Patent Description:
Steel for mechanical structure with high strength and high resistance to delayed fracture has been proposed that also has good recyclability due to the simple composition and does not require a complex heat treatment (see Patent Document <NUM>).

The steel for mechanical structure described in Patent Document <NUM> has a composition in wt% of C: from <NUM>% to <NUM>%, Si: from <NUM> to <NUM>, Mn: from <NUM>% to1. <NUM>%, Cr: from <NUM>% to <NUM>%, Mo from <NUM>% to <NUM>% and balance Fe and inevitable impurities, in which the total amount of alloy elements satisfies Si+Mn+Cr+Mo <_ <NUM> wt%. The steel for mechanical structure is tempered at a temperature within the range of <NUM> to Ae1 in the condition of the tempering parameter λ of λ = T (<NUM>+log t) ≥ <NUM> (T being temperature (K), and t being time (h)). The steel for mechanical structure has a tensile strength of <NUM> MPa or more. Patent Document <NUM> teaches a steel for high-strength bolts which has excellent workability and good delayed fracture resistance and which can attain a tensile strength of 1550MPa or more; and a process for the production of high-strength bolts using the steel. A high-strength bolt that exhibits a tensile strength of 1550MPa or more can be produced by using a specific steel as the raw material and subjecting the specific steel to quenching and tempering. The specific steel contains <NUM> to <NUM> mass% of C, <NUM> to <NUM> mass% of Si, <NUM> to <NUM> mass% of Mn, either <NUM> to <NUM> mass% of Mo and/or <NUM> to <NUM> mass% of W, and, if necessary, either <NUM> to <NUM> mass% of Cr and/or <NUM> to <NUM> mass% of Ni, or at least one selected from the group consisting of <NUM> to <NUM> mass% of V, <NUM> to <NUM> mass% of Ti and <NUM> to <NUM> mass% of Nb. Further, the specific steel has an a value of <NUM> or less as calculated from the contents of C and Si according to the formula: a = <NUM> + <NUM>×C + <NUM>×vSi- <NUM>×C×vSi. Patent Document <NUM> teaches to provide a high tensile bolt, which has a tensile strength of ≥<NUM>,<NUM> MPa, and is excellent in tensile deformation performance and a delayed fracture resistance characteristic, and a method for manufacturing the same. The steel contains by mass: C:<NUM>-<NUM>%, Si:<NUM>-<NUM>%, Mn:<NUM>-<NUM>%, Cr:<NUM>-<NUM>%, Mo:<NUM>-<NUM>%, Al:<NUM>-<NUM>%, and the residue comprising Fe and inevitable impurities is used as a material. The steel is formed in a bolt shape having a thread section shape at the thread section in which an angle of a flank face is set to <NUM>°, the height of fundamental triangle of the thread section is set as H, and a transition point between the flank face and the bottom of the thread facing each other is set as H/<NUM> from a bottom side of the fundamental triangle, and in which an arc-shaped curve is formed, the arc-shaped curve drawing a small abutting circle that abuts on the flank face at the transition point and that has a radius of curvature R of ≥H/<NUM> and ≤H/<NUM>. The bolt-shaped steel is hardened by austenitizing treatment in a temperature range of <NUM>-<NUM>,<NUM>, and then is subjected to tempering treatment in a temperature range of <NUM>-<NUM>. Patent Document <NUM> discloses a high tensile bolt having a tensile strength of <NUM>,<NUM> MPa or more and excellent tensile deformation performance and delayed fracture resistance, and a method of manufacturing the same. The steel contains by mass: <NUM>-<NUM>% C, <NUM>-<NUM>% Si, <NUM>-<NUM>% Mn, <NUM>-<NUM>% Cr, <NUM>-<NUM>% Mo, <NUM>-<NUM>% Al and the balance Fe with inevitable impurities, and is formed into a bolt shape in such a manner that a shape parameter S (=Ab/Ae, wherein, Ab is an effective cross-sectional area in a shaft part <NUM> and Ae is an effective cross-sectional area in a screw part) in the shaft part <NUM>, becomes a value of <NUM> to <<NUM>, and after quenching the steel by austenization in the temperature range of <NUM>-<NUM>,<NUM>, the steel is subjected to a tempering treatment in the temperature range of <NUM>-<NUM>. Patent Document <NUM> teaches to provide a new high-strength steel for a machine structure, which is superior in a recycling property because of having a simple composition, does not require complex processing and heat treatment, and is superior in delayed fracture resistance. The high-strength steel for the machine structure has a composition of, by wt. %: <NUM>-<NUM>% C, <NUM>-<NUM>% Si, <NUM>-<NUM>% Mn, <NUM>-<NUM>% Cr, and <NUM>-<NUM>% Mo, has a total amount of alloy elements, which satisfies Si+Mn+Cr+Mo ≤ <NUM> wt. %, includes the balance Fe with unavoidable impurities, is tempered in a range of <NUM> to Ae1 temperature or lower, and in a condition that a tempering parameter, λ, satisfies λ=T(<NUM>+logt)≥<NUM>,<NUM>, where T indicates a temperature (K), and t indicates a time (h), and has a tensile strength of <NUM>,<NUM> MPa or higher.

However, a problem with a bolt that is formed using the steel for mechanical structure described in Patent Document <NUM> is that it does not sufficiently meet a request for a further improvement in resistance to delayed fracture.

The present invention has been made in view of the problem with the prior art. It is therefore an object of the present invention to provide steel for a high-strength bolt with high resistance to delayed fracture and a high-strength bolt that is formed using the steel.

The present inventors have made a keen study in order to achieve the above-described object. As a result, they found that the above-described object can be achieved by using steel for a high-strength bolt having a predetermined composition. The present invention, which is defined in the appended claims, has been thus completed.

In the present invention, the steel for a high-strength bolt contains from <NUM> mass% to <NUM>. 65mass% carbon, from <NUM> mass% to <NUM>. 5mass% silicon, <NUM> mass% to <NUM> mass% chromium, greater than <NUM> mass% to <NUM> mass% manganese, greater than <NUM> mass% to <NUM> mass% molybdenum, <NUM> mass% or less phosphorus and sulfur combined, and balance iron and inevitable impurities.

Therefore, it is possible to provide the steel for a high-strength bolt with high resistance to delayed fracture and the high-strength bolt that is formed using the steel.

Hereinafter, the steel for a high-strength bolt and the high-strength bolt that is formed using the steel of the present invention will be described in detail.

First, steel for a high-strength bolt according to a first embodiment of the present invention will be described in detail. The steel for a high-strength bolt of the embodiment contains from <NUM> mass% to <NUM> mass% carbon, from <NUM> mass% to <NUM>. 5mass% silicon, <NUM> mass% to <NUM> mass% chromium, greater than <NUM> mass% to <NUM> mass% manganese, greater than <NUM> mass% to <NUM> mass% molybdenum, <NUM> mass% or less phosphorus and sulfur combined, and balance iron and inevitable impurities.

This configuration enables a bolt with high resistance to delayed fracture and high strength. Further, a bolt that is formed using the steel has high resistance to delayed fracture and high strength.

When the carbon content is less than <NUM> mass%, the resistance to temper softening is insufficient. Since high-temperature tempering described below cannot be performed, high resistance to delayed fracture is not achieved.

When the carbon content is greater than <NUM> mass%, the amount of cementite, which traps hydrogen, is remarkably increased. Accordingly, high resistance to delayed fracture is not achieved.

When the silicon content is less than <NUM> mass%, the resistance to temper softening is insufficient. Since high-temperature tempering described below cannot be performed, high resistance to delayed fracture is not achieved.

When the silicon content is greater than <NUM> mass%, the forgeability is remarkably degraded. Accordingly, it is impossible to form a predetermined bolt.

When the chromium content is less than <NUM> mass%, the resistance to temper softening is insufficient. Since high-temperature tempering described below cannot be performed, high resistance to delayed fracture is not achieved. The chromium content is equal to or less than <NUM> mass%.

When the manganese content is greater than <NUM> mass%, intergranular segregation of an intergranular segregating component is promoted, and the intergranular strength is thereby remarkably degraded. Accordingly, high resistance to delayed fracture is not achieved. The manganese content is preferably equal to or greater than <NUM> mass%, although it is not particularly limited as long as it is greater than <NUM> mass%.

When the molybdenum content is less than <NUM> mass%, molybdenum-based carbide, which neutralizes hydrogen, is not produced to a sufficient amount. Accordingly, high resistance to delayed fracture is not achieved. The molybdenum content is <NUM> mass% or less.

When the total content of phosphorus and sulfur is greater than <NUM> mass%, the intergranular strength is remarkably degraded due to intergranular segregation. Accordingly, high resistance to delayed fracture is not achieved. The total content of phosphorus and sulfur is preferably equal to or less than <NUM>%, although it is not particularly limited.

Next, a high-strength bolt according to a second embodiment of the present invention will be described in detail. The high-strength bolt of the embodiment is formed using steel for a high-strength bolt according to an embodiment of the present invention.

With this configuration, high resistance to delayed fracture and high strength are achieved.

For example, the high-strength bolt can be produced by cold-forging and then rolling the previously-described steel for a high-strength bolt, and thereafter performing a heat treatment that involves quenching at <NUM> or more and tempering at <NUM> or more. However, the production is not particularly limited. The high-strength bolt can also be produced even when the order of the rolling and the heat treatment (quenching and tempering) is changed.

For example, the high-temperature tempering at <NUM> or more spheroidizes and finely disperses intergranular cementite, which can improve the intergranular strength.

In the embodiment, it is preferred that the bolt has one or both of iron phosphate coating and chromium plate coating on the surface.

Providing such coating on the surface can further improve the resistance to delayed fracture.

An example of the high-strength bolt of the embodiment will be described in more detail referring to the drawings. <FIG> is a partial cross sectional view of an example of a reciprocating engine with multi-link mechanism. <FIG> is a schematic cross sectional view of an example of a lower link in <FIG>.

As illustrated in <FIG>, the reciprocating engine <NUM> includes an upper link <NUM> coupled to a piston pin <NUM> of a piston <NUM>, a lower link <NUM> that couples the upper link <NUM> to a crankpin <NUM> of a crankshaft <NUM>, and a control link <NUM> that is swingably supported by the engine <NUM> at one end and is coupled to the lower link <NUM> at the other end. The upper link <NUM> and the lower link <NUM> are rotatably coupled to each other via an upper pin <NUM>. The control link <NUM> and the lower link <NUM> are rotatably coupled to each other by a control pin <NUM>.

The lower link <NUM> receives a combustion pressure received by the piston <NUM> from the upper pin <NUM> via the upper link <NUM> and transmits the force to the crankpin <NUM> as a movement pivoted on the control pin <NUM>. Accordingly, a large combustion pressure received by the piston <NUM> and an inertial load of the piston <NUM> are input to the lower link <NUM> from an upper pin bearing 108a via the piston pin <NUM>, the upper link <NUM> and the upper pin <NUM>. At the same time, a crankpin bearing 105a and a control pin bearing 109a are also subjected to such a load that balances the input load. In this way, each of the bearings (108a, 105a, 109a) are subjected to higher interface pressure compared to a typical reciprocating engine with mono-link mechanism.

Therefore, higher strength is required for the lower link. Further, it is also desired to reduce the size and weight of the lower link in terms of improving the fuel efficiency.

As illustrated in <FIG>, the structure of the lower link <NUM> is typically such that split lower link parts 106A, 106B are fastened to each other by high-strength bolts <NUM>. To achieve the above-described performance required for the lower link, high resistance to delayed fracture is required for the high-strength bolts. The high-strength bolt of the embodiment is particularly suitable for fastening such lower link parts, although the usage is not particularly limited. The reference signs 108a, 105a, 109a represent bearings.

Hereinafter, the present invention will be described in more detail with examples.

High-strength bolts of the examples, Reference Examples, and Comparative Examples were produced by cold-forging and then rolling the respective steels for a high-strength bolt having the composition listed in Table <NUM> and thereafter performing a heat treatment that involves quenching at <NUM> or more and tempering at <NUM> or more.

The bolts of the examples, Reference Examples, and Comparative Examples were immersed in hydrochloric acid for a specified time. It was observed whether the bolts were broken. The results are shown in Table <NUM> as the results of Test <NUM>. In the item Test <NUM>, "OK" represents that a bolt was not broken, and "NG" represents that a bolt was broken.

The tensile strength of the bolts of the examples, Reference Examples, and Comparative Example were evaluated by a tensile test. It was confirmed that the bolts of the examples, Reference Examples, and Comparative Examples had a tensile strength of <NUM> MPa or more.

A specified number of bolts were practically produced as a trial, and it was observed whether the die was broken or worn. The results are shown in Table <NUM> as the results of Test <NUM>. In the item Test <NUM>, "OK" represents that a die was neither broken nor worn, and "NG" represents that a die was broken or worn. Test <NUM> was conducted only for the Examples, Reference Examples, and Comparative Examples that were "OK" in Test <NUM>.

As can be seen in Table <NUM>, Examples <NUM>, <NUM>, and <NUM>, which fall within the scope of the present invention, are high-strength bolts with high resistance to delayed fracture. In contrast, non-inventive examples Comparative Example <NUM> to <NUM> and <NUM> to <NUM> are inferior in resistance to delayed fracture. Further, Comparative Example <NUM> is remarkably inferior in forgeability, and it is therefore not applicable as steel for a high-strength bolt.

Claim 1:
A steel for a high-strength bolt containing: from <NUM> mass% to <NUM> mass% carbon, from <NUM> mass% to <NUM> mass% silicon, from <NUM> mass% to <NUM> mass% chromium, greater than <NUM> mass% to <NUM> mass% manganese, greater than <NUM> mass% to <NUM> mass% or less molybdenum, <NUM> mass% or less phosphorus and sulfur combined, and balance iron and inevitable impurities.