CN115917027A - Near net shape steel material and its producing method - Google Patents

Near net shape steel material and its producing method Download PDF

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CN115917027A
CN115917027A CN202180042287.2A CN202180042287A CN115917027A CN 115917027 A CN115917027 A CN 115917027A CN 202180042287 A CN202180042287 A CN 202180042287A CN 115917027 A CN115917027 A CN 115917027A
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steel material
content
net
precipitates
steel
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CN115917027B (en
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铃木崇久
宫西庆
江头诚
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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Abstract

A near net shape steel material having high fatigue strength and high tensile strength is provided. The chemical composition of the near-net-shape steel material contains, in mass%, C:0.03 to 0.25%, si:0.02 to 0.50%, mn: more than 0.70% and 2.50% or less, P:0.035% of the following, S:0.050% or less, al: 0.005-0.050%, V: more than 0.10% and 0.40% or less, and N:0.003 to 0.030%, polygonal ferrite having an area ratio of 20 to 90%, and a hard phase having an area ratio of 10 to 80%, and satisfies the formula (1), and the amount of diffusible hydrogen when hydrogen is charged by a cathodic hydrogen charging method is 0.10ppm or more. [ V ]/[ V ] in the precipitate is not less than 0.30 (1).

Description

Near net shape steel material and its producing method
Technical Field
The present invention relates to a near-net-shape steel material formed of steel, and a method for manufacturing the same.
Background
Structural steel materials are used as materials for machine structural parts such as automobile parts, industrial machine parts, and construction machine parts. The structural steel material is, for example, a carbon steel material for machine structures, an alloy steel material for machine structures, or the like.
High fatigue strength is required for mechanical structural parts. For this reason, the following manufacturing method is known as a method for manufacturing a component for a machine structure having high fatigue strength using a steel material blank. First, a steel material is subjected to a working such as hot forging to produce a steel material having a desired part shape. A steel near-net-shape material is produced by subjecting a steel material having a desired part shape to age hardening treatment. The steel near net shape material is subjected to cutting processing to manufacture a machine structural member as a final product. In the above manufacturing process, the fatigue strength of the machine structural member can be improved by subjecting the worked steel material to age hardening treatment.
A steel material for a machine structural member produced by subjecting the steel material to an age hardening treatment is disclosed in, for example, japanese patent application laid-open publication No. 2011-236452 (patent document 1).
The steel material described in patent document 1 contains, in mass%, C:0.14 to 0.35%, si:0.05 to 0.70%, mn:1.10 to 2.30%, S:0.003 to 0.120%, cu:0.01 to 0.40%, ni:0.01 to 0.40%, cr:0.01 to 0.50%, mo:0.01 to 0.30% and V:0.05 to 0.45%, the balance being Fe and inevitable impurities, and satisfying 13[ C ] +8[ Si ] +10[ Mn ] +3[ Cu ] +3[ Ni ] +22[ Mo ] +11[ V ] ≦ 30, 5[ C ] + [ Si ] +2[ Mn ] +3[ Cr ] +2[ Mo ] +4[ V ] ≦ 7.3, 2.4 ≦ 0.3[ C ] +1.1[ Mn ] +0.2[ Cu ] +0.2[ Ni ] +1.2[ Cr ] +1.1[ Mo ] +0.2[ V ] + 3.1, 2.5 ≦ [ C ] + 1[ Si ] +4[ Mo ] +9 ]/16 [ V ]/16. In the steel material disclosed in patent document 1, the chemical composition is adjusted to satisfy the above parameter formula, whereby the microstructure becomes bainite, the hot forgeability is improved, and the hardness after hot forging is improved. Patent document 1 describes that the steel material disclosed in patent document 1 has excellent machinability because it has a bainite structure. In patent document 1, an intermediate member is manufactured by hot forging a steel material having the above-described structure. Then, the intermediate member is cut to form a member having a desired shape. Then, age hardening treatment is performed. Patent document 1 describes that the member thus manufactured can obtain high strength.
However, when a member is manufactured by hot forging, the intermediate member after hot forging is likely to be strained in a cooling step. Therefore, the shape of the intermediate member is likely to be slightly deformed with respect to a desired shape. That is, it is difficult to bring the intermediate member after hot forging close to the final shape due to the influence of thermal strain. Therefore, the intermediate member after hot forging is subjected to cutting processing to approximate the final shape.
As described above, when the intermediate member after hot forging is subjected to cutting, the yield is lowered. For this reason, recently, hot forging has been intentionally replaced with cold working typified by cold forging for the purpose of improving the yield. When cold working is used instead of hot forging, the intermediate product can achieve near net shape (substantially the same shape as the final shape). In this case, the amount of cutting in the cutting process of the intermediate member can be reduced. Thus, the yield is improved. In addition, the cutting process itself may be omitted. In this case, productivity is improved.
However, cold working represented by cold forging tends to increase the working load as compared with hot forging. Therefore, it is necessary to improve the workability (hereinafter referred to as cold workability) of the steel material at the time of cold working. Specifically, it is required that the steel sheet can be processed into a desired shape with a small load and that the occurrence of cracks during cold working be suppressed. Therefore, when the age hardening treatment is performed after the cold working, excellent cold workability and excellent fatigue strength after the age hardening treatment are required for the steel material to be subjected to the age hardening treatment.
A blank steel material for a part manufactured by applying an age hardening treatment after cold forging is disclosed in japanese patent application laid-open No. 2019-173168 (patent document 2).
The steel material disclosed in patent document 2 is C:0.02 to 0.25%, si:0.005 to 0.50%, mn: more than 0.70% and 2.50% or less, P:0.035% of the following, S:0.050% or less, al:0.005 to 0.050%, cr:0.02 to 0.70%, V:0.02 to 0.30%, N:0.003 to 0.030%, nb:0 to 0.10%, B:0 to 0.005%, ca:0 to 0.005%, bi:0 to 0.10%, pb:0 to 0.20%, and the balance: fe and impurities. The steel material of patent document 2 has the following chemical composition: the total content of Cu, ni and Mo in the impurities is 0.05 mass% or less, the content of Ti in the impurities is 0.005 mass% or less, and the formula (1) is satisfied. In the formula (1), the [ V precipitate ]/[ V content ] ≦ 0.50. The microstructure of the steel material of patent document 2 contains ferrite, pearlite and/or bainite. The area ratio of ferrite in the microstructure is 10 to 90%. Patent document 2 describes that the steel material having the above-described structure has high cold forgeability, and high fatigue strength can be obtained when age hardening treatment is performed after cold forging.
Documents of the prior art
Patent literature
Patent document 1: japanese patent laid-open publication No. 2011-236452
Patent document 2: japanese patent laid-open publication No. 2019-173168
Disclosure of Invention
Problems to be solved by the invention
The parts made of the steel material disclosed in patent document 2 have high fatigue strength. However, the parts are sometimes required to have not only high fatigue strength but also high tensile strength. In patent document 2, it is not studied to achieve both high fatigue strength and high tensile strength.
The invention aims to provide a near net shape steel material with high fatigue strength and high tensile strength and a manufacturing method thereof.
Means for solving the problems
The present application relates to a near-net-shape steel material having a chemical composition of mass%
C：0.03～0.25％、
Si：0.02～0.50％、
Mn: more than 0.70% and not more than 2.50%,
P: less than 0.035%,
S: less than 0.050 percent,
Al：0.005～0.050％、
V: more than 0.10% and not more than 0.40%,
N：0.003～0.030％、
Cr：0～0.70％、
Nb：0～0.100％、
B：0～0.0100％、
Cu：0～0.30％、
Ni：0～0.30％、
Ca：0～0.0050％、
Bi：0～0.100％、
Pb：0～0.090％、
Mo：0～0.05％、
Ti：0～0.005％、
Zr：0～0.010％、
Se：0～0.10％、
Te：0～0.10％、
Rare earth elements: 0 to 0.010 percent,
Sb：0～0.10％、
Mg：0～0.0050％、
W:0 to 0.050%, and
the balance is as follows: fe and impurities in the iron-based alloy, and the impurities,
the microstructure of the steel near net shape material comprises the following phases:
polygonal ferrite with an area ratio of 20-90%; and
a hard phase comprising pearlite and/or bainite and having an area ratio of 10 to 80%,
satisfying formula (1) when the content of V in the chemical composition is defined as [ V ] (mass%), the total content of V in V precipitates in the near-net-shape steel material is defined as [ V in precipitates ] (mass%),
the diffusible hydrogen amount when charging hydrogen by the cathodic charging method is 0.10ppm or more.
[ V ]/[ V ] in the precipitate is not less than 0.30 (1)
The method for producing the near net shape steel material according to the present invention includes the steps of:
a steel material preparation step of preparing a steel material having a chemical composition of mass%
C：0.03～0.25％、
Si：0.02～0.50％、
Mn: more than 0.70% and not more than 2.50%,
P: less than 0.035%,
S: less than 0.050%,
Al：0.005～0.050％、
V: more than 0.10% and not more than 0.40%,
N：0.003～0.030％、
Cr：0～0.70％、
Nb：0～0.100％、
B：0～0.0100％、
Cu：0～0.30％、
Ni：0～0.30％、
Ca：0～0.0050％、
Bi：0～0.100％、
Pb：0～0.090％、
Mo：0～0.05％、
Ti：0～0.005％、
Zr：0～0.010％、
Se：0～0.10％、
Te：0～0.10％、
Rare earth elements: 0 to 0.010 percent,
Sb：0～0.10％、
Mg：0～0.0050％、
W:0 to 0.050%, and
and the balance: fe and impurities, the microstructure of the steel material consists of the following phases:
polygonal ferrite with an area ratio of 20-90%; and
a hard phase comprising pearlite and/or bainite and having an area ratio of 10 to 80%,
when the content of V in the chemical composition is defined as [ V ] (mass%), and the total content of V in V precipitates in steel is defined as [ V in precipitates ] (mass%), V/[ V in precipitates ] is 0.05 or more and less than 0.30;
a cold working step of cold working the steel material; and (c) a second step of,
subjecting the cold-worked steel material to a treatment temperature of 500 to A c1 An age hardening treatment step of age hardening treatment in which the holding time at the treatment temperature is set to 15 to 150 minutes,
the cold working process comprises
A cold working step in the 1 st direction of cold working the steel material from the 1 st direction with a work strain amount of 0.05 or more, and
a 2 nd direction cold working step of cold working the steel material from a 2 nd direction different from the 1 st direction with a work strain amount of 0.05 or more,
the sum of the amount of work strain generated in the steel material in the 1 st direction cold working step and the amount of work strain generated in the steel material in the 2 nd direction cold working step is 0.20 or more.
ADVANTAGEOUS EFFECTS OF INVENTION
The steel near net shape material of the present application has high fatigue strength and high tensile strength. The method for manufacturing a near-net-shape steel material according to the present application can manufacture the near-net-shape steel material.
Drawings
Fig. 1 is a graph showing a hydrogen release curve obtained when hydrogen is charged by a cathodic hydrogen charging method for a steel near-net shape material.
Detailed Description
The present inventors have made various studies to obtain high fatigue strength and high tensile strength in a near-net shape steel material, and have obtained the following findings.
The present inventors have first studied a near-net shape steel material that can achieve both high fatigue strength and high tensile strength from the viewpoint of chemical composition. As a result, it is considered that if the chemical composition of the steel near net shape material is C:0.03 to 0.25%, si:0.02 to 0.50%, mn: more than 0.70% and 2.50% or less, P:0.035% of the following, S:0.050% or less, al: 0.005-0.050%, V: more than 0.10% and 0.40% or less, N:0.003 to 0.030%, cr:0 to 0.70%, nb:0 to 0.100%, B:0 to 0.0100%, cu:0 to 0.30%, ni:0 to 0.30%, ca:0 to 0.0050%, bi:0 to 0.100%, pb:0 to 0.090%, mo:0 to 0.05%, ti:0 to 0.005%, zr:0 to 0.010%, se:0 to 0.10%, te:0 to 0.10%, rare earth elements: 0 to 0.010%, sb:0 to 0.10%, mg:0 to 0.0050%, W:0 to 0.050%, and the balance: fe and impurities, it is possible to obtain high fatigue strength and high tensile strength.
Here, the inventors of the present invention considered that if the microstructure of the near-net-shape steel material is a structure mainly composed of martensite, the tensile strength is improved. However, when the microstructure of the steel near-net shape material having the above chemical composition is a structure mainly composed of martensite, it is necessary to perform thermal refining (quenching and tempering). In the quenching treatment, it is necessary to heat the steel material to A c3 High temperatures above the point. In addition, in the quenching and tempering treatment, tempering treatment is also performed after quenching treatment, and thus the number of steps in the manufacturing process is increased. Therefore, when the microstructure of the steel near-net shape material having the above chemical composition is a structure mainly composed of martensite, the production cost increases. Here, the structure mainly composed of martensite means a structure in which the martensite area ratio is 90% or more.
When the microstructure of the steel near-net shape material having the above chemical composition is a structure mainly composed of martensite, the hardness of the steel near-net shape material may be excessively high. In this case, even if a high tensile strength is obtained, the fatigue strength of the near-net-shape steel material may decrease.
Therefore, the present inventors have studied means for achieving both high fatigue strength and high tensile strength in a near-net shape steel material having the above chemical composition even if the microstructure is not a microstructure mainly composed of martensite but polygonal ferrite and a phase composed of pearlite and/or bainite (hereinafter referred to as a hard phase). As a result, the present inventors considered that both high fatigue strength and high tensile strength can be achieved by precipitation strengthening by V precipitates even though the microstructure is not a martensite main body but a microstructure including polygonal ferrite and a hard phase.
The precipitation strengthening by the V precipitates generates many nano-sized fine V precipitates in the steel material, thereby improving the fatigue strength. In the present specification, V carbonitride (V (C, N)), V Carbide (VC), and V Nitride (VN) are collectively defined as "V precipitate". The V precipitates in the near net shape steel are mostly V carbonitrides. However, there may be a case where a part of the V precipitates is precipitated as V carbides and/or V nitrides. The V carbide and V nitride also have the same effect as the V carbonitride. Therefore, in the present specification, "V precipitates" include V carbonitrides, V carbides, and V nitrides.
The present inventors have also studied how much the V precipitates are present in the near-net shape steel material having the chemical composition in which the contents of the respective elements are within the above ranges to improve the fatigue strength. In the near-net shape steel material having the chemical composition in which the contents of the respective elements are within the above ranges, the content of V in the chemical composition is defined as [ V ] (mass%). Further, the total content of V in V precipitates in the steel near-net-shape material in the case where the chemical composition of the steel near-net-shape material is 100% by mass is defined as [ V in precipitates ] (mass%). As a result of the studies by the present inventors, it has been found that, in a near-net-shape steel material having a chemical composition in which the contents of the respective elements are within the above ranges, when the formula (1) is satisfied, the fatigue strength of the near-net-shape steel material is sufficiently improved even if the microstructure of the near-net-shape steel material is a structure including polygonal ferrite and a hard phase.
[ V ]/[ V ] in the precipitate is not less than 0.30 (1)
As described above, in the near-net shape steel material having the chemical composition in which the contents of the respective elements are within the above ranges, when the formula (1) is satisfied, the fatigue strength of the near-net shape steel material is sufficiently improved even if the microstructure of the near-net shape steel material is a structure including polygonal ferrite and a hard phase. However, it was found that the fatigue strength of the near-net shape steel material was sufficiently improved, but the tensile strength of the near-net shape steel material could not be sufficiently obtained. Therefore, the present inventors have further studied means capable of achieving both high fatigue strength and high tensile strength. As a result, the present inventors have found that, in a near-net shape steel material having a chemical composition in which the content of each element is within the above range, a microstructure including polygonal ferrite and a hard phase, and satisfying the formula (1), if the amount of diffusible hydrogen in hydrogen charging by a cathodic hydrogen charging method is 0.10ppm or more, both high fatigue strength and high tensile strength can be achieved. This point will be explained below.
It is considered that the amount of diffusible hydrogen upon charging by the cathodic charging method is correlated with the shape of V precipitates in the near net shape steel material. Among the V precipitates, spherical V precipitates and plate-like V precipitates are present. In the following description, spherical V precipitates are referred to as spherical V precipitates. The plate-like V precipitates are referred to as plate-like V precipitates.
The spherical V precipitates form a noncoherent interface with the matrix phase (. Alpha.). In this case, the spherical V precipitates function only as simple obstacles. Specifically, the spherical V precipitates only hinder the dislocation motion directly colliding with the spherical V precipitates. Therefore, the spherical V precipitates have relatively small resistance to dislocation movement.
On the other hand, the plate-like V precipitates have a NaCl-type crystal structure which forms coherent interfaces or semi-coherent interfaces in a Baker-Nutting (B-N) relationship with respect to the parent phase (. Alpha.). Specifically, the plate-like V precipitates form coherent interfaces or semi-coherent interfaces in which {100} of the plate-like V precipitates is parallel to {100} of the parent phase and the < 100 > direction of the plate-like V precipitates is parallel to the < 110 > direction of the parent phase. The coherent interface or semi-coherent interface forms a coherent strain field around the precipitates of the plate-like V. The coherent strain field retards dislocation motion. That is, the plate-like V precipitates inhibit not only dislocation movement directly colliding with the plate-like V precipitates but also dislocation movement passing through the periphery of the plate-like V precipitates. Therefore, the plate-like V precipitates have a larger resistance to dislocation movement than the spherical V precipitates.
Therefore, in the near-net shape steel material satisfying the formula (1) in which the content of each element in the chemical composition is within the above range, the microstructure is a structure including polygonal ferrite and a hard phase, and the resistance to dislocation motion can be further increased as long as the proportion of the plate-like V precipitates in the V precipitates is large, and as a result, not only high fatigue strength but also high tensile strength can be obtained.
However, the size of V precipitates (spherical V precipitates and plate-like V precipitates) is in the order of nanometers. Therefore, it is extremely difficult to identify plate-like V precipitates and spherical V precipitates by microscopic observation and to determine the proportion of the plate-like V precipitates in the V precipitates. On the other hand, hydrogen is easily trapped at coherent and semi-coherent interfaces, and hydrogen is not easily trapped at noncoherent interfaces. That is, hydrogen is easily captured by the plate-shaped V precipitates, and hydrogen is not easily captured by the spherical V precipitates. Therefore, it means that in the near net shape steel in which the content of each element in the chemical composition is in the above range, the microstructure is a structure including polygonal ferrite and a hard phase, and V precipitates in an amount satisfying the formula (1) are precipitated, the larger the hydrogen trapping amount (i.e., the diffusible hydrogen amount) in the hydrogen charging by the cathodic hydrogen charging method is, the larger the proportion of the plate-like V precipitates capable of improving the tensile strength among the V precipitates capable of improving the fatigue strength is.
For the above reasons, it is considered that in a near-net shape steel material having a chemical composition in which the contents of the respective elements are within the above ranges and the microstructure is a structure including polygonal ferrite and a hard phase and satisfying the formula (1), if the amount of diffusible hydrogen in hydrogen charging by the cathodic hydrogen charging method is 0.10ppm or more, high fatigue strength and high tensile strength can be obtained. The above reason is presumed. However, the following examples demonstrate that in a near-net shape steel material having a chemical composition in which the contents of the respective elements are within the above ranges, and a microstructure including polygonal ferrite and a hard phase and satisfying the formula (1), high fatigue strength and high tensile strength can be obtained if the amount of diffusible hydrogen in hydrogen charging by the cathodic hydrogen charging method is 0.10ppm or more.
The steel billet and the method for manufacturing the same according to the present embodiment completed based on the above-described findings have the following features.
[1]
A near net-shape steel material, which is made of a high-strength steel,
the chemical composition of which is calculated by mass percent
C：0.03～0.25％、
Si：0.02～0.50％、
Mn: more than 0.70% and not more than 2.50%,
P: less than 0.035%,
S: less than 0.050%,
Al：0.005～0.050％、
V: more than 0.10% and not more than 0.40%,
N：0.003～0.030％、
Cr：0～0.70％、
Nb：0～0.100％、
B：0～0.0100％、
Cu：0～0.30％、
Ni：0～0.30％、
Ca：0～0.0050％、
Bi：0～0.100％、
Pb：0～0.090％、
Mo：0～0.05％、
Ti：0～0.005％、
Zr：0～0.010％、
Se：0～0.10％、
Te：0～0.10％、
Rare earth elements: 0 to 0.010 percent,
Sb：0～0.10％、
Mg：0～0.0050％、
W:0 to 0.050%, and
and the balance: fe and impurities in the iron-based alloy, wherein the impurities are,
the microstructure of the steel near net shape material comprises the following phases:
polygonal ferrite with an area ratio of 20-90%; and
a hard phase comprising pearlite and/or bainite and having an area ratio of 10 to 80%,
satisfying formula (1) when the content of V in the chemical composition is defined as [ V ] (mass%), the total content of V in V precipitates in the near-net-shape steel material is defined as [ V in precipitates ] (mass%),
the diffusible hydrogen amount when charging hydrogen by the cathodic charging method is 0.10ppm or more.
[ V ]/[ V ] in the precipitate is not less than 0.30 (1)
[2]
The near net-shape steel material according to [1], wherein,
said chemical composition containing, in place of a portion of Fe, a compound selected from the group consisting of
Cr：0.01～0.70％、
Nb：0.001～0.100％、
B：0.0001～0.0100％、
Cu：0.01～0.30％、
Ni：0.01～0.30％、
Ca：0.0001～0.0050％、
Bi：0.001～0.100％、
Pb：0.001～0.090％、
Mo：0.01～0.05％、
Ti：0.001～0.005％、
Zr：0.002～0.010％、
Se：0.01～0.10％、
Te：0.01～0.10％、
Rare earth elements: 0.01 to 0.010 percent,
Sb：0.01～0.10％、
Mg：0.0005～0.0050％、
W: 0.001-0.050% of more than 1 element.
[3]
[1] The method for producing a near-net-shape steel material according to [ 3 ] or [2], comprising the steps of: a steel material preparation step of preparing a steel material having a chemical composition of mass%
C：0.03～0.25％、
Si：0.02～0.50％、
Mn: more than 0.70% and not more than 2.50%,
P: less than 0.035%,
S: less than 0.050%,
Al：0.005～0.050％、
V: more than 0.10% and not more than 0.40%,
N：0.003～0.030％、
Cr：0～0.70％、
Nb：0～0.100％、
B：0～0.0100％、
Cu：0～0.30％、
Ni：0～0.30％、
Ca：0～0.0050％、
Bi：0～0.100％、
Pb：0～0.090％、
Mo：0～0.05％、
Ti：0～0.005％、
Zr：0～0.010％、
Se：0～0.10％、
Te：0～0.10％、
Rare earth elements: 0 to 0.010 percent,
Sb：0～0.10％、
Mg：0～0.0050％、
W:0 to 0.050%, and
and the balance: fe and impurities in the iron-based alloy, and the impurities,
the microstructure of the steel material consists of the following phases:
polygonal ferrite with an area ratio of 20-90%; and
a hard phase comprising pearlite and/or bainite and having an area ratio of 10 to 80%,
when the content of V in the chemical composition is defined as [ V ] (mass%), and the total content of V in V precipitates in steel is defined as [ V in precipitates ] (mass%), V/[ V in precipitates ] is 0.05 or more and less than 0.30;
a cold working step of cold working the steel material; and the number of the first and second groups,
subjecting the cold-worked steel material to a treatment temperature of 500 to A c1 At the treatment temperatureAn age hardening treatment step of age hardening treatment with a holding time of 15 to 150 minutes,
the cold working process comprises
A cold working step in the 1 st direction of subjecting the steel material to cold working with a work strain amount of 0.05 or more from the 1 st direction, and
a 2 nd direction cold working step of cold working the steel material from a 2 nd direction different from the 1 st direction with a work strain amount of 0.05 or more,
the sum of the amount of work strain produced in the steel material in the 1 st direction cold working step and the amount of work strain produced in the steel material in the 2 nd direction cold working step is 0.20 or more.
Hereinafter, the near net shape steel material and the method of manufacturing the same according to the present embodiment will be described in detail. The% of the element means mass% unless otherwise specified.
[ near net shape steel materials ]
In the present specification, a near-net-shape steel material refers to a steel material to which a shape is imparted by applying a working or a heat treatment by an external force. The steel near net shape material may be a final article. Further, the near-net-shape steel material may be further subjected to machining such as cutting to produce a final product.
[ chemical composition ]
The chemical composition of the near net shape steel material of the present embodiment contains the following elements.
C：0.03～0.25％
Carbon (C) bonds with V of the steel material to form V precipitates. The V precipitates improve the fatigue strength and tensile strength of the near net shape steel material by precipitation strengthening. When the C content is less than 0.03%, the above-described effects cannot be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the C content exceeds 0.25%, the cold workability of the steel material as the near-net-shape steel material is lowered even if the contents of other elements are within the ranges of the present embodiment. Therefore, the C content is 0.03 to 0.25%. The lower limit of the C content is preferably 0.04%, more preferably 0.05%, even more preferably 0.06%, even more preferably 0.07%, and even more preferably 0.08%. The upper limit of the C content is preferably 0.24%, more preferably 0.23%, even more preferably 0.22%, even more preferably 0.21%, and even more preferably 0.20%.
Si：0.02～0.50％
Silicon (Si) improves the fatigue strength of steel near net shape materials. Further, si deoxidizes the steel. If the Si content is less than 0.02%, the above-described effects cannot be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the Si content exceeds 0.50%, the cold workability of the steel material as the near-net-shape steel material is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Si content is 0.02 to 0.50%. The lower limit of the Si content is preferably 0.03%, more preferably 0.04%, further preferably 0.05%, further preferably 0.06%, further preferably 0.07%. The upper limit of the Si content is preferably 0.45%, more preferably 0.40%, further preferably 0.35%, further preferably 0.30%, and further preferably 0.25%.
Mn: more than 0.70% and not more than 2.50%
Manganese (Mn) improves the fatigue strength of near net shape steel materials. When the Mn content is 0.70% or less, the above-described effects cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mn content exceeds 2.50%, the cold workability of the steel material as the near-net-shape steel decreases even if the contents of other elements are within the ranges of the present embodiment. Therefore, the Mn content exceeds 0.70% and is 2.50% or less. The lower limit of the Mn content is preferably 0.75%, more preferably 0.80%, further preferably 1.00%, further preferably 1.20%, further preferably 1.40%, further preferably 1.50%. The upper limit of the Mn content is preferably 2.40%, more preferably 2.30%, further preferably 2.20%, further preferably 2.10%, further preferably 2.00%, further preferably 1.90%.
P: less than 0.035%
Phosphorus (P) is an impurity inevitably contained. That is, the P content exceeds 0%. P segregates at grain boundaries, and decreases the fatigue strength and tensile strength of the near-net-shape steel material. Therefore, the P content is 0.035% or less. The upper limit of the P content is preferably 0.030%, more preferably 0.025%, and still more preferably 0.020%. The P content is preferably as low as possible. However, excessively lowering the P content increases the production cost. Therefore, in view of the conventional industrial production, the preferable lower limit of the P content is 0.001%, more preferably 0.005%, even more preferably 0.008%, and even more preferably 0.010%.
S:0.050% or less
Sulfur (S) is an impurity inevitably contained. That is, the S content exceeds 0%. S combines with Mn to form MnS, thereby improving the machinability of the steel. However, when the S content exceeds 0.050%, coarse MnS is formed. Coarse MnS is likely to become a starting point of cracks at the time of cold working. Therefore, the cold workability of the steel material as a near-net shape steel material is lowered. Therefore, the S content is 0.050% or less. The upper limit of the S content is preferably 0.045%, more preferably 0.040%, still more preferably 0.030%, and still more preferably 0.020%. The S content is preferably as low as possible. However, excessively lowering the S content increases the production cost. Therefore, in view of the conventional industrial production, the preferable lower limit of the S content is 0.001%, more preferably 0.005%, further preferably 0.006%.
Al：0.005～0.050％
Aluminum (Al) deoxidizes steel. If the Al content is less than 0.005%, the above-described effects cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Al content exceeds 0.050%, coarse Al inclusions such as Al oxides are formed in the steel material even if the content of other elements is within the range of the present embodiment. Coarse Al inclusions are likely to be the starting points of cracks during cold working. Therefore, the cold workability of the steel material as a near-net shape steel material is lowered. Therefore, the Al content is 0.005 to 0.050%. The lower limit of the Al content is preferably 0.005%, more preferably 0.006%, further preferably 0.007%, further preferably 0.008%, further preferably 0.009%, further preferably 0.010%, further preferably 0.015%. The upper limit of the Al content is preferably 0.045%, more preferably 0.040%, still more preferably 0.030%, still more preferably 0.025%, and yet more preferably 0.020%. In the near-net shape steel of the present embodiment, the Al content means the total Al content.
V: more than 0.10% and not more than 0.40%
Vanadium (V) combines with C and/or N in the steel to form V precipitates. The V precipitates improve the fatigue strength and tensile strength of the near net shape steel material by precipitation strengthening. When the content of V is 0.10% or less, the above-described effects cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the V content exceeds 0.40%, the cold workability of the steel material as the near-net-shape steel material is lowered even if the contents of other elements are within the ranges of the present embodiment. Therefore, the V content exceeds 0.10% and is 0.40% or less. The lower limit of the V content is preferably 0.11%, more preferably 0.12%, even more preferably 0.13%, even more preferably 0.14%, and even more preferably 0.15%. The upper limit of the V content is preferably 0.38%, more preferably 0.35%, still more preferably 0.33%, still more preferably 0.30%, still more preferably 0.28%, and still more preferably 0.25%.
N：0.003～0.030％
Nitrogen (N) bonds with V in the steel material to form V precipitates. The V precipitates improve the fatigue strength and tensile strength of the near-net shape steel material by precipitation strengthening. If the N content is less than 0.003%, the above-described effects cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, when the N content exceeds 0.030%, the number ratio of spherical V precipitates among the V precipitates increases even when the content of other elements is within the range of the present embodiment. In this case, the fatigue strength and tensile strength of the steel near-net shape material decrease. Therefore, the N content is 0.003 to 0.030%. The lower limit of the N content is preferably more than 0.003%, more preferably 0.004%, and still more preferably 0.005%. The upper limit of the N content is preferably 0.028%, more preferably 0.025%, even more preferably 0.023%, even more preferably 0.020%, even more preferably 0.018%, and even more preferably 0.015%.
The balance of the chemical composition of the steel near-net shape material of the present embodiment is composed of Fe and impurities. Here, the impurities are elements which are not intentionally contained in the steel near-net shape material, and which are mixed from ores and scraps as raw materials or from a production environment or the like in industrial production of a steel material serving as a steel near-net shape material. Conceivable impurities are, for example, oxygen (O). The effect of the near net shape steel of the present embodiment can be obtained even if 0.040% or less of O is contained as an impurity. The element that may be contained in the impurity may be an element other than O.
[ optional elements ]
The chemical composition of the near net shape steel of the present embodiment may further contain, in place of a part of Fe, 1 or more elements selected from the group consisting of Cr, nb, B, cu, ni, ca, bi, pb, mo, ti, zr, se, te, rare earth elements (REM), sb, mg, and W. These elements are all optional elements. Hereinafter, each optional element will be described.
[ group 1]
The chemical composition of the near-net shape steel of the present embodiment may further contain 1 or more elements selected from the group consisting of Cr, nb, B, cu, and Ni in place of a part of Fe within the following content range. These elements all improve the fatigue strength and tensile strength of the near net shape steel material.
Cr：0～0.70％
Chromium (Cr) is an optional element, and may not be contained. That is, the Cr content may be 0%. When it is contained, that is, when the Cr content exceeds 0%, cr improves the hardenability of the steel material, and the fatigue strength and tensile strength of the near-net-shape steel material are improved. The above effects can be obtained to some extent by containing a small amount of Cr. However, if the Cr content exceeds 0.70%, the cold workability of the steel material as the near-net-shape steel material is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Cr content is 0 to 0.70%. When it is contained, the Cr content is 0.70% or less. The lower limit of the Cr content is preferably 0.01%, more preferably 0.03%, further preferably 0.05%, further preferably 0.07%, further preferably 0.09%, further preferably 0.10%. The upper limit of the Cr content is preferably 0.65%, more preferably 0.60%, further preferably 0.50%, further preferably 0.45%, further preferably 0.40%, further preferably 0.35%, further preferably 0.30%.
Nb：0～0.100％
Niobium (Nb) is an optional element, and may not be contained. That is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content exceeds 0%, nb bonds with C and/or N in the steel material to form Nb precipitates. The Nb precipitates improve the fatigue strength and tensile strength of the near net shape steel material by precipitation strengthening. The above effects can be achieved to some extent by containing a small amount of Nb. However, if the Nb content exceeds 0.100%, the cold workability of the steel material as the near-net-shape steel decreases even if the content of other elements is within the range of the present embodiment. Therefore, the Nb content is 0 to 0.100%. When contained, the content of Nb is 0.100% or less. The lower limit of the Nb content is preferably more than 0%, more preferably 0.001%, even more preferably 0.010%, and even more preferably 0.020%. The preferable upper limit of the Nb content is 0.080%, and more preferably 0.060%.
B：0～0.0100％
Boron (B) is an optional element and may be absent. That is, the B content may be 0%. When contained, that is, when the B content exceeds 0%, B strengthens the grain boundary of the near-net shape steel. As a result, the fatigue strength and tensile strength of the near net shape steel material are improved. The above-mentioned effects can be obtained to some extent by containing B in a small amount. However, if the content of B exceeds 0.0100%, the above effect is saturated. If the B content exceeds 0.0100%, the raw material cost further increases and the productivity also decreases. Therefore, the B content is 0 to 0.0100%. When contained, the content of B is 0.0100% or less. The lower limit of the B content is preferably more than 0%, more preferably 0.0001%, still more preferably 0.0010%, still more preferably 0.0020%, and yet more preferably 0.0030%. The upper limit of the B content is preferably 0.0080%, more preferably 0.0070%, and still more preferably 0.0060%.
Cu：0～0.30％
Copper (Cu) is an optional element, and may or may not be contained. That is, the Cu content may be 0%. When contained, that is, when the Cu content exceeds 0%, cu improves the hardenability of the steel material, and the fatigue strength and the tensile strength of the near-net-shape steel material are improved. The above-described effects can be obtained to some extent by containing a small amount of Cu. However, if the Cu content exceeds 0.30%, the cold workability of the steel material as a near-net-shape steel material is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Cu content is 0 to 0.30%. When contained, the Cu content is 0.30% or less. The lower limit of the Cu content is preferably more than 0%, more preferably 0.01%, still more preferably 0.05%, and still more preferably 0.10%. The upper limit of the Cu content is preferably 0.29%, more preferably 0.28%, and still more preferably 0.25%.
Ni：0～0.30％
Nickel (Ni) is an optional element, and may be absent. That is, the Ni content may be 0%. When Ni is contained, that is, when the Ni content exceeds 0%, ni improves the hardenability of the steel material, and the fatigue strength and tensile strength of the near-net-shape steel material are improved. The above-mentioned effects can be obtained to some extent by containing a small amount of Ni. However, if the Ni content exceeds 0.30%, the cold forgeability of the steel blank as the near-net-shape steel decreases even if the content of other elements is within the range of the present embodiment. Therefore, the Ni content is 0 to 0.30%. When contained, the Ni content is 0.30% or less. The lower limit of the Ni content is preferably more than 0%, more preferably 0.01%, still more preferably 0.05%, and still more preferably 0.10%. The upper limit of the Ni content is preferably 0.29%, more preferably 0.28%, even more preferably 0.27%, and even more preferably 0.25%.
[ group 2]
The chemical composition of the near-net shape steel of the present embodiment may further contain 1 or more elements selected from the group consisting of Ca, bi, and Pb in a range of the following contents in place of a part of Fe. These elements all improve the machinability of the near net shape steel material.
Ca：0～0.0050％
Calcium (Ca) is an optional element, and may or may not be contained. That is, the Ca content may be 0%. When contained, that is, when the Ca content exceeds 0%, ca improves the machinability of the near net shape steel. The above-mentioned effects can be obtained to some extent by containing a small amount of Ca. However, when the Ca content exceeds 0.0050%, coarse CaO is generated even if the content of other elements is within the range of the present embodiment. In this case, the cold workability of the steel material as the near-net shape steel material is lowered. Therefore, the Ca content is 0 to 0.0050%. When contained, the Ca content is 0.0050% or less. The lower limit of the Ca content is preferably more than 0%, more preferably 0.0001%, further preferably 0.0010%, further preferably 0.0020%. The upper limit of the Ca content is preferably 0.0045%, and more preferably 0.0040%.
Bi：0～0.100％
Bismuth (Bi) is an optional element, and may or may not be contained. That is, the Bi content may be 0%. When contained, that is, when the Bi content exceeds 0%, bi improves the machinability of the near net shape steel. The above-mentioned effects can be obtained to some extent by containing Bi in a small amount. However, if the Bi content exceeds 0.100%, the cold workability of the steel material, which is a near-net-shape steel material, is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Bi content is 0 to 0.100%. When contained, the Bi content is 0.100% or less. The lower limit of the Bi content is preferably more than 0%, more preferably 0.001%, even more preferably 0.010%, even more preferably 0.020%, and even more preferably 0.030%. The upper limit of the Bi content is preferably 0.090%, more preferably 0.080%, still more preferably 0.070%, and yet more preferably 0.065%.
Pb：0～0.090％
Lead (Pb) is an optional element, and may or may not be contained. That is, the Pb content may be 0%. In the case of inclusion, i.e., in the case where the Pb content exceeds 0%, pb improves the machinability of the near net shape material of steel. The above-described effect can be obtained to some extent by containing a small amount of Pb. However, if the Pb content exceeds 0.090%, the cold workability of the steel material, which is a near-net-shape steel material, is lowered even if the contents of other elements are within the ranges of the present embodiment. Therefore, the Pb content is 0 to 0.090%. When contained, the Pb content is 0.090% or less. The lower limit of the Pb content is preferably more than 0%, more preferably 0.001%, even more preferably 0.010%, even more preferably 0.020%, and even more preferably 0.040%. The upper limit of the Pb content is preferably 0.080%, and more preferably 0.070%.
[ group 3 ]
The chemical composition of the near-net shape steel material of the present embodiment may further contain 1 or more elements selected from the group consisting of Mo, ti, zr, se, te, rare earth elements (REM), sb, mg, and W, instead of a part of Fe. These elements are impurities.
Mo：0～0.05％
Molybdenum (Mo) is an impurity, and may not be contained. That is, the Mo content may be 0%. Mo reduces cold workability of a steel material blank as a near-net-shape steel material. If the Mo content exceeds 0.05%, the cold workability of the steel material decreases even if the content of other elements is within the range of the present embodiment. Therefore, the Mo content is 0 to 0.05%. When contained, the content of Mo is 0.05% or less. The upper limit of the Mo content is preferably 0.04%, more preferably 0.03%, and still more preferably 0.02%. The Mo content is preferably as low as possible. However, excessively lowering the Mo content increases the production cost. Therefore, the lower limit of the Mo content is preferably more than 0%, and more preferably 0.01%.
Ti：0～0.005％
Titanium (Ti) is an impurity, and may not be contained. That is, the Ti content may be 0%. Ti combines with N in the near net shape steel to form Ti-based inclusions. The Ti-based inclusions serve as starting points of cracks during cold working. Therefore, ti-based inclusions deteriorate the cold workability of a steel material as a near-net shape steel. If the Ti content exceeds 0.005%, the cold workability of the steel material decreases even if the content of other elements is within the range of the present embodiment. Therefore, the Ti content is 0 to 0.005%. When it is contained, the Ti content is 0.005% or less. The upper limit of the Ti content is preferably 0.004%, more preferably 0.003%, and still more preferably 0.002%. The Ti content is preferably as low as possible. However, excessively lowering the Ti content increases the production cost. Therefore, the lower limit of the Ti content is preferably more than 0%, and more preferably 0.001%.
Zr：0～0.010％
Zirconium (Zr) is an impurity, and may not be contained. That is, the Zr content may be 0%. If the Zr content exceeds 0.010%, the Zr forms coarse inclusions and deteriorates the fatigue characteristics of the steel material even if the content of other elements is within the range of the present embodiment. Therefore, the Zr content is 0 to 0.010%. When contained, the Zr content is 0.010% or less. The upper limit of the Zr content is preferably 0.008%, more preferably 0.006%, and still more preferably 0.005%. The Zr content is preferably as low as possible. However, excessively lowering the Zr content increases the production cost. Therefore, the lower limit of the Zr content is preferably more than 0%, and more preferably 0.002%.
Se：0～0.10％
Selenium (Se) is an impurity, and may or may not be contained. That is, the Se content may be 0%. If the Se content exceeds 0.10%, the steel material is embrittled by Se and the strength and fatigue characteristics of the steel material are lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Se content is 0 to 0.10%. When contained, the Se content is 0.10% or less. The upper limit of the Se content is preferably 0.08%, more preferably 0.06%, and still more preferably 0.05%. The Se content is preferably as low as possible. However, excessively lowering the Se content increases the production cost. Therefore, the lower limit of the Se content is preferably more than 0%, more preferably 0.01%.
Te：0～0.10％
Tellurium (Te) is an impurity, and may not be contained. That is, the Te content may be 0%. When the Te content exceeds 0.10%, the Te embrittles the steel material to lower the strength and fatigue strength of the steel material even if the contents of other elements are within the ranges of the present embodiment. Therefore, the Te content is 0 to 0.10%. When it is contained, the Te content is 0.10% or less. The upper limit of the Te content is preferably 0.08%, more preferably 0.06%, and still more preferably 0.05%. The Te content is preferably as low as possible. However, excessively lowering the Te content increases the production cost. Therefore, the lower limit of the Te content is preferably more than 0%, and more preferably 0.01%.
Rare earth element (REM): 0 to 0.010 percent
The rare earth element (REM) is an impurity, and may or may not be contained. That is, the REM content may be 0%. When the REM content exceeds 0.010%, the REM forms coarse inclusions and deteriorates the fatigue characteristics of the steel material even if the content of other elements is within the range of the present embodiment. Therefore, the REM content is 0 to 0.010%. When contained, the REM content is 0.010% or less. The upper limit of the REM content is preferably 0.008%, more preferably 0.006%, and still more preferably 0.005%. The REM content is preferably as low as possible. However, excessively lowering the REM content increases the production cost. Therefore, the lower limit of the REM content is preferably more than 0%, and more preferably 0.001%.
REM in the present specification means 1 or more elements selected from the group consisting of scandium (Sc) having an atomic number of 21, yttrium (Y) having an atomic number of 39, and lanthanum (La) having an atomic number of 57 to lutetium (Lu) having an atomic number of 71, which are lanthanoids. The REM content in the present specification means the total content of these elements.
Sb：0～0.10％
Antimony (Sb) is an impurity, and may not be contained. That is, the Sb content may be 0%. If the Sb content exceeds 0.10%, the steel material is embrittled by Sb, and the strength and fatigue characteristics of the steel material are lowered, even if the content of other elements is within the range of the present embodiment. Therefore, the Sb content is 0 to 0.10%. When contained, the content of Sb is 0.10% or less. The upper limit of the Sb content is preferably 0.08%, more preferably 0.06%, and still more preferably 0.05%. The Sb content is preferably as low as possible. However, excessively lowering the Sb content increases the production cost. Therefore, the lower limit of the Sb content is preferably more than 0%, and more preferably 0.01%.
Mg：0～0.0050％
Magnesium (Mg) is an impurity, and may or may not be contained. That is, the Mg content may be 0%. If the Mg content exceeds 0.0050%, mg forms coarse inclusions and deteriorates the fatigue characteristics of the steel material even if the content of other elements is within the range of the present embodiment. Therefore, the Mg content is 0 to 0.0050%. When contained, the Mg content is 0.0050% or less. The upper limit of the Mg content is preferably 0.0040%, more preferably 0.0030%, and still more preferably 0.0025%. The Mg content is preferably as low as possible. However, excessively lowering the Mg content increases the production cost. Therefore, the lower limit of the Mg content is preferably more than 0%, and more preferably 0.0005%.
W：0～0.050％
Tungsten (W) is an impurity, and may not be contained. That is, the W content may be 0%. When the W content exceeds 0.050%, the cold workability as the steel material is lowered by W even if the contents of other elements are within the ranges of the present embodiment. Therefore, the W content is 0 to 0.050%. When contained, the W content is 0.040% or less. The upper limit of the W content is preferably 0.030%, more preferably 0.025%, and still more preferably 0.020%. The W content is preferably as low as possible. However, excessively lowering the W content increases the production cost. Therefore, the lower limit of the W content is preferably more than 0%, and more preferably 0.001%.
[ microscopic Structure ]
The microstructure of the near-net shape steel material of the present embodiment contains polygonal ferrite, and pearlite and/or bainite. In the present specification, pearlite and/or bainite are referred to as "hard phases". In the present specification, bainite includes martensite. In subsequent microstructure observation, it is extremely difficult to distinguish bainite from martensite after the time hardening treatment. For this reason, bainite and martensite are not distinguished in the present specification and are collectively referred to as "bainite".
The area ratio of polygonal ferrite in the microstructure is 20 to 90%. As mentioned above, the balance of the microstructure is the hard phase. That is, the microstructure of the steel near net shape material contains: polygonal ferrite with an area ratio of 20 to 90%, and a hard phase with a total area ratio of 10 to 80%.
When the area ratio of polygonal ferrite is 20 to 90%, the near-net shape steel material can obtain high fatigue strength and high tensile strength on the premise that the content of each element in the chemical composition is within the range of the present embodiment, the formula (1) is satisfied, and the amount of diffusible hydrogen at the time of hydrogen charging by the cathodic hydrogen charging method is 0.10ppm or more.
The lower limit of the area ratio of polygonal ferrite in the microstructure of the near-net-shape steel material is preferably 25%, more preferably 30%, and still more preferably 35%. The upper limit of the polygonal ferrite area ratio is preferably 80%, more preferably 75%, and still more preferably 70%.
As described above, the area ratio of the hard phase in the microstructure is 10 to 80%. The lower limit of the pearlite area ratio in the microstructure is preferably 5%, and more preferably 10%. The upper limit of the pearlite area ratio is preferably 50%, and more preferably 40%. The lower limit of the area ratio of bainite in the microstructure is preferably 5%, and more preferably 10%. The upper limit of the area ratio of bainite in the microstructure is preferably 80%, and more preferably 70%.
[ method for measuring the area ratio of polygonal ferrite and the total area ratio of pearlite and bainite in the microstructure ]
The area ratio of polygonal ferrite and the total area ratio of pearlite and bainite in the microstructure of the near-net-shape steel material were measured by the following methods.
A test piece for microstructure observation was selected from an arbitrary position of the steel near-net shape material. Any of the surfaces of the test piece was determined as an observation surface. And performing mirror polishing on the observation surface. The polished observation surface was etched using a 3% nital etching solution (ethanol +3% nitric acid solution). Any 5 observation fields in the etched observation surface were observed with an optical microscope of 400 magnifications to generate photographic images. At this time, the position of each observation field was set to a position at least 3mm deep from the surface of the raw steel near net shape material. The size of each observation field was 200. Mu. M.times.200. Mu.m. In the photographic image of each field, polygonal ferrite is determined. Specifically, the phase having a lamellar structure may be determined as pearlite. A region having a higher brightness than pearlite (white region) may be determined as polygonal ferrite. A region having a lower brightness than polygonal ferrite and pearlite (dark region) may be determined as bainite. The polygonal ferrite area ratio (%) is determined from the total area of polygonal ferrite determined in 5 fields of view and the total area of 5 fields of view. Similarly, the total area ratio (%) of pearlite and bainite was determined from the total area of pearlite and bainite determined in 5 fields and the total area of 5 fields. In the microstructure, when pearlite is 0%, the total area ratio of pearlite and bainite corresponds to the area ratio of bainite. Similarly, in the microstructure, when bainite is 0%, the total area ratio of pearlite and bainite corresponds to the area ratio of pearlite.
[ concerning the formula (1) ]
In the near-net shape steel material of the present embodiment, the V content in the chemical composition of the near-net shape steel material is defined as [ V ] (mass%). Further, the total content of V in V precipitates in the near-net shape steel material is defined as [ V in precipitates ] (mass%) when the chemical composition of the near-net shape steel material is 100%. In this case, the near-net shape steel material of the present embodiment satisfies formula (1).
[ V ]/[ V ] in the precipitate is not less than 0.30 (1)
VP = [ V in precipitate ]/[ V ] is defined. VP represents the precipitation ratio of V precipitates in the near-net shape steel. Even if the contents of the respective elements in the chemical composition of the near-net shape steel are within the range of the present embodiment and the microstructure includes polygonal ferrite having an area ratio of 20 to 90% and a hard phase having an area ratio of 10 to 80%, if the VP is less than 0.30, the V precipitates in the near-net shape steel are not sufficiently formed. In this case, the fatigue strength and the tensile strength in the steel near-net-shape material are decreased.
On the other hand, when VP is 0.30 or more, V precipitates are sufficiently precipitated in the near-net shape steel on the premise that the content of each element in the chemical composition of the near-net shape steel is within the range of the present embodiment, the microstructure includes polygonal ferrite having an area ratio of 20 to 90% and a hard phase having an area ratio of 10 to 80%, and the diffusible hydrogen amount is 0.10ppm or less. Therefore, the fatigue strength and tensile strength of the near-net shape steel material are improved by precipitation strengthening by the V precipitates.
The lower limit of VP is preferably 0.31, more preferably 0.32. The upper limit of VP is not particularly limited. The upper limit of VP is preferably 0.60, more preferably 0.55, and still more preferably 0.52.
[ method of measuring the Total content of V in V precipitates in a near-net shape Steel ([ V in precipitates ]) ]
The V content of V precipitates in the near net shape steel (i.e., [ V in precipitates ]) was determined by extraction residue analysis.
In particular, about 1000mm is selected from the near net shape steel material 3 (about 7.8 g) of a sample. An aa-based solution (a mixture of tetramethylammonium chloride, acetylacetone, and methanol in a ratio of 1. Dipping the selected sample into 10% AA based solution. The impregnated sample was subjected to constant current electrolysis.
First, a sample is subjected to pre-electrolysis. This removes the deposits on the surface of the sample. The conditions of pre-electrolysis are current: 1000mA, time: 28 min at room temperature (25 ℃). The pre-electrolyzed sample was taken out. The sample taken out was subjected to ultrasonic cleaning in alcohol. This removes the deposits on the surface of the sample. The mass of the sample from which the deposit was removed (the mass of the sample before the constant current electrolysis) was measured.
Then, the sample after the pre-electrolysis was subjected to constant current electrolysis. The conditions of electrolysis were current: 173mA, time: 142 minutes at room temperature (25 ℃). The electrolyzed sample was taken out. The sample taken out was subjected to ultrasonic cleaning in alcohol. Thereby removing the deposits (residues) on the surface of the sample. The electrolyzed solution and the solution used for ultrasonic cleaning were filtered with a filter. The mesh size of the filter was set to 0.2 μm. Thereby, the residue is extracted.
The mass of the sample from which the deposit (residue) was removed (the mass of the sample after constant current electrolysis) was measured. And the "mass of the sample electrolyzed at constant current" was determined from the difference between the measured values of the mass of the sample before and after the constant current electrolysis.
The residue extracted on the filter was transferred to a petri dish and dried. The mass of the dried residue was measured. Then, the residue was analyzed by an ICP emission spectrometer (high frequency inductively coupled plasma emission spectrometry) in accordance with JIS G1258 (2014) to determine "the mass of V in the residue".
The value obtained by dividing the "mass of V in the residue" by the "mass of the sample electrolyzed at constant current" in percentage was defined as [ V in precipitate ] (mass%).
[ amount of diffusible hydrogen ]
In the near-net shape steel material of the present embodiment, the amount of diffusible hydrogen when hydrogen is charged by the cathodic hydrogen charging method is 0.10ppm or more, provided that the content of each element in the chemical composition is within the range of the present embodiment, the microstructure includes 20 to 90% of polygonal ferrite and 10 to 80% of hard phase, and the formula (1) is satisfied. More specifically, the steel near net shape material of the present embodiment is used in an amount of 3% NaCl-3g/LNH 4 The current density in the SCN aqueous solution is 0.1mA/cm 2 And the amount of diffusible hydrogen in the case of cathodic hydrogen charging for 72 hours is 0.10ppm or more.
In the near-net shape steel material of the present embodiment, when the diffusible hydrogen is 0.10ppm or more, the microstructure includes 20 to 90% of polygonal ferrite and 10 to 80% of hard phases within the range of the present embodiment based on the content of each element in the chemical composition, and high fatigue strength and high tensile strength can be obtained on the premise that the formula (1) is satisfied.
The reason is not clear, but is considered to be due to the shape of V precipitates as described above. Among the V precipitates, spherical V precipitates form a noncoherent interface with the matrix phase. In this case, the spherical V precipitates themselves constitute an obstacle to dislocation movement. However, the parent phase in contact with the spherical precipitates at the noncoherent interface hardly constitutes an obstacle to the movement of dislocations. On the other hand, the peripheries of the plate-like V precipitates form coherent interfaces or semi-coherent interfaces. In this case, not only the plate-like V precipitates but also the coherent strain field of the parent phase around the plate-like V precipitates in contact with the plate-like V precipitates at the coherent interface or the semi-coherent interface constitutes an obstacle to the dislocation motion. Therefore, the plate-like V precipitates have a larger resistance to dislocation movement than the spherical V precipitates. Therefore, when VP is the same value (that is, even if the amount of precipitated V precipitates is the same), the fatigue strength and tensile strength are improved in which the amount of precipitated V precipitates in the form of plates is large.
However, hydrogen is easily trapped at coherent interfaces and semi-coherent interfaces, while hydrogen is not easily trapped at noncoherent interfaces. That is, hydrogen is easily captured by the plate-shaped V precipitates, and hydrogen is not easily captured by the spherical V precipitates. Therefore, it is considered that, in the near-net shape steel containing V precipitates in an amount satisfying the formula (1), when the amount of the diffusible hydrogen is large, the ratio of the plate-like V precipitates is high. As a result, high fatigue strength and high tensile strength can be obtained in the near-net shape steel.
The above mechanism is presumed. However, the following examples demonstrate that in a near-net shape steel material having a chemical composition in which the contents of the respective elements are within the above ranges, the microstructure is composed of 20 to 90% of polygonal ferrite and 10 to 80% of a hard phase, and the formula (1) is satisfied, and further, when the amount of diffusible hydrogen upon hydrogen charging by the cathodic hydrogen charging method is 0.10ppm or more, high fatigue strength and high tensile strength can be obtained.
The lower limit of the diffusible hydrogen amount is preferably 0.11ppm, more preferably 0.12ppm, still more preferably 0.13ppm, and still more preferably 0.14ppm. The upper limit of the amount of diffusible hydrogen is not particularly limited, but is, for example, 0.50ppm, more preferably 0.45ppm, still more preferably 0.40ppm, still more preferably 0.35ppm, and still more preferably 0.30ppm.
[ measuring method of diffusible Hydrogen quantity ]
The amount of diffusible hydrogen was measured as follows. A round bar test piece with the diameter of 7mm and the length of 40mm is selected from any position of the steel near net shape material. For the selected round bar test piece, hydrogen was introduced by using a cathodic hydrogen charging method.
Specifically, the round bar test piece was immersed in 3% NaCl-3g/LNH 4 SCN in aqueous solution. Then, at a current density: 0.1mA/cm 2 And energization time: hydrogen was introduced into the round bar test piece by cathodic hydrogen charging under the condition of 72 hours. The timing at which the energization was stopped was the timing at which the introduction of hydrogen into the round bar test piece was completed.
After the introduction of hydrogen into the round bar test piece was completed, the amount of hydrogen in the round bar test piece was measured using a temperature-programmed gas chromatograph. The following process is performed based on the time from completion of introduction of hydrogen into the round bar test piece to the start of measurement of the amount of hydrogen in the round bar test piece by the temperature-programmed gas chromatograph (hereinafter referred to as "gap time").
When the gap time was within 30 minutes, the measurement of the amount of hydrogen was started by using the round bar test piece into which hydrogen was introduced as it is. On the other hand, when the gap time exceeds 30 minutes, after the introduction of hydrogen into the round bar test piece is completed, the round bar test piece is stored in a state of being immersed in liquid nitrogen until the measurement of the amount of hydrogen is started. This is to suppress the release of hydrogen introduced into the round bar test piece to the outside of the round bar test piece until the start of the measurement of the amount of hydrogen.
The amount of hydrogen in the round bar test piece subjected to the above-described treatment according to the gap time was measured by the following method using a temperature programmed gas chromatograph. Specifically, the round bar test piece was heated from room temperature to 400 ℃ at a temperature rising rate of 100 ℃/hr. The amount of hydrogen generated by the temperature rise was measured at 5-minute intervals. From the obtained hydrogen amount, a hydrogen release curve shown in fig. 1 can be obtained. The cumulative amount of hydrogen released between room temperature and 350 ℃ was determined using the obtained hydrogen release curve. The cumulative hydrogen amount obtained was the diffusible hydrogen amount (ppm).
As described above, the near-net shape steel material of the present embodiment has a chemical composition in which the contents of the respective elements are within the range of the present embodiment, a microstructure composed of polygonal ferrite having an area ratio of 20 to 90% and a hard phase having an area ratio of 10 to 80%, satisfies formula (1), and has a diffusible hydrogen amount of 0.10ppm or more when hydrogen is charged by the cathodic hydrogen charging method. Therefore, the near-net shape steel material of the present embodiment can obtain not only high fatigue strength but also high tensile strength.
[ production method ]
Hereinafter, a method for producing the near net shape steel material of the present embodiment will be described. The manufacturing method described later is an example of a manufacturing method of a near net shape steel material, and is not limited to this. That is, if the contents of the respective elements in the chemical composition are within the range of the present embodiment, the microstructure is composed of polygonal ferrite having an area ratio of 20 to 90% and a hard phase having an area ratio of 10 to 80%, and the formula (1) is satisfied, and the amount of diffusible hydrogen when hydrogen is charged by the cathodic hydrogen charging method is 0.10ppm or more, the method for producing the near-net-shape steel is not limited to the production method described later. However, the production method described later is a preferable production method of the near net shape steel material of the present embodiment.
The method for producing a near-net-shape steel material according to the present embodiment includes: a step of preparing a steel material blank as a near-net-shape steel material (steel material preparation step); and a step of producing a near-net-shape steel material from the steel material (a near-net-shape steel material production step). Hereinafter, each step will be described in detail.
[ Steel Material preparation Process ]
In the steel material preparation step, a steel material blank as a near-net-shape steel material is prepared. The shape of the steel material is not particularly limited, and is, for example, a steel rod or a wire rod. The composition of the steel material of the present embodiment as a near-net-shape steel material is as follows.
[ constitution of blank Steel Material as Steel near-net-shape Material ]
The composition of the steel material of the present embodiment as a near-net-shape steel material is as follows. The chemical composition of the steel material as the steel near-net shape material is the same as that of the steel near-net shape material. That is, the chemical composition of the steel material is, in mass%, C:0.03 to 0.25%, si:0.02 to 0.50%, mn: more than 0.70% and 2.50% or less, P:0.035% or less, S:0.050% or less, al:0.005 to 0.050%, V: more than 0.10% and 0.40% or less, N:0.003 to 0.030%, cr:0 to 0.70%, nb:0 to 0.100%, B:0 to 0.0100%, cu:0 to 0.30%, ni:0 to 0.30%, ca:0 to 0.0050%, bi:0 to 0.100%, pb:0 to 0.090%, mo:0 to 0.05%, ti:0 to 0.005%, zr:0 to 0.010%, se:0 to 0.10%, te:0 to 0.10%, rare earth elements: 0 to 0.010%, sb:0 to 0.10%, mg:0 to 0.0050%, W:0 to 0.050%, and the balance: fe and impurities.
In the steel material of the near net shape steel of the present embodiment, V in the chemical composition of the steel material is defined as [ V ] (mass%). The total content of V in V precipitates in the steel material when the chemical composition of the steel material is 100% is defined as "V in precipitates" (mass%). In this case, [ V in precipitate ]/[ V ] in the steel material is 0.05 or more and less than 0.30.
VP0= [ V in precipitate ]/[ V ] in steel material. Among the steel materials used as the near-net shape steel materials, steel materials containing as much as possible no V precipitates and a large amount of dissolved V are preferred. When a large amount of V precipitates have already been formed in a steel material, the V precipitates already present in the steel material are coarsened without forming fine V precipitates in the subsequent age hardening treatment in the process of producing a near-net-shape steel material as a billet from the steel material. In this case, the steel near net shape material is hard to form plate-like V precipitates, and the ratio of spherical V precipitates is too large. In this case, the amount of diffusible hydrogen in the near-net shape steel material when hydrogen is charged by the cathodic hydrogen charging method is low. Therefore, sufficient fatigue strength and tensile strength cannot be obtained in the near-net shape steel material.
When VP0 of the steel material is 0.30 or more, the amount of diffusible hydrogen in the near-net shape steel material when hydrogen is charged by the cathodic hydrogen charging method is low. Therefore, sufficient fatigue strength and tensile strength cannot be obtained in the near-net shape steel material. Therefore, VP0 of the steel material is less than 0.30. The lower limit of VP0 of the steel material is not particularly limited, but is, for example, 0.05.
The VP0 of the steel material can be determined by the same measurement method as the measurement method of VP in the near-net shape steel material.
The microstructure of the steel material as the near-net-shape steel material according to the present embodiment is the same as the microstructure of the near-net-shape steel material. That is, the microstructure of the steel material is composed of 20 to 90% of polygonal ferrite and 10 to 80% of a hard phase.
When the area ratio of polygonal ferrite in the microstructure of the steel material exceeds 90% or more, excessive V precipitates are generated in the steel material. Therefore, VP0 is 0.30 or more. In this case, the diffusible hydrogen amount of the steel near-net shape material when hydrogen is charged by the cathodic hydrogen charging method is low in this case. Therefore, sufficient fatigue strength and tensile strength cannot be obtained in the near-net shape steel material.
On the other hand, when the area percentage of polygonal ferrite in the microstructure of the steel material is less than 10%, the cold workability of the steel material is too low. In this case, cracks may be generated in the steel material in the cold working step in the subsequent manufacturing step of the near-net-shape steel material. Therefore, the microstructure of the steel material is composed of 20 to 90% of polygonal ferrite and 10 to 80% of a hard phase.
The area fraction of polygonal ferrite and the area fraction of hard phase in the microstructure of the steel material can be measured by the same measurement method as the area fraction of polygonal ferrite and the area fraction of hard phase in the microstructure of the near-net shape steel material.
The steel material for the nitrided steel member according to the present embodiment may be supplied from a third party or may be manufactured. In the case of manufacturing a steel material, the steel material preparation step includes a step of preparing a billet (billet preparation step) and a step of hot-working the billet to manufacture a steel material (hot-working step). These steps will be explained below.
[ blank preparation Process ]
Molten steel having the above chemical composition is produced. A billet is prepared using the molten steel. For example, molten steel having the above chemical composition is produced using a converter, an electric furnace, or the like. A cast slab is manufactured by a continuous casting method using molten steel. Alternatively, an ingot is produced by an ingot casting method using molten steel.
[ Hot working Process ]
The prepared billet is subjected to hot working to produce a steel material. When hot rolling is performed as hot working, for example, the following methods are available. The hot rolling includes a rough rolling step of roughly rolling the billet to form a billet, and a finish rolling step of finish rolling the billet to form a steel material. The rough rolling step is performed, for example, as follows. After heating the billet (casting blank, ingot) it is first rolled using a blooming mill. If necessary, the billet is further rolled by a continuous rolling mill after the initial rolling to manufacture a billet. In a continuous rolling mill, horizontal roll stands and vertical roll stands are arranged in a row in a staggered manner, and a billet is rolled using a hole die formed in a roll of each stand to form a billet. The billet may be directly manufactured by a continuous casting method.
The finish rolling step is performed, for example, as follows. The billet produced in the rough rolling step is charged into a heating furnace and heated. The heated billet is subjected to finish rolling (hot rolling) by a finishing mill train to form a rod wire having a predetermined diameter. The finishing train includes a plurality of stands arranged in a train. Each stand includes a plurality of rolls arranged around the pass line. The billet is rolled using a hole die formed in a roll of each stand to produce a steel material (wire rod).
The hot working process is not limited to hot rolling. In the hot working step, hot forging or hot extrusion may be performed instead of the hot rolling.
[ regarding the heating temperature ]
In the hot working step, the heating temperature of the steel material immediately before the final hot working is, for example, 1000 to 1300 ℃. For example, when the hot working step includes a rough rolling step and a finish rolling step, the heating temperature in the heating furnace in the finish rolling step is 1000 to 1300 ℃. When the heating temperature in the heating furnace in the finish rolling step is 1000 to 1300 ℃, the V precipitates formed before the hot working step are sufficiently dissolved in a solid state on the premise that other production conditions are satisfied.
[ with respect to the end temperature ]
In the hot working step, the temperature of the steel material after the final reduction is defined as the finish temperature (. Degree. C.). When the hot working step includes a rough rolling step and a finish rolling step, the finish temperature is a temperature of the steel material (surface temperature of the steel material) on the exit side of the stand where the finish rolling is performed last in the finish rolling step. The finishing temperature is, for example, 800 to 1200 ℃. When the finishing temperature is 800 to 1200 ℃, re-precipitation of V dissolved in the heating furnace can be sufficiently suppressed on the premise that other production conditions are satisfied.
[ with respect to the cooling rate ]
In the hot working step, the cooling rate after hot working is, for example, 0.4 to 4.0 ℃/s. Here, the cooling rate after hot working is defined as follows. The average cooling rate at which the temperature of the steel material after the hot working was decreased from the end temperature to 200 ℃ was defined as the cooling rate (. Degree.C./s) after the hot working. When the cooling rate after hot working is 0.4 to 4.0 ℃/s, the steel material has an area fraction of polygonal ferrite of 20 to 90%, an area fraction of hard phases of 10 to 80%, and [ V in precipitates ]/[ V ] of 0.05 or more and less than 0.30, on the premise that other production conditions are satisfied.
A steel material blank as a near-net-shape steel material is produced by the above production method. The steel material after the hot working step may be subjected to a normalizing treatment step for the purpose of adjusting the microstructure.
[ normalizing treatment Process ]
The normalizing treatment step is an optional step and may not be performed. In the case of the normalizing treatment, the heat treatment temperature is 1000 to 1300 ℃ and the cooling rate after the holding at the heat treatment temperature may be 0.4 to 4.0 ℃/s. That is, the heat treatment temperature and the cooling rate in the normalizing treatment are in the same ranges as the heating temperature and the cooling rate in the hot working step.
[ Process for producing near-net-shape Steel Material ]
An example of a method for producing a near-net-shape steel material using the above steel material will be described. The process for producing a near-net-shape steel material comprises: a step of cold working a steel material (cold working step), a step of applying age hardening treatment to the cold-worked steel material (age hardening treatment step), and a step of cutting the age-hardened steel material (cutting treatment step). Here, the cutting process is an optional process. That is, the cutting process may not be performed. Hereinafter, each step will be explained.
[ Cold working Process ]
The cold working step includes a 1 st direction cold working step and a 2 nd direction cold working step. In the 1 st direction cold working step, the steel material is subjected to cold working with a work strain amount of 0.05 or more from the 1 st direction. In the cold working step in the 2 nd direction, the steel material is subjected to cold working with a work strain amount of 0.05 or more from the 2 nd direction. In the cold working step, the sum of the amount of work strain produced in the steel material in the 1 st direction cold working step and the amount of work strain produced in the steel material in the 2 nd direction cold working step is 0.20 or more.
The 1 st direction and the 2 nd direction are not particularly limited as long as they are different directions. For example, the 1 st and 2 nd directions may intersect. Further, the 1 st direction and the 2 nd direction may be perpendicular to each other as in the subsequent drawing and upsetting.
In short, in the cold working step, the steel material receives loads from two different directions (the 1 st direction and the 2 nd direction). By the loads applied from two directions, the movement directions of dislocations in the grains of the steel material are not only in a fixed direction but also in a plurality of directions. Therefore, cross slip is more likely to occur in the steel material than in the case where cold working is performed only from one direction. When the cross slip is generated, the dislocations easily collide with each other. Therefore, dislocations that gradually become immobile with the collision (immobile dislocations) increase, and dislocations remaining in the grains increase. As a result, the dislocation density in the crystal grains increases. As the dislocation density increases, strain is formed. In the subsequent age hardening treatment step, the plate-like V precipitates are likely to precipitate at the portion where strain is formed in order to eliminate the strain. That is, the formed strain becomes nuclei of plate-like V precipitates. When the plate-like V precipitates are precipitated, the fatigue strength and tensile strength of the near-net shape steel to be produced are improved by precipitation strengthening.
That is, it is considered that there is a positive correlation between the amount of strain formed in the steel material and the amount of plate-like V precipitates precipitated in the steel material. In the cold working step, strain is generated in the steel material by performing cold working from two different directions (the 1 st direction and the 2 nd direction). In the present specification, the sum of the working strains generated by cold working from two different directions is referred to as a total working strain. The total work strain is accumulated in the steel material, and the plate-like V precipitates can be sufficiently precipitated so that the diffusible hydrogen content becomes 0.10ppm or more in the subsequent age hardening treatment step.
Here, the total of the strain amount generated in the 1 st cold working step (1 st direction work strain amount) and the strain amount generated in the 2 nd cold working step (2 nd direction work strain amount) is defined as a total work strain amount. More specifically, in the present embodiment, the 1 st direction machining strain amount is 0.05 or more, and the 2 nd direction machining strain amount is 0.05 or more. The total processing strain amount is 0.20 or more.
When the total work strain amount is 0.20 or more, dislocations moving in a plurality of directions in the steel material increase, and as a result, the dislocation density in the crystal grains increases. Therefore, in the subsequent age hardening treatment step, the plate-like V precipitates can be sufficiently precipitated so as to satisfy formula (1) and the diffusible hydrogen amount is 0.10ppm or more. As a result, a sufficiently high fatigue strength and a sufficiently high tensile strength can be obtained in the near-net-shape steel material. When the total processing strain amount is less than 0.20, the above-described effects cannot be sufficiently obtained. Therefore, the total work strain amount is 0.20 or more. The preferred lower limit of the total processing strain amount is 0.23, more preferably 0.25, and still more preferably 0.28.
The upper limit of the total processing strain amount is not particularly limited. However, if the total work strain amount is too large, the deformation resistance of the steel material in the cold working step increases excessively, and this places an excessive burden on the manufacturing equipment. Therefore, the preferable upper limit of the total processing strain amount is 1.50, more preferably 1.20, and still more preferably 0.80.
In the cold working step of the present embodiment, as described above, not only the total work strain amount, which is the sum of the 1 st direction work strain amount and the 2 nd direction work strain amount, but also the 1 st direction work strain amount is required to be 0.05 or more and the 2 nd direction work strain amount is required to be 0.05 or more. When at least one of the 1 st direction work strain amount and the 2 nd direction work strain amount is less than 0.05, the diffusible hydrogen amount of the near-net shape steel material when hydrogen is charged by the cathode charging method is too low, although the formula (1) may be satisfied even if the total work strain amount is 0.20 or more. When at least one of the 1 st direction process strain amount and the 2 nd direction process strain amount is less than 0.05, the movement directions of dislocations within the crystal grains are unbalanced. In this case, cross slip is difficult to occur. Therefore, the dislocation density in the crystal grains is insufficient. Therefore, it is considered that the formation of plate-like V precipitates in the age hardening treatment step is insufficient.
The upper limits of the 1 st direction work strain and the 2 nd direction work strain are not particularly limited. The upper limit of the 1 st direction work strain amount is, for example, 0.40, and more preferably 0.30. The lower limit of the 1 st direction work strain amount is preferably 0.06, and more preferably 0.08. The upper limit of the 2 nd direction processing strain amount is, for example, 0.80, and more preferably 0.50. The lower limit of the 2 nd direction work strain amount is preferably 0.06, and more preferably 0.08.
[ preferred Cold working step in 1 st direction and Cold working step in 2 nd direction ]
Preferably, the cold working in the 1 st direction is drawing and the cold working in the 2 nd direction is upsetting.
In the drawing (the 1 st direction cold working step), wire drawing is performed. The drawing may be performed only once or may be performed several times such as twice. After the drawing (after the 1 st direction cold working step), the steel material can be cut into an appropriate length depending on the near-net shape of the steel to be produced.
In the upsetting process (the 2 nd direction cold working step), the steel material is compressed in the longitudinal direction. The upsetting may be performed 1 time or a plurality of times.
When drawing is used as the 1 st cold working step and upsetting is used as the 2 nd cold working step, the steel material receives a load from both the direction perpendicular to the longitudinal direction of the steel material and the longitudinal direction of the steel material by the drawing and upsetting. In this case, cross slip is more likely to occur, and immobile dislocation is more likely to increase in the steel material. As a result, the dislocation density in the crystal grains increases, and a large amount of strain is likely to be formed in the steel material as nuclei of the plate-like V precipitates.
The amount of work strain generated in the steel material by drawing (the 1 st direction work strain amount) is defined as a drawing strain amount. The work strain amount (2 nd direction work strain amount) generated in the steel material by upsetting is defined as an upset strain amount. The drawing strain amount and the upsetting strain amount are calculated by the true strain amount ε (-) based on an approximate cylinder defined by the formula (2).
ε(-)＝|ln{1+(L-L0)/L0} | (2)
Specifically, in calculating the drawing strain amount, L in equation (2) is the length of the steel material after the drawing process in the drawing direction (longitudinal direction). L0 in the formula (2) is the length of the steel material in the wire drawing direction (longitudinal direction) before the drawing step. From the above definitions, the drawing strain amount (true strain amount ∈) is obtained by equation (2). When the drawing is performed a plurality of times, the drawing strain amount (true strain amount ∈) in each drawing is obtained, and the total value of these amounts is the drawing strain amount (true strain amount ∈) in the drawing step.
On the other hand, in calculating the upset strain, L in equation (2) is the length of the steel material in the wire drawing direction (longitudinal direction) after the upset working step. L0 in the formula (2) indicates the length of the steel material in the wire drawing direction (longitudinal direction) before the upsetting step. From the above definition, the upset strain (true strain ε) was determined by using equation (2). When upsetting is performed a plurality of times, the upset strain (true strain ∈) in each upsetting is determined, and the total value of these amounts is the upset strain (true strain ∈) in the upsetting step.
The sum of the drawing strain and the upsetting strain obtained is the total machining strain (-).
As described above, in the cold working step of the manufacturing method according to the present embodiment, the 1 st direction cold working step and the 2 nd direction cold working step are performed to give the steel material different amounts of work strain in two directions. In this case, the 1 st direction work strain amount is 0.05 or more, the 2 nd direction work strain amount is 0.05 or more, and the total work strain amount is 0.20 or more. As a result, in the subsequent age hardening treatment step, nuclei in which many plate-like V precipitates are formed in the steel material. As a result, in the age hardening treatment step, the plate-like V precipitates can be sufficiently precipitated so as to satisfy formula (1) and the diffusible hydrogen amount is 0.10ppm or more.
[ age hardening treatment Process ]
And an age hardening treatment step for the steel material after the cold working step. The treatment temperature (. Degree. C.) and the holding time (minutes) at the treatment temperature in the age hardening treatment step are as follows.
Treatment temperature: 500 ℃ to A c1 Dot
Treatment temperature in the age hardening treatment step (hereinafter also referred to as age hardening treatment temperature)Is 500 ℃ to A c1 At this time, V precipitates can be precipitated in the steel material so that the diffusible hydrogen content of the near-net-shape steel material upon hydrogen charging by the cathodic hydrogen charging method satisfies the formula (1) is 0.10ppm or more. As a result, high fatigue strength and high tensile strength can be obtained in the near-net shape steel material.
When the age hardening treatment temperature is less than 500 ℃, the amount of V precipitates precipitated is insufficient. In this case, the steel near-net shape material does not satisfy formula (1). On the other hand, the age-hardening treatment temperature exceeds A c1 In this case, it is considered that the change from the plate-like V precipitates to the spherical V precipitates is promoted. Therefore, the amount of diffusible hydrogen in the near-net-shape steel material when hydrogen is charged by the cathodic hydrogen charging method is too low. Therefore, the age hardening treatment temperature is 500 ℃ to A c1 And (4) point. The lower limit of the age-hardening treatment temperature is preferably 520 ℃, more preferably 540 ℃, and still more preferably 560 ℃. The upper limit of the age hardening treatment temperature is preferably 700 ℃, more preferably 680 ℃, and still more preferably 660 ℃.
The retention time is as follows: 15 to 150 minutes
The retention time at the age hardening treatment temperature is 15 to 150 minutes. When the holding time is 15 to 150 minutes, V precipitates can be precipitated in the steel material so that the diffusible hydrogen content of the near-net shape steel material when hydrogen is charged by the cathodic hydrogen charging method satisfies the formula (1) is 0.10ppm or more. As a result, high fatigue strength and high tensile strength can be obtained in the near-net shape steel material.
When the holding time is less than 15 minutes, the amount of V precipitates precipitated is insufficient. In this case, the steel near-net shape material does not satisfy formula (1). On the other hand, when the retention time exceeds 150 minutes, it is considered that the change from the plate-like V precipitates to the spherical V precipitates is promoted. Therefore, the amount of diffusible hydrogen in the near-net-shape steel material when hydrogen is charged by the cathodic hydrogen charging method is too low. Therefore, the holding time is 15 to 150 minutes. The preferred lower limit of the holding time is 20 minutes, more preferably 30 minutes. The preferred upper limit of the holding time is 120 minutes, more preferably 100 minutes.
Through the above-described manufacturing steps, the near-net-shape steel material of the present embodiment can be manufactured. The above-described manufacturing method is an example of the manufacturing method of the near net shape steel material according to the present embodiment. Therefore, as long as the contents of the respective elements in the chemical composition are within the range of the present embodiment, the microstructure is composed of polygonal ferrite having an area ratio of 20 to 90% and a hard phase having an area ratio of 10 to 80%, and satisfies the formula (1), and the amount of diffusible hydrogen when hydrogen is charged by the cathodic hydrogen charging method is 0.10ppm or more, the method for producing a near-net-shape steel material is not limited to the production method described later. However, the production method described later is a preferable production method of the near net shape steel of the present embodiment.
[ optional Process ]
As described above, the cutting process may be performed on the steel material after the time hardening process.
[ cutting Process ]
The cutting process is an optional process. In the case of the cutting process, the steel material after the time hardening treatment is subjected to cutting to produce a near net shape steel material having a desired shape.
As described above, the near-net shape steel material according to the present embodiment can be manufactured by the above-described manufacturing steps (cold working step-age hardening step-cutting working step, or cold working step-age hardening working step) instead of the conventional manufacturing steps (hot forging step-cutting working step). Since the hot forging step can be omitted, the yield can be improved and the productivity can be improved. Hereinafter, the near net shape steel material according to the present embodiment will be specifically described by examples.
Examples
Molten steel having each test number of the chemical compositions shown in tables 1-1 and 1-2 was produced by vacuum melting. A150 kg ingot was produced using the molten steel. The "-" in the column of "chemical composition" in tables 1-1 and 1-2 means that the content of the corresponding element is less than the detection limit. In the steels of any of the test numbers shown in tables 1-1 and 1-2, the O content was 0.0040% or less.
[ tables 1-1]
Figure BDA0003994352890000381
[ tables 1-2]
Figure BDA0003994352890000391
Using the produced ingot, a steel charge as a near-net-shape steel material was produced. Specifically, the ingot was hot worked (hot forged) to produce a round bar having a diameter of 42mm (phi 42). The heating temperature in hot forging was 1200 ℃ and the finishing temperature was 1000 ℃. In test Nos. other than test Nos. 76 to 79, the cooling rate after hot forging was 0.5 ℃/sec. In test No. 76, the cooling rate after hot forging was 0.1 ℃/sec. In test nos. 77 and 78, the cooling rate after hot forging was 6.0 ℃/sec. In test No. 79, the cooling rate after hot forging was 0.2 ℃/sec. By the above-described manufacturing process, a round bar material (steel material) as a near-net-shape steel material is manufactured.
The manufactured round bar is subjected to a cold working step. Specifically, the round bar materials of the respective test numbers were subjected to drawing as the 1 st direction cold working step, and then to upsetting as the 2 nd direction cold working step. The drawing strain and the upsetting strain in the drawing and the upsetting and the total processing strain are shown in tables 2-1 and 2-2. When a crack was observed in the round bar material in the cold working step, the production was immediately stopped, and the test number was judged to have low cold workability.
[ Table 2-1]
Figure BDA0003994352890000411
[ tables 2-2]
Figure BDA0003994352890000421
And (4) performing an age hardening treatment process on the round bar material after the cold working process to manufacture a near-net-shape steel material. In the age hardening treatment, the age hardening treatment temperature (. Degree. C.) and the holding time (minutes) are shown in tables 2-1 and 2-2.
Through the above-described manufacturing process, near-net-shape steel materials of the respective test numbers were manufactured.
[ evaluation test ]
The following evaluation tests were carried out on the round bar stock (steel material) as the near-net-shape steel material and the near-net-shape steel material of each test number.
[ microscopic Structure Observation test of round bar stock (Steel Material) as a near-net-shape Steel Material ]
The microstructures of the round bars of the respective test numbers were observed by the following methods. A test piece was taken from the center of the round bar of each test number including the center axis. Of the surfaces of the test piece, the surface perpendicular to the longitudinal direction of the round bar was set as an observation surface. And performing mirror polishing on the observation surface. The polished observation surface was etched with a 3% nital etching solution (ethanol +3% nitric acid solution). Any 5 observation fields in the etched observation surface were observed by an optical microscope of 400 × to generate a photographic image. The size of the observation field was set to 200. Mu. M.times.200. Mu.m. In the photographic images of the respective fields, polygonal ferrite and hard phases (pearlite and/or bainite) were determined by the above method. The polygonal ferrite area ratio (%) is determined from the total area of polygonal ferrite determined in 5 fields of view and the total area of 5 fields of view. Similarly, the total area ratio (%) of the hard phase (pearlite and bainite) was determined from the total area of pearlite and bainite determined in 5 fields and the total area of 5 fields. The obtained polygonal ferrite area ratio (%) is shown in the column of "polygonal ferrite area ratio (%)" in the column of "round bar (steel material)" in tables 2-1 and 2-2. The obtained hard phase area ratios (%) are shown in the column "hard phase area ratio (%)" in "round bar (steel material)" in tables 2-1 and 2-2.
[ VP0 measurement test of round bar stock (steel product) as a near-net-shape steel Material ]
VP0 (= [ V in precipitate ]/[ V ]) of the round bar of each test number was determined by the following extraction residue analysis method.
In particular, about 1000mm is cut from a round bar 3 (about 7.8 g) of a sample. Using the cut sample, the [ V in precipitate ] in the round bar was determined by the same method as the VP measurement method (extraction residue analysis method)](mass%). V content according to chemical composition of round bar ([ V ]]) And [ V in precipitate]VP0 was determined. The obtained VP0 is shown in the "VP0" column of the "round steel material (steel material)" column in tables 2-1 and 2-2.
[ microscopic Structure Observation test of near-net-shape Steel Material ]
The microstructure of the near-net shape steel material of each test number was observed by the following method. A test piece was taken from the center portion including the center axis of the near-net-shape steel material of each test number. Of the surfaces of the test pieces, the surface perpendicular to the longitudinal direction of the steel near net shape material was set as an observation surface. And performing mirror polishing on the observation surface. The polished observation surface was etched with a 3% nital etching solution (ethanol +3% nitric acid solution). The area ratio (%) of polygonal ferrite and the area ratio (%) of hard phase in the near-net-shape steel were determined by the same method as in the microstructure observation of round bar (steel material) using the etched observation surface. Note that the positions of the 5 observation fields are all positions at least 3mm deep from the surface of the steel near net shape material. The obtained polygonal ferrite area ratios (%) are shown in the column "polygonal ferrite area ratio (%)" in the column "steel near-net shape material" in tables 2-1 and 2-2. The obtained area ratio (%) of the hard phase is shown in "area ratio (%) of hard phase" in the column of "near net shape steel" in tables 2-1 and 2-2.
[ VP measurement test for near-net-shape Steel Material ]
VP (= [ V in precipitate ]/[ V ]) of the near-net shape steel of each test number was determined by the following extraction residue analysis method.
Specifically, approximately 1000mm is cut from a near net shape steel material 3 (about 7.8 g) of a sample. Using the cut samples, the value of [ V in precipitate in ] near net shape steel was determined by the VP measurement method (extracted residue analysis method)](mass%). V content according to chemical composition of steel near net shape material ([ V ]]) And [ V in precipitate]VP was determined. Found VP is inThe "VP" column of the "near net shape Steel" column in tables 2-1 and 2-2 shows.
[ measurement test of amount of diffusible Hydrogen ]
The diffusible hydrogen amount of the near-net shape steel material of each test number was determined by the following method. A round bar test piece having a diameter of 7mm and a length of 40mm was cut from a portion of the near-net-shape steel material including the central axis. For the cut round bar test piece, hydrogen was introduced by a cathodic hydrogen charging method. Specifically, the round bar test piece was immersed in 3% NaCl-3g/LNH 4 SCN in aqueous solution. Then, at a current density: 0.1mA/cm 2 And energization time: hydrogen was introduced into the round bar test piece by cathodic hydrogen charging under the condition of 72 hours. The timing at which the energization was stopped was the timing at which the introduction of hydrogen into the round bar test piece was completed. After the introduction of hydrogen into the round bar test piece was completed, the amount of hydrogen in the round bar test piece was measured rapidly (i.e., during a gap time of 30 minutes or less) by the following method using a temperature programmed gas chromatograph. Specifically, the round bar test piece was heated from room temperature to 400 ℃ at a temperature rising rate of 100 ℃/hr. The amount of hydrogen generated by the temperature rise was measured at 5-minute intervals. From the obtained hydrogen amount, a hydrogen release curve shown in fig. 1 can be obtained. The cumulative amount of hydrogen released between room temperature and 350 ℃ was determined using the obtained hydrogen release curve. The cumulative hydrogen amount obtained was the diffusible hydrogen amount (ppm). The amounts of diffusible hydrogen obtained are shown in the columns of "diffusible hydrogen amount (ppm)" in the columns of "near net shape steel" in tables 2-1 and 2-2.
[ fatigue test ]
The fatigue strength (bending fatigue strength) of the near-net-shape steel material of each test number was measured by the following method. A plurality of small field type rotary bending fatigue test pieces based on JIS Z2274 (1978) were selected from the steel near net shape material. The central axis of the small-field type rotating bending fatigue test piece is coaxial with the central axis of the steel near net-shape material. The small field type rotary bending fatigue test was carried out according to JIS Z2274 (1978) at room temperature in an atmospheric atmosphere using a small field type rotary bending fatigue test piece. In the fatigue test, the number of rotations was 3000rpm, and the number of stress load repetitions was 10 7 The maximum stress without fracture after the cycle was taken as the fatigue strength (MPa). The fatigue obtainedThe strength is shown in the column "fatigue strength (MPa)" in the column "Steel near-Final shape Material" in tables 2-1 and 2-2. In the present example, when the fatigue strength is 480MPa or more, it is judged that the fatigue strength is high. On the other hand, when the fatigue strength is less than 480MPa, the fatigue strength is judged to be low.
[ tensile test ]
The tensile strength of the near-net shape steel material of each test number was measured by the following method. From the position including the central axis of the near-net-shape steel material, a 14A test piece defined in JIS Z2241 (2011) was selected. The longitudinal direction of the test piece substantially coincides with the longitudinal direction of the steel near net shape material. The diameter of the parallel portion of the test piece was 6mm, and the dot distance was 10mm. Tensile test was carried out at room temperature (25 ℃) in the atmosphere using the test piece to determine the tensile strength (MPa). The tensile strengths obtained are shown in the column "tensile strength (MPa)" in the column "Steel near-Final shape Material" in tables 2-1 and 2-2. In this example, when the tensile strength is 845MPa or more, it is judged that the tensile strength is high. On the other hand, when the tensile strength is less than 845MPa, the tensile strength is judged to be low.
[ test results ]
The test results are shown in tables 2-1 and 2-2. Referring to tables 1-1, 1-2, 2-1, and 2-2, the contents of the respective elements in the chemical compositions of the near-net-shape steel materials of test nos. 1 to 48 are within the range of the present embodiment. The microstructure is composed of 20 to 90% of polygonal ferrite and 10 to 80% of a hard phase, and VP satisfies formula (1). In addition, the diffusible hydrogen content of the near-net-shape steel material when hydrogen is charged by the cathodic hydrogen charging method is 0.10ppm or more. Therefore, the near net shape steels of test nos. 1 to 48 exhibited high fatigue strengths, with a fatigue strength of 480MPa or more. Further, the near net shape steel materials of test nos. 1 to 48 exhibited high tensile strength, since the tensile strength was 845MPa or more.
On the other hand, in test No. 49, the C content was too high. Therefore, cracks were observed in the round bar material (steel material) in the cold working step, and the cold workability was low.
In test No. 50, the C content was too low. Therefore, the polygonal ferrite area ratio of the steel near-net shape material is too high. Further, the VP of the near-net-shape steel material does not satisfy formula (1), and the diffusible hydrogen content of the near-net-shape steel material upon charging hydrogen by the cathodic charging method is less than 0.10ppm. As a result, the fatigue strength and tensile strength were low.
In test No. 51, the Si content was too low. Therefore, the fatigue strength and tensile strength of the steel near-net shape material are low.
In test No. 52, the Si content was excessively high. Therefore, the round bar rotary forging material has low cold forgeability, and a near net shape steel material cannot be produced.
In test nos. 53 and 54, the Mn content was too high. Therefore, cracks were observed in the round bar material (steel material) in the cold working step, and the cold workability was low.
In test No. 55, the V content was too high. Therefore, cracks were observed in the round bar material (steel material) in the cold working step, and the cold workability was low.
In test No. 56, the V content was too low. Therefore, the VP of the steel near-net shape material does not satisfy formula (1), and the diffusible hydrogen content of the steel near-net shape material when hydrogen is charged by the cathodic hydrogen charging method is less than 0.10ppm. Therefore, the fatigue strength and the tensile strength of the steel near net shape material are low.
In test Nos. 57 and 58, the Cr content was too high. Therefore, cracks were observed in the round bar material (steel material) in the cold working step, and the cold workability was low.
In test No. 59, the N content was too high. Therefore, VP0 is 0.30 or more in the round bar material as a near-net-shape steel material. That is, coarse V precipitates which become starting points of fatigue fracture excessively form in the round bar material. Therefore, the diffusible hydrogen amount of the steel near-net shape material when hydrogen is charged by the cathodic hydrogen charging method is less than 0.10ppm. Therefore, the fatigue strength and the tensile strength of the steel near net shape material are low.
In test No. 60, the Ti content was excessively high. Therefore, cracks were observed in the round bar material (steel material) in the cold working step, and the cold workability was low.
In test No. 61, the Mo content was excessively high. Therefore, cracks were observed in the round bar material (steel material) in the cold working step, and the cold workability was low.
In test nos. 62, 63 and 80, the total work strain amount in the cold working process was too low. Therefore, VP of the steel near-net shape material does not satisfy formula (1). In addition, the diffusible hydrogen amount of the steel near-net-shape material when hydrogen is charged by the cathodic hydrogen charging method is less than 0.10ppm. Therefore, the fatigue strength and tensile strength of the steel near-net shape material are low.
In test nos. 64, 65 and 81, the total work strain amount in the cold working step was 0.20 or more, but the upset strain amount was less than 0.05. Therefore, the diffusible hydrogen amount of the steel near-net shape material when hydrogen is charged by the cathodic hydrogen charging method is less than 0.10ppm. As a result, the fatigue strength and tensile strength of the near net shape steel material are low.
In test nos. 66, 67, and 82, the total work strain amount in the cold working process was 0.20 or more, but the drawing strain amount was less than 0.05. Therefore, the diffusible hydrogen content of the near-net-shape steel material when hydrogen is charged by the cathodic hydrogen charging method is less than 0.10ppm. As a result, the fatigue strength and tensile strength of the near net shape steel material are low.
In test Nos. 68 and 83, the age hardening treatment temperature was too low. Therefore, VP of the near-net-shape steel material does not satisfy formula (1). In addition, the diffusible hydrogen amount of the steel near-net-shape material when hydrogen is charged by the cathodic hydrogen charging method is less than 0.10ppm. Therefore, the fatigue strength and the tensile strength of the steel near net shape material are low.
In test Nos. 69, 70 and 84, the age hardening treatment temperature was too high. Therefore, the diffusible hydrogen content of the near-net-shape steel material when hydrogen is charged by the cathodic hydrogen charging method is less than 0.10ppm. As a result, the fatigue strength and tensile strength of the near net shape steel material are low.
In test Nos. 71 and 85, the retention time at the age hardening treatment temperature was too short. Therefore, VP of the near-net-shape steel material does not satisfy formula (1). In addition, the diffusible hydrogen amount of the steel near-net-shape material when hydrogen is charged by the cathodic hydrogen charging method is less than 0.10ppm. Therefore, the fatigue strength and the tensile strength of the steel near net shape material are low.
In test Nos. 72 to 75 and 86, the retention time at the age hardening treatment temperature was too long. Therefore, the diffusible hydrogen content of the near-net-shape steel material when hydrogen is charged by the cathodic hydrogen charging method is less than 0.10ppm. Therefore, the fatigue strength and the tensile strength of the steel near net shape material are low.
In test No. 76, the content of each element in the chemical composition was within the range of the present embodiment, but the area ratio of polygonal ferrite in the round bar material as a near-net-shape steel material was too high. Further, VP0 is 0.30 or more. Therefore, the polygonal ferrite area ratio of the steel near-net shape material is too high. In addition, the diffusible hydrogen amount of the steel near-net-shape material when hydrogen is charged by the cathodic hydrogen charging method is less than 0.10ppm. Therefore, the fatigue strength and tensile strength of the steel near-net shape material are low.
In test nos. 77 and 78, the content of each element in the chemical composition was within the range of the present embodiment, but the area ratio of polygonal ferrite in the round bar stock as the near-net-shape steel material was too low. Therefore, cracks were observed in the round bar material (steel material) in the cold working step, and the cold workability was low.
In test No. 79, although the contents of the respective elements in the chemical composition were within the range of the present embodiment, VP0 of the round bar material as a near-net-shape steel material was 0.30 or more. Therefore, the diffusible hydrogen content of the near-net-shape steel material when hydrogen is charged by the cathodic hydrogen charging method is less than 0.10ppm. Therefore, the fatigue strength and the tensile strength of the steel near net shape material are low.
In test No. 87, the age hardening treatment was not performed although the contents of the respective elements in the chemical composition were within the ranges of the present embodiment. Therefore, VP of the near-net-shape steel material does not satisfy formula (1). In addition, the diffusible hydrogen amount of the steel near-net-shape material when hydrogen is charged by the cathodic hydrogen charging method is less than 0.10ppm. Therefore, the fatigue strength and the tensile strength of the steel near net shape material are low.
The embodiments of the present invention have been described above. However, the above embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments may be appropriately modified and implemented within a scope not departing from the gist thereof.

Claims (3)

1. A near net-shape steel material, which is made of a high-strength steel,
the chemical composition of the material is C:0.03 to 0.25 percent,
Si：0.02～0.50％、
Mn: more than 0.70% and 2.50% or less, P: less than 0.035%,
S: less than 0.050%,
Al：0.005～0.050％、
V: more than 0.10% and 0.40% or less, N:0.003 to 0.030 percent,
Cr：0～0.70％、
Nb：0～0.100％、
B：0～0.0100％、
Cu：0～0.30％、
Ni：0～0.30％、
Ca：0～0.0050％、
Bi：0～0.100％、
Pb：0～0.090％、
Mo：0～0.05％、
Ti：0～0.005％、
Zr：0～0.010％、
Se：0～0.10％、
Te：0～0.10％、
Rare earth elements: 0 to 0.010 percent,
Sb：0～0.10％、
Mg：0～0.0050％、
W:0 to 0.050%, and the balance: fe and impurities in the iron-based alloy, and the impurities,
the microstructure of the steel near net shape material comprises the following phases:
polygonal ferrite with an area ratio of 20-90%; and
a hard phase comprising pearlite and/or bainite and having an area ratio of 10 to 80%,
when the V content in the chemical composition is defined as [ V ] (mass%), and the total content of V in V precipitates in the near-net shape steel is defined as [ V in precipitates ] (mass%), the formula (1) is satisfied, and the amount of diffusible hydrogen when hydrogen is charged by the cathodic hydrogen charging method is 0.10ppm or more,
[ V ]/[ V ] in the precipitate is not less than 0.30 (1).
2. The steel near net shape material of claim 1,
the chemical composition comprises a chemical composition selected from the group consisting of
Cr：0.01～0.70％、
Nb：0.001～0.100％、
B：0.0001～0.0100％、
Cu：0.01～0.30％、
Ni：0.01～0.30％、
Ca：0.0001～0.0050％、
Bi：0.001～0.100％、
Pb：0.001～0.090％、
Mo：0.01～0.05％、
Ti：0.001～0.005％、
Zr：0.002～0.010％、
Se：0.01～0.10％、
Te：0.01～0.10％、
Rare earth elements: 0.01 to 0.010 percent,
Sb：0.01～0.10％、
Mg：0.0005～0.0050％、
W: 0.001-0.050% of more than 1 element of the group to replace part of Fe.
3. A method for producing a near-net-shape steel material according to claim 1 or claim 2, comprising the steps of:
a steel material preparation step for preparing a steel material having a chemical composition consisting of, in mass%, C:0.03 to 0.25 percent,
Si：0.02～0.50％、
Mn: more than 0.70% and not more than 2.50%,
P: less than 0.035%,
S: less than 0.050%,
Al：0.005～0.050％、
V: more than 0.10% and not more than 0.40%,
N：0.003～0.030％、
Cr：0～0.70％、
Nb：0～0.100％、
B：0～0.0100％、
Cu：0～0.30％、
Ni：0～0.30％、
Ca：0～0.0050％、
Bi：0～0.100％、
Pb：0～0.090％、
Mo：0～0.05％、
Ti：0～0.005％、
Zr：0～0.010％、
Se：0～0.10％、
Te：0～0.10％、
Rare earth elements: 0 to 0.010 percent,
Sb：0～0.10％、
Mg：0～0.0050％、
W:0 to 0.050%, and
the balance is as follows: fe and impurities in the iron-based alloy, and the impurities,
the microstructure of the steel material consists of the following phases:
polygonal ferrite with an area ratio of 20-90%; and
a hard phase comprising pearlite and/or bainite and having an area ratio of 10 to 80%,
when the content of V in the chemical composition is defined as [ V ] (mass%), and the total content of V in V precipitates in steel is defined as [ V in precipitates ] (mass%), V/[ V in precipitates ] is 0.05 or more and less than 0.30;
a cold working step of cold working the steel material;
subjecting the cold-worked steel material to a treatment temperature of 500 to A c1 An age hardening treatment step of age hardening treatment in which the holding time at the treatment temperature is set to 15 to 150 minutes,
the cold working procedure comprises
A cold working step in the 1 st direction of cold working the steel material from the 1 st direction with a work strain amount of 0.05 or more, and
a 2 nd direction cold working step of cold working the steel material from a 2 nd direction different from the 1 st direction with a work strain amount of 0.05 or more,
the sum of the amount of work strain produced in the steel material in the 1 st direction cold working step and the amount of work strain produced in the steel material in the 2 nd direction cold working step is 0.20 or more.
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