JP2010285677A - Steel for induction hardening - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for induction hardening which can suppress the generation of fatigue crack with impurities as a starting point, and is suitable for use as the stock for a component used in an environment subjected to repeated stress such as a crankshaft. <P>SOLUTION: The steel for induction hardening has a composition comprising, by mass, 0.35 to 0.45% C, 0.20 to 0.60% Si, 0.60 to 1.50% Mn, &le;0.030% P, 0.040 to 0.070% S, 0.10 to 0.40% Cr, &lt;0.010% Al, 0.020 to 0.100% Ti, &le;0.0100% N, 0.0005 to 0.0030% B, 0.0001 to 0.0004% Ca and &le;0.0030% O, further satisfying Ca/Al of 0.015 to 0.20, and the balance Fe with impurities. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a steel for induction hardening. More specifically, the present invention relates to a steel for induction hardening that can suppress fatigue failure due to inclusion starting points and can be suitably used as a material for parts used in an environment such as a crankshaft subjected to repeated stress.

  Parts used in environments subject to repeated stress, such as crankshafts of automobiles and construction vehicles, are generally made of mechanical structural steel by hot forging and cutting, and then improved in wear resistance and fatigue strength In order to make it, it is manufactured by applying a surface hardening treatment.

Among various surface hardening treatments, induction hardening is
・ Only necessary parts can be cured.
・ Since it is heated for a short time, there are few adverse effects such as grain coarsening,
・ Because it can be processed in-line, the degree of freedom of the process is high compared to batch type surface treatment such as soft nitriding.
It has the merit that. For this reason, the above-mentioned parts are often manufactured by induction hardening.

On the other hand, the steel for machine structural use, which is a raw material, is refined in a converter or electric furnace, then added with Al, and deoxidized to form a slab or a steel ingot. However, in Al killed steel that is deoxidized with Al, since Al ₂ O ₃ in the steel material, which is a deoxidation product, is very hard, the tool life during cutting tends to be shortened.

  For this reason, for example, Patent Document 1 proposes a Ca-treated steel that improves the tool life by lowering the melting point of the oxide in the steel material, that is, by softening.

That is, in Patent Document 1,
“In mass%, C: 0.35 to 0.45%, Si: 0.20 to 0.60%, Mn: 0.40 to 0.80%, S: 0.040 to 0.070%, Cr : 0.10 to 0.40%, Ti: 0.020 to 0.100%, Ca: 0.0005 to 0.0050%, B: 0.0005 to 0.0030%, O (oxygen): 0.0. 0015 to 0.0050%, Mo: 0 to 0.05%, P: 0.025% or less, V: 0.03% or less, Al: 0.009% or less, and N: 0.0100% or less The balance consists of Fe and impurities,
The value of Fn1 represented by the following formula (1) is 0.63 or less,
The value of Fn2 represented by the following formula (2) is 1.0 or less,
A hot forged non-tempered steel for induction hardening, wherein the value of Fn3 represented by the following formula (3) is 5.7 or more.

Formula (1): Fn1 = C + (Si / 10) + (Mn / 5) + (5Cr / 22) + 1.65V− (5 / 7S) + 1.51 × (Ti-3.4N)
Formula (2): Fn2 = Ca / O
Formula (3): Fn3 = 25.9 × Fn1 + 27.5 × (Ti-3.4N) −7.9 ”
Has been proposed.

JP 2005-68518 A

  The technology proposed in Patent Document 1 described above can achieve both good fatigue characteristics and machinability as steel for crankshafts of automobiles at present. However, as a future steel for crankshafts of automobiles that require high levels of weight reduction and high output, its fatigue characteristics are still not sufficient.

  This invention is made | formed in view of the said present condition, and it aims at providing the steel for induction hardening which can suppress the fatigue failure by an inclusion origin in the environment which receives a repeated stress.

  In order to improve the fatigue strength as compared with the Ca-treated steel proposed in Patent Document 1, the present inventors first studied to increase the hardenability of the steel by increasing the Mn content.

  However, when the hardenability of the steel is increased, the hardness increases, but the intended effect of improving fatigue strength cannot always be obtained.

As a result of detailed examination there,
-Fracture occurs starting from non-metallic inclusions present in the surface hardened by induction hardening, and fatigue strength may decrease,
-The coarse oxide produced | generated by there being much Ca content becomes a starting point of destruction,
Became clear.

  From the above, the present inventors have come to the conclusion that it is necessary to refine individual oxides produced in steel in order to increase fatigue strength.

  Then, next, the present inventors conducted various studies on oxides and obtained the following knowledge.

In Al killed steel that is deoxidized with Al, fine Al ₂ O ₃ is formed in a cluster in the steel material. And when the oxide of Al killed steel is rolled, individual Al ₂ O ₃ is not stretched, but the cluster is extended while collapsing in the rolling direction, and the size of the cluster is small. It will become.

On the other hand, the oxide of the Ca-treated steel is a ternary oxide of CaO, SiO ₂ and Al ₂ O ₃ , and the size of each oxide generated by solidification is the oxide of Al killed steel. It is larger than Al ₂ O ₃ and does not form in a cluster. The ternary oxides of CaO, SiO ₂ and Al ₂ O ₃ are softer than Al ₂ O ₃ which is an oxide of Al killed steel, but are difficult to stretch in the rolling direction. Therefore, when compared after rolling, the oxide size of the Ca-treated steel is larger than the size of the Al ₂ O ₃ cluster of Al killed steel that collapses by rolling.

However, if Ca is not contained, it is destroyed starting from a clustered hard Al ₂ O ₃ oxide.

Therefore, after further examination,
Although Ca is contained, its content is extremely small, and by limiting the content of Al, a ternary oxide of CaO, SiO ₂ and Al ₂ O ₃ can be oxidized more softly with SiO ₂ richness. It becomes possible to change to a product, and individual oxides can be drawn and refined by rolling. If the form of the oxide is changed as described above, the occurrence of fatigue failure starting from inclusions can be suppressed.
New knowledge was obtained.

  The present invention has been completed based on the above findings, and the gist thereof is the induction-hardened steel shown below.

  “In mass%, C: 0.35 to 0.45%, Si: 0.20 to 0.60%, Mn: 0.60 to 1.50%, P: 0.030% or less, S: 0.00. 040 to 0.070%, Cr: 0.10 to 0.40%, Al: less than 0.010%, Ti: 0.020 to 0.100%, N: 0.0100% or less, B: 0.0005 -0.0030%, Ca: 0.0001-0.0004% and O: 0.0030% or less, and satisfying Ca / Al: 0.015-0.20, with the balance being Fe and impurities Induction hardening steel characterized by that. "

  The induction hardening steel of the present invention does not shorten the tool life at the time of cutting, can suppress the occurrence of fatigue failure starting from inclusions, and has excellent fatigue characteristics. It is suitable for use as a material for parts used in an environment that receives repeated stress such as.

It is a figure which shows the shape of the roughing test piece for ultrasonic fatigue tests which performed induction hardening and tempering. It is a figure which shows the shape of the test piece which performed the finishing process after performing induction hardening and tempering to the rough processing test piece of FIG. 1, and used for the ultrasonic fatigue test.

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

C: 0.35-0.45%
C has the effect of increasing the hardenability and the base material hardness. In order to obtain the minimum hardenability and strength, it is necessary to contain 0.35% or more of C. However, if the C content is too high, the hardness of the base material increases significantly, and the machinability decreases. Therefore, the C content is set to 0.35 to 0.45%. In order to secure better machinability, the C content is preferably 0.40% or less.

Si: 0.20 to 0.60%
Si is necessary as a deoxidizer for steel and has the effect of strengthening ferrite and increasing fatigue strength. In order to obtain these effects, it is necessary to contain 0.20% or more of Si. However, if the Si content is too large, decarburization during hot forging is promoted, and fatigue strength is reduced. Therefore, the Si content is set to 0.20 to 0.60%. Note that the Si content is preferably 0.30% or more, and more preferably 0.50% or less.

Mn: 0.60 to 1.50%
Mn is necessary as a deoxidizer for steel and has the effect of improving hardenability and improving the strength of steel. In order to obtain such an effect, it is necessary to contain 0.60% or more of Mn. On the other hand, if the Mn content is too large, the hardness is remarkably increased and the machinability is lowered. Therefore, the content of Mn is set to 0.60 to 1.50%. Note that the Mn content is preferably 0.80% or more, and preferably 1.20% or less.

P: 0.030% or less P is an impurity element, and when its content increases, hot forgeability is reduced. Therefore, the content of P is set to 0.030% or less. A more preferable content of P is 0.020% or less.

S: 0.040-0.070%
S has the effect of improving machinability by forming MnS together with Mn. In order to obtain this effect, it is necessary to contain 0.040% or more of S. However, if the S content is too large, the hot forgeability of the steel is lowered and the fatigue strength is lowered. For this reason, the content of S is set to 0.040 to 0.070%. In addition, it is preferable that S content shall be 0.060% or less.

Cr: 0.10 to 0.40%
Cr has the effect of increasing the hardenability of the steel and increasing the strength. In order to obtain such an effect, it is necessary to contain 0.10% or more of Cr. However, when the content of Cr is too large, the hot forgeability of steel is lowered and the machinability is also lowered. For this reason, the content of Cr is set to 0.10 to 0.40%. The Cr content is preferably 0.20% or less.

Al: less than 0.010% Al combines with O to form hard Al ₂ O ₃ inclusions. The above hard Al ₂ O ₃ inclusions not only cause fatigue fracture as a starting point of fatigue strength, but also reduce machinability and increase the Al content. When it is 010% or more, the fatigue strength and machinability are significantly reduced due to hard Al ₂ O ₃ inclusions. Therefore, the Al content is less than 0.010%. The Al content is preferably less than 0.008%.

Ti: 0.020 to 0.100%
In the steel according to the present invention having a small Al content, Ti has an action of suppressing the generation of BN that fixes N and lowers the effect of improving the hardenability of B. Further, solute Ti in steel has an effect of strengthening steel. In order to obtain such an effect, it is necessary to contain 0.020% or more of Ti. On the other hand, if the Ti content is too large, the machinability of the steel is lowered. Therefore, the Ti content is set to 0.020 to 0.100%. The Ti content is preferably 0.030% or more, and preferably 0.060% or less.

N: 0.0100% or less In the present invention, in order to suppress the formation of BN that reduces the effect of improving the hardenability of B, the N content needs to be lowered. Further, N is fixed as TiN by containing Ti, but if the content of N is large, coarse TiN is generated, and fatigue strength is reduced. Therefore, the N content is set to 0.0100% or less. In addition, the preferable range of N content is 0.0060% or less.

B: 0.0005 to 0.0030%
In the present invention, in order to reduce the hardness of the base material and improve the machinability, the content of elements that enhance the hardenability such as C and Cr is limited, and Mo and V are not included. Therefore, in order to secure the quenching depth during induction hardening, it is necessary to contain B, and in order to obtain the effect of improving hardenability, the content of B needs to be 0.0005% or more. There is. However, even if the content of B is increased, the above effect is saturated and the cost is increased. For this reason, content of B was made into 0.0005 to 0.0030%.

Ca: 0.0001 to 0.0004%
By containing 0.0001% or more of Ca, it is possible to suppress the breakage starting from the clustered hard Al ₂ O ₃ oxide. However, when the Ca content increases and exceeds 0.0004%, the proportion of coarse oxides increases, and the breakage starting from the oxides is promoted. Therefore, the content of Ca is set to 0.0001 to 0.0004%. The Ca content is preferably 0.0003% or less.

O: 0.0030% or less O is an impurity element, and its content increases. In particular, if it exceeds 0.030%, a coarse oxide may be generated, which may reduce fatigue strength. Therefore, the O content is set to 0.0030% or less. Note that the O content is preferably less than 0.0025%.

Ca / Al: 0.015 to 0.20
When the value of Ca / Al, which is the ratio of the content of Ca and Al, exceeds 0.20, coarse oxides are generated and fatigue strength is reduced. On the other hand, when the value of Ca / Al is less than 0.015, cluster-like Al ₂ O ₃ inclusions are generated and the fatigue strength is reduced. For this reason, the value of Ca / Al was set to 0.015 to 0.20. The Ca / Al value is preferably 0.15 or less.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  A 300 mm × 400 mm slab manufactured by a continuous casting machine of steels A1 to A3 and steels B1 to B3 adjusted to the chemical composition shown in Table 1 by secondary refining and a 70 ton converter into a 180 mm × 180 mm steel slab The slab was rolled into pieces, and then heated to 1200 ° C. and hot rolled into a steel bar having a diameter of 70 mm.

  Steels A1 to A3 in Table 1 are steels whose chemical compositions are within the range specified by the present invention, while Steels B1 to B3 are steels of comparative examples whose chemical compositions deviate from the conditions specified by the present invention. It is.

  Steels A1 to A3, Steel B1 and Steel B3 were deoxidized by adding a small amount of Al at the time of steel leaving the converter, while Steel B2 was subjected to normal Al deoxidation treatment.

  In the secondary refining, a small amount of Ca was added to the steels A1 to A3 and the steel B3, while the same level of Ca as the normal Ca-treated steel was added to the steel B1.

  From the R / 2 part ("R" represents the radius of the steel bar) of a steel bar having a diameter of 70 mm produced as described above, a rough test piece having the shape shown in FIG. 1 was cut out in parallel with the longitudinal direction of the steel bar. .

  Next, the test piece was subjected to induction hardening under the conditions of a frequency of 40 kHz, a voltage of 6 kV, and a heating time of 3.0 s, and then further tempered by heating at 150 ° C. for 1 hour and then allowing to cool in the atmosphere.

  The above-mentioned roughened test pieces subjected to induction hardening and tempering are finished to produce test pieces having the shape shown in FIG. 2, and the total number of test pieces is set to 15 for each steel, and an ultrasonic fatigue test is performed. It was used for.

  1 and 2 are both “mm”.

As the ultrasonic fatigue tester, an ultrasonic fatigue tester USF-2000 manufactured by Shimadzu Corporation was used, and the test was performed up to 10 ⁷ cycles at a frequency of 20 kHz and a load stress of 900 MPa.

In the case of the test piece that did not break due to the load stress of 900 MPa, the load stress was further increased by 100 MPa and retested at 1000 MPa, and the test was repeated up to 10 ⁷ cycles as in the case of the load stress of 900 MPa. Carried out.

In the case of a test piece that did not break at a load stress of 1000 MPa, the load stress was further increased by 100 MPa and tested again at 1100 MPa. When this load stress was 1100 MPa, all the test pieces broke before reaching 10 ⁷ cycles.

  As described above, the fracture surface of all the 15 fracture specimens subjected to the ultrasonic fatigue test was observed with a scanning electron microscope at a magnification of 500 times. In addition, when inclusions were observed on the fracture surface by the above fracture surface observation, the inclusion composition was further analyzed by EPMA, and the ratio of the test pieces fractured at the oxide starting point was determined for each steel. .

  Further, the fatigue characteristics were evaluated by the number of repetitions with a 10% fracture probability at a load stress of 900 MPa. As for the 10% fracture probability, the test result (number of repetitions of fracture) at a load stress of 900 MPa was plotted on the Weibull distribution probability paper to obtain the 10% fracture probability.

  Table 2 shows the EPMA investigation results of the fracture surface subjected to the ultrasonic fatigue test and the number of repetitions of 10% fracture probability at a load stress of 900 MPa.

As shown in Table 2, in the case of steels for induction hardening of steels A1 to A3 that satisfy the conditions specified in the present invention, no breakage occurred due to inclusion starting points. Further, the number of repetitions of 10% fracture probability at a load stress of 900 MPa was 1 × 10 ⁶ times or more.

On the other hand, in the case of the steel for induction hardening of steels B1 to B3 whose chemical composition deviated from the conditions specified in the present invention, the breakage due to the inclusion starting point occurred. Further, the number of repetitions of 10% fracture probability at a load stress of 900 MPa was less than 1 × 10 ⁶ times, and the fatigue life was low.

  The induction hardening steel of the present invention does not shorten the tool life at the time of cutting, can suppress the occurrence of fatigue failure starting from inclusions, and has excellent fatigue characteristics. It is suitable for use as a material for parts used in an environment that receives repeated stress such as.

Claims (1)

  In mass%, C: 0.35 to 0.45%, Si: 0.20 to 0.60%, Mn: 0.60 to 1.50%, P: 0.030% or less, S: 0.040 To 0.070%, Cr: 0.10 to 0.40%, Al: less than 0.010%, Ti: 0.020 to 0.100%, N: 0.0100% or less, B: 0.0005 to 0.0030%, Ca: 0.0001 to 0.0004%, and O: 0.0030% or less, and satisfying Ca / Al: 0.015 to 0.20, with the balance being Fe and impurities Induction-hardening steel.

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