JP7587124B2 - Steel for carburizing, carburizing steel parts, and method for manufacturing carburizing steel parts - Google Patents

Description

The present invention relates to carburized steel, carburized steel parts, and a method for manufacturing carburized steel parts.

2. Description of the Related Art Machine parts such as gears, shafts, and pulleys (pulleys used in CVTs (Continuously Variable Transmissions)) used in automobiles, ships, industrial machinery, etc. require high bending fatigue strength and pitting strength, so carburized and quenched low alloy steels are used.
Carburizing and quenching produces a harder surface layer than induction hardening, another surface hardening heat treatment, and can form a deeper hardened layer than soft nitriding, so it can effectively improve fatigue properties.

However, there is a drawback in that the parts are distorted and deformed because the entire steel is quenched from the austenite region to transform into martensite. These deformations can deteriorate the fatigue properties and assembly precision of the parts. If finishing is performed to reduce the adverse effects of deformation, this leads to increased manufacturing costs.

As a means of reducing the adverse effects of deformation during carburizing, there is a method of designing parts on the assumption that a certain amount of deformation will occur. In order to design parts on the assumption that a certain amount of deformation will occur, it is necessary to reduce the variability so that the amount of deformation is always constant. Therefore, various technologies have been disclosed for reducing the variability in the amount of deformation of mechanical parts during carburizing.

For example, Patent Document 1 discloses a technique that finds that the hardenability of steel is correlated with the amount of deformation and the variation in the amount of deformation, and that optimizes the amount of alloy elements to stabilize the deformation at a low level.
Patent Document 2 discloses a technology for optimizing the electromagnetic stirring conditions during continuous casting to reduce the variation in the equiaxed crystal ratio and C concentration, thereby making the amount of deformation constant when carburizing after molding into a hollow part.

Japanese Patent Application Publication No. 2-277744 JP 2003-320439 A

The technology described in Patent Document 1 stabilizes deformation at a low level by keeping the hardenability of the steel material within a certain range, but its main effect is to reduce the amount of deformation. In Patent Document 1, the variation in the amount of deformation in the invention example is about the same as the variation in the comparative example using chromium steel (SCr420), so the effect of reducing the variation in the amount of deformation is small.

The technology described in Patent Document 2 optimizes the electromagnetic stirring conditions during continuous casting, making it possible to keep the amount of deformation constant when carburizing hollow shaft-shaped parts after molding. However, there are restrictions on the shape of the parts, and there is no mention of keeping the amount of deformation constant for parts with shapes other than hollow shaft shapes.

In addition, before the carburizing process, carburizing steel is cut to obtain carburized steel parts of the desired shape. Therefore, carburizing steel is required to have high machinability and to have its hardness sufficiently reduced by softening heat treatment so that it can be cut using normal processing equipment.

The object of the present invention is to provide a carburized steel that sufficiently reduces hardness after softening heat treatment and reduces the variation in the amount of deformation of carburized steel parts after carburizing treatment, and that exerts this effect even on parts with complex shapes, as well as a carburized steel part and a method for manufacturing a carburized steel part using the same.

The means for solving the problems include the following aspects.
＜1＞
In mass percent,
C: 0.06-0.25%,
Si: 0.01-0.90%,
Mn: 3.51-6.00%,
P: 0.05% or less,
S: 0.001-0.100%,
Cr: 0.03-1.49%,
Al: 0.001-0.050%,
N: 0.0030 to 0.0250%;
The balance is Fe and impurities,
A carburizing steel which, when carburized, has an Ms point (°C) at a depth of 25 µm from the carburized surface of 30°C or higher.
Ms=219.2-1.3×Si-31.7×Mn-11.6×Cr
In the formula expressing the index Ms, each element symbol indicates the content (mass %) of each element.
＜2＞
In mass percent,
Ti: 0.05% or less,
Nb: 0.05% or less Mo: 0.25% or less,
V: 0.15% or less,
Cu: 0.50% or less,
Ni: 0.50% or less and B: 0.005% or less,
The carburizing steel according to <1>, which contains one or more of the following:
＜3＞
In mass percent,
C: 0.06-0.25%,
Si: 0.01-0.90%,
Mn: 3.51-6.00%,
P: 0.05% or less,
S: 0.001-0.100%,
Cr: 0.03-1.49%,
Al: 0.001-0.050%,
N: 0.0030 to 0.0250%;
The balance is Fe and impurities,
The following index Ms, which indicates the Ms point (°C) at a depth of 25 μm from the carburized surface, is 30°C or higher,
The amount of C at a depth of 25 μm from the carburized surface is 0.40 to 0.70% by mass,
A carburized steel part having a hardness of 630 HV or more at a depth of 25 μm from the carburized surface.
Ms=219.2-1.3×Si-31.7×Mn-11.6×Cr
In the formula expressing the index Ms, each element symbol indicates the content (mass %) of each element.
＜4＞
In mass percent,
Ti: 0.05% or less,
Nb: 0.05% or less Mo: 0.25%,
V: 0.15% or less,
Cu: 0.50% or less,
Ni: 0.50% or less and B: 0.005% or less,
The carburized steel part according to <3>, which contains one or more of the following:
＜5＞
A method for producing a carburized steel part according to <3> or <4>,
The carburizing steel is subjected to a forming process by hot forging, a softening heat treatment process at 650 to 750°C for 30 to 640 minutes, and a cutting process in this order to obtain a steel part.
A method for manufacturing a carburized steel part, comprising carburizing the steel part so that the amount of C at a position 25 μm deep from the surface of the carburized steel part is 0.40 to 0.70% by mass, and then cooling from the carburizing temperature from 800°C to 200°C at a cooling rate of 0.01 to 2°C/s.

The present invention provides a carburized steel that sufficiently reduces hardness after softening heat treatment and reduces the variation in the amount of deformation of carburized steel parts after carburizing treatment, and that exerts this effect even on parts with complex shapes, as well as a carburized steel part and a method for manufacturing a carburized steel part using the same.

FIG. 2 is a schematic diagram showing a rotating bending fatigue test piece and a test piece for measuring deformation amount, which were prepared in the examples. FIG. 2 is a schematic diagram showing how to hang a test piece.

Hereinafter, an embodiment of the present invention will be described in detail.
The "%" for the content of each element means "mass %."
The content of each element in a chemical composition may be expressed as the “element amount.” For example, the content of C may be expressed as the C amount.
A numerical range expressed using "to" means a range that includes the numerical values before and after "to" as the lower and upper limits.
When the numerical range described before and after "to" is followed by "greater than" or "less than," it means that the numerical range does not include the numerical value as the lower limit or upper limit.

<Carburizing steel>
The carburizing steel according to this embodiment (hereinafter also simply referred to as "carburizing steel") has a specified chemical composition and, when carburized, has an index Ms, shown below, of 30°C or higher, which indicates the Ms point (°C) at a depth of 25 μm from the carburized surface.
Ms=219.2-1.3×Si-31.7×Mn-11.6×Cr
In the formula expressing the index Ms, each element symbol indicates the content (mass %) of each element.

The carburizing steel according to this embodiment has a sufficiently reduced hardness after softening heat treatment and reduces the variation in the amount of deformation of carburized steel parts after carburizing treatment, and can exert this effect even on parts with complex shapes.
The carburizing steel according to this embodiment was discovered based on the following findings.

The present inventors have investigated the conditions under which differences in the variation in the amount of deformation after carburizing are caused by changing the chemical composition of the steel and the cooling conditions after carburizing in various ways, and have obtained the following findings (a) and (b).
(a) The more complex the shape of a steel part, the greater the deformation is likely to be. This is because the difference in cooling speed between different parts of a part with a complex shape becomes greater.
(b) One of the major causes of differences in cooling speed is the variation in the cooling speed of each steel part when it is cooled after carburizing. This variation is caused by the difference in the boiling state of the oil depending on the position when quenching in oil after carburizing, and is caused by the fact that the positional relationship between the nozzle and each part is not constant when quenching with high-pressure gas such as helium or nitrogen.

Based on the above findings, the inventors of the present invention considered that, instead of the conventional oil cooling or gas cooling, cooling from the carburizing temperature can be performed in a vacuum, in the atmosphere, or in an atmospheric gas at a cooling rate equivalent to air cooling or air cooling by a fan, thereby reducing the temperature difference within the part and suppressing deformation. Hereinafter, the oil cooling, high-pressure gas cooling, or cooling at a rate equal to or higher than those performed in the general carburizing process will be referred to as quenching, and the air cooling, air cooling by a fan, or cooling at a rate equal to or lower than those will be referred to as slow cooling. Therefore, the inventors further studied the conditions for applying these steels as carburized steel parts by using steels with increased alloy component amounts so that martensitic transformation occurs even when cooling after carburizing is slow. As a result, the following findings (c) and (d) were obtained.
(c) It is known that by adding a large amount of Mn, martensitic transformation can be induced even if the cooling from a temperature equivalent to carburizing is slow. However, when such steel is formed by hot forging and subsequent cutting, the structure after hot forging becomes martensite, and the machinability deteriorates.
(d) In order to soften the structure after hot forging, it is sufficient to carry out softening heat treatment in the ferrite region after hot forging. However, depending on the steel composition and the softening heat treatment conditions, the hardness of the carburized layer after subsequent vacuum carburization may vary.
(e) The variation in hardness during vacuum carburization is due to the fact that alloying elements are concentrated in cementite during softening heat treatment, which stabilizes the cementite, and do not completely dissolve during the subsequent vacuum carburization. In order to suppress the stabilization of cementite during softening heat treatment, it is necessary to appropriately control the content of alloying elements with high carbide forming ability.

In addition, by increasing the amount of Mn and decreasing the amount of Cr, the hardness of the steel after softening heat treatment is sufficiently reduced, improving machinability.

Based on the above findings, it has been discovered that the carburizing steel according to this embodiment has a sufficiently reduced hardness after softening heat treatment, reduces the variation in the amount of deformation of carburized steel parts after carburizing treatment, and can exert this effect even on parts with complex shapes.
Furthermore, since the carburizing steel according to this embodiment can significantly reduce the variation in the amount of deformation of carburized steel parts after carburizing treatment, by performing a design assuming a certain amount of deformation, it is possible to improve the dimensional accuracy of the finished product or omit finishing processing.
In addition, the carburizing steel according to this embodiment has a sufficiently reduced hardness after softening heat treatment, so that it has high machinability and improves the degree of freedom in the shape of the resulting carburized steel part.
Furthermore, carburized steel parts obtained from the carburizing steel according to this embodiment are suitable for use as machine parts for automobiles, industrial machinery, construction machinery, and the like.

The carburizing steel according to this embodiment is described in detail below.

[Chemical composition (essential elements)]
The chemical composition of the carburizing steel according to this embodiment contains the following elements.

C: 0.06-0.25%
C increases the hardness of martensite that is formed during cooling after carburizing. The amount of C in the carburized layer is determined by the carburizing conditions, so the amount of C in the core does not have a significant effect on the amount of C in the carburized layer. Increasing the C content does not promote the formation of retained austenite in the carburized layer. On the other hand, if the C content is too high, the hardness does not decrease sufficiently even with softening heat treatment, and the cutting resistance increases, resulting in poor machinability. Therefore, the C content is 0.06 to 0.25%.
The lower limit of the C content is preferably 0.08%, more preferably 0.10%, and further preferably 0.12%.
The upper limit of the C content is preferably 0.23%, and more preferably 0.20%.

Si: 0.01~0.90%
Silicon increases the hardness of martensite through solid solution strengthening, improving fatigue properties. On the other hand, if the amount of silicon is too high, oxides are formed on the surface layer depending on the cooling atmosphere, deteriorating machinability. Therefore, The Si content is 0.01 to 0.90%.
The lower limit of the Si content is preferably 0.03%, and more preferably 0.05%.
The upper limit of the Si content is preferably 0.50%, more preferably 0.35%, and further preferably 0.25%.

Mn: 3.51-6.00%
Mn is an important element for transforming the structure into martensite even if the cooling after carburizing is equivalent to natural cooling. Mn also forms MnS in the steel material, which makes the steel material harder to cut after softening heat treatment. On the other hand, if the Mn content is too high, even if the C concentration during carburization is appropriately controlled, a large amount of retained austenite is generated, which reduces hardness and deteriorates fatigue properties.
Therefore, the Mn content is 3.51 to 6.00%.
The lower limit of the Mn content is preferably 3.80%, and more preferably 4.00%.
The upper limit of the Mn content is preferably 5.80%, and more preferably 5.65%.

P: 0.05% or less P is an impurity. P segregates at grain boundaries and causes grain boundary embrittlement cracking. Therefore, the P content is preferably as low as possible.
Therefore, the upper limit of the P content is 0.05% or less, and the preferred upper limit of the P content is 0.02% or less.
The lower limit of the P content is preferably 0% (i.e., it is preferable that the P content is not contained), but from the viewpoint of reducing the dephosphorization cost, it is preferable that the P content is more than 0% (or 0.0001% or more).

S: 0.001-0.100%
S combines with Mn in steel to form MnS, improving the machinability of the steel. On the other hand, if the S content is too high, coarse MnS is formed, which reduces the fatigue strength of the steel. Therefore, The S content is 0.001 to 0.100%.
The lower limit of the S content is preferably 0.003%, more preferably 0.005%, and further preferably 0.010%.
The upper limit of the S content is preferably 0.080%, more preferably 0.060%, and further preferably 0.050%.

Cr: 0.03-1.49%
Cr has the effect of transforming the structure into martensite even if the cooling after carburizing is equivalent to natural cooling. On the other hand, Cr has a very high affinity with carbon, so when steel containing Cr is subjected to softening heat treatment, Cr is concentrated in the cementite. The cementite with concentrated Cr becomes very stable, and if it is subsequently carburized, it may take a long time to dissolve. Especially when the type of carburization is vacuum carburization, the cementite However, depending on the conditions, cementite may remain even after treatment. Even if the cementite dissolves, the Cr that was once concentrated remains in an uneven distribution even after the cementite dissolves. This can cause variations in the structure and properties after cooling.
In addition, if the Cr content is large, the resistance to temper softening increases, so that the hardness of the steel material after softening heat treatment is unlikely to decrease sufficiently, and the machinability cannot be sufficiently improved.
Therefore, the Cr content is 0.03 to 1.49%.
The lower limit of Cr is preferably 0.10%, and more preferably 0.30%.
The upper limit of Cr is preferably 1.00%, more preferably 0.60%, and further preferably 0.45%.

Al: 0.001-0.050%
Al combines with N to form AlN, and the pinning effect of this AlN suppresses the formation of coarse grains. Al is also used for deoxidation during steel manufacturing. On the other hand, if the Al content is too high, Coarse oxides are easily formed, which deteriorates fatigue properties. Therefore, the Al content is 0.001 to 0.050%.
The lower limit of the Al content is preferably 0.005%, and more preferably 0.010%. The upper limit of the Al content is preferably 0.035%, and more preferably 0.030%.

N: 0.0030-0.0250%
N combines with Al to form AlN, and the pinning effect of this element suppresses the formation of coarse grains. On the other hand, if the amount of N is too high, bubbles are generated in the steel material. The bubbles become defects, so the bubbles are It is preferable to suppress the occurrence of N. Therefore, the N content is 0.0030 to 0.0250%. The preferable lower limit of the N content is 0.0050. The preferable upper limit of the N content is 0.0200%. , more preferably 0.0180%, and even more preferably 0.0170%.

In the chemical composition of the carburizing steel according to this embodiment, the balance consists of Fe and impurities.
Here, impurities refer to substances that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment, during industrial production of steel, and are acceptable within a range that does not adversely affect the carburizing steel of this embodiment.

[Chemical composition (arbitrary elements)]
The carburizing steel according to this embodiment may contain optional elements. The lower limit of the content of the optional elements is 0%.

In the carburizing steel according to this embodiment, among the optional elements, the group consisting of Ti, Nb, Mo, V, Cu, and Ni has the effect of improving fatigue properties, and the carburizing steel may contain one or more of these elements.

Ti: 0.050% or less Ti combines with N to form TiN, which suppresses the coarsening of crystal grains during hot forging and carburization, thereby improving fatigue properties. However, if the Ti content is too high, TiC is generated in the core during carburization, reducing the amount of solute C. If the amount of solute C decreases, hardness and fatigue properties may deteriorate. Therefore, when Ti is contained, the Ti content is 0.050% or less.
The lower limit of the Ti content is preferably 0.005%, and more preferably 0.010%.
The upper limit of the Ti content is preferably 0.040%, and more preferably 0.030%.

Nb: 0.050% or less Nb combines with N to form NbN, which suppresses the coarsening of crystal grains during hot forging and carburizing, thereby improving fatigue properties. However, if the Nb content is too high, NbC is generated in the core during carburizing, reducing the amount of solute C. If the amount of solute C decreases, hardness and fatigue properties may deteriorate. Therefore, when Nb is contained, the Nb content is 0.050% or less.
The lower limit of the Nb content is preferably 0.005%, and more preferably 0.010%.
The upper limit of the Nb content is preferably 0.040%, and more preferably 0.030%.

Mo: 0.25% or less Mo has the effect of transforming the structure into martensite even if the cooling rate after carburizing is slowed. On the other hand, Mo stabilizes cementite, so if a steel material with too much Mo is softened by heat treatment, cementite may remain during the subsequent vacuum carburizing, causing deterioration of fatigue properties. Mo is expensive, and if a large amount of Mo is added, it will increase manufacturing costs. Therefore, when Mo is added, the Mo amount is 0.25% or less.
The preferred lower limit of the Mo content is 0.05%.
The upper limit of the Mo content is preferably 0.20%, and more preferably 0.15%.

V: 0.15% or less V has the effect of transforming the structure into martensite even if the cooling rate after carburizing is slowed. On the other hand, V stabilizes cementite, so if a steel material with too much V is softened by heat treatment, cementite may remain during the subsequent vacuum carburizing, causing deterioration of fatigue properties. Therefore, when V is contained, the V content is 0.15% or less.
The lower limit of the V content is preferably 0.05%.
The upper limit of the V content is preferably 0.13%, and more preferably 0.10%.

Cu: 0.50% or less Cu has the effect of transforming the structure into martensite even if the cooling rate after carburizing is slowed. On the other hand, if the amount of Cu is excessively large, it segregates at the grain boundaries of the steel during hot forging, inducing hot cracks. Therefore, when Cu is contained, the amount of Cu is 0.50% or less.
The lower limit of the Cu content is preferably 0.05%, and more preferably 0.10%.
The upper limit of the Cu content is preferably 0.40%, and more preferably 0.30%.

Ni: 0.50% or less Ni has the effect of transforming the structure into martensite even if the cooling rate after carburizing is slowed. On the other hand, Ni is expensive, and the inclusion of a large amount of Ni leads to an increase in manufacturing costs. Therefore, when Ni is included, the Ni content is 0.50% or less.
The lower limit of the Ni content is preferably 0.05%, and more preferably 0.10%.
The upper limit of the Ni content is preferably 0.40%, and more preferably 0.30%.

Of the optional elements in the carburizing steel according to this embodiment, B has the effect of improving fatigue properties, and the carburizing steel may contain B.

B: 0.0050% or less B has the effect of stabilizing grain boundaries and converting the structure to martensite even if the cooling rate after carburizing is slowed. On the other hand, if the amount of B is excessively large, it will form coarse carboborides with other elements, deteriorating fatigue properties. Therefore, the amount of B is 0.0050% or less. The preferable lower limit of the amount of B to stably obtain the above effect is 0.0005%. The preferable upper limit of the amount of B is 0.0040% or less.

[Ms point (°C) at a depth of 25 μm from the carburized surface: 30°C or higher]
When the carburizing steel according to this embodiment is carburized, the index Ms, which indicates the Ms point (° C.) at a depth of 25 μm from the carburized surface, is 30° C. or higher.
Details of the index Ms representing the Ms point (° C.) will be explained later in relation to the carburized steel part according to this embodiment.
Here, "when carburized" means that the steel is carburized so that the C content at a depth of 25 μm from the surface of the carburized steel part is 0.40 to 0.70% by mass, and then cooled from the carburizing temperature from 800°C to 200°C at a cooling rate of 0.01 to 2°C/s.

[Hardness at a depth of 8.75 μm from the softening heat-treated surface: 400 HV or less]
When the carburizing steel according to this embodiment is subjected to softening heat treatment, the hardness at a depth of 8.75 mm from the softening heat treated surface (hereinafter also referred to as "hardness of the steel material after softening heat treatment") is preferably 400 HV or less.
If the hardness of the steel material after the softening heat treatment is 400 HV or less, it can be cut using ordinary processing equipment.
The hardness of the steel material after the softening heat treatment is more preferably 350 HV or less. However, from the viewpoint of ensuring strength, the lower limit of the hardness of the steel material after the softening heat treatment is, for example, 120 HV or more.

Here, the hardness of the steel material after softening heat treatment refers to the hardness of the steel material after softening heat treatment of the carburizing steel (i.e., the carburizing steel part raw material) formed by hot forging and before carburizing treatment.
The softening heat treatment means a heat treatment at 650 to 750° C. for 30 to 640 minutes.

The hardness of the steel after softening heat treatment is Vickers hardness, and is measured by the method described in the examples below.

<Carburized steel parts>
The carburized steel part according to this embodiment (hereinafter also simply referred to as "carburized steel part") has a chemical composition at a position deeper than 3 mm from the surface, which is not affected by carburization, that is the chemical composition of the carburized steel according to this embodiment, and satisfies the following characteristics. The carburized steel part is a part that is carburized but not subsequently quenched.
(1) The index Ms, which indicates the Ms point (°C) at a depth of 25 µm from the carburized surface (hereinafter also referred to as the "surface layer"), is 30°C or higher.
(2) The amount of C at a depth of 25 μm from the carburized surface (hereinafter also referred to as the "surface layer") is 0.40 to 0.70% by mass.
(3) The hardness at a depth of 25 μm from the carburized surface (hereinafter also referred to as the “surface layer”) is 630 HV or more.

The carburized steel parts according to this embodiment are described in detail below.

[Ms point (°C) at a depth of 25 μm from the carburized surface: 30°C or higher]
When steel with a high content of Mn and Cr is carburized, a larger amount of retained austenite is generated than when a typical case-hardened chrome steel (such as SCr420) is carburized, resulting in a decrease in hardness. In order to suppress the generation of retained austenite, it is sufficient that the index Ms, which represents the Ms point (°C) of the surface layer (i.e., the carburized layer), is 30°C or higher, even when the C content of the surface layer (i.e., the carburized layer) becomes 0.70% after carburizing steel.
The index Ms representing the Ms point (°C) of the surface layer portion is preferably 50°C or higher, and more preferably 60°C or higher. However, the upper limit of the index Ms representing the Ms point (°C) of the surface layer portion is set to, for example, 300°C or lower from the viewpoint of suppressing the generation of pro-eutectoid ferrite.
The index Ms representing the Ms point (° C.) is defined as the Ms point of a steel when the C content at a depth of 25 μm from the carburized surface is 0.70%.

Ms=219.2-1.3×Si-31.7×Mn-11.6×Cr
In the formula expressing the index Ms, each element symbol indicates the content (mass %) of each element.

The index Ms, which indicates the Ms point (°C) of the surface layer (25 μm deep from the carburized surface), is calculated using the method described in the examples below.

[C content at a depth of 25 μm from the carburized surface: 0.40 to 0.70%]
Even if a steel can be used to convert the structure to martensite without significantly lowering the Ms point of the surface layer, if that steel is carburized to have a carbon content in the surface layer that is the same as that of a typical case-hardened part, the amount of retained austenite produced may increase, resulting in a decrease in hardness.
In order to sufficiently harden the surface layer of carburizing steel, the C content in the surface layer must be 0.70% or less. On the other hand, if the C content in the surface layer is too low, the hardness of martensite decreases and fatigue properties deteriorate, so the C content in the surface layer must be 0.40% or more.
The lower limit of the surface layer C content is preferably 0.45% or more, more preferably 0.50% or more, and even more preferably 0.55% or more.
The upper limit of the surface layer C content is preferably set to 0.65% or less, and more preferably set to 0.62% or less.

The amount of carbon in the surface layer (25 μm deep from the carburized surface) is measured using the method described in the examples below.

[Hardness at a depth of 25 μm from the carburized surface: 630 HV or more]
In order to impart high fatigue properties to carburized steel parts, the hardness of the surface layer must be 630 HV or more. The lower limit of the hardness of the surface layer is preferably 650 HV or more.
Although there is no particular upper limit to the hardness of the surface layer, the hardness obtained when the steel is carburized is usually 1000 HV or less. In addition, when the possibility that the mating material may be worn when the parts slide, the upper limit of the hardness may be 900 HV or less, 850 HV or less, or 800 HV or less.

The hardness of the surface layer (25 μm deep from the carburized surface) is Vickers hardness, and is measured by the method described in the examples below.

The structure of the surface layer (25 μm deep from the carburized surface) is mainly martensite, and is a mixture of retained austenite, non-metallic inclusions such as oxides and carbides, and incompletely hardened structure. As long as the hardness of the surface layer meets the regulations, there are no restrictions on the ratio of the mixed structure, but to achieve high surface hardness, the area rate on the cross section at the same depth should be less than 30% for retained austenite, less than 5% for non-metallic inclusions, and less than 15% for incompletely hardened structure. The incompletely hardened structure is a mixed structure of bainite and pearlite.

[Production method]
An example of a method for manufacturing the carburized steel and the carburized steel part according to this embodiment will be described.

The carburized steel parts according to this embodiment are manufactured by sequentially carrying out a hot forging forming process, a softening heat treatment process at 650-750°C for 30-640 minutes, and a cutting process on the carburized steel to obtain a steel part, which is then carburized so that the C content at a depth of 25 μm from the surface of the carburized steel part is 0.40-0.70% by mass, and then cooling from the carburizing temperature from 800°C to 200°C at a cooling rate of 0.01-2°C/s. Specifically, it is as follows.

The manufacturing method for carburized steel parts according to this embodiment includes, for example, a steel material preparation process, a molding process, a softening heat treatment process, a cutting process, and a carburizing process, and also includes a heat treatment process to adjust the structure as necessary. Each process is described below.

[Steel material preparation process]
Molten steel satisfying the chemical composition of the carburizing steel according to the present embodiment is manufactured. The manufactured molten steel is made into a slab (or bloom) by a general continuous casting method. Alternatively, the molten steel is made into an ingot by an ingot casting method. The slab or ingot is hot worked to manufacture a billet. The hot working may be hot rolling or hot forging. The billet is then heated, rolled, and cooled under general conditions to manufacture a steel bar. The manufactured steel bar is used as the steel material for the carburizing steel part.

[Molding process]
The produced steel bars are then hot forged into carburized steel part blanks. If the heating temperature for hot forging is too low, excessive load is placed on the forging equipment. On the other hand, if the heating temperature is too high, there is a large scale loss. Therefore, the preferred heating temperature is 1000 to 1300°C.
The preferred finishing temperature for hot forging is 900° C. or higher. If the finishing temperature is too low, the burden on the die increases. On the other hand, the preferred upper limit of the finishing temperature is 1250° C. In order to facilitate subsequent machining, the cooling rate after hot forging may be slow to reduce the hardness.

[Softening heat treatment process]
The carburized steel part raw material is softened by heat treatment. Specifically, the hot forged raw material is heated at 650-750°C for 30-640 minutes to reduce the hardness to a level where sufficient machinability is obtained. Cooling after heating may be performed by any method, including cooling in the air or water cooling.

[Cutting process]
The carburized steel part blank after the soft heat treatment is cut to form a desired carburized steel part shape. In addition to the cutting, grinding may also be performed.

[Carburizing process]
The machined steel parts are subjected to a carburizing treatment, after which they are cooled at a specified cooling rate.

The carburizing treatment is carried out so that the carbon content at a depth of 25 μm from the surface of the carburized steel part is 0.40 to 0.70% by mass.
The carburizing treatment may be gas carburizing or vacuum carburizing. Any type of treatment may be used as long as the carburizing temperature and time are within the specified ranges for the amount of carbon (the amount of carbon at a depth of 25 μm from the carburized surface) and the cooling rate from the carburizing temperature, which will be described later, is within the specified ranges.
For example, when obtaining a carburized steel part according to this embodiment containing 0.15% C and 5.0% Mn, gas carburizing with a surface carbon potential of 0.6 may be performed at 920°C for 3 hours, or vacuum carburizing may be performed at 950°C with acetylene flowing for 22 minutes, followed by holding for 148 minutes.

After carburizing, the steel part is cooled from the carburizing temperature from 800° C. to 200° C. at a cooling rate of 0.01-2° C./s.
In general carburizing, the steel is quenched by cooling from the holding temperature using oil cooling or forced air cooling. The average cooling rate during quenching is 5°C/s or more, which increases the temperature gradient in the steel part and the amount of deformation. In order to reduce the temperature gradient in the steel part and the amount of deformation, the cooling rate must be 2°C/s or less. On the other hand, if the cooling rate is slower than 0.01°C/s, an incompletely quenched structure is formed, which reduces the hardness and deteriorates the fatigue properties. Therefore, the cooling rate from 800°C to 200°C must be 0.01 to 2°C/s.
The lower limit of the cooling rate is preferably 0.05° C./s or more, more preferably 0.10° C./s or more, and even more preferably 0.20° C./s or more.
The upper limit of the cooling rate is preferably 1.5° C./s or less, more preferably 1.2° C./s or less, and even more preferably 1.0° C./s or less.
Here, the average cooling rate is the time required for the surface temperature of the steel part to cool from 800°C to 200°C divided by the temperature difference of 600°C.

[Post-carburizing process]
Steel parts after carburizing may be subjected to various post-treatments in the same manner as general carburized and quenched parts. Specifically, they may be subjected to low-temperature tempering at 200°C or less to increase toughness, sub-zero treatment to reduce retained austenite, shot peening to increase hardness and compressive residual stress, or a combination of multiple post-treatments.

The present invention will be specifically described below with reference to examples. However, these examples do not limit the present invention.

First, a vacuum melting furnace was used to produce 50 kg ingots of steels B to O and S, and 150 kg ingots of A, P to R, each having the chemical composition shown in Table 1. Each ingot was heated to 1250°C and then hot forged into steel bars with a diameter of φ35 mm. In order to soften the forged steel bars, they were subjected to a softening heat treatment in which they were heated at 700°C for 2 hours and then allowed to cool.

A test piece was prepared from a part of the steel bar after the softening heat treatment, with the cross section (i.e., a plane perpendicular to the length direction of the steel bar) being the test surface. The Vickers hardness of the test piece was measured based on JIS Z 2244 (2009) on the test surface. Specifically, the Vickers hardness was measured at any five points at a depth of 8.75 mm from the surface of the steel bar on the test surface of the test piece with a test force of 9.8 N. The average value of the five Vickers hardness values obtained was defined as the Vickers hardness of the steel material after the softening treatment. When the Vickers hardness of the steel material after the softening heat treatment was 400 HV or less, the Vickers hardness of the steel material after the softening heat treatment was deemed to be sufficiently low and high machinability.
Further, from another part of the steel bar after the softening heat treatment, a rotating bending fatigue test piece shown in FIG. 1 and a test piece for measuring the deformation amount were prepared.
In the chemical compositions shown in Table 1, the notations "<0.01" and "-" indicate that the corresponding element is not intentionally added or is contained at an impurity level.

The prepared test specimens were subjected to carburizing treatment while suspended by wire in a steel basket. The rotating bending fatigue test specimens were hung vertically with the axial direction facing vertically. Of the test specimens used to measure deformation, three of each test number were hung horizontally with the keyway facing vertically upwards (Figure 2(a)). Three of each test number were hung horizontally with the keyway facing vertically downwards (Figure 2(b)). Three of each test number were hung vertically with the keyway extending vertically (Figure 2(c)). An overview of how the test specimens were hung is shown in Figure 2.

These test pieces were vacuum carburized using acetylene at a carburizing temperature of 950° C. The carburizing conditions for the test pieces of each test number were as follows:
For test pieces Nos. 1, 2, 5 to 11, 13, 14, 17, and 26, acetylene was introduced for 22 minutes, and then the specimens were held in a vacuum state for 148 minutes.
For test pieces Nos. 3 and 4, acetylene was introduced for 18 minutes and then the specimens were held in a vacuum state for 1528 minutes.
For test pieces Nos. 12, 15, and 16, acetylene was introduced for 16 minutes and then the specimens were held under vacuum for 154 minutes.
For test pieces Nos. 18 to 24, acetylene was introduced for 53 minutes and then the specimens were held in a vacuum state for 127 minutes.
For the test piece of test number 25, acetylene was introduced for 10 minutes and then the specimen was held in a vacuum state for 170 minutes.

After the carburizing treatment, the test pieces of test numbers 1 to 9, 11 to 17, and 21 to 25 were taken out of the furnace and allowed to cool to 50° C. or less in a nitrogen atmosphere. The average cooling rate from 800° C. to 200° C. during cooling from the carburizing temperature was 0.4° C./sec.
The test pieces of test numbers 18 to 20 were carburized, removed from the furnace, and then placed in oil to cool. The average cooling rate from the carburizing temperature of 800°C to 200°C was about 10°C/sec.
The test piece of test number 10 was carburized, removed from the furnace, and then cooled by blowing air on it with a fan. The average cooling rate from the carburizing temperature of 800°C to 200°C was about 1.0°C/sec.
The test piece of test number 26 was carburized, removed from the furnace, and then transferred to an electric furnace heated to 800°C. At this time, the test piece was hung on a small hanger so that the top and bottom directions of the test piece were the same as those at the time of carburization. The atmosphere in the furnace was an Ar atmosphere. After the temperature of the test piece reached 820°C, the furnace temperature was controlled to decrease at a rate of 0.005°C/sec while cooling to 200°C. When the temperature of the test piece reached 200°C, the electric furnace was turned off and the test piece was cooled in the furnace. At this time, the average cooling rate from 800°C to 200°C was 0.005°C/sec, which was the same as the rate of decrease in the furnace temperature.

[Hardness measurement, carbon content measurement, index Ms calculation]
Using a part of the rotary bending fatigue test piece after carburization, the hardness and C amount of the surface layer (at a depth of 25 μm from the carburized surface) were measured, and the index Ms was calculated as follows.
First, a sample was prepared with the vertical cross section of the notch bottom of the test piece as the test surface, embedded in resin and polished.
The Vickers hardness was measured based on JIS Z 2244 (2009) at five arbitrary points at a depth of 25 μm from the surface near the notch bottom on the test surface. The test force was 2.9 N. The average value of the five obtained Vickers hardness values was defined as the Vickers hardness of the surface layer. When the Vickers hardness of the surface layer was 630 HV or more, the Vickers hardness of the surface layer was considered to be sufficiently high.

Next, the test pieces after hardness measurement were polished again and then removed from the resin and subjected to carbon content analysis using EPMA. Starting from an arbitrary position 25 μm deep from the surface, line analysis was performed in a direction parallel to the surface. The measurement step was 2 μm, the measurement time for each measurement point was 2.0 seconds, and the measurement line length was 50 μm. Although the surface of the test piece on the cross section is strictly arc-shaped, the measurement line length was sufficiently short compared to the radius of the arc, so the surface of the test piece was considered to be a straight line and the measurement line was considered to be a straight line. This type of line analysis was performed twice at different positions, and the average value was considered to be the C content of the surface layer of the test piece for each test number.

Since the diffusion distance of substitutional alloying elements during carburizing is extremely short, the Si, Mn, and Cr contents in the surface layer were considered to be equal to the bulk values, and the index Ms, which represents the Ms point (°C), was calculated.

[Rotating bending fatigue test]
The rotating bending fatigue test specimens after the carburizing treatment were subjected to a fatigue test after the gripping portion was machined from φ12.4 mm to φ12.0 mm for axial alignment.
The rotating bending fatigue test was carried out in an air atmosphere at room temperature (25°C) in accordance with JIS Z2274 (1978). The rotation speed during the test was 3000 rpm, and the highest stress among the test pieces that did not break up to 1.0 x ¹⁰⁷ repetitions was defined as the fatigue strength (MPa) of that test number. If the fatigue strength was 500 MPa or more, it was determined that the fatigue strength was excellent.

[Investigation of variation in deformation amount]
Among test numbers 1 to 26, the test pieces for measuring the amount of deformation after carburizing treatment (test pieces with test numbers 1 to 10, 12, 15, 16, and 18 to 20) had high hardness in the surface layer and therefore sufficiently high fatigue strength, and were used to investigate the variation in the amount of deformation during carburizing treatment. Specifically, a contact type roughness meter was used to measure the cross-sectional curve in the axial direction on the back side of the keyway of the test pieces. The measurement range was 95 mm, excluding 2.5 mm from both ends of the test pieces.
The obtained cross-sectional profile was approximated by a circular arc to eliminate the influence of measurement errors and surface defects, and the difference between the straight line connecting both ends of the circular arc and the circular arc at the maximum was defined as the bending amount of the test piece. The bending amount of each of the three test pieces hung in the basket with the same orientation was averaged to be the bending amount for each hanging direction. The absolute value of the difference between the maximum and minimum bending amounts for each of the three hanging directions was taken as the variation in the deformation amount. When the variation in the deformation amount was 20 μm or less, it was considered that the variation in the deformation amount was sufficiently small.

[Test Results]
Test numbers 1 to 10 are examples within the scope of the present invention, with fatigue strength of 500 MPa or more and deformation variation of 14 μm or less, showing that the fatigue strength is increased while the deformation variation is reduced. It is also clear that the hardness of the steel material after softening heat treatment is low and the machinability is high.

In contrast, it can be seen that the comparison examples of test numbers 11 to 26, which deviate from the provisions of the present invention, do not achieve the target performance.

Specifically, in test numbers 11 and 13, the Mn content was high or the index Ms was low, so that retained austenite was formed, sufficient hardness was not obtained in the surface layer after carburization, and the fatigue strength was also low.
Test No. 12 has a large C content, and therefore the steel material after softening heat treatment has high hardness and low machinability.
In test numbers 14 and 17, the Mn content was small, so that the martensitic transformation was incomplete, sufficient hardness was not obtained, and the fatigue strength was also low.
Test Nos. 15 and 16 had a large Cr content, and the steel materials after softening heat treatment had high hardness and low machinability.
Test Nos. 18 to 20 were made of general JIS steel that was carburized under general carburizing conditions and then oil-cooled, and therefore had sufficiently high hardness and fatigue strength, but the amount of deformation varied greatly depending on the hanging direction during carburization.
Test numbers 21 to 23 are typical JIS steels that have been carburized and then naturally cooled in nitrogen. However, because the chemical composition is not within the scope of the present invention and is not optimized, if the steel is naturally cooled after carburization, martensitic transformation does not occur, and the hardness of the surface layer after carburization is extremely low. Therefore, the fatigue strength is also low.
Test No. 24 has a chemical composition within the range of the present invention, but the surface layer has a high C content. This C content value is close to the carbon content of the surface layer of a typical carburized steel part. This test No. has a large amount of retained austenite, and the surface layer does not have sufficient hardness after carburizing, and the fatigue strength is also low.
Test No. 25 had a chemical composition within the range specified by the present invention, but had a low carbon content in the surface layer, low hardness, and low fatigue strength.
Test Example 26 had a chemical composition within the range specified by the present invention, but the hardness of the surface layer after carburizing treatment was low, and the fatigue strength was also low.

Claims (3)

In mass percent,
C: 0.06-0.25%,
Si: 0.01-0.90%,
Mn: 3.51-6.00%,
P: 0.05% or less,
S: 0.001-0.100%,
Cr: 0.03-1.49%,
Al: 0.001-0.050%,
N: 0.0030 to 0.0250%;
The balance is Fe and impurities,
The following index Ms, which indicates the Ms point (°C) at a depth of 25 μm from the carburized surface, is 30°C or higher,
The amount of C at a depth of 25 μm from the carburized surface is 0.40 to 0.70% by mass,
A carburized steel part having a hardness of 630 HV or more at a depth of 25 μm from the carburized surface.
Ms=219.2-1.3×Si-31.7×Mn-11.6×Cr
In the formula expressing the index Ms, each element symbol indicates the content (mass %) of each element. In mass percent,
Ti: 0.05% or less,
Nb: 0.05% or less Mo: 0.25%,
V: 0.15% or less,
Cu: 0.50% or less,
Ni: 0.50% or less and B: 0.005% or less,
2. The carburized steel part according to claim 1, further comprising one or more of the following: A method for manufacturing a carburized steel part according to claim 1 or 2 , comprising the steps of:
The carburizing steel is subjected to a forming process by hot forging, a softening heat treatment process at 650 to 750°C for 30 to 640 minutes, and a cutting process in this order to obtain a steel part.
A method for manufacturing a carburized steel part, comprising carburizing the steel part so that the amount of C at a position 25 μm deep from the surface of the carburized steel part is 0.40 to 0.70% by mass, and then cooling from the carburizing temperature from 800°C to 200°C at a cooling rate of 0.01 to 2°C/s.