JP5825157B2 - Induction hardening steel - Google Patents

Description

  The present invention relates to a steel material for induction hardening, and more particularly to a steel material for induction hardening excellent in rolling fatigue life used for mechanical structural parts such as constant velocity joints and hub units.

  Among automotive parts, parts such as “rolling bearings”, “constant velocity joints” and “hub units” (hereinafter also referred to as “rolling parts”), which are repeatedly subjected to high surface pressure, have excellent rolling fatigue. Characteristics are required. Among these, “carbon steel for machine structure” described in JIS G 4051 (2009) is mainly used as the material of “constant velocity joint” and “hub unit”, and the rolling fatigue characteristics are Only necessary portions are cured by induction hardening.

  Note that the merits of induction hardening include that only necessary portions can be cured, and that in-line treatment is possible, so that the degree of freedom of the process is higher than other batch type surface treatments.

  It is known that rolling fatigue characteristics (rolling fatigue life) of rolling parts are reduced by non-metallic inclusions in steel (hereinafter also simply referred to as “inclusions”), particularly oxide inclusions. Yes. Therefore, conventionally, attempts have been made to reduce the oxygen content in steel by a steelmaking process, and as a result, the rolling fatigue life has been improved with a decrease in the oxygen content.

  As a method for evaluating inclusions in steel materials, for example, Non-Patent Document 1 and Non-Patent Document 2 propose a method using extreme value statistical processing.

  However, as described in Non-Patent Document 3, for example, the oxide inclusions (hereinafter sometimes simply referred to as “oxide”) have become smaller in diameter as a result of recent progress in steelmaking technology. In some cases, the size of sulfide inclusions (hereinafter sometimes referred to simply as “sulfides”) may increase, so that measures using only oxide as an index will increase the variation in rolling fatigue life. There is.

  Therefore, for example, Patent Documents 1 to 3 disclose techniques for improving the rolling fatigue life.

In Patent Document 1, in mass%, C: 0.45 to 0.60%, Si: 0.01 to 0.15%, Mn: 0.20 to 0.60%, S: 0.012% or less, Al: 0.015-0.040%, Ti: 0.005-0.050%, B: 0.0005-0.0050%, N: 0.010% or less, O: 0.0010% or less And, if necessary, further containing any one or more of Cr: 1.00% or less, Mo: 0.50% or less and Ni: 1.50% or less, the balance Fe and inevitable impurities The maximum dimension of the non-metallic inclusions contained is the equivalent circular diameter, which is 15 μm or less for oxide inclusions, 5 μm or less for nitride inclusions, and sulfide inclusions, respectively. The number of non-metallic inclusions having a diameter of 15 μm or less and an equivalent circular diameter of 1 μm or more. The number per 1 mm ² is 5 or less for oxide inclusions, 10 or less for nitride inclusions, and 5 or less for sulfide inclusions. The characteristic “steel for induction hardening excellent in cold workability, rolling fatigue strength and torsional fatigue strength” is disclosed.

In Patent Document 2, in mass%, C: 0.35 to 0.75%, Si: 0.15 to 1.1%, Mn: 0.2 to 2.0%, P: 0.020% or less, S: 0.06% or less, Al: 0.005 to 0.25%, Cr: 0.2% or less and Mo: 0.05 to 0.6%, if necessary, further Cu: 1 0.0% or less, Ni: 0.05 to 3.5%, Co: 0.01 to 1.0%, Nb: 0.005 to 0.1%, V: 0.01 to 0.5%, Ti : 0.1% or less and B: containing one or more selected from 0.006% or less, the balance is a steel composition consisting of inevitable impurities, and the predicted maximum intervention corresponding to 181.4 mm ³ “Mechanical structure excellent in rolling fatigue characteristics” characterized by having an object diameter of 11 μm or less and an average prior austenite particle size of the hardened layer after quenching of 12 μm or less. Use parts "is disclosed.

Patent Document 3 discloses a steel for machine structural use in which the surface hardness of the steel when used for machine parts is 58 HRC or more, and by mass ratio, O is 20 ppm or less and Al is less than 0.010%. When the inclusion diameter is defined as (longitudinal x lateral) ^1/2 , the oxide inclusion having the maximum inclusion diameter existing in the speculum area 3000 mm ² existing in the steel or the inclusion diameter of 15 μm or more Disclosed is a “steel used for machine parts having excellent rolling fatigue life”, characterized in that the composition of all oxide inclusions having a mass% is SiO ₂ : 30% or more. Yes.

JP 2001-26836 A JP 2009-242923 A JP 2008-240019 A

Takayoshi Murakami: Metal fatigue Effect of minute defects and inclusions (1993), [Yokendo] Zhou Seei et al .: Iron and Steel Vol. 87 (2001) No. 12, p. 22 Misao Nagao et al .: Sanyo Technical Report Vol. 12 (2005) No. 1 1, p. 38

  Although the steel disclosed in the above-mentioned Patent Document 1 is locally excellent in rolling fatigue life, there is a possibility that coarse inclusions may exist under a large dangerous volume such as an actual part. It may cause peeling.

  The mechanical structural component disclosed in Patent Document 2 may have a coarse oxide stretched or dotted in a longitudinal longitudinal section, and further a stretched coarse sulfide. In some cases, the rolling fatigue life is short.

  The steel disclosed in Patent Document 3 may not have an excellent rolling fatigue life because stretched coarse oxides and sulfides may exist.

  This invention is made | formed in view of the said present condition, and it aims at providing the steel material for induction hardening excellent in the rolling fatigue life.

  The main form in which defects occur in rolling parts is that repeated loads are applied to inclusions present in steel, and cracks generated by stress concentration gradually develop due to repeated loads, eventually leading to delamination. .

  Therefore, the inventors investigated the influence of inclusions on the rolling fatigue life. As a result, in order to detect inclusions that affect the rolling fatigue life, a two-dimensional evaluation, not a two-dimensional evaluation, as shown in Non-Patent Document 2, a sufficient volume was secured in three dimensions. In addition to confirming that the evaluation is necessary, as shown in Non-Patent Document 1, in order to improve the rolling fatigue life, prediction is made by an evaluation in which a sufficient volume is secured by using an ultrasonic fatigue test. It has been confirmed that it is effective to reduce the maximum inclusion diameter √AREA, and among them, it is important to shorten the length of inclusions observed in the longitudinal direction of the steel material.

  Therefore, the influence of inclusions on rolling fatigue was investigated in detail, and as a result, the following findings (a) and (b) were obtained.

  (A) By controlling the composition of the oxide and sulfide, that is, by controlling the composition so as to contain an appropriate amount of CaO in the oxide and CaS in the sulfide, the length of each inclusion , So that the rolling fatigue life is remarkably improved.

  (B) The rolling fatigue life has a correlation with the types and average compositions of oxides and sulfides that are the starting points of fracture in the ultrasonic fatigue fracture test.

  This invention is completed based on said knowledge, The summary exists in the steel materials for induction hardening shown to following (1)-(3). Hereinafter, each may be simply referred to as “present invention (1)” to “present invention (3)”. Further, the present invention (1) to the present invention (3) may be collectively referred to as “the present invention”.

(1) By mass%, C: 0.40 to 0.60%, Si: 0.15 to 0.35%, Mn: 0.60 to 0.90%, P: 0.030% or less, S: 0.008% or less, Cr: 0.01% or more and less than 0.50%, Al: 0.010 to 0.040%, O: 0.0015% or less and N: 0.020% or less, the balance Consists of the chemical composition of Fe and impurities,
In the longitudinal section of the steel material, it is the maximum inclusion diameter predicted in 144 mm ³ , which is the critical volume of the rolling fatigue test, by subjecting the inclusion diameter, which is the starting point of the ultrasonic fatigue test, to extreme statistical processing. Prediction √ AREA is 47.5 μm or less,
And the average aspect ratio of the inclusion which is the fracture starting point of the ultrasonic fatigue test is 5.8 or less,
Furthermore, when the inclusion which is the fracture starting point of the ultrasonic fatigue test is an oxide, the content in mass% in the average composition is CaO: 2.0 to 20%, MgO: 0 to 20%, and SiO ₂ : 0 to 10%, the balance being Al ₂ O ₃ , a binary oxide of CaO and Al ₂ O _3, a ternary oxide of CaO, MgO and Al ₂ O ₃ , CaO, SiO ₂ and ternary oxide of Al ₂ O ₃ and becomes CaO, MgO, from any of 4 elemental oxides of SiO ₂ and Al ₂ O _3,
And when the inclusion which is a fracture starting point of the ultrasonic fatigue test is a sulfide, the CaS: 100% CaS unidirectional sulfide containing CaS: 100% in the average composition, or CaS: 1. 0% or more, MgS: 0 to 20%, and the balance is MnS, consisting of a binary sulfide of CaS and MnS or a ternary sulfide of CaS, MgS and MnS.
This is a steel for induction hardening.

  (2) Inductive quenching according to the above (1), which contains one or more of V: 0.30% or less and Nb: 0.10% or less in mass% instead of part of Fe Steel material.

  (3) The steel material for induction hardening according to the above (1), which contains B: 0.005% or less and Ti: 0.05% or less in mass% instead of a part of Fe.

  “Impurity” means an element mixed from ore, scrap, or environment as a raw material when manufacturing steel materials industrially, and is allowed within a range that does not adversely affect the present invention.

  The “longitudinal longitudinal section” of the steel material in the present invention refers to a surface cut in parallel to the rolling direction of the steel material.

  Further, the aspect ratio of inclusions in the present invention refers to the ratio of the major axis (L) to the minor axis (W) of the inclusions, that is, L / W.

Hereinafter, “estimated maximum inclusion √AREA in 144 mm ³ , which is a dangerous volume in the rolling fatigue test” may be omitted and referred to as “predicted √AREA”.

  The steel for induction hardening according to the present invention can stably obtain a good rolling fatigue life. Therefore, it is suitable for use as a material for rolling parts such as “constant velocity joint” and “hub unit”.

It is a figure explaining the area | region (46 mm < ³ >) used as the test | inspection volume for the ultrasonic fatigue test piece used in the Example which becomes a stress range 90% or more of the maximum stress. It is a figure which illustrates typically the method which extract | collected the ultrasonic fatigue test piece used in the Example from the steel bar of diameter 80mm and the steel bar of diameter 70mm. “R” represents the radius of the steel bar. It is a figure which illustrates typically the method which extract | collected the ultrasonic fatigue test piece used in the Example from the steel bar of diameter 120mm, the steel bar of diameter 100mm, the steel piece of 180mmx180mm, and the steel piece of 160mmx160mm. “R” and “T” represent the radius of the steel bar and the thickness of the steel piece, respectively. It is a figure which shows the rough shape as cut out from the board | plate material of the ultrasonic fatigue test piece used in the Example. The unit of the dimension in the figure is “mm”. It is a figure which shows the finishing shape of the ultrasonic fatigue test piece used in the Example. The unit of the dimension in the figure is “mm”. It is a figure which illustrates typically the method which extract | collected the raw material used for the rolling fatigue test of the Example from the steel piece of 180 mm x 180 mm and the steel piece of 160 mm x 160 mm.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, "%" of content of each element, oxide, and sulfide means "mass%".

(A) Chemical composition of steel:
C: 0.40 to 0.60%
C is an element that ensures the hardness required for the rolling part after induction hardening, and the content needs to be 0.40% or more. However, if the C content exceeds 0.60%, the base material becomes hard, the forgeability is remarkably lowered and the tool life at the time of cutting is reduced. It becomes. Therefore, the content of C is set to 0.40 to 0.60%. A desirable C content range is 0.50 to 0.58%.

Si: 0.15-0.35%
Si is an element necessary for ensuring the depth of the hardened layer necessary for the rolling part after induction hardening, and must be contained by 0.15% or more. However, even if Si exceeds 0.35%, the effect of improving the hardenability is saturated, the base material becomes hard, the forgeability is remarkably reduced, and the tool life at the time of cutting is reduced. End up. Therefore, the Si content is set to 0.15 to 0.35%. The range of preferable Si content is 0.17 to 0.30%.

Mn: 0.60 to 0.90%
Mn is an element necessary for ensuring the depth of the hardened layer necessary for the rolling part after induction hardening, and must be contained at 0.60% or more. However, even if Mn is contained exceeding 0.90%, the effect of improving hardenability is saturated, the base material becomes harder, the forgeability is remarkably reduced, and the tool life during cutting is reduced. End up. Therefore, the Mn content is set to 0.60 to 0.90%. The range of preferable Mn content is 0.70 to 0.85%.

P: 0.030% or less P segregates at the grain boundary and shortens the rolling fatigue life. In particular, when the content exceeds 0.030%, the rolling fatigue life is significantly reduced. Therefore, the content of P is set to 0.030% or less. The range of preferable P content is 0.020% or less. The smaller the P content, the better.

S: 0.008% or less S is an element that forms sulfides. If the content exceeds 0.008%, coarse sulfides remain, so that the rolling fatigue life is shortened. Therefore, the content of S is set to 0.008% or less. A preferable S content from the viewpoint of improving the rolling fatigue life is 0.006% or less. The smaller the S content, the better.

Cr: 0.01% or more and less than 0.50% Cr is an element necessary for ensuring the depth of the hardened layer necessary for the rolling part after induction hardening, and must be contained by 0.01% or more. However, if Cr is contained in an amount of 0.50% or more, the tool life during cutting is reduced. Therefore, the Cr content is set to 0.01% or more and less than 0.50%. The range of preferable Cr content is 0.05 to 0.40%.

Al: 0.010 to 0.040%
Al is an element used for deoxidizing in the refining process, and must be contained by 0.010% or more. However, if the Al content exceeds 0.040%, it tends to remain as a coarse oxide, which may lead to a reduction in rolling fatigue life. Therefore, the content of Al is set to 0.010 to 0.040%. The range of preferable Al content is 0.015-0.035%.

O: 0.0015% or less O is an undesirable impurity element. If the content of O increases, and particularly exceeds 0.0015%, it remains as a coarse oxide after rolling, leading to a reduction in rolling fatigue life. Therefore, the content of O is set to 0.0015% or less. In addition, the range of preferable O content is 0.0012% or less. The smaller the O content, the better.

N: 0.020% or less N is an element that, when contained excessively, produces coarse nitrides and may cause a decrease in fatigue strength. Therefore, an upper limit is set and the N content is set to 0.020% or less.

  In order to obtain the effect of suppressing the coarsening of crystal grains during high-frequency induction heating (hereinafter simply referred to as “high-frequency heating”), when one or more of V and Nb described later are contained, It is necessary to generate elemental nitrides. Therefore, when it contains 1 or more types of V and Nb, it is desirable to control N content highly, keeping it in the range of 0.020% or less. In this case, a desirable range of N content is 0.010 to 0.020%, and a more desirable range is 0.012 to 0.020%.

  In addition, in order to obtain the effect of improving the hardenability during induction hardening, it is necessary to suppress the formation of BN in which B is bonded to N as much as possible when B and Ti described later are contained. Therefore, when B and Ti are contained, it is desirable that the N content be further lower than 0.020% or less. In this case, the desirable range of N content is less than 0.010%, and the more desirable range is 0.008% or less.

  For the above reasons, the steel for induction hardening according to the present invention (1) contains C, Si, Mn, P, S, Cr, Al, O, and N in the above-mentioned ranges, and the balance is Fe and impurity chemistry. It was defined as consisting of composition.

  Since the induction heating is heated to a high temperature even for a short time, the induction hardening steel according to the present invention is the same as that of the induction hardening steel according to the present invention (1) from the viewpoint of preventing coarsening of crystal grains. Instead of a part of Fe, it may contain one or more of V: 0.30% or less and Nb: 0.10% or less (steel material for induction hardening of the present invention (2)).

  Hereinafter, the above-described V and Nb which are optional elements will be described.

V: 0.30% or less Since V combines with N to form a nitride, it has an effect of suppressing crystal grain coarsening during high-frequency heating. V also has the effect of increasing the strength of the base material by combining with C. However, even if it contains V exceeding 0.30%, the effect of preventing crystal grain coarsening during high-frequency heating is saturated. Furthermore, in the above case, the strength of the base material becomes too high, and the machinability may be reduced. Therefore, when V is included, the content of V is set to 0.30% or less. In addition, in order to more effectively exhibit the effect of suppressing the coarsening of crystal grains during high-frequency heating and the effect of increasing the strength of the base material, and in order to ensure sufficient machinability, the V content when contained is , 0.01 to 0.20% is desirable.

Nb: 0.10% or less Since Nb combines with N to form a nitride, it has the effect of suppressing crystal grain coarsening during high-frequency heating. Nb also has the effect of increasing the strength of the base material by combining with C. However, even if Nb is contained exceeding 0.10%, the effect of preventing crystal grain coarsening during high-frequency heating is saturated. Furthermore, in the above case, the strength of the base material becomes too high, and the machinability may be reduced. Therefore, when Nb is contained, the content of Nb is set to 0.10% or less. In order to more effectively exhibit the effect of suppressing the coarsening of crystal grains during high-frequency heating and the effect of increasing the strength of the base material, and in order to ensure sufficient machinability, the Nb content in the case of inclusion is 0.01 to 0.10% is desirable.

  Said V and Nb can be contained only in any 1 type or 2 types of composite. The total amount when these elements are contained in combination may be 0.4%, but is preferably 0.3% or less.

  For the above reasons, the steel for induction hardening according to the present invention (2) is replaced with a part of Fe of the steel for induction hardening according to the present invention (1), V: 0.30% or less and Nb: 0 It was defined as containing one or more of 10% or less.

  The steel for induction hardening according to the present invention is replaced with a part of Fe of the steel for induction hardening according to the present invention (1) from the viewpoint of ensuring better induction hardenability, B: 0.005% And Ti: 0.05% or less (steel material for induction hardening of the present invention (3)).

  Hereinafter, the above-described B and Ti which are optional elements will be described.

B: 0.005% or less B is an element that can greatly improve the hardenability of the steel in a small amount and can further increase the depth of the hardened layer necessary for the rolling part after induction hardening. However, even if the B content exceeds 0.005%, the effect is saturated. Therefore, when B is included, the content of B is set to 0.005% or less. In order to stably exhibit the hardenability improving effect and ensure better induction hardenability, the B content in the case of inclusion is preferably 0.0003 to 0.005%.

Ti: 0.05% or less By containing B, the hardenability is improved when B is present in a solid solution B state. Therefore, when B is combined with N to form BN, the effect of improving hardenability by B cannot be expected. For the above reason, when B is contained, it is necessary to contain Ti having a larger affinity with N than B. However, even if Ti is contained in an amount exceeding 0.05%, not only the effect of fixing N is saturated, but also a large amount of coarse TiN is generated, so that rolling fatigue characteristics may be reduced. is there. Therefore, when Ti is included, the content of Ti is set to 0.05% or less. In order to stably exhibit the hardenability improving effect of B, the Ti content when contained together with the above-described amount of B is preferably 0.01 to 0.05%.

  For the above reasons, the steel for induction hardening according to the present invention (3) is replaced with a part of Fe of the steel for induction hardening according to the present invention (1), B: 0.005% or less and Ti: 0 .05% or less is specified.

(B) Prediction of inclusions in longitudinal section of steel material √ AREA and aspect ratio In order to use as a material for rolling parts such as “constant velocity joint” and “hub unit”, the steel material for induction hardening of the present invention is: In the longitudinal section of the steel material, the inclusion diameter, which is the fracture starting point of the ultrasonic fatigue test, is subjected to extreme statistical processing, and the maximum inclusion diameter is predicted to be 144 mm ³ , which is the dangerous volume of the rolling fatigue test. AREA must be 50 μm or less, and the average aspect ratio of inclusions, which is the starting point of destruction in ultrasonic testing, must be 7 or less.

Regarding the method for measuring the inclusion diameter, referring to the measurement method using an optical microscope described in Non-Patent Document 3, in the present invention, the inclusion that is the starting point of fracture in the ultrasonic fatigue test is observed with a scanning electron microscope (SEM). Then, the major axis (L) and minor axis (W) of the inclusion are measured, and the inclusion diameter √AREA = (L × W) ^1/2 and the aspect ratio (L / W) are obtained.

  In other words, the major axis of the inclusion is the largest side connecting the ends of inclusions existing in a single or plural group, and the maximum width of the inclusion sandwiched by a line parallel to the major axis side is short. The diameter. For inclusions existing in groups, the distance between inclusions and the size of the smaller inclusion (√AREA) are compared, and the smaller inclusion diameter √AREA is greater than the distance between inclusions. Is larger, the two inclusions are judged as one, and when the value of √AREA of the smaller inclusion is smaller than the distance between the inclusions, both are judged as separate inclusions.

In the rolling fatigue test conditions using the thrust type rolling fatigue tester in the examples described later, that is, the maximum contact surface pressure is 5230 MPa and the diameter of the opposing ball is 9.53 mm, the contact ellipse in Hertz contact The diameter is about 0.7 mm, the diameter of the rolling track is 38.5 mm, and the depth at which stress of 90% or more of the maximum shear stress is applied is about 0.17 mm. In the rolling fatigue test, the above-described region can be determined as a dangerous volume that may cause separation, and the specific volume is π × 38.5 × 0. 7 × 0.17 = 14.4 mm ³ . The rolling fatigue characteristics in the present invention, was evaluated in L ₁₀ life showing a 10% failure probability, time, the number of tested as 10, that is, performs the rolling fatigue test using 10 test pieces. For this reason, the dangerous volume of the rolling fatigue test is 144 mm ³ described above.

Further, in the ultrasonic fatigue test, as shown in FIG. 1, it is possible to determine a region in which the stress range is 90% or more of the maximum stress as an inspection reference volume at which there is a risk of fatigue failure starting from inclusions. it can. The cross section of the test piece at the position where 90% of the maximum stress is applied has a radius of about 1.58 mm, and the length of the range is about 6.14 mm. When the volume of this region is obtained by integration, the critical volume of one ultrasonic fatigue test piece is 46 mm ³ .

  Therefore, the prediction √AREA of inclusions by “extreme value statistical processing” can be estimated by the following procedure, for example.

<1> In the produced ultrasonic fatigue test piece, a region in which the stress range is 90% or more of the maximum stress is set as an inspection reference volume V ₀ (mm ³ ) at which there is a risk of fatigue failure starting from inclusions.
<2> the V ₀ inclusions became a starting point of fatigue fracture in the maximum inclusions in V _0, measuring its √AREA (μm).
<3> The above-described measurement is performed with n test pieces. However, n may be an integer of 10 or more.
<4> The measured √AREA are rearranged in ascending order, and set as √AREA _j (j = 1 to n).
<5> The following normalized variable y _j is calculated for each _j .
y _j = −ln [−ln {j / (n + 1)}].
<6> The coordinate horizontal axis of the extreme value statistical paper is √AREA and the vertical axis is the standardized variable y, and plotted for j = 1 to n, and an approximate straight line is obtained by the method of least squares.
<7> Assuming that the volume to be evaluated is V (mm ³ ) and T = (V + V ₀ ) / V ₀ , the value of y is obtained from the following equation, and using the above approximate curve, √AREA at the value of y can be obtained. For example, this is the prediction √AREA of inclusions in the evaluation volume.
y = -ln [-ln {(T-1) / T}].

In the case of the present invention, as described above, since the inspection reference volume V ₀ of the ultrasonic fatigue test is 46 mm ³ and the dangerous volume V of the rolling fatigue test is 144 mm ³ , T = (144 + 46) / 46 = 4. If 13 is substituted into the above equation, the value of y is 1.28. Therefore, if √AREA in the above approximate curve when the value of y is 1.28 is obtained, this is the predicted √AREA in 144 mm ³ , which is the dangerous volume of the rolling fatigue test.

By the above-described method, the inclusion diameter, which is the fracture starting point of the ultrasonic fatigue test, is subjected to extreme statistical processing in the longitudinal longitudinal section of the steel material, and is predicted to be 144 mm ³ , which is the dangerous volume of the rolling fatigue test. If the predicted √AREA that is the maximum inclusion diameter exceeds 50 μm, the rolling fatigue life is reduced by coarse inclusions, as shown in an example in the examples described later. The desired prediction √AREA is 45 μm or less.

  In addition, when the aspect ratio (L / W) of the inclusion, which is the fracture starting point of the ultrasonic fatigue test, exceeds 7, stretched or point-row-like coarse oxides, and stretched coarse sulfides As a result, the rolling fatigue life is reduced. Therefore, the average aspect ratio of inclusions, which is the fracture starting point of the ultrasonic fatigue test, was set to 7 or less.

(C) When the inclusion that is the fracture starting point of the ultrasonic fatigue test is an oxide In order to use as a material for rolling parts such as “constant velocity joint” and “hub unit”, the steel for induction hardening of the present invention is: In the longitudinal cross-section in the longitudinal direction of the steel material, when the inclusion that is the fracture starting point of the ultrasonic fatigue test is an oxide, the content (hereinafter sometimes referred to as “concentration”) in terms of mass% in the average composition is CaO. : 2.0~20%, MgO: 0~20% and SiO _2: 0-10% and the balance is a Al ₂ O _3, 2 elemental oxides of CaO and Al ₂ O _3, CaO, Any of ternary oxides of MgO and Al ₂ O ₃ , ternary oxides of CaO, SiO ₂ and Al ₂ O ₃ , and quaternary oxides of CaO, MgO, SiO ₂ and Al ₂ O ₃ It must consist of

  The steel material for induction hardening according to the present invention is a coarse oxide that is long stretched or point-stringed by satisfying the above condition when the inclusion that is the fracture starting point of the ultrasonic fatigue test is an oxide. Is suppressed, and an excellent rolling fatigue life can be secured.

  For oxides, the concentration of each oxide is calculated by the following method.

  First, the content of Ca, Mg, Al, Si, Mn, Cr, Fe, and S in the oxide is quantified by “mol%” using an energy dispersive spectrometer (EDS).

  Here, in an inclusion in which an oxide and a sulfide are combined, S may be detected from constituent elements of the oxide. A method for obtaining the oxide concentration in that case will be described.

First, regarding Ca, it is detected by either CaS or CaO. In the situation where CaS is present, since MnO does not exist thermodynamically, all Mn is assumed to be bonded to S. Therefore, when [S] is detected from the oxide, that is, when [S]> [Mn], all [Mn] constitutes MnS, and no oxide of Mn exists. Then, [Ca] corresponding to [S] surplus with respect to [Mn] is Ca constituting CaS, and further, [Ca] surplus to constitute CaS is CaO concentration. Next, the concentrations of MgO, SiO ₂ and Al ₂ O ₃ are determined from [Mg], [Si] and [Al], respectively.

On the other hand, in the case of [S] ≦ [Mn], all [S] constitutes MnS, and from [Ca], [Mg], [Si] and [Al], CaO, MgO, The concentrations of SiO ₂ and Al ₂ O ₃ are determined.

Note that the Fe _x O and Cr ₂ O ₃ is lower oxide, there is only a trace amount in the range of O content is specified in the present invention. Therefore, even when [Cr] and / or [Fe] are detected in a small amount from the oxide, elements excluding detected [Cr] and [Fe], that is, [Ca], [Mg] , [Si] and [Al] are set to 100%, and the concentration of each oxide is obtained. Finally, the concentrations of CaO, MgO, SiO ₂ and Al ₂ O ₃ in the respective samples obtained as described above are obtained. From this, the average composition in mass% of the oxide, which is the fracture starting point of the ultrasonic fatigue test, is calculated.

  In the above, [X] refers to a value quantified in “mol%” unit of element X.

  Hereafter, each oxide prescribed | regulated by this invention is demonstrated in detail.

CaO: 2.0-20%
CaO, which is a basic oxide, is one of the main components of slag and is used as a solvent medium during desulfurization. When the CaO concentration is 2.0% or more, an effect of suppressing the generation of Al ₂ O ₃ and spinel that are long stretched or dot-like is obtained. On the other hand, when the CaO concentration exceeds 20%, coarse oxides mainly composed of large-scale CaO are generated. Therefore, the CaO concentration in the average composition of the oxide is set to 2.0 to 20%.

MgO: 0 to 20%
Since MgO is a basic oxide and has low solubility, it is crystallized as a hard MgO (periclase) phase, and further together with Al ₂ O ₃ as an MgO.Al ₂ O ₃ (spinel) phase. Since these may become coarse oxides in the form of point sequences and remain in the steel material and reduce the rolling fatigue life, an upper limit is set for the MgO concentration and limited to 20% or less. Note that MgO may not be present in the oxide. For this reason, the MgO concentration in the average composition of the oxide is set to 0 to 20%.

SiO ₂ : 0 to 10%
SiO ₂ , which is an acidic oxide, is one of the main components of slag, and may be contained in the oxide. Up to 10% is acceptable, but if it exceeds 10%, the oxide stretches. It may become coarse and a rolling fatigue life may fall. Note that SiO ₂ may not be present in the oxide. Therefore, the SiO ₂ concentration in the average composition of the oxide is set to 0 to 10%.

When the CaO concentration is 2.0% or more, Al ₂ O ₃ which is the remainder of the oxide is suppressed from being stretched for a long time or to be in a dotted line shape. Therefore, the concentration of Al ₂ O ₃ as the balance, and a binary oxide of CaO and Al ₂ O _3, may be 98.0% for CaO concentration of 2.0%.

(D) When the inclusion that is the starting point of fracture in the ultrasonic fatigue test is sulfide, the steel for induction hardening according to the present invention is used as a material for rolling parts such as “constant velocity joint” and “hub unit”. In the longitudinal cross-section in the longitudinal direction of the steel material, when the inclusion that is the fracture starting point of the ultrasonic fatigue test is a sulfide, the content (hereinafter sometimes referred to as “concentration”) in mass% in the average composition. , CaS: 100% CaS monosulfide, or CaS: 1.0% or more, MgS: 0 to 20%, the balance being MnS, and CaS and MnS binary sulfide It must consist of ternary sulfides of CaS, MgS and MnS.

  The steel for induction hardening according to the present invention is excellent in that when the inclusion that is the fracture starting point of the ultrasonic fatigue test is a sulfide, the formation of a stretched coarse sulfide is suppressed by satisfying the above conditions. It is possible to ensure a rolling fatigue life.

  For sulfides, the concentration of each sulfide is calculated by the following method.

  First, the content of Ca, Mg, Mn and S in the sulfide is quantified by “mol%” by an energy dispersive spectrometer (EDS). Next, the concentrations of CaS, MgS, and MnS are obtained from [Ca], [Mg], and [Mn], respectively.

  Finally, from the CaS, MgS, and MnS concentrations in the respective samples obtained as described above, the average composition in mass% of sulfide that is the fracture starting point of the ultrasonic fatigue test is calculated.

  In the above, [X] refers to a value quantified in “mol%” unit of element X.

  Hereafter, each sulfide prescribed | regulated by this invention is demonstrated in detail.

CaS: 1.0 to 100%
CaS is a sulfide produced by a desulfurization reaction. When the CaS concentration is 1.0% or more, an effect of suppressing the formation of stretched coarse sulfide is obtained. Even if only CaS exists as a sulfide, that is, the CaS concentration may be 100%. Therefore, the CaS concentration in the average composition of sulfide is set to 1.0 to 100%.

  Note that the CaS concentration in the case where the sulfide is made of a binary sulfide of CaS and MnS or a ternary sulfide of CaS, MgS and MnS may be a value close to 100%.

MgS: 0 to 20%
In the refining stage, Mg may be taken into the steel and MgS may be mixed into the sulfide. If the MgS concentration exceeds 20%, the MgO concentration in the above-described oxide increases, leading to the generation of coarse oxides in the form of dotted lines, so the MgS concentration is limited to 20% or less. Note that MgS may not be present in the sulfide. Therefore, the MgS concentration in the average composition of sulfides is set to 0 to 20%.

  Note that when the CaS concentration is 1.0% or more, the formation of stretched coarse sulfides is suppressed. Therefore, the concentration of MnS as the balance may be 99.0% when the CaS concentration is 1.0% in the case of a binary sulfide of CaS and MnS. In the case of ternary sulfides of CaS, MgS and MnS, the value may be close to 99.0% when the CaS concentration is 1.0% and the MgO concentration is close to 0%. .

  The oxide composition and sulfide composition described above can be obtained, for example, by the production method described below.

  First, after performing oxidation refining in a converter, Al is added at the time of steel output from the converter, deoxidation treatment is performed, and then further demetalization treatment is performed.

_{Then, CaO: 30~70%, Al 2} O 3: 5~40%, SiO 2: 10% or less (not including 0%), MgO: 0~10% , CaF 2: 0~30%, CaO / A slag containing SiO ₂ : 6 or more and CaO / Al ₂ O ₃ : 1.5 to 15 is adjusted in a range of 5 to 20 kg per ton of molten steel, and a vacuum molten steel stirring device with an arc heating device (hereinafter, "VAD")), stirring and refining treatment with Ar gas is performed, treatment is performed for 30 minutes with an RH vacuum degassing apparatus, and continuous casting is performed to obtain a slab having a cross section of 300 mm x 400 mm.

  Further, the slab is subjected to hot rolling with a rolling reduction ratio of 10 or more by split rolling and bar rolling.

  The slag composition of the above components and the treatment with VAD are intended to control the composition of oxides and sulfides. And the process in RH vacuum degassing apparatus is a process implemented in order to reduce the total amount of oxide inclusions.

  The reduction ratio is a value obtained by dividing the cross-sectional area of the slab by the cross-sectional area of the steel for induction hardening obtained by the final reduction. Increasing the reduction ratio to 10 or more increases the distance between inclusions present in a plurality of groups, and reduces the inclusions that are judged as integral by dividing the stretched oxide or sulfide. This is to make it happen.

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

  Steels 1-21 having the chemical composition shown in Table 1 were produced by the following process.

  Steels 1 to 11 and Steels 15 to 21 in Table 1 are steels whose chemical compositions are within the range specified by the present invention, while Steels 12 to 14 are out of the conditions specified by the chemical composition of the present invention. Steel.

  First, after oxidative refining in a 70-t converter, Al was added at the time of steel removal from the converter to perform a deoxidation treatment, and then a denitrification treatment was further performed.

  Next, slag was adjusted to the conditions shown in Table 2 under the Ar atmosphere by VAD, and stirring with Ar gas was performed under the conditions shown in Table 3.

  Then, after further processing by the RH vacuum degassing apparatus was carried out for the time shown in Table 4, continuous casting was performed to obtain a slab of 300 mm × 400 mm.

  After soaking the cast slab obtained as described above at 1250 ° C., the steel slab having a size shown in Table 5 is obtained by split rolling in a temperature range of 1100 to 1050 ° C., and the steel slab is further heated to 1200 ° C. After heating, the steel bar was rolled in a temperature range of 1100 to 1020 ° C. to produce a steel bar having a diameter of 120 to 70 mm.

  In addition, about the steel 15 and the steel 16, only the partial rolling was implemented and it was set as the steel slab and the bar rolling was not performed.

  After cutting the steel bars of test numbers 1 to 14 and test numbers 17 to 21 and the steel pieces of test numbers 15 and 16 to 200 mm, coarse ultrasonic fatigue test pieces were collected.

  Specifically, for a steel bar having a diameter of 80 mm of test number 3 and a steel bar having a diameter of 70 mm of test numbers 4 to 14 and test numbers 17 to 21, as shown in FIG. 14 mm in thickness and 45 mm in width in a direction parallel to the rolling direction on the basis of R / 2 part (“R” represents the radius of the steel bar) which is an intermediate position between the surface and the center with respect to the surface cut perpendicularly to A plate material having a length of 200 mm was cut out.

  Next, both ends in the width direction of the plate material were “flattened” by milling, and then the same steel material as the plate material was welded to both ends by electron beam welding to finish a plate having a width of 85 mm.

  As shown in FIG. 3, the steel bar of test number 1 120 mm in diameter, the steel bar of test number 2 of 100 mm in diameter, the steel piece of test number 15 of 180 mm × 180 mm and the steel piece of test number 16 of 160 mm × 160 mm are as shown in FIG. A plate material having a thickness of 14 mm, a width of 75 mm, and a length of 200 mm is cut out in a direction parallel to the rolling direction on the basis of R / 2 part or T / 4 part (“T” represents the thickness of the steel slab). No welding was performed.

  Next, in order to eliminate the heat effect during welding, all the above plate materials were first kept at 900 ° C. for 60 minutes and then air-cooled to room temperature in the atmosphere. As shown in FIG. 2 and FIG. 3, 19 pieces of coarse ultrasonic fatigue test pieces shown in FIG. 4 were collected from the direction perpendicular to the rolling direction.

  The coarse-shaped ultrasonic fatigue test piece collected as described above was subjected to induction hardening under the conditions of a frequency of 200 kHz, an output of 50 kW, and a heating time of 2.3 seconds, and then heated at 150 ° C. for 1 hour in the atmosphere. Tempering to cool was performed. Next, finishing was performed to produce an ultrasonic fatigue test piece shown in FIG.

  In addition, the unit of the dimension in the above-mentioned each test piece shown in FIG. 4 and FIG. 5 is all “mm”.

  An ultrasonic fatigue test was carried out using the ultrasonic fatigue test piece that had been finished.

Specifically, using an ultrasonic fatigue tester USF-2000 manufactured by Shimadzu Corporation, a fatigue test was performed until failure occurred under the conditions of a frequency of 20 kHz, a stress amplitude of 700 MPa, and a stress ratio of -1. In addition, when it did not break even if the number of repetitions became 1.0 × 10 ⁷ , the fatigue test was carried out until the fracture occurred by increasing the stress by 20 MPa.

The destroyed specimen was observed with a scanning electron microscope (SEM) for inclusions at the origin of destruction, the major axis and minor axis of the inclusions were measured, and the inclusion diameter √AREA = (major axis x minor axis) ^1/2 . Inclusion diameter √AREA was determined for each of 19 steels. In addition, the aspect ratio (L / W) was obtained by dividing the major axis (L) of the inclusion at the starting point measured by the minor axis (W), and the aspect ratio of the 19 inclusions at the fracture starting point was arithmetically averaged. The average aspect ratio was determined.

  The major axis of the inclusion is the largest side connecting the ends of inclusions existing in a single or plural group, and the maximum width of the inclusion sandwiched by a line parallel to the major axis side is short. The diameter. For inclusions existing in groups, the distance between inclusions and the size of the smaller inclusion (√AREA) are compared, and the smaller inclusion diameter √AREA is greater than the distance between inclusions. Is larger, the two inclusions are judged as one, and when the √AREA value of the smaller inclusion is smaller than the distance between the inclusions, they are judged as separate inclusions.

As shown in FIG. 1, the ultrasonic fatigue test piece is 46 mm ^3, which is a stress range of 90% or more of the maximum stress, is set as the inspection reference volume V _0, and V, which is the dangerous volume of the rolling fatigue test, is 144 mm ³ . With reference to Patent Document 1, predicted √AREA that is the maximum inclusion diameter predicted at y = 1.28 was obtained by the extreme value statistical processing described above.

  About the inclusion composition which is a fracture starting point of the ultrasonic fatigue test, each composition was measured by energy dispersive X-ray spectroscopy. First, it is determined whether the inclusions at the fracture starting point of each of the 19 steels are oxides or sulfides. For oxides, the average composition of the oxides, and for sulfides, the sulfides are sulfided. The average composition of each product was determined by arithmetic average.

  Moreover, about each steel, the steel bar of the test number 15 and the test number 17 and the steel number of the test number 15 and the test number 16 and the steel number of the test number 15 and the test number 16 was cut | disconnected to 250 mm. Thereafter, from the center of a steel bar having a diameter of 70 to 120 mm, a raw material having a diameter of 60 mm and a thickness of 10 mm was sliced and collected so that the longitudinal direction of the steel bar was the thickness of the raw material. Further, in order to avoid the influence of center segregation as much as possible for the 180 mm × 180 mm steel pieces and the 160 mm × 160 mm steel pieces, as shown in FIG. 6, the T / 4 portion is the center of the raw material, and the longitudinal direction of the steel pieces is A raw material having a diameter of 60 mm and a thickness of 10 mm was collected so as to obtain the thickness of the raw material.

  One side of the above-mentioned shaped material having a diameter of 60 mm and a thickness of 10 mm was induction-quenched under the conditions of a frequency of 20 kHz, an output of 150 kW, and a heating time of 5 seconds, and then heated at 150 ° C. for 1 hour and allowed to cool in the atmosphere. Tempered.

  In addition, the effective hardened layer depth was measured according to the prescription | regulation of JISG0559 (2008) about the shaped material induction-hardened-tempered as mentioned above. As a result, the effective hardened layer depth of each shaped material was 2 to 3 mm.

  The surface of the shaped material thus quenched and tempered was lapped to produce a rolling fatigue test piece, which was subjected to a rolling fatigue test.

  The rolling fatigue test was performed using a thrust type rolling fatigue tester with a maximum contact surface pressure of 5230 MPa and a repetition rate of 1800 cpm (cycle per minute), with 10 tests. Table 6 shows the detailed conditions of the rolling fatigue test.

The rolling fatigue test results were plotted on a Weibull distribution probability paper, and the rolling fatigue characteristics were evaluated by setting the L ₁₀ life indicating 10% failure probability as “rolling fatigue life”.

Table 7 shows the average composition of inclusions, the predicted maximum inclusion diameter √AREA, the average aspect ratio (average L / W), and rolling fatigue life, which are the starting points of the ultrasonic fatigue test. In Test No. 12, since no sulfide was present at the fracture starting point of the ultrasonic fatigue test, the columns of CaS, MgS, and MnS of the average sulfide composition were all represented as “−”. Similarly, in the test number 13, since no oxide was present at the fracture starting point of the ultrasonic fatigue test, the columns of the average oxide composition CaO, MgO, SiO ₂ and Al ₂ O ₃ were all “-”. Indicated.

As shown in Table 7, in the case of test numbers 1 , 3 and 5 to 11 of the present invention examples, the chemical compositions of steels 1 , 3 and 5 to 11 satisfy the conditions defined in the present invention, and the longitudinal profile of the steel material The predicted maximum inclusion diameter √AREA of the inclusion which is the fracture starting point of the ultrasonic fatigue test on the surface is as small as 47.5 μm or less, the average aspect ratio is 5.8 or less, and satisfies the conditions defined in the present invention. The average composition of the oxide that is the fracture starting point and the average composition of the sulfide that is the fracture starting point all satisfy the conditions defined in the present invention. For this reason, a rolling fatigue life of 15.7 × 10 ⁶ or more was obtained.

  On the other hand, in the case of test numbers 12 to 14 of comparative examples in which the chemical composition of the steel deviates from the conditions specified in the present invention, the rolling fatigue life is short.

That is, in the test number 12, since the O content of the steel 12 is 0.0019% and exceeds the value specified in the present invention, the oxide becomes coarse, and the L ₁₀ life is 3.6 × 10 6. ⁶ and short.

In Test No. 13 and Test No. 14, the S content of Steel 13 and Steel 14 is 0.013% and 0.011%, respectively, exceeding the values specified in the present invention, and CaS in sulfides. Since the concentration is lower than the value specified in the present invention, it becomes a stretched coarse sulfide, and the L ₁₀ life is as short as 2.5 × 10 ⁶ and 4.2 × 10 ⁶ , respectively.

  Next, even if the chemical composition of the steel satisfies the conditions specified in the present invention, the prediction √AREA of the inclusion that is the fracture start point of the ultrasonic fatigue test in the longitudinal longitudinal section of the steel material, and the average aspect ratio of the present invention In the case of test number 15 and test number 16 that deviate from the conditions specified in (1), the rolling fatigue life is short.

That is, since test number 15 and test number 16 have a predicted √AREA and an average aspect ratio exceeding the value specified in the present invention, the L ₁₀ life is 5. As short as 5 × 10 ⁶ and 5.0 × 10 ⁶ .

  Moreover, even if the chemical composition of the steel satisfies the conditions specified in the present invention, the average composition of oxides and the average composition of sulfides that are the fracture starting points of the ultrasonic fatigue test in the longitudinal section of the steel material When at least one of the test numbers 17 to 21 deviates from the conditions specified in the present invention, the rolling fatigue life is short.

That is, in the test number 17, since the CaO concentration in the oxide exceeds the value specified in the present invention, a large oxide is generated, and the L ₁₀ life is as short as 6.2 × 10 ⁶ .

In test number 18, since the SiO ₂ concentration in the oxide exceeds the value specified in the present invention, it becomes a stretched coarse oxide, and the L ₁₀ life is as short as 6.7 × 10 ⁶ .

In test number 19, since the MgO concentration in the oxide exceeds the value specified in the present invention, the dot-sequence oxide becomes coarse, and the L ₁₀ life is as short as 7.9 × 10 ^6. .

In Test No. 20, since the CaS concentration in the sulfide is lower than the value specified in the present invention, the sulfide that became the fracture starting point becomes a coarse sulfide that is stretched, and the L ₁₀ life is 8. As short as 8 × 10 ⁶ .

In Test No. 21, since the MgS concentration in the sulfide exceeds the value specified in the present invention, the MgO concentration in the oxide increases, and the dotted oxide becomes coarse, and L ₁₀ Life is as short as 5.4 × 10 ⁶ .

  The steel for induction hardening according to the present invention can stably obtain a good rolling fatigue life. Therefore, it is suitable for use as a material for rolling parts such as “constant velocity joint” and “hub unit”.

Claims (3)

In mass%, C: 0.40 to 0.60%, Si: 0.15 to 0.35%, Mn: 0.60 to 0.90%, P: 0.030% or less, S: 0.008 %: Cr: 0.01% or more and less than 0.50%, Al: 0.010 to 0.040%, O: 0.0015% or less and N: 0.020% or less, with the balance being Fe and Consisting of chemical composition of impurities,
In the longitudinal section of the steel material, it is the maximum inclusion diameter predicted in 144 mm ³ , which is the critical volume of the rolling fatigue test, by subjecting the inclusion diameter, which is the starting point of the ultrasonic fatigue test, to extreme statistical processing. Prediction √ AREA is 47.5 μm or less,
And the average aspect ratio of the inclusion which is the fracture starting point of the ultrasonic fatigue test is 5.8 or less,
Furthermore, when the inclusion which is the fracture starting point of the ultrasonic fatigue test is an oxide, the content in mass% in the average composition is CaO: 2.0 to 20%, MgO: 0 to 20%, and SiO ₂ : 0 to 10%, the balance being Al ₂ O ₃ , a binary oxide of CaO and Al ₂ O _3, a ternary oxide of CaO, MgO and Al ₂ O ₃ , CaO, SiO ₂ and ternary oxide of Al ₂ O ₃ and becomes CaO, MgO, from any of 4 elemental oxides of SiO ₂ and Al ₂ O _3,
And when the inclusion which is a fracture starting point of the ultrasonic fatigue test is a sulfide, the CaS: 100% CaS unidirectional sulfide containing CaS: 100% in the average composition, or CaS: 1. 0% or more, MgS: 0 to 20%, and the balance is MnS, consisting of a binary sulfide of CaS and MnS or a ternary sulfide of CaS, MgS and MnS.
This is a steel for induction hardening.   The steel material for induction hardening according to claim 1, which contains at least one of V: 0.30% or less and Nb: 0.10% or less in mass% instead of a part of Fe.   The steel material for induction hardening according to claim 1, which contains B: 0.005% or less and Ti: 0.05% or less in mass% instead of a part of Fe.

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