JP2020015939A - Steel material for surface hardening, and surface hardening component - Google Patents

Abstract

To provide a steel material for surface hardening capable of providing excellent fatigue property even when variations of a heating condition is generated during surface hardening, and a surface hardening component.SOLUTION: There is provided a steel material for surface hardening containing, by mass%, C:0.30 to 0.80%, Si:0.01 to 0.40%, Mn:1.40 to 3.00%, P:0.05% or less, S:0.001 to 0.100%, Cr:0.03 to 0.40%, Al:0.001 to 0.080%, N:0.003 to 0.025%, and the balance Fe with impurities, and having A3 point temperature represented by the following formula (1) of 760°C or lower, and (Mnθ+Crθ)/Feθ, which is a weight ratio of sum of Mn and Cr in θ carbide observed in a metal structure to Fe of 0.18 or less. A3(°C)=854-180×C-20×Mn+40×Si-10×Cr-200×N (1).SELECTED DRAWING: None

Description

  The present invention relates to a surface-hardened steel material and a surface-hardened part having high fatigue strength.

  BACKGROUND ART For the purpose of improving fatigue strength, a mechanical component used for an automobile, an industrial machine, a construction machine, or the like may be subjected to a process of hardening the surface of the component by partial quenching such as induction hardening or laser quenching. The fatigue strength of the surface quenched component subjected to such treatment is affected by the hardness profile and the structure near the surface of the component after quenching. Therefore, by improving the hardenability by optimizing the steel composition and detoxifying harmful inclusions, or by controlling the manufacturing conditions of the steel material and refining the structure, a high quality is obtained in the hardened part. Techniques for obtaining fatigue strength have been developed.

  Patent Document 1 reports a technique of optimizing steel components and mixing bainite or martensite in a matrix structure to reduce the grain size during high-frequency heating and improve the fatigue strength of surface hardened parts. Have been.

  Patent Literature 2 reports a technique for improving the fatigue strength of a surface-hardened part by optimizing steel components and detoxifying inclusion forms that are harmful to fatigue.

  Further, machine parts used for automobiles, industrial machines, construction machines, and the like are manufactured by cutting a steel plate as a material. Therefore, steel materials for machine parts are also required to have high machinability (machinability).

JP 2006-45678 A International Publication No. 2014/061782

According to the above-described conventional technology, excellent fatigue characteristics can be obtained when the surface layer of the component is uniformly heated and quenched by high-frequency heating and a deep hardened layer is obtained. However, when steel is processed into various component shapes and subjected to surface quenching under various conditions, the fatigue characteristics vary, and high fatigue strength may not always be obtained. This phenomenon is caused by the fact that even if the heating conditions such as the output of the equipment for surface quenching are fixed, if the shape of the part is complicated, depending on the part, the heating conditions such as the time to be exposed to high temperature and the ultimate temperature This causes a difference in austenite structure and quenched structure during heating.
The magnitude of the degree of variation in the fatigue characteristics when the substantial heating conditions are changed is affected by the steel composition and the structure, but even when the technology reported in the above-described invention is applied, the variation in the fatigue characteristics is reduced. Could not be reduced.
In addition, even if the shape of the part after processing is not complicated, even when the surface quenching equipment to be used, or the heating conditions in the first place due to changes in conditions, use environment, etc., the above-mentioned variation in the structure occurs, It has been confirmed that variations in fatigue characteristics occur.

  The present invention has been made in view of the above-described problems, and its purpose is to obtain excellent fatigue characteristics even when the heating conditions during surface quenching vary. Moreover, it is an object of the present invention to provide a surface hardened steel material excellent in machinability, which can be easily cut, and a surface hardened part excellent in fatigue characteristics.

  The present inventors quenched the surface of steel 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) When the surface quenching conditions vary, the steel structure in which the fatigue strength is greatly deteriorated has a non-uniform steel structure immediately before quenching, and as a result, the structure after quenching (quenched structure) is also non-uniform. .

(B) Even if the surface quenching conditions vary, one of the requirements necessary to make the steel structure just before surface quenching uniform austenite is to lower the A3 point of the steel. In particular, by containing Mn, the A3 point can be effectively reduced, and uniform austenite can be obtained even at relatively low temperature and for a short time of heating.

(C) Another requirement necessary for obtaining high fatigue properties even when the surface quenching conditions vary is to reduce the difference between the substitutional alloy element concentration of cementite and the substitutional alloy element concentration of the parent phase. This is because, when the concentration difference between the substitutional alloy element of cementite and the parent phase becomes large, the diffusion of the substitutional alloy element is required prior to dissolution of the cementite during heating during the quenching treatment. This is because even at a temperature, cementite does not easily decompose and a uniform austenite structure cannot be obtained. That is, in order to obtain uniform austenite during heating, it is important to suppress the concentration of the substitutional alloy element in cementite and to facilitate the dissolution of cementite.

(D) Cr is an element that easily concentrates in cementite, and cementite containing a large amount of Cr is difficult to dissolve even if it exceeds the A3 point. Mn is also an element that concentrates in cementite, but the degree of concentration is smaller than that of Cr.

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

  The present invention has been completed based on the above findings, and the gist is as shown in the following (1) to (5).

(1) 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 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 The point A3 represented by the following formula (1) is 760 ° C. or less, and the ratio (Mnθ + Crθ) / Feθ, which is the weight ratio of the sum of Mn and Cr in Fe carbide to Fe in Fe, is 0.18. A steel material for surface quenching characterized by the following.
A3 (° C.) = 854-180 × C-20 × Mn + 40 × Si-10 × Cr-200 × N (1)
Here, each element symbol in the 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. Represents%.
(2) The steel material for surface quenching according to (1), further comprising one or more of Ti: 0 to 0.050% and Nb: 0 to 0.050%.
(3) Further, it is characterized by containing at least one of V: 0 to 0.15%, Mo: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%. The steel material for surface quenching according to the above (1) or (2).
(4) The above (1) to (3), further containing one or more of B: 0 to 0.0050%, Ca: 0 to 0.0050%, and Bi: 0 to 0.4%. The steel material for surface quenching according to any one of the above 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 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, and the surface of the component including the component surface has a quenched layer having a Vickers hardness of more than 500 Hv and a thickness of 0.90 mm or more; In the region where the total area ratio of proeutectoid ferrite and pearlite in the metal structure at a depth of 0.90 mm from the component surface is 10% or less and 0.5 mm or more deeper than the quenched layer, It is the weight ratio of the sum of Mn and Cr to Fe (Mnθ + Crθ) / Feθ is 0.18 or less.
A3 (° C.) = 854-180 × C-20 × Mn + 40 × Si-10 × Cr-200 × N (2)
Here, each element symbol in the formula (2) is the content (% by mass) in the component.
(6) The surface-hardened part according to (5), further comprising one or more of Ti: 0 to 0.050% and Nb: 0 to 0.050%.
(7) Further, it is characterized by containing at least one of V: 0 to 0.15%, Mo: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%. (5) or (6).
(8) Further, B: 0 to 0.0050%, Ca: 0 to 0.0050%, and Bi: 0 to 0.4%, characterized by containing one or more of (5) to (7). A surface-hardened part according to any one of the preceding claims.

  ADVANTAGE OF THE INVENTION According to this invention, even if the heating conditions at the time of surface quenching vary, it is possible to obtain excellent fatigue characteristics, and it is also easy to machine and has excellent machinability. And a surface-hardened part having excellent fatigue properties.

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

  Hereinafter, the surface-hardened steel material and the surface-hardened parts according to one embodiment of the present invention will be described in detail.

<Steel hardened steel>
[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 the following description, “%” of the content of each element means “% by mass”.

C: 0.30 to 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 in a region deeper than the surface layer to improve the strength. In order to obtain this effect sufficiently, the content of C needs to be 0.30% or more. When the content of C exceeds 0.80%, the hardness after hot forging, particularly the core hardness becomes too large, and the machinability deteriorates. Therefore, the content of C needs to be 0.80% or less. The C content is preferably 0.35% or more, and more preferably 0.38% or more. Further, the C content is preferably 0.75% or less, and more preferably 0.70% or less.

Si: 0.01 to 0.40%
The sliding parts such as gears are heated and tempered during use, thereby decreasing hardness and deteriorating fatigue strength. Si has the effect of suppressing the generation and growth of cementite and reducing the amount of softening during tempering. In order to obtain this effect sufficiently, the content of Si needs to be 0.01% or more. When the content of Si exceeds 0.40%, it becomes difficult to lower the A3 point, and as a result, the low-temperature heat treatment during the surface quenching treatment and the uniformization of the steel structure in the short-time treatment become insufficient, and the quenched parts It is difficult to suppress variations in the fatigue characteristics of the steel. Therefore, the content of Si needs to be 0.40% or less. Note that the Si content is preferably 0.03% or more. Further, 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 to 3.00%
Mn has the effect of lowering the A3 point and promoting the reverse transformation from ferrite to austenite during low-temperature heat treatment and short-time heat treatment, thereby reducing the variation in fatigue characteristics. 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 increasing hardenability than other substitutional alloy elements having the same effect. In order to obtain the above effects, the Mn content needs to be 1.40% or more. On the other hand, if the content of Mn exceeds 3.00%, the hardness after hot forging becomes hard, and the machinability deteriorates. Therefore, the Mn content is 1.40 to 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. Further, the Mn content is preferably at most 2.80%, more preferably at most 2.60%.

P: 0.050% or less P is an impurity. P segregates at crystal grain boundaries, causing grain boundary embrittlement. Therefore, the P content is preferably as low as possible. The P content is 0.050% or less. The preferred P content is 0.040% or less.

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

Cr: 0.03 to 0.40%
Cr has the effect of improving the quenchability, suppressing the formation of an incompletely quenched structure such as proeutectoid ferrite and pearlite, and reducing the variation in hardness after quenching. To obtain this effect, the Cr content needs to be 0.03% or more. On the other hand, if the Cr content exceeds 0.40%, the cementite in which Cr is dissolved as a solid becomes difficult to dissolve at a temperature of the A3 point or more, the steel structure immediately before quenching becomes uneven, and the hardened steel after quenching becomes hard. It causes the variation of the height. Therefore, the Cr content is 0.03 to 0.40%. Note that the Cr content is preferably 0.05% or more. Further, the Cr content is preferably at most 0.35%, more preferably at most 0.30%.

Al: 0.001 to 0.080%
Aluminum deoxidizes steel. In order to obtain this effect, the Al content needs to be 0.001% or more. On the other hand, if the Al content is too high, coarse oxides are easily formed in the steel, and the fatigue strength deteriorates. Therefore, the Al content is 0.001 to 0.080%. Note that the Al content is preferably 0.005% or more, and more preferably 0.010% or more. Further, the Al content is preferably 0.060% or less, and more preferably 0.050% or less.

N: 0.0030 to 0.0250%
N contributes to the uniformization of the steel structure immediately before quenching by lowering the A3 point of steel, and also has the effect of reducing the variation in hardness after quenching. In order to obtain this effect, the N content needs to be 0.0030% or more. On the other hand, if the N content is too high, bubbles may be generated in the steel material and the fatigue strength may be deteriorated. Therefore, the N content is 0.0030 to 0.0250%. Note that the N content is preferably 0.0050% or more. Further, the N content is preferably 0.0220% or less, and more preferably 0.0200% or less. N is also an element mixed into steel as an impurity.

  The remainder of the steel material for surface quenching according to the present embodiment comprises Fe and impurities. Here, the impurities are those that are mixed in from the ore, scrap, or the production environment as raw materials when the steel material is industrially manufactured, and within a range that does not adversely affect the effects and characteristics of the present invention. Means acceptable.

  The steel material for surface hardening according to the present embodiment may further contain one or two of Ti and Nb.

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

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

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

V: 0 to 0.15%
V may be included in steel to strengthen the parent phase by solid solution strengthening or by precipitation strengthening by the formation of carbonitrides. In that case, a preferable lower limit of the V amount is 0.01% or more. On the other hand, if the V content is too high, the hardness of the matrix may increase, and may be excessively strengthened to deteriorate the machinability. Further, V consumes C and N in the base material by forming carbonitride, and as a result, has an 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, and more preferably 0.10% or less.

Mo: 0 to 0.50%
Mo may be included in the steel to strengthen the parent phase by solid solution strengthening or by precipitation strengthening by the formation of carbonitrides. In that case, a preferable lower limit of the Mo amount 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 in machinability. In addition, Mo consumes C and N in the base material by forming carbonitride, and as a result, has an effect of raising the A3 point and lowering the hardenability. Therefore, the Mo content is 0.50% or less. The Mo content is preferably at most 0.35%, more preferably at most 0.25%.

Cu: 0 to 0.50%
Cu may be contained in steel to strengthen the parent phase by solid solution strengthening. In that case, a 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 and induces hot cracking. Therefore, the Cu content is 0.50% or less. The Cu content is preferably at most 0.30%, more preferably at most 0.20%.

Ni: 0 to 0.50%
Ni may be contained in steel to strengthen the parent phase by solid solution strengthening. Ni further suppresses hot cracking caused by Cu when the steel material contains Cu. To obtain these effects, Ni may be contained at 0.005% or more. However, if the Ni content is too high, the effect is saturated and the production cost increases. Therefore, the Ni content is 0.50% or less. The Ni content is preferably at most 0.35%, more preferably at most 0.25%.

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

B: 0 to 0.0050%
B may be contained in steel to enhance hardenability. In that case, a preferable lower limit of the B amount is 0.0005% or more. On the other hand, if the content of B is too high, the effect is saturated and the cost is increased. Therefore, the B content is 0.0050% or less. The B content is preferably 0.0030% or less. When B is contained, it is preferable to simultaneously contain Ti in order to suppress the generation of BN and enhance the function of B.

Ca: 0 to 0.0050%
Ca has the effect of extending the tool life. For this reason, Ca may be contained as necessary. However, when the content of Ca is large, a coarse oxide is formed and the toughness is deteriorated. Therefore, when Ca is contained, the amount of Ca is set to 0.0050% or less. When Ca is contained, the amount of Ca is preferably set to 0.0030% or less.
On the other hand, the amount of Ca is desirably 0.0005% or more in order to stably obtain the effect of extending the tool life by the above-described Ca content.

Bi: 0 to 0.4%
Bi has the effect of reducing the cutting resistance and extending the tool life. For this reason, Bi may be contained as needed. However, when the Bi content increases, the hot workability decreases. Therefore, the content of Bi when contained is set to 0.4% or less. The content of Bi when contained 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 inclusion of Bi as described above, the amount of Bi is desirably 0.01% or more.

A3 point: 760 ° C. or lower The surface hardening steel material according to the present embodiment has the above chemical composition, and the A3 point represented by the following formula (1) is 760 ° C. or lower.
When quenching a steel material, it is necessary to heat the steel material to a temperature range of at least the A3 point or more in order to make the steel material an austenitic single phase. Therefore, in order to obtain a uniform austenite structure, a composition having a low A3 point is desirable, and from the viewpoint of heating cost and productivity, a composition having a low A3 point is desirable. That is, in order to make the steel surface layer uniform austenite even at a relatively low temperature and for a short time, the A3 point represented by the following equation (1) is set to 760 ° C. or less. In addition, it is preferable that A3 point is 750 degreeC or less.

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

[Ratio of (Mn + Cr) to Fe in θ carbide: 0.18 or less]
In the present embodiment, in order to dissolve the θ carbide (cementite) quickly at the time of heating in the quenching process and to make the steel structure immediately before quenching a homogeneous austenite structure, the concentration of Mn and Cr in the cementite in the steel material is reduced. It is important that
Among the substitutional alloy elements contained in cementite, Mn and Cr are easily concentrated. When the amount of Mn and Cr concentrated in cementite increases, the difference in concentration between Mn and Cr between cementite and the matrix increases, and during heating, diffusion of Mn and Cr in cementite is required prior to dissolution of cementite. . Then, even if the heating temperature during the quenching treatment is a temperature of A3 point or more, cementite does not easily decompose and a uniform austenite structure cannot be obtained. As a result, the quenched structure becomes non-uniform, and the This leads to variations in fatigue characteristics. That is, in order to make the steel structure at the time of heating in the quenching process a uniform austenite structure, the concentration of Mn and Cr in the cementite in the steel material is suppressed before heating, and the dissolution of cementite at the time of heating is performed. It is important to facilitate

From the above viewpoints, in the present embodiment, the ratio of the sum of the Mn amount (Mnθ) and the Cr amount (Crθ) to the Fe amount (Feθ) in the cementite of the surface hardening steel material ((Mnθ + Crθ) / Feθ) is 0.18. The following is assumed. The ratio of the sum of Mn and Cr to Fe in cementite may be as low as possible. For example, in a steel material containing 0.3% of C, 1.4% of Mn, and 0.03% of Cr, when Mn and Cr do not concentrate at all into cementite, the sum of Mn and Cr with respect to Fe in cementite is used. 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, and more preferably 0.12 or less.
The above “Mnθ”, “Crθ”, and “Feθ” indicate the weight percentages of Mn, Cr, and Fe in cementite, respectively.

  The reduction of (Mnθ + Crθ) / Feθ as described above may be achieved in the entire steel material, but it is sufficient that at least cementite observed in a range from the steel material surface to a depth of 0.9 mm is achieved. That is, when the steel material according to the present embodiment is subjected to a surface quenching treatment to form a quenched part, the fatigue characteristics are enhanced by homogenization and hardening of the surface layer of the part, and thus the concentration of Mn and Cr in the cementite is increased. It is sufficient that the formation control is achieved in the surface layer of the steel material (range from the steel surface to a depth of 0.9 mm). However, in practice, the degree of Mn and Cr enrichment in cementite does not change depending on the location of the steel material. Therefore, the degree of Mn and Cr enrichment in cementite is evaluated in the vicinity of the surface of the steel material, R / 2, at the center, or at any other position.

Mnθ, Crθ, and Feθ in the θ carbide of the steel material can be measured, for example, by the following method.
First, a part (for example, in the case of a steel material having a diameter of 30 mm, a depth of 3 mm to 12 mm deep from the surface) that is at least 1/10 of the diameter from each of the surface and the center of the steel material is set as the surface to be measured. Make a test piece. Further, the test piece is energized in a 10% by volume acetylacetone-1% by weight tetramethylammonium chloride-methanol solution (10% AA-based electrolyte) to electrolyze 0.4 g of the surface layer of the test surface of the test piece. The obtained electrolytic solution was filtered through a filter having a mesh roughness of 0.2 μm, and the residue (precipitate or inclusion) remaining on the filter was dissolved in an acid solution and analyzed by ICP emission spectroscopy. The amounts of Fe, Cr and Mn that are not in a solid solution state, that is, the amounts of Fe, Cr and Mn existing as precipitates and inclusions are determined. Of these elements that are not in a solid solution state, the total amount of Fe and Cr can be considered to exist as θ carbide. However, since Mn exists not only as θ carbide but also as MnS, the amount of Mn obtained by subtracting 1.7 times the amount of S in the residue from the obtained value exists as θ carbide. Can be considered.

As described above, Mnθ, Crθ, and Feθ in θ carbide in steel can be determined.
The test piece may be plate-shaped or column-shaped as long as the test surface is located at a distance of at least 1/10 of the diameter from each of the surface and the center of the steel material.
In the case where there is a remarkable center segregation region or a region where the chemical component partially deviates from the regulation of the present invention due to the diffusion of the alloy element into the scale in the steel material, the above test piece is collected avoiding these regions. I decided to.

  One of the requirements for reducing the ratio of (Mnθ + Crθ) to Feθ in cementite to 0.18 or less is that Mn and Cr are contained in a complex manner, the Mn content is relatively large, and the Cr content is small. That is, it is to be within the component range specified in the present invention. 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 a subsequent heat treatment among steel manufacturing methods. Will be described later.

The steel material for surface quenching 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 carbide. This is because the steel material is heated to the austenite temperature range when performing the surface quenching treatment, and thus the mechanical properties at room temperature of the steel material before the heating are not so important.
However, the structure and hardness of the quenched part after the surface quenching treatment is performed on the steel material are important for improving the fatigue characteristics.
Including this point, the surface hardened component according to the present embodiment will be described in detail below.

<Surface hardened parts>
The surface quenched part according to the present embodiment has the same chemical composition as the above-described steel material component, and has a point A3 that satisfies the expression (1). Further, the component surface layer including the component surface has a quenched layer having a Vickers hardness of more than 500 Hv and a thickness of 0.90 mm or more, proeutectoid ferrite occupying the metal structure at a depth of 0.90 mm from the component surface, and In the region where the total area ratio of pearlite is 10% or less and 0.5 mm or more deeper than the quenched layer, the weight ratio (Mnθ + Crθ) / Feθ, which is the weight ratio of the sum of Mn and Cr in the θ carbide to Fe, is 0.1%. 18 or less.

[Hardened layer]
In order to obtain excellent fatigue properties in the quenched component (surface-quenched component) of the present embodiment, in the region of the component surface layer including the component surface, the Vickers hardness is more than 500 Hv and the thickness is 0.90 mm. It is important to provide a quenched layer as described above. In other words, the hardness in the range from the component surface to a depth of at least 0.90 mm needs to be more than 500 HV.
When the hardness profile is measured in the depth direction from the quenched component surface, the hardness tends to decrease in the depth direction. Although the same tendency occurs in the present embodiment, it is important that the position (hardening depth) where the hardness exceeds 500 HV is a depth position of 0.90 mm or more. In the present embodiment, a region having a Vickers hardness of more than 500 Hv is defined as a quenched layer, and the hardening depth is defined as a thickness of the quenched layer.
As described above, although hardening hardly occurs in the depth direction from the steel surface, when 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 deepened. And the fatigue characteristics can be improved.
As described above, from the viewpoint of improving the fatigue characteristics, it is desirable to increase the hardening depth exceeding 500 HV, that is, to sufficiently secure the thickness of the quenched layer. ) Is also preferably larger. However, no matter how large this surface hardness is, if the hardening depth is shallow (the thickness of the quenched layer is small), improvement in fatigue characteristics cannot be achieved. Therefore, in this embodiment, in addition to the surface hardness of the component, 0.90 mm It is important that the hardness up to the depth exceeds 500 HV. That is, it is important that the curing depth exceeding 500 HV is 0.90 mm or more.

The above-described Vickers hardness and hardening depth (thickness of the quenched layer) of the quenched layer can be measured by the following methods.
First, the quenched component is cut at an arbitrary cross section so as to include the component surface, the measurement region is divided into a plurality of portions along the direction of the cross section length, and a hardness profile is measured in each region with a Vickers hardness meter. Specifically, at the 現, １ ／, and ８ positions of the cross-sectional length in the direction orthogonal to the cross-sectional thickness direction of the exposed cross-section of the component, the depth is set with the component surface as the starting point. Measure the hardness profile in the vertical direction. 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, among the obtained depths, the average of the remaining depths excluding the maximum value and the minimum value can be determined as the “hardening depth (thickness of the quenched layer)” of the component.

[Surface structure: the total area ratio 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 characteristics in the part (surface quenched part) after quenching the steel material for surface quenching of the present embodiment, as described above, the thickness of the quenched layer is set to 0.90 mm or more. In addition, it is necessary to limit the amounts of proeutectoid ferrite, which is an incompletely quenched structure (non-quenched structure), and pearlite contained in the metal structure (surface structure) of the quenched layer. Generally, hardening hardly proceeds from the steel surface toward the depth direction. In the present embodiment, in the range from the component surface to a depth of 0.90 mm, the hardest part is the position at a depth of 0.90 mm. Therefore, the total sum of the area ratios of proeutectoid ferrite and pearlite at a depth of 0.90 mm from the component surface is limited to 10% or less. If the total of pro-eutectoid ferrite and pearlite at a depth of 0.90 mm from the component surface is 10% or less, it can be said that the surface layer of the component has been sufficiently baked. That is, the fact that the sum of the incompletely quenched structure at a depth of 0.90 mm where hardening does not easily occur is 10% or less means that the amount of the incompletely quenched structure increases as the part is more easily quenched than the position. Tends to decrease. From these facts, if the total of proeutectoid ferrite and pearlite at a position 0.90 mm deep from the component surface is 10% or less, the entire region from the component surface to 0.90 mm depth, that is, the entire quenched layer is incomplete. It can be said that the sum of the area ratios of the quenched structure satisfies 10% or less. In addition, from the viewpoint of further improving the fatigue characteristics of the component, the sum of the area ratios of the proeutectoid ferrite and the pearlite at a position 0.90 mm deep from the component surface is preferably 5% or less, and even if it is 0%. Good.

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

The surface structure of the surface hardened part can be observed and measured by the following method.
First, one or a plurality of samples are cut out so as to include the surface of the part in the cross section of the part after quenching. Next, the sample embedded with the resin was etched with nital, and using an optical microscope, a photograph of the structure at a magnification of 200 times centered at a depth of 0.90 mm from the surface of the quenched portion of the sample was taken in a plurality of fields, The area ratio of proeutectoid ferrite and pearlite in each sample and each visual field is determined by image analysis. The average value of the area ratio of proeutectoid ferrite in each of the obtained samples and each visual field and the average value of the area ratio of pearlite were calculated, and the sum was "occupied in the metal structure at a depth of 0.90 mm." The sum of the area ratios of proeutectoid ferrite and pearlite ". The number of samples is not particularly limited, and may be appropriately set depending on the shape of the component and quenching conditions. However, the field of view used for image analysis may be, for example, 3 to 5 fields.

[Ratio of (Mn + Cr) and Fe in θ carbide inside the part: 0.18 or less]
After a steel material is subjected to surface quenching to form a quenched component, the metal structure of the surface layer of the component is significantly changed by the quenching. However, the region inside the surface of the part, that is, the region deeper than the quenched layer is not affected by the quenching, and therefore, hardly changes from the metal structure before quenching, that is, the metal structure of the steel material. From this, even after the surface quenching treatment is performed using the steel material of the present embodiment to form a quenched component, the ratio of (Mnθ + Crθ) and Feθ in the cementite inside the quenched component is 0 in the same manner as the steel material. .18 or less. Specifically, since it can be said that the effect of quenching has not been exerted on a region 0.5 mm or more deeper than the quenched layer, the cementite in the cementite is deeper than the quenched layer thickness +0.5 mm depth position from the component surface. The ratio of (Mnθ + Crθ) to Feθ is 0.18 or less.
When measuring Mnθ, Crθ, and Feθ in the cementite after the surface-hardened part, the test piece was placed so that the surface to be inspected was not affected by quenching (position deviated from the quenched layer). The other methods and conditions can be measured by the same method as the method for measuring Mnθ, Crθ, and Feθ in a steel material as described above.

<Production method>
An example of a method for manufacturing a surface-hardened steel material and a surface-hardened component according to the present embodiment will be described.
The method for producing a steel material for surface quenching according to the present embodiment includes a material preparation step and a hot working step, and a surface quenched component is thereafter produced by a step including a cutting step and a surface quenching step. Hereinafter, each step will be described.

[Material preparation process]
A molten steel satisfying the above chemical composition is manufactured. A slab (slab, bloom) is formed by a casting method using the manufactured molten steel. Alternatively, an ingot is formed by ingot making method using molten steel. A billet is manufactured by hot working a slab or ingot.
The raw material used in the next hot working step may be the above-mentioned slab or ingot, or may be a billet.

[Hot working process]
The material is subjected to hot working. The hot working may be hot rolling or hot forging.
First, the manufactured material is heated. Specifically, when performing surface quenching with hot working (for example, forging) after hot working (for example, forging), the heating temperature in hot working is desirably 1100 ° C. or higher.
When a steel material is made using a large billet or ingot as a material, the concentration of Mn and Cr in cementite tends to proceed during cooling during casting or rolling. Such a material containing cementite in which Mn or Cr is concentrated does not sufficiently homogenize the distribution of Mn and Cr even if the cementite is melted by heating before hot working. Cementite having a large ratio of Mn + Cr is likely to be generated during cooling during the subsequent heat treatment. Furthermore, when Mn and Cr remain concentrated in the cementite in the steel material, it is necessary to diffuse Mn and Cr into the parent phase prior to dissolution of the cementite at the time of heating in the subsequent quenching treatment. And a uniform tissue cannot be obtained. Therefore, when performing surface quenching with hot working (for example, forging) after hot working (for example, forging) using steel having the specified composition of the present invention, the heating temperature in hot working should be 1100 ° C. or higher. Is desirable.
As described above, if the heating temperature during the hot working is too low or the heating time is too short, Cr and Mn concentrated in the cementite in the previous step are difficult to disperse, and the Cr in the cementite after cooling is hardly dispersed. And the Mn concentration may increase. On the other hand, if the heating temperature is too high or the heating time is too long, the scale loss is large. Therefore, a preferable heating temperature is 1100 to 1300 ° C, and a preferable heating time is 30 minutes to 120 minutes. The “heating temperature” refers to the surface temperature of a material or a steel material.

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

[Heat treatment]
Before performing the cutting process in the next step, if necessary, the structure may be adjusted by heat treatment or a heat treatment for releasing strain may be performed. For example, normalizing, quenching / tempering, or low-temperature annealing may be performed.
In the case where the surface quenching is performed after normalizing after the hot forging or performing the heat treatment in a temperature range where a phase transformation occurs such as annealing, the purpose is to reduce (Mnθ + Crθ) / Feθ. And heating at a heating temperature higher than the point A3 by 100 ° C. or more for 30 minutes or more, and in the subsequent cooling, the temperature range from the point A1 to 500 ° C. may be cooled at an average cooling rate of 0.1 ° C./s or more. . After hot forging or after heat treatment in a temperature range in which phase transformation occurs, annealing or tempering at a low temperature at which phase transformation does not occur may be further performed. In this case, however, the parameter P represented by the following formula (A) may be set to 18.7 or less as a processing condition in the heat treatment at a low temperature at which no phase transformation occurs.
P = {(T + 273) / 1000} × (Log (t) +20) (A)
Here, T in the formula (A) represents the processing temperature (° C.), and t represents time (h).

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

Further, after the hot working step or the heat treatment step, a cutting step may be further performed.
The steel material for surface quenching according to the present embodiment includes a member processed into a desired shape before quenching in the next step, in addition to a steel plate, a steel pipe, a shaped steel, and the like.

[Cutting]
The above-mentioned steel material for surface quenching may be subjected to cutting to obtain a steel material having a predetermined shape.
Next, in order to obtain a surface quenched part, a quenching process described below is performed on the obtained surface quenched steel material.

[Surface quenching treatment]
A surface hardening treatment is performed on the cut surface hardening steel material. The surface quenching may be performed by any process that heats only the vicinity of the surface of the steel material and rapidly cools the heating unit after the heating. Heating of the surface may be high-frequency induction heating, heating by laser irradiation, or heating by directly applying a flame to the surface of the steel. The cooling of the heating part may be water cooling, gas cooling, or self-cooling by heat 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 is softened and the strength is reduced. Therefore, the preferred tempering temperature for tempering is 500 ° C or lower. In order to sufficiently obtain the effect of increasing the toughness by tempering, the preferable tempering temperature in the case of performing tempering is 100 ° C. or higher. In order to sufficiently obtain the effect of increasing the toughness by tempering, the tempering time is preferably 0.5 hours or more. If the tempering time is long, the effect is saturated despite the increase in the manufacturing cost. Therefore, the tempering time is preferably 5 hours or less.

  When a surface quenching treatment is performed using the steel material for surface quenching manufactured by the above manufacturing process, a quenched part having excellent fatigue strength can be manufactured even when surface quenching conditions vary.

  Next, an example of the present invention will be described. The conditions in the example are one condition example adopted to confirm the operability and effects of the present invention, and the present invention is based on this one condition example. It is not limited. The present invention can employ various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

  Ingots of steel types A to Q having the chemical components shown in Table 1 were heated to 1250 ° C. The heated ingot was hot forged to produce a square bar having a 75 mm square cross section. 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 under the target condition of a finishing temperature of 1000 ° C., and allowed to cool to room temperature. At this time, the range from the point A1 to 500 ° C. was cooled at an average cooling rate of 0.1 ° C./s or more. Table 1 also shows point A3 (° C.).

A part of the forged steel type K round bar was subjected to quenching and tempering treatment (Test Nos. 12, 18). In the quenching step, the round bar was kept at 850 ° C. for 1 hour and then cooled with water. In the subsequent tempering process, the test No. In Test No. 12 at 600 ° C. In No. 18, the sample was heated to 650 ° C., kept at those temperatures for 3 hours, and then cooled to room temperature. In addition, the test No. The parameters P, which are the conditions in the tempering processes of Test Nos. 12 and 18, are respectively the same as those of Test No. No. 12 is “17.9”, and test No. 18 was "18.9".
In addition, another part of the forged steel type K (A1 point: 713 ° C.) other round bar was subjected to a normalizing treatment of maintaining the temperature at 850 ° C. for 1 hour and cooling to room temperature (Test Nos. 11, 19). . In addition, the test No. In Test No. 11, the cooling from 850 ° C. to room temperature was allowed to cool in the air (average cooling rate from point A1 to 500 ° C. of 0.1 ° C./s or more). In 19, the sheet was covered with a ceramic sheet and slowly cooled to room temperature (average cooling rate from point A1 to 500 ° C. less than 0.1 ° C./s).
The following test was performed using the round bar of each test number.

[Core hardness measurement]
From the round bar of each test number, a test piece for accuracy investigation having a length of 50 mm and a square cross section with one side of 10 mm 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 at the position of R / 2 (R is the radius of the round bar) of the round bar. It was made to match. Next, the test piece was mounted on a resin and then polished. The Vickers hardness at a load of 98 N was measured at any five points near the center of the cross section of the test piece, and the arithmetic average of the measured values at the five points was used as the test number of the test number. Core hardness was used. When the core hardness is 280 HV or less, it is determined that the core hardness is sufficiently low and the machinability is excellent, and when the core hardness is more than 280 HV, it is determined that the machinability has deteriorated. did.
The results are shown in Tables 2 and 3.

[Preparation of test piece for measuring rotational bending fatigue strength after induction hardening]
From the R / 2 of the round bar of each test number, an Ono-type rotary bending fatigue test piece having a notch with a depth of 1 mm and a tip R of 3 mm in a parallel portion of φ10 as shown in FIG. Created.
Next, with respect to the Ono-type rotating bending fatigue test piece, a parallel portion including a notch portion was heated at a frequency of 100 kHz and heated for 1.5 seconds (high-frequency condition A) or heated for 1 second (high-frequency condition B). And subjected to induction hardening treatment immediately after cooling with water. A part of the rotary bending fatigue test piece after the induction hardening was subjected to a low-temperature tempering treatment at 150 ° C. × 2 h. Table 2 shows each condition. After the induction hardening or the low-temperature tempering, the grip of the Ono-type rotating bending fatigue test piece was polished to center the test piece.

[Ono-type rotating bending fatigue test]
Using the Ono-type rotating bending fatigue test piece after the above-mentioned induction hardening or low-temperature tempering, it was subjected to the Ono-type rotating bending fatigue test under the condition of a rotation speed of 3000 rpm. It was determined 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 quenching]
Next, from the center of the round bar of each test number, it has a cross section of 30 mm x 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 having a notch was prepared.
Next, the feed rate (irradiation) of the area including the notch portion of the notched test piece was adjusted under the condition of an output of 3.0 kW (laser irradiation condition A) or the condition of 2.6 kW (laser irradiation condition B). (Velocity) was set to 6.5 mm / s, and heating was performed by the laser. When only the surface layer portion of the test piece is heated by the laser, the surface layer portion is rapidly cooled by heat diffusion to a non-heated layer inside the test piece, and burning occurs. A part of the test piece after laser irradiation was subjected to a low-temperature tempering treatment at 150 ° C. × 2 h. Table 3 shows each condition.
Next, from the test piece after laser quenching or after low temperature tempering, the test piece was moved as close as possible to the center of the width of 32 mm so as to include the notch, and the four points with a cross section of 13 mm × 13 mm and a length of 100 mm were used. A bending fatigue test piece was prepared (see FIG. 2). Note that, during a four-point bending fatigue test described later, the notch end was chamfered in order to prevent fatigue failure from both ends of the notch having a unique stress state. Specifically, chamfering was performed so that the value of the chamfer C of the corner 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 distance between the fulcrums was 80 mm, and the fatigue test piece was set so that the notch was placed at the center 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 four-point bending fatigue test with a test frequency of 20 Hz and a stress ratio of 0.05 was performed on the fatigue test pieces thus supported. Among the load amplitudes (1/2 of the difference between the maximum load and the minimum load) that did not break until the number of repetitions of 1.0 × 10 ⁷ times, 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 any of the laser irradiation conditions A and B, it was determined that the fatigue strength was excellent. Table 5 shows the results.

[Measurement of electrolytic extraction residue]
From R / 2 of the round bar of each test number, a round bar test piece of φ10 × 35 was prepared. The produced 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 electrolyte solution was filtered through a filter having a mesh roughness of 0.2 μm, the residue remaining on the filter was dissolved in an acid solution, and analyzed by ICP emission spectroscopy. Was determined. Of these elements that are not in a solid solution state, Fe and Cr are assumed to be present in total in the form of [theta] carbides. On the other hand, Mn is a value obtained by subtracting 1.7 times the S content from the obtained value. Assuming that they exist as carbides, the ratio between (Mnθ + Crθ) and Feθ in θ carbide in steel material (round bar) was determined.

In addition, (Mnθ + Crθ) / Feθ after quenching was determined as follows.
First, the center of the rotary bending fatigue test specimen after induction hardening or low-temperature tempering, or the depth of (0.5 + 5) mm from the notch bottom of the four-point bending fatigue test specimen after laser hardening or low-temperature tempering. A φ3 × 35 round bar test piece was prepared such that the length direction was parallel to the length direction of each fatigue test piece. The subsequent measurement method was performed by the same method as the above-described electrolytic extraction residue measurement on steel materials (round bars), and the ratio of (Mnθ + Crθ) and Feθ in the internal θ carbide after quenching was determined.
In Tables 2 to 5, (Mnθ + Crθ) / Feθ in θ carbide is simply expressed as (Mn + Cr) / Fe.

[Microstructure observation after quenching]
After induction hardening or low-temperature tempering, the longitudinal bending fatigue test specimen was cut longitudinally to include the notch bottom, a sample was collected so that the cut surface could be observed, the sample was mounted on resin, and a sample for structure observation was obtained. Was prepared.
Similarly, from the four-point bending fatigue test specimen after laser quenching or after low-temperature tempering, a longitudinal section was taken along the plane passing through the center in the width direction so as to include the notch bottom, and a sample was collected so that the cut surface could be observed. 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 visual fields at a magnification of 200 times at a depth of 0.90 mm in the direction perpendicular to the surface in three visual fields, and image analysis of the structural photographs in each visual field was performed. The area ratio of proeutectoid ferrite and pearlite was determined. Then, the average value of the area ratio of the proeutectoid ferrite and the average value of the area ratio of the pearlite obtained from the micrographs of the three visual fields were calculated, and the sum of the area ratios of the proeutectoid ferrite and the pearlite was calculated. When the total area ratio of proeutectoid ferrite and pearlite was 10% or less, it was determined that quenching was sufficiently performed. In Tables 4 and 5, the combination of proeutectoid ferrite and pearlite is described as “non-quenched structure”.

[Measurement of hardening depth after quenching]
After induction hardening, or rotating bending fatigue test specimens after low-temperature tempering, and after laser quenching, or from each of the four-point bending fatigue test specimens after low-temperature tempering, include the notch bottom in the same manner as the above-described sample observation for microstructure observation. A sample for the depth of cure was taken longitudinally. Next, among the exposed cross sections of each sample, at each of the 位置, 起, and ８ positions of the cross-sectional length in the direction orthogonal to the thickness direction, the notch bottom of each sample is defined as a starting point. The hardness profile in the depth direction was measured. The “depth at which the hardness exceeds 500 HV” at each position was determined from the obtained hardness profile results at each position. Then, among the obtained “depths exceeding 500 HV” at each position, the average of the remaining depths excluding the maximum value and the minimum value is calculated as the “hardening depth” of the sample (fatigue test piece). Thickness). The results are shown in Tables 4 and 5. In Tables 4 and 5, the hardness at a position at a depth of 0.05 mm from the bottom of the notch is also described as “surface hardness”.

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

  In Test Nos. 13 and 23 (steel type L), the quenchability could not be sufficiently increased and the hardening depth could not be sufficiently ensured because the amount of Mn was too small. As a result, the fatigue strength of Test No. 13 deteriorated under the condition B, and that of Test No. 23 deteriorated under both the conditions A and B. In particular, in the case of Test No. 23, since a large amount of non-quenched structure was generated, the fatigue strength was significantly reduced.

  In Test Nos. 14 and 24 (steel type M), since the amount of Si was too large, the A3 point could not be lowered sufficiently, and as a result of uneven structure, the variation in fatigue characteristics could not be suppressed. Was.

  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, variations in fatigue characteristics could not be suppressed.

  In Test No. 16 (steel type O), the core amount was excessively increased due to an excessive amount of carbon, and the machinability was deteriorated.

  In Test No. 17 (steel type P), since the amount of Cr was too large, the cementite in which Cr was dissolved was difficult to dissolve, and the hardening depth was reduced by the unevenness of the structure before quenching and the accompanying variation in hardness after quenching. We could not secure enough. As a result, although the fatigue strength was good under the high frequency condition A, the fatigue strength was deteriorated under the condition B, and the variation in the fatigue characteristics could not be suppressed.

  In Test Nos. 18 and 19, although the steel type was within the range of the invention, the heat treatment conditions after forging were unsuitable, so that (Mnθ + Crθ) / Feθ in θ carbide increased both before and after quenching. As a result, it became difficult to decompose cementite, a uniform austenite structure was not obtained, and variations in fatigue characteristics after quenching could not be suppressed.

  In Test No. 20 (steel type Q), the A3 point was too large, resulting in non-uniform structure, and as a result, it was not possible to suppress variations in fatigue characteristics.

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

Claims (8)

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 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 760 ° C. or less,
A steel material for surface hardening, wherein (Mnθ + Crθ) / Feθ, which is the weight ratio of Fe to the sum of Mn and Cr in θ carbide in the metal structure, is 0.18 or less.
A3 (° C.) = 854-180 × C-20 × Mn + 40 × Si-10 × Cr-200 × N (1)
Here, each element symbol in the 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. Represents%.   The steel material for surface quenching according to claim 1, further comprising one or more of Ti: 0 to 0.050% and Nb: 0 to 0.050%.   Further, it contains at least one of V: 0 to 0.15%, Mo: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%. Item 3. The steel material for surface hardening according to item 1 or 2.   Furthermore, B: 0 to 0.0050%, Ca: 0 to 0.0050%, and Bi: 0 to 0.4% are contained. The steel material for surface quenching according to 1. 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 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,
A component surface layer including a component surface has a quenched layer having a Vickers hardness of more than 500 Hv and a thickness of 0.90 mm or more,
The total area ratio of proeutectoid ferrite and pearlite in the metal structure at a depth of 0.90 mm from the component surface is 10% or less,
A surface quenched part characterized in that (Mnθ + Crθ) / Feθ, which is the weight ratio of the sum of Mn and Cr in θ carbide to Fe, is 0.18 or less in a region 0.5 mm or more deeper than the quenched layer.
A3 (° C.) = 854-180 × C-20 × Mn + 40 × Si-10 × Cr-200 × N (2)
Here, each element symbol in the formula (2) is the content (% by mass) in the component.   The surface-hardened part according to claim 5, further comprising one or more of Ti: 0 to 0.050% and Nb: 0 to 0.050%.   Further, V: 0 to 0.15%, Mo: 0 to 0.50%, Cu: 0 to 0.50%, and Ni: 0 to 0.50%. Item 7. A surface-hardened part according to item 5 or 6.   Furthermore, B: 0 to 0.0050%, Ca: 0 to 0.0050%, and Bi: 0 to 0.4% are contained, and any one or more of them is characterized by the above-mentioned. Surface hardened parts as described in.

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