Steel Material

The present disclosure relates to a steel material, and more particularly to a steel material that will serve as a starting material for a component for machine structural use.

Components for machine structural use are used as automobile parts, such as crankshafts for automobiles and construction vehicles. Machine structural parts are required to have high fatigue strength.

A technique for improving the fatigue strength of components for machine structural use is disclosed in, for example, Japanese Patent Application Publication No. 2008-169411 (Patent Literature 1).

The steel material disclosed in Patent Literature 1, used as a material for machine structural parts, contains, in 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. For this steel material, the Larson-Miller Parameter (LMP) that gives the maximum HRC hardness at room temperature is 17.66 or more after the steel material is held at 1010 to 1050° C., cooled to 500 to 550° C. at a cooling rate of 200° C./min or more, subsequently cooled to 150° C. or less at a cooling rate of 100° C./min or more, and then heated in a temperature range of 550 to 700° C. According to this literature, the LMP that gives the maximum HRC hardness at room temperature after heat treatment performed under the above conditions is 17.66 or more to improve softening resistance and thereby improve fatigue properties.

Patent Literature 1: Japanese Patent Application Publication No. 2008-169411

In order to improve the fatigue strength of components for machine structural use, surface hardening treatment may be performed on the components for machine structural use.

One of the various surface hardening treatments is induction hardening. With induction hardening, only the areas that need to be hardened can be hardened. Induction hardening involves heating the steel material at a high temperature and thereafter cooling the steel material to form a hardened layer on the surface thereof. Induction hardening provides a greater depth of a hardened layer and a higher fatigue strength than other surface hardening treatments such as soft nitriding.

The induction hardening treatment applied to a component for machine structural use is described below based on an example in which the component for machine structural use is a crankshaft. When a crankshaft, which is a component for machine structural use, has the shape shown in, induction hardening is applied, for example, to a filleted round portion 1 of the crankshaft to improve the fatigue strength of the crankshaft. In this case, a hardened layer is formed at the surface layer of the filleted round portion 1.

In order to increase the depth of the hardened layer, the output of high frequency power may be increased to raise the heating temperature of induction hardening. However, when induction hardening is performed at a high temperature, the heating temperature tends to become excessively high at edge portions of the component for machine structural use. For example, when the component for machine structural use is the crankshaft shown in, the heating temperature becomes excessively high at edge portions 2. In particular, when the heating rate during induction hardening is high, the heating temperature is likely to become excessively high.

For example, if the heating temperature during induction hardening becomes excessively high and reaches 1350° C. or higher, a portion of the steel material may melt and crack. Such cracks are hereinafter referred to as “melting cracks” in the present description. It is preferable to suppress the occurrence of such melting cracks. That is to say, the steel material is required to have excellent melting cracks resistance.

In addition, hot working is applied to the steel material that will serve as the starting material for the component for machine structural use, during the process for producing the component for machine structural use or during the process for producing the steel material. Therefore, for the steel material, it is desired to suppress the occurrence of cracking caused by hot working (hot working cracks). That is to say, the steel material is required to have excellent hot working cracks resistance.

Furthermore, the steel material that will serve as the starting material for the component for machine structural use is subjected to machining during the process for producing the component for machine structural use. There are cases where excellent machinability is required, especially inside the steel material. For example, when the component for machine structural use is a crankshaft, machining such as drilling is performed on the central portions of both end faces thereof. Therefore, the steel material is required to have excellent machinability inside the steel material.

In the above Patent Literature 1, melting cracks resistance, hot working cracks resistance, and machinability of the steel material are not considered.

An objective of the present disclosure is to provide a steel material that has excellent melting cracks resistance, excellent hot working cracks resistance, and excellent machinability, and that, when served as a starting material for a component for machine structural use, enables the component for machine structural use to have high fatigue strength.

A steel material according to the present disclosure is a steel material whose cross section perpendicular to an axial direction thereof is circular, consisting of, by mass %,

A steel material of the present disclosure has excellent melting cracks resistance, excellent hot working cracks resistance, and excellent machinability, and when served as a starting material for a component for machine structural use, enables the component for machine structural use to have high fatigue strength.

The present inventors first conducted studies regarding a chemical composition of a steel material that, when served as a starting material for a component for machine structural use, would improve the fatigue strength of the component for machine structural use. As a result, the present inventors concluded that the component for machine structural use may have high fatigue strength when made of a steel material having a chemical composition consisting of, in percent by mass, C: more than 0.30 to 0.60%, Si: 0.01 to 0.90%, Mn: 0.50 to 1.70%, P: 0.030% or less, S: 0.200% or less, Al: 0.001 to 0.100%, N: 0.0250% or less, O: 0.0050% or less, Cr: 0 to 1.30%, V: 0 to 0.200%, Sn: 0 to 0.1000%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Pb: 0 to 0.09%, Mg: 0 to 0.0100%, Ti: 0 to 0.0400%, Nb: 0 to 0.0500%, 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%, rare earth metal: 0 to 0.0100%, Co: 0 to 0.0100%, Se: 0 to 0.0100%, In: 0 to 0.0100%, Mo: 0 to 0.30%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, and with the balance being Fe and impurities.

Next, the present inventors conducted studies regarding means to improve the machinability of the steel material in which the content of each element in the chemical composition is within the range described above. As a result, the present inventors discovered that excellent fatigue strength of the component for machine structural use can be obtained and the machinability of the steel material, which is a starting material for the component for machine structural use, can be improved by setting Fn defined in formula (1) to 0.45 to 1.05:

Furthermore, the present inventors conducted studies regarding means to improve the melting cracks resistance of the steel material during induction hardening.

The C content affects the occurrence of melting cracks in the steel material during induction hardening. Specifically, the melting point at the grain boundaries is lowered by the C that segregates at the grain boundaries. As a result, melting cracks are more likely to occur. Therefore, the above chemical composition also contains Bi: 0.0051 to 0.2500%. If the Bi content is within the above range, Bi particles (inclusions) are generated in the steel material. The fine Bi particles have a pinning effect, which suppress the coarsening of crystal grains (austenite grains) in the steel material during induction hardening. If the crystal grains can be kept fine during induction hardening, the reduction of the grain boundary area can be suppressed. If the reduction of the grain boundary area can be suppressed and a certain level of grain boundary area can be ensured, the concentration per unit area of the C that segregates at the grain boundaries decreases. As a result, the occurrence of melting cracks is suppressed.

However, when the steel material contains Bi in the above range, not only fine Bi particles but also coarse Bi particles may be generated. Coarse Bi particles serve as the starting points of hot working cracks. Therefore, excessive coarse Bi particles reduce the hot working cracks resistance of the steel material.

On the other hand, the coarse Bi particles improve the machinability of the steel material. Considering the above, it seems difficult to simultaneously obtain excellent melting cracks resistance, excellent hot working cracks resistance, and excellent machinability by Bi in the above range contained in a steel material having the above chemical composition.

However, the present inventors thought that it would be possible to simultaneously obtain excellent melting cracks resistance, excellent hot working cracks resistance, and excellent machinability by varying the number density of fine Bi particles and the number density of coarse Bi particles depending on the regions in the steel material. Specifically, in the steel material, the region where melting cracks and hot working cracks are likely to occur is the surface layer region of the steel material. Therefore, the surface layer region of the steel material needs to have excellent melting cracks resistance and excellent hot working cracks resistance. On the other hand, in the internal region of the steel material, melting cracks and hot working cracks are less likely to occur. Therefore, the internal region of the steel material only needs to have high machinability.

Based on the above technical concept, the present inventors investigated and studied the relationship between the number density of fine Bi particles and the number density of coarse Bi particles in the surface layer regions and internal regions of the steel material; and the melting cracks resistance, hot working cracks resistance, and machinability of the steel material. As a result, the present inventors have discovered that in a steel material satisfying the above chemical composition and having an Fn of 0.45 to 1.05, excellent melting cracks resistance, excellent hot working cracks resistance, and excellent machinability can be simultaneously satisfied when the number density of fine Bi particles is 15.00 pieces/mmor more and the number density of coarse Bi particles is 0.25 pieces/mmor less in the surface layer region, and the number density of fine Bi particles is less than 15.00 pieces/mmand the number density of coarse Bi particles is more than 0.25 pieces/mmin the internal region, and further, when served as a starting material for a component for machine structural use, such a steel material enables the component for machine structural use to have excellent fatigue strength.

The steel material according to the present embodiment, which has been completed based on the above technical concept, has the following configuration:

[1]

A steel material whose cross section perpendicular to an axial direction thereof is circular, consisting of, by mass %,

The steel material according to [1], containing one or more elements selected from a group consisting of:

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 Feature 1 to Feature 4.

(Feature 1) The chemical composition consists of, in percent by mass, C: more than 0.30 to 0.60%, Si: 0.01 to 0.90%, Mn: 0.50 to 1.70%, P: 0.030% or less, S: 0.200% or less, Bi: 0.0051 to 0.2500%, Al: 0.001 to 0.100%, N: 0.0250% or less, O: 0.0050% or less, Cr: 0 to 1.30%, V: 0 to 0.200%, Sn: 0 to 0.1000%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Pb: 0 to 0.09%, Mg: 0 to 0.0100%, Ti: 0 to 0.0400%, Nb: 0 to 0.0500%, 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%, rare earth metal: 0 to 0.0100%, Co: 0 to 0.0100%, Se: 0 to 0.0100%, In: 0 to 0.0100%, Mo: 0 to 0.30%, Cu: 0 to 0.50%, Ni:0 to 0.50%, and with the balance being Fe and impurities.

Fn defined by formula (1) is 0.45 to 1.05.

At the depth of 0.08R from the surface of the steel material, where R defines the radius of the steel material, the number density of fine Bi particles having an equivalent circular diameter of 0.1 to 1.0 μm, is 15.00 pieces/mmor more, and the number density of coarse Bi particles having an equivalent circular diameter of 10.0 or more, is 0.25 pieces/mmor less.

At the depth of 0.65R from the surface of the steel material, the number density of fine Bi particles is less than 15.00 pieces/mm, and the number density of coarse Bi particles is more than 0.25 pieces/mm.

Hereunder, Feature 1 to Feature 4 are described.

The chemical composition of the steel material of the present embodiment contains the following elements.

C: more than 0.30 to 0.60%

Carbon (C) improves the hardness of the component for machine structural use produced using the steel material, and improves the fatigue strength of the component for machine structural use. If the content of C is 0.30% or less, 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, C lowers the melting point of the steel material. Therefore, if the content of C is more than 0.60%, melting cracks will be likely to occur in the steel material when induction hardening is performed on the steel material during the process for producing the component for machine structural use made of the steel material, even if the contents of other elements are within the range of the present embodiment.

Therefore, the content of C is to be more than 0.30 to 0.60%.

A preferable lower limit of the content of C is 0.31%, more preferably is 0.35%, further preferably is 0.37%, and further preferably is 0.38%.

A preferable upper limit of the content of C is 0.55%, more preferably is 0.50%, and further preferably is 0.45%.

Silicon (Si) deoxidizes the steel in the steelmaking process. Si also improves the hardness of the component for machine structural use produced using the steel material, and improves the fatigue strength of the component for machine structural use. 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, Si has a weak affinity with C. Therefore, if the content of Si is more than 0.90%, C will become more likely to segregate at the grain boundaries during heating than within the grains where Si is dissolved even if the contents of other elements are within the range of the present embodiment. As a result, melting cracks is likely to occur in the steel material when induction hardening is performed on the steel material during the process for producing the component for machine structural use made of the steel material.

Therefore, the content of Si is to be 0.01 to 0.90%.

A preferable lower limit of the content of Si is 0.02%, more preferably is 0.05%, further preferably is 0.08%, and further preferably is 0.10%.

A preferable upper limit of the content of Si is 0.70%, more preferably is 0.65%, further preferably is 0.55%, and further preferably is 0.50%.