CN116234938A - Steel material - Google Patents

Steel material Download PDF

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Publication number
CN116234938A
CN116234938A CN202180066574.7A CN202180066574A CN116234938A CN 116234938 A CN116234938 A CN 116234938A CN 202180066574 A CN202180066574 A CN 202180066574A CN 116234938 A CN116234938 A CN 116234938A
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ferrite
less
steel material
cross
content
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志贺聪
根石丰
河原木雄介
江头诚
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/60Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/06Surface hardening
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/06Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/28Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for plain shafts
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/32Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for gear wheels, worm wheels, or the like

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)
  • Heat Treatment Of Steel (AREA)
  • Heat Treatment Of Articles (AREA)

Abstract

Provided is a steel material which has excellent machinability, has excellent bending fatigue strength and surface fatigue strength even after gas carburization treatment or the like, and can suppress heat treatment deformation. The steel material according to the present embodiment has a chemical composition containing, in mass%, C:0.20 to 0.25 percent of Si:0.40 to 0.70 percent of Mn:0.50 to 0.90 percent of Cr: 1.00-2.00%, S: 0.005-0.050%, N:0.0050 to 0.0200 percent, al:0.001 to 0.100 percent of O:0.0050% or less, and P: less than 0.030%, and the balance being Fe and impurities, and satisfying the formula (1) in the specification. Further, the microstructure of the cross section and the longitudinal section contains ferrite, the balance being pearlite and/or bainite, the area fraction of ferrite being 50 to 70%, the standard deviation of the area fraction of ferrite being 4.0% or less, and the average grain size ratio of ferrite being 2.00 or less.

Description

Steel material
Technical Field
The present invention relates to a steel material, and more particularly, to a steel material suitable for a blank of a machine structural member manufactured by performing gas carburizing treatment.
In the present specification, the gas carburizing treatment also includes a gas nitrocarburizing treatment. In the present specification, the gas carburizing treatment includes a gas carburizing step (including a gas carburizing and nitriding step) and a quenching step after the gas carburizing step.
Background
The machine structural members are represented by gears and shafts of automobiles, construction vehicles, and the like, for example. As the machine structural member, an alloy steel material for machine structural use represented by SCr420, SCM420, and SNCM420 specified in JIS G4053 (2016) is used.
These steels are produced into parts for machine structural use, for example, by the following production steps. Forging (hot forging or cold forging) and/or cutting work are performed on the steel material to produce an intermediate product having a desired shape. The intermediate product is subjected to heat treatment (quenching and tempering, carburizing treatment, carbonitriding treatment, etc.), and the hardness and microstructure of the intermediate product are adjusted. Through the above manufacturing steps, a component for a mechanical structure is manufactured.
As described above, in the manufacturing process of the mechanical structure member, the steel material may be subjected to cutting processing. Therefore, high machinability is required for steel materials as blanks for mechanical structural members.
In recent years, weight reduction and miniaturization of parts for mechanical structures have been advanced for the purpose of improving fuel consumption of automobiles, construction vehicles, and the like. Therefore, excellent bending fatigue strength and surface fatigue strength are required for the mechanical structural member.
As a method for improving the bending fatigue strength and the surface fatigue strength of a mechanical structural member, gas carburizing treatment and gas carbonitriding treatment are known. In the gas carburizing treatment and the gas carburizing treatment, a hardened layer (carburized layer or carburized layer) is formed on the surface layer of the mechanical structural member. By the hardened layer, the bending fatigue strength and the surface fatigue strength of the mechanical structure member are improved.
However, when the gas carburizing treatment (gas carburizing treatment and gas carburizing treatment) is performed, the mechanical structural member is easily deformed. In the present specification, deformation of the mechanical structure member at the time of gas carburizing treatment is referred to as heat treatment deformation. The shape of the mechanical structure member is deformed by the heat treatment deformation. The deformation of the shape of the mechanical structural member causes noise and vibration when an automobile, a construction vehicle, or the like is running. Therefore, a steel material capable of suppressing deformation by heat treatment when gas carburizing treatment is performed is demanded.
Techniques for suppressing deformation in heat treatment are proposed in Japanese patent application laid-open No. 9-137266 (patent document 1) and Japanese patent application laid-open No. 11-50191 (patent document 2).
The steel disclosed in patent document 1 contains C:0.03 to 0.40 percent of Mn: below 2.0%, si: less than 2.0%, al: 0.015-0.06%, N: 0.005-0.03%, P:0 to 0.030 percent, the balance: fe and unavoidable impurities. In the steel material, the chemical composition is also adjusted so that the quenching start temperature in the quenching performed after the surface hardening treatment of the part formed by using the steel material is set to T A At the time T A T1 (=788-117× [ C) ]+29×[Si]-14×[Mn]) Above and T2 (=900-387× [ C]+63×[Si]-18×[Mn]) Hereinafter, further Heq 1 (=[C]+0.12×[Si]+0.13×[Mn]) And the ratio is 0.33 or more.
In patent document 1, the above-described structure makes the area fraction of pro-eutectoid ferrite in the structure of the core of the mechanical structure member after carburizing 20 to 80%. As a result, heat treatment deformation of the steel material is suppressed.
However, in the mechanical structural member of patent document 1, the bending fatigue strength of the mechanical structural member is sometimes low.
The carburized shaft-like member disclosed in patent document 2 contains, in mass%, C:0.10 to 0.35 percent of Si:0.02 to 0.50 percent of Mn: 0.30-1.80%, S: 0.005-0.15%, al:0.015 to 0.040 percent, nb: 0.005-0.040%, N: 0.0060-0.0200%, P: less than 0.025%, ti: less than 0.01%, O: less than 0.0025%, and Cr:0.40 to 1.80 percent of Mo:0.02 to 1.0 percent of Ni:0.1 to 3.5 percent of V:0.03 to 0.5 percent of 1 or more than 2 percent, and the balance of iron and unavoidable impurities. In the carburized shaft-like member, a total of 80/100 μm are dispersed 2 The above Nb (CN), alN or a composite precipitate of Nb (CN) and AlN having a diameter of 0.1 μm or less, and the austenite grain size is 8 or more.
In patent document 2, coarse crystal grains at the time of carburizing treatment are suppressed. As a result, heat treatment deformation of the steel material is suppressed.
In the carburized shaft-like member of patent document 2, it is considered that the heat treatment deformation is suppressed to some extent at the time of the gas carburization treatment. However, in order to suppress noise and vibration during running of automobiles, construction vehicles, and the like, steel materials that can suppress heat treatment deformation by other means may be used.
Techniques for suppressing heat treatment deformation by means different from those of patent document 1 and patent document 2 are proposed in international publication nos. 2014/038548 (patent document 3), 2013-108144 (patent document 4), and 2013-151719 (patent document 5).
The steel material disclosed in patent document 3 contains C in mass%: 0.20 to 0.30 percent of Si:0.10 to 1.50 percent of Mn:0.10 to 1.20 percent, P: less than 0.030%, S: less than 0.030%, cr:1.30 to 2.50 percent of Cu: less than 0.30%, al:0.008 to 0.300 percent of O: less than 0.0030%, N:0.0020 to 0.0300 percent, and the balance of Fe and unavoidable impurities. In the steel material, the martensite start temperature (Ms point) is 460 ℃ or lower. Further, when the hardness at a position of 1.5mm from the quenched end of the steel material measured by the Qiao Mini tip quenching method is defined as J1.5, the hardness at a position of 9mm from the quenched end is defined as J9, and the hardness at a position of 11mm from the quenched end is defined as J11, (J9/J1.5) is 0.70 to 0.85, and (J11/J1.5) is 0.67 to 0.78.
In the steel material disclosed in patent document 3, the Ms point is set low, and the hardness is adjusted to a predetermined range. Thereby, heat treatment deformation of the steel material is suppressed.
The steel material disclosed in patent document 4 contains C in mass%: 0.10 to 0.25 percent of Si:0.01 to 0.10 percent of Mn: 0.40-1.00%, S:0.003 to 0.050 percent, cr:1.60 to 2.00 percent of Mo:0 to 0.10 percent of Al:0.025 to 0.050 percent, N: 0.0100-0.0250%, the balance being Fe and impurities, P, ti and O (oxygen) in the impurities being P: less than 0.025%, ti: less than 0.003%, O: less than 0.0020 percent. In the steel material, fn (=cr+2×mo) is further 1.82 to 2.10. In the steel material, the difference between the maximum value and the minimum value of the Ms point at the position measured at 17 points on the cross section perpendicular to the longitudinal direction of the steel material is 10 or less.
The steel material disclosed in patent document 5 contains C:0.1 to 0.3 percent of Si:0.01 to 0.6 percent of Mn:0.4 to 1.0 percent, S:0.003 to 0.05 percent of Cr:0.80 to 2.00 percent of Mo:0 to 0.50 percent of Al:0.01 to 0.05 percent and N:0.010 to 0.025 percent, the balance being Fe and impurities, wherein P, ti and O in the impurities are P respectively: less than 0.025%, ti: less than 0.003%, and O: less than 0.002%. In the steel material, the difference between the maximum value and the minimum value of the Ms point at the position measured at 17 points in the cross section perpendicular to the longitudinal direction of the steel material is 10 or less. The microstructure of the steel material is composed of ferrite, pearlite and/or bainite. In the steel material, the ratio of the maximum value to the minimum value of the average grain size of ferrite in the cross section is 2.0 or less.
In patent documents 4 and 5, the variation of the Ms point in the cross section of the steel material is suppressed. As a result, heat treatment deformation of the steel material is suppressed.
Prior art literature
Patent literature
Patent document 1 Japanese patent laid-open No. 9-137266
Patent document 2 Japanese patent laid-open No. 11-50191
Patent document 3 International publication No. 2014/038548
Patent document 4 Japanese patent application laid-open No. 2013-108144
Patent document 5 Japanese patent application laid-open No. 2013-151719
Disclosure of Invention
Problems to be solved by the invention
However, it is also possible to improve the bending fatigue strength and the surface fatigue strength by means other than patent documents 3 to 5, and to suppress the heat treatment deformation.
The purpose of the present application is to provide a steel material that has excellent machinability, has excellent bending fatigue strength and surface fatigue strength after gas carburization treatment, and can suppress heat treatment deformation after gas carburization treatment.
Solution for solving the problem
The steel material of the present application has the following constitution.
A steel material, which is used for the production of steel,
the chemical composition of the composition comprises in mass percent
C:0.20~0.25%、
Si:0.40~0.70%、
Mn:0.50~0.90%、
Cr:1.00~2.00%、
S:0.005~0.050%、
N:0.0050~0.0200%、
Al:0.001~0.100%、
O:0.0050% or less
P: the content of the catalyst is less than or equal to 0.030 percent,
the balance being Fe and impurities, and satisfying the formula (1),
in a circular cross section having a radius R as a cross section perpendicular to the longitudinal direction of the steel material,
When the center position of the cross section and 8 positions of R/2 arranged at 45 ° intervals around the center of the cross section as positions of R/2 from the center of the cross section in the radial direction are defined as 9 cross-section observation positions,
the microstructure at each of the cross-sectional view locations comprises ferrite, the balance being pearlite and/or bainite,
the arithmetic average value of the area fraction of ferrite at the cross-sectional view position at 9 is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less,
the ratio of the maximum average grain size to the minimum average grain size of the average grain size of ferrite at the cross-sectional view position of 9 is 2.00 or less,
in a longitudinal section including a central axis of the steel material as a section parallel to the longitudinal direction of the steel material,
when defining 3 central axis positions arranged at R/2 intervals on the central axis and 6R/2 positions arranged at R/2 intervals from the respective central axis positions in the radial direction as 9 longitudinal section observation positions,
the microstructure at each longitudinal section observation position contains ferrite, and the balance is pearlite and/or bainite,
The arithmetic average value of the area fraction of ferrite at the position of observation of the longitudinal section at 9 is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less,
the ratio of the maximum average grain size to the minimum average grain size of the average grain size of ferrite at the 9 vertical section observation positions is 2.00 or less.
1-(0.5C+0.03Si+0.06Mn+0.01Cr+0.05Mo)<0.800 (1)
Wherein the content (mass%) of the corresponding element is substituted at each element symbol in the formula (1). When the corresponding element is not contained, a "0" is substituted at the symbol of the element.
ADVANTAGEOUS EFFECTS OF INVENTION
The steel material according to the present invention has excellent machinability, and after gas carburization, has excellent bending fatigue strength and surface fatigue strength, and can suppress heat treatment deformation after gas carburization.
Drawings
Fig. 1 is a graph showing the relationship between the value of F1 (=1- (0.5c+0.03si+0.06mn+0.01cr+0.05mo)) and the maximum deformation ratio (%).
Fig. 2 is a schematic view of a cross section perpendicular to the longitudinal direction of the steel material according to the present embodiment.
Fig. 3 is a schematic view of a longitudinal section, which is a section parallel to the longitudinal direction and includes a central axis, of the steel material according to the present embodiment.
Fig. 4 is a schematic view of a band tissue.
Fig. 5 is a diagram showing an example of the heating pattern of the gas carburizing step and the quenching step.
FIG. 6 is a top view of a small field type rotary bending test piece fabricated in the example.
Fig. 7 is a plan view of a test piece for roll pitting fatigue test prepared in the example.
Fig. 8 is a schematic diagram for explaining a roll pitting fatigue test.
Fig. 9 is a front view of the large-roll test piece fabricated in the example.
Fig. 10A is a perspective view of a gear simulation test piece fabricated in the example.
Fig. 10B is a perspective view of the through hole in fig. 10A.
Detailed Description
The present inventors have studied and studied a steel material which has excellent machinability, and when a gas carburization treatment is performed to produce a part for a machine structure, has excellent bending fatigue strength and surface fatigue strength, and can suppress heat treatment deformation after the gas carburization treatment.
The present inventors have studied a steel material having excellent machinability, and further having excellent bending fatigue strength and surface fatigue strength after gas carburization treatment, from the viewpoint of chemical composition.
The results of the study suggest that the composition of the composition, in mass%, contains C:0.20 to 0.25 percent of Si:0.40 to 0.70 percent of Mn:0.50 to 0.90 percent of Cr: 1.00-2.00%, S: 0.005-0.050%, N:0.0050 to 0.0200 percent, al:0.001 to 0.100 percent of O: less than 0.0050%, P: less than 0.030%, mo:0 to 0.30 percent of Nb:0 to 0.050 percent, ti:0 to 0.020%, cu:0 to 0.50 percent of Ni:0 to 0.80 percent, V:0 to 0.30 percent of Mg:0 to 0.0035 percent of Ca:0 to 0.0030 percent and rare earth elements: 0 to 0.0050% and the balance of Fe and impurities, the steel material has excellent machinability, and further may have excellent bending fatigue strength and surface fatigue strength after gas carburization treatment.
The present inventors have also studied means for suppressing deformation of the gas carburized layer after heat treatment. The inventors focused on the microstructure of the steel material. If the microstructure of each part in the steel is as uniform as possible, specifically, if the phase structure unevenness and the crystal grain unevenness of the microstructure of each part in the steel are suppressed, the occurrence timing unevenness of the martensitic transformation at the time of gas carburizing and quenching is suppressed. As a result, heat treatment deformation can be suppressed. For this reason, the inventors have studied the phase structure and crystal grain size of each part of the steel material.
The inventors of the present invention first focused on the variation of microstructure in a cross section perpendicular to the longitudinal direction of the steel material. In order to quantify the unevenness of the microstructure on the cross section, the observation position of the microstructure on the cross section, i.e., the cross-section observation position, is defined as follows.
When the radius of the cross section of the steel material is set to R, the center position of the cross section, and 8R/2 positions disposed at 45 ° intervals around the center of the cross section as positions R/2 from the center of the cross section in the radial direction, are defined as 9 cross section observation positions.
The inventors have investigated and studied the microstructure at each cross-sectional view position. As a result of the study, it was found that the heat treatment deformation after the gas carburizing treatment was suppressed if the microstructure at the cross-sectional view position satisfied the following condition.
(1) The microstructure at each cross-sectional view location contains ferrite, the balance being pearlite and/or bainite.
(2) The arithmetic average value of the area fraction of ferrite at the cross-sectional view position at 9 is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less.
(3) Of the average grain sizes of ferrite at the cross-sectional view position at 9, the ratio of the maximum average grain size to the minimum average grain size is 2.00 or less.
However, even when the steel material has the above chemical composition and satisfies the above microstructure, the heat treatment deformation cannot be sufficiently suppressed, and particularly, it has been found that the noise and vibration at the time of running of an automobile, a construction vehicle, or the like cannot be sufficiently suppressed. For this reason, the present inventors have further studied.
As a result, the following was found. In order to suppress noise and vibration during operation, it is effective to three-dimensionally suppress heat treatment deformation of the steel material. As described above, if the phase structure and the crystal grain unevenness of the microstructure of the cross section of the steel material are suppressed, the heat treatment deformation of the steel material in the direction perpendicular to the longitudinal direction can be suppressed.
However, the inhibition of the deformation by the heat treatment in two dimensions is achieved by only inhibiting the variation in the microstructure of the cross section of the steel material. That is, even if the variation in the microstructure of the cross section of the steel material is suppressed, there is a case where the microstructure of the cross section including the central axis of the steel material, that is, the longitudinal section, is not uniform in the cross section of the steel material, which is parallel to the longitudinal direction. In this case, unevenness occurs in heat treatment deformation. As a result, noise and vibration during operation cannot be sufficiently suppressed.
For this reason, the present inventors have paid attention to not only the unevenness of the microstructure of the cross section of the steel material but also the unevenness of the microstructure of the longitudinal section of the steel material. In order to quantify the unevenness of the microstructure in the vertical section, the observation position of the microstructure in the vertical section, that is, the vertical section observation position is defined as follows.
3 central axis positions disposed at R/2 intervals on the central axis of the steel material and 6R/2 positions disposed at R/2 intervals from the respective central axis positions in the radial direction are defined as 9 longitudinal section observation positions.
The present inventors have studied and studied the microstructure at each longitudinal section observation position. As a result of the study, it was found that, if the microstructure at the cross-sectional view position satisfies the above condition and further the microstructure at the longitudinal-sectional view position satisfies the following condition, the heat treatment deformation after the gas carburizing treatment is sufficiently suppressed.
(4) The microstructure at each longitudinal section observation position contains ferrite, and the balance is pearlite and/or bainite.
(5) The arithmetic average value of the area fraction of ferrite at the longitudinal section observation position at 9 is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less.
(6) The ratio of the maximum average grain size to the minimum average grain size of the average grain size of ferrite at the vertical section observation position at 9 is 2.00 or less.
However, even if the steel material has the chemical composition and the microstructure at the cross-sectional view position and the longitudinal-sectional view position satisfies the above conditions (1) to (6), the heat treatment deformation may not be sufficiently suppressed. For this reason, the present inventors have further studied.
Here, the inventors focused on the martensitic transformation after the gas carburizing treatment. The present inventors have studied the mechanism of the martensitic transformation during the gas carburizing and quenching in detail.
In each of patent documents 3 to 5, deformation by heat treatment is suppressed by suppressing the occurrence of uneven phase transition timing of martensite at the time of gas carburizing and quenching, due to each part in the steel material. Specifically, in patent document 3, by setting the Ms point to 460 ℃ or lower, the martensite transformation timing is suppressed from greatly deviating in each part of the steel material. In patent documents 4 and 5, by suppressing the variation of the Ms points in each part of the steel material, the martensite transformation timing is suppressed from greatly deviating in each part of the steel material.
For this reason, the present inventors first tried to use a steel material having the chemical composition described above, and as in patent documents 3 to 5, suppressed heat treatment deformation by making the martensitic transformation timings in each part of the steel material the same. Specifically, attempts have been made to suppress heat treatment deformation by suppressing the variation in microstructure in each portion (cross-sectional view position, longitudinal-sectional view position) of the steel material and suppressing the variation in Ms points in each portion as much as possible.
However, as a result of investigation by the present inventors, it was found that even if the variation in microstructure of each part of the steel material having the chemical composition is suppressed, the martensitic transformation timing of each part of the steel material is slightly deviated in any way, and it is extremely difficult to perform martensitic transformation at the same timing for each part. Specifically, it was found that when the time at the time of quenching in the gas carburizing treatment is divided into minute times, even if the unevenness of the microstructure at the cross-sectional view position and the longitudinal-sectional view position of the steel material is suppressed to the limit, a minute period in which a portion (hereinafter also referred to as "martensite phase change portion") where martensite phase change occurs anyway and a portion (hereinafter also referred to as "martensite non-phase change portion") where non-martensite phase change occurs in the steel material are mixed exists.
The microstructure change of the steel material at the time of the vacuum carburization treatment is considered to occur as follows.
When the quenching time (quenching time) is divided into minute times, first, martensitic transformation starts in a part of the inside of the steel material. Then, as time goes by, the martensitic transformation proceeds from the central portion toward the surface layer portion. That is, the martensitic transformation does not occur from the surface layer of the steel material but occurs from the inside of the steel material.
The carbon concentration of the surface layer of the steel material becomes higher than that of the inside of the steel material by the gas carburizing treatment. Therefore, the Ms point of the steel surface layer is lower than the Ms point of the steel inside. Further, even if the Ms points can be made uniform at each portion inside the steel material, the cooling rates at each portion are not completely the same due to the shape of the steel material. Therefore, when the quenching time is divided into minute times, the martensitic transformation starts from a portion having a high cooling rate inside the steel material among the portions of the steel material. Therefore, at the time of quenching by the gas carburizing treatment, a minute period in which the martensite transformation portion and the martensite non-transformation portion exist in a mixed manner is necessarily generated.
Based on the above findings, the present inventors studied a means for suppressing the heat treatment deformation on the premise that a minute period in which the martensite phase transformation portion and the martensite non-transformation portion are mixed together is necessarily present at the time of the gas carburizing treatment, instead of suppressing the heat treatment deformation by making the martensite phase transformation timing the same as much as possible as in the technical ideas disclosed in patent documents 3 to 5.
Upon quenching, the martensite non-transformed portion is softer than the martensite transformed portion. Further, the martensite phase-change portion having the body-centered cubic lattice structure is larger in volume than the martensite non-phase-change portion having the body-centered cubic lattice structure. Therefore, when a portion of the steel material undergoes martensitic transformation during quenching, and a martensitic transformation portion and a martensitic non-transformation portion are mixed, strain is generated in the martensitic non-transformation portion. The strain is considered to cause heat treatment deformation.
For this reason, the inventors believe that if the strength of the martensite non-transformation portion at the time of the formation of the martensite transformation portion can be kept high if the precondition is that there is a minute period in which the martensite transformation portion and the martensite non-transformation portion are mixed at the time of the gas carburizing treatment, the occurrence of strain in the martensite non-transformation portion can be suppressed, and as a result, the heat treatment deformation can be suppressed.
For this reason, the present inventors have studied further on means for keeping the strength of the martensite non-transformation portion high when the martensite transformation portion is generated during quenching in the gas carburizing treatment. In the steel material having the chemical composition described above, it is effective to appropriately contain an element that strengthens the martensite non-transformation portion in the temperature region where the martensite transformation portion is generated in order to increase the strength of the martensite non-transformation portion in the temperature region where the martensite transformation portion is generated.
The present inventors considered that C, si, mn, cr and Mo are effective as elements for increasing the strength of the martensite non-transformation portion in the temperature region where the martensite transformation portion is generated in the above chemical composition. For this reason, the relationship between these elements and the heat treatment deformation amount at the time of quenching by gas carburizing treatment was further studied. As a result, it was found that, in the steel material having the chemical composition described above, heat treatment deformation was significantly suppressed by further satisfying the following formula (1).
1-(0.5C+0.03Si+0.06Mn+0.01Cr+0.05Mo)<0.800(1)
Wherein the content (mass%) of the corresponding element is substituted at each element symbol in the formula (1). When the corresponding element is not contained, a "0" is substituted at the symbol of the element.
Defined as f1=1- (0.5c+0.03si+0.06mn+0.01cr+0.05mo). Fig. 1 is a graph showing the relationship between the F1 value and the maximum deformation ratio (%) in a steel material in which the content of each element in the chemical composition is within the above-described range and the unevenness of the microstructure at the cross-sectional view position and the longitudinal-sectional view position satisfies the above-described condition. The maximum deformation ratio is an index of heat treatment deformation. The larger the maximum deformation ratio is, the larger the heat treatment deformation of the steel material is. The maximum deformation ratio is obtained by a method described later.
Referring to fig. 1, in a steel material in which the content of each element in the chemical composition is within the above-described range and the microstructure at the cross-sectional view position and the longitudinal-sectional view position satisfies the above-described conditions (1) to (6), as F1 decreases, the maximum deformation ratio decreases. Further, when F1 is less than 0.800, the maximum deformation ratio is significantly reduced. That is, the maximum deformation ratio has an inflection point in the vicinity of f1=0.800 with respect to F1. Therefore, if F1 is less than 0.800, heat treatment deformation of the steel material during carburizing and quenching can be sufficiently suppressed.
As described above, in the steel material having the above chemical composition, the inventors have found that, by making F1 less than 0.800, excellent machinability, excellent bending fatigue strength and excellent plane fatigue strength after gas carburizing treatment, and heat treatment deformation after gas carburizing treatment can be sufficiently suppressed, while suppressing the variation in occurrence timing of martensitic transformation at the time of quenching to some extent and also generating a minute period in which the martensitic transformation portion and the martensitic non-transformation portion are mixed at any time at the time of quenching.
The steel material according to the present embodiment, which has been completed based on the above findings, has the following structure.
[1]
A steel material, which is used for the production of steel,
the chemical composition of the composition comprises in mass percent
C:0.20~0.25%、
Si:0.40~0.70%、
Mn:0.50~0.90%、
Cr:1.00~2.00%、
S:0.005~0.050%、
N:0.0050~0.0200%、
Al:0.001~0.100%、
O:0.0050% or less
P: the content of the catalyst is less than or equal to 0.030 percent,
the balance being Fe and impurities, and satisfying the formula (1),
in a circular cross section having a radius R as a cross section perpendicular to the longitudinal direction of the steel material,
when the center position of the cross section and 8 positions of R/2 arranged at 45 ° intervals around the center of the cross section as positions of R/2 from the center of the cross section in the radial direction are defined as 9 cross-section observation positions,
The microstructure at each of the cross-sectional view locations comprises ferrite, the balance being pearlite and/or bainite,
the arithmetic average value of the area fraction of ferrite at the cross-sectional view position at 9 is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less,
the ratio of the maximum average grain size to the minimum average grain size of the average grain size of ferrite at the cross-sectional view position of 9 is 2.00 or less,
in a longitudinal section including a central axis of the steel material as a section parallel to the longitudinal direction of the steel material,
when defining 3 central axis positions arranged at R/2 intervals on the central axis and 6R/2 positions arranged at R/2 intervals from the respective central axis positions in the radial direction as 9 longitudinal section observation positions,
the microstructure at each longitudinal section observation position contains ferrite, and the balance is pearlite and/or bainite,
the arithmetic average value of the area fraction of ferrite at the position of observation of the longitudinal section at 9 is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less,
the ratio of the maximum average grain size to the minimum average grain size of the average grain size of ferrite at the 9 vertical section observation positions is 2.00 or less.
1-(0.5C+0.03Si+0.06Mn+0.01Cr+0.05Mo)<0.800 (1)
Wherein the content (mass%) of the corresponding element is substituted at each element symbol in the formula (1). When the corresponding element is not contained, a "0" is substituted at the symbol of the element.
[2]
The steel product according to [1], wherein,
the chemical composition further contains 1 or more elements selected from the group consisting of:
mo: less than 0.30 percent,
Nb:0.050% or less,
Ti: less than 0.020%,
Cu: less than 0.50 percent,
Ni: less than 0.80 percent,
V: less than 0.30 percent,
Mg: less than 0.0035 percent,
Ca:0.0030% or less
Rare earth element: less than 0.0050%.
Hereinafter, the steel material according to the present embodiment will be described in detail. Unless otherwise specified, "%" related to an element represents mass%.
The steel material of the present embodiment has the following chemical composition.
C:0.20~0.25%
Carbon (C) improves the strength of the steel. When the content of C is less than 0.20%, 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 C content is more than 0.25%, the hardenability becomes too high even if the other element content is within the range of the present embodiment. In this case, the hardness of the steel material after the gas carburization treatment becomes hard. As a result, the machinability of the steel material is reduced.
Therefore, the C content is 0.20 to 0.25%.
The preferable lower limit of the C content is 0.21%, and more preferably 0.22%.
The preferable upper limit of the C content is 0.24%, and more preferably 0.23%.
Si:0.40~0.70%
Silicon (Si) improves the hardenability of the steel material and improves the strength of the steel material. Si also increases the temper softening resistance of the hardened layer of the mechanical structure component. Therefore, the surface fatigue strength of the mechanical structural member is improved. When the Si content is less than 0.40%, 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 Si content is more than 0.70%, the grain boundary oxide layer after the gas carburizing treatment becomes too deep even if the content of other elements is within the range of the present embodiment. In this case, the bending fatigue strength of the mechanical structure member decreases.
Therefore, the Si content is 0.40 to 0.70%.
The preferable lower limit of the Si content is 0.41%, more preferably 0.42%, still more preferably 0.45%, still more preferably 0.47%, still more preferably 0.50%.
The preferable upper limit of the Si content is 0.69%, more preferably 0.67%, still more preferably 0.65%, still more preferably 0.63%.
Mn:0.50~0.90%
Manganese (Mn) improves the hardenability of the steel material, and improves the bending fatigue strength and the surface fatigue strength of the mechanical structural member. When the Mn content is less than 0.50%, 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 Mn content is more than 0.90%, even if the content of other elements is within the range of the present embodiment, the formation of Mn oxide is promoted in the surface layer of the mechanical structural member at the time of the gas carburizing treatment. In this case, the grain boundary oxide layer of the mechanical structure member becomes too deep. Therefore, the bending fatigue strength of the mechanical structural member is reduced. Mn oxide formed on the surface layer of the mechanical structural member also becomes a cause of peeling of the surface origin. When the surface origin is peeled off, the surface fatigue strength of the mechanical structural member is reduced.
Therefore, the Mn content is 0.50 to 0.90%.
The preferable lower limit of the Mn content is 0.51%, more preferably 0.52%, still more preferably 0.55%, still more preferably 0.60%.
The preferable upper limit of the Mn content is 0.89%, more preferably 0.87%, still more preferably 0.85%.
Cr:1.00~2.00%
Chromium (Cr) improves the hardenability of the steel material, and improves the bending fatigue strength and the surface fatigue strength of the mechanical structural member. When the Cr content is less than 1.00%, 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 Cr content is more than 2.00%, the steel becomes too hard even if the content of other elements is within the range of the present embodiment. In this case, the machinability of the steel material is reduced.
Therefore, the Cr content is 1.00 to 2.00%.
The preferable lower limit of the Cr content is 1.10%, more preferably 1.20%, more preferably 1.40%, more preferably 1.60%, more preferably 1.70%, more preferably 1.80%.
The preferable upper limit of the Cr content is less than 2.00%, more preferably 1.95%, still more preferably 1.90%.
S:0.005~0.050%
Sulfur (S) bonds with Mn to form MnS. MnS improves machinability of steel. When the S 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, when the S content is more than 0.050%, mnS is excessively formed even if the content of other elements is within the range of the present embodiment. In this case, the bending fatigue strength and the surface fatigue strength of the mechanical structural member decrease.
Therefore, the S content is 0.005 to 0.050%.
The preferable lower limit of the S content is 0.007%, more preferably 0.010%, still more preferably 0.013%, still more preferably 0.015%.
The preferable upper limit of the S content is 0.049%, more preferably 0.045%, more preferably 0.040%, more preferably 0.035%, more preferably 0.025%.
N:0.0050~0.0200%
Nitrogen (N) will bond with Al and Nb to form AlN and NbN. AlN and NbN suppress coarsening of crystal grains during heating of the gas carburizing treatment by the pinning effect. When the N content is less than 0.0050%, 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 is more than 0.0200%, defects are likely to occur on the surface of the cast slab or ingot produced in the steel production step even if the content of other elements falls within the range of the present embodiment.
Therefore, the N content is 0.0050 to 0.0200%.
The preferable lower limit of the N content is 0.0100%, more preferably 0.0120%, still more preferably 0.0130%.
The preferable upper limit of the N content is less than 0.0200%, more preferably 0.0190%, still more preferably 0.0180%, still more preferably 0.0150%.
Al:0.001~0.100%
Aluminum (Al) deoxidizes the steel. Al also bonds with N to form AlN. AlN suppresses coarsening of crystal grains during heating in the gas carburizing treatment by the pinning effect. When the Al content is less than 0.001%, 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 Al content is more than 0.100%, the formation of coarse Al oxide is promoted even if the content of other elements is within the range of the present embodiment. The coarse Al oxide reduces the bending fatigue strength of the mechanical structural member.
Therefore, the Al content is 0.001 to 0.100%.
The preferable lower limit of the Al content is 0.020%, more preferably 0.025%, still more preferably 0.027%, still more preferably 0.030%.
The preferable upper limit of the Al content is 0.090%, more preferably 0.070%, more preferably 0.050%, more preferably 0.045%, more preferably 0.040%, more preferably 0.035%.
O (oxygen): less than 0.0050%
Oxygen (O) is an impurity. O bonds with other elements in the steel material to form coarse oxide inclusions. The coarse oxide inclusions reduce the bending fatigue strength of the mechanical structural member. When the O content is more than 0.0050%, the bending fatigue strength of the mechanical structural member is significantly reduced even if the content of other elements is within the range of the present embodiment.
Therefore, the O content is 0.0050% or less.
The O content is preferably as low as possible. However, excessive reduction in the O content increases the production cost. Therefore, considering the usual industrial production, the preferable lower limit of the O content is more than 0%, more preferably 0.0001%, still more preferably 0.0005%, still more preferably 0.0010%.
The preferable upper limit of the O content is 0.0040%, more preferably 0.0030%, further preferably 0.0020%, further preferably 0.0015%.
P: less than 0.030 percent
Phosphorus (P) is an impurity. P segregates to grain boundaries, reducing the grain boundary strength. If the P content is more than 0.030%, P is excessively segregated in the grain boundaries to lower the grain boundary strength even if the content of other elements is within the range of the present embodiment. As a result, the bending fatigue strength and the surface fatigue strength of the mechanical structural member are reduced.
Therefore, the P content is 0.030% or less.
The P content is preferably as low as possible. However, excessive reduction in the P content increases the production cost. Therefore, considering the usual industrial production, the lower limit of the P content is preferably more than 0%, more preferably 0.001%, even more preferably 0.005%, even more preferably 0.010%.
The upper limit of the P content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.
The balance of the chemical composition of the steel material according to the present embodiment is Fe and impurities. Here, the impurities are substances mixed from ores, scraps, a manufacturing environment, or the like as raw materials in the industrial production of the steel material, and are allowed within a range that does not adversely affect the steel material of the present embodiment. The impurities herein are B, pb, W, sb, bi, co, ta, sn, in, zr, te, se, zn, etc. The total content of impurities other than O and P is 0.01% or less. In the above impurities, the B content was 0.0003% or less.
[ concerning optional element (optional elements) ]
The steel material according to the present embodiment may further contain 1 or more elements selected from the group consisting of:
mo: less than 0.30 percent,
Nb:0.050% or less,
Ti: less than 0.020%,
Cu: less than 0.50 percent,
Ni: less than 0.80 percent,
V: less than 0.30 percent,
Mg: less than 0.0035 percent,
Ca:0.0030% or less
Rare earth element: less than 0.0050%.
These elements are optional elements, and both improve the flexural fatigue strength and the surface fatigue strength of the mechanical structural member.
Mo: less than 0.30%
Molybdenum (Mo) is an optional element, and may not be contained. That is, the Mo content may be 0%.
When contained, mo improves the hardenability of the steel material, and improves the bending fatigue strength and the surface fatigue strength of the mechanical structural member. The above effects can be obtained to some extent by only containing a small amount of Mo.
However, when the Mo content is more than 0.30%, the steel becomes too hard even if the content of other elements is within the range of the present embodiment. In this case, the machinability of the steel material is reduced.
Therefore, the Mo content is 0 to 0.30%, and in the case of being contained, 0.30% or less (i.e., more than 0 and 0.30% or less).
The lower limit of the Mo content is preferably 0.01%, more preferably 0.02%, even more preferably 0.05%, and even more preferably 0.10%.
The upper limit of the Mo content is preferably less than 0.30%, more preferably 0.25%, even more preferably 0.20%, and even more preferably 0.15%.
Nb: less than 0.050%
Niobium (Nb) is an optional element, and may be absent. That is, the Nb content may be 0%.
When Nb is contained, nb is bonded to C and/or N to form Nb precipitates (NbC, nbN, nb (CN) and the like). The Nb precipitates suppress coarsening of crystal grains in the gas carburizing treatment by the pinning effect, similarly to AlN. Therefore, the bending fatigue strength and the surface fatigue strength of the mechanical structural member are improved. The above effects can be obtained to some extent by incorporating a small amount of Nb.
However, when the Nb content is more than 0.050%, nb precipitates coarsen even if the content of other elements falls within the range of the present embodiment. In this case, the coarsening of crystal grains in the gas carburizing treatment cannot be sufficiently suppressed. Therefore, the bending fatigue strength and the surface fatigue strength of the mechanical structural member are reduced.
Accordingly, the Nb content is 0 to 0.050%, and in the case of being contained, 0.050% or less (i.e., greater than 0 and 0.050% or less).
The preferable lower limit of the Nb content is 0.001%, more preferably 0.010%, still more preferably 0.015%, still more preferably 0.020%, still more preferably 0.025%.
The preferable upper limit of the Nb content is less than 0.050%, more preferably 0.045%, still more preferably 0.040%, still more preferably 0.035%.
Ti: less than 0.020%
Titanium (Ti) is an optional element, and may be absent. That is, the Ti content may be 0%.
When Ti is contained, ti precipitates (TiC, tiN, ti (CN) and the like) are formed similarly to Nb. The Ti precipitates inhibit coarsening of crystal grains in the gas carburizing treatment by the pinning effect. Therefore, the bending fatigue strength and the surface fatigue strength of the mechanical structural member are improved. The above-described effects can be obtained to some extent by only containing Ti in a small amount.
However, when the Ti content is more than 0.020%, ti precipitates coarsen even if the content of other elements falls within the range of the present embodiment. In this case, the coarsening of crystal grains in the gas carburizing treatment cannot be sufficiently suppressed. Therefore, the bending fatigue strength and the surface fatigue strength of the mechanical structural member are reduced.
Accordingly, the Ti content is 0 to 0.020%, and in the case of being contained, 0.020% or less (that is, more than 0 and 0.020% or less).
The lower limit of the Ti content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%.
The preferable upper limit of the Ti content is 0.019%, more preferably 0.017%, still more preferably 0.015%.
Cu: less than 0.50%
Copper (Cu) is an optional element, and may not be contained. That is, the Cu content may be 0%.
When included, cu improves the hardenability of the steel material, and improves the bending fatigue strength and the surface fatigue strength of the mechanical structural member. The effect described above can be obtained to some extent by only containing a small amount of Cu.
However, when the Cu content is more than 0.50%, the steel becomes too hard even if the content of other elements is within the range of the present embodiment. In this case, the machinability of the steel material is reduced.
Therefore, the Cu content is 0 to 0.50%, and in the case of being contained, 0.50% or less (i.e., more than 0 and 0.50% or less).
The lower limit of the Cu content is preferably 0.01%, more preferably 0.05%, and even more preferably 0.10%. The upper limit of the Cu content is preferably 0.45%, more preferably 0.40%, still more preferably 0.30%, and still more preferably 0.25%.
Ni: less than 0.80%
Nickel (Ni) is an optional element, and may not be contained. That is, the Ni content may be 0%.
When contained, ni improves the hardenability of the steel material, and improves the bending fatigue strength and the surface fatigue strength of the mechanical structural member. The above effects can be obtained to some extent by only containing Ni in a small amount.
However, when the Ni content is more than 0.80%, the steel becomes too hard even if the content of other elements is within the range of the present embodiment. In this case, the machinability of the steel material is reduced.
Therefore, the Ni content is 0 to 0.80%, and in the case of being contained, 0.80% or less (i.e., more than 0 and 0.80% or less).
The preferable lower limit of the Ni content is 0.01%, more preferably 0.05%, still more preferably 0.10%.
The upper limit of the Ni content is preferably 0.70%, more preferably 0.60%, even more preferably 0.40%, and even more preferably 0.20%.
V: less than 0.30%
Vanadium (V) is an optional element, and may not be contained. That is, the V content may be 0%.
When V is contained, V precipitates (VC, VN, V (CN), etc.) are formed similarly to Nb. The V precipitates inhibit coarsening of crystal grains in the gas carburizing treatment by the pinning effect. Therefore, the bending fatigue strength and the surface fatigue strength of the mechanical structural member are improved. The above effects can be obtained to some extent if V is contained in a small amount.
However, when the V content is more than 0.30%, the steel becomes too hard even if the content of other elements is within the range of the present embodiment. In this case, the machinability of the steel material is reduced.
Accordingly, the V content is 0 to 0.30%, and in the case of being contained, 0.30% or less (i.e., more than 0 and 0.30% or less).
The preferable lower limit of the V content is 0.01%, more preferably 0.03%, still more preferably 0.04%.
The preferable upper limit of the V content is 0.20%, more preferably 0.15%, still more preferably 0.10%.
Mg: less than 0.0035 percent
Magnesium (Mg) is an optional element, and may be absent. That is, the Mg content may be 0%.
When contained, mg deoxidizes steel in the same manner as Al. In this case, the formation of coarse oxides is suppressed. Therefore, the bending fatigue strength and the surface fatigue strength of the mechanical structural member are improved. The above effects can be obtained to some extent if Mg is contained in a small amount.
However, when the Mg content is more than 0.0035%, the formation of coarse Mg oxides is promoted in the steel material even if the content of other elements is within the range of the present embodiment. In this case, the limiting working rate at the time of hot working is lowered.
Therefore, the Mg content is 0 to 0.0035%, and in the case of being contained, 0.0035% or less (i.e., more than 0 and 0.0035% or less).
The preferable lower limit of the Mg content is 0.0001%, more preferably 0.0003%, still more preferably 0.0005%.
The upper limit of the Mg content is preferably 0.0030%, more preferably 0.0028%, further preferably 0.0025%, further preferably 0.0020%.
Ca: less than 0.0030 percent
Calcium (Ca) is an optional element, and may not be contained. That is, the Ca content may be 0%.
When contained, ca refines sulfides in the steel material. Ca also promotes spheroidization of sulfides in steel. Therefore, the bending fatigue strength and the surface fatigue strength of the mechanical structural member are improved. The above effects can be obtained to some extent if Ca is contained in a small amount.
However, when the Ca content is more than 0.0030%, coarse Ca oxide is generated in the steel even if the content of other elements is within the range of the present embodiment. In this case, the bending fatigue strength and the surface fatigue strength of the mechanical structural member decrease.
Therefore, the Ca content is 0 to 0.0030%, and if included, 0.0030% or less (i.e., greater than 0 and 0.0030% or less).
The preferable lower limit of the Ca content is 0.0001%, more preferably 0.0002%, still more preferably 0.0005%, still more preferably 0.0007%, still more preferably 0.0010%.
The preferable upper limit of the Ca content is 0.0025%, more preferably 0.0022%, still more preferably 0.0020%.
Rare earth element (REM): less than 0.0050%
The rare earth element (REM) is an optional element, and may not be contained. That is, the REM content may be 0%. When contained, REM is dissolved in sulfide in the steel material, and MnS elongation is suppressed. As a result, the bending fatigue strength and the surface fatigue strength of the mechanical structural member are improved. The REM can be contained in a small amount to achieve the above-mentioned effects to some extent.
However, when the REM content is more than 0.0050%, coarse oxides are generated even if the content of other elements is within the range of the present embodiment. In this case, the bending fatigue strength and the surface fatigue strength of the mechanical structural member decrease.
Accordingly, the REM content is 0 to 0.0050%, and in the case of being contained, 0.0050% or less (that is, more than 0 and 0.0050% or less).
The preferable lower limit of the REM content is 0.0001%, more preferably 0.0010%, still more preferably 0.0020%.
The upper limit of the REM content is preferably 0.0045%, more preferably 0.0040%, further preferably 0.0035%, further preferably 0.0030%.
REM in the present specification means 1 or more elements selected from the group consisting of scandium (Sc) having an atomic number 21, yttrium (Y) having an atomic number 39, and lanthanum (La) having an atomic number 57 to lutetium (Lu) having an atomic number 71, which are lanthanoids. REM content in the present specification refers to the total content of these elements.
[ microstructure of Steel Material ]
The microstructure of the steel material of the present embodiment contains ferrite, and the balance is pearlite and/or bainite, and the area fraction of ferrite is 50 to 70%.
When the area fraction of ferrite is less than 50%, the area fraction of pearlite and/or bainite in the steel is too high. In this case, the hardness of the steel material is excessively increased. As a result, the machinability of the steel material is reduced.
On the other hand, when the area fraction of ferrite is more than 70%, the crystal grain size tends to be uneven during the gas carburizing treatment. Therefore, heat treatment deformation is excessively generated at the time of gas carburizing treatment.
When the area fraction of ferrite is 50 to 70%, the balance other than ferrite in the microstructure is pearlite and/or bainite, the machinability of the steel is sufficiently improved. Further, heat treatment deformation at the time of gas carburizing treatment can be suppressed.
In the steel material according to the present embodiment, the microstructure in each cross-sectional view position and each longitudinal-sectional view position contains 50 to 70% ferrite in terms of area fraction, and the balance pearlite and/or bainite.
The preferable lower limit of the area fraction of ferrite in each observation position (each cross-sectional observation position, each longitudinal-sectional observation position) is 52%, more preferably 55%, still more preferably 57%.
The preferable upper limit of the area fraction of ferrite in each observation position (each cross-sectional observation position, each longitudinal-sectional observation position) is 68%, more preferably 65%, still more preferably 63%.
[ unevenness of microstructure in Cross section of Steel Material ]
In the steel material according to the present embodiment, the variation in microstructure is further sufficiently suppressed in the cross section perpendicular to the longitudinal direction of the steel material. This will be described below.
Fig. 2 is a schematic view of a cross section perpendicular to the longitudinal direction of the steel material according to the present embodiment. Referring to fig. 2, the cross section CS of the steel material is circular with a radius R. In this cross section CS, the center position C1 of the cross section CS, and 8 positions C2 to C9 of R/2 arranged at 45 ° intervals around the center of the cross section CS as positions R/2 from the center position C1 of the cross section CS in the radial direction are defined as 9 "cross section observation positions" C1 to C9.
The microstructures at the cross-sectional view positions C1 to C9 satisfy the following (a) and (B).
(A) The arithmetic average of the area fractions of ferrite at the cross-sectional view positions C1 to C9 is 50 to 70%, and the standard deviation of the area fractions of ferrite is 4.0% or less.
(B) Among the average grain sizes of ferrite at the cross-sectional view positions C1 to C9, the ratio of the maximum average grain size to the minimum average grain size is 2.00 or less.
Hereinafter, (a) and (B) will be described in detail.
[ about (A) ]
As described in (a) above, in the steel material of the present embodiment, the arithmetic average value of the area fraction of ferrite at the cross-sectional view positions C1 to C9 is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less.
Since the standard deviation of the ferrite area fraction is 4.0% or less, the variation in the phase fraction of the microstructure at each of the cross-sectional observation positions C1 to C9 is sufficiently suppressed. Therefore, in the gas carburizing treatment, the occurrence timing of the martensitic transformation at each of the cross-sectional view positions C1 to C9 can be suppressed from being uneven.
When the standard deviation of the area fraction of ferrite in the cross-sectional view positions C1 to C9 is greater than 4.0%, the phase fraction at each of the cross-sectional view positions C1 to C9 is not uniform. In this case, the heat treatment deformation at the time of the gas carburizing treatment cannot be sufficiently suppressed.
Therefore, the standard deviation of the area fraction of ferrite at the cross-sectional view positions C1 to C9 is 4.0% or less.
The upper limit of the standard deviation of the area fraction of ferrite is preferably 3.8%, more preferably 3.5%, and even more preferably 3.0%. The lower limit of the standard deviation of the area fraction of ferrite is not particularly limited.
The lower limit of the standard deviation of the area fraction of ferrite is not particularly limited. The lower limit of the standard deviation of the area fraction of ferrite is preferably 0.1%, more preferably 0.5%, further preferably 1.0%, further preferably 1.5%.
[ concerning (B) ]
Among the average grain sizes of ferrite at the cross-sectional view positions C1 to C9, the ratio of the maximum average grain size to the minimum average grain size is referred to as "ferrite average grain size ratio". The ferrite average grain size ratio in the cross section is defined by the following formula.
Ferrite average grain size ratio= (maximum value of ferrite average grain size at C1 to C9)/(minimum value of ferrite average grain size at C1 to C9)
In the steel material of the present embodiment, the ferrite average grain size ratio at the cross-sectional view positions C1 to C9 is 2.00 or less. In this case, the variation in the average grain size of ferrite at each of the cross-sectional view positions C1 to C9 is sufficiently suppressed. That is, ferrite grains at each position are concentrated. Therefore, the occurrence of unevenness of martensitic transformation during carburizing treatment can be suppressed. Therefore, heat treatment deformation of the steel material during carburization can be suppressed.
When the ferrite average grain size ratio is more than 2.00, ferrite grains are not uniform at each of the cross-sectional view positions C1 to C9. In this case, the heat treatment deformation of the steel material during the gas carburizing treatment cannot be suppressed.
Therefore, the ferrite average grain size ratio is 2.00 or less.
The upper limit of the ferrite average grain size ratio is preferably 1.95, more preferably 1.90, and still more preferably 1.80.
The lower limit of the ferrite average grain size ratio is not particularly limited. The preferable lower limit of the ferrite average grain size ratio is 1.10, more preferably 1.20, still more preferably 1.30, still more preferably 1.40.
[ unevenness of microstructure in longitudinal section in Steel Material ]
In the steel material according to the present embodiment, the variation in microstructure is sufficiently suppressed not only in the cross section but also in a longitudinal section which is parallel to the longitudinal direction of the steel material and includes the central axis of the steel material. In the steel material according to the present embodiment, the variation in microstructure of not only the cross section but also the longitudinal section is sufficiently suppressed, and therefore, the heat treatment deformation occurring in three dimensions can be sufficiently suppressed. Hereinafter, suppression of unevenness of a microstructure in a longitudinal section will be described.
Fig. 3 is a schematic view of a longitudinal section, which is a section parallel to the longitudinal direction and includes a central axis, of the steel material according to the present embodiment. Referring to fig. 3, in a longitudinal section LS of the steel material, 3 center axis positions L1 to L3 arranged at R/2 intervals on a center axis CL1 of the steel material and 6R/2 positions L4 to L9 arranged at positions R/2 from each center axis position in the radial direction are defined as 9 "longitudinal section observation positions" L1 to L9.
The following (C) and (D) are satisfied in the microstructure at the vertical section observation positions L1 to L9 at the total 9.
(C) The arithmetic average value of the area fraction of ferrite at the longitudinal section observation positions L1 to L9 is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less.
(D) Among the average grain sizes of ferrite at the vertical section observation positions L1 to L9, the ratio of the maximum average grain size to the minimum average grain size (ferrite average grain size ratio) is 2.00 or less.
Hereinafter, (C) and (D) will be described in detail.
[ about (C) ]
As described in (C) above, in the steel material of the present embodiment, the arithmetic average value of the area fraction of ferrite at the longitudinal section observation positions L1 to L9 is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less.
Since the standard deviation of the area fraction of ferrite is 4.0% or less, the phase fraction of the microstructure at each of the longitudinal section observation positions L1 to L9 is sufficiently suppressed from being uneven. Therefore, in the gas carburizing treatment, the occurrence timing of the martensitic transformation at each of the longitudinal section observation positions L1 to L9 can be suppressed from being uneven.
When the standard deviation of the area fraction of ferrite in the longitudinal section observation positions L1 to L9 is greater than 4.0%, the phase fraction at each of the longitudinal section observation positions L1 to L9 is not uniform. In this case, the heat treatment deformation at the time of the gas carburizing treatment cannot be sufficiently suppressed.
Therefore, the standard deviation of the area fraction of ferrite at the longitudinal section observation positions L1 to L9 is 4.0% or less.
The upper limit of the standard deviation of the area fraction of ferrite is preferably 3.8%, more preferably 3.5%, and even more preferably 3.0%.
The lower limit of the standard deviation of the area fraction of ferrite is not particularly limited. The lower limit of the standard deviation of the area fraction of ferrite is preferably 0.1%, more preferably 0.5%, further preferably 1.0%, further preferably 1.5%.
[ about (D) ]
Among the average grain sizes of ferrite at the vertical section observation positions L1 to L9, the ratio of the maximum average grain size to the minimum average grain size is referred to as "ferrite average grain size ratio". The ferrite average grain size ratio in the longitudinal section is defined by the following formula.
Ferrite average grain size ratio = (maximum value of ferrite average grain size at L1 to L9)/(minimum value of ferrite average grain size at L1 to L9)
In the steel material according to the present embodiment, the ferrite average grain size ratio at the vertical section observation positions L1 to L9 is 2.00 or less. In this case, the variation in the average grain size of ferrite at each of the longitudinal section observation positions L1 to L9 is sufficiently suppressed. That is, ferrite grains at each position are concentrated. Therefore, the occurrence of unevenness of martensitic transformation during carburizing treatment can be suppressed. Therefore, heat treatment deformation of the steel material during carburization can be suppressed.
When the ferrite average grain size ratio is more than 2.00, ferrite grains are not uniform at each of the longitudinal section observation positions L1 to L9. In this case, the heat treatment deformation of the steel material during the gas carburizing treatment cannot be suppressed. Therefore, the ferrite average grain size ratio at the longitudinal section observation positions L1 to L9 at 9 is 2.00 or less.
The upper limit of the ferrite average grain size ratio is preferably 1.95, more preferably 1.90, and still more preferably 1.80.
The lower limit of the ferrite average grain size ratio is not particularly limited. The preferable lower limit of the ferrite average grain size ratio is 1.10, more preferably 1.20, still more preferably 1.30, still more preferably 1.40.
[ method of observing microstructure, area fraction of ferrite and method of measuring average ferrite grain size ratio at each observation position ]
The method for observing the microstructure at the cross-sectional observation positions C1 to C9 and the longitudinal-sectional observation positions L1 to L9, the area fraction of ferrite, and the method for measuring the average grain size ratio of ferrite of the steel material according to the present embodiment are as follows.
[ method of observing microstructure of Cross section CS ]
The microstructure of the cross section CS is observed as follows. Samples including each of the cross-sectional view positions C1 to C9 were collected from the steel material. The surface corresponding to the cross section CS of the surface of the sample was taken as the observation surface. In the observation plane, the observation field including the cross-sectional observation position was set to 0.5mm×1.0mm.
After polishing the observation surface of the sample, the observation surface was etched using 3% nitrate alcohol (nitrate-ethanol etching solution). The observation field of view (0.5 mm. Times.1.0 mm) of the etched observation surface was observed with an optical microscope at 100 times.
The contrast of each phase, such as ferrite, pearlite, and bainite, is different in the observation field. Specifically, in the observation field, ferrite is observed as white, and bainite and pearlite are observed as darker than ferrite. Therefore, ferrite can be easily distinguished from other phases (pearlite and bainite). Ferrite is determined based on the contrast.
[ about (A) ]
The area (. Mu.m) of ferrite in each observation field (each cross-sectional observation position) was determined 2 ). The area fraction (%) of ferrite in each observation field (each cross-sectional observation position) was obtained using the area of ferrite and the area of the observation field.
[ arithmetic mean of ferrite area fraction ]
The arithmetic average of ferrite area fractions (%) in 9 observation fields (cross-sectional observation positions) is defined as the arithmetic average (%) of ferrite area fractions at the cross-sectional observation positions C1 to C9 at 9.
[ method of obtaining standard deviation of area fraction of ferrite ]
The standard deviation (%) of the ferrite area fractions at the cross-sectional view positions C1 to C9 at 9 was calculated from the ferrite area fractions (%) in the 9 view fields (cross-sectional view positions). The standard deviation referred to herein is the sample standard deviation.
[ concerning (B) ]
The area (. Mu.m) of each ferrite grain observed in each of the above-mentioned observation fields (each of the cross-sectional observation positions C1 to C9) was measured 2 ). An arithmetic average of the areas of the ferrite grains was obtained. The equivalent circle diameter of the arithmetic average of the obtained areas was defined as the average particle diameter (μm) of ferrite at each of the cross-sectional view positions C1 to C9. Here, the equivalent circle diameter refers to the diameter (μm) of a circle having the same area as the arithmetic average of the areas of ferrite grains.
The average grain size of ferrite at the cross-sectional view positions C1 to C9 at 9 was obtained. Then, among the average grain sizes of these ferrite, the maximum average grain size (μm) and the minimum average grain size (μm) of the ferrite are determined. The ratio of the determined maximum average grain size to the minimum average grain size (ferrite average grain size ratio) was determined.
[ method of observing microstructure of longitudinal section LS ]
The microstructure of the longitudinal section LS is observed as follows. Samples including each of the longitudinal section observation positions L1 to L9 were collected from the steel material. The surface corresponding to the longitudinal section LS of the surface of the sample was used as the observation surface. In the observation plane, the observation field including the observation position of the longitudinal section was set to 0.5mm×1.0mm. More specifically, the length of 0.5mm of the observation field was defined as the radial direction of the steel material, and 1.0mm was defined as the longitudinal direction of the steel material.
After polishing the observation surface of the sample, the observation surface was etched using 3% nitrate alcohol (nitrate-ethanol etching solution). The observation field of view (0.5 mm. Times.1.0 mm) of the etched observation surface was observed with an optical microscope at 100 times. The phases in the field of view of the observation are determined in the same way as for the microscopic observation of the cross section CS.
[ about (C) ]
In the phase determined by the above method, each observation field (each longitudinal sectionArea of ferrite (μm) in the surface observation position) 2 ). The area fraction (%) of ferrite in each observation field (each vertical section observation position) was obtained using the area of ferrite and the area of the observation field.
[ arithmetic mean of area fraction of ferrite ]
The arithmetic average of ferrite area fractions (%) in the 9 observation fields (longitudinal section observation positions) is defined as the arithmetic average (%) of ferrite area fractions at the 9 longitudinal section observation positions L1 to L9.
[ method of obtaining standard deviation of area fraction of ferrite ]
The standard deviation (%) of the ferrite area fractions at the longitudinal section observation positions L1 to L9 at 9 was calculated from the ferrite area fractions (%) in the 9 observation fields (longitudinal section observation positions).
[ about (D) ]
The area (. Mu.m) of each ferrite grain observed in each of the above-mentioned observation fields (each of the longitudinal section observation positions L1 to L9) was measured 2 ). An arithmetic average of the areas of the ferrite grains was obtained. The equivalent circle diameter of the arithmetic average of the obtained areas is defined as the average grain diameter (μm) of ferrite at each of the longitudinal section observation positions L1 to L9.
The average grain size of ferrite at 9 longitudinal section observation positions L1 to L9 was obtained. Then, among the average grain sizes of these ferrite, the maximum average grain size (μm) and the minimum average grain size (μm) of the ferrite are determined. The ratio of the determined maximum average grain size to the minimum average grain size (ferrite average grain size ratio) was determined.
[ concerning (1) ]
As described above, in the steel material according to the present embodiment, the microstructure in the longitudinal section LS is made as uniform as possible in addition to the cross section CS. In the steel material according to the present embodiment, further, on the premise that a minute period of time is generated in which the martensite transformation portion and the martensite non-transformation portion are mixed at the time of quenching, the occurrence of heat treatment strain in the martensite non-transformation portion is suppressed by satisfying the formula (1). As a result, it is possible to sufficiently suppress heat treatment deformation while maintaining sufficient machinability, and sufficient bending fatigue strength and sufficient surface fatigue strength in the mechanical structural member. The following describes formula (1).
The chemical composition of the steel product according to the present embodiment further satisfies the following formula (1) on the premise that the content of each element in the chemical composition is within the above-described range.
1-(0.5C+0.03Si+0.06Mn+0.01Cr+0.05Mo)<0.800 (1)
Wherein the content (mass%) of the corresponding element is substituted at each element symbol in the formula (1). When the corresponding element is not contained, a "0" is substituted at the symbol of the element.
In the steel material of the present embodiment, the microstructure of not only the cross section CS but also the longitudinal section LS is uniform. However, when the microstructures at the cross-section observation positions C1 to C9 in the cross section CS satisfy (a) and (B), and the microstructures at the longitudinal-section observation positions L1 to L9 in the longitudinal section LS satisfy (C) and (D), even if the microstructures are made uniform, as described above, a minute period in which the martensite transformation portion and the martensite non-transformation portion exist in a mixed state is inevitably generated at the time of quenching by the gas carburizing treatment. If the amount of heat treatment strain in the martensite non-transformation portion is large in this minute period, heat treatment deformation occurs. For this reason, the steel material according to the present embodiment further satisfies the formula (1).
Defined as f1=1- (0.5c+0.03si+0.06mn+0.01cr+0.05mo). F1 is an index concerning the heat treatment deformation amount of the steel material when the steel material is subjected to the gas carburizing treatment. Among the elements in the above chemical composition, C, si, mn, cr and Mo contained in F1 increase the strength of the martensite non-transformation portion particularly in a minute period in which the martensite transformation portion and the martensite non-transformation portion exist in a mixed state at the time of quenching.
Referring to fig. 1, as F1 decreases, the maximum deformation ratio decreases, and the heat treatment deformation decreases. Further, when F1 is less than 0.800, the maximum deformation ratio is significantly reduced. That is, the maximum deformation ratio has an inflection point in the vicinity of f1=0.800 with respect to F1.
Therefore, if the content of each element falls within the range of the present embodiment, and if F1 satisfies the formula (1), that is, if F1 is less than 0.800, the heat treatment deformation of the steel material during the gas carburizing treatment can be sufficiently suppressed.
The upper limit of F1 is preferably 0.799, more preferably 0.797, and still more preferably 0.795. The lower limit of F1 is not particularly limited. However, in view of the upper limit of the content of each element in the chemical composition of the present embodiment, the preferable lower limit of F1 is 0.765, more preferably 0.770, and still more preferably 0.775. The value of F1 is obtained by rounding the 4 th bit after the decimal point.
The steel material of the present embodiment having the above-described structure has the content of each element in the chemical composition within the scope of the present embodiment, and the microstructure at the cross-sectional view positions C1 to C9 and the longitudinal-sectional view positions L1 to L9 satisfies the formula (1) within the scope of the present embodiment. Therefore, the steel material according to the present embodiment is excellent in machinability after hot working. Further, when the steel material according to the present embodiment is subjected to the gas carburization treatment, the mechanical structural member has excellent bending fatigue strength and surface fatigue strength, and heat treatment deformation can be sufficiently suppressed.
[ microstructure of Steel Material ]
The steel material of the present embodiment is a so-called rolled material (as-rolled material). Therefore, in the steel material according to the present embodiment, a so-called band-like structure is observed in the above-described observation fields of view at the longitudinal section observation positions L1 to L9. The band structure is a known microstructure, and as shown in fig. 4, it is a structure in which ferrite (ferrite band) F extending in the longitudinal direction of the steel material and non-ferrite (non-ferrite band) NF extending in the longitudinal direction of the steel material are alternately laminated in the radial direction. The non-ferrite is pearlite and/or bainite.
[ use of Steel material ]
The steel material according to the present embodiment is suitable for a blank for a mechanical structural member as described above. The steel material according to the present embodiment is particularly suitable for applications such as gears and shafts of automobiles, construction machines, industrial machines, and the like.
[ method for producing Steel material ]
An example of the method for producing a steel material according to the present embodiment will be described. The method for producing a steel material described below is an example for producing the steel material of the present embodiment. Therefore, the steel material having the above-described structure may be produced by a production method other than the production method described below. However, the manufacturing method described below is a preferred example of the manufacturing method of the steel material according to the present embodiment.
An example of the method for producing a steel material according to the present embodiment includes the following steps.
(step 1) step of preparing a blank (blank preparation step)
(step 2) a step of producing a steel material by hot working the blank (hot working step)
Hereinafter, each step will be described.
[ (Process 1) blank preparation Process ]
In the billet preparation step, a billet of the steel material according to the present embodiment is prepared. Specifically, molten steel having a chemical composition in which the content of each element is within the range of the present embodiment and F1 satisfies formula (1) is produced. The refining method is not particularly limited, and a known method may be used. For example, refining (primary refining) by a converter is performed on molten iron manufactured by a known method. The molten steel tapped from the converter is subjected to known secondary refining. In the secondary refining, the content of the alloy element in the molten steel is adjusted, and molten steel having a chemical composition in which the content of each element is within the range of the present embodiment and in which F1 satisfies the formula (1) is produced.
Using the molten steel produced by the above-described refining method, a billet is produced by a known casting method. For example, an ingot can be produced from molten steel by an ingot casting method. In addition, a bloom or a billet may be produced by a continuous casting method using molten steel. By the above method, a charge (ingot, bloom, or billet) is produced. In the case of using the continuous casting method, the casting blank during solidification can be subjected to reduction.
[ (Process 2) Hot working Process ]
In the hot working step, the billet (ingot, bloom, or billet) prepared in the billet preparation step is subjected to hot working to produce the steel material according to the present embodiment. The shape of the steel material is not particularly limited, and is, for example, a steel bar or a wire rod. In the following description, a case where the steel material is a steel bar will be described as an example. However, even if the steel material is shaped other than the bar steel, it can be produced in the same hot working process.
The hot working process includes the following steps.
(step 21) blooming step
Heating temperature: 1250-1300 DEG C
Holding time: for more than 10 hours
(step 22) finish rolling step
Heating temperature: 1150-1200 DEG C
Holding time: 1.5 to 3.0 hours
Final temperature: 950-1000 DEG C
(step 23) temperature maintaining step
Average cooling rate at 900-800 ℃): less than 0.05 ℃/sec
(step 24) Cooling step
Average cooling rate at 800-300 ℃): 0.10 to 1.00 ℃/s
Hereinafter, each step will be described.
[ (Process 21) preliminary rolling Process ]
In the initial rolling step, the billet is hot rolled to produce a billet. Specifically, in the blooming step, the slab is hot rolled (blooming) by a blooming mill to produce a billet. When a continuous rolling mill is provided downstream of the blooming mill, the bloomed billet can be hot rolled by the continuous rolling mill to produce a small-sized billet. In the tandem mill, a horizontal mill having a pair of horizontal rolls and a vertical mill having a pair of vertical rolls are alternately arranged in a row. As described above, in the blooming step, the billet is formed into a billet using a blooming mill or using a blooming mill and a tandem mill.
The conditions in the blooming step are as follows.
Heating temperature: 1250-1300 DEG C
Holding time: for more than 10 hours
The heating temperature in the heating furnace in the initial rolling process is 1250-1300 ℃. The holding time at the heating temperature (1250-1300 ℃) is more than 10 hours. If the heating temperature in the heating furnace in the initial rolling step is 1250 to 1300 ℃, and further if the holding time at the heating temperature (1250 to 1300 ℃) is 10 hours or more, the solidification segregation in the billet generated in the billet preparation step can be sufficiently alleviated on the premise that other manufacturing conditions are satisfied. Therefore, the standard deviation of the area fraction of ferrite at each of the cross-sectional view positions C1 to C9 and each of the longitudinal-sectional view positions L1 to L9 is 4.0% or less. The upper limit of the holding time at the heating temperature is not particularly limited. However, in view of production cost, the preferable upper limit of the holding time at the heating temperature is 30 hours.
The billet produced in the initial rolling step is naturally cooled (air-cooled) to room temperature before the finish rolling step.
The reduction of area in the initial rolling step was 30% or more. Here, the cross-sectional shrinkage (%) is defined by the following formula.
Area of cross section shrinkage (%) = (1-cross section of steel after blooming (cross section perpendicular to the longitudinal direction)/area of cross section of billet before blooming (cross section perpendicular to the longitudinal direction)) ×100
If the reduction of area in the initial rolling step is 30% or more, the standard deviation of the area fraction of ferrite at each of the cross-sectional view positions C1 to C9 and each of the longitudinal-sectional view positions L1 to L9 becomes 4.0% or less on the premise that other manufacturing conditions are satisfied.
[ (Process 22) finish-rolling Process ]
In the finish rolling step, first, the billet cooled to the normal temperature is heated by a heating furnace. The heated billet was hot-rolled by using a continuous rolling mill to produce a steel bar.
The conditions in the finish rolling step are as follows.
Heating temperature: 1150-1200 DEG C
Holding time: 1.5 to 3.0 hours
Final temperature: 950-1000 DEG C
[ heating temperature and holding time ]
The heating temperature in the heating furnace in the finish rolling step is 1150-1200 ℃. The holding time at the heating temperature (1150-1200 ℃) is 1.5-3.0 hours. If the heating temperature of the heating furnace in the finish rolling step is 1150 to 1200 ℃ and the holding time at the heating temperature (1150 to 1200 ℃) is 1.5 to 3.0 hours, it is possible to sufficiently suppress the temperature variation in the steel material (steel bar) on the premise that other manufacturing conditions are satisfied. Therefore, the ferrite average grain size ratio at the cross-sectional view positions C1 to C9 and the longitudinal-sectional view positions L1 to L9 is 2.00 or less.
[ final temperature ]
In the finish rolling step, hot rolling (finish rolling) is performed by a continuous rolling mill including a plurality of rolling mills arranged in a row. In hot rolling using a continuous rolling mill, the temperature of steel on the outlet side of the mill from which the steel is finally rolled is defined as the final temperature (. Degree. C.). The steel temperature refers to the surface temperature of the steel.
The final temperature is 950-1000 ℃. If the final temperature is 950 to 1000 ℃, the unevenness of austenite grain size in the steel material (steel bar) is sufficiently suppressed on the premise that other manufacturing conditions are satisfied. Therefore, when austenite is transformed into ferrite in the temperature maintaining step and the cooling step, which will be described later, the variation in average grain size of ferrite is sufficiently suppressed. Therefore, the ferrite average grain size ratio at the cross-sectional view positions C1 to C9 and the longitudinal-sectional view positions L1 to L9 is 2.00 or less.
[ (step 23) temperature maintenance step ]
In the temperature maintaining step, the temperature of the steel material is maintained after the finish rolling step and before the cooling step. The conditions in the temperature maintaining step are as follows.
The average cooling speed of the steel at 900-800℃: less than 0.05 ℃/sec
After the finish rolling step, the average cooling rate of the steel material at 900-800 ℃ is suppressed to 0.05 ℃/sec or less. For example, after the finish rolling step, the average cooling rate of a steel material having a steel material temperature of 900 to 800 ℃ is controlled to 0.05 ℃/sec or less by using a slow cooling hood, a heat retaining hood or a temperature retaining furnace.
If the average cooling rate of the steel material at 900 to 800 ℃ is 0.05 ℃/sec or less, the temperature unevenness in the axial direction (longitudinal direction) of the steel material can be suppressed on the premise that other production conditions are satisfied. Therefore, the variation in ferrite transformation timing in the axial direction of the steel material can be suppressed. Therefore, in particular, the uneven growth of ferrite grains in the axial direction (longitudinal section) of the steel material can be suppressed. Specifically, the following mechanism works.
In the steel material after the finish rolling step, austenite gradually changes into ferrite as the temperature of the steel material decreases. If there is a temperature unevenness in the axial direction of the steel material in the range of 900 to 800 ℃, the ferrite produced at an earlier stage and the ferrite produced at a later stage are mixed after the finish rolling step. In this case, ferrite grains generated at an earlier stage are more likely to become coarse than ferrite grains transformed at a later stage. As a result, the ferrite grains become more uneven particularly in the axial direction (longitudinal section) of the steel.
If the average cooling rate of the steel material is high at 900 to 800 ℃, the temperature unevenness in the axial direction (longitudinal section) of the steel material becomes large. Therefore, the ferrite average grain size ratio at the vertical section observation positions L1 to L9 becomes large.
Therefore, in the present embodiment, the average cooling rate of the steel material at 900 to 800 ℃ is suppressed to 0.05 ℃/sec or less. In this case, temperature unevenness in the axial direction (longitudinal section) of the steel material can be suppressed. Therefore, the deviation of the timing of ferrite generation (transformation) in the longitudinal section of the steel is suppressed. As a result, the ferrite average grain size ratio at the vertical section observation positions L1 to L9 can be made to be 2.00 or less on condition that other manufacturing conditions are satisfied.
[ (procedure 24) Cooling procedure ]
In the cooling step, the steel material temperature after the temperature maintaining step is cooled. The conditions in the cooling step are as follows.
The average cooling speed of the steel at 800-300℃:0.10 to 1.00 ℃/s
The steel material with the steel material temperature of 800-300 ℃ is cooled at the average cooling speed of 0.10-1.00 ℃/s. If the average cooling rate at 800 to 300 ℃ of the steel material is set to 0.10 to 1.00 ℃/sec, the arithmetic average of the area fractions of ferrite at the cross-sectional view positions C1 to C9 and the longitudinal-sectional view positions L1 to L9 becomes 50 to 70% on the premise that other manufacturing conditions are satisfied.
The steel material according to the present embodiment having the above-described configuration can be produced by the above-described production steps. As described above, the steel material according to the present embodiment is a rolled material.
The steel material according to the present embodiment is suitable for a blank of a machine structural member manufactured by performing gas carburizing treatment. However, the steel material according to the present embodiment may be subjected to a surface hardening heat treatment other than the gas carburization treatment to produce a component for a machine structure. Other case hardening heat treatments are, for example, quenching and tempering, induction hardening tempering, nitriding treatment (nitriding quenching and tempering), and the like.
[ concerning parts for machine structural use ]
Parts for machine structural use are used for example in automobiles, construction vehicles and the like. The mechanical structural member is, for example, a gear, a rotary shaft, or the like for a steering mechanism.
The mechanical structural member using the steel material according to the present embodiment as a blank is manufactured by a known manufacturing method. The machine structural member is manufactured, for example, by the following method.
An example of a method for manufacturing a mechanical structure member includes the following steps.
Thermal working procedure
Cutting process
Heat treatment step
Hereinafter, each step will be described.
[ thermal working procedure ]
In the hot working step, the steel material according to the present embodiment is subjected to hot working. The hot working is, for example, known hot forging. The heating temperature in the heat treatment step is, for example, 1000 to 1300 ℃. The steel after the hot working is naturally cooled (air-cooled). The naturally cooled steel material may be annealed as needed.
[ cutting Process ]
The steel material after the hot working step is subjected to a cutting working step to produce an intermediate product of a predetermined shape. In this cutting process, high machinability of the steel material is required. In the cutting process, a known cutting process is performed. By the cutting process, a precision-shaped part for a mechanical structure which is difficult to manufacture by only the hot working process can be manufactured.
[ Heat treatment Process ]
And (3) performing heat treatment on the intermediate product after cutting. Here, the "heat treatment" includes a known gas carburizing treatment and a known tempering step. As described above, the gas carburizing treatment also includes a gas nitrocarburizing treatment.
The gas carburizing treatment includes a gas carburizing process and a quenching process. In the gas carburizing treatment, the known conditions may be appropriately adjusted, so that the surface hardness, core hardness, and surface carbon concentration of the mechanical structural member may be appropriately adjusted, which are technical matters known to those skilled in the art.
Hereinafter, a known gas carburizing process will be described as an example of the heat treatment step. It is known to those skilled in the art that the known gas carbonitriding treatment is also performed in the same process as the gas carbonitriding treatment.
[ gas carburizing treatment ]
The gas carburizing treatment includes a gas carburizing process and a quenching process. Hereinafter, a gas carburizing step and a quenching step will be described.
[ gas carburizing Process ]
Fig. 5 is a diagram showing an example of the heating pattern in the gas carburizing step S10 and the quenching step S20. In FIG. 5, the vertical axis represents the treatment temperature (. Degree. C.) at the time of gas carburizing treatment, and the horizontal axis represents time (minutes). Referring to fig. 5, the gas carburizing process S10 includes a heating process S0, a carburizing process S1, and a diffusion process S2.
In the heating step S0, the intermediate product charged into the furnace is heated to a carburizing temperature Tc. The carburizing temperature Tc in the heating step S0 is, for example, 830 to 1100 ℃.
In the carburizing step S1, the intermediate product is held for a predetermined time (holding time t 1) at a predetermined carbon potential Cp1 atmosphere and a carburizing temperature Tc. The carbon potential Cp1 in the carburizing step S1 is, for example, 0.5 to 1.2%, and the holding time t1 at the carburizing temperature Tc is, for example, 60 minutes or longer.
In the diffusion step S2, the diffusion step is carried out in an atmosphere having a predetermined carbon potential Cp2 at a carburizing temperature Tc for a predetermined time (a holding time t 2). Here, the carbon potential Cp2 in the diffusion step S2 is, for example, 0.5 to 1.2%, and the holding time t2 at the carburizing temperature Tc is, for example, 30 minutes or longer. The carbon potential Cp2 in the diffusion step S2 is preferably lower than the carbon potential Cp1 in the carburization step S1.
[ quenching Process ]
And a quenching step S20 for quenching the intermediate product after the gas carburizing step S10. In the quenching step S20, A is r3 The quenching temperature Ts of the spot or more is kept at the intermediate product after the gas carburizing step S10, and then the intermediate product is quenched and quenched. The holding time t3 at the quenching temperature Ts is not particularly limited, and is, for example, 30 to 60 minutes. The quenching temperature Ts is preferably lower than the carburizing temperature Tc. The cooling method in the quenching treatment is oil cooling or water cooling. Specifically, the intermediate product held at the quenching temperature is immersed in a cooling bath to which oil or water is added as a cooling medium, and quenched. The temperature of the oil or water as the cooling medium is, for example, room temperature to 200 ℃. Further, if necessary, a cryogenic treatment may be performed.
[ tempering step ]
And (3) performing a known tempering process on the intermediate product after the quenching process. The tempering temperature is, for example, 100 to 200 ℃. The holding time at the tempering temperature is, for example, 90 to 150 minutes.
Through the above steps, the steel material according to the present embodiment is used as a blank to manufacture a machine structural member.
[ other procedures ]
The intermediate product after the tempering step may be further subjected to grinding or shot blasting, if necessary. In the case of performing grinding, cutting is performed to give a shape to the steel material. By performing the cutting process, a precise shape can be imparted to the steel material. In the case of performing shot peening, compressive residual stress may be introduced into the surface layer portion of the intermediate product after gas carburizing. Compressive residual stress inhibits the generation and development of fatigue cracks. Therefore, the bending fatigue strength and the surface fatigue strength of the mechanical structural member are improved. The shot blasting may be performed by a known method. For example, it is desirable to use shot having a diameter of 0.7mm or less and to perform the shot blasting under the condition that the arc height is 0.4mm or more.
Example 1
Steels having chemical compositions shown in table 1 were prepared. The steel grade a corresponds to SCr420H defined in JIS G4052 (2016). Further, as the reference steel material, a steel material having a chemical composition corresponding to SCM420H specified in JIS G4052 (2016) was used.
TABLE 1
TABLE 1
Figure BDA0004149284200000391
The "-" in table 1 indicates that the content of the corresponding element is 0% in the significant figures (values up to the minimum number of digits) specified in the embodiment. In other words, the number of mantissas of significant digits (a number up to the minimum number of digits) specified in the above embodiment is 0% when rounded in the corresponding element content.
For example, the Mo content specified in the present embodiment is specified by a numerical value up to the 2 nd position after the decimal point. Thus, in steel grade A in Table 1, M to be measured is represented o The content was 0% when rounded at position 3 after the decimal point.
The Nb content specified in the present embodiment is specified by a numerical value up to the 3 rd bit after the decimal point. Therefore, in steel grade a in table 1, it is indicated that the measured Nb content was 0% when the 4 th bit was rounded after the decimal point.
The rounding means rounding off if the number of bits (mantissa) subsequent to the predetermined minimum number of bits is smaller than 5, and rounding off if the number is 5 or more.
The steel material was produced by the following method. Molten steel is continuously cast to produce a cast slab (bloom) as a billet. The hot working process was performed on the bloom as a blank under the conditions shown in table 2.
TABLE 2
TABLE 2
Figure BDA0004149284200000401
The temperatures indicated in the columns "heating temperature (. Degree. C.) in the columns" initial rolling step "and" finish rolling step "are heating temperatures (. Degree. C.). The "holding time (hours)" in the columns of the "initial rolling step" and "finish rolling step" are the holding times (hours) at the heating temperature. The "final temperature (. Degree. C.) in the column of the" finish rolling step "is the temperature of the steel material at the outlet side of the rolling mill (the surface temperature of the steel material) in the finish rolling step in the continuous rolling mill. The "cooling rate (. Degree. C./second)" in the column of the "temperature maintaining step" is the average cooling rate (. Degree. C./second) at a steel temperature of 900 to 800 ℃. The "cooling rate (. Degree. C./second)" in the column of "cooling step" refers to the average cooling rate (. Degree. C./second) at a steel temperature of 800 to 300 ℃.
In the initial rolling step of the hot working step, the produced bloom was heated at the heating temperature and holding time shown in table 2. Then, the bloom is bloomed by using a blooming mill to produce a billet. The produced billets were water cooled to room temperature (25 ℃). The square billet has a rectangular cross section of 162mm×162mm perpendicular to the longitudinal direction. The heating temperature and holding time in the blooming step are shown in table 2. The reduction of area in the initial rolling step was 30% or more under any production conditions.
The billets after the initial rolling step were subjected to a finish rolling step under the conditions shown in Table 2 to produce steel materials (bars) having a diameter of 50 mm. Specifically, the billets were heated at the heating temperatures (. Degree. C.) and holding times (hours) shown in the finishing step column of Table 2. And (3) performing finish rolling on the heated billet to manufacture the bar steel. At this time, the final temperature (. Degree. C.) is shown in Table 2.
The steel (bar) after the finish rolling step is subjected to a temperature maintaining step. In the production conditions a to i, the average cooling rate of a steel material having a steel material temperature of 900 to 800 ℃ is adjusted to 0.05 ℃/sec or less by using a slow cooling jacket. On the other hand, in the production condition j, the steel material having a steel material temperature of 900 to 800 ℃ is naturally cooled without using a slow cooling jacket. Therefore, the average cooling rate of the steel material at 900-800 ℃ is more than 0.05 ℃/s.
After the temperature maintaining step, a cooling step is performed. Specifically, the average cooling rate (DEG C/sec) of the steel material at 800 to 300 ℃ under each production condition is shown in the column of the cooling process in Table 2.
Naturally cooling (air cooling) the steel material with the steel material temperature below 300 ℃ to normal temperature. Through the above production steps, steels (bar steels) of test numbers 1 to 31 shown in table 3 were produced. Test No. 31 is an example using SCM420H as a reference steel, and the production condition a, which is one of production methods generally used for steel materials having the chemical composition of SCM420H, was used.
TABLE 3
TABLE 3 Table 3
Figure BDA0004149284200000421
[ evaluation test ]
The following matters were obtained for the steel materials (steel bars) of each test number manufactured by the above manufacturing steps.
(A1) Microscopic structure observation of cross section
(A11) Arithmetic mean of area fractions of ferrite of cross section
(A12) Standard deviation of area fraction of ferrite of cross section
(A13) Ferrite average grain size ratio of cross section
(B1) Observation of microstructure of longitudinal section
(B11) Arithmetic mean value of area fraction of ferrite of longitudinal section
(B12) Standard deviation of area fraction of ferrite of longitudinal section
(B13) Ferrite average grain size ratio of longitudinal section
Further, the following evaluation tests were performed on the steel materials of the respective test numbers.
(C1) Machinability evaluation test
(C2) Flexural fatigue Strength evaluation test
(C3) Evaluation test of fatigue Strength of surface
(C4) Evaluation test of deformation amount by heat treatment
The following is a detailed description.
[ (A1) microscopic structural observations of Cross section ]
From each test-numbered steel material, 9 samples including each of the cross-sectional view positions C1 to C9 were collected. The surface corresponding to the cross section CS of the surface of each sample was taken as the observation surface. In the observation plane, the observation field including the cross-sectional observation position was set to 0.5mm×1.0mm.
After polishing the observation surface of the sample, the observation surface was etched using 3% nitrate alcohol (nitrate-ethanol etching solution). The observation field of view (0.5 mm. Times.1.0 mm) of the etched observation surface was observed with an optical microscope at 100 times. In the field of view, the phase is determined by contrast.
The observed phases are shown in the column "phases" of the column "cross section" of table 3. In the column "phase" of the column "cross section" of table 3, the microstructure at all the cross-sectional observation positions contains ferrite, and the balance is pearlite and/or bainite, denoted by ". In any test number, the microstructure of the cross section contains ferrite, and the balance is pearlite and/or bainite.
[ (A11) arithmetic mean of area fraction of ferrite of cross section ]
The area (. Mu.m) of ferrite in each observation field (each cross-sectional observation position) was determined 2 ). The area fraction (%) of ferrite in each observation field (each cross-sectional observation position) was obtained using the area of ferrite and the area of the observation field.
The arithmetic average of ferrite area fractions (%) in 9 observation fields (cross-sectional observation positions) is defined as the arithmetic average (%) of ferrite area fractions at the cross-sectional observation positions C1 to C9 at 9. The arithmetic average of the obtained ferrite area fractions is shown in the column "ferrite area fraction (%)" of the column "cross section" of table 3.
[ (A12) standard deviation of area fraction of ferrite of cross section ]
The standard deviation (%) of the ferrite area fractions at the cross-sectional view positions C1 to C9 at 9 was calculated from the ferrite area fractions (%) in the 9 view fields (cross-sectional view positions). The standard deviation obtained is shown in the column of "standard deviation (%)" of ferrite area fraction "in the column of" cross section "of table 3.
[ (A13) ferrite average particle diameter ratio of Cross section ]
Further, the area (. Mu.m) of each ferrite grain observed in each of the above-mentioned observation fields (each of the cross-sectional observation positions C1 to C9) was measured 2 ). An arithmetic average of the areas of the ferrite grains was obtained at each of the cross-sectional observation positions C1 to C9. The equivalent circle diameter of the arithmetic average of the obtained areas was defined as the average particle diameter (μm) of ferrite at each of the cross-sectional view positions C1 to C9.
The average grain size of ferrite at the cross-sectional view positions C1 to C9 at 9 was obtained. Then, among the average grain sizes of these ferrite, the maximum average grain size (μm) and the minimum average grain size (μm) of the ferrite are determined. The ratio of the determined maximum average grain size to the minimum average grain size (ferrite average grain size ratio) was determined. The obtained ferrite average grain size ratio is shown in the column "ferrite grain size ratio" of the column "cross section" of table 3.
[ (B1) observation of microstructure of longitudinal section ]
9 samples including each of the longitudinal section observation positions L1 to L9 were collected from each test-numbered steel material. The surface corresponding to the longitudinal section LS of the surface of each sample was used as the observation surface. In the observation plane, the observation field including the observation position of the longitudinal section was set to 0.5mm×1.0mm.
After polishing the observation surface of the sample, the observation surface was etched using 3% nitrate alcohol (nitrate-ethanol etching solution). The observation field of view (0.5 mm. Times.1.0 mm) of the etched observation surface was observed with an optical microscope at 100 times. In the field of view, the phase is determined by contrast.
The observed phases are shown in the column "phases" of the column "vertical section" of table 3. In the column "phase" of the column "vertical section" in table 3, when the microstructure at all the vertical section observation positions contains ferrite and the balance is pearlite and/or bainite, it is indicated as "o". In any test number, the microstructure of the longitudinal section contains ferrite, and the balance is pearlite and/or bainite.
[ (B11) arithmetic mean of area fraction of ferrite of longitudinal section ]
The area (. Mu.m) of ferrite in each observation field (each longitudinal section observation position) was obtained 2 ). The area fraction (%) of ferrite in each observation field (each vertical section observation position) was obtained using the area of ferrite and the area of the observation field.
The arithmetic average of ferrite area fractions (%) in the 9 observation fields (longitudinal section observation positions) is defined as the arithmetic average (%) of ferrite area fractions at the 9 longitudinal section observation positions L1 to L9. The arithmetic average of the obtained ferrite area fractions is shown in the column "ferrite area fraction (%)" of the column "vertical section" of table 3.
[ (B12) standard deviation of area fraction of ferrite of longitudinal section ]
The standard deviation (%) of the ferrite area fractions at the longitudinal section observation positions L1 to L9 at 9 was calculated from the ferrite area fractions (%) in the 9 observation fields (longitudinal section observation positions). The standard deviation obtained is shown in the column of "standard deviation (%) of ferrite area fraction" in the column of "vertical section" in table 3.
[ (B13) ferrite average particle diameter ratio of longitudinal section ]
Further, the area (. Mu.m) of each ferrite grain observed in each of the above-mentioned observation fields (each of the longitudinal section observation positions L1 to L9) was measured 2 ). An arithmetic average of the areas of the ferrite grains was obtained at each of the longitudinal section observation positions L1 to L9. The equivalent circle diameter of the arithmetic average of the obtained areas is defined as the average grain diameter (μm) of ferrite at each of the longitudinal section observation positions L1 to L9.
The average grain size of ferrite at 9 longitudinal section observation positions L1 to L9 was obtained. Then, among the average grain sizes of these ferrite, the maximum average grain size (μm) and the minimum average grain size (μm) of the ferrite are determined. The ratio of the determined maximum average grain size to the minimum average grain size (ferrite average grain size ratio) was determined. The obtained ferrite average grain size ratio is shown in the column "ferrite grain size ratio" of the column "vertical section" in table 3.
[ evaluation test ]
[ (C1) test for evaluation of machinability ]
The machinability evaluation test was performed as follows. A steel bar having a diameter of 50mm was subjected to a heat treatment simulating hot forging and a constant temperature annealing treatment. Specifically, the steel bars were heated at 1200 ℃ and held at 1200 ℃ for 30 minutes. Then, the steel bar is naturally cooled to room temperature. Further, the mixture was heated at 950℃and kept at 950℃for 1 hour. And then kept at 650 ℃ for 2 hours, and naturally cooled to room temperature. The naturally cooled steel bars were subjected to machining (cutting) to prepare test pieces for evaluating machinability, each having a diameter of 45mm and a length of 400 mm.
The test pieces of each test number were subjected to peripheral turning, and the tool life was evaluated. Specifically, the test pieces of each test number were subjected to outer periphery turning under the following conditions. The cutting tool used was an uncoated cemented carbide corresponding to P20 specified in JIS B4053 (2013). The cutting speed was 250 m/min, the feed speed was 0.35mm/rev, and the cut-in was 1.0mm. Water-soluble cutting oil is used in turning.
Peripheral turning was performed for 20 minutes under the above cutting conditions. Then, the flank wear (mm) of the cutting tool was measured.
The obtained flank wear (mm) is shown in the "wear (mm)" column of table 3. When the flank wear (mm) is less than 0.25mm, it is determined that the machinability of the steel material is high. When the obtained flank wear (mm) was 0.25mm or more, it was determined that the machinability of the steel material was low.
[ (C2) flexural fatigue Strength evaluation test ]
From each test-numbered steel material (steel bar having a diameter of 50 mm), an intermediate product of the small-field rotary bending test piece for bending fatigue strength evaluation shown in FIG. 6 was processed. The values in fig. 6 represent dimensions (in mm). "phi" in fig. 6 indicates the diameter. "R1" means that the radius of curvature of the bottom of the incision is 1mm.
Specifically, the steel materials (steel bars having a diameter of 50 mm) of each test number were heated at a heating temperature of 1200 ℃ for a holding time of 30 minutes. Then, hot working (hot forging) was performed at a final temperature of 950 ℃ or higher to produce a steel bar having a diameter of 35 mm. The steel bars with the diameter of 35mm were machined (cut) to obtain intermediate products of the small-field rotary bending test pieces. The diameter of the cross section of the intermediate product at the bottom of the cut was 8mm. The intermediate product was subjected to carburizing treatment (gas carburizing treatment and tempering), and a small-field rotary bending test piece shown in fig. 6 was produced. The conditions of the carburizing treatment performed are as follows.
[ gas carburizing treatment and tempering ]
The test piece was heated at 950℃for 240 minutes in an atmosphere having a carbon potential Cp1 of 0.8%. Then, the mixture was heated at 950℃for 60 minutes in an atmosphere having a carbon potential Cp2 of 0.8%. Then, the mixture was heated at 850℃for 30 minutes, and oil-cooled with 130℃oil. The oil-cooled test piece was tempered at a tempering temperature of 180℃for 120 minutes. Air cooling is performed after the holding time has elapsed.
The C concentration on the surface of the steel material (round bar) was adjusted to 0.80 mass% by the above gas carburizing method.
The small field type rotary bending fatigue test was performed using the carburized small field type rotary bending test piece. For each test number, a plurality of test pieces were prepared. The fatigue test was performed by changing the stress applied to each test piece and repeating the test 1000 ten thousand times (10 7 Secondary), the highest stress without fracture was taken as bending fatigue strength (MPa). In the small-field type rotating bending fatigue test, the rotating speed is 3000rpm, and the stress ratio is cyclically alternating.
The ratio of the bending fatigue strength of each test number to the bending fatigue strength of the reference steel was defined as the bending fatigue strength ratio. That is, the bending fatigue strength ratio (%) was determined by the following formula.
Bending fatigue strength ratio (%) = (bending fatigue strength (MPa) of each test number/bending fatigue strength (MPa) of the reference steel)) ×100
The obtained bending fatigue strength ratio (%) is shown in the column of "bending fatigue strength ratio (%)" in table 3. When the bending fatigue strength ratio obtained was 110% or more, it was determined that a sufficient bending fatigue strength could be obtained. On the other hand, if the bending fatigue strength ratio is less than 110%, it is determined that the bending fatigue strength is low.
[ (C3) surface fatigue Strength evaluation test ]
From each test-numbered steel (steel bar having a diameter of 50 mm), an intermediate product of a test piece for roll pitting fatigue test for use in the evaluation test of surface fatigue strength shown in FIG. 7 was processed. The values in fig. 7 represent dimensions (in mm). "phi" in the figure indicates the diameter.
Specifically, the steel materials (steel bars having a diameter of 50 mm) of each test number were heated at a heating temperature of 1200 ℃ for a holding time of 30 minutes. Then, hot working (hot forging) was performed at a final temperature of 950 ℃ or higher to produce a steel bar having a diameter of 35 mm. A35 mm diameter steel bar was machined (cut) to obtain an intermediate product of a test piece for roll pitting fatigue test. The intermediate product was subjected to carburizing treatment (gas carburizing treatment and tempering) under the above-described carburizing treatment conditions, and a test piece for roll pitting fatigue test (small roll test piece) shown in fig. 7 was produced.
Fig. 8 is a schematic diagram of a roll pitting fatigue test. As shown in fig. 8, the small roll test piece 200 is rotated while the large roll test piece 100 is pressed against the small roll test piece 200 by a surface pressure described later. The small roll test piece 200 is a test piece for roll pitting fatigue test manufactured by the method of the test piece described above. The large roll test piece has the shape shown in fig. 9. The values in fig. 9 represent dimensions (in mm). In the figure, "R700" represents that the radius of curvature of the outer peripheral surface is 700mm.
The large roll test piece 100 was a steel having a chemical composition corresponding to SCM420H specified in JIS G4053 (2016), and a test piece having been subjected to surface polishing after gas carburization under the same conditions as the small roll test piece 200. The diameter of the large roll test piece 100 was 130mm.
In the roll pitting fatigue test, the large roll test piece 100 was pressed against the small roll test piece 2 with surface pressures of various hertz stresses00. The two-roll test pieces at the contact portion were rotated with the circumferential speed direction of the two-roll test pieces being the same and the slip ratio being-40% (the circumferential speed of the contact portion of the large-roll test piece 100 being 40% greater than that of the small-roll test piece 200), and the test was performed. The oil temperature of the ATF (AT lubricating oil) supplied as the lubricating oil to the contact portion was 90 ℃, and the maximum surface pressure of the contact stress between the large roll test piece 100 and the small roll test piece 200 was 4000MPa. The test cut-off number was set to 2000 ten thousand times (2.0X10) 7 Secondary). For each test number, 2.0X10 were repeated for multiple test pieces 7 After that, the highest stress at which pitting did not occur was regarded as the surface fatigue strength (MPa).
The ratio of the surface fatigue strength of each test number to the surface fatigue strength of the reference steel was defined as the surface fatigue strength ratio. That is, the surface fatigue strength ratio (%) was determined by the following formula.
Surface fatigue strength ratio (%) = (surface fatigue strength (MPa) of each test number)/surface fatigue strength (MPa) of the reference steel) ×100
The obtained surface fatigue strength ratio (%) is shown in the column of "surface fatigue strength ratio (%)" in table 3. When the obtained surface fatigue strength ratio is 120% or more, it is determined that sufficient surface fatigue strength can be obtained. On the other hand, if the surface fatigue strength ratio is less than 120%, it is determined that the surface fatigue strength is low.
[ (C4) evaluation test of deformation by heat treatment ]
Gear simulation test pieces shown in FIG. 10A were produced from steel materials (steel bars having a diameter of 50 mm) of each test number. Specifically, the steel materials (steel bars having a diameter of 50 mm) of each test number were heated at a heating temperature of 1200 ℃ for a holding time of 30 minutes. Then, hot working (hot forging) was performed at a final temperature of 950 ℃ or higher to produce a steel bar having a diameter of 35 mm. A bar steel having a diameter of 35mm was machined (cut) to prepare a gear simulation test piece before gas carburizing treatment.
The numerical value in "mm" in FIG. 10A represents the dimension (in mm). "phi" in the figure indicates the diameter. The gear simulation test piece has a truncated cone shape. The gear simulation test piece had a circular upper surface with a diameter of 22mm and a circular lower surface with a diameter of 34 mm. The gear simulation test piece has a cylindrical through hole TH including a center axis CL 2. The diameter (inner diameter) of the through hole TH was 15mm, and the center axis of the through hole TH was aligned with the center axis of the gear simulation test piece.
The inner diameters (diameters) of the gear wheel simulation test pieces before carburization were measured at the respective positions in the longitudinal direction of the through-holes TH by a three-dimensional measuring instrument. As the three-dimensional measuring instrument, CNC three-dimensional measuring instrument (trade name: crystal-Apex) manufactured by Sanfeng, co., ltd was used.
Specifically, as shown in fig. 10B, a total of 16 inner diameters are measured at 1.0mm pitch positions from the upper end to the lower end in the longitudinal direction of the through hole TH in a range of 1.0 to 16.0mm from the upper end. Further, from the upper end toward the lower end in the longitudinal direction of the through-hole TH, the inner diameter was measured at a position of 0.5mm from the upper end and at a position of 16.5mm from the upper end. That is, the inner diameter of the through-hole TH is measured at 18 measurement positions in the longitudinal direction of the through-hole TH. Further, the inner diameters of a total of 18 parts (points P1 to P18 in fig. 10B) were measured at 10 ° intervals around the central axis CL2 at each measurement position. Accordingly, the inner diameter of the through-hole TH is measured at 18 measurement positions×18 sites=324 points.
The gear simulation test piece after the inner diameter measurement was subjected to gas carburizing treatment and tempering under the same conditions as those of the gas carburizing treatment and tempering in the small-field rotary bending fatigue test piece. The inner diameter of the through hole TH was measured for the gear simulation test piece after carburization in the same manner as the inner diameter measurement method for the through hole TH of the gear simulation test piece before carburization.
[ maximum deformation ratio of Heat treatment ]
Among the points P1 to P18 at each measurement position of the through hole TH, a value obtained by subtracting the inner diameter (μm) after carburization from the inner diameter (μm) before carburization is used as the heat treatment deformation amount at each point P1 to P18 at each measurement position. In each test number, the maximum heat treatment deformation amount was obtained from the measurement results of 324 points in total.
The ratio of the maximum heat treatment deformation amount of each test number to the maximum heat treatment deformation amount of the reference steel was defined as "maximum deformation amount ratio". That is, the maximum deformation ratio (%) was obtained by the following formula.
Maximum deformation ratio (%) = (maximum heat treatment deformation (mm) of each test number/maximum heat treatment deformation (mm) of the reference steel)) ×100
The obtained maximum deformation ratio (%) is shown in the column of "maximum deformation ratio (%)" in table 3. If the obtained maximum deformation ratio is 90% or less, it is determined that the maximum deformation ratio is small. On the other hand, if the maximum deformation ratio is more than 90%. It is determined that the maximum deformation ratio is large.
[ deformation ratio of Heat treatment ]
In each test number, the maximum heat treatment deformation amount and the minimum heat treatment deformation amount in the measurement results of 324 points in total were obtained. The value obtained by subtracting the minimum heat treatment deformation amount from the maximum heat treatment deformation amount was defined as the deformation amount difference (μm).
The ratio of the deformation difference of each test number to the deformation difference of the reference steel was defined as the deformation difference ratio. That is, the deformation ratio is obtained by the following equation.
Deformation ratio (%) = (deformation difference (μm) of each test number)/deformation difference (μm) of the reference steel ×100
The obtained deformation ratio (%) is shown in the column of "deformation ratio (%)" in table 3. If the obtained deformation ratio is 90% or less, it is determined that the deformation ratio is small. On the other hand, if the deformation ratio is greater than 90%, it is determined that the deformation ratio is large.
When the maximum deformation ratio and the deformation amount difference ratio are both 90% or less, it is determined that the heat treatment deformation can be sufficiently suppressed in three dimensions. If the maximum deformation amount ratio and/or the deformation amount difference ratio is greater than 90%, it is determined that the heat treatment deformation cannot be sufficiently suppressed.
[ test results ]
The test results are shown in table 3. Referring to table 3, the chemical compositions of the steels of test numbers 5 to 9 have suitable contents of each element, and F1 satisfies the formula (1). Furthermore, the steel products of test numbers 5 to 9 were also suitable for production conditions. Therefore, the microstructure of the steel material is suitable. Specifically, the microstructures of the cross sections of test nos. 5 to 9 were ferrite-containing microstructures, and the balance was pearlite and/or bainite. Further, the arithmetic average value of the area fraction of ferrite is 50 to 70%, the standard deviation of the area fraction of ferrite is 4.0% or less, and the average grain size ratio of ferrite is 2.00 or less. Further, the microstructures of the longitudinal sections of test numbers 5 to 7 were structures containing ferrite and the balance pearlite and/or bainite. Further, the arithmetic average value of the area fraction of ferrite is 50 to 70%, the standard deviation of the area fraction of ferrite is 4.0% or less, and the average grain size ratio of ferrite is 2.00 or less.
Therefore, in the machinability evaluation test, the flank wear amount was less than 0.25mm, and the machinability was high. Further, the bending fatigue strength ratio was 110% or more, the surface fatigue strength ratio was 120% or more, and both the bending fatigue strength and the surface fatigue strength were excellent. Further, the maximum deformation ratio and the deformation difference ratio in the heat treatment are 90% or less, and the heat treatment deformation is sufficiently suppressed in three dimensions.
On the other hand, in test No. 1, the Si content was too low. Further, F1 does not satisfy formula (1). Further, the holding time in the blooming step is less than 10 hours. Therefore, the standard deviation of the area fraction of ferrite of the cross section and the longitudinal section is greater than 4.0%. As a result, the bending fatigue strength and the surface fatigue strength are low. Further, the maximum deformation amount ratio and the deformation amount difference ratio in the heat treatment are more than 90%, and the heat treatment deformation cannot be sufficiently suppressed.
In test numbers 2 to 4, the holding time in the blooming step was less than 10 hours. Therefore, the standard deviation of the area fraction of ferrite of the cross section and the longitudinal section is greater than 4.0%. As a result, the deformation ratio in the heat treatment was more than 90%, and the heat treatment deformation could not be sufficiently suppressed.
In test nos. 10 to 12, the heating temperature in the blooming step was too low. Therefore, the standard deviation of the area fraction of ferrite of the cross section and the longitudinal section is greater than 4.0%. As a result, the deformation ratio in the heat treatment was more than 90%, and the heat treatment deformation could not be sufficiently suppressed.
In test nos. 13 to 15, the heating temperature in the finish rolling step was too low. Therefore, the ferrite average grain size ratio of the cross section and the longitudinal section is more than 2.00. As a result, the deformation ratio in the heat treatment was more than 90%, and the heat treatment deformation could not be sufficiently suppressed.
In test numbers 16 to 18, the holding time of the finish rolling step was too short. Therefore, the ferrite average grain size ratio of the cross section and the longitudinal section is more than 2.00. As a result, the deformation ratio in the heat treatment was more than 90%, and the heat treatment deformation could not be sufficiently suppressed.
In test numbers 19 to 21, the final temperature in the finish rolling step was too high. Therefore, the ferrite average grain size ratio of the cross section and the longitudinal section is more than 2.00. As a result, the deformation ratio in the heat treatment was more than 90%, and the heat treatment deformation could not be sufficiently suppressed.
In test numbers 22 to 24, the final temperature in the finish rolling step was too low. Therefore, the ferrite average grain size ratio of the cross section and the longitudinal section is more than 2.00. As a result, the deformation ratio in the heat treatment was more than 90%, and the heat treatment deformation could not be sufficiently suppressed.
In test nos. 25 to 27, the cooling rate in the cooling step was too low. Therefore, the arithmetic average of the area fractions of ferrite of the cross section and the longitudinal section is more than 70%. Therefore, the deformation ratio in the heat treatment is more than 90%, and the heat treatment deformation cannot be sufficiently suppressed.
In test nos. 28 and 29, the cooling rate in the cooling process was too high. Therefore, the arithmetic average of the area fractions of ferrite of the cross section and the longitudinal section is less than 50%. Therefore, the flank wear is 0.25mm or more. As a result, the machinability of the steel material is low.
In test No. 30, the cooling rate in the temperature maintaining step was too high. Therefore, in the microstructure of the longitudinal section, the ferrite average grain size ratio is more than 2.00. Therefore, the deformation ratio in the heat treatment is more than 90%, and the heat treatment deformation cannot be sufficiently suppressed.
Example 2
Steels having chemical compositions shown in table 4 were prepared in the same manner as in example 1.
TABLE 4
TABLE 4 Table 4
Figure BDA0004149284200000531
The steel material was produced by the following method. Steels (bar steels) of test numbers 1 to 32 shown in table 5 were produced in the same manner as in example 1 using production conditions b of table 2 for molten steel.
TABLE 5
TABLE 5
Figure BDA0004149284200000541
[ evaluation test ]
The steel (steel bar) produced in the above production steps was subjected to the same measurement and evaluation tests as in example 1 in the same manner as in example 1. In the (C2) bending fatigue strength evaluation test, the (C3) plane fatigue strength evaluation test, and the (C4) heat treatment deformation amount evaluation test, the reference steels in table 1 were used.
[ test results ]
The test results are shown in table 5. Referring to table 5, the chemical compositions of the steels of test numbers 1 to 17 have suitable contents of each element, and F1 satisfies the formula (1). Furthermore, the steel products of test numbers 1 to 17 were also suitable for production conditions. Therefore, the microstructure of the steel material is suitable. Specifically, the microstructures of the cross sections of test numbers 1 to 17 were composed of ferrite, pearlite and/or bainite, the arithmetic average value of the area fraction of ferrite was 50 to 70%, the standard deviation of the area fraction of ferrite was 4.0% or less, and the ferrite average particle diameter ratio was 2.00 or less. The microstructures of the longitudinal sections of test numbers 1 to 17 were composed of ferrite, pearlite and/or bainite, the arithmetic average value of the area fraction of ferrite was 50 to 70%, the standard deviation of the area fraction of ferrite was 4.0% or less, and the average grain diameter ratio of ferrite was 2.00 or less. Therefore, the flank wear is less than 0.25mm and the machinability is high. Further, the bending fatigue strength ratio was 110% or more, the surface fatigue strength ratio was 120% or more, and both the bending fatigue strength and the surface fatigue strength were excellent. Further, the maximum deformation amount ratio and the deformation amount difference ratio in the heat treatment are 90% or less, and the heat treatment deformation is sufficiently suppressed.
On the other hand, in test numbers 18 to 23, F1 was too high. Therefore, the maximum deformation ratio in the heat treatment is more than 90%, and the heat treatment deformation cannot be sufficiently suppressed.
In test No. 24, the C content was too high. Therefore, the flank wear is 0.25mm or more, and the machinability is low.
The Si content of test No. 25 was too low. Therefore, the ratio of the surface fatigue strength is less than 120%, and the surface fatigue strength is low.
The Si content of test No. 26 was too high. Therefore, the bending fatigue strength ratio is less than 110%, and the bending fatigue strength is low.
The Mn content of test No. 27 was too low. Therefore, the bending fatigue strength ratio is less than 110%, and the surface fatigue strength ratio is less than 120%. As a result, the bending fatigue strength and the surface fatigue strength are low.
The Mn content of test No. 28 was too high. Therefore, the bending fatigue strength ratio is less than 110%, and the surface fatigue strength ratio is less than 120%. As a result, the bending fatigue strength and the surface fatigue strength are low.
The Cr content of test No. 29 was too low. Therefore, the bending fatigue strength ratio is less than 110%, and the surface fatigue strength ratio is less than 120%. As a result, both the bending fatigue strength and the surface fatigue strength are insufficient.
The Cr content of test No. 30 was too high. Therefore, the flank wear is 0.25mm or more, and the machinability is low.
The Mo content of test No. 31 was too high. Therefore, the flank wear is 0.25mm or more, and the machinability is low.
The Nb content of test No. 32 was too high. Therefore, the bending fatigue strength ratio is less than 110%, and the bending fatigue strength is low.
The embodiments of the present application are described above. However, the above-described embodiments are merely examples for implementing the present application. Therefore, the present application is not limited to the above-described embodiment, and can be implemented with appropriate modifications within the scope of the gist thereof.

Claims (2)

1. A steel material, which is used for the production of steel,
the chemical composition of the composition comprises in mass percent
C:0.20~0.25%、
Si:0.40~0.70%、
Mn:0.50~0.90%、
Cr:1.00~2.00%、
S:0.005~0.050%、
N:0.0050~0.0200%、
Al:0.001~0.100%、
O:0.0050% or less
P: the content of the catalyst is less than or equal to 0.030 percent,
the balance being Fe and impurities, and satisfying the formula (1),
in a circular cross section having a radius R as a cross section perpendicular to the longitudinal direction of the steel material,
when the center position of the cross section and 8 positions of R/2 arranged at 45 ° intervals around the center of the cross section as positions of R/2 from the center of the cross section in the radial direction are defined as 9 cross-section observation positions,
the microstructure at each of the cross-sectional view locations comprises ferrite, the balance being pearlite and/or bainite,
the arithmetic average value of the area fraction of ferrite at the cross-sectional view position at 9 is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less,
The ratio of the maximum average grain size to the minimum average grain size of the average grain size of ferrite at the cross-sectional view position of 9 is 2.00 or less,
in a longitudinal section including a central axis of the steel material as a section parallel to the longitudinal direction of the steel material,
when defining 3 central axis positions arranged at R/2 intervals on the central axis and 6R/2 positions arranged at R/2 intervals from the respective central axis positions in the radial direction as 9 longitudinal section observation positions,
the microstructure at each longitudinal section observation position contains ferrite, and the balance is pearlite and/or bainite,
the arithmetic average value of the area fraction of ferrite at the position of observation of the longitudinal section at 9 is 50 to 70%, and the standard deviation of the area fraction of ferrite is 4.0% or less,
the ratio of the maximum average grain size to the minimum average grain size of the average grain size of ferrite at the position of the 9 longitudinal section observation is 2.00 or less,
1-(0.5C+0.03Si+0.06Mn+0.01Cr+0.05Mo)<0.800 (1)
wherein the content in mass% of the corresponding element is substituted at each element symbol in the formula (1), and when the corresponding element is not contained, 0 is substituted at the element symbol.
2. The steel product as claimed in claim 1, wherein,
The chemical composition further contains 1 or more elements selected from the group consisting of:
mo: less than 0.30 percent,
Nb:0.050% or less,
Ti: less than 0.020%,
Cu: less than 0.50 percent,
Ni: less than 0.80 percent,
V: less than 0.30 percent,
Mg: less than 0.0035 percent,
Ca:0.0030% or less
Rare earth element: less than 0.0050%.
CN202180066574.7A 2020-09-30 2021-09-29 Steel material Pending CN116234938A (en)

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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2011225897A (en) * 2010-04-15 2011-11-10 Sumitomo Metal Ind Ltd Hot-rolled steel bar or wire rod for cold forging
CN102597290A (en) * 2009-11-05 2012-07-18 住友金属工业株式会社 Hot-rolled steel bar or wire rod
JP2013151719A (en) * 2012-01-25 2013-08-08 Nippon Steel & Sumitomo Metal Corp Rolled steel bar or wire rod for hot forging
US20140363329A1 (en) * 2011-08-31 2014-12-11 Nippon Steel & Sumitomo Metal Corporation Rolled steel bar or wire rod for hot forging
CN105492644A (en) * 2013-08-26 2016-04-13 新日铁住金株式会社 Rolled round steel material for steering rack bar, and steering rack bar

Family Cites Families (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP3770291B1 (en) 2018-03-23 2024-01-17 Nippon Steel Corporation Steel

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN102597290A (en) * 2009-11-05 2012-07-18 住友金属工业株式会社 Hot-rolled steel bar or wire rod
JP2011225897A (en) * 2010-04-15 2011-11-10 Sumitomo Metal Ind Ltd Hot-rolled steel bar or wire rod for cold forging
US20140363329A1 (en) * 2011-08-31 2014-12-11 Nippon Steel & Sumitomo Metal Corporation Rolled steel bar or wire rod for hot forging
JP2013151719A (en) * 2012-01-25 2013-08-08 Nippon Steel & Sumitomo Metal Corp Rolled steel bar or wire rod for hot forging
CN105492644A (en) * 2013-08-26 2016-04-13 新日铁住金株式会社 Rolled round steel material for steering rack bar, and steering rack bar

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