the MC-type carbides. As a result, Mo lowers the hydrogen embrittlement susceptibility of a bolt that has high strength. If the content of Mo is 0.70% or more, even when the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effects will be obtained to a certain extent.
On the other hand, if the content of Mo is less than 1.50%, even when the contents of other elements are within the range of the present embodiment, sufficient cold forgeability of the steel material will be obtained.
Therefore, the content of Mo is to be 0.70 to less than 1.50%.
A preferable lower limit of the content of Mo is 0.75%, and more preferably is 0.80%.
A preferable upper limit of the content of Mo is 1.40%, and more preferably is 1.30%.
Vanadium (V) forms MC-type carbides together with Mo, and thereby lowers the hydrogen embrittlement susceptibility of the bolt. If the content of V is less than 0.01%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
On the other hand, if the content of V is more than 0.50%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of V is to be 0.01 to 0.50%.
A preferable lower limit of the content of V is 0.03%, and more preferably is 0.05%.
A preferable upper limit of the content of V is 0.45%, more preferably is 0.40%, and further preferably is 0.35%.
Aluminum (Al) functions as a deoxidizer during steel refining. In addition, Al combines with N to form Al nitrides. The Al nitrides suppress coarsening of grains by a pinning effect in a quenching process during production of the bolt. As a result, the hydrogen embrittlement susceptibility of the bolt is lowered. If the content of Al is less than 0.005%, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
On the other hand, if the content of Al is more than 0.100%, coarse Al nitrides will form even if the contents of other elements are within the range of the present embodiment. The coarse Al nitrides will act as starting points for cracks. Consequently, the cold forgeability of the steel material will decrease.
Therefore, the content of Al is to be 0.005 to 0.100%.
A preferable lower limit of the content of Al is 0.006%, more preferably is 0.007%, and further preferably is 0.008%.
A preferable upper limit of the content of Al is 0.090%, more preferably is 0.080%, and further preferably is 0.070%.
In the chemical composition of the steel material of the present embodiment, the content of Al means the total Al content.
Nitrogen (N) combines with Al to form Al nitrides. The Al nitrides suppress coarsening of grains by a pinning effect in a quenching process during production of the bolt. As a result, the hydrogen embrittlement susceptibility of the bolt is lowered. If the content of N is less than 0.0010%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
On the other hand, if the content of N is more than 0.0300%, coarse nitrides will form even if the contents of other elements are within the range of the present embodiment. The course nitrides will act as starting points for cracks. Consequently, the cold forgeability of the steel material will decrease.
Therefore, the content of N is to be 0.0010 to 0.0300%.
A preferable lower limit of the content of N is 0.0020%, more preferably is 0.0025%, and further preferably is 0.0030%.
A preferable upper limit of the content of N is 0.0290%, more preferably is 0.0280%, further preferably is 0.0270%, further preferably is 0.0250%, further preferably is 0.0200%, further preferably is 0.0150%, and further preferably is 0.0100%.
The balance of the chemical composition of the steel material of the present embodiment is Fe and impurities. Here, the term “impurities” with respect to the chemical composition means substances which are mixed in from ore and scrap as the raw material or from the production environment or the like when industrially producing the steel material, and which are permitted within a range that does not adversely affect the steel material of the present embodiment.
Impurities other than the impurities described above are, for example, as follows.
The content of each of these impurity elements may be 0%.
The chemical composition of the steel material of the present embodiment may further contain one or more elements selected from the group consisting of Cu. Ni, B, Zr, Hf, Ta, W, Ti, Nb, Ca, Mg, Bi, and Te in lieu of a part of Fe. Each of these elements is an optional element, and does not have to be contained.
Hereunder, these optional elements are described.
The chemical composition of the steel material of the present embodiment may further contain one or more elements selected from the group consisting of Cu, Ni, B, Zr, HI, Ta and W in lieu of a part of Fe. Each of these elements is an optional element, and does not have to be contained. When contained, Cu, Ni, B, Zr, Hf, Ta and W each increases the hardenability of the steel material, and thereby increases the strength of the produced bolt.
Copper (Cu) is an optional element, and does not have to be contained. That is, the content of Cu may be 0%.
When Cu is contained, that is, in a case where the content of Cu is more than 0%, Cu increases the hardenability of the steel material, and thereby increases the strength of the bolt. If even a small amount of Cu is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Cu is more than 0.40%, even if the contents of other elements are within the range of the present embodiment, the steel material will become brittle. As a result, the hot workability of the steel material will decrease.
Therefore, the content of Cu is to be 0 to 0.40%, and when contained, the content of Cu is to be 0.40% or less (more than 0 to 0.40%).
A preferable lower limit of the content of Cu is 0.01%, more preferably is 0.03%, and further preferably is 0.05%.
A preferable upper limit of the content of Cu is 0.35%, more preferably is 0.30%, and further preferably is 0.25%.
Nickel (Ni) is an optional element, and does not have to be contained. That is, the content of Ni may be 0%.
When Ni is contained, that is, in a case where the content of Ni is more than 0%, Ni increases the hardenability of the steel material, and thereby increases the strength of the bolt. If even a small amount of Ni is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Ni is more than 0.40%, even if the contents of other elements are within the range of the present embodiment, the hardenability will be too high. As a result, the hot workability of the steel material will decrease.
Therefore, the content of Ni is to be 0 to 0.40%, and when contained, the content of Ni is to be 0.40% or less (more than 0 to 0.40%).
A preferable lower limit of the content of Ni is 0.01%, more preferably is 0.02%, and further preferably is 0.03%
A preferable upper limit of the content of Ni is 0.35%, more preferably is 0.30%, and further preferably is 0.25%.
Boron (B) is an optional element, and does not have to be contained. That is, the content of B may be 0%.
When B is contained, that is, in a case where the content of B is more than 0%, B increases the hardenability of the steel material, and thereby increases the strength of the bolt. In addition, B suppresses grain-boundary segregation of P, thereby lowering the hydrogen embrittlement susceptibility of the bolt. If even a small amount of B is contained, the aforementioned advantageous effects will be obtained to a certain extent.
However, if the content of B is more than 0.0100%, coarse B nitrides will form even if the contents of other elements are within the range of the present embodiment. The coarse B nitrides will act as starting points for cracks. Consequently, the cold forgeability of the steel material will decrease.
Therefore, the content of B is to be 0 to 0.0100%, and when contained, the content of B is to be 0.0100% or less (more than 0 to 0.0100%).
A preferable lower limit of the content of B is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0008%.
A preferable upper limit of the content of B is 0.0090%, more preferably is 0.0080%, further preferably is 0.0070%, further preferably is 0.0060%, and further preferably is 0.0050%.
Zirconium (Zr) is an optional element, and does not have to be contained. That is, the content of Zr may be 0%.
When Zr is contained, that is, in a case where the content of Zr is more than 0%, Zr increases the hardenability of the steel material, and thereby increases the strength of the bolt. If even a small amount of Zr is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Zr is more than 0.300%, coarse Zr nitrides will form even if the contents of other elements are within the range of the present embodiment. The coarse Zr nitrides will act as starting points for cracks. Consequently, the cold forgeability of the steel material will decrease.
Therefore, the content of Zr is to be 0 to 0.300%, and when contained, the content of Zr is to be 0.300% or less (more than 0 to 0.300%).
A preferable lower limit of the content of Zr is 0.001%, more preferably is 0.010%, and further preferably is 0.020%.
A preferable upper limit of the content of Zr is 0.280%, more preferably is 0.250%, further preferably is 0.200%, further preferably is 0.150%, and further preferably is 0.100%.
Hafnium (Hf) is an optional element, and does not have to be contained. That is, the content of Hf may be 0%.
When Hf is contained, that is, in a case where the content of Hf is more than 0%, Hf increases the hardenability of the steel material, and thereby increases the strength of the bolt. If even a small amount of Hf is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Hf is more than 0.100%, coarse Hf nitrides will form even if the contents of other elements are within the range of the present embodiment. The coarse Hf nitrides will act as starting points for cracks. Consequently, the cold forgeability of the steel material will decrease.
Therefore, the content of Hf is to be 0 to 0.100%, and when contained, the content of Hf is to be 0.100% or less (more than 0 to 0.100%).
A preferable lower limit of the content of Hf is 0.001%, more preferably is 0.005%, and further preferably is 0.010%.
A preferable upper limit of the content of Hf is 0.080%, more preferably is 0.070%, further preferably is 0.060%, and further preferably is 0.050%.
Tantalum (Ta) is an optional element, and does not have to be contained. That is, the content of Ta may be 0%.
When Ta is contained, that is, in a case where the content of Ta is more than 0%, Ta increases the hardenability of the steel material, and thereby increases the strength of the bolt. If even a small amount of Ta is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Ta is more than 0.100%, coarse Ta nitrides will form even if the contents of other elements are within the range of the present embodiment. The coarse Ta nitrides will act as starting points for cracks. Consequently, the cold forgeability of the steel material will decrease.
Therefore, the content of Ta is to be 0 to 0.100%, and when contained, the content of Ta is to be 0.100% or less (more than 0 to 0.100%).
A preferable lower limit of the content of Ta is 0.001%, more preferably is 0.005%, and further preferably is 0.010%.
A preferable upper limit of the content of Ta is 0.080%, more preferably is 0.070%, further preferably is 0.060%, and further preferably is 0.050%.
Tungsten (W) is an optional element, and does not have to be contained. That is, the content of W may be 0%.
When W is contained, that is, in a case where the content of W is more than 0%, W increases the hardenability of the steel material, and thereby increases the strength of the bolt. If even a small amount of W is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of W is more than 0.200%, even if the contents of other elements are within the range of the present embodiment, the hardenability will be too high. Consequently, the cold forgeability of the steel material will decrease.
Therefore, the content of W is to be 0 to 0.200%, and when contained, the content of W is to be 0.200% or less (more than 0 to 0.200%).
A preferable lower limit of the content of W is 0.001%, more preferably is 0.002%, and further preferably is 0.010%.
A preferable upper limit of the content of W is 0.150%, more preferably is 0.120%, and further preferably is 0.100%.
The chemical composition of the steel material of the present embodiment may further contain one or more elements selected from the group consisting of Ti and Nb in lien of a part of Fe. Each of these elements is an optional element, and does not have to be contained. When contained, Ti and Nb form precipitates and refine the grains. As a result, the hydrogen embrittlement susceptibility of the bolt becomes lower.
Titanium (Ti) is an optional element, and does not have to be contained. That is, the content of Ti may be 0%.
When Ti is contained, that is, in a case where the content of Ti is more than 0%, Ti forms fine precipitates such as Ti carbides, and thereby refines the grains. As a result, the hydrogen embrittlement susceptibility of the bolt is lowered. If even a small amount of Ti is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Ti is more than 0.100%, coarse Ti nitrides will form even if the contents of other elements are within the range of the present embodiment. The coarse Ti nitrides will act as starting points for cracks. Consequently, the cold forgeability of the steel material will decrease.
Therefore, the content of Ti is to be 0 to 0.100%, and when contained, the content of Ti is to be 0.100% or less (more than 0 to 0.100%).
A preferable lower limit of the content of Ti is 0.001%, more preferably is 0.003%, and further preferably is 0.005%
A preferable upper limit of the content of Ti is 0.090%, more preferably is 0.080%, and further preferably is 0.075%.
Niobium (Nb) is an optional element, and does not have to be contained. That is, the content of Nb may be 0%.
When Nb is contained, that is, in a case where the content of Nb is more than 0%, Nb forms fine precipitates such as Nb carbides, and thereby refines the grains. As a result, the hydrogen embrittlement susceptibility of the bolt is lowered. If even a small amount of Nb is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Nb is more than 0.100%, coarse Nb carbides and the like will form even if the contents of other elements are within the range of the present embodiment. The coarse Nb carbides and the like will act as starting points for cracks. Consequently, the cold forgeability of the steel material will decrease.
Therefore, the content of Nb is to be 0 to 0.100%, and when contained, the content of Nb is to be 0.100% or less (more than 0 to 0.100%).
A preferable lower limit of the content of Nb is 0.001%, more preferably is 0.003%, further preferably is 0.010%, and further preferably is 0.020%.
A preferable upper limit of the content of Nb is 0.090%, more preferably is 0.080%, and further preferably is 0.070%.
The chemical composition of the steel material of the present embodiment may further contain one or more elements selected from the group consisting of Ca and Mg in lieu of a part of Fe. Each of these elements is an optional element, and does not have to be contained. When contained, Ca and Mg refine MnS in the steel material, and thereby lower the hydrogen embrittlement susceptibility of the bolt.
Calcium (Ca) is an optional element, and does not have to be contained. That is, the content of Ca may be 0%.
When Ca is contained, that is, in a case where the content of Ca is more than 0%, Ca refines MnS in the steel material, thereby lowering the hydrogen embrittlement susceptibility of the bolt. If even a small amount of Ca is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Ca is more than 0.0050%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of Ca is to be 0 to 0.0050%, and when contained, the content of Ca is to be 0.0050% or less (more than 0 to 0.0050%).
A preferable lower limit of the content of Ca is 0.0001%, more preferably is 0.0003%, and further preferably is 0.0005%.
A preferable upper limit of the content of Ca is 0.0040%, and more preferably is 0.0030%.
Magnesium (Mg) is an optional element, and does not have to be contained. That is, the content of Mg may be 0%.
When Mg is contained, that is, in a case where the content of Mg is more than 0%, Mg refines MnS in the steel material, thereby lowering the hydrogen embrittlement susceptibility of the bolt. If even a small amount of Mg is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Mg is more than 0.0050%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of Mg is to be 0 to 0.0050%, and when contained, the content of Mg is to be 0.0050% or less (more than 0 to 0.0050%).
A preferable lower limit of the content of Mg is 0.0001%, more preferably is 0.0002%, and further preferably is 0.0005%.
A preferable upper limit of the content of Mg is 0.0040%, and more preferably is 0.0030%.
The chemical composition of the steel material of the present embodiment may further contain one or more elements selected from the group consisting of Bi and Te in lieu of a part of Fe. Each of these elements is an optional element, and does not have to be contained. When contained, Bi and Te increase the machinability of the steel material.
Bismuth (Bi) is an optional element, and does not have to be contained. That is, the content of Bi may be 0%.
When Bi is contained, that is, in a case where the content of Bi is more than 0%, Bi increases the machinability of the steel material. If even a small amount of Bi is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Bi is more than 0.020%, the hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of Bi is to be 0 to 0.020%, and when contained, the content of Bi is to be 0.020% or less (more than 0 to 0.020%).
A preferable lower limit of the content of Bi is 0.001%, more preferably is 0.005%, and further preferably is 0.010%.
A preferable upper limit of the content of Bi is 0.018%, and more preferably is 0.015%.
Tellurium (Te) is an optional element, and does not have to be contained. That is, the content of Te may be 0%.
When Te is contained, that is, in a case where the content of Te is more than 0%, Te increases the machinability of the steel material. If even a small amount of Te is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Te is more than 0.010%, the hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of Te is to be 0 to 0.010%, and when contained, the content of Te is to be 0.010% or less (more than 0 to 0.010%).
A preferable lower limit of the content of Te is 0.001%, more preferably is 0.002%, and further preferably is 0.003%, A preferable upper limit of the content of Te is 0.009%, and more preferably is 0.008%
The microstructure of the steel material of the present embodiment has the following characteristic.
(Characteristic 2) The area fraction of hard structure is 90% or more, and the Vickers hardness of the steel material is 220 to 400 HV.
Here, the hard structure is a structure other than pro-eutectoid ferrite and pearlite, and is a structure composed of ferrite that contains carbides. This kind of hard structure is sometimes also referred to as “bainite”.
The microstructure of the steel material of the present embodiment is substantially a hard structure. The hard structure and those structures (pro-cutectoid ferrite and pearlite) other than the hard structure can easily be distinguished by microstructure observation that is described later.
In a case where the content of each element in the chemical composition is within the range of the present embodiment, if the area fraction of hard structure in the microstructure is 90% or more and the Vickers hardness of the microstructure of the steel material is 220 to 400 HV, the microstructure is substantially composed of a hard structure.
During a process for producing a bolt, the steel material of the present embodiment is subjected to spheroidizing. As mentioned above, the microstructure of the steel material of the present embodiment is substantially a hard structure. Therefore, in the steel material after the spheroidizing, the spheroidization ratio of cementite particles increases. In this case, the cold forgeability of the steel material can be increased.
A preferable lower limit of the area fraction of hard structure in the microstructure of the steel material is 92%, more preferably is 94%, and further preferably is 96%.
A preferable lower limit of the Vickers hardness of the microstructure of the steel material is 230 HV, more preferably is 240 HV, and further preferably is 250 HV. A preferable upper limit of the Vickers hardness of the microstructure of the steel material is 395 HV, more preferably is 390 HV, further preferably is 380 HV, and further preferably is 370 HV.
The microstructure of the present embodiment can be measured by the following method. A cross section perpendicular to the longitudinal direction of the steel material is defined as a transverse section. In the transverse section, the center position of a radius connecting the central axis and the surface of the steel material is defined as a “D/4” position. Here, “D” represents the diameter of the transverse section.
A sample that includes the D/4 position of the steel material is taken from the steel material. Among the surfaces of the sample, a surface that corresponds to the aforementioned transverse section is defined as an observation surface. The observation surface of the sample is mirror-polished. The mirror-polished observation surface is then subjected to etching for 10 seconds at normal temperature using a 3% nitric acid-alcohol solution (nital etching reagent). On the etched observation surface, the D/4 position of the steel material is set as an observation visual field P1. On the observation surface, observation visual fields P2 to P5 are specified at a pitch of 500 μm from the observation visual field P1 in a direction (corresponding to the circumferential direction of the steel material) that is perpendicular to the radial direction of the steel material. The area of each of the observation visual fields P1 to P5 is to be set to 90 μm×120 μm. Each of the observation visual fields P1 to P5 is observed with an optical microscope at a magnification of 1000×.
In the observation visual fields, a hard structure can be easily distinguished from pro-eutectoid ferrite and pearlite by contrast. Pro-eutectoid ferrite is observed as a white region. When observed at a magnification of 1000×, pearlite is observed as a phase having a lamellar structure. A hard structure is observed as a region that has a lower brightness than ferrite. Therefore, pro-cutectoid ferrite and pearlite are identified based on the contrast. Specifically, in the observation at a magnification of 1000×, a white-colored structure is identified as pro-eutectoid ferrite. In the observation al a magnification of 1000×, a structure in which lamellae can be confirmed is identified as pearlite. In each observation visual field, any structure other than pro-eutectoid ferrite and pearlite is regarded as a hard structure.
The total area of pro-eutectoid ferrite and pearlite in the five observation visual fields P1 to P5 is determined based on the identified pro-eutectoid ferrite and identified pearlite. The total area of the hard structure in the five observation visual Gelds P1 to P5 is then determined by subtracting the total area of pro-eutectoid ferrite and pearlite in the five observation visual fields P1 to P5 from the total area of the five observation visual fields P1 to P5. The area fraction of hard structure (%) is determined based on the total area of the hard structure in the five observation visual fields and the total area of the five observation visual fields.
The Vickers hardness of the microstructure of the steel material is determined by the following method. A sample is taken in which a cross section perpendicular to the longitudinal direction of the steel material (a transverse section) is adopted as an observation surface. The observation surface of the sample is taken as the entire cross section perpendicular to the longitudinal direction of the steel material. That is, the diameter of the observation surface of the sample is D. The observation surface is mirror-polished. On the observation surface after the mirror polishing, an arbitrary measurement position at a depth of D/4 in the radial direction from the surface of the steel material is defined as a measurement position P1. From the measurement position P1, measurement positions P2 to P12 are determined at intervals of 30° around the center of the observation surface of the sample (that is, a location corresponding to the center of the transverse section of the steel material). The Vickers hardness is measured at these 12 measurement positions P1 to P12 which are D/4 depth positions.
A Vickers hardness test in conformity with JIS Z 2244:2020 is carried out at each of the measurement positions P1 to P12. The test force is to be set to 0.98 N. The arithmetic mean value of the hardness values obtained at the 12 measurement positions is defined as the Vickers hardness (HV) of the microstructure of the steel material.
In the steel material of the present embodiment, a number density ND of cementite particles having an area of 0.0005 μm2 or more in the hard structure is 4.0 pieces/μm2 or more, a number ratio NR of the cementite particles having an area of 0.0005 to 0.0100 μm2 among a plurality of the cementite particles having an area of 0.0005 μm2 or more in the hard structure is 50.0% or more, and a sample standard deviation σ of areas of a plurality of the cementite particles having an area of 0.0005 μm2 or more in the hard structure is 0.070 μm or less.
In the steel material of the present embodiment, the content of V and the content of Mo are high. Therefore, in a case where a bolt is produced using this steel material as a starting material, a large number of fine MC-type carbides containing Mo are formed by tempering after quenching in the production process. These fine MC-type carbides tend to trap hydrogen. Therefore, in the produced bolt, the threshold amount of hydrogen that will lead to a fracture increases. As a result, the hydrogen embrittlement susceptibility of the bolt is lowered.
In order to exert the aforementioned advantageous effect, in the steel material that will serve as the starting material for a bolt: (1) the number density of cementite particles in the hard structure is increased, (2) the ratio of fine cementite particles among the plurality of cementite particles in the hard structure is increased, and (3) variations in the size of the cementite particles are suppressed to make the particle size distribution of the cementite particles a sharp distribution. These points are described hereunder.
In the process for producing a bolt that uses the steel material as a starting material, the higher the number density of cementite particles in the hard structure of the steel material is, the higher the number density of MC-type carbides in the bolt will be, and the more uniformly MC-type carbides will be dispersed in the bolt. As a result, the hydrogen embrittlement susceptibility of the bolt will be sufficiently lowered. Specifically, in a steel material that satisfies aforementioned Characteristic 1 and Characteristic 2, if the number density ND of cementite particles in the hard structure is 4.0 pieces/μm2 or more, MC-type carbides will be finely dispersed to a sufficient extent in the bolt, and the hydrogen embrittlement susceptibility of the bolt will be sufficiently low.
A preferable lower limit of the number density ND of cementite particles in the hard structure is 4.2 pieces/μm2, more preferably is 4.5 pieces/μm2, and further preferably is 4.7 pieces/μm2.
The upper limit of the number density ND of cementite particles in the hard structure is not particularly limited. A preferable upper limit of the number density ND of cementite particles in the hard structure is 25.0 pieces/μm2, more preferably is 20.0 pieces/μm2, further preferably is 15.0 pieces/μm2, and further preferably is 10.0 pieces/μm2.
Even if the microstructure of a steel material that satisfies Characteristic 1 is made substantially bainite, and the cementite particles are made a granular shape, the amount of fine MC-type carbides formed in the bolt after tempering is influenced by the cementite particle size distribution. Specifically, even when the average particle diameter of a plurality of cementite particles in the steel material is small, if the particle size distribution of the cementite particles is broad, the sizes of the cementite particles in the steel material will vary. In such a case, the hydrogen embrittlement susceptibility of a bolt produced using the steel material as a starting material cannot be lowered sufficiently.
If the number ratio NR of cementite particles having an area of 0.0005 to 0.0100 μm2 among a plurality of cementite particles having an area of 0.0005 μm or more in the hard structure is 50.0% or more, and the sample standard deviation σ of the areas of a plurality of cementite particles having an area of 0.0005 μm2 or more in the hard structure is 0.070 μm2 or less, the cementite particles in the steel material will be sufficiently fine and the particle size distribution of the cementite particles will be sufficiently sharp. In this case, during quenching in a production process for producing a bolt that uses the steel material as a starting material, cementite particles will sufficiently dissolve and it will also be difficult for segregation regions of V and Mo to occur. As a result, the hydrogen embrittlement susceptibility of the bolt will be sufficiently low.
A preferable lower limit of the number ratio NR is 52.5%, more preferably is 55.0%, and further preferably is 57.5%.
The upper limit of the number ratio NR is not particularly limited. The upper limit of the number ratio NR is, for example, 100.0%, for example is 90.0%, for example is 85.0%, or for example is 80.0%.
A preferable upper limit of the sample standard deviation σ is 0.065 μm2, more preferably is 0.063 μm2, and further preferably is 0.061 μm2.
The number density ND of cementite particles having an area of 0.0005 μm2 or more in the hard structure, the number ratio NR of the cementite particles having an area of 0.0005 to 0.0100 μm2 among a plurality of the cementite particles having an area of 0.0005 μm2 or more in the hard structure, and the sample standard deviation σ of areas of a plurality of the cementite particles having an area of 0.0005 μm2 or more in the hard structure can be measured by the following method.
A sample is taken that includes a D/4 position in a cross section perpendicular to the longitudinal direction of the steel material (a transverse section). Among the surfaces of the sample, a surface corresponding to the aforementioned transverse section is defined as an observation surface. After polishing the observation surface of the sample, the observation surface is subjected to etching for 20 seconds at normal temperature using a 6% picral etching reagent (an etching reagent composed of 6 g of picric acid dissolved in 94 mL of ethanol). Using a field emission scanning electron microscope (FE-SEM), the D/4 position on the etched observation surface is defined as an observation visual field P11. On the observation surface, observation visual fields P12 to P15 are specified at a pitch of 100 μm in a direction (corresponding to the circumferential direction of the steel material) that is perpendicular to the radial direction of the steel material from the observation visual field P11. The area of each of the observation visual fields P11 to P15 is to be made 9 μm×12 μm. Each of the observation visual fields P11 to P15 is observed at an observation magnification of 10000×, and particles (precipitates and inclusions) within each observation visual field are photographed to generate photographic images. Each observation visual field is to be a region within the hard structure.
In addition, in each of the aforementioned observation visual fields, oxides and sulfides are identified by a well-known method using an EDS (energy dispersive X-ray spectroscope) attached to an SEM. Among the particles in the respective observation visual fields, particles excluding oxides and sulfides are identified as cementite particles. The areas of the identified cementite particles are determined. For example, well-known image processing can be used to determine the areas of the identified cementite particles.
The number density ND of cementite particles (pieces/μm2) in the hard structure is then determined based on the total number of cementite particles having an area of 0.0005 μm2 or more identified in the five observation visual fields, and the total area of the five observation visual fields.
Further, among the identified cementite particles, the ratio (%) of the total number of cementite particles having an area of 0.0005 to 0.0100 μm2 to the total number of identified cementite particles is defined as a number ratio NR (%). In addition, the sample standard deviation σ is determined based on the area of each identified cementite particle. Here, the sample standard deviation σ is calculated by the following formula.
In the formula, n in the formula represents the total number of cementite particles having an area of 0.0005 μm2 or more identified in the five observation visual fields. Further, xi represents the area of each identified cementite particle having an area of 0.0005 μm2 or more. Further, μ represents the arithmetic mean value of the areas of identified cementite particles having an area of 0.0005 μm2 or more. Note that, the above formula for the sample standard deviation σ is a well-known formula.
As described above, the steel material of the present embodiment is composed as follows.
When a steel material composed as described above is used as a starting material for a bolt, the steel material can lower the hydrogen embrittlement susceptibility of the bolt.
In the present description, hydrogen embrittlement susceptibility can be evaluated, for example, by the following method. The steel material of the present embodiment is subjected to wire drawing. After being subjected to wire drawing, the steel material is then subjected to well-known spheroidizing annealing. In the spheroidizing annealing, the steel material is heated to 760° C. The steel material is then held at 760° C. for 5.0 hours. After the holding time elapses, the steel material is slow cooled to 650° C. at a rate of 7° C./hr. The steel material is then air-cooled from 650° C. to normal temperature.
The steel material that underwent the spheroidizing annealing is then subjected to cold forging (bolt forming) to produce a bolt-shaped intermediate product. The produced intermediate product is subjected to quenching and tempering under well-known conditions. The quenching temperature is to be set to 920° C., and the holding time at the quenching temperature is to be set to one hour. After the holding time elapses, the intermediate product is water-cooled, After quenching, the intermediate product is subjected to tempering. The tempering temperature is to be set to 620° C., and the holding time at the tempering temperature is to be set to 2.0 hours. A bolt is produced by the above process.
A round bar specimen with an annular notch having a parallel portion with a diameter of 7 mm and a length of 70 mm is taken from the produced bolt at a position that is at a depth of 1 mm or more on the inner side from the surface of the bolt. The annular notch is to be formed at a central position in the longitudinal direction of the specimen. With regard to the notch shape, the notch depth is to be 1.4 mm, the notch angle is to be 60°, and the radius of curvature of the notch root is to be 0.175 mm.
The round bar specimen with an annular notch is charged with hydrogen by a cathodic hydrogen charging method. Specifically, a cathodic hydrogen charging solution at normal temperature is prepared. An aqueous solution obtained by adding 3 g of ammonium thiocyanate to 1 L of a 3% by mass sodium chloride aqueous solution is adopted as the cathodic hydrogen charging solution.
In a state in which the round bar specimen with an annular notch is immersed in the cathodic hydrogen charging solution, a constant current with a cathodic current density controlled to 0.05 mA/cm2 is generated for 18 hours to introduce hydrogen into the round bar specimen with an annular notch. After performing the cathodic hydrogen charging method, the round bar specimen with an annular notch is left to stand at normal temperature for 96 hours. Thereafter, a galvanizing coating is formed under the same conditions on the surface of the round bar specimen with an annular notch charged with hydrogen, to thereby prevent the hydrogen in the round bar specimen with an annular notch from leaking to outside.
The round bar specimen with an annular notch on which the galvanizing coating was formed is subjected to a constant load test that applies a constant load at normal temperature and atmospheric pressure so that a load equivalent to the cross-sectional area×tensile strength×0.90 is applied to the round bar specimen with an annular notch. Here, the term “cross-sectional area” means the cross-sectional area of a cross section (transverse section) perpendicular to the longitudinal direction of the specimen at the notch root. Further, the term “tensile strength” means a tensile strength (MPa) determined by performing a tensile test, which is described below, using a smooth round bar tensile test specimen.
The tensile test using a smooth round bar tensile test specimen is performed by the following method. A smooth round bar tensile test specimen is taken from the bolt at a position that is at a depth of 1 mm or more on the inner side from the surface of the bolt. The diameter of a parallel portion of the smooth round bar tensile test specimen is to be 6 mm, and the length of the parallel portion is to be 70 mm. The central axis of the parallel portion of the smooth round bar tensile test specimen is to be coaxial with the central axis of the bolt. A tensile test in conformity with JIS Z 2241:2022 is performed in air at normal temperature (20±15° C.) using the smooth round bar tensile test specimen to determine the tensile strength (MPa).
In the aforementioned constant load test, if a round bar specimen with an annular notch endures the load without rupturing for 100 hours or more, it is determined that the hydrogen embrittlement susceptibility is sufficiently low. With respect to the steel material of the present embodiment, in the aforementioned constant load test, a round bar specimen with an annular notch of the steel material endures the load without rupturing for 100 hours or more. Therefore, the hydrogen embrittlement susceptibility of a bolt produced using the steel material of the present embodiment as a starting material is sufficiently low.
The steel material of the present embodiment is a steel material in which a cross section perpendicular to the axial direction (longitudinal direction) is a round shape. Specifically, the steel material of the present embodiment is a steel bar or a wire rod in which a cross section perpendicular to the axial direction is a round shape. The steel material may be a material that is wound in a coiled shape, or may be a material that is cut to a predetermined length. Although not particularly limited, the cross-sectional diameter of the steel material of the present embodiment is, for example, 5 to 30 mm.
As described above, the steel material of the present embodiment is suitable for use as a starting material for a bolt. In particular, the steel material of the present embodiment is suitable for use as a starting material for a bolt which has a tensile strength of 1300 MPa or more and which is required to have low hydrogen embrittlement susceptibility. However, the steel material of the present embodiment may also be used for other uses in addition to the aforementioned bolt.
An example of a method for producing the steel material of the present embodiment will now be described. The method for producing the steel material described hereinafter is one example for producing the steel material of the present embodiment. Accordingly, a steel material composed as described above may also be produced by a production method other than the production method described hereinafter. However, the production method described hereinafter is a preferable example of the method for producing the steel material of the present embodiment.
One example of the method for producing the steel material of the present embodiment includes the following processes.
The main production conditions in the aforementioned process 1 and process 2 are as follows.
In the above production process, the grains in the steel material are maintained as coarse grains as much as possible until the finish rolling is completed. By this means, during cooling after finish rolling is completed, the bainite nose in the continuous cooling transformation diagram (CCT diagram) shifts to the longer time side. As a result, the bainite transformation start temperature decreases. When the bainite transformation start temperature decreases, the formation temperature with respect to cementite particles that precipitate accompanying bainite transformation is also lowered. When the formation temperature of the cementite particles is lowered, the cementite particles remain fine. Therefore, a steel material that satisfies Characteristic 1 to Characteristic 3 is produced. Each process is described hereunder.
[(Process 1) Direct Hot Working after Casting Process]
In the direct hot working after casting process, a cast material is produced, and hot working is started at a time when the surface temperature of the cast material is within the range of 1100 to 900° C. during cooling (during solidification), to thereby produce a billet. By this means, a billet in which the grains are coarse grains can be produced.
The direct hot working after casting process includes the following processes.
Each process is described hereunder.
In the casting process, a cast material having a chemical composition satisfying Characteristic 1 is produced. Specifically, a molten steel in which the content of each element in the chemical composition satisfies Characteristic 1 is prepared. The prepared molten steel is used to produce a starting material by a well-known casting process. For example, an ingot is produced by an ingot-making process. Alternatively, a bloom is produced by a continuous casting process. A cast material (ingot or bloom) is produced by the above process.
In the direct hot working process, while the cast material is cooling (is solidifying) after the casting process, hot working is performed on the cast material to produce a billet. Specifically, hot working is started at a timing at which the surface temperature of the cast material that is in the process of cooling (during solidification) is within the range of 1100 to 900° C., to thereby produce a billet.
In the present description, performing hot working at a timing at which the surface temperature of the produced cast material is within the range of 1100 to 900° C. during cooling and without the produced cast material being cooled to normal temperature in this way is referred to as “direct hot working”. The hot working method may be hot forging or may be hot rolling. Although not particularly limited, the accumulative rolling ratio in the direct hot working process is, for example, 35% or more.
Generally, a cast material produced by casting is cooled once to normal temperature. Then, the cast material cooled to normal temperature is heated in a reheating furnace to the Ac transformation point or more, and thereafter blooming (rough rolling) is performed. In the present description, such kind of conventional production process is referred to as “hot working after reheating”. In the case of hot working after reheating, reverse transformation (transformation from a phase to y phase) occurs in the cast material during heating before blooming. The grains are refined by this reverse transformation.
On the other hand, in the present embodiment, refining of grains by reverse transformation is avoided. Specifically, at a time point at which the surface temperature of the cast material is within the range of 1100 to 900° C. during cooling in the casting process, that is, in a state in which the surface temperature of the cast material is at the Ar3 transformation point or higher, the cast material is subjected to hot forging or hot rolling to produce a billet. In this case, the cast material is subjected to hot working without reverse transformation occurring. Therefore, refinement of the grains in the produced billet can be suppressed.
Note that, although not particularly limited, the upper limit of the accumulative rolling ratio in the direct hot working process is, for example, 80%,
In the finish rolling process, the billet produced in the direct hot working process is heated. The heated billet is subjected to finish rolling. The billet that underwent the finish rolling is then cooled to produce a steel material. In the finish rolling process, the following production conditions are to be satisfied.
The conditions in the finish rolling process are as follows.
Each production condition is described hereunder.
The heating temperature HT in the reheating furnace in the finish rolling process is to be 1000 to 1200° C.
If the heating temperature HT in the reheating furnace in the finish rolling process is less than 1000° C., precipitates in the billet will not sufficiently dissolve. In such a case, coarsening of grains in the billet will be suppressed because undissolved precipitates will exert a pinning effect. In such a case, cementite particles in the steel material after the finish rolling process will be coarse, and Characteristic 3 will not be satisfied.
On the other hand, if the heating temperature HT is more than 1200° C., the production cost will increase. In addition, a crack will easily form in the billet during finish rolling.
Accordingly, the heating temperature HT is to be 1000 to 1200° C.
A preferable lower limit of the heating temperature HT is 1020° C., more preferably is 1040° C., and further preferably is 1060° C.
A preferable upper limit of the heating temperature HT is 1180° C., more preferably is 1160° C., and further preferably is 1140° C.
Note that, the holding time at the heating temperature HT is not particularly limited. The holding time is, for example, 0.5 to 4.0 hours.
In the finish rolling process, hot rolling (finish rolling) is performed by a continuous mill equipped with a plurality of roll stands arranged in a row. In the hot rolling using a continuous mill, the steel material temperature on the exit side of the stand which last rolls the steel material is defined as the rolling finishing temperature FT (° C.). Note that, the term “steel material temperature” means the surface temperature of the steel material.
The rolling finishing temperature FT is to be 900° C. or more.
If the rolling finishing temperature FT is less than 900° C., coarsening of grains in the steel material will be suppressed by a pinning effect exerted by precipitates. In such a case, cementite particles in the steel material after the finish rolling process will be coarse, and Characteristic 3 will not be satisfied.
If the rolling finishing temperature is 900° C. or more, the grains after the finish rolling can be maintained in a coarse state. As a result, fine cementite particles can be formed in a cooling process that is the next process, and Characteristic 3 will be satisfied.
[Regarding Cooling after Finish Rolling]
After the finish rolling is completed, the steel material subjected to the finish rolling is cooled. By satisfying the production conditions in the aforementioned process, the grains in the steel material subjected to the finish rolling are maintained as they are as coarse grains. Therefore, in a steel material having a chemical composition satisfying Characteristic 1, the bainite nose in the CCT diagram shifts to the longer time side. That is, the bainite transformation start temperature becomes lower.
Therefore, in the cooling after the finish rolling, when the steel material temperature is in the range from the rolling finishing temperature FT to 820° C., the cooling rate (average cooling rate CR1) is made as slow as possible to maintain the grains as they are as coarse grains as much as possible. Subsequently, when the steel material temperature is in the range from 820 to 400° C., because this range corresponds to the bainite transformation temperature region, the cooling rate (average cooling rate CR2) is increased to suppress growth of cementite particles that form accompanying bainite transformation. The average cooling rates CR1 and CR2 are described hereunder.
The term “average cooling rate CR1” means the arithmetic mean value of the cooling rates when the steel material temperature is in a range from the rolling finishing temperature FT to 820° C.
If the average cooling rate CR1 is 2.0° C./sec or more, the cooling rate in this temperature region will be too fast. In this case, the grains in the steel material will be refined. Consequently, in a CCT diagram of a steel material having a chemical composition satisfying Characteristic 1, the bainite nose will shift to the shorter time side. In this case, the bainite transformation temperature will increase. Therefore, cementite particles that are formed accompanying bainite transformation will grow during cooling. As a result, the steel material after the finish rolling process will not satisfy Characteristic 3.
The lower limit of the average cooling rate CR1 is not particularly limited. When taking into consideration the equipment capacity, the lower limit of the average cooling rate CR1 is 1.0° C./sec.
A preferable lower limit of the average cooling rate CR1 is 1.1° C./sec, and more preferably is 1.2° C./sec. A preferable upper limit of the average cooling rate CR1 is 1.9° C./sec, and more preferably is 1.8° C./sec.
The term “average cooling rate CR2” means the arithmetic mean value of the cooling rates when the steel material temperature is in a range from 820° C. to 400° C.
The temperature region of the steel material temperature from 820 to 400° C. includes a temperature region in which bainite transformation occurs in a steel material produced in a manner satisfying the aforementioned production conditions using a cast material having a chemical composition satisfying Characteristic 1. Therefore, in the temperature range of 820 to 400° C., the cooling rate is increased to an extent such that martensite does not excessively form. By this means, growth of cementite particles that form during bainite transformation is suppressed, and the cementite particles are maintained in a fine state.
In a case where the average cooling rate CR2 is less than 2.0° C./sec, the cooling rate in the temperature range of 820 to 400° C. is too slow. In this case, the formed cementite particles will coarsen. Therefore, in the steel material after the finish rolling process, Characteristic 3 will not be satisfied.
On the other hand, in a case where the average cooling rate CR2 is more than 4.0° C./sec, the cooling rate in the temperature region of 820 to 400° C. is too fast. In this case, martensite will be excessively formed. As a result, in the microstructure of the steel material after the finish rolling process, although the area fraction of hard structure will be 90% or more, the Vickers hardness will be more than 400 HV.
If the average cooling rate CR2 is 2.0 to 4.0° C./sec, on the precondition that the other production conditions are satisfied, the steel material after the finish rolling process will satisfy Characteristic 1 to Characteristic 3.
A preferable lower limit of the average cooling rate CR2 is 2.2° C./sec, more preferably is 2.4° C./sec, and further preferably is 2.6° C./sec. A preferable upper limit of the average cooling rate CR2 is 3.8° C./sec, more preferably is 3.6° C./sec, and further preferably is 3.4° C./sec.
Note that, the average cooling rate CR1 and the average cooling rate CR2 can be determined by the following method. Cooling of the steel material is performed using a cooling equipment line installed downstream of the continuous mill that performs the finish rolling. Thermometers are arranged at a plurality of locations from the upstream side towards the downstream side along the cooling equipment line: The steel material temperature when passing each thermometer as well as the transit times are detected. The average cooling rate CR1 and the average cooling rate CR2 are determined based on the obtained steel material temperatures and transit times.
The steel material of the present embodiment is produced by the production processes described above.
For reference, a process for producing a bolt that uses the steel material of the present embodiment as a starting material will be described. One example of the process for producing a bolt that uses the steel material of the present embodiment as a starting material includes the following processes. Note that, a process for producing a bolt is well known.
Each process is described hereunder.
In the wire drawing process, the aforementioned steel material is subjected to well-known wire drawing to produce a steel wire. The wire drawing may be only primary wire drawing, or may be wire drawing that is carried out multiple times such as a primary wire drawing, a secondary wire drawing and the like.
In the spheroidizing annealing process, the steel wire after the wire drawing process is subjected to spheroidizing annealing. It suffices to perform the spheroidizing annealing under well-known conditions. For example, the steel wire is heated to 720 to 800° C. Thereafter, the steel wire is held at 720 to 800° C. for 1.0 to 6.0 hours. Next, the steel wire is slow cooled to 650° C. at a cooling rate of 3 to 10° C./hr. Thereafter, the steel wire is cooled to normal temperature.
In the cold forging process, the steel wire after the spheroidizing annealing process is subjected to well-known cold forging to produce a bolt-shaped intermediate product.
In the quenching and tempering process, the intermediate product is subjected to quenching and tempering.
The intermediate product after the cold forging process is subjected to quenching by a well-known method. The quenching temperature and the holding time at the quenching temperature are not particularly limited. The quenching temperature is, for example, 840 to 970° C. The holding time at the quenching temperature is, for example, 15 minutes to 360 minutes (6 hrs). After the holding time elapses, the intermediate product is rapidly cooled. Specifically, the intermediate product is subjected to water cooling or oil cooling.
The intermediate product after quenching is subjected to well-known tempering. The tempering conditions are, for example, as follows. The tempering temperature is, for example, 570 to 660° C. The holding time at the tempering temperature is, for example, 0.5 to 6.0 hours.
A bolt for which the steel material of the present embodiment is used as the Starting material can be produced by the above production method.
The process for producing a bolt may include other processes in addition to the processes described above. For example, at a timing that is after the cold forging process and is before the quenching and tempering process, a thread rolling process may be performed to form threads. In addition, after the quenching and tempering process, a compressive residual stress imparting process may be performed. Each of these processes is an optional process, and does not have to be performed.
The advantageous effects of the steel material of the present embodiment are described more specifically hereunder by way of examples. The conditions adopted in the following examples are one example of conditions adopted for confirming the feasibility and advantageous effects of the steel material of the present embodiment. Accordingly, the steel material of the present embodiment is not limited to this one example of conditions.
Steel materials (round bars) to serve as the starting materials for bolts were prepared that bad the chemical compositions shown in Table 1-1 and Table 1-2. In the tables, the symbol “-” indicates that the relevant element was intentionally not added.
The steel material of each test number was produced by the following method. Ingots having the chemical compositions described in Table 1-1 and Table 1-2 were produced by casting (casting process).
In Test Nos. 1 to 26 and 29 to 36, at a time at which the surface temperature of the cast material during cooling after the casting process was 1100 to 900° C., the cast material was subjected to hot forging to produce a billet. That is, for these test numbers, direct hot working was performed (described as “Direct” in the “Hot Working” column in Table 2). On the other hand, in Test Nos. 27 and 28, after cooling the cast material to normal temperature, the cast material was reheated to 1200° C. in a reheating furnace and thereafter was subjected to hot forging to produce a billet (described as “After reheating” in the “Hot Working” column in Table 2). In each of the hot working methods, the accumulative rolling ratio in the direct hot working process was within the range of 35 to 60%.
Each produced billet (cross section of 162 mm×162 mm) was subjected to a finish rolling process to produce a steel material (round bar) with a diameter of 20 mm.
In the finish rolling process, the heating temperature HT (° C.), the rolling finishing temperature FT (° C.), the average cooling rate CR1 (° C./sec) when the steel material temperature was cooled from the rolling finishing temperature FT to 820° C., and the average cooling rate CR2 (° C./sec) when the steel material temperature was cooled from the 820° C. to 400° C. were as described in Table 2. The steel material (round bar) of each test number was produced by the above production process.
The following evaluation tests were carried out using the produced steel materials.
Each of these evaluation tests is described hereunder.
The area fraction of hard structure (%) in the microstructure of the steel material of each test number was determined based on [Method for measuring microstructure of steel material] that is described above. The obtained results are shown in the column “Hard Structure Area Fraction (%)” of the column “Microstructure” in Table 2.
The Vickers hardness (HV) of the steel material of each test number was determined based on [Vickers hardness measurement method] that is described above. The obtained Vickers hardness (HV) is shown in the column “Vickers Hardness (HV)” of the column “Microstructure” in Table 2.
For the steel material of each test number, the number density ND (pieces/μm2) of cementite particles having an area of 0.0005 μm2 or more in the hard structure, the number ratio NR (%) of cementite particles having an area of 0.0005 to 0.0100 μm2 among a plurality of cementite particles in the hard structure, and the sample standard deviation σ (μm3) of the areas of a plurality of the cementite particles having an area of 0.0005 μm2 or more in the hard structure were determined based on [Method for measuring number density ND of cementite particles, number ratio NR of cementite particles, and sample standard deviation σ of areas of cementite particles] that is described above. The obtained number density ND (pieces/μm2), number ratio NR (%), and sample standard deviation σ (μm2) are shown in the columns “Number Density ND (pieces/μm2)”, “Number Ratio NR (%)”, and “Sample Standard Deviation σ (μm2)”, respectively, of the column “Cementite Particles” in Table 2.
The steel material of each test number was subjected to the following wire drawing. 250 kg of the steel material of each test number was subjected to a lubrication treatment under the same conditions, and thereafter was subjected to wire drawing. The cumulative reduction of area of the die in the wire drawing was set to 25%. If wire breakage did not occur during wire drawing, the steel material was evaluated as being excellent in wire drawability (indicated by “P (Pass)” in the column “Wire Drawability” in Table 2). On the other hand, if wire breakage occurred during wire drawing, the wire drawability was evaluated as being low (indicated by “F (Fail)” in the column “Wire Drawability” in Table 2). Note that. Tests 5 and 6 were not performed on the steel materials of the test numbers whose wire drawability was low.
The cold forgeability of the steel material of each test number was evaluated by the following method.
First, the steel material was subjected to spheroidizing annealing. In the spheroidizing annealing the steel material was heated to 760° C. The steel material was then held at 760° C. for 5.0 hours. After the holding time elapsed, the steel material was slow cooled to 650° C. at a cooling rate of 7° C./hr. The steel material temperature was then air-cooled from 650° C. to normal temperature.
A smooth test specimen was taken from a D/4 depth position in a cross section perpendicular to the longitudinal direction of the steel material after the spheroidizing annealing. The smooth specimen was cylindrical, with a diameter of 8 mm and a length of 12 mm. The longitudinal direction of the smooth test specimen was parallel to the longitudinal direction of the steel material.
The smooth test specimen was subjected to a critical compression test in accordance with a test method described in the material research group of cold forging subcommittee, Journal of the Japan Society for Technology of Plasticity, vol. 22, No. 241 (1981-2), pp. 139-144, Specifically, the smooth test specimen was subjected to cold compression in air at normal temperature at a rate of 10 mm/min using an end-face restraining die. Compression was stopped when a microcrack of 0.5 mm or more occurred in the smooth test specimen, and the compression ratio (%) at such time was calculated. This measurement was performed a total of 10 times, and the compression ratio (%) at which the cumulative failure probability was 50% was determined, and the thus-determined compression ratio was defined as the critical compression ratio (%).
If the critical compression ratio was 70% or more, it was determined that the steel material was excellent in cold forgeability (indicated by “70 or more” in the column “Critical Compression Ratio (%)” in Table 2). On the other hand, if the critical compression ratio was less than 70%, it was determined that the cold forgeability was low (critical compression ratio (%) is shown in the column “Critical Compression Ratio (9%)” in Table 2). Note that, Test 6 was not performed on a test number whose cold forgeability was low.
A test to evaluate the hydrogen embrittlement susceptibility of bolts produced using the steel material of each test number as a starting material was performed by the following method.
First, the steel material of each test number was subjected to wire drawing under the same conditions to produce a steel wire with a diameter of 16 mm. The steel wire of each test number was subjected to spheroidizing annealing under the same conditions as the spheroidizing annealing performed in [(Test 5) Test to evaluate cold forgeability of steel material].
Each steel wire after spheroidizing annealing was subjected to cold forging (bolt forming) under the same conditions to produce a bolt-shaped intermediate product with a thread root diameter of 14 mm that was the same for each test number.
Each produced intermediate product was subjected to quenching and tempering. In the quenching, for each test number, the quenching temperature was set to 920° C. and the holding time at the quenching temperature was set to one hour. After the holding time elapsed, each intermediate product was water-cooled.
The intermediate product after the quenching was subjected to tempering. The tempering temperature was set to 620° C., and the holding time at the tempering temperature was set to 2.0 hours.
A bolt of each test number was produced by the above production process.
The bolt of each test number was subjected to the following hydrogen embrittlement susceptibility evaluation test.
A round bar specimen with an annular notch having a parallel portion with a diameter of 7 mm and a length of 70 mm was taken from the bolt at a position that was at a depth of 1 mm or more on the inner side from the surface of the bolt. The annular notch was formed at a central position in the longitudinal direction of the specimen. The notch had a shape in which the notch depth was 1.4 mm, the notch angle was 60°, and the radius of curvature of the notch root was 0.175 mm.
The round bar specimen with an annular notch was charged with hydrogen by a cathodic hydrogen charging method. Specifically, a cathodic hydrogen charging solution at normal temperature was prepared. An aqueous solution obtained by adding 3 g of ammonium thiocyanate to 1 L of a 3% by mass sodium chloride aqueous solution was adopted as the cathodic hydrogen charging solution.
In a state in which the round bar specimen with an annular notch was immersed in the cathodic hydrogen charging solution, a constant current with a cathodic current density controlled to 0.05 mA/cm2 was generated for 18 hours to introduce hydrogen into the round bar specimen with an annular notch.
After performing the cathodic hydrogen charging method, the round bar specimen with an annular notch was left to stand at normal temperature for 96 hours. Thereafter, a galvanizing coating was formed under the same conditions on the surface of the round bar specimen with an annular notch that had been charged with hydrogen, to thereby prevent the hydrogen in the round bar specimen with an annular notch from leaking to outside.
The round bar specimen with an annular notch on which the galvanizing coating was formed was subjected to a constant load test that applied a constant load at normal temperature and atmospheric pressure so that a load equivalent to the cross-sectional area×tensile strength×0.90 was applied thereto. Here, the term “cross-sectional area” means the cross-sectional area of a cross section (transverse section) perpendicular to the longitudinal direction of the specimen at the notch root. The term “tensile strength” means a tensile strength (MPa) obtained by performing a tensile test, which is described later, using a smooth round bar tensile test specimen. The test time was set to a maximum of 100 hours. In the constant load test, if the round bar specimen with an annular notch endured the load without rupturing for 100 hours or more, the hydrogen embrittlement susceptibility was evaluated as being sufficiently low (indicated by “100 or more” in the column “Endurance Time (hrs)” of the column “Hydrogen Embrittlement Susceptibility” in Table 2). On the other hand, in the constant load test, if the round bar specimen with an annular notch ruptured in less than 100 hours, the hydrogen embrittlement susceptibility was evaluated as being high (the endurance time (hrs) is shown in the column “Endurance Time (hrs)” of the column “Hydrogen Embrittlement Susceptibility” in Table 2).
Note that, the tensile test using a smooth round bar tensile test specimen was performed by the following method. A smooth round bar tensile test specimen was taken from the bolt of each test number at a position that was at a depth of 1 mm or more on the inner side from the surface of the bolt. The diameter of a parallel portion of the smooth round bar tensile test specimen was 6 mm, and the length of the parallel portion was 70 mm. The central axis of the parallel portion of the smooth round bar tensile test specimen was made coaxial with the central axis of the bolt.
A tensile test in conformity with JIS Z 2241:2022 was carried out in air at normal temperature (20±15° C.) using the smooth round bar tensile test specimen, and the tensile strength (MPa) was determined. The obtained tensile strength is shown in the column “Tensile Strength (MPa)” of the column “Hydrogen Embrittlement Susceptibility” in Table 2.
Referring to Table 1-1, Table 1-2, and Table 2, for each of the steel materials of Test Nos. 1 to 23, the chemical composition was appropriate, and the production conditions were also appropriate. Therefore, the area fraction of hard structure was 90% or more, and the Vickers hardness was 220 to 400 HV. In addition, the number density ND of cementite particles having an area of 0.0005 μm2 or more in the hard structure of the steel material was 4.0 pieces/μm2 or more, the number ratio NR of cementite particles having an area of 0.0005 to 0.0100 μm2 among a plurality of cementite particles in the hard structure was 50.0% or more, and the sample standard deviation σ of the areas of a plurality of the cementite particles having an area of 0.0005 μm2 or more in the hard structure was 0.070 μm2 or less. Therefore, these steel materials were excellent in wire drawability. In addition, these steel materials were excellent in cold forgeability after spheroidizing. Furthermore, the hydrogen embrittlement susceptibility of the bolts produced using these steel materials as a starting material was sufficiently low.
On the other hand, in Test No. 24, the content of Mo was too low. Consequently, the hydrogen embrittlement susceptibility of the bolt produced using this steel material as a starting material was high.
In Test No. 25, the content of V was too low. Consequently, the hydrogen embrittlement susceptibility of the bolt produced using this steel material as a starting material was high.
In Test No. 26, the content of V was too high. Consequently, the cold forgeability after spheroidizing was low.
In Test Nos. 27 and 28, although the chemical composition was appropriate, the cast material was subjected to hot working after reheating. Consequently, the number density ND of cementite particles was less than 4.0 pieces/μm2, the number ratio NR of cementite particles was less than 50.0%, and the sample standard deviation σ of the areas of the cementite particles was more than 0.070 μm2. Therefore, the hydrogen embrittlement susceptibility of the bolts produced using these steel materials as a starting material was high.
In Test No. 29, although the chemical composition was appropriate, the beating temperature HT in the finish rolling process was too low, and the rolling finishing temperature FT was too low. Consequently, the number density ND of cementite particles was less than 4.0 pieces/μm2, the number ratio NR of cementite particles was less than 50.0%, and the sample standard deviation σ of the areas of the cementite particles was more than 0.070 μm2. Therefore, the hydrogen embrittlement susceptibility of the bolt produced using this steel material as a starting material was high.
In Test Nos. 30 and 31, although the chemical composition was appropriate, the average cooling rate CR1 was too fast, Consequently, the number density ND of cementite particles was less than 4.0 pieces/μm2, the number ratio NR of cementite particles was less than 50.0%, and the sample standard deviation σ of the areas of the cementite particles was more than 0.070 μm2. Therefore, the hydrogen embrittlement susceptibility of the bolts produced using these steel materials as a starting material was high.
In Test Nos. 32 and 33, although the chemical composition was appropriate, the average cooling rate CR2 was too slow. Consequently, the number density ND of cementite particles was less than 4.0 pieces/μm2, the number ratio NR of cementite particles was less than 50.0%, and the sample standard deviation σ of the areas of the cementite particles was more than 0.070 μm2. Therefore, the hydrogen embrittlement susceptibility of the bolts produced using these steel materials as a starting material was high.
In Test Nos. 34 and 35, although the chemical composition was appropriate, the average cooling rate CR2 was too fast. Therefore, although the area fraction of hard structure was 90% or more, the Vickers hardness was more than 400 HV. Consequently, the wire drawability was low.
In Test No. 36, the content of C was too low. Consequently, the number density ND of cementite particles was less than 4.0 pieces/μm2. Therefore, the hydrogen embrittlement susceptibility of the bolt produced using this steel material as a starting material was high.
An embodiment of the present disclosure has been described above. However, the embodiment described above is merely an example for carrying out the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above embodiment within a range that does not depart from the gist of the present disclosure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-033982 | Mar 2022 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2023/008073 | 3/3/2023 | WO |
