Patent Application
20240352566
The present invention relates to a steel material, and more particularly relates to a steel material that will serve as a starting material for a component for machine structural use.
Components for machine structural use are utilized for suspension components and axles and the like of automobiles and construction vehicles. Such components for machine structural use are required to have high fatigue strength.
In a process for producing a component for machine structural use, in some cases the steel material that will serve as the starting material for the component for machine structural use is subjected to a cutting process. Therefore, the steel material that will serve as the starting material for the component for machine structural use is required to have high machinability.
Steel materials that will serve as starting materials for components for machine structural use is disclosed, for example, in Japanese Patent Application Publication No. 57-19366 (Patent Literature 1), Japanese Patent Application Publication No. 2004-18879 (Patent Literature 2), and Japanese Patent Application Publication No. 2008-169411 (Patent Literature 3).
The steel material disclosed in Patent Literature 1 contains 0.001 to 0.05% of Ca, and 0.02 to 0.15% of Pb and Bi alone or in combination, and in this steel material an amount of S is controlled to 0.005% or less, and inclusions are converted to CaS—CaO-, Pb-, and Bi-based inclusions, and an amount of Al2O3 inclusions is suppressed to less than 0.001%. According to the technique disclosed in Patent Literature 1, a large amount of Ca is continuously added to molten steel to change dissolved S into CaS. Further, Al2O3 is eliminated or is made an extremely small amount by a reduction reaction with Ca. Therefore, inclusions become CaS—CaO-based inclusions. Thereafter, a small amount of one or both of Pb and Bi is added to the molten steel to form single inclusions of Pb or Bi. It is disclosed in Patent Literature 1 that, by this means, the machinability of the steel material is enhanced.
The steel material disclosed in Patent Literature 2 contains, in percent by mass, B: 0.001 to 0.010%, N: 0.002 to 0.010%, and Bi: 0.005 to 0.10%. In this steel for cold forging, a total of 15 or more BN having a diameter of 0.7 μm or more and Bi precipitates containing B are present per visual field area of 0.5 mm×0.5 mm in transverse section. In this steel material, by fixing N as BN, an amount of dissolved N is reduced and work hardening is decreased. In addition, it is disclosed in Patent Literature 2 that Bi precipitates containing B are formed, which improves chip treatability.
The steel material disclosed in Patent Literature 3 consists of, in percent by mass, C: 0.15 to 0.55%, Si: 0.01 to 2.0%, Mn: 0.01 to 2.5%, Cu: 0.01 to 2.0%, Ni: 0.01 to 2.0%, Cr: 0.01 to 2.5%, Mo: 0.01 to 3.0%, and at least one kind of element selected from a group consisting of V and W in a total amount of 0.01 to 1.0%, with the balance being Fe and unavoidable impurities. This steel material is held at 1010° C. to 1050° C., is then cooled to 500° C. to 550° C. at a cooling rate of 200° C./min or more, and then cooled to 150° C. or less at a cooling rate of 100° C./min or more, and thereafter heated to a temperature range of 550° C. to 700° C. LMP that gives a maximum value of HRC hardness at room temperature of the steel material on which the aforementioned heat treatment and cooling treatment were performed is 17.66 or more. It is disclosed in Patent Literature 3 that in this steel material, because the LMP is 17.66 or more, softening resistance is enhanced and thermal fatigue strength is increased.
In this connection, one example of a process for producing a component for machine structural use using a steel material as a starting material is as follows. The steel material that will serve as the starting material is subjected to hot working to produce an intermediate product having a rough shape of the component for machine structural use. The hot working is, for example, hot forging. The produced intermediate product is subjected to machining (a cutting process) to make the intermediate product into a predetermined shape. The intermediate product after the cutting process is subjected to quenching and tempering. The component for machine structural use is produced by the above production process.
In the quenching in the production process described above, in some cases the intermediate product (steel material) is subjected to induction hardening to increase strength of a region in one part of the component for machine structural use. In such a case, high frequency induction heating is performed on the region of the intermediate product (steel material) where it is desired to increase the strength, and thereafter rapid cooling (quenching) is performed.
However, in some cases the steel material may be excessively heated locally during the high frequency induction heating due to the shape of the intermediate product (steel material). Further, in some cases an outer layer and some of an interior of the steel material melt and a crack occurs. In the present description, such cracks are also referred to as “melting cracks”. When performing induction hardening in the process for producing the component for machine structural use, it is required to suppress the occurrence of melting cracks in the steel material.
In addition, hot working (for example, hot rolling, hot forging, or the like) is performed during a process for producing a steel material, and during the process for producing the component for machine structural use using the steel material. Therefore, when producing the steel material that will serve as the starting material for the component for machine structural use, it is required to not only suppress melting cracks, but to also suppress the occurrence of cracks during hot working. Here, in the present description, cracks that occur during hot working are also referred to as “hot working cracks”. Accordingly, in addition to the requirement for the steel material that will serve as the starting material for the component for machine structural use to have excellent machinability and for high fatigue strength to be obtained when the steel material is formed into the component for machine structural use, there is also a requirement to suppress hot working cracks and suppress melting cracks.
In the aforementioned Patent Literature 1 to Patent Literature 3, at least the suppression of hot working cracks and the suppression of melting cracks have not been investigated.
An objective of the present invention is to provide a steel material which is excellent in machinability, which can suppress cracks during hot working, which can suppress melting cracks during induction hardening, and with which excellent fatigue strength is obtained when the steel material is formed into a component for machine structural use.
A steel material of the present disclosure consists of, in percent by mass,
A steel material of the present disclosure contains, in percent by mass,
The steel material of the present disclosure is excellent in machinability, can suppress cracks during hot working, and can suppress melting cracks during induction hardening, and is a steel material with which excellent fatigue strength is obtained when formed into a component for machine structural use.
First, the present inventors conducted studies regarding a chemical composition of a steel material that is excellent in machinability and with which excellent fatigue strength is obtained when the steel material is formed into a component for machine structural use. As a result, the present inventors considered that if a steel material has a chemical composition consisting of, in percent by mass, C: 0.05 to 0.30%, Si: 0.05 to 0.45%, Mn: 0.30 to 2.00%, P: 0.030% or less, S: 0.010 to 0.095%, Cr: 0.01 to 2.00%, N: 0.0030 to 0.0250%, Al: 0 to 0.060%, Mg: 0 to 0.0100%, Ti: 0 to 0.1500%, Nb: 0 to 0.0800%, W: 0 to 0.4000%, Zr: 0 to 0.2000%, Ca: 0 to 0.0100%, Te: 0 to 0.0100%, B: 0 to 0.0050%, Sn: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Co: 0 to 0.0100%, Se: 0 to 0.0100%, Sb: 0 to 0.0100%, In: 0 to 0.0100%, V: 0 to 0.200%, Mo: 0 to 1.00%, Cu: 0 to 0.20%, and Ni: 0 to 0.20%, with the balance being Fe and impurities, there is a possibility that excellent machinability will be obtained and also that excellent fatigue strength will be obtained when the steel material is formed into a component for machine structural use.
However, it is not necessarily the case that simply with the steel material having the aforementioned chemical composition, excellent machinability will be obtained and excellent fatigue strength will be obtained when the steel material is formed into a component for machine structural use. Even if a content of each element in the chemical composition is within the range described above, the machinability of the steel material will decrease if the hardness of the steel material is high. On the other hand, even if the content of each element in the chemical composition is within the range described above, the fatigue strength of a component for machine structural use produced using the steel material as a starting material will be low if the hardness of the steel material is low. Therefore, in order to compatibly achieve both fatigue strength that is required for a component for machine structural use and machinability of the steel material, it is effective to make the hardness of the steel material that is the starting material for the component for machine structural use fall within a suitable range.
Therefore, the present inventors conducted studies regarding the content of elements that affect the hardness of a steel material in which the content of each element in the chemical composition is within the range described above. Among the elements in the chemical composition described above, in particular C, Si, Mn, Cr and V increase the internal hardness of a component for machine structural use produced using the steel material as a starting material, and consequently increase the fatigue strength of the component for machine structural use. On the other hand, S causes the internal hardness to decrease. Therefore, the present inventors considered that setting the contents of these elements within a suitable range would enable both enhancement of the machinability of the steel material and also an increase in the fatigue strength of a component for machine structural use that is produced using the steel material as a starting material. As a result of further studies, the present inventors discovered that in a steel material in which the content of each element in the chemical composition is within the range described above, if Formula (1) is satisfied, excellent machinability is obtained in the steel material, and furthermore, excellent fatigue strength is obtained when the steel material is formed into a component for machine structural use:
where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in the formula. If an element is not contained, “0” is substituted for the corresponding symbol of an element.
Next, the present inventors conducted studies regarding means for suppressing melting cracks during induction hardening in a steel material in which the content of each element in the chemical composition is within the range described above and which satisfies Formula (1). First, in order to identify a cause of occurrence of melting cracks in the steel material during induction hardening, the present inventors observed a microstructure of regions where melting cracks occurred. As a result, it was found that decarburization had not occurred in the regions where melting cracks occurred. On the other hand, melting cracks had not occurred in decarburized regions.
Based on these results, the present inventors considered that the content of C affects melting cracks which occur in a steel material during induction hardening. Specifically, melting cracks are liable to occur due to C segregating to grain boundaries. Therefore, the present inventors conducted investigations regarding means for suppressing the segregation of C to grain boundaries.
As a result of such investigations, the present inventors discovered that, in addition to the chemical composition described above, by also containing 0.0051 to 0.1500% of Bi in lieu of a part of Fe, melting cracks in the steel material during induction hardening can be suppressed. It is considered that the reason for this is as follows. If an appropriate amount of Bi is contained, the Bi will be present as inclusions in the steel material. Hereunder, inclusions composed of Bi are referred to as “Bi particles”. Bi particles suppress coarsening of austenite grains in the steel material during induction hardening by the pinning effect. If the Bi particles are fine, the pinning effect will be enhanced. When austenite grains are kept fine during induction hardening, the grain boundary area of the austenite grains increases. As the grain boundary area increases, the concentration of C segregating to austenite grain boundaries per unit area decreases. As a result, the occurrence of melting cracks is suppressed.
As described above, the occurrence of melting cracks during induction hardening is suppressed by containing an appropriate amount of Bi. However, it was revealed that when hot working is performed on the steel material, cracks may occur in some cases. Here, the term “hot working” refers to, for example, hot rolling performed during a process of producing the steel material, or hot forging or the like performed during a process of producing the component for machine structural use. Therefore, the present inventors investigated a cause of such cracks during hot working. As a result, the present inventors obtained the following new finding.
When Bi is contained in the steel material to suppress the occurrence of melting cracks, in some cases coarse Bi particles having an equivalent circular diameter of 10.0 μm or more are formed in the steel material together with fine Bi particles (Bi inclusions) having an equivalent circular diameter of 1.0 μm or less. The coarse Bi particles are liable to become starting points for cracks during hot working. Therefore, if a number density of coarse Bi particles is too high, cracks (hot working cracks) will easily occur during hot working.
As described above, in a steel material that contains Bi, although melting cracks during induction hardening are easy to suppress, hot working cracks that are attributable to coarse Bi particles are liable to occur. If the Bi particles in a steel material are fine, the occurrence of melting cracks during induction hardening will be suppressed. On the other hand, if the Bi particles in a steel material are coarse, hot working cracks will easily occur.
Based on the results of the investigations described above, the present inventors considered that by suppressing the number density of coarse Bi particles in the steel material to as low a number as possible while at the same time securing a number density of fine Bi particles in the steel material to a certain extent, melting cracks during induction hardening can be suppressed and hot working cracks can also be suppressed. Therefore, the present inventors conducted further investigations and studies regarding the number density of fine Bi particles and the number density of coarse Bi particles at which these effects are sufficiently exerted. As a result, the present inventors discovered that in a steel material having the chemical composition described above, on the precondition that the aforementioned Formula (1) is satisfied, if the number density of fine Bi particles having an equivalent circular diameter of 0.1 to 1.0 μm is 80 to 8000 pieces/mm2, and the number density of coarse Bi particles having an equivalent circular diameter of 10.0 μm or more is 10 pieces/mm2 or less, melting cracks during induction hardening can be suppressed and hot working cracks can also be suppressed.
The steel material according to the present embodiment that has been completed based on the above findings is as follows.
[1]
A steel material consisting of, in percent by mass,
[2]
A steel material containing, in percent by mass,
[3]
The steel material according to [2], wherein:
[4]
The steel material according to [2] or [3], wherein:
[5]
The steel material according to any one of [2] to [4], wherein:
[6]
The steel material according to any one of [2] to [5], wherein:
[7]
The steel material according to any one of [2] to [6], wherein:
Hereunder, the steel material of the present embodiment is described in detail. The symbol “%” in relation to an element means “mass percent” unless specifically stated otherwise.
The steel material of the present embodiment satisfies the following Characteristic 1 to Characteristic 4.
The chemical composition consists of, in percent by mass, C: 0.05 to 0.30%, Si: 0.05 to 0.45%, Mn: 0.30 to 2.00%, P: 0.030% or less, S: 0.010 to 0.095%, Cr: 0.01 to 2.00%, Bi: 0.0051 to 0.1500%, N: 0.0030 to 0.0250%, Al: 0 to 0.060%, Mg: 0 to 0.0100%, Ti: 0 to 0.1500%, Nb: 0 to 0.0800%, W: 0 to 0.4000%, Zr: 0 to 0.2000%, Ca: 0 to 0.0100%, Te: 0 to 0.0100%, B: 0 to 0.0050%, Sn: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Co: 0 to 0.0100%, Se: 0 to 0.0100%, Sb: 0 to 0.0100%, In: 0 to 0.0100% or less, V: 0 to 0.200%, Mo: 0 to 1.00%, Cu: 0 to 0.20%, and Ni: 0 to 0.20% or less, with the balance being Fe and impurities.
On the precondition that the content of each element is within the range of Characteristic 1, the steel material satisfies Formula (1).
Where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in the formula. If an element is not contained, “0” is substituted for the corresponding symbol of an element.
In the steel material, the number density of fine Bi particles having an equivalent circular diameter of 0.1 to 1.0 μm is 80 to 8000 pieces/mm2.
In the steel material, the number density of coarse Bi particles having an equivalent circular diameter of 10.0 μm or more is 10 pieces/mm2 or less. Hereunder, Characteristic 1 to Characteristic 4 are each described.
The chemical composition of the steel material of the present embodiment contains the following elements.
Carbon (C) increases the hardness of a component for machine structural use produced using the steel material as a starting material, and increases the fatigue strength of the component for machine structural use. If the content of C is less than 0.05%, 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 C is more than 0.30%, C will segregate to grain boundaries even if the contents of other elements are within the range of the present embodiment. In such a case, the C concentration at the grain boundaries will be high. If the C concentration is high, the fusing point will be lowered. As a result, melting cracks will easily occur during induction hardening.
Therefore, the content of C is to be 0.05 to 0.30%.
A preferable lower limit of the content of C is 0.08%, more preferably is 0.10%, and further preferably is 0.13%.
A preferable upper limit of the content of C is 0.28%, more preferably is 0.25%, and further preferably is 0.23%.
Silicon (Si) deoxidizes the steel in the steelmaking process. Si also increases the hardness of the component for machine structural use, and increases the fatigue strength of the component for machine structural use. If the content of Si is less than 0.05%, 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, Si has a weak affinity to C. Therefore, if the content of Si is more than 0.45%, even if the contents of other elements are within the range of the present embodiment, C will be more likely to segregate to grain boundaries during high frequency induction heating compared to within grains in which Si is dissolved. As a result, melting cracks will easily occur during induction hardening.
Therefore, the content of Si is to be 0.05 to 0.45%.
A preferable lower limit of the content of Si is 0.07%, more preferably is 0.10%, and further preferably is 0.13%.
A preferable upper limit of the content of Si is 0.43%, more preferably is 0.40%, and further preferably is 0.38%.
Manganese (Mn) deoxidizes the steel in the steelmaking process. Mn also has a strong affinity to C. Therefore, during heating, C remains in grains in which Mn is dissolved. Consequently, segregation of C to grain boundaries is suppressed, and the occurrence of melting cracks during induction hardening is suppressed. If the content of Mn 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 Mn is more than 2.00%, the hardness of the steel material will excessively increase even if the contents of other elements are within the range of the present embodiment. As a result, the machinability of the steel material will decrease.
Therefore, the content of Mn is to be 0.30 to 2.00%.
A preferable lower limit of the content of Mn is 0.35%, more preferably is 0.40%, further preferably is 0.50%, and further preferably is 0.60%.
A preferable upper limit of the content of Mn is 1.90%, more preferably is 1.70%, further preferably is 1.50%, and further preferably is 1.40%.
Phosphorus (P) is an impurity. P segregates to grain boundaries. Therefore, P lowers the fusing point of the steel material. As a result, melting cracks easily occur during induction hardening.
Therefore, the content of P is to be 0.030% or less.
The content of P is preferably as low as possible. However, excessively reducing the content of P will raise the production cost. Therefore, when ordinary industrial productivity is taken into consideration, a preferable lower limit of the content of P is more than 0%, more preferably is 0.001%, and further preferably is 0.002%.
A preferable upper limit of the content of P is 0.028%, more preferably is 0.026%, further preferably is 0.023%, and further preferably is 0.020%.
Sulfur(S) forms sulfide-based inclusions and thereby enhances the machinability of the steel material. If the content of S is less than 0.010%, 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, S lowers the fusing point of the steel material. Therefore, if the content of S is more than 0.095%, melting cracks will easily occur during induction hardening even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of S is to be 0.010 to 0.095%.
A preferable lower limit of the content of S is 0.012%, more preferably is 0.015%, further preferably is 0.018%, and further preferably is 0.020%.
A preferable upper limit of the content of S is 0.080%, more preferably is 0.070%, and further preferably is 0.060%.
Chromium (Cr) increases the hardenability of the steel material. Hence, the internal hardness of the component for machine structural use increases. As a result, the fatigue strength of the component for machine structural use increases. Cr also has a strong affinity to C. Therefore, during heating, C remains in grains in which Cr is dissolved. Consequently, segregation of C to grain boundaries is suppressed, and the occurrence of melting cracks during induction hardening is suppressed. Cr also combines with S to form Cr sulfides. In this case, formation of coarse FeS is suppressed. As a result, the ductility of the steel material during hot working increases, and hot working cracks are suppressed. 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 more than 2.00%, the hardness of the steel material will excessively increase even if the contents of other elements are within the range of the present embodiment. As a result, the machinability of the steel material will decrease.
Therefore, the content of Cr is to be 0.01 to 2.00%.
A preferable lower limit of the content of Cr is 0.02%, more preferably is 0.04%, further preferably is 0.06%, further preferably is 0.08%, and further preferably is 0.10%.
A preferable upper limit of the content of Cr is 1.90%, more preferably is 1.70%, further preferably is 1.50%, and further preferably is 1.20%.
Bismuth (Bi) forms inclusions (Bi particles) in the steel material. Therefore, the occurrence of melting cracks during induction hardening is suppressed. In addition, Bi enhances the machinability of the steel material. If the content of Bi is less than 0.0051%, 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 Bi is more than 0.1500%, coarse Bi particles will form even if the contents of other elements are within the range of the present embodiment. The coarse Bi particles will be liable to serve as starting points for cracks during hot working in the process for producing the steel material, or during hot working in the process for producing the component for machine structural use that is produced using the steel material as a starting material. Consequently, hot working cracks will easily occur.
Therefore, the content of Bi is to be 0.0051 to 0.1500%.
A preferable lower limit of the content of Bi is 0.0080%, more preferably is 0.0100%, further preferably is 0.0120%, further preferably is 0.0140%, and further preferably is 0.0160%.
A preferable upper limit of the content of Bi is 0.1400%, more preferably is 0.1350%, and further preferably is 0.1300%.
Nitrogen (N) forms nitrides and/or carbo-nitrides in a cooling process after hot working during the process for producing the component for machine structural use, and thereby contributes to precipitation strengthening of the steel material. As a result, the fatigue strength of the component for machine structural use increases. If the content of N is less than 0.0030%, 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 0.0250%, 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 N is to be 0.0030 to 0.0250%.
A preferable lower limit of the content of N is 0.0035%, more preferably is 0.0040%, further preferably is 0.0050%, and further preferably is 0.0080%.
A preferable upper limit of the content of N is 0.0240%, more preferably is 0.0230%, further preferably is 0.0200%, further preferably is 0.0180%, and further preferably is 0.0150%.
The balance of the chemical composition of the steel material according to the present embodiment is Fe and impurities. Here, the term “impurities” refers to elements 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 not intentionally contained but are permitted within a range not adversely affecting the steel material according to the present embodiment.
All elements other than the aforementioned impurities (P and S) may be mentioned as examples of such impurities. The balance may contain only one kind of impurity or may include two or more kinds of impurity. The impurities other than the aforementioned impurities are, for example, as follows: O: 0.0050% or less, Ta and Zn: 0 to 0.01% in total, and Pb: 0 to 0.09%.
The chemical composition of the present embodiment may further contain one or more kinds of element selected from a group consisting of the following first group to fifth group in lieu of a part of Fe.
One or more kinds of element selected from a group consisting of:
Hereunder, each optional element is described.
The chemical composition of the steel material of the present embodiment may further contain the above-described first group in lieu of a part of Fe. These elements are optional elements, and each of these elements deoxidizes the steel. Hereunder, each element of the first group is described.
Aluminum (Al) is an optional element, and does not have to be contained. That is, the content of Al may be 0%.
When contained, that is, when the content of Al is more than 0%, Al deoxidizes the steel. If even a small amount of Al is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Al is more than 0.060%, Al will form coarse oxides even if the contents of other elements are within the range of the present embodiment. The coarse oxides will reduce the fatigue strength of the component for machine structural use.
Therefore, the content of Al is to be 0 to 0.060%. When contained, the content of Al is to be 0.060% or less.
A preferable lower limit of the content of Al is 0.001%, more preferably is 0.002%, further preferably is 0.003%, further preferably is 0.005%, and further preferably is 0.010%.
A preferable upper limit of the content of Al is 0.055%, more preferably is 0.050%, and further preferably is 0.045%.
Magnesium (Mg) is an optional element, and does not have to be contained. That is, the content of Mg may be 0%.
When contained, that is, when the content of Mg is more than 0%, Mg deoxidizes the steel. 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 0.0100%, Mg will form coarse oxides even if the contents of other elements are within the range of the present embodiment. The coarse oxides will reduce the fatigue strength of the component for machine structural use.
Therefore, the content of Mg is to be 0 to 0.0100%. When contained, the content of Mg is to be 0.0100% or less.
A preferable lower limit of the content of Mg is 0.0001%, more preferably is 0.0003%, and further preferably is 0.0005%.
A preferable upper limit of the content of Mg is 0.0090%, more preferably is 0.0070%, further preferably is 0.0050%, and further preferably is 0.0040%.
The chemical composition of the steel material of the present embodiment may further contain the above-described second group in lieu of a part of Fe. These elements are optional elements, and each of these elements forms precipitates and thereby increases the toughness of the component for machine structural use. Hereunder, each element of the second group is described.
Titanium (Ti) is an optional element, and does not have to be contained. That is, the content of Ti may be 0%.
When contained, that is, when the content of Ti is more than 0%, Ti forms carbides and/or carbo-nitrides in a cooling process of a hot working process during the process for producing the component for machine structural use, and thereby refines the grains. As a result, the toughness of the component for machine structural use increases. 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 0.1500%, even if the contents of other elements are within the range of the present embodiment, the aforementioned effect will be saturated, and the production cost will increase.
Therefore, the content of Ti is to be 0 to 0.1500%. When contained, the content of Ti is to be 0.1500% or less.
A preferable lower limit of the content of Ti is 0.0001%, more preferably is 0.0010%, further preferably is 0.0050%, and further preferably is 0.0080%.
A preferable upper limit of the content of Ti is 0.1400%, more preferably is 0.1200%, further preferably is 0.1000%, further preferably is 0.0500%, further preferably is 0.0200%, and further preferably is 0.0150%.
Niobium (Nb) is an optional element, and does not have to be contained. That is, the content of Nb may be 0%.
When contained, that is, when the content of Nb is more than 0%, Nb forms carbides and/or carbo-nitrides in the cooling process of the hot working process during the process for producing the component for machine structural use, and thereby refines the grains. As a result, the toughness of the component for machine structural use increases. 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 0.0800%, even if the contents of other elements are within the range of the present embodiment, the aforementioned effect will be saturated, and the production cost will increase.
Therefore, the content of Nb is to be 0 to 0.0800%. When contained, the content of Nb is to be 0.0800% or less.
A preferable lower limit of the content of Nb is 0.0001%, more preferably is 0.0010%, further preferably is 0.0050%, and further preferably is 0.0080%.
A preferable upper limit of the content of Nb is 0.0700%, more preferably is 0.0600%, further preferably is 0.0500%, further preferably is 0.0200%, and further preferably is 0.0150%.
Tungsten (W) is an optional element, and does not have to be contained. That is, the content of W may be 0%.
When contained, that is, when the content of W is more than 0%, W forms carbides and/or carbo-nitrides in the cooling process of the hot working process during the process for producing the component for machine structural use, and thereby refines the grains. As a result, the toughness of the component for machine structural use increases. 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 0.4000%, even if the contents of other elements are within the range of the present embodiment, the aforementioned effect will be saturated, and the production cost will increase.
Therefore, the content of W is to be 0 to 0.4000%. When contained, the content of W is to be 0.4000% or less.
A preferable lower limit of the content of W is 0.0001%, more preferably is 0.0050%, and further preferably is 0.0500%.
A preferable upper limit of the content of W is 0.3500%, more preferably is 0.3000%, and further preferably is 0.2000%.
Zirconium (Zr) is an optional element, and does not have to be contained. That is, the content of Zr may be 0%.
When contained, that is, when the content of Zr is more than 0%, Zr forms carbides and/or carbo-nitrides in the cooling process of the hot working process during the process for producing the component for machine structural use, and thereby refines the grains. As a result, the toughness of the component for machine structural use increases. 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 0.2000%, even if the contents of other elements are within the range of the present embodiment, the aforementioned effect will be saturated, and the production cost will increase.
Therefore, the content of Zr is to be 0 to 0.2000%. When contained, the content of Zr is to be 0.2000% or less.
A preferable lower limit of the content of Zr is 0.0001%, more preferably is 0.0010%, further preferably is 0.0020%, and further preferably is 0.0050%.
A preferable upper limit of the content of Zr is 0.1500%, more preferably is 0.1000%, further preferably is 0.0500%, and further preferably is 0.0100%.
The chemical composition of the steel material of the present embodiment may further contain the above-described third group in lieu of a part of Fe. These elements are optional elements, and each of these elements enhances the machinability of the steel material. Hereunder, each element of the third group is described.
Ca: 0.0100% or less
Calcium (Ca) is an optional element, and does not have to be contained. That is, the content of Ca may be 0%.
When contained, that is, when the content of Ca is more than 0%, Ca enhances the machinability of the steel material. 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 0.0100%, Ca will form coarse oxides even if the contents of other elements are within the range of the present embodiment. In such a case, the fatigue strength of the component for machine structural use will decrease.
Therefore, the content of Ca is to be 0 to 0.0100%. When contained, the content of Ca is to be 0.0100% or less.
A preferable lower limit of the content of Ca is 0.0001%, more preferably is 0.0010%, and further preferably is 0.0015%.
A preferable upper limit of the content of Ca is 0.0090%, more preferably is 0.0070%, further preferably is 0.0050%, further preferably is 0.0030%, and further preferably is 0.0020%.
Te: 0.0100% or less
Tellurium (Te) is an optional element, and does not have to be contained. That is, the content of Te may be 0%.
When contained, that is, when the content of Te is more than 0%, Te enhances 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 0.0100%, hot working cracks will easily occur in the steel material 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 0.0100%. When contained, the content of Te is to be 0.0100% or less.
A preferable lower limit of the content of Te is 0.0001%, more preferably is 0.0003%, and further preferably is 0.0010%.
A preferable upper limit of the content of Te is 0.0090%, more preferably is 0.0085%, and further preferably is 0.0080%.
B: 0.0050% or less
Boron (B) is an optional element, and does not have to be contained. That is, the content of B may be 0%.
When contained, that is, when the content of B is more than 0%, B enhances the machinability of the steel material. If even a small amount of B is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of B is more than 0.0050%, hot working cracks will easily occur in the steel material even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of B is to be 0 to 0.0050%. When contained, the content of B is to be 0.0050% or less.
A preferable lower limit of the content of B is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0010%.
A preferable upper limit of the content of B is 0.0040%, more preferably is 0.0035%, and further preferably is 0.0030%.
Tin (Sn) is an optional element, and does not have to be contained. That is, the content of Sn may be 0%.
When contained, that is, when the content of Sn is more than 0%, Sn enhances the machinability of the steel material. If even a small amount of Sn is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Sn is more than 0.0100%, hot working cracks will easily occur in the steel material even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of Sn is to be 0 to 0.0100%. When contained, the content of Sn is to be 0.0100% or less.
A preferable lower limit of the content of Sn is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0010%.
A preferable upper limit of the content of Sn is 0.0095%, more preferably is 0.0090%, further preferably is 0.0085%, and further preferably is 0.0080%.
Rare earth metal (REM) is an optional element, and does not have to be contained. That is, the content of REM may be 0%.
When contained, that is, when the content of REM is more than 0%, REM enhances the machinability of the steel material. If even a small amount of REM is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of REM is more than 0.0100%, hot working cracks will easily occur in the steel material even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of REM is to be 0 to 0.0100%. When contained, the content of REM is to be 0.0100% or less.
A preferable lower limit of the content of REM is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0010%.
A preferable upper limit of the content of REM is 0.0090%, more preferably is 0.0070%, and further preferably is 0.0055%.
The term “REM” as used in the present description means one or more kinds of element selected from the group consisting of scandium (Sc) which is the element with atomic number 21, yttrium (Y) which is the element with atomic number 39, and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 that are lanthanoids. Further, in the present description the term “content of REM” refers to the total content of these elements.
The chemical composition of the steel material of the present embodiment may further contain the above-described fourth group in lieu of a part of Fe. These elements are optional elements, and each of these elements suppresses decarburization of the steel material. Hereunder, each element of the fourth group is described.
Cobalt (Co) is an optional element, and does not have to be contained. That is, the content of Co may be 0%.
When contained, that is, when the content of Co is more than 0%, Co suppresses decarburization of the steel material during hot working. If even a small amount of Co is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Co is more than 0.0100%, hot working cracks will easily occur in the steel material even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of Co is to be 0 to 0.0100%. When contained, the content of Co is to be 0.0100% or less.
A preferable lower limit of the content of Co is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0010%.
A preferable upper limit of the content of Co is 0.0090%, more preferably is 0.0080%, and further preferably is 0.0070%.
Selenium (Se) is an optional element, and does not have to be contained. That is, the content of Se may be 0%.
When contained, that is, when the content of Se is more than 0%, Se suppresses decarburization of the steel material during hot working. If even a small amount of Se is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Se is more than 0.0100%, hot working cracks will easily occur in the steel material even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of Se is to be 0 to 0.0100%. When contained, the content of Se is to be 0.0100% or less.
A preferable lower limit of the content of Se is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0010%.
A preferable upper limit of the content of Se is 0.0090%, more preferably is 0.0080%, and further preferably is 0.0070%.
Antimony (Sb) is an optional element, and does not have to be contained. That is, the content of Sb may be 0%.
When contained, that is, when the content of Sb is more than 0%, Sb suppresses decarburization of the steel material during hot working. If even a small amount of Sb is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Sb is more than 0.0100%, hot working cracks will easily occur in the steel material even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of Sb is to be 0 to 0.0100%. When contained, the content of Sb is to be 0.0100% or less.
A preferable lower limit of the content of Sb is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0010%.
A preferable upper limit of the content of Sb is 0.0090%, more preferably is 0.0080%, and further preferably is 0.0070%.
Indium (In) is an optional element, and does not have to be contained. That is, the content of In may be 0%.
When contained, that is, when the content of In is more than 0%, In suppresses decarburization of the steel material during hot working. If even a small amount of In is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of In is more than 0.0100%, hot working cracks will easily occur in the steel material even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of In is to be 0 to 0.0100%. When contained, the content of In is to be 0.0100% or less.
A preferable lower limit of the content of In is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0010%.
A preferable upper limit of the content of In is 0.0090%, more preferably is 0.0080%, and further preferably is 0.0070%.
The chemical composition of the steel material of the present embodiment may further contain the above-described fifth group in lieu of a part of Fe. These elements are optional elements, and each of these elements increases the fatigue strength of the component for machine structural use. Hereunder, each element of the fifth group is described.
Vanadium (V) is an optional element, and does not have to be contained. That is, the content of V may be 0%.
When contained, that is, when the content of V is more than 0%, V forms precipitates and increases the fatigue strength of the component for machine structural use. In addition, V combines with C and thereby fixes C in the austenite grains. Hence, V suppresses the occurrence of melting cracks during induction hardening. If even a small amount of V is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of V is more than 0.200%, the hardness of the steel material will excessively increase even if the contents of other elements are within the range of the present embodiment. As a result, the machinability of the steel material will decrease.
Therefore, the content of V is to be 0 to 0.200%. When contained, the content of V is to be 0.200% or less.
A preferable lower limit of the content of Vis 0.001%, more preferably is 0.005%, further preferably is 0.010%, and further preferably is 0.015%.
A preferable upper limit of the content of Vis 0.195%, more preferably is 0.190%, further preferably is 0.185%, and further preferably is 0.150%.
Molybdenum (Mo) is an optional element, and does not have to be contained. That is, the content of Mo may be 0%.
When contained, that is, when the content of Mo is more than 0%, Mo increases the fatigue strength of the component for machine structural use. If even a small amount of Mo is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Mo is more than 1.00%, the hardness of the steel material will excessively increase even if the contents of other elements are within the range of the present embodiment. As a result, the hot working property of the steel material will decrease.
Therefore, the content of Mo is to be 0 to 1.00%. When contained, the content of Mo is to be 1.00% or less.
A preferable lower limit of the content of Mo is 0.01%, more preferably is 0.05%, and further preferably is 0.10%.
A preferable upper limit of the content of Mo is 0.90%, more preferably is 0.80%, further preferably is 0.60%, and further preferably is 0.40%.
Copper (Cu) is an optional element, and does not have to be contained. That is, the content of Cu may be 0%.
When contained, that is, when the content of Cu is more than 0%, Cu increases the fatigue strength of the component for machine structural use. If even a small amount of Cu is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, similarly to Si, Cu promotes the occurrence of melting cracks during induction hardening. Therefore, if the content of Cu is more than 0.20%, melting cracks will easily occur during induction hardening even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of Cu is to be 0 to 0.20%. When contained, the content of Cu is to be 0.20% or less.
A preferable lower limit of the content of Cu is 0.01%, more preferably is 0.02%, and further preferably is 0.03%.
A preferable upper limit of the content of Cu is 0.15%, more preferably is 0.13%, and further preferably is 0.10%.
Nickel (Ni) is an optional element, and does not have to be contained. That is, the content of Ni may be 0%.
When contained, that is, when the content of Ni is more than 0%, Ni increases the fatigue strength of the component for machine structural use. If even a small amount of Ni is contained, the aforementioned advantageous effect will be obtained to a certain extent.
However, similarly to Si and Cu, Ni promotes the occurrence of melting cracks during induction hardening. Therefore, if the content of Ni is more than 0.20%, melting cracks will easily occur during induction hardening even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of Ni is to be 0 to 0.20%. When contained, the content of Ni is to be 0.20% or less.
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 0.15%, more preferably is 0.13%, and further preferably is 0.10%.
The chemical composition of the steel material of the present embodiment can be measured by a well-known composition analysis method in accordance with JIS G0321: 2017. Specifically, a machined chip is collected from a position that is at a depth of 1 mm or more on the inner side of the steel material from the surface using a drill. The collected machined chip is dissolved in acid to obtain a liquid solution. The liquid solution is subjected to ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) to perform elementary analysis of the chemical composition. The content of C and the content of S are determined by a well-known high-frequency combustion method (combustion-infrared absorption method). The content of N is determined using a well-known inert gas fusion-thermal conductivity method. The content of O is determined using a well-known inert gas fusion-infrared absorption method.
Note that, the content of each element is taken as a numerical value up to the least significant digit of the content of each element defined in the present embodiment that is obtained by rounding off a fraction of the measured numerical value based on the significant figures defined in the present embodiment. For example, the content of C in the steel material of the present embodiment is defined as a numerical value up to the second decimal place. Therefore, the content of C is taken as a numerical value up to the second decimal place that is obtained by rounding off the third decimal place of the measured numerical value.
Similarly, for the content of each element other than the content of C in the steel material of the present embodiment also, a value obtained by rounding off a fraction of the numerical value of the measured value up to the least significant digit defined in the present embodiment is taken as the content of the relevant element.
Note that, the term “rounding off” means rounding down if the fraction is less than 5, and rounding up if the fraction is 5 or more.
On the precondition of the content of each element being within the range described above, the steel material of the present embodiment also satisfies Formula (1).
Here, a content in percent by mass of a corresponding element is substituted for each symbol of an element in the formula. If an element is not contained, “0” is substituted for the corresponding symbol of an element. That is, in a case where V that is an optional element is not contained, Formula (1) is as follows.
Let fn1 be defined as follows.
Note that, in a case where V that is an optional element is not contained, fn1 is as follows.
fn1 is an index of the hardness of the steel material. C, Si, Mn, Cr and V increase the internal hardness of the component for machine structural use that is produced using the steel material as a starting material. On the other hand, S reduces the internal hardness of the component for machine structural use.
Even when the content of each element in the steel material is within the range of the present embodiment, if fn1 is less than 0.25, the internal hardness of the component for machine structural use will excessively decrease. As a result, the fatigue strength of the component for machine structural use will decrease. On the other hand, even when the content of each element in the steel material is within the range of the present embodiment, if fn1 is more than 1.00, the hardness of the steel material will excessively increase. As a result, the machinability of the steel material will decrease.
Therefore, fn1 is to be within the range of 0.25 to 1.00.
A preferable lower limit of fn1 is 0.28, more preferably is 0.30, and further preferably is 0.33.
A preferable upper limit of fn1 is 0.98, more preferably is 0.95, and further preferably is 0.90.
In the steel material of the present embodiment, on the precondition that the steel material satisfies Characteristic 1 and Characteristic 2, the number density of fine Bi particles having an equivalent circular diameter of 0.1 to 1.0 μm (hereinafter, also referred to as simply “fine Bi particles”) is 80 to 8000 pieces/mm2. When the number density of fine Bi particles is 80 to 8000 pieces/mm2, the occurrence of melting cracks during induction hardening is suppressed.
Bi is present in the steel material in the form of particles of Bi alone or particles that contain Bi. In the present description, particles of Bi alone and particles that contain Bi are collectively defined as “Bi particles”. In the present description, the term “fine Bi particles” refers to Bi particles having an equivalent circular diameter of 0.1 to 1.0 μm. Because Bi is a heavy element, Bi particles are observed with higher brightness than their surroundings in a backscattered electron image. The respective fine Bi particles may be present independently in the steel material without contacting other particles (precipitates or inclusions). Further, the fine Bi particles may be present in the steel material in a state in which the fine Bi particles adhere to or contact other particles.
As described above, Bi particles pin the austenite grain boundaries during high frequency induction heating. If the equivalent circular diameter of the Bi particles is 0.1 to 1.0 μm, the effect of pinning the austenite grain boundaries increases. If the austenite grains are kept fine during induction hardening, the grain boundary area of the austenite grains will increase. As the grain boundary area increases, the concentration of C segregating to the grain boundaries decreases. As a result, the occurrence of melting cracks is suppressed. Even when the steel material satisfies Characteristic 1 and Characteristic 2, and also satisfies Characteristic 4 which is described later, if the number density of fine Bi particles is less than 80 pieces/mm2, the aforementioned advantageous effect will not be sufficiently obtained.
On the other hand, even when the steel material satisfies Characteristic 1, Characteristic 2, and Characteristic 4, if the number density of fine Bi particles is more than 8000 pieces/mm2, the aforementioned effect will be saturated and, in addition, the production cost will increase.
Therefore, in the steel material of the present embodiment, the number density of fine Bi particles having an equivalent circular diameter of 0.1 to 1.0 μm is 80 to 8000 pieces/mm2.
A preferable lower limit of the number density of fine Bi particles is 90 pieces/mm2, more preferably is 95 pieces/mm2, and further preferably is 100 pieces/mm2. A preferable upper limit of the number density of fine Bi particles is 7900 pieces/mm2, more preferably is 6000 pieces/mm2, further preferably is 3000 pieces/mm2, further preferably is 1000 pieces/mm2, further preferably is 900 pieces/mm2, and further preferably is 800 pieces/mm2.
In the steel material of the present embodiment, the number density of coarse Bi particles that are Bi particles having an equivalent circular diameter of 10.0 μm or more (hereunder, also referred to simply as “coarse Bi particles”) is 10 pieces/mm2 or less. When the number density of coarse Bi particles is 10 pieces/mm2 or less, cracks during hot working (hot working cracks) in the process of producing the steel material, and cracks during hot working in the process for producing the component for machine structural use that uses the steel material as a starting material can be suppressed. The hot working is, for example, hot rolling, hot forging, or the like.
In the present description, the term “coarse Bi particle” refers to a Bi particle that has an equivalent circular diameter of 10.0 μm or more. If the equivalent circular diameter of a particle measured by a method for measuring the number density of coarse Bi particles that is described later is 10.0 μm or more, and the relevant particle is observed with a higher brightness than its surroundings in a backscattered electron image, it is determined that the particle is a coarse Bi particle.
The respective coarse Bi particles may be present independently in the steel material without contacting other particles (precipitates or inclusions). Further, the coarse Bi particles may be present in the steel material in a state in which the coarse Bi particles adhere to or contact other particles. Although an upper limit of the equivalent circular diameter of the coarse Bi particles is not particularly limited, in the case of the chemical composition of the present embodiment, the upper limit of the equivalent circular diameter of the coarse Bi particles is 50.0 μm.
As described above, the occurrence of melting cracks during induction hardening is suppressed by fine Bi particles in the steel material. However, in some cases the Bi in the steel material forms coarse Bi particles, and not fine Bi particles. The coarse Bi particles can become starting points for hot working cracks in the steel material.
Even when the steel material satisfies Characteristic 1 to Characteristic 3, if the number density of coarse Bi particles is more than 10 pieces/mm2, in some cases hot working cracks may occur in the steel material. Therefore, in the steel material of the present embodiment, the number density of coarse Bi particles having an equivalent circular diameter of 10.0 μm or more is 10 pieces/mm2 or less. A preferable upper limit of the number density of coarse Bi particles is 8 pieces/mm2, more preferably is 7 pieces/mm2, further preferably is 6 pieces/mm2, and further preferably is 5 pieces/mm2.
The number density of coarse Bi particles is preferably as low as possible. In other words, preferably the number density of coarse Bi particles is 0 pieces/mm2. However, excessively reducing the number density of coarse Bi particles will raise the production cost. Therefore, when ordinary industrial productivity is taken into consideration, a preferable lower limit of the number density of coarse Bi particles is 1 piece/mm2, and more preferably is 2 pieces/mm2.
Note that, intermediate Bi particles of more than 1.0 μm to less than 10.0 μm (hereunder, also referred to simply as “intermediate Bi particles”) may also be present in the steel material of the present embodiment, and not just the aforementioned fine Bi particles and coarse Bi particles. However, the intermediate Bi particles do not affect hot working cracks and melting cracks during induction hardening. Therefore, the intermediate Bi particles do not need to be taken into consideration with regard to suppressing hot working cracks and suppressing melting cracks.
The number densities of fine Bi particles and coarse Bi particles can be measured by the following method.
A test specimen including an R/2 portion is taken from a cross section perpendicular to an axial direction (rolling direction) of the steel material (steel bar). Here, the term “R/2 portion” refers to a central part of a radius R in a cross section perpendicular to the axial direction of the steel material. Among the surfaces of the test specimen that is taken, a surface corresponding to the aforementioned cross section perpendicular to the axial direction of the steel material is adopted as an observation surface. The observation surface is mirror-polished. An R/2 portion of the observation surface after the mirror-polishing is observed in 20 visual fields using a scanning electron microscope (SEM) at a magnification of 1000×. The area of each visual field is set to 100 μm×120 μm.
Based on a backscattered electron image of each visual field obtained by the SEM observation, the number density of coarse Bi particles and the number density of fine Bi particles are investigated using a well-known particle analysis method that uses image analysis. Specifically, particles in the steel material are identified based on the interface between a parent phase and the particles in the steel material. Here, the term “particles” refers to inclusions and precipitates. Image analysis is performed to determine the equivalent circular diameter of the identified particles. Specifically, the area of each identified particle is determined. The diameter of a circle having the same area as the determined area is defined as the equivalent circular diameter (μm) of the relevant particle.
Since Bi is a heavy element, Bi particles are observed with a high brightness in a backscattered electron image. Therefore, among the particles observed in the backscattered electron images obtained by the aforementioned SEM observation, particles which have an equivalent circular diameter of 0.1 to 1.0 μm and which are observed with a higher brightness than their surroundings are identified as fine Bi particles. Further, among the particles observed in the backscattered electron images obtained by the SEM observation, particles which have an equivalent circular diameter of 10.0 μm or more and which are observed with a higher brightness than their surroundings are identified as coarse Bi particles.
Note that, in Examples that are described later, as a result of using an energy dispersive X-ray spectroscope (EDX) attached to a SEM to analyze the chemical composition of fine Bi particles and coarse Bi particles that were identified by the above-described method, it could be confirmed that the fine Bi particles and coarse Bi particles were each Bi particles. Note that, a beam diameter of the EDX used when confirming the chemical composition was 0.1 to 1.0 μm.
Fine Bi particles and coarse Bi particles are identified by the method described above. The number of fine Bi particles per unit area (pieces/mm2) is determined based on the total number of fine Bi particles identified in the respective visual fields and the total area (0.24 mm2) of the 20 visual fields. Further, the number of coarse Bi particles per unit area (pieces/mm2) is determined based on the total number of coarse Bi particles identified in the respective visual fields and the total area (0.24 mm2) of the 20 visual fields.
As described above, the steel material of the present embodiment satisfies Characteristic 1 to Characteristic 4. Therefore, the steel material of the present embodiment is excellent in machinability, can suppress cracks during hot working and melting cracks during induction hardening, and has excellent fatigue strength when the steel material is formed into the component for machine structural use.
The steel material of the present embodiment, for example, can be widely applied as a starting material for the component for machine structural use. In particular, the steel material of the present embodiment is suitable for a case where induction hardening is performed in the process for producing the component for machine structural use. However, even in a case where induction hardening will not be performed, the steel material of the present embodiment can be applied as a starting material for the component for machine structural use.
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 according to 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 preferred example of a method for producing the steel material according to the present embodiment.
One example of a method for producing the steel material according to the present embodiment includes the following steps.
Note that, the hot working process is an optional step. Hereunder, each step is described.
In the refining process, a molten steel having the chemical composition that satisfies the aforementioned Characteristic 1 and Characteristic 2 is produced. The refining process includes a primary refining process and a secondary refining process.
In the primary refining process, molten iron produced by a well-known method is subjected to refining using a converter. In the secondary refining process, alloying elements are added to the molten steel so as to make the chemical composition of the molten steel satisfy Characteristic 1 and Characteristic 2. Specifically, in the secondary refining process, adjustment of the composition of the molten steel other than Bi is performed while stirring the molten steel by a well-known refining method. Thereafter, while stirring the molten steel, Bi is added to the molten steel using wire and adjustment of the content of Bi is performed.
In the secondary refining process, the following condition is satisfied.
After Bi is added to the molten steel, a time period T until the end of stirring in the secondary refining process is to be more than 15 minutes and less than 60 minutes.
In the secondary refining process, after adding Bi, the time period until the end of stirring in the secondary refining process is to be within a range of more than 15 minutes to less than 60 minutes. After adding Bi, if the time period until the end of stirring in the secondary refining process is 15 minutes or less, Bi will not sufficiently diffuse in the molten steel. In such a case, an excessively large amount of coarse Bi particles will form in the steel material. After adding Bi, if the time period until the end of stirring in the secondary refining process is 60 minutes or more, fine Bi particles will be liable to agglomerate together. Consequently, the number density of fine Bi particles will decrease.
In the secondary refining process, after adding Bi, if the time period until the end of stirring in the secondary refining process is more than 15 minutes, Bi will sufficiently diffuse in the molten steel. Therefore, fine Bi particles will sufficiently form in the steel material. In addition, in the secondary refining process, after adding Bi, if the time period until the end of stirring in the secondary refining process is less than 60 minutes, the agglomeration of fine Bi particles can be sufficiently suppressed. Therefore, the number density of fine Bi particles will be 80 pieces/mm2 or more, and the number density of coarse Bi particles will be 10 pieces/mm2 or less.
A preferable upper limit of the time period until the end of stirring in the secondary refining process after adding Bi to the molten steel is 50 minutes, and more preferably is 40 minutes. A preferable lower limit of the time period until the end of stirring in the secondary refining process after adding Bi to the molten steel is 20 minutes, and more preferably is 30 minutes.
Note that, after adding Bi, a temperature of the molten steel until the end of stirring in the secondary refining process is to be 1510 to 1560° C.
In the casting process, a cast piece (slab or bloom) or an ingot is produced by a well-known casting method using the molten steel. The casting method is, for example, a continuous casting process or an ingot-making process.
The hot working process is an optional step. That is, the hot working process may be performed or need not be performed.
In the case of performing the hot working process, in the hot working process the cast piece or ingot produced in the aforementioned casting process is subjected to hot working to produce the steel material of the present embodiment. The steel material of the present embodiment is, for example, a steel bar. The hot working process may be, for example, hot rolling or may be hot forging.
In the case of performing hot rolling in the hot working process, for example, only a rough rolling process may be performed, or the rough rolling process and a finish rolling process may be performed. The rough rolling process is, for example, blooming. The finish rolling process is, for example, finish rolling using a continuous mill. In the continuous mill, for example, horizontal stands each of which has a pair of horizontal rolls and vertical stands each of which has a pair of vertical rolls are arranged alternately in a row. A heating temperature in the rough rolling process and the finish rolling process is, for example, 1000 to 1300° C.
The steel material of the present embodiment is produced by the production process described above. As mentioned above, in the present production method, the hot working process may be omitted. That is, the steel material of the present embodiment may be a cast product (a cast piece or an ingot). Further, the steel material of the present embodiment may be produced by also performing the hot working process.
As described above, the steel material of the present embodiment serves as a starting material for the component for machine structural use. The component for machine structural use is, for example, a component for use in an automobile. The component for machine structural use is, for example, a suspension component, an axle, or a crankshaft.
The component for machine structural use which uses the steel material of the present embodiment as a starting material is produced, for example, by the following well-known production method.
First, the steel material of the present embodiment is subjected to hot working to produce an intermediate product having a rough shape of the component for machine structural use. The hot working is, for example, hot forging. The produced intermediate product is cut into a predetermined shape by machining. The intermediate product after performing the cutting is subjected to induction hardening and tempering. The component for machine structural use is produced by the above process.
In the steel material of the present embodiment, the content of each element in the chemical composition is within the range of the present embodiment and satisfies Formula (1). In addition, the number density of fine Bi particles having an equivalent circular diameter of 0.1 to 1.0 μm is 80 to 8000 pieces/mm2, and the number density of coarse Bi particles having an equivalent circular diameter of 10.0 μm or more is 10 pieces/mm2 or less. That is, the steel material of the present embodiment satisfies Characteristic 1 to Characteristic 4. Therefore, the steel material of the present embodiment has excellent machinability. In addition, excellent fatigue strength is obtained in the component for machine structural use produced using the steel material of the present embodiment as a starting material. Further, hot working cracks are suppressed in the process for producing the steel material, and in the process for producing the component for machine structural use. In addition, when producing the component for machine structural use using the steel material of the present embodiment as a starting material, melting cracks are suppressed even if induction hardening is performed.
Hereunder, the advantageous effects of the steel material of the present embodiment are described more specifically by using 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 having the chemical compositions shown in Table 1 to Table 4 were produced. Table 2 is a continuation of Table 1, and Table 4 is a continuation of Table 3. Note that, in each test number, the content of O (oxygen) was 0.0050% by mass or less. Further, the total content of Ta and Zn was 0 to 0.01% by mass. In addition, the content of Pb was 0 to 0.09% by mass.
Specifically, the refining process (primary refining process and secondary refining process) was performed using a 70-ton converter. In the primary refining process, molten irons produced by a well-known method were subjected to refining under the same conditions in the converter. In the secondary refining process, elements were added to produce molten steels having the chemical compositions shown in Table 1 to Table 4. Specifically, secondary refining was performed by a well-known method, and adjustment of the composition of the elements other than Bi was performed while stirring the molten steel. Thereafter, while stirring the molten steel, Bi was further added using wire, and adjustment of the content of Bi in the molten steel was performed.
A time period T (min) until the end of stirring in the secondary refining after adding Bi to the molten steel was as shown in Table 5 and Table 6. Note that, after Bi was added to the molten steel, the molten steel temperature until the end of stirring in the secondary refining was 1510 to 1560° C. Molten steels having the chemical compositions shown in Table 1 to Table 4 were produced by the above process.
Each molten steel was used to produce a cast piece (bloom) having a transverse section of 300 mm×400 mm by a continuous casting process. After heating the cast piece, the cast piece was subjected to blooming to produce a billet having a transverse section of 180 mm×180 mm. Note that, a heating temperature of the cast piece during blooming was 1250° C.
In addition, hot forging corresponding to finish rolling was performed on the billet to produce a steel material (steel bar) with a diameter of 80 mm. Note that, a heating temperature of the billet during the hot forging was 1250° C. Steel materials were produced by the above production process.
The steel material of each test number was subjected to the following evaluation tests.
Each evaluation test is described hereunder.
The chemical composition of the steel material of each test number was measured by the well-known composition analysis method described in the foregoing [Method for measuring chemical composition of steel material]. As a result, it was found that the chemical compositions of the steel materials of the respective test numbers were as described in Table 1 to Table 4.
A surface of the produced steel material of each test number was observed by visual observation. As a result of the observation by visual observation, if three or more clear cracks in the surface of the steel material were observed per meter in a longitudinal direction of the steel material, it was determined that hot working cracks had occurred. As the result of the observation by visual observation, if three or more clear cracks in the surface of the steel material were not observed per meter in the longitudinal direction of the steel material, it was determined that hot working cracks were suppressed.
Results of the evaluation of hot working cracks are shown in the column “Hot Working Cracks” in Table 5 and Table 6. A case where hot working cracks were suppressed is indicated by “E” (excellent). A case where hot working cracks occurred is indicated by “NA” (Not Accepted).
A heat treatment that simulated hot forging during the process for producing the component for machine structural use using the steel material of each test number as a starting material was performed. Specifically, the steel material was heated, and the steel material was held at 1100° C. for 30 minutes. Thereafter, the steel material was allowed to cool in the air. Hereinafter, the steel material that was subjected to the foregoing heat treatment is referred to as a “simulated intermediate product of the component for machine structural use (or simply a “simulated intermediate product”). The simulated intermediate product of the component for machine structural use was a steel bar with a diameter of 80 mm.
The steel material of each test number was used to determine the number density of fine Bi particles (pieces/mm2) and the number density of coarse Bi particles (pieces/mm2) of the steel material of each test number based on the method described in the foregoing [Method for measuring number densities of fine Bi particles and coarse Bi particles]. A test specimen including an R/2 portion was taken from a cross section (transverse section) perpendicular to the axial direction (rolling direction) of the steel material of each test number. The number density of fine Bi particles (pieces/mm2) and the number density of coarse Bi particles (pieces/mm2) of each test number were determined using these test specimens. Results obtained for the number density of fine Bi particles are shown in the column “Fine Bi Particles Number Density (pieces/mm2)” of the column “Steel Material” in Table 5 and Table 6. Results obtained for the number density of coarse Bi particles are shown in the column “Coarse Bi Particles Number Density (pieces/mm2)” of the column “Steel Material” in Table 5 and Table 6.
Using the simulated intermediate product of the component for machine structural use of each test number, the number density of fine Bi particles (pieces/mm2) and the number density of coarse Bi particles (pieces/mm2) of each test number were determined based on the method described in the foregoing [Method for measuring number densities of fine Bi particles and coarse Bi particles]. A test specimen including an R/2 portion was taken from a cross section (transverse section) perpendicular to the axial direction (rolling direction) of the simulated intermediate product (steel bar) of each test number. The number density of fine Bi particles (pieces/mm2) and the number density of coarse Bi particles (pieces/mm2) of each test number were determined using these test specimens. Results obtained for the number density of fine Bi particles are shown in the column “Fine Bi Particles Number Density (pieces/mm2)” of the column “Simulated Intermediate Product” in Table 5 and Table 6. Results obtained for the number density of coarse Bi particles are shown in the column “Coarse Bi Particles Number Density (pieces/mm2)” of the column “Simulated Intermediate Product” in Table 5 and Table 6.
Note that, each simulated intermediate product of the component for machine structural use was produced by subjecting the steel material that was the starting material to a heat treatment that simulated hot forging. Performing a heat treatment at 1100° C. that simulates the hot forging does not affect the number density of fine Bi particles and the number density of coarse Bi particles. Therefore, the number density of fine Bi particles and the number density of coarse Bi particles in the simulated intermediate product of the component for machine structural use are substantially the same as the number density of fine Bi particles and the number density of coarse Bi particles in the steel material.
A test specimen having a width of 10 mm, a thickness of 3 mm, and a length of 10 mm was taken from an R/2 portion of a cross-section perpendicular to the axial direction (rolling direction) of each simulated intermediate product of the component for machine structural use. The longitudinal direction of the test specimen was parallel to the axial direction (rolling direction) of the simulated intermediate product of the component for machine structural use. Further, a central axis parallel to the longitudinal direction of the test specimen coincided with the R/2 portion.
The test specimen was subjected to a simulation test of induction hardening using a thermal cycle testing device manufactured by Fuji Dempa Kogyo Co., Ltd. Specifically, a high-frequency coil was used to heat the test specimen to 1390° C. at a heating rate of 100° C./sec. The test specimen was then held at 1390° C. for 15 seconds. Thereafter, the test specimen was subjected to water cooling.
After the test specimen was water-cooled, the test specimen was cut in a direction perpendicular to the longitudinal direction at a center position in the longitudinal direction of the test specimen. The cut surface was adopted as an observation surface. The observation surface was subjected to mechanical polishing. After undergoing the mechanical polishing, the observation surface was etched with a picral reagent. The etched observation surface was observed under an optical microscope with a magnification of 400×, and the presence or absence of melting cracks was checked by visual observation. Two visual fields were set as observation visual fields. The area of each observation visual field was 250 μm×400 μm.
In a case where a region (corroded region) having a width of 5 μm or more that was clearly corroded at a grain boundary was observed in at least one of the two observation visual fields of the observation surface, it was determined that a melting crack occurred. The phrase “region having a width of 5 μm or more that was clearly corroded at a grain boundary” refers to a region in which a maximum width is 5 μm or more such as, for example, as illustrated in
Results of the evaluation of melting cracks are shown in the column “Melting Cracks” in Table 5 and Table 6. A case where melting cracks were suppressed is indicated by “E”. A case where melting cracks occurred is indicated by “NA”. The symbol “-” in the column “Melting Cracks” in Table 6 means that the melting cracks evaluation test was not performed.
A test specimen for a machinability evaluation test was cut out from the simulated intermediate product of the component for machine structural use. Specifically, a drill was used to drill a hole in a cross section perpendicular to the axial direction (rolling direction) of the simulated intermediate product having a diameter of 80 mm, at a position at a depth of 21 mm in a radial direction from an outer surface thereof. A drill with the model number SD 3.0 manufactured by Nachi-Fujikoshi Corp. was used as the tool. As a drilling condition, a feed was set to 0.25 mm/rev. Further, a drilling depth of a hole was set to 9 mm. During drilling, a water-soluble cutting oil was continuously supplied as a lubricant to a drilling location.
Drilling was performed under the aforementioned drilling conditions, and the machinability of the steel material was evaluated. A maximum cutting velocity VL1000 (m/min) was used as an evaluation index. The term “maximum cutting velocity VL1000” refers to a fastest cutting velocity of the drill at which drilling of a hole of 1000 mm in length is possible.
In a case where the maximum cutting velocity VL1000 was 35 m/min or more, it was determined that excellent machinability was obtained (indicated by “E” in the column “Machinability” in Table 5 and Table 6). On the other hand, in a case where the maximum cutting velocity VL1000 was less than 35 m/min, it was determined that sufficient machinability was not obtained (indicated by “NA” in the column “Machinability” in Table 5 and Table 6). Note that, the symbol “-” in the column “Machinability” in Table 6 means that the machinability evaluation test was not performed.
Fatigue strength was evaluated by the following test method using a fatigue test specimen assuming the component for machine structural use produced using the steel material as a starting material.
A rotating bending fatigue test specimen was collected from the simulated intermediate product of the component for machine structural use.
The fatigue test specimen was a round bar specimen having a parallel portion with a diameter of 8 mm, and a grip portion with a diameter of 12 mm. A longitudinal direction of the fatigue test specimen was parallel to the axial direction of the simulated intermediate product. Specifically, the simulated intermediate product was cut to a depth of 3.5 mm from the surface thereof by turning to create the parallel portion. Therefore, a surface of the parallel portion was found to be at least within a depth range of 5 mm from the surface of the steel bar. The fatigue test specimen was assumed to be an equivalent of the component for machine structural use after performing a cutting process on the intermediate product after hot working in the process for producing the component for machine structural use using the steel material. The parallel portion of the fatigue test specimen was subjected to finishing polishing to adjust a surface roughness. Specifically, in conformity with JIS B 0601 (2001), polishing was performed so that a center line average roughness (Ra) of the surface was 3.0 μm or less, and a maximum height (Rmax) was 9.0 μm or less.
Note that, it is common technical knowledge which is well known by those skilled in the art that if the fatigue strength is sufficiently high in a test using the rotating bending fatigue test specimen collected from the simulated intermediate product of the component for machine structural use prior to induction hardening, excellent fatigue strength will also be obtained in the component for machine structural use after induction hardening is performed. Therefore, an Ono type rotating bending fatigue test was performed using the fatigue test specimen at room temperature (23° C.) in atmospheric air under a condition of fully-reversed tension-compression at a rotational speed of 3600 rpm. The fatigue test was performed on a plurality of test specimens in which the applied stress was changed, and the highest stress that did not result in rupture of the test specimen after 107 cycles was adopted as the fatigue strength (MPa).
If the obtained fatigue strength was 230 MPa or more, it was determined that sufficient fatigue strength was obtained. Results of the fatigue strength evaluation are shown in the column “Fatigue Strength” in Table 5 and Table 6. In a case where the fatigue strength was 230 MPa or more, it was determined that excellent fatigue strength was obtained (indicated by “E”). On the other hand, in a case where the fatigue strength was less than 230 MPa, it was determined that sufficient fatigue strength was not obtained (indicated by “NA”). Note that, the symbol “-” in the column “Fatigue Strength” in Table 6 means that the fatigue strength evaluation test was not performed.
Referring to Table 1 to Table 6, the steel materials of Test Nos. 1 to 46 satisfied Characteristic 1 to Characteristic 4. Therefore, hot working cracks and melting cracks were sufficiently suppressed. In addition, the maximum cutting velocity VL1000 was 35 m/min or more, and thus excellent machinability was obtained. Furthermore, the fatigue strength was 230 MPa or more, and thus excellent fatigue strength was obtained.
On the other hand, in Test No. 47, the content of C was too high. Therefore, melting cracks occurred.
In Test No. 48, the content of C was too low. Therefore, the fatigue strength was low.
In Test No. 49, the content of Si was too high. Therefore, melting cracks occurred.
In Test No. 50, the content of Mn was too high. Therefore, the machinability was low.
In Test No. 51, the content of Mn was too low. Therefore, melting cracks occurred.
In Test No. 52, the content of P was too high. Therefore, melting cracks occurred.
In Test No. 53, the content of S was too high. Therefore, melting cracks occurred.
In Test No. 54, the content of S was too low. Therefore, the machinability was low.
In Test No. 55, the content of Cr was too high. Therefore, the machinability was low
In Test No. 56, the content of Bi was too high. Consequently, the number density of coarse Bi particles was more than 10 pieces/mm2. Therefore, hot working cracks occurred.
In Test No. 57, the content of Bi was too low. Therefore, the machinability was low. In addition, the number density of fine Bi particles was less than 80 pieces/mm2. As a result, melting cracks occurred.
In Test No. 58, the content of N was too high. Therefore, hot working cracks occurred.
In Test Nos. 59 and 60, the value of fn1 was too high. That is, fn1 did not satisfy Formula (1). Therefore, sufficient machinability was not obtained.
In Test Nos. 61 and 62, the value of fn1 was too low. That is, fn1 did not satisfy Formula (1). Therefore, sufficient fatigue strength was not obtained.
In Test Nos. 63 to 65, in the refining process, the time period T (min) until the end of stirring after Bi was added was too short. Therefore, the number density of coarse Bi particles was more than 10 pieces/mm2. Therefore, hot working cracks occurred.
In Test Nos. 66 to 68, in the refining process, the time period T (min) until the end of stirring after Bi was added was too long. Therefore, the number density of fine Bi particles was less than 80 pieces/mm2, and thus melting cracks occurred.
The embodiment of the present disclosure has been described above. However, the foregoing embodiment is merely an example for implementing the present disclosure. Accordingly, the present disclosure is not limited to the above embodiment, and the above embodiment can be appropriately modified and implemented within a range that does not deviate from the gist of the present disclosure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-154999 | Sep 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/035434 | 9/22/2022 | WO |
