JP4581966B2 - Induction hardening steel - Google Patents

Description

  The present invention relates to a steel material for induction hardening. More specifically, the present invention relates to a steel material for induction hardening that can ensure fatigue strength equal to or higher than that of carburizing and quenching, in particular bending fatigue strength, by induction hardening.

  Automotive parts such as axle shafts, drive shafts, and outer races for constant velocity joints, and parts for construction machinery are subjected to surface hardening treatment to provide the desired mechanical properties after machining to a predetermined shape. As the treatment, “carburizing and quenching treatment” is often performed. And in order to perform this "carburization quenching process", in general, in addition to about 0.2% C by mass%, alloy elements such as Si, Mn, Ni, Cr and Mo are used as the material of the parts. Including low carbon alloy steels have been used.

  However, “carburization” is a process that uses the diffusion phenomenon. Therefore, in order to diffuse sufficient C in the steel material, it is necessary to perform a heat treatment for several hours to several tens of hours in an austenite region of about 800 to 1050 ° C. For this reason, when performing “carburizing and quenching” as the surface hardening treatment, it is difficult to inline the manufacturing process from molding to heat treatment, so it is difficult to increase the production efficiency, and the manufacturing cost of the parts is also high. It becomes bulky.

  Although the “gas carburizing method” is usually adopted as the carburizing process, a so-called “grain boundary oxide layer” is formed on the surface of the steel material to be processed during gas carburizing. Deterioration of strength and impact characteristics will occur.

  In order to suppress the above-mentioned grain boundary oxide layer, it is conceivable to reduce the contents of Si, Mn and Cr in the steel material, but to contain Mo, Ni, etc. instead, but a large amount of expensive alloy elements Since it will be included, the steel material cost rises.

  Furthermore, since the carburizing process is a process of heating for a long time of several hours to several tens of hours in a high temperature region of about 800 to 1050 ° C. as described above, austenite crystal grains generally referred to as “abnormal grain growth” The abnormal grain growth may cause the occurrence of heat treatment distortion after carburizing and quenching, fatigue strength, and impact characteristics.

  For this reason, application of induction hardening treatment as a surface hardening treatment instead of carburizing hardening treatment has been studied, and Patent Documents 1 to 9 propose steel materials suitable for induction hardening parts and induction hardening processes.

  In Patent Document 1, “machine structural steel” excellent in cold forgeability, induction hardenability and rolling fatigue characteristics, specifically, C: 0.40 to 0.80% by weight ratio, Si: More than 0.05 to 1.20%, Mn: less than 0.40%, S: 0.020% or less, Al: 0.01 to less than 0.05%, Cr: 0.30% or less, B: 0.0. 0003-0.0030%, Ti: 0.005-0.05%, N: 0.007% or less, O: 0.0020% or less, and Mo: 0.05- “Mechanical structural steel” excellent in cold forgeability, induction hardenability and rolling fatigue characteristics, characterized by containing 0.50% and the balance being Fe and inevitable impurities, is disclosed.

  In Patent Document 2, “high-strength induction hardening steel” excellent in machinability, specifically, the alloy element content is mass%, C: 0.50 to 0.80%, Si: 0.00. 15% or less, Mn: 0.60% or less, B: 0.0005 to 0.0050%, Ti: 0.05% or less. Further, if necessary, Cr: 2.0% or less, Ni: 3.0% or less, Mo: 1.0% or less, Nb: 0.10% or less, V: one or more elements of 0.05 to 0.50%, and / or S: It contains one or more machinability improving elements of 0.20% or less, Te: 0.20% or less, Ca: 0.0050% or less, and the balance consisting of Fe and unavoidable impurities. “High-strength induction hardening steel” having excellent machinability is disclosed.

  Patent Document 3 discloses a “steel material for induction hardening” that has excellent cold workability at the time of manufacturing parts manufactured by cold forging and cutting processes, and has excellent strength characteristics after induction hardening, and a manufacturing method thereof, specifically In weight percent, C: more than 0.38 to 0.58%, Si: 0.01 to 0.15%, Mn: 0.2 to 0.6%, S: 0.005 to 0.15 %, Cr: 0.15-0.6%, B: 0.0005-0.005%, Al: 0.015-0.05%, N: less than 0.007% (including 0%) And Ti is contained in the range of 0.015- (3.4N + 0.02)% depending on the N content, and Mo: 0.02-0.3%, Ni: 0.0. One or two of 02 to 1.0% and / or Nb: 0.002 to 0.035%, P: 0.025% or less (0% And O: 0.0025% or less (including 0%), the balance is composed of iron and inevitable impurities, and the microstructure is substantially a ferrite-pearlite structure, and the ferrite crystal grain size Is a steel material for induction hardening having both cold workability and high strength characteristics, characterized in that the ferrite band of the cross-sectional structure parallel to the hot rolling direction has a score of 1 to 5; The manufacturing method is disclosed.

  Patent Document 4 discloses a high-strength, high-frequency hardened gear steel with excellent machinability, specifically, C: 0.50 to 0.65%, Si: 0.50% or less, based on weight. , Mn: 2.0% or less, P: 0.020% or less, S: 0.010 to 0.040%, Mo: 0.05 to 0.50%, Ti + Zr + REM: 0.01% or less, B + N and / or Or Te: B: 0.0040 to 0.020%, N: 0.0050 to 0.020%, Te: 0.003 to 0.030%, and, if necessary, Cr, Ni , V, Nb, in an amount of Cr: 1.00% or less, Ni: 3.0% or less, V: 0.50% or less, Nb: 0.50% or less, based on weight. "High-frequency hardened gear steel" with excellent high-strength machinability characterized by containing and the balance being substantially made of Fe is disclosed. That.

  Patent Document 5 states that “steel for induction hardening” having high strength and excellent delayed fracture resistance and rolling fatigue life characteristics and its manufacturing method, specifically, by weight, C: 0.30-0. 80%, Si: 0.05 to 0.50%, Mn: 0.2 to 2.0%, Ti: 0.05 to 0.20%, Al: 0.010 to 0.050%, N: 0 0.120% or less, O: 12 ppm or less, and further, if necessary, Ni: 0.1-2.0%, Cr: 0.20-2.0%, Mo: 0.05-1. 1 type or 2 types or more selected from 0%, the balance Fe and inevitable impurities, Ti carbide and Ti carbonitride having a size of 70 nm or less are finely dispersed in steel High-strength, induction-hardened steel with excellent delayed fracture resistance and rolling fatigue life characteristics and its manufacturing method are disclosed To have.

In Patent Document 6, “high-strength induction-hardening steel” having excellent induction hardenability and high-strength characteristics and its manufacturing method, specifically, by mass%, C: 0.35 to 0.6% , Si: 0.01 to 1%, Mn: 0.2 to 2%, S: 0.005 to 0.15%, Cr: 0.35% or less (including 0%), B: 0.0005 It contains 0.005%, Al: 0.015-0.05%, Ti: 0.05-0.2%, and, if necessary, Mo: 1% or less, Ni: 2.5% or less V: One or more of 0.4% or less, N: less than 0.007% (including 0%), P: 0.025% (including 0%), and O: The content is limited to 0.0025% or less (including 0%), the balance is Fe and inevitable impurities, and the diameter is 0.2 μm in the matrix of the structure after hot rolling. The precipitate below the Ti has five / [mu] m ² or more, the ferrite grain size is not more 20μm or less, the ferrite fraction is 30% decarburization depth specified in JIS G 0558: DM- A “high-strength induction-hardening steel material” characterized by T 0.2 mm or less and its manufacturing method are disclosed.

  In Patent Document 7, “carbon steel for mechanical structure” having low deformation resistance, excellent cold forgeability and excellent induction hardenability, specifically, by weight percentage, C: 0.40 to 0.60. %, Si: 0.05% or less, Mn: 0.30 to 0.75%, Cr: 0.15% or less, S: 0.005 to 0.020%, and Mo: 0.05 to 0.30% with P restricted to 0.015% or less, O restricted to 0.0020% or less, and N restricted to 0.0080% or less, with the balance being substantially Fe. “Carbon steel for machine structure” that is excellent in cold forgeability and induction hardenability, characterized by having a composition, is disclosed.

  Patent Document 8 discloses that “steel for induction hardening” in which quenching cracks hardly occur and excellent torsional characteristics are obtained. Specifically, by weight ratio, C: 0.45 to 0.60%, Si: 2. 00 to 2.50%, Mn: 0.40 to 1.20%, P: 0.020% or less, S: 0.010% or less, Ni: 0.10 to 1.00%, Cr: 0.00. 20 to 0.40%, Mo: 0.20 to 0.40%, O: 0.0020% or less, N: 0.0050 to 0.0200%, V: 0.05 to 0.40%, Nb : Induction hardening steel having excellent torsional characteristics containing one or two of 0.03 to 0.40%, the balance being Fe and impurity elements is disclosed.

  Patent Document 9 discloses a “drive transmission system component” having excellent torsional characteristics, specifically, C: 0.45 to 0.60%, Si: 1.50 to 2.50%, and Mn: 0.40 to 1.20%, P: 0.020% or less, S: 0.010% or less, Ni: 0.10 to 1.00%, Cr: 0.20 to 0.40%, Mo: 0 20 to 0.40%, V: 0.05 to 0.40%, Nb: one or two of 0.03 to 0.40%, O: 0.0020% or less, N: 0.0. Disclosed is a “drive transmission system component” with excellent torsional characteristics, characterized in that induction-quenching was performed on steel containing 0050-0.0200%, the balance being Fe and impurity elements, and then tempered. ing.

JP-A-9-272946 JP-A-8-81733 JP 11-217649 A JP-A-5-271868 JP 2000-319748 A JP 2004-183065 A Japanese Patent Laid-Open No. 2-145744 JP-A-6-336646 JP-A-6-336650

  The techniques proposed in the above-mentioned Patent Documents 1 to 3 have a low Mn content, and when induction hardening is performed, incomplete quenching such as residual ferrite may occur, and always good bending fatigue strength is obtained. Was not limited.

  The technique proposed in Patent Document 4 is to positively bond B with N in steel to form BN, which is utilized for improving machinability. For this reason, Ti, Zr and REM that bind to N and inhibit the formation of BN need to be 0.01% or less in total amount, so the effect of improving the hardenability of B in induction hardening is not ensured, Incomplete quenching such as residual ferrite may occur and good bending fatigue strength may not be obtained.

  The techniques proposed in Patent Document 5 and Patent Document 6 have a large Ti content and combine with C in the steel to form many carbides, so the amount of C in the steel material is reduced and the proportion of ferrite is reduced. This increases the induction hardenability and also reduces the toughness of the induction-hardened hardened layer. For this reason, good bending fatigue strength is not always obtained.

  The technique proposed in Patent Document 7 is to subject the material to a spheroidizing heat treatment, and then forming it into a predetermined shape by cold forging and then induction hardening. For this reason, the microstructure before induction hardening is “ferrite + spherical cementite”, and even hard martensite may not be obtained by induction hardening, and good bending fatigue strength is not always obtained. .

Technique proposed in Patent Document 8 and Patent Document 9, much content of Si, causes deterioration of the high-frequency hardenability by A ₃ transformation point at the time of induction hardening is increased, further, many content of Cr Also, stabilization of the carbides in the steel causes a decrease in induction hardenability, so that a good bending fatigue strength is not always obtained.

  Therefore, the object of the present invention is to ensure fatigue strength equal to or higher than that of carburizing and quenching, especially bending fatigue strength, even when the conventional carburizing and quenching is changed to induction quenching to improve production efficiency. Another object of the present invention is to provide a “steel material for induction hardening” suitable as a material for automobile parts and construction machine parts.

  Induction hardening is performed in a rapidly heated state, unlike carburizing and quenching, in which carbon is diffused from the surface during long-time heating and hardened from a state where sufficient hardenability is ensured.

  Therefore, the present inventors first investigated various relationships with alloy elements in order to suppress incomplete quenching in the induction-quenched portion. As a result, the following findings (a) to (f) were obtained.

  (A) In the case of quenching after heating for a long period of time, the hardenability is improved by increasing the content of alloy elements such as C, Si, Mn, etc. There is an element that causes a decrease in hardenability when the content is increased.

(B) Both Si and Al are elements that increase the A ₃ transformation point as the content increases. For this reason, in the case of rapid and short-time heating such as induction hardening, a steel material having a high Si and Al content does not provide a single-phase structure of austenite, but a two-phase structure of ferrite and austenite. Even if quenching is performed, a homogeneous martensite structure cannot be obtained. Therefore, the Si and Al contents should be low.

  (C) Cr has the effect of stabilizing cementite. For this reason, Cr, which is one of the elements that increase the hardenability in the case of normal quenching, may cause a decrease in hardenability if the content is high in the case of induction hardening. Therefore, the Cr content should be low.

  (D) In order to prevent incomplete quenching during induction quenching, it is preferable to increase the contents of C, Mn, Mo, and B to utilize the effect of improving the hardenability of these elements.

  (E) In order to secure the effect of improving the hardenability of B, it is preferable to contain Ti for fixing N, but when the content is too large, the amount of C in the steel material is reduced and the proportion of ferrite Increases the induction hardenability. For this reason, it is good to adjust content of Ti to an appropriate range.

  (F) In the case where V effective for strength improvement is contained, if the amount is too large, the amount of C in the steel material decreases and the ratio of ferrite increases as in the case of Ti described above, and the induction hardenability decreases. Invite. For this reason, when it contains V, it is good to adjust the quantity to an appropriate range.

  Then, in order to suppress the incomplete quenching in the induction hardening part, the present inventors investigated various relations with the structure of the steel material before induction hardening. As a result, the following important finding (g) was obtained.

  (G) If the structure of the steel material before induction hardening is a structure in which martensite occupies 70% or more of the area fraction, induction hardening can be enhanced. As used herein, the term “martensite” refers to so-called “fresh martensite” and “martensite subjected to self-tempering” obtained by isothermal transformation and continuous cooling transformation, and those Of the “tempered martensite” obtained by tempering, it refers to a structure characterized by a “lass-like structure morphology”, and a structure in which carbides such as ε or θ are precipitated in the “lass-like structure” This includes things. Therefore, even when the above-mentioned “fresh martensite” and “martensite subjected to self-tempering” are tempered, tempering at a high temperature, for example, at a high temperature exceeding 700 ° C. When tempering is performed and the “lass-like structure” is recrystallized into equiaxed ferrite, it is not included in “tempered martensite”.

  The present inventors further conducted various investigations in order to improve the bending fatigue strength, in order to improve the static strength and fatigue strength when induction-quenched. As a result, the following findings (h) to (k) were obtained.

  (H) There is a good correlation between bending fatigue strength and static bending strength.

  (I) In order to improve the static bending strength, it is important to increase the toughness of the hardened layer in addition to increasing the hardness after making the hardened layer a uniform hard martensite structure after induction hardening. .

  (J) By optimizing the chemical composition of the steel material and making the structure before induction quenching the above-described martensite account for 70% or more of the area fraction, the hardened layer is uniformly hard martens after induction quenching. Can be a site organization.

  (K) The structure of the steel material before induction hardening affects the toughness of the hardened layer formed by induction hardening. That is, the toughness of the hardened layer after induction hardening when the structure of the steel material before induction hardening is a structure in which martensite occupies 70% or more of the area fraction by quenching or quenching and tempering is the steel material before induction hardening. This structure becomes extremely larger than the toughness of the hardened layer when it is simply subjected to normalization or spheroidizing annealing.

  Generally, hardness and toughness exhibit contradictory characteristics, and it is considered that an increase in the hardness of the cured layer and an increase in the depth of the cured layer cause a decrease in toughness. Further, both the hardness and the toughness are greatly affected by the C content of the steel material.

  Therefore, the present inventors further scrutinized contradictory toughness and hardness. As a result, the following findings (1) and (m) were obtained.

  (L) It is possible to ensure toughness without reducing hardness by refining the prior austenite crystal grain size of the hardened layer formed by induction hardening to 20 μm or less.

  (M) A steel material having an optimized chemical composition and a structure in which martensite occupies 70% or more of the area fraction before induction hardening is induction hardened with a prior austenite grain size of 20 μm by induction hardening. The following can be refined. In addition, in this case, precipitation of carbides on the prior austenite grain boundaries or lath interface generated by induction hardening is greatly suppressed, so that toughness is improved.

  In the process of processing automobile parts and construction machine parts, there is always “cutting” into a predetermined shape. For this reason, it is necessary to consider machinability also when applying induction hardening as an alternative to carburizing quenching. Therefore, the present inventors also examined a technique for improving machinability. As a result, the following findings (n) to (p) were obtained.

  (N) The machinability can be increased by reducing the content of O (oxygen) in the steel and refining the form of MnS that affects the machinability.

  (O) If a low melting point oxide is formed by containing an appropriate amount of Ca, the tool life can be increased.

  (P) If an appropriate amount of Pb, Bi or Te, which is a low melting point machinability improving element, is contained, it is melted by machining heat during cutting, and chip breaking is improved by grain boundary embrittlement. Machinability can be improved.

  This invention is completed based on said knowledge, The summary exists in the steel materials for induction hardening shown to following (1)-(4).

  (1) By mass%, C: 0.35 to 0.65%, Si: 0.50% or less, Mn: 0.65 to 2.00%, P: 0.015% or less, S: 0.003 -0.080%, Mo: 0.05-0.50%, Al: 0.10% or less, N: 0.0070% or less and O (oxygen): 0.0020% or less, the remainder is Fe And a steel material for induction hardening, characterized in that the martensite has a structure with an area fraction of 70% or more.

(2) Instead of part of Fe, Cu: 0.20% or less, Ni: 0.20% or less, Cr: 0.20% or less, Nb: 0.30% or less, and V: 0.20% or less The steel material for induction hardening as described in said (1) characterized by containing 1 type, or 2 or more types .

(3) Instead of part of Fe, one or two of Ca: 0.01% or less, Pb: 0.30% or less, Bi: 0.03% or less, and Te: 0.10% or less The steel for induction hardening as described in (1) or (2) above, which contains the above.

(4) Si content is 0.20% or less, and in place of part of Fe, B: 0.0005-0.0050% and Ti: 0.045% or less, and 3.4N The steel for induction hardening according to any one of (1) to (3) above, which contains ~ (3.4N + 0.02)% .

  As described above, “martensite” in the present specification is so-called “fresh martensite” and “martensite that has undergone self-tempering”, and is obtained by tempering them. Among the “tempered martensite” obtained, it refers to a structure characterized by a “lass-like structure form”.

  Hereinafter, the inventions relating to the steel materials for induction hardening according to the above (1) to (4) are referred to as “present invention (1)” to “present invention (4)”, respectively. Also, it may be collectively referred to as “the present invention”.

  The steel material for induction hardening according to the present invention can ensure fatigue strength equal to or higher than that in the case of carburizing and quenching, even when the conventional carburizing and quenching is changed to induction hardening to improve production efficiency. Therefore, it can be used as a material for automobile parts such as axle shafts, drive shafts, outer races for constant velocity joints, and parts for construction machinery.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the chemical component content means “mass%”.

(A) Chemical composition C: 0.35 to 0.65%
C has the effect | action which ensures the intensity | strength of steel, and the effect | action which ensures the hardened layer hardness after induction hardening. However, if the content is less than 0.35%, the desired effect due to the above action cannot be obtained. On the other hand, if the C content exceeds 0.65%, the toughness of the steel deteriorates. Therefore, the content of C is set to 0.35 to 0.65%. In addition, in order to acquire the said effect stably, it is preferable that content of C shall be 0.40 to 0.55%.

Si: 0.50% or less (in the case of the present invention (1) to the present invention (3)) or Si: 0.20% or less (in the case of the present invention (4))
Although Si is a deoxidizing element, it raises the A ₃ transformation point during induction hardening and causes induction hardenability to deteriorate. In particular, when the content exceeds 0.50%, the deoxidation effect can be expected, but the induction hardenability is significantly lowered.
Therefore, in the present invention (1) to the present invention (3), the Si content is set to 0.50% or less .
However, and the Si, when tempering the martensite structure, suppressing a decrease in hardness with the rise of the tempering temperature, has the effect of ensuring the so-called "temper softening resistance", the effect of Si If the content is 0.05% or less, it cannot be expected.
In addition, it is preferable to reduce Si content as much as possible from the point of ensuring favorable induction hardenability.
For this reason, when it is desired to provide both good induction hardenability and temper softening resistance, the Si content is preferably more than 0.05% and 0.20% or less.
For the above reason, in the present invention (4), the Si content is set to 0.20% or less.

Mn: 0.65 to 2.00%
Mn is an effective element that improves induction hardenability. However, when the content of Mn is less than 0.65%, the desired effect due to the above action cannot be obtained. On the other hand, even if it contains Mn exceeding 2.00%, the said effect will be saturated and cost will only increase. Therefore, the content of Mn is set to 0.65 to 2.00%. In order to stably obtain the above effect while keeping the alloy cost low, the content of Mn is preferably adjusted to 0.75 to 1.50%.

P: 0.015% or less P deteriorates the toughness of the hardened layer at the time of induction hardening. In particular, when the content exceeds 0.015%, the toughness of the hardened layer is significantly reduced. Therefore, the content of P is set to 0.015% or less. The P content is preferably 0.010% or less.

S: 0.003-0.080%
S combines with Mn to form MnS, and has an effect of improving machinability, in particular, chip disposal. However, if the content is less than 0.003%, the above effect cannot be obtained. On the other hand, S segregates at the grain boundaries to deteriorate the grain boundary strength, leading to a reduction in steel strength. In particular, when the S content exceeds 0.080%, the strength reduction of steel becomes significant. Therefore, the content of S is set to 0.003 to 0.080%. The S content is preferably 0.006 to 0.060%. .

Mo: 0.05 to 0.50%
Mo, like C and Mn, has the effect of improving the hardenability of the steel and improving the strength. Mo also has the effect of increasing the temper softening resistance. However, a content of 0.05% or more is necessary in order to reliably obtain the strength improvement effect and the temper softening resistance improvement effect. On the other hand, even if the Mo content exceeds 0.50%, the above effect is saturated and the cost is increased. Therefore, the Mo content is set to 0.05 to 0.50%. The Mo content is preferably 0.10 to 0.30%.

Al: 0.10% or less Al, like Si, albeit at a deoxidizing element, raise the A ₃ transformation point at the time of induction hardening, lowering the induction hardening properties. In particular, when the content exceeds 0.10%, the deoxidation effect and the effect of preventing grain coarsening during induction hardening of AlN formed by combining Al with N in steel can be expected. However, the induction hardenability is significantly reduced. Therefore, the Al content is set to 0.10% or less. In addition, it is preferable to reduce Al content as much as possible from the point of ensuring favorable induction hardenability.

N: 0.0070% or less N has a large affinity with B, Al, Ti and the like, and when it is combined with B in steel to form BN, induction hardenability cannot be improved. In particular, when the N content increases and exceeds 0.0070%, the effect of preventing the coarsening of grains during induction hardening of AlN or TiN can be expected, but the effect of improving the induction hardenability by forming BN cannot be expected. Moreover, when the amount of solute N in steel increases, the hot deformability is lowered. Therefore, the N content is set to 0.0070% or less. Note that the content of N as an impurity in the steel is preferably reduced as much as possible.

O (oxygen): 0.0020% or less O combines with the elements in steel to form oxides, leading to a decrease in strength, particularly a decrease in fatigue strength. In particular, when the O content exceeds 0.0020%, the amount of oxide formed increases and MnS coarsens, resulting in a significant decrease in fatigue strength. Therefore, the content of O as an impurity element is set to 0.0020% or less. The O content is preferably 0.0015% or less.

  For the above reason, the chemical composition of the steel for induction hardening according to the present invention (1) is defined to contain elements from C to O (oxygen) in the above-mentioned range, with the balance being Fe and impurities.

In addition, in the steel for induction hardening according to the present invention, if necessary, instead of a part of Fe,
First group: B: 0.0005 to 0.0050% and Ti: 0.045% or less, and 3.4N to (3.4N + 0.02)%,
Second group: Cu: 0.20% or less, Ni: 0.20% or less, Cr: 0.20% or less, Nb: 0.30% or less, and V: 0.20% or less More than species,
Third group: Ca: 0.01% or less, Pb: 0.30% or less, Bi: 0.03% or less, and Te: one or more of 0.10% or less,
One or more elements of each group can be contained. That is, one or more elements of the three groups of the first group to the third group may be contained as an optional additive element instead of a part of Fe.

  Hereinafter, the above optional additive elements will be described.

First group: B: 0.0005 to 0.0050% and Ti: 0.045% or less, and 3.4N to (3.4N + 0.02)%
B has the effect | action which improves induction hardenability and the effect is remarkable when content of B is 0.0005% or more. However, even if B is contained in excess of 0.0050%, the above effect is saturated and the cost is increased. Therefore, the content of B is set to 0.0005 to 0.0050%. The B content is preferably 0.0010 to 0.0020%.

  Even when B is contained in an amount in the above range, induction hardenability cannot be improved when BN is formed by combining B with N present as an impurity in steel. Therefore, as already described, in order to exhibit the effect of improving the induction hardenability of B, it is necessary to reduce N present as an impurity in the steel.

  Ti is an element effective in suppressing the formation of BN by preferentially bonding with N present as an impurity in steel and ensuring the effect of improving the induction hardenability of B. In order to obtain this effect, it is necessary to contain 3.4 N% or more of Ti. However, when the Ti content is too high, it combines with C in the steel to form carbides, so the amount of C in the steel decreases and the proportion of ferrite increases, on the other hand, induction hardenability decreases. In addition, the toughness of the induction-hardened hardened layer is also reduced. In particular, when the Ti content increases and exceeds 0.045%, the induction hardenability and the toughness of the hardened layer are significantly reduced. And even if content of Ti is 0.045% or less, when it exceeds (3.4N + 0.02)%, induction hardenability and the toughness of a hardened layer will be reduced. Therefore, the Ti content is set to 0.045% or less and 3.4N to (3.4N + 0.02)%.

Second group: Cu: 0.20% or less, Ni: 0.20% or less, Cr: 0.20% or less, Nb: 0.30% or less, and V: 0.20% or less Cu: 0.20% or less Cu has the effect | action which improves the intensity | strength of steel. That is, Cu has the effect | action which raises the hardenability of steel like C and Mn, and improves intensity | strength. However, if the Cu content exceeds 0.20%, the hardenability is improved, but the fatigue strength may be lowered. Therefore, the Cu content is set to 0.20% or less. The Cu content is preferably 0.008 to 0.015%.

Ni: 0.20% or less Ni has the effect | action which improves the intensity | strength of steel. That is, Ni has the effect | action which raises the hardenability of steel like C and Mn, and improves intensity | strength. However, if the Ni content exceeds 0.20%, the hardenability is improved, but quenching may occur during induction hardening. Therefore, the Ni content is set to 0.20% or less. Note that the Ni content is preferably 0.008 to 0.015%.

Nb: 0.30% or less Nb has the effect | action which improves the intensity | strength of steel. That is, Nb has the effect | action which forms carbide | carbonized_material, nitride, or carbonitride, refines | miniaturizes a crystal grain, and raises the intensity | strength of steel. However, even if Nb is contained in excess of 0.30%, the above effects are saturated and the cost is increased. Therefore, the Nb content is set to 0.30% or less. Note that the Nb content is preferably 0.01 to 0.30%.

V: 0.20% or less V has an effect of improving the strength of steel. That is, V, like C and Mn, has the effect of enhancing the hardenability of the steel, forming carbonitrides to refine crystal grains, and improving the strength. However, if the content of V exceeds 0.20%, carbonitride precipitates excessively, causing toughness deterioration of the hardened layer induction-hardened, leading to a decrease in fatigue life, and bonding with C in the steel. As a result, a large amount of carbide is formed, so that the amount of C in the steel material is reduced and the proportion of ferrite is increased, which causes a decrease in induction hardenability. Therefore, the content of V is set to 0.20% or less. The V content is preferably 0.01 to 0.20%.

  In addition, said Cu, Ni, Nb, and V can be contained only in one of them, or 2 or more types of composites.

Third group: Ca: 0.01% or less, Pb: 0.30% or less, Bi: 0.03% or less and Te: 0.10% or less Ca: 0.01% or less Ca enhances machinability. There is an effect. That is, Ca has a function of forming a low melting point oxide effective for improving the tool life and improving the machinability. Ca also has a deoxidizing action. However, if the Ca content exceeds 0.01%, the yield of Ca addition is low, and thus the production cost increases, and on the contrary, the formation of a low melting point oxide becomes difficult. Furthermore, since MnS that dissolves Ca increases and coarse MnS is easily formed, even if machinability can be ensured, the fatigue strength is reduced. Therefore, the Ca content is set to 0.01% or less. In addition, in order to acquire the machinability improvement effect more stably without reducing fatigue strength, the Ca content is preferably set to 0.0005 to 0.005%.

Pb: 0.30% or less Pb has an effect of improving machinability. However, if its content exceeds 0.30%, the above-mentioned effects are saturated and the economic efficiency is impaired, and the bending fatigue strength is deteriorated. Furthermore, the hot workability is deteriorated, and cracks are generated during hot rolling or hot forging. Therefore, the Pb content is set to 0.30% or less. In addition, when cutting an automotive part or a construction machine part into a predetermined shape, in order to surely obtain an effect of improving the machinability of Pb, the content thereof should be 0.02% or more. preferable. Therefore, the more preferable content of Pb is 0.02 to 0.30%.

Bi: 0.03% or less Bi has an effect of improving machinability. However, if the content exceeds 0.03%, the above effects are saturated and the cost is increased, and the bending fatigue strength is deteriorated. Furthermore, the hot workability is deteriorated, and cracks are generated during hot rolling or hot forging. Therefore, the Bi content is set to 0.03% or less. In order to reliably obtain the effect of improving the machinability of Bi when cutting into a predetermined part shape, the content is preferably set to 0.005% or more. Therefore, the more preferable Bi content is 0.005 to 0.03%.

Te: 0.10% or less Te has an effect of improving machinability. However, if its content exceeds 0.10%, the above-mentioned effects are saturated and the economy is impaired, and the bending fatigue strength is deteriorated. Furthermore, the hot workability is deteriorated, and cracks are generated during hot rolling or hot forging. Therefore, the Te content is set to 0.10% or less. In addition, when cutting into a predetermined part shape, the content is preferably set to 0.01% or more in order to surely obtain an effect of improving the machinability of Te. Therefore, the more preferable Te content is 0.01 to 0.10%.

  In addition, said Ca, Pb, Bi, and Te can be contained only in any 1 type in them, or 2 or more types of composites.

For the above reason, the chemical composition of 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), Cu: 0.20% or less, Ni: It was specified to contain one or more of 0.20% or less, Cr: 0.20% or less, Nb: 0.30% or less, and V: 0.20% or less .

Further, the chemical composition of the steel for induction hardening according to the present invention (3) is replaced with a part of Fe of the steel for induction hardening in the present invention (1) or the present invention (2), Ca: 0.01% or less , Pb: 0.30% or less, Bi: 0.03% or less and Te: 0.10% or less .

Furthermore, the chemical composition for induction hardening steel according to the present invention (4), in place of part of Fe of the steel product for induction hardening in any one of the present invention (1) to the present invention (3), B: 0 It is specified that the content of .0005 to 0.0050% and Ti: 0.045% or less and 3.4N to (3.4N + 0.02)% .

(B) Structure The structure of the steel for induction hardening according to the present invention having the chemical composition described in the above section (A) must be a structure in which martensite occupies 70% or more in area fraction.

  This is because, in addition to the chemical composition, the steel structure is the above structure, incomplete quenching during induction hardening is suppressed, and the hardened layer formed by induction hardening becomes a uniform hard martensite structure, The hardness of the hardened layer can be increased, and further, the prior austenite crystal grain size of the hardened layer after induction hardening is refined to 20 μm or less, and the carbide to the prior austenite grain boundary or lath interface generated by induction hardening. This is because the toughness of the hardened layer can be ensured and good static bending strength, and hence good bending fatigue strength can be ensured.

  In addition, the “martensite” referred to in the present specification is the so-called “fresh martensite” and “martensite subjected to self-tempering”, and “tempered martensite” obtained by tempering them. Among them, it refers to a structure characterized by a “lass-like structure form”, and the “lass-like structure” includes a structure in which carbides such as ε or θ are precipitated as described above.

  If the martensite occupies an area fraction of 85% or more, incomplete quenching during induction hardening is more stably suppressed, and the toughness of the hardened layer is improved. For this reason, it is preferable that the structure of the steel for induction hardening according to the present invention having the chemical composition described in the above section (A) is a structure in which martensite occupies 85% or more of the area fraction. The structure of the steel for induction hardening may be a structure in which martensite occupies 100% of the area fraction, that is, a martensite single-phase structure.

  As a method for adjusting the structure of the steel material before induction hardening, for example, after processing into a predetermined shape of an automobile part or a construction machine part, heat treatment may be performed before induction hardening. As heat treatment conditions, for example, after heating to a temperature range of 850 to 1200 ° C., water cooling or oil cooling may be performed. Moreover, after water cooling or oil cooling, you may cool, after hold | maintaining for 15 minutes or more in the temperature range of 150-700 degreeC.

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

  Steels A1 to A18 and steels B1 to B5 having the chemical composition shown in Table 1 were melted in a vacuum furnace to produce a 150 kg steel ingot.

  Steels A1 to A18 in Table 1 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, steel B1-B5 is steel of the comparative example from which the chemical composition remove | deviated from the conditions prescribed | regulated by this invention.

  The steel ingot thus obtained was heated to 1200 to 1300 ° C. and then hot forged into a round bar having a diameter of 30 mm. The cooling after hot forging was allowed to cool in the atmosphere.

  Next, the above round bar with a diameter of 30 mm obtained by hot forging was heated to 900 ° C. for 30 minutes to form an austenite single phase structure, and then cooled or cooled at the cooling rate shown in Table 2 and further tempered. The structure of the round bar before induction hardening was changed.

  The microstructure was investigated using a round bar with a diameter of 30 mm subjected to the above heat treatment. In other words, the heat-treated round bar was cut so that the state of the cross section could be observed, embedded in the resin, mirror-polished, then corroded with nital to reveal the microstructure, and the martensite was developed using a scanning electron microscope. The site was discriminated and the area fraction of the part discriminated as martensite was measured.

  To determine the martensite structure, it is difficult to determine the lath structure or carbide precipitation with a scanning electron microscope by confirming whether or not the characteristic “laser-like structure morphology” is exhibited. In such cases, martensite was also determined using a transmission electron microscope.

  Even when tempering so-called “fresh martensite” and “martensite that has undergone self-tempering”, tempering at a high temperature, for example, tempering at a high temperature exceeding 700 ° C. is performed. As described above, when the “lass-like structure” is recrystallized into equiaxed ferrite, it is not called “tempered martensite”.

  In addition, the scanning electron micrograph of magnification 2000 times taken arbitrarily 10 times was image-processed, and the area fraction of the martensite was calculated from the area fraction of the lath-like martensite structure discriminated as described above.

  In addition, a three-point bend in which a semicircular cutout having a radius of 2 mm is provided on one surface of a longitudinal central portion of a rectangular parallelepiped having a cross section of 10 mm × 10 mm and a length of 30 mm from a round bar having a diameter of 30 mm subjected to the heat treatment. The test piece was cut out by machining. Next, the surface provided with the semicircle notch of the above three-point bending test piece was subjected to induction hardening in which the surface was heated for 3.0 seconds under the conditions of a frequency of 20 kHz and an output of 50 kW, and then water-cooled. The prior austenite grain size was investigated.

  Induction hardening is embedded in resin so that the cross section at the semicircle notch of the three-point bending test piece after induction hardening can be examined, and the Vickers hardness test is performed by the method specified in JIS G 2244 (2003). And evaluated. That is, the hardness is obtained by performing a Vickers hardness test with a test force of 2.94 N, and the distance (mm) from the notch bottom to a position where the Vickers hardness (hereinafter referred to as “HV hardness”) is 500. ) In the following, the distance from the notch bottom to the position where the HV hardness is 500 is referred to as “ECD”.

  The target of induction hardenability is that ECD is 0.85 mm or more.

  The prior austenite grain size of the hardened layer was corroded with resin so that the cross section of the three-point bend specimen after induction hardening can be observed, mirror-polished, and then corroded with a saturated aqueous solution of picric acid to which a surfactant was added. Using an optical micrograph of magnification 400 times taken from 5 fields of view to reveal the prior austenite grain boundaries, the “average section length” L determined by the cutting method was determined, and “Heat Treatment, Vol. 24, No. 6” was obtained. (1984) ”,“ 1.128 × L ”was used as the prior austenite grain size.

  The target is the prior austenite grain size of 20 μm or less.

  Further, the three-point bending test piece induction-hardened under the above conditions was tempered at 180 ° C. for 1 hour to investigate the static bending strength and bending fatigue strength.

  The static bending strength test was performed at a fulcrum distance of 45 mm and a strain rate at the bottom of the notch of the test piece of 0.01 / second, and the static bending strength was calculated from the maximum ultimate load. The target is to have a static bending strength of 1800 MPa or more.

The bending fatigue strength test was also performed at a fulcrum distance of 45 mm, and the crack initiation strength at the number of repetitions of 1 × 10 ⁴ was evaluated as the bending fatigue strength. The goal is to have a bending fatigue strength of 950 MPa or more.

  Tables 2 and 3 show the results of the above tests together.

  From Table 2 and Table 3, in the case of Test Nos. 1 to 18 that satisfy the conditions specified in the present invention (1) to (4), it is excellent in induction hardenability and has good bending fatigue strength. it is obvious.

  On the other hand, it is clear that the bending fatigue strengths of Test Nos. 19 to 28 deviating from the conditions defined in the present invention are low, and the induction hardenability may be inferior.

  In addition, after subjecting the SCr420 steel material separately described in JIS G 4053 (2003) to a normalizing treatment that is heated at 925 ° C. for 1 hour and then air-cooled, a three-point bending test piece having the above-described shape was collected, and The three-point bending test piece was held at 940 ° C. for 3 hours under a carbon potential of 0.8%, then cooled to 870 ° C., held for another hour, and then carburized and quenched under oil quenching conditions. After tempering at 180 ° C. for 1 hour, static bending strength and bending fatigue strength were investigated under the same conditions as described above. As a result, carburized and quenched SCr420 had a static bending strength of 1690 MPa and a bending fatigue strength of 700 MPa. Therefore, it was confirmed that the bending fatigue strengths of Test Nos. 1 to 18 satisfying the conditions defined in the present inventions (1) to (4) have a bending fatigue strength equal to or higher than that of the carburized and quenched material.

  Further, a turning test was performed using a round bar having a diameter of 30 mm obtained by hot forging of steels A1 to A18, and the amount of tool wear was measured to investigate machinability. That is, each round bar with a diameter of 30 mm as hot forged was turned for 10 minutes under the following conditions using a P20 type carbide tool with TiN coating treatment, and the average flank wear amount of the carbide tool was determined. Measurement was made and the machinability was evaluated using this as the amount of tool wear.

・ Cutting speed: 120 m / min,
・ Incision amount: 1.5 mm,
-Feed amount: 0.40 mm / rev. ,
・ Lubrication: Wet (uses water-soluble lubricant).
As a result, the tool wear amounts of steels A1 to A18 were all 100 μm or less, and it was confirmed that all the steels had good machinability.

The steel material for induction hardening according to the present invention can ensure fatigue strength equal to or higher than that of conventional carburizing and quenching, in particular bending fatigue strength. Therefore, automobiles such as axle shafts, drive shafts, outer races for constant velocity joints, etc. It can be used as a material for parts and construction machine parts.

Claims (4)

  In mass%, C: 0.35-0.65%, Si: 0.50% or less, Mn: 0.65-2.00%, P: 0.015% or less, S: 0.003-0. 080%, Mo: 0.05 to 0.50%, Al: 0.10% or less, N: 0.0070% or less, and O (oxygen): 0.0020% or less, with the balance being Fe and impurities Furthermore, the steel material for induction hardening characterized by martensite being the structure | tissue which occupies 70% or more in an area fraction. In place of a part of Fe, Cu: 0.20% or less, Ni: 0.20% or less, Cr: 0.20% or less, Nb: 0.30% or less, and V: 0.20% or less The steel material for induction hardening according to claim 1, comprising one kind or two or more kinds . Instead of a part of Fe, Ca: 0.01% or less, Pb: 0.30% or less, Bi: 0.03% or less, and Te: 0.10% or less , containing one or more The steel material for induction hardening according to claim 1, wherein the steel material is induction hardened. The content of Si is 0.20% or less. Further, in place of part of Fe, B: 0.0005 to 0.0050% and Ti: 0.045% or less, and 3.4 N to (3 The steel material for induction hardening according to any one of claims 1 to 3, characterized by containing .4N + 0.02)% .