JP3941806B2 - Induction hardening steel - Google Patents

Description

  The present invention relates to steel for induction hardening, and more specifically, after being formed into a predetermined shape such as a hub unit or a constant velocity joint for automobiles and various industrial machines, a part thereof is subjected to induction hardening to be kept in induction hardening. The present invention relates to a steel for induction hardening suitable as a material for parts used in the above, or as a material for parts used in the state of further tempering if necessary after induction hardening.

  Conventionally, parts such as hub units and constant velocity joints of automobiles and various industrial machines have been manufactured by combining steel materials with excellent rolling fatigue characteristics and steel materials with excellent rotational bending fatigue strength and toughness. Has been used bearing steel such as SUJ2 steel of JIS standard, and non-tempered steel or tempered steel for other parts.

  However, in recent years, there has been a great demand from the industry to give a specific steel material a lot of functions from the viewpoint of weight reduction and cost reduction, and to make a component with only that steel material. Even in constant velocity joints, there are steel materials that have both rolling fatigue characteristics required for bearing steel and properties such as tensile strength, rotational bending fatigue strength and toughness required for non-heat treated steel or tempered steel. It has come to be required.

  For this reason, in some cases, a steel material having a high C content such as S55C of JIS G 4051 or 1065 of SAE J 403 is used as a raw material, and induction hardening or further tempering is performed as necessary. In response to requests from

  However, with the progress of miniaturization of parts, even higher rotational bending fatigue strength has been demanded. In the case of using JIS G 4051 S55C or SAE J 403 1065 as a material, the desired rotational bending fatigue strength is required. The service life cannot be secured.

  Therefore, in Patent Document 1, by setting the Si content to 0.5 to 1.0 mass%, the fatigue strength, machinability, and the like are reduced while minimizing the increase in the hardness of the non-hardened portion. In addition, by increasing the content of V to 0.01 to 0.15 mass%, the fatigue strength and machinability of the non-hardened part is improved, and the increase in hardness after forging is minimized. While improving the machinability and cold workability, the fatigue strength of the non-hardened part, the rolling strength of the hardened part, the pitting resistance, the wear resistance, the fatigue strength, etc. have been improved. An excellent high strength induction hardening steel has been proposed.

  Patent Document 2 investigates the relationship between the content of C, Mn, and Cr and the pro-eutectoid ferrite area ratio for steel containing V, and finds a formula that can obtain a uniform hardened layer even in an extremely short time. After hot forging, it is processed into the desired part shape, and a uniform hardened layer structure is obtained by high-frequency contour quenching with ultra-short heating without performing tempering treatment, and high bending or torsional fatigue strength and rolling contact fatigue strength "Non-tempered steel for induction hardening" has been proposed.

JP 2002-332535 A Japanese Patent Laid-Open No. 10-317095

  An object of the present invention is to use a state in which induction hardening is performed on a part of the product after it has been molded into a predetermined shape and used in the state of induction hardening, or after further tempering as necessary after the induction hardening. In parts such as hub units and constant velocity joints, excellent toughness can be secured even if the tensile strength and rotational bending fatigue strength of the parts that are not subjected to induction hardening are improved, and the parts are kept induction-hardened. As a material for the above parts, it is possible to secure not only good rolling fatigue life but also stable rotational bending fatigue strength in mass production on an industrial scale for the parts of this part and parts tempered after induction hardening. It is to provide a suitable induction hardening steel.

  The techniques disclosed in Patent Documents 1 and 2 described above do not necessarily have sufficient toughness, and cannot always stably obtain excellent characteristics in a hardened portion that has been induction-hardened.

  That is, in the case of the technique disclosed in Patent Document 1, consideration is not sufficiently given to toughness reduction accompanying an increase in strength, and since it does not contain Al, the amount of fine particles that suppress the growth of crystal grains and Since the number is insufficient and the austenite grain size tends to be large when subjected to high-frequency heating, the characteristics of the cured portion are unstable.

  In the case of the technique disclosed in Patent Document 2, consideration is not sufficiently given to toughness reduction due to the increase in strength, and since no consideration is given to the N content, fine particles that suppress the growth of crystal grains. It is difficult to ensure the amount and number of particles stably, and since the austenite particle size is likely to vary when heated at high frequency, the characteristics of the cured portion are unstable.

  In order to solve the above-mentioned problems, the present inventors heated to general hot forging conditions, that is, 1200 to 1250 ° C., performed hot forging, finished at 1050 ° C. or higher, and after finishing Conducted various investigations and research on methods to improve the toughness and fatigue strength characteristics of steels treated under conditions of cooling in the air. As a result, first, the following findings (a) to (e) were obtained.

  (A) One of the most effective means for improving toughness is to refine the austenite grain size (hereinafter referred to as “old austenite grain size”) during high-frequency heating and quenching treatment. It is necessary to reduce the austenite grain size during heating to 1200 to 1250 ° C. in hot forging.

  (B) In order to reduce the austenite particle size at the time of heating to 1200 to 1250 ° C., particles such as TiN and MnS that do not dissolve even in this temperature range may be dispersed. Since properties such as rotational bending fatigue strength and rolling fatigue life are greatly reduced, it cannot be actively used.

  (C) The growth rate of austenite grains at high temperatures can be suppressed if it contains an element that dissolves in austenite but has a strong tendency to segregate at grain boundaries and does not have a very large diffusion coefficient.

  (D) Among elements satisfying the above-mentioned requirement (c), a combination of Cu and Sn, which are relatively easy to add, contains a trace amount of Group 5B elements in the periodic table, so that the growth rate of austenite grains at high temperatures Can be greatly suppressed.

  (E) It is known that it is effective to contain Si and V in order to improve rotational bending fatigue strength. However, V is easier to bond with N than C. In order to effectively utilize V for the purpose of improving the rotational bending fatigue strength, it is necessary to precipitate it as a carbide. For that purpose, the content of N and the contents of Ti and Al that are more easily bonded to N than V are controlled. There is a need to.

  Next, the present inventors have repeatedly investigated and studied the factors affecting the characteristics of the as-quenched portion or the portion tempered as necessary after the induction hardening. As a result, the following (f) and The knowledge of (g) was obtained.

  (F) Hardness has the greatest influence on the properties of the part that has been induction-hardened or the part that has been tempered as required after induction hardening, but in addition to the hardness, the prior austenite grain size also has a great influence. . In order to refine the prior austenite grain size, it is necessary that many fine particles remain during high-frequency heating.

  (G) As the fine particles remaining during high-frequency heating, it is most effective to use AlN. And in order to deposit AlN reliably enough before high frequency heating, it is necessary to manage the content of Al and N.

  The present invention has been completed based on the above findings.

  The gist of the present invention resides in the steel for induction hardening shown in the following (1) to (3).

(1) By mass%, C: 0.45-0.60%, Si: 0.4-0.9%, Mn: 0.2-1.0%, Cr: 0.03-0.6% , V: 0.02-0.15%, S: 0.003-0.05%, Al: 0.012-0.05%, N: 0.007-0.025% and Sn: 0.003 -0.05%, and Sb: 0.001-0.01% and As: One or two of 0.001-0.01%, and Ni: 0-0.2% And the balance is Fe and impurities, Ti in the impurities is 0.005% or less, P is 0.03% or less, O (oxygen) is 0.0015% or less, and the following formula (1) A steel for induction hardening, wherein the value of fn1 represented is 1.5 × 10 ⁻⁴ or more and the value of fn2 represented by the following formula (2) is −0.005 or less.
fn1 = Al × {N− (14/48) × Ti} (1),
fn2 = N− (14/27) × Al− (14/48) × Ti− (14/51) × V (2).
In addition, the element symbol in (1) Formula and (2) Formula represents content in steel in the mass% of the element.

(2) By mass%, C: 0.45-0.60%, Si: 0.4-0.9%, Mn: 0.2-1.0%, Cr: 0.03-0.6% V: 0.02-0.15%, S: 0.003-0.05%, Al: 0.012-0.05%, N: 0.007-0.025% and Cu: 0.04 And 0.2%, and Sb: 0.001 to 0.01% and As: one or two of 0.001 to 0.01%, and Ni: (1/2) × [ Cu content (%)] to 0.2%, with the balance being Fe and impurities, Ti in the impurities is 0.005% or less, P is 0.03% or less, and O (oxygen) is 0 The value of fn1 represented by the following formula (1) is 1.5 × 10 ⁻⁴ or more and the value of fn2 represented by the following formula (2) is −0.005 or less. It is characterized by Induction hardening steel.
fn1 = Al × {N− (14/48) × Ti} (1),
fn2 = N− (14/27) × Al− (14/48) × Ti− (14/51) × V (2).
In addition, the element symbol in (1) Formula and (2) Formula represents content in steel in the mass% of the element.

(3) By mass%, C: 0.45-0.60%, Si: 0.4-0.9%, Mn: 0.2-1.0%, Cr: 0.03-0.6% , V: 0.02-0.15%, S: 0.003-0.05%, Al: 0.012-0.05%, N: 0.007-0.025%, Cu: 0.04 -0.2% and Sn: 0.003-0.05%, and Sb: 0.001-0.01% and As: 1 type or 2 types of 0.001-0.01% are included. In addition, Ni: (1/2) × [Cu content (%)] to 0.2% is contained, the balance is made of Fe and impurities, Ti in the impurities is 0.005% or less, and P is 0 0.03% or less and O (oxygen) is 0.0015% or less, and the value of fn1 represented by the following formula (1) is 1.5 × 10 ⁻⁴ or more, and is represented by the following formula (2). The value of fn2 is -0.0 Induction hardening steel characterized by satisfying 05 or less.
fn1 = Al × {N− (14/48) × Ti} (1),
fn2 = N− (14/27) × Al− (14/48) × Ti− (14/51) × V (2).
In addition, the element symbol in (1) Formula and (2) Formula represents content in steel in the mass% of the element.

  In the present invention, the above-mentioned Al refers to acid-soluble Al (so-called “sol. Al”).

  Hereinafter, the inventions related to induction hardening steels (1) to (3) above are referred to as “present invention (1)” to “present invention (3)”, respectively. Also, it may be collectively referred to as “the present invention”.

  Using the steel material of the present invention, after being formed into a predetermined shape, a part thereof is subjected to induction hardening and used in a state of induction hardening, or a state of further tempering if necessary after induction hardening The parts used in are excellent in tensile strength, rotational bending fatigue strength, and toughness of parts that are not induction-hardened. Also, rolling fatigue life and rotational bending fatigue strength of parts that have been subjected to induction hardening and parts that have been tempered after induction hardening. Therefore, it can be used for hub units, constant velocity joints, etc., which are parts of automobiles and industrial machines.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of the chemical component means “mass%”. Further, in the following description, “a portion that is not induction-hardened” is referred to as “base material”, and “a portion that has been subjected to induction hardening” and “a portion that has been tempered after induction hardening” are referred to as “quenched portion”.

(A) Chemical composition C: 0.45 to 0.60%
In the present invention, a material such as a hot-rolled steel bar is formed by, for example, hot forging or warm forging into a predetermined shape and then cooling to obtain a desired “base material”. The mechanical properties are imparted. On the other hand, the material that has been cooled after the molding is induction-quenched, or further tempered as necessary, thereby imparting desired mechanical properties to the “quenched portion”. . However, if the C content is less than 0.45%, the hardness of the “quenched part” becomes insufficient, and not only the rolling fatigue life and wear resistance are greatly reduced, but also other requirements are satisfied. In the “quenched portion”, a desired rotational bending fatigue strength (rotational bending fatigue strength of 710 MPa or more in an Ono type rotational bending fatigue test in a normal temperature atmosphere using a test piece with an annular semicircular groove described later) cannot be obtained. On the other hand, if the C content exceeds 0.60%, the tensile strength of the “base material” becomes too high, and the cold workability and the machinability are greatly reduced. Therefore, the content of C is set to 0.45 to 0.60%. In addition, when better toughness is required for the “base material”, the upper limit of the C content is preferably 0.52%.

Si: 0.4-0.9%
Si is an element effective for increasing the tensile strength and rotational bending fatigue strength of the “base material” and the rolling fatigue life of the “quenched portion”. Si also has an effect as a deoxidizer and an effect of improving machinability. However, if the content is less than 0.4%, even if other requirements are met, the “base metal” has a desired rotational bending fatigue strength (in a normal temperature atmosphere using a smooth specimen described later). Rotational bending fatigue strength of 390 MPa or more in the Ono type rotating bending fatigue test) cannot be obtained. On the other hand, if the Si content exceeds 0.9%, the tensile strength of the “base material” becomes too high, and cold workability and machinability are greatly reduced. Therefore, the Si content is set to 0.4 to 0.9%. In addition, when a better rolling fatigue life is required for the “quenched part”, the lower limit of the Si content is preferably 0.6%.

Mn: 0.2 to 1.0%
Mn has the effect of improving the tensile strength, rotational bending fatigue strength, and hardenability of the “base material”. Mn also has the effect of preventing hot brittleness due to S. In order to exert these effects, it is necessary to contain 0.2% or more of Mn. On the other hand, if the Mn content exceeds 1.0%, the tensile strength of the “base material” becomes too high, and cold workability and machinability are greatly reduced. Therefore, the Mn content is set to 0.2 to 1.0%. In order to more effectively use the effect of Mn described above, the lower limit of the Mn content is preferably 0.6%.

Cr: 0.03-0.6%
Cr has the effect of reducing the lamella spacing of pearlite and increasing the toughness of the “base material”. In order to exhibit this effect, the Cr content needs to be 0.03% or more. On the other hand, if the Cr content exceeds 0.6%, the tensile strength of the “base metal” becomes too high, and the cold workability and the machinability are greatly reduced. Therefore, the Cr content is set to 0.03 to 0.6%. In order to make more effective use of the effect of Cr described above, the lower limit of the Cr content is preferably 0.1%, and when cold workability and machinability are important. The upper limit of the Cr content is preferably 0.3%.

V: 0.02-0.15%
V precipitates as fine nitrides or carbides in the “base material”. Of these precipitates, carbides are particularly effective in improving the tensile strength and rotational bending fatigue strength of the “base material”. In order to ensure these effects, in particular, the desired rotational bending fatigue strength (rotary bending fatigue strength of 390 MPa or more in an Ono-type rotational bending fatigue test in a normal temperature atmosphere using a smooth specimen described later) In order to obtain the effect of increasing the rotational bending fatigue strength of “2”, the V content should be 0.02% or more, and the value of fn2 represented by the formula (2) should satisfy −0.005 or less. is necessary. On the other hand, if the V content exceeds 0.15%, the toughness of the “base metal” is greatly reduced, and even if other requirements are satisfied, a desired impact value (a U-notch with a notch height of 8 mm described below) is obtained. In a Charpy impact test at room temperature using a test piece, an impact value of 41 J / cm ² or more cannot be obtained. Therefore, the content of V is set to 0.02 to 0.15%. In addition, when better toughness is required for the “base material”, the upper limit of the V content is preferably 0.07%.

S: 0.003-0.05%
S combines with Mn to form MnS and has an effect of improving machinability. However, if the content is less than 0.003%, it is difficult to obtain the above effect. On the other hand, coarse MnS tends to reduce the rolling fatigue life. In particular, when the S content exceeds 0.05%, the rolling fatigue life is greatly reduced. Therefore, the content of S is set to 0.003 to 0.05%. In addition, when favorable machinability is requested | required, it is preferable that content of S is 0.01 to 0.05% and the minimum is 0.01%. When a good rolling fatigue life is required, the S content is preferably 0.003 to 0.02% and the upper limit is preferably 0.02%. Furthermore, when both good machinability and good rolling fatigue life are required, the S content is preferably 0.01 to 0.02%.

Al: 0.012-0.05%
Al is an element that is easily bonded to N to form AlN. AlN is effective for refining the prior austenite grain size of the “quenched part”, and the rotational bending fatigue strength of the “quenched part” is improved when the prior austenite grain size is refined. In order to reliably obtain this effect, the Al content must be 0.012% or more, and the value of fn1 represented by the above formula (1) must satisfy 1.5 × 10 ⁻⁴ or more. is there. Further, the effect of improving the tensile strength and rotational bending fatigue strength of the above-mentioned “carbide” of the carbide of V, in particular, the desired rotational bending fatigue strength (ono in a normal temperature atmosphere using a smooth test piece described later). In order to obtain the effect of increasing the rotational bending fatigue strength of the “base material” for ensuring the rotational bending fatigue strength of 390 MPa or more in the rotational bending fatigue test, the Al content is expressed by the above formula (2). It is also necessary that the value of fn2 satisfies −0.005 or less. On the other hand, Al tends to form oxide inclusions, and if the Al content exceeds 0.05%, it becomes easy to form coarse oxide inclusions, so the rolling fatigue life of the “quenched part” decreases. In addition to satisfying other requirements, the "quenched part" has the desired rotational bending fatigue strength (in the Ono type rotational bending fatigue test in the ambient temperature atmosphere using a test piece with an annular semicircular groove described later) Rotational bending fatigue strength of 710 MPa or higher) cannot be obtained. Therefore, the Al content is set to 0.012 to 0.05%. In order to more actively utilize the effect of improving the rotational bending fatigue strength of the “quenched part” by the formation of AlN, the Al content is preferably 0.025 to 0.05%. As already described, Al in the present invention refers to acid-soluble Al (so-called “sol.Al”), and is solid-solved in Al contained in inclusions other than oxides and in the matrix. It is Al.

N: 0.007 to 0.025%
N is an element that easily forms TiN, AlN, and VN by combining with Ti, Al, and V. In the present invention, the effect of increasing the rotational bending fatigue strength of the “quenched portion” by refining the prior austenite grain size of the “quenched portion” of AlN of the above nitrides is utilized. In order to obtain this effect with certainty, it is necessary that the N content is 0.007% or more and that the value of fn1 represented by the above formula (1) satisfies 1.5 × 10 ⁻⁴ or more. . Further, the effect of improving the tensile strength and rotational bending fatigue strength of the above-mentioned “carbide” of the carbide of V, in particular, the desired rotational bending fatigue strength (ono in a normal temperature atmosphere using a smooth test piece described later). In order to obtain the effect of increasing the rotational bending fatigue strength of the “base material” for ensuring the rotational bending fatigue strength of 390 MPa or more in the rotational bending fatigue test, the N content is expressed by the above formula (2). It is also necessary that the value of fn2 satisfies −0.005 or less. On the other hand, when the N content increases, coarse TiN is formed, which tends to be a fracture starting point of rotational bending fatigue in the “quenched part”, and in particular, when the N content exceeds 0.025%, other requirements are satisfied. However, in the “quenched part”, a desired rotational bending fatigue strength (rotational bending fatigue strength of 710 MPa or more in an Ono type rotational bending fatigue test in a normal temperature atmosphere using a test piece with an annular semicircular groove described later) is secured. I can't do that. Therefore, the N content is set to 0.007 to 0.025%. In order to more actively utilize the effect of improving the rotational bending fatigue strength of the “quenched portion” by the formation of AlN, the N content is preferably 0.012 to 0.025%.

Sn: 0.003-0.05%, Cu: 0.04-0.2%, Sb: 0.001-0.01% and As: 0.001-0.01%
In the steel for induction hardening according to the present invention, a desired impact value in the “base metal” (41 J / cm ² or more in a Charpy impact test at room temperature using a U-notch test piece having a notch height of 8 mm described later) ) And the desired rotational bending fatigue strength in the “quenched part” (Rotational bending fatigue strength of 710 MPa or more in an Ono-type rotational bending fatigue test in a normal temperature atmosphere using a test piece with an annular semicircular groove described later) ), It is necessary to include the following elements as essential constituent elements.

  That is, the present invention (1) includes Sn: 0.003 to 0.05%, Sb: 0.001 to 0.01%, and As: 0.001 in addition to the elements from C to N. It is necessary to contain one or two of 0.01%.

  In addition to the elements from C to N, the present invention (2) includes Cu: 0.04 to 0.2%, Sb: 0.001 to 0.01%, and As: 0.001 to It is necessary to contain one or two of 0.01%.

  Furthermore, the present invention (3) includes Cu: 0.04 to 0.2%, Sn: 0.003 to 0.05%, and Sb: 0.001 to 0.001 in addition to the elements from C to N. It is necessary to contain one or two of 0.01% and As: 0.001 to 0.01%.

  In order for the “base material” to have the desired impact value, in addition to the elements from C to N, one of 0.04% or more of Cu and 0.003% or more of Sn is used. Or two and one or two of 0.001% or more of Sb and 0.001% or more of As, while the contents of Cu, Sn, Sb and As are respectively If it exceeds 0.2%, 0.05%, 0.01%, and 0.01%, the rotational bending fatigue strength of the “quenched portion” is lowered, and the desired rotational bending fatigue strength cannot be obtained. It is.

  Hereinafter, among the elements that the present inventors refine the prior austenite grain size and increase the toughness with regard to the above, pay attention to those that have a high tendency to segregate at the grain boundaries although they are dissolved in austenite when heated at high temperatures. This will be explained in detail based on the contents of the study conducted.

  Periodic table 1B group Cu, 3B group B, 4B group C and Sn, 5B group N, P, As and Sb, 6B group O, S and Se It has been known. However, among these elements, B, C, and N have a large diffusion coefficient at high temperature, and O has a small amount of solid solution in austenite. Further, S and Se are easily bonded to Mn, and thus austenite. Since the amount of the solid solution in the inside is small, almost none of them can be expected to suppress grain growth. Then, Cu, Sn, P, As, and Sb were examined.

  That is, as shown in Table 1, 0.54% C-0.7% Si-0.9% Mn-0.014% S-0.16% Cr-0.02% Al-0.001% Ti -50% vacuum melting furnace for steel ax with varying contents of Cu, Sn, P, As and Sb based on chemical composition of -0.05% V-0.0140% N-0.0008% O Was melted and cast into an ingot. In the above, the Ni content was also changed depending on the Cu content of the steel.

  These steel ingots were heated to 1250 ° C. and subjected to homogenization heat treatment for 10 hours, and then hot forging was performed to produce a round bar having a diameter of 40 mm. Next, each of the round bars was heated to 1250 ° C., hot forged, and finished at 1050 ° C. or higher to produce a steel plate having a thickness of 20 mm.

  From each steel plate having a thickness of 20 mm obtained in this way, the length specified in JIS Z 2202 is 55 mm, and the height and width are both 10 mm so that the length direction of the test piece is parallel to the forging axis. A U-notch test piece having a height under the notch of 8 mm was collected, and a Charpy impact test was performed at room temperature by an ordinary method, and an average value of impact values in three tests was obtained.

  Further, from each of the steel plates having a thickness of 20 mm, the diameter and length of the parallel portions are 10 mm and 21 mm and the radius is 1 mm so that the length direction of the test piece is parallel to the forging axis. Ono-type rotating bending fatigue test pieces with grooves were collected. The above Ono-type rotating bending fatigue test piece was induction-quenched so that the effective hardened layer depth at the bottom of the annular semicircular groove was 1.3 ± 0.2 mm, and then tempered at 160 ° C. for 1 hour. Furthermore, grinding was performed to remove the scale using No. 800 emery paper. The “effective hardened layer depth” in the induction hardening mentioned above refers to a distance from the surface of the hardened layer to a position of 500 in terms of Vickers hardness (hereinafter referred to as “Hv hardness”).

Here, the number of tests in the Ono-type rotating bending fatigue test was 7 each, and the test was performed in a normal temperature atmosphere by a normal method, and the highest stress among those that did not break until the number of repetitions was 1.0 × 10 ⁷ times. “Rotating bending fatigue strength”.

  Table 2 summarizes the above test results. In addition, the said impact test piece is what was extract | collected from the forged steel plate, and this has not been induction-hardened. For this reason, in Table 2, the impact value is expressed as “base material impact value”. On the other hand, since the Ono type rotating bending fatigue test piece was subjected to the above-mentioned induction hardening, “Rotating bending fatigue strength” in Table 2 was expressed as “Quenched portion rotating bending fatigue strength”.

  The following (h) to (m) were found from the results of the impact value of the “base metal” and the rotational bending fatigue strength of the “quenched part” shown in Table 2.

  (H) When the P content is increased, the impact value of the “base metal” and the rotational bending fatigue strength of the “quenched portion” are reduced (comparison of steels a to c).

(I) Increasing the content of Cu together with Ni slightly improves the impact value of the “base material”, but uses the above-mentioned desired impact value, that is, the U-notch test piece having a notch height of 8 mm. The impact value of 41 J / cm ² or more in the Charpy impact test at room temperature was not obtained. On the other hand, when Cu is added together with Ni, if the Cu content exceeds 0.2%, the rotational bending fatigue strength of the “quenched portion” decreases (comparison of steel a, steel d, and steel e).

  (J) When the Sn content is increased, the impact value of the “base material” is slightly improved, but the desired impact value described above cannot be obtained. On the other hand, if the Sn content exceeds 0.05%, the rotational bending fatigue strength of the “quenched portion” decreases (comparison of steel a, steel f, and steel g).

  (K) When the As content is increased, the impact value of the “base material” is slightly improved, but the desired impact value described above cannot be obtained. On the other hand, when the content of As exceeds 0.01%, the rotational bending fatigue strength of the “quenched portion” decreases (comparison of steel a, steel h, and steel i).

  (L) When the Sb content is increased, the impact value of the “base material” is slightly improved, but the desired impact value described above cannot be obtained. On the other hand, when the Sb content exceeds 0.01%, the rotational bending fatigue strength of the “quenched portion” decreases (comparison of steel a, steel j, and steel k).

(M) The above-mentioned desired rotational bending fatigue strength (rotational bending fatigue strength of 710 MPa or more in the Ono type rotational bending fatigue test in the normal temperature atmosphere using the above-mentioned annular semicircular grooved test piece) in the “quenched part” And the desired impact value (41 J / cm ² or more in the Charpy impact test at room temperature using the above-notched U-notch specimen having an under-notch height of 8 mm) is improved by increasing the toughness of the “base material”. In order to obtain an impact value), the upper limit of the content of Cu, Sn, As and Sb is 0.2%, 0.05%, 0.01% and 0.01%, respectively. In addition to increasing the content of one or two of Sn and Sn, the content of one or two of Sb and As may be increased (of steel a and steel mw) Comparison).

  Therefore, the present invention (1) includes Sn: 0.003-0.05%, and Sb: 0.001-0.01% and As: 0.001-0.01% or 2 It shall contain seeds. Moreover, this invention (2) is Cu: 0.04-0.2%, and Sb: 0.001-0.01% and 1 type or 2 of As: 0.001-0.01% It shall contain seeds. Furthermore, the present invention (3) includes Cu: 0.04-0.2% and Sn: 0.003-0.05%, and Sb: 0.001-0.01% and As: 0.001- One or more of 0.01% were contained.

Ni: 0 to 0.2% (present invention (1)), (1/2) × [Cu content (%)] to 0.2% (present invention (2), present invention (3))
In the case of steel not containing Cu, addition of Ni is optional. If added, it has the effect of increasing toughness. In order to reliably obtain this effect, the Ni content is preferably 0.02% or more. However, since Ni is an expensive element, its content needs to be 0.2% or less in consideration of alloy costs. Therefore, in the present invention (1), the Ni content is set to 0 to 0.2%.

  On the other hand, in addition to the above-described effect of increasing toughness, Ni has an effect of suppressing a decrease in hot workability caused by Cu. In order to obtain this effect, the Ni content needs to be (1/2) × [Cu content (%)] or more. However, as described above, since Ni is an expensive element, its content needs to be 0.2% or less in consideration of alloy costs. Therefore, in the present invention (2) and the present invention (3), the Ni content is set to (1/2) × [Cu content (%)] to 0.2%.

  In the present invention, the contents of P, Ti and O (oxygen) as impurity elements are limited as follows.

P: 0.03% or less P is an element that segregates at the grain boundary and easily embrittles the grain boundary. As described in the section (h) above, the impact value of the “base material” and “quenching” The rotational bending fatigue strength of the “part” is reduced. In particular, when the P content exceeds 0.03%, the impact value of the “base metal” and the rotational bending fatigue strength of the “quenched part” are significantly reduced. The desired impact value (in the above-mentioned Charpy impact test at room temperature using a U-notch test piece having an under-notch height of 8 mm, an impact value of 41 J / cm ² or more) and the desired rotational bending in the “quenched part” Neither fatigue strength (rotational bending fatigue strength of 710 MPa or more in the Ono-type rotational bending fatigue test in the normal temperature atmosphere using the above-mentioned test piece with an annular semicircular groove) cannot be obtained. Therefore, the content of P is set to 0.03% or less.

Ti: 0.005% or less Ti combines with N to form TiN, and coarse TiN tends to be a fracture starting point of rotational bending fatigue in the “quenched part”, and in particular, the Ti content is 0.005%. Exceeding the above, even if the other requirements are satisfied, the desired rotational bending fatigue strength in the “quenched part” (710 MPa in the Ono type rotational bending fatigue test in the normal temperature atmosphere using the above-mentioned test piece with the annular semicircular groove) The above rotating bending fatigue strength) cannot be obtained. Therefore, the Ti content is set to 0.005% or less. Note that the content of Ti as an impurity element is desirably as small as possible, and is preferably 0.002% or less in consideration of costs for raw materials and steelmaking.

O (oxygen): 0.0015% or less O forms oxide inclusions, and coarse oxide inclusions tend to be the starting point of fracture of rotational bending fatigue and rolling fatigue in the “quenched part”. When the content of O exceeds 0.0015%, even if other requirements are satisfied, the desired rotational bending fatigue strength in the “quenched part” (in the normal temperature atmosphere using the above-mentioned test piece with an annular semicircular groove) Rotational bending fatigue strength of 710 MPa or more in the Ono-type rotational bending fatigue test at 1) cannot be obtained. Therefore, the content of O is set to 0.0015% or less. In addition, it is desirable to reduce the content of O as an impurity element as much as possible, and considering the cost of raw materials and steelmaking, it is preferable to make it 0.0009% or less.

In the steel for induction hardening according to the present invention, the old austenite grain size in the “quenched part” is made fine, and the desired rotational bending fatigue strength (Ono in the normal temperature atmosphere using the above-mentioned test piece with the annular semicircular groove is used. In order to ensure (rotating bending fatigue strength of 710 MPa or more in the type rotating bending fatigue test), the value of fn1 represented by the above formula (1) needs to be 1.5 × 10 ⁻⁴ or more.

  Hereinafter, the above rules will be described in detail.

  Generally, the heating temperature at the time of induction hardening is 900 to 1050 ° C., which is lower than the heating temperature at the time of hot forging, and the heating time is also short. Therefore, in order to reduce the prior austenite grain size without adding special elements to the steel for the purpose of cost reduction, AlN particles should be dispersed and precipitated in the steel before heat treatment at high frequency. Is effective.

  In addition, it is known that precipitation of AlN is more likely to occur when the product of the amount of Al and the amount of N in steel, that is, the solubility product is larger. However, Al in steel is more likely to be associated with O (oxygen) than N. For this reason, about Al, it is necessary to consider the amount of so-called “sol.Al” as the amount of Al other than oxide, that is, the amount of Al referred to in the present invention. On the other hand, since N in steel has a greater affinity for Ti than Al, it is necessary to consider N with an amount of N other than N that binds to Ti.

  From the above, it is considered that the precipitation state of AlN can be evaluated by the following fn1 expressed by the above equation (1) in consideration of the atomic weight.

  fn1 = Al × {N− (14/48) × Ti}.

  Therefore, the present inventors, as shown in Table 3, 0.54% C-0.7% Si-0.9% Mn-0.011% P-0.013% S-0.15% Cr -0.07% Ni-0.001% Ti-0.06% V-0.0008% O-0.10% Cu-0.008% Sn-0.003% As based on the chemical composition, Al Further, steels α to δ in which the value of fn1 represented by the above formula (1) was changed by changing the content of N and N were melted using a 50 kg vacuum melting furnace and cast into an ingot.

  These steel ingots were heated to 1250 ° C. and subjected to homogenization heat treatment for 10 hours, and then hot forging was performed to produce a round bar having a diameter of 40 mm. Next, each of the round bars was heated to 1250 ° C., hot forged, and finished at 1050 ° C. or higher to produce a steel plate having a thickness of 20 mm.

  From each of the 20 mm thick steel plates obtained in this way, the diameter and length of the parallel part are 10 mm and 21 mm, respectively, and the radius is 1 mm so that the length direction of the specimen is parallel to the forging axis. Eight Ono rotary bending fatigue test pieces with semicircular grooves were collected. The effective hardening layer depth at the bottom of the annular semicircular groove in the Ono type rotating bending fatigue test piece described above, that is, the distance from the surface of the hardening layer to the position of 500 in Hv hardness is 1.3 ± 0.2 mm. After induction hardening as described above, tempering was performed at 160 ° C. for 1 hour, and further, grinding was performed to remove the scale using No. 800 emery paper.

Then, as the seven test number, performs Ono-type rotating bending fatigue test in a normal temperature atmosphere by conventional methods, "rotating bending the highest stress among that did not break up repetitive number 1.0 × 10 ⁷ times Fatigue strength ". In addition, for each of the remaining test pieces, the cross section of the bottom of the annular semicircular groove (that is, a cut surface perpendicular to the length direction of the test piece) is mirror-polished and then added with a surfactant. Corrosion with acid-saturated aqueous solution reveals prior austenite grain boundaries, and the range from the surface layer to 1 mm is observed with an optical microscope. I asked for a number.

  Table 4 summarizes the above test results. The austenite grain size numbers listed in Table 4 correspond to “old austenite grain size”.

  The following (n) was found from the results of the rotational bending fatigue strength and the austenite grain number of the “quenched part” shown in Table 4.

(N) In the “quenched part”, the above-mentioned desired rotational bending fatigue strength (rotational bending fatigue strength of 710 MPa or more in the Ono-type rotational bending fatigue test in the normal temperature atmosphere using the above-mentioned annular semicircular grooved test piece) In order to ensure, the value of fn1 represented by the above formula (1) may be 1.5 × 10 ⁻⁴ or more (comparison of steels α to δ).

Therefore, the induction hardening steel according to the present invention is such that the value of fn1 represented by the above formula (1) satisfies 1.5 × 10 ⁻⁴ or more.

  In the steel for induction hardening according to the present invention, in addition to the above-mentioned rules, in particular, the desired rotational bending fatigue strength in the “base metal” (Ono-type rotational bending in a normal temperature atmosphere using a smooth specimen described later) In order to ensure (rotating bending fatigue strength of 390 MPa or more in a fatigue test), the value of fn2 expressed by the above equation (2) needs to be −0.005 or less.

  Hereinafter, the above rules will be described in detail.

  V precipitates as “VC” (carbide) in the “base material” and has an action of increasing the tensile strength and the rotational bending fatigue strength of the “base material”. However, the affinity of V for C is smaller than the affinity for N. For this reason, the effect of the above-mentioned V, in particular, the desired rotational bending fatigue strength (rotary bending fatigue strength of 390 MPa or more in the Ono type rotational bending fatigue test in a normal temperature atmosphere using a smooth specimen described later) is ensured. In order to obtain the effect of increasing the rotational bending fatigue strength of the “base metal” for this purpose, it is necessary to make the amount of V larger than the amount stoichiometrically combined with N. On the other hand, N is easier to bond with Ti and Al than V.

  From the above, it was concluded that the value of the next fn2 expressed by the above equation (2) needs to be at least 0 or less in order to ensure the effect of V precipitated as VC.

  fn2 = N− (14/27) × Al− (14/48) × Ti− (14/51) × V.

  Therefore, the present inventors, as shown in Table 5, 0.54% C-0.7% Si-0.9% Mn-0.013% P-0.014% S-0.15% Cr -0.07% Ni-0.02% Al-0.001% Ti-0.0008% O-0.09% Cu-0.008% Sn-0.004% As based on the chemical composition V The steels ε to θ, in which the value of fn2 represented by the formula (2) was changed by changing the N content, were melted in a 50 kg vacuum melting furnace and cast into an ingot.

  These steel ingots were heated to 1250 ° C. and subjected to homogenization heat treatment for 10 hours, and then hot forging was performed to produce a round bar having a diameter of 40 mm. Next, each of the round bars was heated to 1250 ° C., hot forged, and finished at 1050 ° C. or higher to produce a steel plate having a thickness of 20 mm.

  From each of the 20 mm thick steel plates obtained in this way, the diameter and length of the parallel part are 10 mm and 21 mm, respectively, so that the length direction of the test piece is parallel to the forging axis. Test specimens were collected.

Then, as the seven test number, performs Ono-type rotating bending fatigue test in a normal temperature atmosphere by conventional methods, "rotating bending the highest stress among that did not break up repetitive number 1.0 × 10 ⁷ times Fatigue strength ".

  Table 6 shows the test results. In addition, the said Ono type | formula rotation bending fatigue test piece is what was extract | collected from the forged steel plate, and this has not been induction-hardened. For this reason, in Table 6, the rotational bending fatigue strength is expressed as “base metal rotational bending fatigue strength”.

  The following (o) was found from the results of the rotational bending fatigue strength of the “base metal” shown in Table 6.

  (O) To ensure the above-mentioned desired rotational bending fatigue strength (rotational bending fatigue strength of 390 MPa or more in the Ono type rotational bending fatigue test in the normal temperature atmosphere using the aforementioned smooth test piece) in the “base material”. The value of fn2 represented by the formula (2) may be set to −0.005 or less (comparison of steels ε to θ).

  Therefore, the induction hardening steel according to the present invention is such that the value of fn2 represented by the formula (2) satisfies −0.005 or less.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  Steels A to Z, steel AA, and steel AB having chemical compositions shown in Tables 7 and 8 were melted in a 50 kg vacuum melting furnace and cast into ingots. Steel C, steel E, steel F, steel H, steel I, steel K, and steel N in Table 7 are steels of the present invention examples whose chemical compositions are within the range defined by the present invention. On the other hand, Steel A, Steel B, Steel D, Steel G, Steel J, Steel L, Steel M, Steels O to Z, Steel AA and Steel AB in Tables 7 and 8 were compared out of the conditions defined in the present invention. Example steel. Of the steels of the comparative examples, steel A is steel corresponding to S55C of JIS G 4051.

  These steel ingots were heated to 1250 ° C. and subjected to homogenization heat treatment for 10 hours, and then hot forging was performed to produce a round bar having a diameter of 40 mm. Next, each of the round bars was heated to 1250 ° C., hot forged, and finished at 1050 ° C. or higher to produce a steel plate having a thickness of 20 mm.

  From each steel plate having a thickness of 20 mm obtained in this way, the length specified in JIS Z 2202 is 55 mm, and the height and width are both 10 mm so that the length direction of the test piece is parallel to the forging axis. A U-notch test piece having a height under the notch of 8 mm was collected, and a Charpy impact test was performed at room temperature by an ordinary method, and an average value of impact values in three tests was obtained.

  Further, from each of the steel plates having a thickness of 20 mm, the diameter and length of the parallel portions are 10 mm and 21 mm and the radius is 1 mm so that the length direction of the test piece is parallel to the forging axis. A grooved Ono-type rotating bending fatigue test piece and a smooth Ono-type rotating bending fatigue test piece having a parallel part diameter and length of 10 mm and 21 mm, respectively, were collected.

  The Ono rotary bending fatigue test piece with an annular semicircular groove has an effective hardened layer depth at the bottom of the annular semicircular groove, that is, a distance from the surface of the hardened layer to a position of 500 in Hv hardness is 1.3 ±. Induction hardening was performed so that the thickness became 0.2 mm, and then tempering was performed at 160 ° C. for 1 hour, and further, grinding was performed to remove scale using No. 800 emery paper.

  Moreover, the rolling fatigue test piece produced the test piece of diameter 12mm and length 22mm so that the length direction of a test piece might become parallel to a training axis. This test piece was subjected to induction hardening so that the effective hardened layer depth, that is, the distance from the surface of the hardened layer to the position of 500 in Hv hardness was 2.0 ± 0.2 mm, and a normal heat treatment furnace After tempering at 160 ° C. for 1 hour, the surface was mirror-polished to prepare a test piece.

Next, in each case of the Ono rotary bending fatigue test piece and the smooth Ono rotary bending fatigue test piece with the annular semicircular groove obtained as described above, the number of test pieces was set to 7 each, and the room temperature was measured by an ordinary method. The Ono-type rotating bending fatigue test was performed in the atmosphere, and the highest stress among the fractures up to the number of repetitions of 1.0 × 10 ⁷ times was defined as “rotating bending fatigue strength”.

  The rolling fatigue life in the rolling fatigue test was measured by the following method.

・ Testing machine: Cylindrical radial type rolling fatigue testing machine,
・ Maximum surface pressure: 6000 MPa,
-Test piece rotation speed: 46000 times / min-Number of test pieces: 12 pieces each.

  The rolling fatigue life is plotted on Weibull probability paper with the rolling fatigue life of 12 test pieces with a diameter of 12 mm for each condition, the cumulative failure probability on the vertical axis and the rolling fatigue life on the horizontal axis. Then, a linear approximation straight line was drawn to obtain a rolling fatigue life (hereinafter referred to as “L10 life”) at which the cumulative frequency failure probability was 10%.

In addition, the target value of each characteristic was set on the basis of each test result of test number 1 using steel A corresponding to S55C of JIS G 4051. That is, the toughness of the “base material” was set to 41 J / cm ² or more, which exceeds 150% of 27 J / cm ² , which is the impact value of the “base material” of Test No. 1. In addition, for the rotational bending fatigue characteristics of the “base metal”, a target was set to 390 MPa or more, which is 30% or more higher than the 300 MPa that is the rotational bending fatigue strength of the “base metal” of Test No. 1. The rotational bending fatigue characteristics of the “quenched part” were set to 710 MPa or more, which exceeds 115% of 610 MPa, which is the rotational bending fatigue strength of the “quenched part” of test number 1. Furthermore, the rolling fatigue life was targeted to be 2.0 × 10 ⁷ or more for the L10 life.

  Table 9 summarizes the above test results. Note that the impact test piece and the smooth Ono type rotating bending fatigue test piece are taken from a forged steel plate and are not induction-hardened. Therefore, the test results using these are shown in Table 9 as the characteristics of the “base material”. On the other hand, the Ono-type rotating bending fatigue test piece with an annular semicircular groove was subjected to the induction hardening described above, and is shown in Table 9 as the characteristics of the “quenched portion”.

  From Table 9, in the case of a test number that deviates from the conditions specified in the present invention, the impact value of the “base material”, the rotational bending fatigue strength of the “base material”, the rotational bending fatigue strength and the rolling of the “quenched part” It is clear that at least one characteristic of fatigue life has not reached the target value.

  On the other hand, in the case of a test number that satisfies the conditions specified in the present invention, the impact value of the “base material”, the rotational bending fatigue strength of the “base material”, the rotational bending fatigue strength and the rolling of the “quenched part” It is clear that the target value has been reached in all fatigue lives.

Using the steel material of the present invention, after being formed into a predetermined shape, a part thereof is subjected to induction hardening and used in a state of induction hardening, or a state of further tempering if necessary after induction hardening The parts used in are excellent in tensile strength, rotational bending fatigue strength and toughness of parts that are not induction-hardened, and also have rolling fatigue life and rotational bending fatigue strength of parts that have been subjected to induction hardening and parts that have been tempered after induction hardening. Therefore, it can be used for hub units, constant velocity joints, etc., which are parts of automobiles and industrial machines.

Claims (3)

In mass%, C: 0.45 to 0.60%, Si: 0.4 to 0.9%, Mn: 0.2 to 1.0%, Cr: 0.03 to 0.6%, V: 0.02-0.15%, S: 0.003-0.05%, Al: 0.012-0.05%, N: 0.007-0.025%, and Sn: 0.003-0. 05%, and Sb: 0.001 to 0.01% and As: one or two of 0.001 to 0.01%, and Ni: 0 to 0.2%, The balance consists of Fe and impurities, Ti in the impurities is 0.005% or less, P is 0.03% or less, O (oxygen) is 0.0015% or less, and is expressed by the following formula (1). A steel for induction hardening, wherein the value of fn1 satisfies 1.5 × 10 ⁻⁴ or more and the value of fn2 represented by the following formula (2) satisfies −0.005 or less.
fn1 = Al × {N− (14/48) × Ti} (1)
fn2 = N− (14/27) × Al− (14/48) × Ti− (14/51) × V (2)
In addition, the element symbol in (1) Formula and (2) Formula represents content in steel in the mass% of the element. In mass%, C: 0.45 to 0.60%, Si: 0.4 to 0.9%, Mn: 0.2 to 1.0%, Cr: 0.03 to 0.6%, V: 0.02-0.15%, S: 0.003-0.05%, Al: 0.012-0.05%, N: 0.007-0.025% and Cu: 0.04-0. 2% and one or two of Sb: 0.001 to 0.01% and As: 0.001 to 0.01%, and Ni: (1/2) × [Cu content Amount (%)] to 0.2%, the balance being Fe and impurities, Ti in the impurities is 0.005% or less, P is 0.03% or less, and O (oxygen) is 0.0015% The fn1 value represented by the following formula (1) is 1.5 × 10 ⁻⁴ or more and the fn2 value represented by the following formula (2) is −0.005 or less. High lap Steel for wave hardening.
fn1 = Al × {N− (14/48) × Ti} (1)
fn2 = N− (14/27) × Al− (14/48) × Ti− (14/51) × V (2)
In addition, the element symbol in (1) Formula and (2) Formula represents content in steel in the mass% of the element. In mass%, C: 0.45-0.60%, Si: 0.4-0.9%, Mn: 0.2-1.0%, Cr: 0.03-0.6%, V: 0.02-0.15%, S: 0.003-0.05%, Al: 0.012-0.05%, N: 0.007-0.025%, Cu: 0.04-0. 2% and Sn: 0.003-0.05%, and Sb: 0.001-0.01% and As: one or two of 0.001-0.01%, : (1/2) × [Cu content (%)] to 0.2%, the balance is made of Fe and impurities, Ti in the impurities is 0.005% or less, and P is 0.03%. And O (oxygen) is 0.0015% or less, and the value of fn1 represented by the following formula (1) is 1.5 × 10 ⁻⁴ or more, and the value of fn2 represented by the following formula (2) Is -0.005 or more Induction hardening steel characterized by satisfying the following.
fn1 = Al × {N− (14/48) × Ti} (1)
fn2 = N− (14/27) × Al− (14/48) × Ti− (14/51) × V (2)
In addition, the element symbol in (1) Formula and (2) Formula represents content in the steel in the mass% of the element.