JP4488228B2 - Induction hardening steel - Google Patents

Description

  The present invention relates to a steel material for induction hardening. Specifically, steel material for induction hardening that has excellent machinability (hereinafter also referred to as “machinability”) and can ensure fatigue strength equal to or higher than that of carburizing and quenching by induction hardening, especially bending fatigue strength. About.

  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 is studied, and Patent Documents 1 to 6 propose steel materials suitable for induction hardening parts and induction hardening processes.

  Patent Document 1 states that “high-frequency quenching that secures the static strength of parts of automobile power transmission systems such as axle shafts, drive shafts, outer races for constant velocity joints, and is excellent in impact bending characteristics and impact torsional resistance characteristics. Parts ", specifically, by weight, C: 0.30-0.60%, Si: 0.50% or less, Mn: 0.20-1.50%, B: 0.0005-0. 0050%, N: 0.015% or less, Ti: 0.10% or less, and if necessary, Cr: 1.0% or less, Mo: 0.5% or less and Ni: 1.0 % Or less, and / or Pb: 0.20% or less, S: 0.10% or less, Bi: 0.20% or less, Te: 0.10% or less, and Ca: 0.0. Containing one or more of 01% or less, the balance consisting of Fe and impurities, It is a uniform martensite structure with a surface hardness after induction hardening of 50 HRC or more and a martensite ratio of the induction hardening structure of 90% or more, and a hardening depth ratio t (effective hardening depth) / r ( An “induction-hardened component” having a component radius or component thickness of 0.2 to 0.7 is disclosed.

  Patent Document 2 discloses a “steel material for induction hardening” that is excellent in machinability and that can secure characteristics equal to or higher than those of, for example, gears manufactured through a carburizing process. : 0.5-0.75%, Si: 0.5-1.8%, Mn: 0.1-0.4%, P: 0.015% or less, S: 0.020% or less, Al: 0.019 to 0.05%, O: 0.0015% or less, N: 0.003 to 0.015%, and Mo: 0.05 to 0.5%, B as necessary. : 0.0003 to 0.005%, Ti: 0.005 to 0.05%, Ni: 0.1 or more elements of the four elements consisting of 0.1 to 1.0%, and / or V: contains 0.05 to 0.5%, Nb: 0.01 to 0.5% of one or more elements, the balance being Fe and unavoidable Characterized by comprising the object "induction hardening steel" is disclosed.

  In Patent Document 3, even if the P content is high, the “steel material for gears for induction hardening” having excellent fatigue strength, impact characteristics and machinability and its manufacturing method, specifically, by mass%, C : 0.5 to 0.75%, Si: 0.5 to 1.8%, Mn: 0.1 to 0.4%, P: more than 0.015 to 0.03%, S: 0.020% Hereinafter, Al: 0.019 to 0.05%, O: 0.0015% or less, N: more than 0.015 to 0.02%, and Mo: 0.05 to 0 as necessary. 0.5%, B: 0.0003 to 0.005%, Ti: 0.005 to 0.05%, Ni: 0.1 to 1.0%, and / or V: It contains at least one selected from 0.05 to 0.5% and Nb: 0.01 to 0.5%, and the balance is composed of Fe and inevitable impurities. That "induction hardening of steel gear" and a manufacturing method thereof are disclosed.

  Patent Document 4 discloses that an “induction-hardened part” having excellent static strength and fatigue resistance, which can be applied to a drive transmission part for an automobile or the like, and a manufacturing method thereof, specifically, by weight%, C: 0.00. 40 to 0.70%, Si: 0.05 to 0.80%, Mn: 0.50 to 2.00%, S: 0.01 to 0.03%, V: 0.30 to 1.00% , Al: 0.010 to 0.050%, N: 0.0050 to 0.0200%, Nb: 0.05 to 0.50%, and / or Cr: 0.1 to 1.50% is contained, P is 0.030% or less, the balance is made of Fe and inevitable impurities, and the martensite grain size of the induction-hardened hardened layer is 14 or more in JIS grain size number. Disclosed is an induction-hardened component with excellent static strength and fatigue resistance, and its manufacturing method. There.

In Patent Document 5, “high-strength induction-quenching steel” that is excellent in induction hardenability and excellent in high-strength characteristics and its manufacturing method, specifically, in 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 6, “high-strength, high-toughness non-refined steel bar” having both high strength and toughness and excellent balance between strength and toughness, and its manufacturing method, specifically, by mass%, C: 0.00. 35 to 0.50%, Si: 0.1 to 0.6%, Mn: 0.5 to 1.5%, Al: 0.005 to 0.02%, V: 0.05 to 0.50% And, if necessary, Cr: 0.60% or less, Mo: 0.5% or less, Ni: 1% or less, Cu: 1% or less, Ti: 0.2% or less, Nb: 0. 1 type or 2 types or more of 10% or less, the remainder consists of Fe and unavoidable impurities, the ferrite fraction in the steel structure is 20 to 40%, the average lamellar spacing of pearlite is 0.05 to 0. A ferrite particle having an average grain size of 2 to 10 μm surrounded by large-angle grain boundaries having a crystal orientation difference of 15 ° or more is 20 μm. Characterized by comprising the light duplex structure "high strength and high toughness non-heat treated steel bar" and a manufacturing method thereof are disclosed.

JP-A-10-36937 Japanese Patent Laid-Open No. 9-41085 JP-A-9-241798 JP-A-11-71633 JP 2004-183065 A JP 2005-133155 A

  The technique proposed in Patent Document 1 described above is induction-hardened only by specifying the surface hardness and hardening depth ratio of the induction-hardened part and that the induction-hardened part of the surface layer part is a uniform martensite structure. No consideration is given to the crystal grain size of the hardened surface layer or the hardness of the portion that was not affected by heat during induction hardening. For this reason, although the impact characteristics of the induction-hardened parts are improved, it is not always possible to obtain good bending fatigue strength.

The technique proposed in Patent Document 2 improves the temper softening resistance of the steel material by setting the lower limit of the Si content to 0.5% by mass, and the upper limit of the Mn content to 0.4% by mass. %, The layering of pearlite is suppressed, and a structure in which cementite is divided is formed to improve machinability. However, since Si raises the A ₃ transformation point, a single-phase structure of austenite cannot be obtained during high-frequency heating, and a two-phase structure of ferrite and austenite is obtained, so that homogeneous martensite cannot be obtained even when quenched. In some cases, soft ferrite remains and the induction hardenability decreases, leading to a decrease in fatigue strength. In addition, reducing the Mn content also increases ferrite in the steel structure before induction hardening, and an austenite single phase structure cannot be obtained in the heating stage during induction hardening, and ferrite and austenite Since it has a two-phase structure, homogeneous martensite cannot be obtained even after quenching, and soft ferrite remains, so even if machinability is improved, good bending fatigue strength is not necessarily obtained. .

  In the technique proposed in Patent Document 3, the lower limit of the Si content is 0.5% by mass, and the temper softening resistance of the steel material is improved by 0.5%, and the upper limit of the Mn content is 0.4% by mass. %, The layering of pearlite is suppressed, and a structure in which cementite is divided is formed to improve machinability. For this reason, as in the case of Patent Document 2, it is difficult to obtain homogeneous martensite after induction hardening, and bending fatigue strength as member strength is reduced. Moreover, even if the austenite grain size is 16 μm or less, it is necessary to contain P in excess of 0.015% by mass. Therefore, a decrease in bending fatigue strength due to P segregated at the grain boundaries cannot be avoided. . And in order to make an austenite particle size 16 micrometers or less, it is necessary to contain N exceeding 0.015% by mass%, but when N content in steel materials increases, hot rolling, hot forging, etc. In the hot working process, the deformability decreases, and after hot working, the surface of the steel material is increased in defects such as cracks.

  The technique proposed in Patent Document 4 is a non-recrystallization temperature range (600 to 800 ° C.) in order to obtain a martensite crystal grain size of the hardened layer obtained by induction quenching of 14 or more (grain size of 3.1 μm or less). Therefore, it is necessary to perform a large strain of 30% or more twice or more. For this reason, for example, when forging, the deformation resistance of the material is increased, the load on the forging die is increased, the die life is reduced, and the manufacturing cost is increased. Furthermore, in order to obtain a microstructure, it is necessary to contain 0.3% or more of V by mass%, and not only an increase in alloy cost is unavoidable, but also carbonitride precipitates excessively, so that hardening occurs. It causes toughness deterioration of the layer and leads to a decrease in bending fatigue strength.

The technique proposed in Patent Document 5 needs to perform so-called “HCR (Hot Charge Rolling)” in which a steel is cast into pieces without being cooled to A ₃ point temperature or less after casting in order to obtain a fine structure during induction hardening. Therefore, capital investment is required and the manufacturing cost increases. Also, since rolling steel bars and wire rods are premised on low-temperature heating rolling, the deformation resistance of the steel materials increases, increasing the load on the rolling roll, resulting in a decrease in roll life, resulting in a reduction in manufacturing costs. Incurs an increase. Furthermore, in order to refine the prior austenite grain size of the hardened layer by induction hardening, 0.05% or more of Ti is contained in mass%, and a large amount of TiC-based precipitates are refined before induction hardening. Since it is necessary to disperse, the toughness of the induction-hardened hardened layer is lowered, and good bending fatigue strength cannot always be obtained.

  When induction hardening is performed using “high-strength, high-toughness, non-tempered steel bar” with a ferrite fraction of 20 to 40% proposed in Patent Document 6, incomplete quenching such as residual ferrite may occur and is not necessarily good. Bending fatigue strength cannot be obtained.

  Therefore, the object of the present invention is excellent in machinability, and even when the conventional carburizing quenching is changed to induction quenching to improve the production efficiency, the fatigue strength is equal to or higher than that of the carburizing quenching, in particular, the bending fatigue strength. It is possible to provide "steel for induction hardening parts" that can be secured and is 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.

  Therefore, the relationship between the incompletely quenched structure and the alloy elements during induction hardening was investigated in more detail. As a result, the following knowledge (g) was obtained.

  (G) In order to suppress incomplete quenching at the time of induction quenching, when V is not included, the value of the formula represented by “C + Mn + 2Mo-2Si-9Cr-2 (Ti-3.4N) -6Al” When V is included, the value of the expression represented by “C + Mn + 2Mo-2Si-9Cr-2 (Ti-3.4N) -6Al-4V” may be adjusted to an appropriate range.

  Furthermore, the present inventors have investigated various relations with the structure of the steel material before induction hardening in order to suppress incomplete hardening in the induction hardening portion. As a result, the following important knowledge (h) was obtained.

  (H) The structure of the steel material before induction hardening is a mixed structure of ferrite and pearlite (hereinafter also referred to as “ferrite / pearlite structure”) having a ferrite fraction of 15% or less and a pearlite lamellar spacing of 1 μm or less. If it is, induction hardenability can be improved.

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

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

  (J) 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. .

  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 (k) and (l) were obtained.

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

  (L) A steel material with an optimized chemical composition and a ferrite-pearlite structure with a pearlite lamellar spacing of 1 μm or less before induction hardening has increased austenite-forming nuclei during austenite transformation, and induction hardening, The prior austenite crystal grain size of the hardened layer can be reduced to 20 μm or less.

  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 that is greatly influenced by the form of inclusions. As a result, the following findings (m) and (n) were obtained.

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

  (N) The tool life can be increased if a low melting point oxide is formed by containing an appropriate amount of Ca.

  This invention is completed based on said knowledge, The summary exists in the steel materials for induction hardening shown to following (1)-(3).

(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: 0.010 -0.080%, Cr: 0.01-0.5%, Mo: 0.05-0.5%, B: 0.0005-0.0050%, Al: 0.10% or less, N: 0 0.150% or less, O (oxygen): 0.0020% or less, and Ti: 3.4N to (3.4N + 0.05)%, the balance being made of Fe and impurities, expressed by the following formula (1) Fn1 value satisfies 1.0 or more, and the structure is a mixed structure of ferrite and pearlite having a ferrite fraction of 15% or less and a pearlite lamellar spacing of 1 μm or less. Steel material.
fn1 = C + Mn + 2Mo-2Si-9Cr-2 (Ti-3.4N) -6Al (1)
However, the element symbol in the formula (1) represents the content in steel in mass% of each element.

(2) 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: 0.010 -0.080%, Cr: 0.01-0.5%, Mo: 0.05-0.5%, B: 0.0005-0.0050%, Al: 0.10% or less, N: 0 0.150% or less, O (oxygen): 0.0020% or less, and Ti: 3.4N to (3.4N + 0.05)%, Cu: 0.20% or less, Ni: 0.20% or less Nb: 0.30% or less and V: 0.20% or less, and the balance is Fe and impurities, and the value of fn2 represented by the following formula (2) is 1.0 or more, and the structure is a ferrite having a ferrite fraction of 15% or less and a pearlite lamellar spacing of 1 μm or less. Induction hardening steel material characterized in that it is a mixed structure of Raito.
fn2 = C + Mn + 2Mo-2Si-9Cr-2 (Ti-3.4N) -6Al-4V (2)
However, the element symbol in the formula (2) represents the steel content in mass% of each element.

  (3) The steel material for induction hardening as described in (1) or (2) above, which contains Ca: 0.01% or less instead of part of Fe.

  Hereinafter, the inventions relating to the steel materials for induction hardening according to the above (1) to (3) are referred to as “present invention (1)” to “present invention (3)”, respectively. Also, it may be collectively referred to as “the present invention”.

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

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of the chemical component 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 0.45-0.55%.

Si: 0.50% or less Si, albeit with 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 0.50%, the deoxidation effect can be expected, but the induction hardenability is significantly lowered. Therefore, the Si content is set to 0.50% or less. In addition, it is preferable to reduce Si content as much as possible from the point of ensuring favorable induction hardenability.

Mn: 0.65 to 2.00%
Mn is an effective element that improves induction hardenability and also an element that suppresses the formation of ferrite. 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 obtain the above effect stably 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: 0.010 to 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 0.010%, 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 0.010 to 0.080%. In addition, it is preferable that content of S shall be 0.012-0.06%.

Cr: 0.01 to 0.5%
Cr, like C and Mn, has the effect of improving the hardenability of steel and improving the strength. In the case of induction hardening, in order to reliably obtain the effect of improving hardenability, the content of 0.01% or more There is a need to. However, if the Cr content exceeds 0.5%, the hardenability deteriorates in the case of induction hardening. Further, the hot workability is deteriorated. Therefore, the Cr content is set to 0.01 to 0.5%. The Cr content is preferably 0.08 to 0.2%.

Mo: 0.05-0.5%
Mo, like C and Mn, has the effect of increasing the hardenability of the steel and improving the strength. However, a content of 0.05% or more is necessary to reliably obtain the strength improvement effect. On the other hand, even if Mo is contained in excess of 0.5%, the above effect is saturated and the cost is increased. Therefore, the Mo content is set to 0.05 to 0.5%. The Mo content is preferably 0.10 to 0.3%.

B: 0.0005 to 0.0050%
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, 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 as much as possible. This will be described later.

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: 0.0150% 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 0.0150%, the effect of preventing the coarsening of crystal 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. Therefore, the content of N is set to 0.0150% 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.

Ti: 3.4N to (3.4N + 0.05)%
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 (3.4N + 0.05)%, the induction hardenability and the toughness of the hardened layer are significantly reduced. Therefore, the Ti content is determined to be 3.4N to (3.4N + 0.05)%.

fn1 value: 1.0 or more By setting the value of fn1 represented by the formula (1) to 1.0 or more, imperfection when induction-hardening the steel material according to the present invention (1) that does not contain V Quenching can be suppressed.

  For the above reason, the chemical composition of the steel for induction hardening according to the present invention (1) contains elements from C to Ti in the above-mentioned range, and the balance is composed of Fe and impurities. It was defined that the value of fn1 expressed satisfies 1.0 or more.

  In addition, the chemical composition of the steel for induction hardening according to the present invention (2) contains elements from C to Ti in the above-described range, Cu: 0.20% or less, Ni: 0.20% or less, One or more of Nb: 0.30% or less and V: 0.20% or less are contained, and the value of fn2 represented by the formula (2) satisfies 1.0 or more. . Hereinafter, the elements from Cu to V and the value of fn2 represented by the above formula (2) will be described.

Cu: 0.20% or less Cu has an effect of improving the 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.

fn2 value: 1.0 or more Incomplete quenching when the steel material according to the present invention (2) containing V is induction-quenched by setting the value of fn1 represented by the formula (2) to 1.0 or more. Can be suppressed.

  For the above reason, the chemical composition of the steel for induction hardening according to the present invention (2) contains elements from C to Ti in the above-mentioned range, Cu: 0.20% or less, Ni: 0.20% Hereinafter, it contains one or more of Nb: 0.30% or less and V: 0.20% or less, and the value of fn2 represented by the formula (2) satisfies 1.0 or more. Stipulated.

  In addition, the chemical composition of the steel for induction hardening according to the present invention may optionally contain an amount of Ca described later instead of a part of Fe of the present invention (1) or the present invention (2) as necessary. May be good. This will be described below.

Ca: 0.01% or less Ca is a deoxidizing element and forms a low-melting-point oxide effective for improving the tool life. 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%.

  For the above reasons, 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 according to the present invention (1) or the present invention (2). It was specified that the content was 01% or less.

(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 ferrite / pearlite structure having a ferrite fraction of 15% or less and a pearlite lamellar spacing of 1 μm or less. I must. 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 by forming a dense pearlite structure with a lamellar spacing of 1 μm or less, the number of austenite transformation nuclei increases, and the old austenite crystal grain size of the hardened layer after induction hardening is increased. This is because the toughness of the cured layer can be ensured because it is miniaturized to 20 μm or less, and good static bending strength, and hence good bending fatigue strength can be ensured.

  When the ferrite fraction is 5% or less and the pearlite lamellar spacing is 0.5 μm or less, incomplete quenching during induction hardening is more stably suppressed. Therefore, 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 ferrite pearlite structure having a ferrite fraction of 5% or less and a pearlite lamellar spacing of 0.5 μm or less. Preferably there is.

  Further, the lower limit of the ferrite fraction is better from the viewpoint of suppressing incomplete quenching during induction quenching, and the lower limit of the ferrite fraction that forms the entire pearlite single phase structure is 0. Further, the lower limit of the pearlite lamellar spacing is better, but since the processing and special heat treatment are required and the cost is increased, the lower limit is preferably 0.05 μm.

  Note that the ferrite fraction does not include ferrite constituting pearlite and ferrite in cementite.

  As a structure adjustment method of the steel material before induction hardening, if induction hardening is performed after hot rolling, in the hot rolling stage, if induction hardening is performed after hot forging, in the hot forging stage, Organizational adjustments should be made. In either case, a heat treatment for adjusting the structure may be performed before the induction hardening process.

  When adjusting the structure of the steel material in the hot rolling stage or the hot forging stage, at the finishing stage, the processing temperature is set to 900 to 1200 ° C., and 5% or more plastic deformation is applied, and then cooling at 10 ° C./second or more. Cool to a temperature range of 500 to 600 ° C. at a rate, and then slowly cool at a cooling rate of 0.5 ° C./second or less, or at a cooling rate of 10 ° C./second or more to a temperature range of 500 to 600 ° C. What is necessary is just to cool, after also hold | maintaining for 5 minutes or more in the temperature range of 500-600 degreeC. In addition, when cooling after hold | maintaining for 5 minutes or more in the said 500-600 degreeC temperature range, the cooling rate is arbitrary.

  When adjusting the structure of the steel material by heat treatment prior to induction hardening, for example, after heating to 900 ° C. so that it becomes an austenite single-phase structure, a temperature range of 500 to 600 ° C. at a cooling rate of 10 ° C./s or higher is desirable. May be cooled to a temperature range of 500 to 550 ° C., held in this temperature range for 5 minutes or longer to cause pearlite transformation, and then allowed to cool in the atmosphere. For example, a part before induction hardening is heated to 900 ° C. in an air atmosphere using an electric furnace, and then quickly put into a salt bath furnace heated to 550 ° C. within 5 seconds. What is necessary is just to cool to room temperature in air | atmosphere after hold | maintaining.

  When adjusting the structure of the steel material, the cooling rate from the austenite single phase region to the temperature range of 500 to 600 ° C. affects the lamellar spacing of pearlite, and the faster the cooling rate, the finer the lamellar spacing. Further, the ferrite fraction changes depending on the temperature range of 500 to 600 ° C., which is the temperature for cooling from the austenite single phase region, and the ferrite fraction decreases as the cooling temperature decreases.

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

  Steels A1 to A11 and steels B1 to B8 having the chemical composition shown in Table 1 were melted in a vacuum furnace to produce a 150 kg steel ingot.

  Steels A1 to A11 in Table 1 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, steel B1-B8 is steel of the comparative example from which the chemical composition remove | deviated from the conditions prescribed | regulated by this invention. In Table 1, the value of fn1 represented by the above formula (1) when the steel does not contain V and the value of fn2 represented by the above formula (2) when V is included are expressed as “value of fn”. It was written.

  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 round bar having a diameter of 30 mm was heated at 900 ° C. for 30 minutes to form an austenite single-phase structure, then cooled at the cooling rate shown in Table 2, and then in a salt bath controlled to the temperature shown in Table 2. The structure of the round bar before induction quenching was changed by performing a heat treatment for holding for a minute and then air cooling.

  The microstructure was investigated using a round bar with a diameter of 30 mm subjected to the above heat treatment. That is, after cutting and embedding in a resin so that the state of the cross-section of the heat-treated round bar can be observed, mirror-polished and then corroded with nital to reveal a microstructure, and the ferrite fraction was determined using an optical microscope. The lamellar spacing of pearlite was measured using a scanning electron microscope (SEM).

  Specifically, the ferrite fraction was calculated by performing image processing on an optical micrograph at a magnification of 100 to 400 times taken arbitrarily at 10 fields of view. Further, the pearlite lamellar interval was measured by measuring the interval between rod-like cementite using an SEM photograph with magnification of 2000 to 5000 times taken arbitrarily from 10 fields of view, and this average value was taken as the pearlite lamellar interval.

  A turning test was carried out using a round bar with a diameter of 30 mm subjected to the heat treatment, and the amount of tool wear was measured to investigate the machinability. That is, each round bar with a diameter of 30 mm after heat treatment was turned for 10 minutes under the following conditions using a P20 type carbide tool with TiN coating treatment, and the average flank wear of the carbide tool was measured. Using this as the amount of tool wear, the machinability was evaluated.

・ Cutting speed: 120 m / min,
・ Incision amount: 1.5 mm,
-Feed amount: 0.40 mm / rev. ,
・ Lubrication: Wet (uses water-soluble lubricant).

  The target of the machinability is a tool wear amount of 100 μm or less.

  Further, from the round bar having a diameter of 30 mm subjected to the heat treatment, 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 100 mm 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 0.8 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 1700 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 800 MPa or more.

  Table 2 also shows the results of the above tests.

  From Table 2, in the case of test numbers 1 to 11 that satisfy the conditions specified in the present invention (1) to (3), the machinability is good, the induction hardenability is excellent, and the bending fatigue strength is good. It is clear that

  On the other hand, it is clear that the bending fatigue strengths of Test Nos. 12 to 23 deviating from the conditions defined in the present invention are low, and the cutting workability and 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 11 satisfying the conditions specified in the present inventions (1) to (3) have a bending fatigue strength equal to or higher than that of the carburized and quenched material.

The steel for induction hardening according to the present invention is excellent in machinability and can ensure fatigue strength equal to or higher than that of conventional carburizing and quenching, especially bending fatigue strength, so that axle shafts, drive shafts, constant velocity joints can be secured. It can be used as a material for automobile parts such as outer races and parts for construction machines.

Claims (3)

In mass%, C: 0.35-0.65%, Si: 0.50% or less, Mn: 0.65-2.00%, P: 0.015% or less, S: 0.010-0. 080%, Cr: 0.01 to 0.5%, Mo: 0.05 to 0.5%, B: 0.0005 to 0.0050%, Al: 0.10% or less, N: 0.0150% Hereinafter, O (oxygen): 0.0020% or less and Ti: 3.4N to (3.4N + 0.05)% are contained, the balance is made of Fe and impurities, and fn1 represented by the following formula (1) A steel material for induction hardening, characterized in that the value satisfies 1.0 or more, and the structure is a mixed structure of ferrite and pearlite having a ferrite fraction of 15% or less and a pearlite lamellar spacing of 1 μm or less.
fn1 = C + Mn + 2Mo-2Si-9Cr-2 (Ti-3.4N) -6Al (1)
However, the element symbol in the formula (1) represents the content in steel in mass% of each element. In mass%, C: 0.35-0.65%, Si: 0.50% or less, Mn: 0.65-2.00%, P: 0.015% or less, S: 0.010-0. 080%, Cr: 0.01 to 0.5%, Mo: 0.05 to 0.5%, B: 0.0005 to 0.0050%, Al: 0.10% or less, N: 0.0150% Hereinafter, O (oxygen): 0.0020% or less and Ti: 3.4N to (3.4N + 0.05)%, Cu: 0.20% or less, Ni: 0.20% or less, Nb: It contains one or more of 0.30% or less and V: 0.20% or less, and the balance consists of Fe and impurities, and the value of fn2 represented by the following formula (2) is 1.0. The ferrite and parlour satisfying the above, and having a ferrite fraction of 15% or less and a pearlite lamellar spacing of 1 μm or less. Induction hardening steel material characterized in that it is a mixed structure of bets.
fn2 = C + Mn + 2Mo-2Si-9Cr-2 (Ti-3.4N) -6Al-4V (2)
However, the element symbol in the formula (2) represents the steel content in mass% of each element.   The steel for induction hardening according to claim 1 or 2, characterized by containing Ca: 0.01% or less instead of part of Fe.