JP7119697B2 - Steel materials for surface hardening and surface hardening parts - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel material for surface hardening and a surface hardened part having high fatigue strength.

Mechanical parts used in automobiles, industrial machines, construction machines, and the like are sometimes subjected to hardening treatment by partial hardening such as induction hardening and laser hardening for the purpose of improving fatigue strength. The fatigue strength of surface-hardened parts subjected to such treatment is affected by the hardness profile and structure in the vicinity of the surface layer of the part after quenching. Therefore, by optimizing the steel composition to improve hardenability and rendering harmful inclusions harmless, or by refining the structure by controlling the steel manufacturing conditions, high Techniques for obtaining fatigue strength have been developed.

Patent Literature 1 reports a technology that optimizes the steel composition and mixes bainite or martensite in the matrix structure to refine the grain size during high-frequency heating and improve the fatigue strength of surface-hardened parts. It is

Patent Literature 2 reports a technique for improving the fatigue strength of surface-hardened parts by optimizing steel material components and rendering inclusions harmful to fatigue harmless.

Machine parts used in automobiles, industrial machines, construction machines, etc. are manufactured by cutting steel sheets as raw materials. Therefore, steel materials for machine parts are required to have high machinability.

JP-A-2006-45678 WO2014/061782

According to the above-described conventional technique, it is possible to obtain excellent fatigue characteristics by uniformly heating and quenching the surface layer of the part by high-frequency heating and obtaining a deep hardened layer. However, when steel is processed into various parts shapes and subjected to surface quenching under various conditions, fatigue characteristics vary, and high fatigue strength is not necessarily obtained. Even if the heating conditions such as the output of the equipment for surface quenching are constant, if the shape of the part is complicated, the heating conditions such as the time to be exposed to high temperature and the temperature reached will vary depending on the part. This is because a difference occurs in the austenite structure and the quenched structure during heating.
The degree of variation in fatigue properties when the actual heating conditions are changed is affected by the steel composition and structure. could not be reduced.
Even if the shape of the part after processing is not complicated, even if the original heating conditions change due to changes in the surface hardening equipment used, the conditions, the usage environment, etc., the variations in the structure described above will not occur. , it has been confirmed that variations in fatigue properties occur.

The present invention was invented in view of the above-mentioned problems, and its object is to obtain excellent fatigue characteristics even when there is variation in the heating conditions during surface quenching. Moreover, it is an object of the present invention to provide a steel material for surface hardening which can be easily machined and which has excellent machinability, and a surface hardened part which has excellent fatigue properties.

The present inventors quenched the steel surface under various heating conditions, measured the hardness distribution of the cross section, and investigated the fatigue strength. As a result, the following findings (a) to (e) were obtained.

(a) In steels whose fatigue strength deteriorates significantly when surface quenching conditions vary, the steel structure in the state immediately before quenching is uneven, and as a result, the structure after quenching (quenched structure) is also uneven. .

(b) Even if the surface hardening conditions vary, one of the requirements for making the steel structure uniform austenite immediately before surface hardening is to lower the A3 point of the steel. In particular, by including Mn, the A3 point can be effectively lowered, and uniform austenite can be obtained even by heating at a relatively low temperature for a short period of time.

(c) Another requirement for obtaining high fatigue properties even if surface hardening conditions vary is to reduce the difference between the concentration of substitutional alloying elements in cementite and the concentration of substitutional alloying elements in the matrix. This is because when the concentration difference between the cementite and the matrix phase substitutional alloying element increases, the substitutional alloying element needs to diffuse prior to the cementite dissolving during heating during quenching treatment. This is because cementite is not easily decomposed even at high temperatures, and a uniform austenitic structure cannot be obtained. That is, in order to obtain uniform austenite during heating, it is important to suppress the concentration of substitutional alloying elements in cementite and facilitate the dissolution of cementite.

(d) Cr is an element that tends to concentrate in cementite, and cementite containing a large amount of Cr is difficult to dissolve even if the A3 point is exceeded. Mn is also an element that concentrates into cementite, but the degree of concentration is smaller than that of Cr.

(e) Si is an element that raises the A3 point of steel, so it is not preferable to contain it in a large amount. However, since Si has the effect of suppressing the coarsening of cementite and facilitating the melting of cementite at a temperature of A3 or higher, Si may be contained in a small amount.

The present invention was completed based on the above findings, and the gist thereof is as shown in (1) to (5) below.

(1) In mass %, C: 0.30 to 0.80%, Si: 0.01 to 0.40%, Mn: 1.50 to 3.00%, P: 0.050% or less, S: 0.001-0.100%, Cr: 0.03-0.40%, Al: 0.001-0.080%, N: 0.0030-0.0250%, the balance being Fe and impurities A3 point represented by the following formula (1) is 750 ° C. or less, and (Mn θ + Cr θ) / Fe θ, which is the weight ratio of the sum of Mn and Cr in θ carbide in the metal structure to Fe, is 0.18 Steel material for surface hardening characterized by the following.
A3 (°C) = 854 - 180 x C - 20 x Mn + 40 x Si - 10 x Cr - 200 x N
... (1)
Here, each element symbol in formula (1) is the content (% by mass) in the steel material, and the Mnθ, Crθ and Feθ are the weights of Mn, Cr and Fe in the θ carbide, respectively. %.
(2) The steel material for surface hardening according to (1) above, which further contains one or more of Ti: 0.050% or less and Nb: 0.050% or less .
( 3) Further , the above ( The steel material for surface hardening according to 1) or (2).
(4) Any one of the above (1) to (3), further comprising one or more of B: 0.0050% or less , Ca: 0.0050% or less , and Bi: 0.4% or less . The steel material for surface hardening according to any one of items.

(5) In mass %, C: 0.30 to 0.80%, Si: 0.01 to 0.40%, Mn: 1.40 to 3.00%, P: 0.050% or less, S: 0.001-0.100%, Cr: 0.03-0.40%, Al: 0.001-0.080%, N: 0.0030-0.0250%, the balance being Fe and impurities The A3 point represented by the following formula (2) is 760 ° C. or less, and the part surface layer including the part surface has a hardened layer with a Vickers hardness of more than 500 Hv and a thickness of 0.90 mm or more, The sum of the area ratios of proeutectoid ferrite and pearlite in the metal structure at a depth of 0.90 mm from the part surface is 10% or less, and in a region deeper than the hardened layer by 0.5 mm or more, A surface-hardened component, wherein (Mnθ+Crθ)/Feθ, which is the weight ratio of the sum of Mn and Cr to Fe, is 0.18 or less.
A3 (°C) = 854 - 180 x C - 20 x Mn + 40 x Si - 10 x Cr - 200 x N
... (2)
Here, each element symbol in formula (2) is the content (% by mass) in the part.
(6) The surface-hardened part according to (5), further comprising one or more of Ti: 0.050% or less and Nb: 0.050% or less .
(7) Further, it contains one or more of V: 0.15% or less , Mo: 0.50% or less , Cu: 0.50% or less , Ni: 0.50% or less (5 ) or the surface-hardened part according to (6).
(8) Any one of (5) to (7), further comprising one or more of B: 0.0050% or less , Ca: 0.0050% or less , and Bi: 0.4% or less The surface hardened part according to item 1.

According to the present invention, even if there is variation in the heating conditions during surface hardening, it is possible to obtain excellent fatigue properties, and surface hardening with excellent machinability that allows easy cutting. It is possible to provide steel materials for steel and surface hardened parts with excellent fatigue properties.

It is a front view of an Ono-type rotary bending fatigue test piece. It is a front view and a side view of a four-point bending fatigue test piece.

Hereinafter, a steel material for surface hardening and a surface hardened part according to one embodiment of the present invention will be described in detail.

<Steel materials for surface hardening>
[Chemical composition]
The chemical composition of the steel material for surface hardening according to the present embodiment (hereinafter also simply referred to as steel material) contains the following elements. In addition, in the following description, "%" of the content of each element means "% by mass".

C: 0.30-0.80%
C has the effect of increasing the hardness of the surface layer of the part after surface quenching and the hardness of the core portion in a region deeper than the surface layer, thereby improving the strength. In order to sufficiently obtain this effect, the C content should be 0.30% or more. If the C content exceeds 0.80%, the hardness after hot forging, especially the hardness of the core portion, becomes too large, and the machinability deteriorates. Therefore, the C content should be 0.80% or less. The C content is preferably 0.35% or more, more preferably 0.38% or more. Also, the C content is preferably 0.75% or less, more preferably 0.70% or less.

Si: 0.01-0.40%
Sliding parts such as gears are tempered due to temperature rise during use, which reduces hardness and deteriorates fatigue strength. Si has the effect of suppressing the formation and growth of cementite and reducing the amount of softening during tempering. In order to sufficiently obtain this effect, the Si content should be 0.01% or more. If the Si content exceeds 0.40%, it becomes difficult to lower the A3 point. It becomes difficult to suppress variations in fatigue characteristics of Therefore, the Si content should be 0.40% or less. Incidentally, the Si content is preferably 0.03% or more. Also, the Si content is preferably 0.30% or less, more preferably 0.23% or less, and even more preferably 0.18% or less.

Mn: 1.40-3.00%
Mn has the effect of reducing variation in fatigue properties by lowering the A3 point and promoting reverse transformation from ferrite to austenite in low-temperature heat treatment and short-time heat treatment. Mn is an important element in the present invention because it has a greater effect of lowering the A3 point per unit amount and an effect of improving hardenability than other substitutional alloying elements having similar effects. In order to obtain the above effects, the Mn content must be 1.40% or more. On the other hand, when the Mn content exceeds 3.00%, the hardness after hot forging increases, and the machinability deteriorates. Therefore, the Mn content is 1.40-3.00%. The Mn content is preferably 1.50% or more, more preferably 1.60% or more, and even more preferably 1.70% or more. Also, the Mn content is preferably 2.80% or less, more preferably 2.60% or less.

P: 0.050% or less P is an impurity. P segregates at grain boundaries and causes grain boundary embrittlement. Therefore, it is preferable that the P content is as low as possible. The P content is 0.050% or less. A preferable P content is 0.040% or less.

S: 0.001 to 0.100%
S combines with Mn in the steel material to form MnS and enhances the machinability of the steel material. To obtain this effect, the S content must be 0.001% or more. On the other hand, when the S content exceeds 0.100%, coarse MnS is formed, degrading the fatigue strength. Therefore, the S content is 0.001-0.100%. The S content is preferably 0.005% or more, more preferably 0.010% or more. Also, the S content is preferably 0.080% or less, more preferably 0.070% or less.

Cr: 0.03-0.40%
Cr has the effect of increasing hardenability, suppressing the formation of incompletely hardened structures such as proeutectoid ferrite and pearlite, and reducing variations in hardness after hardening. To obtain this effect, the Cr content must be 0.03% or more. On the other hand, when the Cr content exceeds 0.40%, the cementite in which Cr is solid-dissolved becomes difficult to dissolve at temperatures of A3 or higher, and the steel structure immediately before quenching becomes uneven, and the hardness after quenching increases. cause variations in thickness. Therefore, the Cr content is 0.03-0.40%. Incidentally, the Cr content is preferably 0.05% or more. Also, the Cr content is preferably 0.35% or less, more preferably 0.30% or less.

Al: 0.001-0.080%
Aluminum deoxidizes steel. In order to obtain this effect, the Al content must be 0.001% or more. On the other hand, if the Al content is too high, coarse oxides are likely to be formed in the steel, degrading the fatigue strength. Therefore, the Al content is 0.001-0.080%. The Al content is preferably 0.005% or more, more preferably 0.010% or more. Also, the Al content is preferably 0.060% or less, more preferably 0.050% or less.

N: 0.0030 to 0.0250%
N lowers the A3 point of steel, thereby contributing to homogenization of the steel structure immediately before quenching, and also has the effect of reducing variations in hardness after quenching. To obtain this effect, the N content must be 0.0030% or more. On the other hand, if the N content is too high, bubbles may be generated in the steel material, degrading the fatigue strength. Therefore, the N content is 0.0030-0.0250%. Note that the N content is preferably 0.0050% or more. Also, the N content is preferably 0.0220% or less, more preferably 0.0200% or less. Note that N is also an element mixed into steel as an impurity.

The balance of the steel material for surface hardening according to this embodiment consists of Fe and impurities. Here, impurities are those that are mixed from ore, scrap, or the manufacturing environment as raw materials when industrially manufacturing steel materials, and are within the range that does not adversely affect the effects and characteristics of the present invention. means acceptable.

The steel material for surface hardening according to this embodiment may further contain one or both of Ti and Nb.

Ti: 0-0.050%
Ti combines with N in the base material to form TiN, which suppresses grain coarsening during hot forging. However, if the Ti content is too high, TiC is generated and the variation in hardness of the steel increases. Therefore, the Ti content is 0.050% or less. When Ti is contained, the lower limit of the Ti content is preferably 0.005% or more, more preferably 0.010% or more. The upper limit of Ti content is preferably 0.040% or less, more preferably 0.030% or less.

Nb: 0-0.050%
Nb combines with N in the base material to form NbN, and suppresses coarsening of crystal grains during hot forging. Nb further retards recrystallization during hot forging and suppresses coarsening of crystal grains. However, if the Nb content is too high, NbC is formed and the variation in hardness of the steel increases. Therefore, the Nb content is 0.050% or less. A preferable lower limit of the Nb content is 0.005% or more, more preferably 0.010% or more. A preferable upper limit of the Nb content is 0.040% or less, more preferably 0.030% or less.

The steel material for surface hardening according to this embodiment may further contain one or more of V, Mo, Cu and Ni.

V: 0-0.15%
V may be included in the steel to strengthen the matrix phase by solid solution strengthening or by precipitation strengthening through the formation of carbonitrides. In that case, the preferable lower limit of the V content is 0.01% or more. On the other hand, if the V content is too high, the hardness of the matrix may increase, resulting in excessive strengthening and deterioration of machinability. Moreover, V consumes C and N in the base material by forming carbonitrides, and as a result, it also has the effect of raising the A3 point and lowering the hardenability. Therefore, the V content is 0.15% or less. The V content is preferably 0.12% or less, more preferably 0.10% or less.

Mo: 0-0.50%
Mo may be included in the steel to strengthen the matrix phase by solid solution strengthening or by precipitation strengthening through the formation of carbonitrides. In that case, the preferable lower limit of Mo content is 0.01% or more. On the other hand, if the Mo content is too high, the hardness of the matrix may increase, resulting in excessive strengthening and deterioration of machinability. In addition, Mo forms carbonitrides to consume C and N in the base material, and as a result, has the effect of raising the A3 point and lowering the hardenability. Therefore, Mo content is 0.50% or less. The Mo content is preferably 0.35% or less, more preferably 0.25% or less.

Cu: 0-0.50%
Cu strengthens the parent phase by solid-solution strengthening, so it may be contained in the steel. In that case, the preferable lower limit of the amount of Cu is 0.005% or more. On the other hand, if the Cu content is too high, it segregates at the grain boundaries of the steel during hot forging to induce hot cracking. Therefore, the Cu content is 0.50% or less. The Cu content is preferably 0.30% or less, more preferably 0.20% or less.

Ni: 0-0.50%
Since Ni strengthens the parent phase by solid-solution strengthening, it may be contained in the steel. Ni further suppresses hot tearing caused by Cu when the steel material contains Cu. In order to obtain these effects, 0.005% or more of Ni may be contained. However, if the Ni content is too high, the effect is saturated and the manufacturing cost increases. Therefore, the Ni content is 0.50% or less. The Ni content is preferably 0.35% or less, more preferably 0.25% or less.

The steel material for surface hardening according to this embodiment may further contain one or more of B, Ca, and Bi.

B: 0 to 0.0050%
B may be contained in the steel to improve hardenability. A preferable lower limit of the amount of B in that case is 0.0005% or more. On the other hand, if the content of B is too high, the effect is saturated, leading to an increase in cost. Therefore, the B content is 0.0050% or less. Incidentally, the B content is preferably 0.0030% or less. When B is contained, it is preferable to contain Ti at the same time in order to suppress the formation of BN and enhance the action of B.

Ca: 0-0.0050%
Ca has the effect of prolonging the tool life. Therefore, Ca may be contained as necessary. However, if the content of Ca increases, it forms coarse oxides and deteriorates toughness. Therefore, the amount of Ca when it is contained is set to 0.0050% or less. When Ca is contained, the amount of Ca is preferably 0.0030% or less.
On the other hand, in order to stably obtain the effect of prolonging the tool life by containing Ca, the amount of Ca is desirably 0.0005% or more.

Bi: 0-0.4%
Bi has the effect of lowering the cutting resistance and extending the tool life. Therefore, Bi may be contained as necessary. However, when the Bi content increases, the hot workability deteriorates. Therefore, the amount of Bi when it is contained is set to 0.4% or less. When Bi is included, the amount of Bi is preferably 0.2% or less.
On the other hand, in order to stably obtain the effect of prolonging the tool life due to the above-described Bi content, the Bi content is desirably 0.01% or more.

A3 point: 760°C or less The steel material for surface hardening according to this embodiment has the chemical composition described above, and the A3 point represented by the following formula (1) is 760°C or less.
When the steel material is quenched, it is necessary to heat the steel material to a temperature range of at least A3 point or higher in order to convert the steel material into an austenite single phase. Therefore, in order to obtain a uniform austenite structure, it is desirable that the composition has a low A3 point, and from the viewpoint of heating cost and productivity, a composition that has a low A3 point is desirable. That is, in order to convert the surface layer of the steel material into homogeneous austenite even by heat treatment at a relatively low temperature for a short period of time, the A3 point represented by the following formula (1) should be 760° C. or lower. Note that the A3 point is preferably 750° C. or less.

A3 point (°C) = 854-180 x C-20 x Mn + 40 x Si-10 x Cr-200 x N (1)
Here, each element symbol in the formula (1) is the content (% by mass) in the steel material.

[Ratio of (Mn + Cr) and Fe in θ carbide: 0.18 or less]
In the present embodiment, the θ carbide (cementite) is quickly dissolved during heating in the quenching process, and the steel structure immediately before quenching is made into a homogeneous austenitic structure, so that the concentration of Mn and Cr in the cementite in the steel material is reduced. It is important to let
Among substitutional alloying elements contained in cementite, Mn and Cr tend to concentrate. When the amount of Mn and Cr that concentrates in cementite increases, the concentration difference of Mn and Cr between the cementite and the matrix increases, and upon heating, the diffusion of Mn and Cr in the cementite is required prior to the dissolution of the cementite. . In this case, even if the heating temperature during the quenching process is A3 point or higher, the cementite does not easily decompose and a uniform austenitic structure cannot be obtained. This leads to variations in fatigue properties. That is, in order to make the steel structure during heating in the quenching process a uniform austenitic structure, the concentration of Mn and Cr in the cementite in the steel material is suppressed in advance before heating, and the cementite is dissolved during heating. It is important to facilitate

From the above point of view, in the present embodiment, the ratio of the sum of the amount of Mn (Mnθ) and the amount of Cr (Crθ) to the amount of Fe (Feθ) in the cementite of the steel material for surface hardening ((Mnθ+Crθ)/Feθ) is 0.18. Below. The ratio of the sum of Mn and Cr to Fe in cementite may be any low. For example, in a steel material containing 0.3% C, 1.4% Mn and 0.03% Cr, if Mn and Cr are not concentrated to cementite at all, the sum of Mn and Cr with respect to Fe in cementite ratio is about 0.015. Therefore, the lower limit of the ratio of the sum of Mn and Cr to Fe in cementite may be 0.015 or more. The ratio of the sum of Mn and Cr to Fe in the carbide is preferably 0.16 or less, more preferably 0.12 or less.
The above "Mnθ", "Crθ", and "Feθ" indicate weight percentages of Mn, Cr, and Fe in cementite, respectively.

The reduction in (Mnθ+Crθ)/Feθ as described above may be achieved in the entire steel material, but at least in the cementite observed in the range from the steel material surface to a depth of 0.9 mm. That is, when the steel material according to the present embodiment is subjected to surface quenching treatment to form a quenched part, the fatigue characteristics are enhanced by homogenization and hardening of the structure of the surface layer of the part. It suffices that the quenching control is achieved in the surface layer of the steel material (the range from the surface of the steel material to a depth of 0.9 mm). However, in reality, the degree of concentration of Mn and Cr in cementite does not change depending on the part of the steel material. /2, center, or anywhere else.

Mnθ, Crθ, and Feθ in the θ carbides of steel can be measured, for example, by the following method.
First, the test surface should be at a distance of 1/10 or more of the diameter from the surface and center of the steel material (for example, if the steel material has a diameter of 30 mm, the position should be 3 mm to 12 mm deep from the surface). Make a test piece. Furthermore, the test piece is energized in a 10% by volume acetylacetone-1% by mass tetramethylammonium chloride-methanol solution (10% AA-based electrolytic solution) to electrolyze 0.4 g of the surface layer of the test surface of the test piece. The resulting electrolytic solution was filtered through a filter with a mesh roughness of 0.2 μm, and the residue (precipitates and inclusions) remaining on the filter was dissolved in an acid solution and analyzed by ICP emission spectrometry. The amounts of Fe, Cr, and Mn that are not in solid solution, that is, the amounts of Fe, Cr, and Mn present as precipitates and inclusions are determined. Of these elements that are not in a solid solution state, Fe and Cr can be considered to exist entirely as θ-carbides. However, since Mn exists as MnS in addition to θ carbide, the amount of Mn present as θ carbide is the value obtained by subtracting 1.7 times the amount of S in the residue from the obtained value. can be regarded as

Mnθ, Crθ, and Feθ in the θ carbides in the steel material can be determined in the manner described above.
The shape of the test piece may be plate-like or cylindrical, as long as the test surface is located at a distance of 1/10 or more of the diameter from the surface and the center of the steel material.
In addition, if the steel material has a region with significant center segregation or a region where the chemical composition partially deviates from the specifications of the present invention due to the diffusion of alloying elements into the scale, the above test pieces are collected while avoiding these regions. I decided to.

One of the requirements necessary to make the ratio of (Mnθ+Crθ) and Feθ in cementite 0.18 or less is to contain Mn and Cr in a composite manner, and to increase the amount of Mn and decrease the amount of Cr. That is, the range of ingredients defined in the present invention is defined. In addition, as a method for efficiently reducing the ratio of (Mnθ+Crθ) and Feθ in cementite in steel, there is a method of controlling each condition of hot forging and subsequent heat treatment among steel manufacturing methods. will be described later.

The steel material for surface hardening according to the present embodiment has been described above, but the metal structure is not particularly limited except for the ratio of (Mnθ+Crθ) and Feθ in the θ carbides. This is because the steel material is heated to the austenite temperature range when the steel material is subjected to surface hardening treatment, so the mechanical properties of the steel material at room temperature before the heating are not so important.
However, the structure and hardness of the quenched part after the surface quenching treatment is applied to the steel material are important for improving the fatigue properties.
Including this point, the surface-hardened component according to the present embodiment will be described in detail below.

<Surface hardened parts>
The surface-hardened component according to the present embodiment has the same chemical composition as the above-described steel material composition, and has a point A3 that satisfies the above formula (1). Furthermore, the part surface layer including the part surface has a hardened layer having a Vickers hardness of more than 500 Hv and a thickness of 0.90 mm or more, and pro-eutectoid ferrite occupying the metal structure at a depth of 0.90 mm from the part surface, and , the total area ratio of pearlite is 10% or less, and (Mnθ+Crθ)/Feθ, which is the weight ratio of the sum of Mn and Cr in the θ carbide to Fe, is 0.5 mm or more in the region deeper than the hardened layer. 18 or less.

[Hardened layer]
In order to obtain excellent fatigue properties in the quenched part (surface quenched part) of the present embodiment, the region of the part surface layer including the part surface must have a Vickers hardness of more than 500 Hv and a thickness of 0.90 mm. It is important to provide the above quenched layer. In other words, the hardness in the range from the surface of the part to a depth of at least 0.90 mm must be greater than 500 HV.
When measuring the hardness profile in the depth direction from the surface of the part after quenching, the hardness tends to decrease in the depth direction. The same tendency is observed in this embodiment, but it is important that the position (curing depth) where the hardness exceeds 500 HV is a depth position of 0.90 mm or more. In this embodiment, a region having a Vickers hardness exceeding 500 Hv is defined as a quenched layer, and the hardening depth is defined as the thickness of the quenched layer.
As described above, it is difficult to harden in the depth direction from the surface of the steel, but if the hardening depth is 0.90 mm or more, it can be said that the hardening depth is sufficiently deep, that is, the hardening is sufficiently applied. , can improve fatigue properties.
As described above, from the viewpoint of improving fatigue characteristics, it is desirable to increase the hardening depth exceeding 500 HV, that is, to ensure a sufficient thickness of the hardened layer, but the hardness of the part surface (surface hardness ) is also desired to be large. However, no matter how large the surface hardness is, if the hardening depth is shallow (the thickness of the quenched layer is thin), the fatigue characteristics cannot be improved. It is important that the depth range hardness is greater than 500 HV. That is, it is important that the hardening depth exceeding 500 HV is 0.90 mm or more.

The Vickers hardness of the quenched layer and the hardening depth (thickness of the quenched layer) described above can be measured by the following methods.
First, the part after quenching is cut at an arbitrary cross section so as to include the part surface, the measurement area is divided into a plurality of areas along the cross-sectional length direction, and the hardness profile is measured in each area with a Vickers hardness tester. Specifically, of the cross section of the exposed part, the depth is measured starting from the surface of the part at 1/2 position, 1/4 position, and 1/8 position of the cross section length in the direction orthogonal to the thickness direction of the cross section. Measure the hardness profile in the direction of thickness. From the obtained results of the hardness profile of each region, the depth at which the hardness in each region exceeds 500 HV is determined. Then, of the obtained depths, the average of the remaining depths excluding the maximum value and the minimum value can be used as the "hardening depth (thickness of hardened layer)" of the part.

[Surface layer structure: the sum of the area ratios of proeutectoid ferrite and pearlite in the metal structure at a depth of 0.90 mm is 10% or less]
In order to obtain excellent fatigue properties in a part after quenching the steel material for surface hardening according to the present embodiment (surface hardened part), as described above, the thickness of the hardened layer is set to 0.90 mm or more. In addition, it is necessary to limit the amounts of proeutectoid ferrite and pearlite, which are incompletely hardened structures (non-hardened structures), contained in the metallographic structure (surface layer structure) of the hardened layer. In general, it becomes difficult to harden from the steel surface toward the depth direction. In this embodiment, within the range of 0.90 mm depth from the surface of the part, hardening is most difficult at the position of 0.90 mm depth. Therefore, the sum of the area ratios of proeutectoid ferrite and pearlite at a depth of 0.90 mm from the part surface is limited to 10% or less. If the sum of proeutectoid ferrite and pearlite at a depth of 0.90 mm from the surface of the part is 10% or less, it can be said that the surface layer of the part is sufficiently hardened. That is, the fact that the sum of the incompletely hardened structure at the position of 0.90 mm depth where hardening is difficult to enter is 10% or less means that the amount of incompletely hardened structure increases toward the surface side of the part where hardening is more likely to occur than at the position of 0.90 mm. tends to decrease. From these facts, if the sum of proeutectoid ferrite and pearlite at a position 0.90 mm deep from the part surface is 10% or less, the entire region from the part surface to 0.90 mm deep, that is, the entire hardened layer is incomplete. It can be said that the total area ratio of the quenched structure satisfies 10% or less. From the viewpoint of further improving the fatigue properties of the part, the sum of the area ratios of the proeutectoid ferrite and pearlite at a depth of 0.90 mm from the part surface is preferably 5% or less, even if it is 0%. good.

In the surface layer structure of the surface hardened part, the structure other than proeutectoid ferrite and pearlite is not particularly limited, but from the viewpoint of ensuring the strength of the part, it is desirable to use a structure having martensite as the main structure. Of course, it may be a mixed structure of martensite, tempered martensite, bainite, and retained austenite.

The surface layer structure of surface hardened parts can be observed and measured by the following methods.
First, one or more samples are cut to include the surface of the part in the cross section of the part after quenching. Next, the resin-filled sample is etched with nital, and an optical microscope is used to photograph the structure at a magnification of 200 times, centering on a depth of 0.90 mm from the surface of the quenched portion of the sample in multiple fields of view, By image analysis, the area ratio of proeutectoid ferrite and pearlite in each sample and each field of view is determined. The average value of the area ratio of proeutectoid ferrite and the average value of the area ratio of pearlite in each sample and each field of view were calculated, and the sum was calculated as "occupying the metal structure at a depth of 0.90 mm sum of area ratios of proeutectoid ferrite and pearlite. The number of samples is not particularly specified, and may be set as appropriate depending on the shape of the part and the hardening conditions, but the number of fields of view used for image analysis can be, for example, 3 to 5.

[The ratio of (Mn + Cr) and Fe in the θ carbide inside the part: 0.18 or less]
After the surface hardening treatment is applied to the steel material to form a hardened part, the metal structure of the surface layer of the part changes greatly due to the hardening treatment. However, since the region inside the part surface layer, that is, the region deeper than the quenched layer is not affected by quenching, it does not substantially change from the metallographic structure before quenching, that is, the metallographic structure of the steel material. From this, even after the steel material of the present embodiment is subjected to surface quenching treatment and made into a quenched part, the ratio of (Mnθ+Crθ) and Feθ in the cementite inside the quenched part is 0, like the same ratio of the steel material. 0.18 or less. Specifically, since it can be said that the region 0.5 mm or more deeper than the quenched layer is not affected by the quenching, the inside of the quenched layer thickness + 0.5 mm depth position from the surface of the part, the cementite The ratio of (Mnθ+Crθ) and Feθ becomes 0.18 or less.
In addition, when measuring Mnθ, Crθ, and Feθ in the cementite after surface hardening, the test piece should be placed at a position where the test surface is not affected by hardening (a position away from the hardening layer). Other methods and conditions can be measured by the same methods as those for measuring Mnθ, Crθ, and Feθ in steel materials as described above.

<Manufacturing method>
An example of a method for manufacturing a steel material for surface hardening and a surface hardened part according to the present embodiment will be described.
The method of manufacturing a steel material for surface hardening according to the present embodiment includes a material preparation step and a hot working step, and then a surface hardened component is manufactured by steps including a cutting step and a surface hardening step. Each step will be described below.

[Material preparation process]
Molten steel that satisfies the above chemical composition is produced. The produced molten steel is used to cast slabs (slabs, blooms) by casting. Alternatively, molten steel is used to make an ingot by an ingot casting method. A slab or ingot is hot worked to produce a billet.
The material used in the subsequent hot working step may be the aforementioned slab or ingot, or may be a billet.

[Hot working process]
The material is subjected to hot working, and the hot working may be hot rolling or hot forging.
First, the manufactured material is heated. Specifically, when surface hardening is performed after hot working (for example, forging) as it is worked (for example, as forged), the heating temperature in the hot working is preferably 1100° C. or higher.
When steel materials are produced using large billets or ingots as raw materials, Mn and Cr tend to concentrate in cementite during cooling during casting or rolling. In such a material containing cementite in which Mn and Cr are concentrated, even if the cementite is melted by heating before hot working, the distribution of Mn and Cr is not sufficiently homogenized. Cementite with a large Mn+Cr ratio is likely to form during the subsequent cooling of the heat treatment. Furthermore, when Mn and Cr remain concentrated in cementite in the steel material, it is necessary to diffuse Mn and Cr into the matrix prior to dissolving the cementite during heating for the subsequent quenching treatment. can not be decomposed into a uniform structure. Therefore, when using the steel of the specified composition of the present invention and performing surface quenching as it is worked (for example, as it is forged) after hot working (for example, forging), the heating temperature in hot working should be 1100 ° C. or higher. is desirable.
Thus, if the heating temperature during hot working is too low or the heating time is too short, it becomes difficult to disperse Cr and Mn concentrated in cementite in the previous step, and Cr in cementite after cooling and the Mn concentration may increase. On the other hand, if the heating temperature is too high or the heating time is too long, scale loss is large. Therefore, the preferred heating temperature is 1100-1300° C., and the preferred heating time is 30-120 minutes. In addition, "heating temperature" refers to the surface temperature of a raw material or steel materials.

Next, hot working is performed on the raw material after heating.
Hereinafter, the hot working in this step will be described as hot forging.
A preferable finishing temperature for hot forging is 900° C. or higher. This is because if the finishing temperature is too low, the load on the die of the hot forging apparatus increases. On the other hand, the preferred upper limit of the finishing temperature is 1250°C. The "finishing temperature" refers to the temperature of the surface of the steel immediately after hot working.
Also, in the cooling step after hot working, it is desirable to shorten the time during which the steel is held in the temperature range from the A1 point to 500°C. If the temperature is maintained in this temperature range for too long, it becomes difficult to suppress the concentration of Mn and Cr in the cementite in the next step. Therefore, in cooling after hot working, the temperature range from the A1 point of steel to 500° C. should be cooled at an average cooling rate of 0.1° C./s or more. The "average cooling rate" mentioned above is obtained by dividing the difference in temperature from point A1 to 500°C (point A1 - 500°C) by the time (s) required for cooling.

[Heat treatment]
If necessary, the structure may be adjusted by heat treatment or heat treatment may be applied to release strain prior to cutting in the next step. For example, normalizing, quenching/tempering, or low-temperature annealing may be applied.
In addition, when surface quenching is performed after performing normalizing after hot forging or heat treatment in a temperature range where phase transformation occurs such as annealing, it is necessary to reduce (Mn θ + Cr θ) / Fe θ. , It is sufficient to heat for 30 minutes or more at a heating temperature higher than the A3 point by 100 ° C. or more, and then cool the temperature range from the A1 point to 500 ° C. at an average cooling rate of 0.1 ° C./s or more. . Further, after hot forging or after heat treatment in a temperature range where phase transformation occurs, annealing at a low temperature where phase transformation does not occur or tempering treatment may be performed. However, in that case, as a treatment condition for heat treatment at a low temperature where phase transformation does not occur, the parameter P represented by the following formula (A) should be 18.7 or less.
P={(T+273)/1000}×(Log(t)+20) (A)
Here, T in the formula (A) represents the treatment temperature (° C.), and t represents the time (h).

As described above, the steel material for surface hardening according to the present embodiment is obtained. The hot working conditions and heat treatment conditions as described above are one of the methods for making the ratio of (Mnθ+Crθ) and Feθ in the cementite of the steel material 0.18 or less. is not limited to the above method and conditions as long as the ratio of (Mnθ+Crθ) and Feθ is 0.18 or less.

Further, after the hot working process or after the heat treatment process, a cutting process may be performed.
In addition to steel plates, steel pipes, shaped steels, etc., the steel materials for surface hardening in this embodiment include members processed into desired shapes before hardening in the next step.

[Cutting]
The steel material for surface hardening described above may be cut into a steel material having a predetermined shape.
Next, in order to obtain a surface-hardened part, the obtained steel material for surface hardening is subjected to the hardening treatment described below.

[Surface hardening treatment]
A surface hardening treatment is performed on the cut steel material for surface hardening. Surface quenching may be any process as long as it heats only the vicinity of the surface of the steel material and rapidly cools the heated portion after heating. The heating of the surface may be high-frequency induction heating, heating by laser irradiation, or heating by applying a flame directly to the surface of the steel. Cooling of the heated portion may be water cooling, gas cooling, or self-cooling by thermal diffusion to the internal non-heating layer.
After surface quenching, tempering may be performed to increase toughness. If the tempering temperature is too high, the steel softens and the strength decreases, so the preferred tempering temperature is 500°C or less. A preferable tempering temperature is 100° C. or higher in order to sufficiently obtain the effect of enhancing toughness by tempering. In order to sufficiently obtain the effect of increasing toughness by tempering, the tempering time is preferably set to 0.5 hours or longer. As the tempering time increases, the effect becomes saturated although the manufacturing cost increases. Therefore, the tempering time is preferably set to 5 hours or less.

When the surface hardening treatment is performed using the steel material for surface hardening manufactured by the above manufacturing process, it is possible to manufacture hardened parts excellent in fatigue strength even if the surface hardening conditions vary.

Next, examples of the present invention will be described. The conditions in the examples are one example of conditions adopted for confirming the feasibility and effect of the present invention, and the present invention is based on this one example of conditions. It is not limited. Various conditions can be adopted in the present invention as long as the objects of the present invention are achieved without departing from the gist of the present invention.

Ingots of steel grades A to Q having chemical compositions shown in Table 1 were heated to 1250°C. The heated ingot was hot forged to produce a square bar with a square cross section of 75 mm per piece. Further, the square bar was heated to 1250° C. (heating time: 30 minutes), hot forged into a round bar having a diameter of 60 mm at a finish temperature of 1000° C., and allowed to cool to room temperature. At this time, the range from A1 point to 500° C. was cooled at an average cooling rate of 0.1° C./s or more. Table 1 also shows the A3 point (°C).

A part of the round bar of steel type K after forging was subjected to quenching and tempering treatment (Test Nos. 12 and 18). In the quenching step, the round bar was held at 850° C. for 1 hour and then cooled with water. In the subsequent tempering process, test No. 12 at 600° C., test no. 18 was heated to 650° C., held at those temperatures for 3 hours, and then allowed to cool to room temperature. In addition, test No. The above parameters P, which are the conditions in the tempering steps of Nos. 12 and 18, are the same as those of Test Nos. 12 is "17.9", Test No. 18 was "18.9".
In addition, another part of the round bar of steel type K (A1 point: 713°C) after forging was subjected to a normalizing treatment in which it was held at 850°C for 1 hour and then cooled to room temperature (Test Nos. 11 and 19). . In addition, test No. In Test No. 11, the cooling from 850°C to room temperature was allowed to cool in the atmosphere (an average cooling rate of 0.1°C/s or more between point A1 and 500°C). 19 was covered with a ceramic sheet and slowly cooled to room temperature (average cooling rate from point A1 to 500°C was less than 0.1°C/s).
The following tests were carried out using round bars of each test number.

[Core hardness measurement]
From the round bar of each test number, a 50-mm-long test piece for reliability investigation having a square cross-section with 10-mm sides was prepared. The length direction of the test piece is parallel to the length direction of the forged material (round bar), and the center of the cross section of the test piece is the position of R / 2 (R is the radius of the round bar) of the round bar I tried to align with Next, the test piece is mounted in resin and then polished, and the Vickers hardness is measured at a load of 98 N at any five points near the center of the cross section of the test piece, and the arithmetic average of the five measured values is the test number. It was taken as core hardness. If the core hardness is 280 HV or less, it is judged that the core hardness is sufficiently low and machinability is excellent, and if the core hardness is over 280 HV, it is judged that the machinability has deteriorated. did.
Tables 2 and 3 show the results.

[Preparation of test piece for measurement of rotating bending fatigue strength after induction hardening]
From R / 2 of the round bar of each test number, an Ono type rotating bending fatigue test piece with an annular notch (notch) with a depth of 1 mm in the parallel part of φ 10 and a tip R of 3 mm as shown in FIG. Created.
Next, for the Ono-type rotating bending fatigue test piece, the parallel part including the notch part was set at a frequency of 100 kHz and a heating time of 1.5 seconds (high frequency condition A), or a heating time of 1 second (high frequency condition B). It was subjected to induction hardening treatment in which high-frequency heating was performed at , followed by water cooling. A part of the rotating bending fatigue test piece after induction hardening was subjected to low temperature tempering treatment at 150°C for 2 hours. Table 2 shows each condition. After induction hardening or after low-temperature tempering, the test piece was aligned by grinding the grip portion of the Ono-type rotary bending fatigue test piece.

[Ono type rotary bending fatigue test]
Using the Ono-type rotary bending fatigue test piece after induction hardening or low-temperature tempering, an Ono-type rotary bending fatigue test was performed at a rotation speed of 3000 rpm. It was judged that the fatigue strength was excellent when the fatigue strength was 500 MPa or more under any of the induction hardening conditions A and B. Table 4 shows the results.

[Preparation of test piece for measuring four-point bending fatigue strength after laser hardening]
Next, from the center of the round bar of each test number, it has a cross section of 30 mm × 32 mm, a length of 100 mm, a depth of 2 mm parallel to the side of 32 mm at the center in the length direction, and a tip R of 2 mm. A test piece with a notch was created.
Next, for this notched test piece, the area including the notch part is subjected to a feed rate (irradiation Both were heated by a laser at a speed of 6.5 mm/s. When only the surface layer of the test piece is heated with a laser, the surface layer is rapidly cooled and quenched due to heat diffusion to the non-heated layer inside the test piece. A part of the test piece after laser irradiation was subjected to a low temperature tempering treatment at 150°C for 2 hours. Table 3 shows each condition.
Next, from the test piece after laser hardening or low-temperature tempering, four points with a cross section of 13 mm × 13 mm and a length of 100 mm were placed as close as possible to the center of the test piece with a width of 32 mm so as to include the notch part. A bending fatigue test piece was prepared (see FIG. 2). During the four-point bending fatigue test to be described later, the notch ends were chamfered in order to prevent fatigue fracture from both ends of the notch, which has a peculiar stress state. Specifically, the corners were chamfered so that the value of the corner chamfer C in JIS B 0701 was 0.5.

[Four-point bending fatigue test]
A four-point bending fatigue test was performed using the four-point bending fatigue test piece prepared in the above-described process. The fatigue specimen was set so that the distance between the fulcrums was 80 mm and the notch was centered between the fulcrums. The distance between the load points was 20 mm, and the center of the load points was the same as the center between the fulcrums. A fatigue test piece supported in this way was subjected to a four-point bending fatigue test with a test frequency of 20 Hz and a stress ratio of 0.05. Among the load amplitudes (one-half of the difference between the maximum load and the minimum load) that did not break up to 1.0×10 ⁷ repetitions, the largest value was defined as the fatigue strength (kN) of the test number. When the fatigue strength was 5.00 kN or more under both laser irradiation conditions A and B, it was determined that the fatigue strength was excellent. Table 5 shows the results.

[Electrolytic extraction residue measurement]
A round bar test piece of φ10×35 was prepared from R/2 of the round bar of each test number. The prepared round-bar test piece was energized in a 10% AA-based electrolytic solution to electrolyze 0.4 g of the surface layer of the test piece. The electrolytic solution was filtered through a filter with a mesh roughness of 0.2 μm, and the residue remaining on the filter was dissolved in an acid solution and analyzed by ICP emission spectrometry. We asked for the amount of Of these elements that are not in a solid solution state, it is assumed that all of Fe and Cr are present as θ carbides. Assuming that they exist as carbides, the ratio of (Mnθ+Crθ) and Feθ in the θ carbides in the steel material (round bar) was determined.

Note that (Mnθ+Crθ)/Feθ after quenching was obtained as follows.
First, the center of the rotating bending fatigue test piece after induction hardening or low temperature tempering, or the (0.5 + 5) mm depth position from the notch bottom of the four-point bending fatigue test piece after laser hardening or low temperature tempering. A round bar test piece of φ3×35 was prepared so that the length direction was parallel to the length direction of each fatigue test piece. The subsequent measurement method was the same as that for measuring the electrolytically extracted residue of the steel material (round bar) described above, and the ratio of (Mnθ+Crθ) and Feθ in the internal θ carbide after quenching was determined.
In Tables 2 to 5, (Mn θ + Cr θ)/Fe θ in the θ carbide is simply expressed as (Mn + Cr)/Fe.

[Observation of structure after quenching]
After high-frequency quenching or low-temperature tempering, a rotating bending fatigue test piece is cut longitudinally so as to include the notch bottom, a sample is taken so that the cut surface can be observed, the sample is mounted in resin, and a sample for structure observation was made.
Similarly, from the four-point bending fatigue test piece after laser hardening or low temperature tempering, a sample was taken so that the cut surface could be observed by longitudinally crossing the plane passing through the center in the width direction so as to include the notch bottom. , the sample was mounted on a resin to prepare a sample for tissue observation.
Next, each sample was etched with nital, and the structure of the surface layer after quenching was observed using an optical microscope. The notch portion of the test piece was photographed in three fields of view at a magnification of 200, centering on the depth position of 0.90 mm in the direction perpendicular to the surface. , the area ratios of pro-eutectoid ferrite and pearlite were obtained. Then, the average value of the area ratio of pro-eutectoid ferrite and the average value of the area ratio of pearlite obtained from the micrographs of the three fields of view were calculated, and the sum of the area ratios of pro-eutectoid ferrite and pearlite was calculated. When the sum of the area ratios of proeutectoid ferrite and pearlite was 10% or less, it was judged that the hardening was sufficiently performed. In Tables 4 and 5, proeutectoid ferrite and pearlite are collectively described as "unquenched structure".

[Measurement of hardening depth after quenching]
Rotating bending fatigue test pieces after induction hardening or low temperature tempering, and four-point bending fatigue test pieces after laser hardening or low temperature tempering, respectively, so as to include notch bottoms in the same manner as the sampling for observing the structure described above. Samples for cure depth were taken along the length. Next, among the exposed cross sections of each sample, the notch bottom of each sample is used as the starting point at each of the 1/2 position, 1/4 position, and 1/8 position of the cross-sectional length in the direction perpendicular to the thickness direction. The hardness profile in the depth direction was measured. From the obtained results of the hardness profile at each position, the "depth at which the hardness exceeds 500 HV" at each position was determined. Then, of the obtained "depth exceeding 500 HV" at each position, the average of the remaining depths excluding the maximum and minimum values is the "hardening depth" (hardened layer's depth) of the sample (fatigue test piece). thickness). Tables 4 and 5 show the results. In Tables 4 and 5, the hardness at a depth of 0.05 mm from the bottom of the notch is also indicated as "surface hardness."

[Test results]
Tables 4 and 5 show the test results. In the invention examples of test numbers 1 to 12, 21, and 22, when the high-frequency heating conditions or the laser irradiation conditions changed and the heating conditions became insufficient, that is, even when there was variation in the heating conditions, the core hardened An excessive increase in thickness was suppressed, and a deep hardened layer could be obtained. As a result, excellent fatigue strength and machinability could be enjoyed.
On the other hand, in the comparative examples of test numbers 13 to 20, 23, and 24, when the high-frequency heating conditions or the laser irradiation conditions changed, the core hardness increased excessively and the machinability deteriorated, and the surface hardness decreased. Although it was sufficient, the hardened layer became shallow and the fatigue strength deteriorated.

In Test Nos. 13 and 23 (steel type L), the Mn content was too small, so that the hardenability could not be sufficiently improved and the hardening depth could not be secured sufficiently. As a result, the fatigue strength deteriorated in test number 13 under condition B, and in test number 23 under both conditions A and B. Especially in the case of Test No. 23, a large amount of non-quenched structure was generated, so the fatigue strength was greatly reduced.

In Test Nos. 14 and 24 (steel type M), the Si content was too large, so the A3 point could not be sufficiently lowered, and as a result of causing nonuniformity of the structure, variation in fatigue characteristics could not be suppressed. rice field.

In Test No. 15 (steel type N), the amount of C was too small, so the hardness of the structure after quenching was low and the hardening depth was shallow. As a result, it was not possible to suppress variations in fatigue characteristics.

Test No. 16 (steel grade O) had an excessive amount of C, so the core hardness increased excessively and the machinability deteriorated.

In Test No. 17 (steel type P), the amount of Cr was too large, so that the cementite in which Cr dissolved as a solid solution was difficult to dissolve. could not secure enough. As a result, although the fatigue strength was good in the case of the high frequency condition A, it deteriorated in the case of the condition B, and the variation in the fatigue characteristics could not be suppressed.

In Test Nos. 18 and 19, the steel type was within the scope of the invention, but the heat treatment conditions after forging were unsuitable. As a result, it was difficult to decompose cementite, a uniform austenite structure could not be obtained, and variations in fatigue properties after quenching could not be suppressed.

In Test No. 20 (steel type Q), the A3 point was too large, which caused the structure to become non-uniform, and as a result, the variation in fatigue properties could not be suppressed.

According to the steel material for surface hardening of the present invention, it is possible to provide a surface hardened part having high fatigue strength even when the shape of the part is complicated or the surface hardening conditions vary. It can be applied to the manufacture of machine parts and has great industrial utility value.

Claims (8)

in % by mass,
C: 0.30 to 0.80%,
Si: 0.01 to 0.40%,
Mn: 1.50-3.00 %,
P: 0.050% or less,
S: 0.001 to 0.100%,
Cr: 0.03 to 0.40%,
Al: 0.001 to 0.080%,
N: 0.0030 to 0.0250%, the balance being Fe and impurities,
A3 point represented by the following formula (1) is 750 ° C. or less,
A steel material for surface hardening, wherein (Mnθ+Crθ)/Feθ, which is the weight ratio of the sum of Mn and Cr in θ carbides in a metal structure to Fe, is 0.18 or less.
A3 (°C) = 854 - 180 x C - 20 x Mn + 40 x Si - 10 x Cr - 200 x N
... (1)
Here, each element symbol in formula (1) is the content (% by mass) in the steel material, and the Mnθ, Crθ and Feθ are the weights of Mn, Cr and Fe in the θ carbide, respectively. %. 2. The steel material for surface hardening according to claim 1, further comprising one or more of Ti: 0.050% or less and Nb: 0.050% or less . Further, it contains one or more of V: 0.15% or less , Mo: 0.50% or less , Cu: 0.50% or less , Ni: 0.50% or less . Steel material for surface hardening according to . 4. The steel according to any one of claims 1 to 3, further comprising one or more of B: 0.0050% or less , Ca: 0.0050% or less , and Bi: 0.4% or less . Steel material for surface hardening. in % by mass,
C: 0.30 to 0.80%,
Si: 0.01 to 0.40%,
Mn: 1.40-3.00%,
P: 0.050% or less,
S: 0.001 to 0.100%,
Cr: 0.03 to 0.40%,
Al: 0.001 to 0.080%,
N: 0.0030 to 0.0250%, the balance being Fe and impurities,
A3 point represented by the following formula (2) is 760 ° C. or less,
The part surface layer including the part surface has a hardened layer with a Vickers hardness of more than 500 Hv and a thickness of 0.90 mm or more,
The sum of the area ratios of proeutectoid ferrite and pearlite in the metal structure at a depth of 0.90 mm from the part surface is 10% or less,
A surface-hardened component, wherein (Mnθ+Crθ)/Feθ, which is the weight ratio of the sum of Mn and Cr in the θ carbide to Fe, is 0.18 or less in a region deeper than the hardened layer by 0.5 mm or more.
A3 (°C) = 854 - 180 x C - 20 x Mn + 40 x Si - 10 x Cr - 200 x N
... (2)
Here, each element symbol in formula (2) is the content (% by mass) in the part. 6. The surface-hardened part according to claim 5, further comprising one or more of Ti: 0.050% or less and Nb: 0.050% or less . Further, one or more of V: 0.15% or less , Mo: 0.50% or less , Cu: 0.50% or less , and Ni: 0.50% or less are contained. The surface hardened part described in . 8. The steel according to any one of claims 5 to 7, further comprising one or more of B: 0.0050% or less , Ca: 0.0050% or less , and Bi: 0.4% or less . Surface hardened parts.

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