JP4900127B2 - Induction hardening steel and manufacturing method thereof - Google Patents

Description

  The present invention relates to a steel material for induction hardening and a method for producing the same, and more particularly to a steel material for induction hardening excellent in rolling fatigue life and a method for producing the same.

  Among automotive parts, parts such as “rolling bearings”, “constant velocity joints” and “hub units” (hereinafter also referred to as “rolling members”) where high surface pressure repeatedly acts have excellent rolling fatigue. Characteristics are required. Among these, “carbon steel for machine structure” described in JIS G 4051 (2005) is mainly used as the material of “constant velocity joint” and “hub unit”, and the rolling fatigue characteristics are Only necessary portions are cured by induction hardening.

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

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

  On the other hand, in recent years, for example, due to the need for higher engine output and lighter parts, the use environment of the above parts has become increasingly severe due to higher surface pressure and higher temperatures. Fatigue life has been demanded.

  However, simply reducing the oxygen content does not ensure the desired good rolling fatigue life. For this reason, reducing the size of the oxide in the steel improves the rolling fatigue life. Has been proposed.

Patent Document 1 includes a rolled steel material that is linear or rod-shaped, and in a longitudinal section that passes through the axis of the rolled steel material, is parallel to the axis and (1/4) · D (D is rolled) The number of composite inclusions having an average particle diameter of 10 μm or more made of an oxide and a sulfide is present in a test area of 100 mm ² including a distant imaginary line as a center line (representing the diameter of the material). There is disclosed a technique of “steel for induction hardening excellent in bending fatigue strength and rolling fatigue strength”, which is characterized.

  Specifically, in mass%, C: more than 0.3% and 0.7% or less, Mn: 0.3 to 2.5%, Si: 2% or less (including 0%), P: 0.0. Contains 03% or less (including 0%), S: 0.1% or less (including 0%), Al: 0.015 to 0.05%, O: 0.002% or less (including 0%) If necessary, (a) at least one selected from the group consisting of a specific amount of Cu, Ni, Cr and Mo, and (b) selected from the group consisting of a specific amount of V, Nb and Ti. At least one selected from the group consisting of at least one, (c) a specific amount of Ca, Pb, Te, Bi and Zr, and (d) at least one of four elements of a specific amount of B and N "Induction hardening steel" has been proposed that contains an element selected from an element group, with the balance being Fe and inevitable impurities.

  Patent Document 1 discloses that the above-mentioned “high-frequency quenching steel” uses the steel material having the above-mentioned composition, suppresses the mixing of oxygen during melting using a vacuum furnace or the like, and solidifies during casting. It is described that it can be easily obtained by taking measures such as increasing the cooling rate of the slab below the temperature.

In Patent Document 2, C: 0.5 to 0.75% by weight, Si: 0.5 to 1.8%, Mn: 0.1 to 1.5%, P: 0.020% or less, S: 0.020% or less, Al: 0.019-0.05%, O: 0.0015% or less, N: 0.003-0.015%, if necessary, (a) One or more of a specific amount of Mo, B, Ti, Ni, (b) a specific amount of at least one of V, Nb, and at least one element group selected from two element groups, with the balance being Fe and It consists of a steel material consisting of inevitable impurities, and through heating in a temperature range of Ac3-100 ° C or higher and Ac3 + 200 ° C, forging with a processing rate of 70% or higher and cooling at a cooling rate of 0.005 ° C / s or higher. "Induction-hardened parts obtained by induction hardening and tempering treatment" There has been disclosed. Furthermore, “induction-hardened parts” have been proposed in which the number of oxide-based nonmetallic inclusions in the steel is 2.5 / mm ² or less and the maximum size is 19 μm or less.

  Patent Document 2 describes that the induction-hardened component can be obtained by using a steel material manufactured by rolling with a cross-sectional reduction rate of 95% or more from a steel piece after casting.

Japanese Patent Laid-Open No. 11-1749 JP-A-11-131176

  Even when the techniques proposed in Patent Document 1 and Patent Document 2 are applied, a sufficient rolling fatigue life is not always obtained under severe usage environments of rolling members in recent years.

  Therefore, the object of the present invention is for induction hardening, which has good durability against damage due to rolling fatigue and can ensure an excellent rolling fatigue life even under the severe usage environment of rolling members in recent years. It is to provide a steel material and a manufacturing method thereof.

  The present inventors have obtained a good rolling fatigue life under the severe use environment of rolling members in recent years even when the oxygen content is reduced and the size of oxides in steel is reduced. Considered no cause.

As a result, even if the oxygen content is reduced, if the oxide is hard, mainly Al ₂ O ₃ , it may remain in the steel as coarse inclusions, which is good It was thought that there was a possibility that a long rolling fatigue life was not obtained. Furthermore, even if the oxygen content is reduced, if the S content is high, the sulfide may be coarser than the oxide, and this may not provide a good rolling fatigue life. I thought that there was sex.

Therefore, as a result of first examining oxides, the main constituents of slag in the process of so-called “secondary refining” of steel are mainly CaO and SiO _2, and strict so that Al ₂ O ₃ is as small as possible. It has been clarified that a soft oxide can be obtained by performing such control, and that the soft oxide can be further refined by applying a reduction.

  However, the rolling fatigue life cannot be improved no matter how much the oxide is miniaturized by the above-described method.

  Therefore, as a result of further studies, in the case of steel manufactured by the above refining method, there is a tendency that an oxide that seems to be MnO tends to be contained in the sulfide, and this sulfide is removed by conventional addition of Al. Unlike the sulfides in acid-treated steel for induction hardening, a very important finding was found that they are difficult to stretch and break by rolling and remain large.

  And it became clear that the sulfide which is difficult to be stretched and divided by the above-described reduction and remains large reduces the rolling fatigue life.

  Then, next, the examination for extending | stretching and parting a sulfide by reduction was performed.

  As a result, in order to limit the amount of sulfide, if the S content is 0.010% or less in mass%, and the reduction conditions such as reduction ratio and processing temperature are properly controlled, only oxides can be obtained. However, it has been found that it is possible to obtain a steel material for induction hardening having an excellent rolling fatigue life even under a severe use environment.

  The present invention has been completed based on the above findings, and the gist of the present invention is that of the steel materials for induction hardening shown in the following (1) to (3) and the steel materials for induction hardening shown in (4) to (7). In the manufacturing method.

(1) By mass%, C: 0.35 to 0.7%, Si: 0.1 to 0.8%, Mn: 0.1 to 1.5%, P: 0.03% or less, S: 0.010% or less, Cr: 0.01% or more and less than 0.50%, Al: 0.005% or less, Ca: 0.0005% or less, O: 0.0020% or less, N: 0.02% or less The balance is composed of Fe and chemical components of impurities, and with respect to non-metallic inclusions, the average composition of oxides is mass%, CaO: 10 to 60%, Al ₂ O ₃ : 20% or less, MnO: 50 % And MgO: 15% or less, the balance being SiO ₂ and impurities, and the arithmetic average value and sulfide of the maximum thickness of oxides existing in 10 areas of 100 mm ^{2 in} the longitudinal longitudinal section of the steel The arithmetic average value of the maximum thickness of each is 8.5 μm or less. Steel for that.

(2) By mass%, C: 0.35 to 0.7%, Si: 0.1 to 0.8%, Mn: 0.1 to 1.5%, P: 0.03% or less, S: 0.010% or less, Cr: 0.01% or more and less than 0.50%, Al: 0.005% or less, Ca: 0.0005% or less, O: 0.0020% or less, N: 0.02% or less In addition, V: not more than 0.3% and Nb: not more than 0.1%, the balance is composed of chemical components of Fe and impurities, and the average of oxides for non-metallic inclusions The composition is by mass%, CaO: 10-60%, Al ₂ O ₃ : 20% or less, MnO: 50% or less and MgO: 15% or less, and the balance SiO ₂ and impurities. The arithmetic average value of the maximum thickness of oxide existing in 10 areas of 100 mm ² and the arithmetic average of the maximum thickness of sulfide A steel material for induction hardening, wherein the average value is 8.5 μm or less, respectively.

(3) By mass%, C: 0.35 to 0.7%, Si: 0.1 to 0.8%, Mn: 0.1 to 1.5%, P: 0.03% or less, S: 0.010% or less, Cr: 0.01% or more and less than 0.50%, Al: 0.005% or less, Ca: 0.0005% or less, O: 0.0020% or less, N: 0.02% or less In addition, B: 0.005% or less and Ti: 0.05% or less, the balance consists of chemical components of Fe and impurities, the non-metallic inclusions, the average composition of the oxide is mass%, _{CaO: 10~60%, Al 2 O} 3: 20% or less, MnO: 50% or less and MgO: with of 15% or less in balance SiO ₂ and impurities, a longitudinal vertical section 10 positions of 100 mm ² of steel The arithmetic average value of the maximum thickness of oxide present in the area and the arithmetic average value of the maximum thickness of sulfide are A steel material for induction hardening, each having a thickness of 8.5 μm or less.

(4) To the slab or steel ingot having the average composition of the chemical components and oxides described in (1) above, a reduction at a total reduction ratio of 15 or more is added, and the reduction is 1000 ° C. or less. A method for producing a steel for induction hardening, wherein the reduction is performed with a reduction ratio in the temperature range of 4 or more.
However, the total reduction ratio refers to the value obtained by dividing the cross-sectional area of the slab or steel ingot by the cross-sectional area of the steel for induction hardening obtained by the final reduction, and the reduction ratio in the temperature range of 1000 ° C or lower. The term refers to a value obtained by dividing the cross-sectional area of the intermediate steel material before the reduction in the temperature range by the cross-sectional area of the steel material for induction hardening obtained by the final reduction.

(5) To the slab or steel ingot having the average composition of the chemical components and oxides described in (2) above, a reduction at a total reduction ratio of 15 or more is added, and the reduction is 1000 ° C. or less. A method for producing a steel for induction hardening, wherein the reduction is performed with a reduction ratio in the temperature range of 4 or more.
However, the total reduction ratio refers to the value obtained by dividing the cross-sectional area of the slab or steel ingot by the cross-sectional area of the steel for induction hardening obtained by the final reduction, and the reduction ratio in the temperature range of 1000 ° C or lower. The term refers to a value obtained by dividing the cross-sectional area of the intermediate steel material before the reduction in the temperature range by the cross-sectional area of the steel material for induction hardening obtained by the final reduction.

(6) To the slab or steel ingot having the average composition of the chemical components and oxides described in (3) above, a reduction at a total reduction ratio of 15 or more is added, and the reduction is 1000 ° C. or less. A method for producing a steel for induction hardening, wherein the reduction is performed with a reduction ratio in the temperature range of 4 or more.
However, the total reduction ratio refers to the value obtained by dividing the cross-sectional area of the slab or steel ingot by the cross-sectional area of the steel for induction hardening obtained by the final reduction, and the reduction ratio in the temperature range of 1000 ° C or lower. The term refers to a value obtained by dividing the cross-sectional area of the intermediate steel material before the reduction in the temperature range by the cross-sectional area of the steel material for induction hardening obtained by the final reduction.

(7) The slab or steel ingot is subjected to secondary refining using a flux that does not substantially contain Al after oxidative refining without performing Al deoxidation treatment, and final slag after completion of secondary refining The basicity CaO / SiO _{2 has} a value of 0.8 to 2.0, and the slag composition is mass%, so that MgO: 15% or less, F: 10% or less, and Al ₂ O ₃ : 15% or less. The method for producing a steel material for induction hardening according to any one of (4) to (6) above, wherein the steel material is controlled to be subsequently cast.

Note that the “impurity” in the average composition of the oxide refers to Cr ₂ O ₃ , Na ₂ O, ZrO _{2 and the} like.

  Further, the “longitudinal longitudinal section” (hereinafter referred to as “L section”) refers to a surface cut in parallel to the longitudinal direction of the steel material.

  Hereinafter, the inventions relating to the steel materials for induction hardening (1) to (3) and the inventions relating to the method for producing the steel materials for induction hardening (4) to (7) are respectively referred to as “present invention (1)” to “ The present invention (7) ". Also, it may be collectively referred to as “the present invention”.

  The steel material for induction hardening according to the present invention has good durability against damage due to rolling fatigue and has a long rolling fatigue life even under severe use environment of rolling members in recent years. It can be used as a material for rolling members that perform induction hardening, such as “constant velocity joints” and “hub units”. This steel for induction hardening can be manufactured by the method of the present invention.

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

(A) Chemical composition of steel:
C: 0.35-0.7%
C is an element that ensures the hardness required for the rolling part after induction hardening, and the content needs to be 0.35% or more. However, if the C content exceeds 0.7%, the base material becomes hard, the forgeability is significantly deteriorated and the tool life at the time of cutting is reduced, and further, the cause of the cracking when induction hardening is performed. It becomes. Therefore, the content of C is set to 0.35 to 0.7%. A desirable C content range is 0.40 to 0.6%.

Si: 0.1 to 0.8%
Si is an element necessary for ensuring the depth of the hardened layer necessary for the rolling part after induction hardening, and must be contained by 0.1% or more. However, even if Si exceeds 0.8%, the effect of improving the hardenability is saturated, the base material becomes harder, the forgeability is remarkably deteriorated, and the tool life at the time of cutting is reduced. End up. Therefore, the Si content is set to 0.1 to 0.8%. The range of preferable Si content is 0.15 to 0.7%.

Mn: 0.1 to 1.5%
Mn is an element necessary for ensuring the depth of the hardened layer necessary for the rolling part after induction hardening, and must be contained by 0.1% or more. However, even if Mn is contained exceeding 1.5%, the effect of improving the hardenability is saturated, the base material becomes harder, the forgeability is remarkably deteriorated, and the tool life at the time of cutting is reduced. End up. Therefore, the Mn content is set to 0.1 to 1.5%. The range of preferable Mn content is 0.2 to 1.15%.

P: 0.03% or less P segregates at the grain boundary and shortens the rolling fatigue life. In particular, when the content exceeds 0.03%, the rolling fatigue life is significantly reduced. Therefore, the content of P is set to 0.03% or less. The range of preferable P content is 0.02% or less.

S: 0.010% or less S is an element that forms sulfides. If the content exceeds 0.010%, coarse sulfides remain and the rolling fatigue life is shortened. Therefore, the content of S is set to 0.010% or less. A preferable S content from the viewpoint of improving the rolling fatigue life is 0.008% or less.

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

Al: 0.005% or less Al is an undesirable element, and in the present invention, it is necessary to reduce Al as much as possible. Therefore, as will be described later, deoxidation treatment by addition of Al after oxidative refining is not performed, and the flux used when stirring the newly produced slag and molten steel by adding flux also has an Al ₂ O ₃ content. A small amount of Al-free material is used. However, when the Al content increases, especially when it exceeds 0.005%, the amount of hard oxides mainly composed of Al ₂ O ₃ increases, and even after the reduction, coarse oxidation occurs. Since it remains as an object, the rolling fatigue life is shortened. Therefore, the Al content is set to 0.005% or less. The Al content is preferably 0.003% or less, and the lower the better.

Ca: 0.0005% or less In the present invention, as described later, after removing slag produced by oxidative refining, a flux whose main component is CaO is introduced, and the newly produced slag and molten steel are strongly stirred. To do. At this time, a very small amount of Ca is mixed into the steel as an oxide softer than the flux. However, if the Ca content increases and exceeds 0.0005%, the ratio of CaO in the oxide composition becomes too high, resulting in a coarse oxide. Therefore, the Ca content is set to 0.0005% or less. The preferable Ca content is 0.0003% or less, and more desirably 0.0002% or less. In addition, the lower limit of the amount of Ca contained is not particularly specified, and it is sufficient that CaO in the average composition of oxides in the steel material is 10% or more.

O: 0.0020% or less O is an undesirable impurity element. When the content of O increases, and particularly exceeds 0.0020%, it remains as a coarse oxide after rolling, leading to a decrease in rolling fatigue life. Therefore, the content of O is set to 0.0020% or less. In addition, the range of preferable O content is 0.0015% or less.

N: 0.02% or less N has an effect of forming a nitride. However, if it is contained excessively, coarse nitrides are produced, and there is a possibility that fatigue strength is reduced. Therefore, the N content is set to 0.02% or less.

  However, in order to obtain the effect of suppressing grain coarsening during high-frequency induction heating (hereinafter simply referred to as “high-frequency heating”), when one or more of V and Nb are contained as described later, It is necessary to form nitrides of these elements. Therefore, when it contains 1 or more types of V and Nb, it is desirable to control N content high, suppressing to the range of 0.02% or less. In this case, a desirable range of N is 0.010 to 0.02%, and a more desirable range is 0.012 to 0.02%.

  Moreover, in order to acquire the hardenability improvement effect at the time of induction hardening, as mentioned later, when containing B and Ti, it is necessary to suppress the coupling | bonding of B and N as much as possible. Therefore, when B and Ti are contained, it is desirable that the N content be further lower than 0.02% or less. In this case, the desirable range of N is less than 0.010%, and the more desirable range is 0.008% or less.

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

  Also in the present invention (4), C, Si, Mn, P, S, Cr, Al, Ca, O, and N are contained in the above-mentioned range, and the balance is a slab made of Fe and impurities chemical components. A steel ingot was used.

  Since the induction heating is heated to a high temperature even for a short time, the induction hardening steel according to the present invention is C, Si, Mn, P, S, Cr, Al from the viewpoint of preventing coarsening of crystal grains. , Ca, O, N in the same range as the steel for induction hardening according to the present invention (1), V, Nb, V: 0.3% or less and Nb: 0.1% or less (Steel material for induction hardening of the present invention (2)).

  Hereinafter, the above V and Nb will be described.

V: 0.3% or less Since V combines with N to form a nitride, it has an effect of suppressing crystal grain coarsening during high-frequency heating. Furthermore, it has the effect | action which raises the intensity | strength of a base material by couple | bonding with C. However, even if it contains V exceeding 0.3%, the effect of preventing crystal grain coarsening during high-frequency heating is saturated, and the strength of the base material becomes too high, and the machinability may be reduced. There is. Therefore, the content of V is set to 0.3% or less. In order to more effectively exhibit the effect of suppressing the coarsening of crystal grains during high-frequency heating and the effect of increasing the strength of the base material, and to ensure sufficient machinability, the V content is 0.01 to It is desirable to be 0.2%.

Nb: 0.1% or less Since Nb combines with N to form a nitride, it has the effect of suppressing crystal grain coarsening during high-frequency heating. Furthermore, it has the effect | action which raises the intensity | strength of a base material by couple | bonding with C. However, even if Nb is contained in excess of 0.1%, the effect of preventing crystal grain coarsening during high-frequency heating is saturated, and the strength of the base material becomes too high, and the machinability may be reduced. There is. Therefore, the Nb content is set to 0.1% or less. In order to more effectively exhibit the effect of suppressing the coarsening of crystal grains during high-frequency heating and the effect of increasing the strength of the base material, and to ensure sufficient machinability, the Nb content is 0.01 to It is desirable to set it as 0.1%.

  Said V and Nb can be contained only in any 1 type or 2 types of composite.

  For the above reasons, the steel for induction hardening according to the present invention (2) has the same range of C, Si, Mn, P, S, Cr, Al, Ca, as the steel for induction hardening according to the present invention (1). In addition to O and N, it contains at least one of V: 0.3% or less and Nb: 0.1% or less, and the balance is defined as consisting of chemical components of Fe and impurities.

  Also in the present invention (5), C, Si, Mn, P, S, Cr, Al, Ca, O, N, V, and Nb are contained in the above-described range, and the balance is derived from the chemical components of Fe and impurities. The slab or steel ingot to be used was used.

  The steel material for induction hardening according to the present invention includes C, Si, Mn, P, S, Cr, Al, Ca, O, and N in the present invention (1) from the viewpoint of ensuring better induction hardenability. After containing in the same range as the steel for induction hardening, B and Ti can further contain B: 0.005% or less and Ti: 0.05% or less (the induction hardening of the present invention (3)) Steel).

  Hereinafter, the above B and Ti will be described.

B: 0.005% or less B can greatly improve the hardenability of steel by adding a trace amount, so that it is possible to further increase the depth of the hardened layer necessary for the rolling part after induction hardening. It can be an element. However, even if the B content exceeds 0.005%, the effect is saturated. Therefore, the B content is set to 0.005% or less. A preferable range of the B content for ensuring the effect of improving the hardenability and ensuring better induction hardenability is 0.0003 to 0.005%.

Ti: 0.05% or less By containing B, the hardenability is improved when B is not a compound but exists alone. Therefore, when B is combined with N to form a nitride, the effect of improving hardenability by B cannot be expected. For the above reasons, when B is contained, it is necessary to contain Ti that has a higher affinity with N than B and a stronger nitride forming ability. However, even if Ti is contained in an amount exceeding 0.05%, not only the effect of fixing N is saturated, but also a large amount of coarse TiN is generated, so that rolling fatigue characteristics may be reduced. is there. Therefore, the Ti content is set to 0.05% or less. A preferable range of Ti when contained together with the above-mentioned amount of B in order to reliably exhibit the hardenability improving effect of B is 0.01 to 0.05%.

  For the above reasons, the steel for induction hardening according to the present invention (3) has the same range of C, Si, Mn, P, S, Cr, Al, Ca, as the steel for induction hardening according to the present invention (1). In addition to O and N, B: 0.005% or less and Ti: 0.05% or less were contained, and the balance was defined as consisting of chemical components of Fe and impurities.

  Also in the present invention (6), C, Si, Mn, P, S, Cr, Al, Ca, O, N, B, and Ti are contained in the above-mentioned range, and the balance is derived from the chemical components of Fe and impurities. The slab or steel ingot to be used was used.

(B) Non-metallic inclusions:
(B-1) Average composition of oxide:
In the present invention, for non-metallic inclusions, first, the average composition of oxides is mass%, CaO: 10 to 60%, Al ₂ O ₃ : 20% or less, MnO: 50% or less, and MgO: 15%. In the following, it must consist of the remainder SiO ₂ and impurities. Hereinafter, the content in the average composition of oxide in mass% is also referred to as “concentration”.

The “oxide” in the present invention is mainly composed of a ternary system of CaO, SiO _2, Al ₂ O ₃ , MnO and MgO, and the average composition of the oxide is in the above range. In general, oxides are soft as a whole, and are easily stretched, broken, and finely processed by rolling and other processes, so that they do not reduce the rolling fatigue life and are therefore excellent even in harsh usage environments. This is because the rolling fatigue life can be secured.

  The reasons for limiting each oxide composition are shown below.

CaO: 10 to 60%
The oxide having a basic composition of SiO ₂ that is an acidic oxide contains CaO that is basic, so that the liquidus temperature of the oxide is lowered and the ductility is exhibited in the rolling temperature range. The effect is obtained when the CaO concentration in the average composition of the oxide is 10% or more. However, when it exceeds 60%, the SiO ₂ concentration is relatively lowered and the ductility is not exhibited. Therefore, the CaO concentration in the average composition of the oxide is set to 10 to 60%. In addition, the desirable upper limit of the said CaO density | concentration for being able to obtain the stable ductility in a rolling temperature range is 50%.

Al ₂ O ₃ : 20% or less When the concentration in the average composition of the amphoteric oxide Al ₂ O ₃ exceeds 20%, an Al ₂ O ₃ (corundum) phase is crystallized in the rolling temperature range, which will be described later. MgO.Al ₂ O ₃ (spinel) phase crystallizes with MgO. These solid phases are hard and maintain the crystallized thickness without stretching even during rolling. Therefore, the Al ₂ O ₃ concentration in the average composition of the oxide needs to be 20% or less. The desirable upper limit of the Al ₂ O ₃ concentration for stably and reliably suppressing the formation of the hard phase is 15%.

MnO: 50% or less MnO has basicity as an oxide and promotes softening of SiO ₂ system, so that it can be allowed to a relatively high concentration. However, MnO is a so-called lower oxide that is stable when the steel is in a weakly deoxidized state. If the MnO concentration is high, the content of O (oxygen) in the steel also increases. That is, if the MnO concentration in the average oxide composition exceeds 50%, the O content cannot be made 0.0020% or less. Therefore, the MnO concentration in the average composition of the oxide is set to 50% or less. In addition, in order to make content of O mentioned above 0.0015% or less, it is preferable that MnO density | concentration in the average composition of an oxide shall be 40% or less.

MgO: 15% or less MgO is a basic oxide, and a small amount can soften a SiO ₂ -based oxide, but its solubility is low, while it has a low MgO (periclase) phase and Al ₂ O ₃ together with MgO. The Al ₂ O ₃ (spinel) phase crystallizes out. If MgO in the average composition of the oxide exceeds 15% in the rolling temperature range, the probability of crystallizing the hard phase described above increases. Therefore, the MgO concentration in the average composition of the oxide is set to 15% or less. In order to more reliably suppress the crystallization of the hard phase described above, the MgO concentration in the average composition of the oxide is preferably 10% or less.

The “oxide” as used in the present invention is mainly composed of a ternary system of CaO, SiO ₂ , Al ₂ O ₃ , MnO and MgO, but is composed of Cr ₂ O ₃ , Na ₂ O and ZrO _2. It is desirable that the total sum of impurities in the oxides is 3% or less.

Incidentally, the average composition of _{oxides, CaO: 10~50%, Al 2} O 3: 15% or less, MnO: 40% or less and MgO: that the balance 10% or less is SiO _2, and 3% of impurities Is preferred.

Further, in the average composition of the oxide, the lower limit of Al ₂ O ₃ , MnO and MgO does not need to be specified.

For the reasons described above, the average composition of oxides of the steel for induction hardening according to the present invention (1) to (3) is, in mass%, CaO: 10 to 60%, Al ₂ O ₃ : 20% or less, MnO: 50% or less and MgO: 15% or less, and the balance was defined as consisting of the remaining SiO ₂ and impurities.

  Also in the present inventions (4) to (6), slabs or steel ingots having the average composition of the oxides are used.

  In addition, the average composition of the oxide is, for example, a plurality of arbitrary oxides having a thickness of 3 μm or more by energy dispersive X-ray spectroscopy after mirror-polishing the L cross-section obtained by cutting the steel material parallel to the longitudinal direction, For example, 20 compositions may be obtained by arithmetically averaging the measured compositions.

  In addition, the average composition of the oxide described above employs, for example, the steelmaking method described in the following <1> and <2>, and subsequently, into a slab or a steel ingot by a conventional continuous casting method or a mold method. It can be obtained by casting.

  <1> Excludes oxygen contained as an impurity after oxidation refining in a so-called “primary refining furnace” such as a converter or electric furnace (hereinafter also simply referred to as “converter”) during the steelmaking process of induction hardening steel Therefore, the deoxidation treatment with Al addition that is usually performed is not performed.

<2> About the final slag after the completion of secondary refining, the basicity (CaO / SiO ₂ ) is 0.8 to 2.0, the composition is mass%, MgO: 15% or less, F: 10% Hereinafter, control is performed so that Al ₂ O ₃ : 15% or less. The above F (fluorine) is derived from CaF ₂ as the main component of the fluorite as Zokasu agent.

In addition, about the final slag after completion of secondary refining, in order to make the composition of the above <2>, slag composition control in “secondary refining” is facilitated after steel is discharged from the converter to the ladle. For this purpose, first, slag generated by oxidative refining flowing out from the converter is removed, and after removal, the main component is CaO, substantially free of Al, Al ₂ O ₃ or MgO content A small amount of flux may be introduced and the newly generated slag and molten steel may be vigorously stirred.

  In addition, as means for obtaining strong stirring, for example, stirring under reduced pressure, stirring by injection, bottom blowing stirring from the bottom of the ladle, or the like may be applied. When stirring by injection, it is desirable to blow in the above-mentioned flux at the same time. Moreover, when implementing a pressure reduction process, it is necessary to stop at the pressure reduction process for stirring to the last. This is because, if the decompression process for a long time is performed, the inclusion of hard inclusions from the refractory and the entrainment of slag are caused, leading to a decrease in cleanliness.

  In addition, as long as the Ca content of the steel does not exceed 0.0005%, Ca may be further added to the molten steel during the secondary refining process.

(B-2) Maximum thickness of oxide and maximum thickness of sulfide:
When both the oxide and sulfide are thick, the rolling fatigue life is reduced. What has the greatest influence on the rolling fatigue life is the coarsest inclusions present under the rolling part. In particular, when oxides and sulfides having a maximum thickness exceeding 8.5 μm in an area of 100 mm ² of the L cross-section of the steel material are present at many sites in the steel material, the probability of being present in the rolling part is high. As a result, the rolling fatigue life is significantly reduced.

For the above-described reasons, the steel materials for induction hardening according to the present inventions (1) to (3) have the arithmetic average value of the maximum thickness of oxides present in the area of 100 mm ² at 10 locations on the L cross section of the steel material. It was defined that the arithmetic average value of the maximum thickness of sulfides was 8.5 μm or less, respectively.

  The arithmetic average value of the maximum thickness of the oxide and the arithmetic average value of the maximum thickness of the sulfide are both preferably 7 μm or less.

  As already described, the “L cross section” refers to a surface cut in parallel to the longitudinal direction of the steel material.

(C) Method of manufacturing steel for induction hardening:
The steel materials for induction hardening according to the present invention (1) to (3) are each composed of, for example, the methods of the present invention (4) to (6), specifically the chemical components described in the above section (A), For the metal inclusions, a reduction to a total reduction ratio of 15 or more is added to the slab or steel ingot having the average composition of the oxide described in the section (B-1), and 1000 ° C. or less of the reduction. It can be manufactured by reducing the reduction ratio in the temperature range of 4 or more.

  Further, the slab or steel ingot having an average composition of the chemical component and oxide according to the present invention (4) to (6), that is, the chemical component described in the item (A), The slab or the steel ingot having the average composition of the oxide described in the section B-1) is, for example, after adopting the method described in the <1> and <2> in the section (B-1). It can be obtained by casting by a conventional continuous casting method or a casting method.

  And the steel material for induction hardening is manufactured by performing steel bar rolling or wire rod rolling, for example, using the steel piece obtained by carrying out the partial rolling of the slab or the steel ingot in the temperature range over 1000 degreeC. .

  In the above process, when the total reduction ratio in processing the slab or the steel ingot into a steel material such as a final bar or wire is less than 15, it is composed of the chemical components described in the above item (A), ( Even when a slab or steel ingot having the average composition of the oxide described in the section B-1) is used, the maximum thickness of the oxide and the maximum of the sulfide described in the section (B-2) are applied to the steel for induction hardening. The thickness condition cannot be satisfied, and therefore a desired excellent rolling fatigue life cannot be ensured in a severe use environment.

  In addition, the larger the total rolling reduction ratio, the smaller the maximum oxide thickness and sulfide maximum thickness described in the above section (B-2), and the rolling fatigue characteristics (rolling fatigue life) are improved. To do. For this reason, the upper limit of the total reduction ratio does not need to be specified in particular, and is the maximum determined by the dimensions of the slab and the steel ingot and the dimensions of the steel material such as the final bar and wire obtained by processing them and the equipment surface. It may be a value.

  In addition, the range of desirable total reduction ratio is 30 or more.

  However, in order to satisfy the conditions for the maximum thickness of the oxide and the maximum thickness of the sulfide described in the above section (B-2) for the induction hardening steel, it is only necessary to satisfy the total reduction ratio of 15 or more. Is not enough.

  This is because when the average composition of the oxide is as described in the above section (B-1), the sulfide present at the same time contains an oxide that seems to be MnO, and it was deoxidized by adding Al. Compared to the case, since the sulfide is hardened, it is difficult to be stretched and divided by processing. Therefore, the steel material for induction hardening should satisfy the condition of the maximum thickness of the sulfide described in the section (B-2). It is because it is not possible.

  Only when the total reduction ratio is 15 or more, and the reduction ratio in the temperature range of 1000 ° C. or lower is 4 or more in the reduction, the induction hardening steel is the first (B-2). The condition of the maximum thickness of the sulfide described in the section can be satisfied.

  That is, since the deformation resistance of the matrix (base) is smaller than that of sulfides, a reduction applied at a high temperature, particularly a reduction applied at a temperature exceeding 1000 ° C., preferentially deforms the matrix. Therefore, under the pressure in the above temperature range, the sulfide is not easily stretched and divided, and the conditions for the maximum thickness of the sulfide described in the above section (B-2) cannot be satisfied. In this case, the desired excellent rolling fatigue life cannot be ensured in a severe use environment.

  On the other hand, if the temperature range to which the reduction is applied is lowered to 1000 ° C. or less, the difference in deformation resistance between the matrix and the sulfide is reduced, and the sulfide is easily stretched and divided (B-2). The condition of the maximum thickness of sulfide described in the section is satisfied.

  Note that the temperature range to which the reduction is applied is desirably 950 ° C. or less, and more desirably 850 ° C. or less.

  The lower the temperature range to which the above reduction is applied, the more the sulfide stretching and breaking effects are promoted, so the maximum thickness of the sulfide described in the section (B-2) becomes smaller. For this reason, the lower limit of the temperature at which the reduction is applied does not need to be specified in particular, and is determined from the viewpoint of equipment and material properties such as the load for processing into the steel material such as the final bar or wire, and the workability at that time. It may be the minimum value.

  Even when the temperature range to which the reduction is applied is 1000 ° C. or less, the reduction ratio in the temperature range is low. In particular, when it is less than 4, the sulfide is not easily stretched and divided, so that the (B− The condition of the maximum thickness of the sulfide described in the item 2) cannot be satisfied.

  The reduction ratio in the temperature range of 1000 ° C. or lower is preferably 6 or more, and more preferably 8 or more.

  The upper limit of the reduction ratio in the temperature range of 1000 ° C. or lower is not particularly specified, and the equipment and material characteristics such as the load for processing into the steel material such as the final bar and wire, and the workability at that time It may be the maximum value determined from the viewpoint.

  As already mentioned, the above total reduction ratio refers to the value obtained by dividing the cross-sectional area of the slab or steel ingot by the cross-sectional area of the steel for induction hardening obtained by the final reduction, and 1000 ° C. The reduction ratio in the following temperature range refers to a value obtained by dividing the cross-sectional area of the intermediate steel material before reduction in the temperature range by the cross-sectional area of the steel for induction hardening obtained by the final reduction.

  For the reasons described above, in the present inventions (4) to (6), slabs or steel ingots having an average composition of chemical components and oxides described in the present inventions (1) to (3), in other words, The total reduction ratio is 15 or more in a slab or steel ingot comprising the chemical composition described in the section (A) and having the average composition of the oxide described in the section (B-1) for the nonmetallic inclusion. It was defined that a reduction was applied, and that the reduction was performed at a reduction ratio of 4 or more in a temperature range of 1000 ° C. or less.

Further, in the present invention (7), in the present invention (4) to (6), after oxidative refining, secondary refining is performed using a flux that does not substantially contain Al, without performing Al deoxidation treatment, The final slag basicity CaO / SiO ₂ value after the completion of secondary refining is 0.8 to 2.0, and the slag composition is mass%, MgO: 15% or less, F: 10% or less, Al ₂ O ₃ : It was regulated to use 15% or less, and to use a subsequently cast slab or steel ingot.

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

Example 1
Ingot-hardened steel slabs 1 to 26 having various chemical compositions shown in Table 1 were produced.

  In Table 1, Steels 1 to 16, Steel 19, Steel 22, and Steel 24 are steels whose chemical compositions are within the range defined by the present invention, and are Steel 17, Steel 18, Steel 20, Steel 21, and Steel. 23, Steel 25, and Steel 26 are comparative steels whose chemical compositions deviate from the conditions defined in the present invention. Of the comparative steels, Steel 25 and Steel 26 are steels equivalent to conventional Al killed steel.

  Among the steels described above, Steels 1 to 24 were subjected to oxidative refining in a converter, then removed and charged with flux. And after blowing the flux by the flux injection method, in a state in which the flux is mixed in the molten steel, in an Ar atmosphere by a vacuum molten steel stirring equipment with an arc heating device (hereinafter referred to as “VAD”), Molten steel stirring was performed for 40 minutes at an Ar flow rate of 40 to 60 L / min. Thereafter, continuous casting was performed to obtain a slab of 300 mm × 400 mm. In addition, about the steel 17 and the steel 21, Al was added slightly at the time of the steel extraction from a converter, and although the deoxidation process was lightly performed, about the steel 1-16, the steel 18-20, and the steel 22-24, No deoxidation treatment with addition of Al was performed.

Steels 25 and 26 were subjected to oxidative refining in a converter, then deoxidized by addition of Al at the time of steel removal from the converter, and then removed and charged with flux. Then, by VAD, molten steel is stirred for 40 minutes under an Ar atmosphere at an Ar flow rate of 40 to 60 L / min, and further processed by an RH vacuum degasser for 30 minutes, and hard oxidation mainly composed of Al ₂ O ₃ is performed. The thing was removed. Thereafter, continuous casting was performed to obtain a slab of 300 mm × 400 mm.

  Table 2 shows the composition of the flux introduced after removing the steels 1 to 26 and the composition of the flux used in the flux injection method for the steels 1 to 24.

Also, it is shown in Table 3, the slag composition and basicity by mass percent after VAD processing of steel 1-26 of (CaO / SiO _2).

  From the T / 4 part of the slabs of steel 1 to 26 obtained in this manner (where “T” represents the thickness of the slab), that is, from the intermediate part between the outer surface and the center of the slab, the oxide After cutting out a block for composition measurement, embedding the block in a resin, and mirror-polishing the L cross section, select 20 arbitrary oxides with a thickness of 3 μm or more by energy dispersive X-ray spectroscopy. It was measured.

  And the composition measured about 20 oxides was arithmetic-averaged, and the "average composition" of the oxide in each slab was calculated | required.

In Table 4, the average composition of the oxide measured as mentioned above about each slab of steel 1-26 is shown. The balance in the average composition of the oxide refers to “impurities”, that is, Cr ₂ O ₃ , Na ₂ O, ZrO _{2 and the} like.

  About the slabs of the above steels 1 to 24, these were soaked at 1250 ° C., and then subjected to split rolling at a temperature range of 1100 to 1050 ° C. to form a steel slab of 160 mm × 160 mm, and the steel slab was further 1050 ° C. Then, the steel bar was rolled in a temperature range of 930 to 800 ° C. to produce a steel bar having a diameter of 70 mm (hereinafter referred to as “φ70 mm”).

  On the other hand, for the slabs of Steel 25 and Steel 26, these were soaked at 1250 ° C., and then rolled at a temperature range of 1100 to 1050 ° C. to obtain a steel slab of 160 mm × 160 mm. After heating to ° C., steel bars were rolled in a temperature range of 1100 to 1020 ° C. to produce φ70 mm steel bars.

  A block for measuring the oxide composition was cut out from the R / 2 part (φ represents the radius of the steel bar) of φ70 mm steel bar of steels 1 to 26 obtained as described above, and the block was made of resin. After embedding in L and mirror-polishing the L cross section, 20 arbitrary oxides having a thickness of 3 μm or more were selected by energy dispersive X-ray spectroscopy, and the respective compositions were measured.

  And the composition measured about 20 oxides was arithmetically averaged, and the "average composition" of the oxide in each steel bar of (phi) 70mm was calculated | required.

Further, 10 blocks of 100 mm ² are cut out from the R / 2 part of φ70 mm steel bars of steel 1 to 26 in the longitudinal direction, embedded in resin so that the L cross section becomes the test surface, and then mirror-polished, and then 100 mm The maximum thickness of the oxide and the maximum thickness of the sulfide existing in each L cross section of ² were measured using an optical microscope, and each was arithmetically averaged.

Specifically, assuming that the magnification of observation with an optical microscope is 400 times, first, oxides and sulfides having the largest thickness are detected in the L cross section of 100 mm ² , and then each magnification is 1000 times. Was measured for 10 blocks, and 10 arithmetic average values were obtained for each of the 10 blocks.

In addition, when the oxide and the sulfide are combined without separation, the thicknesses of the oxide and the sulfide are respectively measured, and when the thicknesses are the largest in the measured L cross section, Was arithmetically averaged as the oxide or sulfide having the largest thickness in the 100 mm ² L cross section.

Table 5 shows the average composition of the oxides measured as described above for each of the steels 1 to 26 with a diameter of 70 mm, and the arithmetic average value of the maximum thickness of the oxides present in the 10 L sections of 100 mm ^2. And the arithmetic mean value of the maximum thickness of sulfide. The balance in the average composition of the oxide refers to “impurities”, that is, Cr ₂ O ₃ , Na ₂ O, ZrO _{2 and the} like. In Table 5, the arithmetic average value of the maximum thickness of the oxide and the arithmetic average value of the maximum thickness of the sulfide are respectively expressed as “maximum oxide thickness” and “maximum sulfide thickness”. "". Also in the following description, the arithmetic average value of the maximum oxide thickness and the arithmetic average value of the maximum sulfide thickness are referred to as “maximum oxide thickness” and “maximum sulfide thickness”, respectively. Sometimes.

  A shaped material having a diameter of 60 mm and a thickness of 10 mm was sliced and collected from the steels 1 to 26 obtained as described above so as to have a thickness of the shaped material in the longitudinal direction.

  One side of a shaped material having a diameter of 60 mm and a thickness of 10 mm is subjected to induction quenching under the conditions of a frequency of 20 kHz, an output of 150 kW, and a heating time of 5 s, and then heated at 150 ° C. for 1 hour and allowed to cool in the air. Tempering was performed.

  In addition, the effective hardened layer depth was measured according to the prescription | regulation of JISG0559 (1996) about the shaped material induction-tempered and tempered as mentioned above. As a result, the effective hardened layer depth of each shaped material was 2 to 3 mm.

  In addition, for the above-described induction-quenched and tempered shaped material, the back surface of the surface that has been induction-hardened is ground, and then the surface of the surface that has been induction-quenched is lapped to provide a thickness of 5.0 mm. A dynamic fatigue test piece was prepared and subjected to a rolling fatigue test.

  The rolling fatigue test was performed using a thrust type rolling fatigue tester under conditions of a maximum contact surface pressure of 5490 MPa and a repetition rate of 1800 cpm (cycle per minute).

  Table 6 shows the detailed conditions of the rolling fatigue test.

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

  Table 5 also shows the rolling fatigue life determined as described above.

From Table 5, the chemical composition of steel and non-metallic inclusions (that is, the average composition of the oxide and the arithmetic average value of the maximum thickness of the oxide present in 10 areas of 100 mm ² of the L section of the steel) When the test numbers 1 to 15 satisfying the requirements of the present invention (1) to (3) are the arithmetic average value of the maximum thickness of sulfide, a long rolling fatigue life of 8.96 × 10 ⁶ or more is obtained. Has been obtained.

  On the other hand, even if the chemical composition of the steel satisfies the provisions of the present invention, the steel is manufactured by a method deviating from the provisions of the present invention (7), and the nonmetallic inclusions are defined by the present inventions (1) to (3). In the case of test number 16, test number 19, test number 22, and test number 24 that deviate from the conditions, the rolling fatigue life is short.

That is, in the case of each of the above test numbers, although the maximum thickness of the sulfide for the nonmetallic inclusions satisfies the conditions specified in the present invention (1) to (3), the average composition of the oxide is the present invention (1 ) To (3) deviate from the conditions specified in (3), the oxide becomes hard, and as a result, the maximum thickness of the oxide increases and deviates from the conditions specified in the present invention (1) to (3). The rolling fatigue life is as short as 4.12 × 10 ⁶ , 3.34 × 10 ⁶ , 4.32 × 10 ⁶ and 4.11 × 10 ⁶ , respectively.

  Also, the rolling fatigue life is short when the chemical composition of steel deviates from the provisions of the present invention.

That is, test number 17 and test number 21 are the non-metallic inclusions, although the maximum thickness of the sulfide satisfies the conditions specified in the present invention (1) to (3), but the steel 17 and steel 21 used Al Since the contents are as high as 0.009% and 0.008%, respectively, exceeding the values defined in the present invention, the average composition of the oxides deviates from the conditions defined in the present invention (1) to (3), As a result, the maximum thickness of the oxide is increased and deviates from the conditions defined in the present invention (1) to (3), so that the rolling fatigue life is 3.49 × 10 ⁶ and It is as short as 4.42 × 10 ⁶ .

Test number 18 and test number 20 are the non-metallic inclusions, although the average composition of the oxides satisfies the conditions defined in the present invention (1) to (3), the S content of steel 18 and steel 20 used is Since the values are as high as 0.015% and 0.022%, respectively, exceeding the values specified in the present invention, the maximum thickness of the sulfide increases and deviates from the conditions specified in the present invention (1) to (3). The rolling fatigue life is as short as 3.97 × 10 ⁶ and 2.81 × 10 ⁶ , respectively.

In test number 23, the average composition of oxides and the maximum thickness of sulfide satisfy the conditions specified in the present invention (1) to (3), but the O content of the steel 23 used is the nonmetallic inclusion. Since it is as high as 0.0027% and exceeds the value specified in the present invention, the maximum thickness of the oxide becomes large and deviates from the conditions specified in the present invention (1) to (3). .63 × 10 ⁶ and short.

Similarly, since the test numbers 25 and 26 used the steel 25 and the steel 26 corresponding to the conventional Al killed steel, the Al contents were as high as 0.021% and 0.018%, respectively. Therefore, for non-metallic inclusions, the maximum thickness of the sulfide satisfies the conditions specified in the present invention (1) to (3), but the average composition of the oxide is the present invention (1) to Since the condition defined in (3) is deviated, it becomes a hard oxide, and as a result, the maximum thickness of the oxide becomes large and deviates from the condition defined in the present invention (1) to (3). The lifetimes are as short as 3.21 × 10 ⁶ and 3.98 × 10 ⁶ , respectively.

(Example 2)
After the 300 mm × 400 mm slabs of Steel 3, Steel 12, Steel 15, Steel 18, Steel 22, and Steel 26 produced in Example 1 were soaked at 1250 ° C., they were rolled in the temperature range of 1100 to 1050 ° C. A steel piece of 160 × 160 mm was obtained.

  Next, using the above steel slab, a steel bar was rolled under the conditions shown in the following [1] to [5] to produce a steel bar having a diameter of 70 mm or 110 mm.

[1] After heating the steel slab to 1200 ° C., the steel bar is rolled in a temperature range of 1100 to 1020 ° C. to produce a φ70 mm steel bar,
[2] After heating the steel slab to 1050 ° C., the steel bar is rolled in a temperature range of 930 to 800 ° C. to produce a φ70 mm steel bar,
[3] After heating the steel slab to 950 ° C, the steel bar is rolled in a temperature range of 850 to 780 ° C to produce a φ70mm steel bar,
[4] After heating the steel slab to 1200 ° C., the steel bar is rolled in a temperature range of 1100 to 1020 ° C. to produce a φ110 mm steel bar,
[5] After the steel slab was heated to 1050 ° C., the steel bar was rolled in a temperature range of 930 to 800 ° C. to produce a φ110 mm bar.

  Further, after 300 mm × 400 mm slabs of Steel 3, Steel 12, Steel 15, Steel 18, Steel 22, and Steel 26 produced in Example 1 were soaked at 1250 ° C., a temperature range of 1100 to 1050 ° C. The steel piece was rolled into a 140 × 140 mm steel piece and further rolled using the steel piece under the conditions shown in [6] below to produce a φ100 mm steel bar.

  [6] After the steel slab was heated to 1050 ° C., the steel bar was rolled at a temperature range of 930 to 800 ° C. to produce a φ100 mm bar.

  Tables 7 and 8 show the details of the manufacturing conditions for the above steel bars.

  A block for measuring the oxide composition was cut out from the R / 2 part of φ70 mm, φ100 mm, and φ110 mm steel bars of Steel 3, Steel 12, Steel 15, Steel 18, Steel 22, and Steel 26 produced as described above. After embedding the block in a resin and mirror-polishing the L cross section, 20 arbitrary oxides having a thickness of 3 μm or more were selected by energy dispersive X-ray spectroscopy, and the respective compositions were measured.

  And the composition measured about 20 oxides was arithmetic-averaged, and the "average composition" of the oxide in the bar steel of (phi) 70mm, (phi) 100mm, and (phi) 110mm was calculated | required.

Further, 10 blocks of 100 mm ² are cut out in the longitudinal direction from the R / 2 part of the steel bars of steel 3, steel 12, steel 15, steel 18, steel 22, and steel 26 of φ70 mm, φ100 mm, and φ110 mm, and the L cross section is cut. It is embedded in a resin so as to be a test surface and mirror-polished, and then the maximum thickness of oxide and the maximum thickness of sulfide existing in each L cross section of 100 mm ² are measured using an optical microscope, , Arithmetic average.

Specifically, assuming that the magnification of observation with an optical microscope is 400 times, first, oxides and sulfides having the largest thickness are detected in the L cross section of 100 mm ² , and then each magnification is 1000 times. Was measured for 10 blocks, and 10 arithmetic average values were obtained for each of the 10 blocks.

In addition, when the oxide and the sulfide are combined without separation, the thicknesses of the oxide and the sulfide are respectively measured, and when the thicknesses are the largest in the measured L cross section, Was arithmetically averaged as the oxide or sulfide having the largest thickness in the 100 mm ² L cross section.

Tables 9 and 10 show the average composition of oxides measured as described above for each of the above steel bars and the arithmetic average value and sulfurization of the maximum thickness of oxides present in 10 100 mm ² L cross sections. Indicates the arithmetic average value of the maximum thickness of the object. As described above, the balance in the average composition of the oxide refers to “impurities”, that is, Cr ₂ O ₃ , Na ₂ O, ZrO _{2 and the} like. Further, “maximum oxide thickness” and “maximum sulfide thickness” refer to the arithmetic average value of the maximum oxide thickness and the arithmetic average value of the maximum sulfide thickness, respectively.

  Also, the steel 3, steel 12, steel 15, steel 18, steel 22, and steel 26 obtained as described above are made of steel bars of φ70 mm, φ100 mm, and φ110 mm so that the longitudinal direction is the thickness of the shaped material. The raw material having a diameter of 60 mm and a thickness of 10 mm was sliced and collected.

  One side of a shaped material having a diameter of 60 mm and a thickness of 10 mm is subjected to induction quenching under the conditions of a frequency of 20 kHz, an output of 150 kW, and a heating time of 5 s, and then heated at 150 ° C. for 1 hour and allowed to cool in the air. Tempering was performed.

  In addition, the effective hardened layer depth was measured according to the prescription | regulation of JISG0559 (1996) about the shaped material induction-tempered and tempered as mentioned above. As a result, the effective hardened layer depth of each shaped material was 2 to 3 mm.

  In addition, for the above-described induction-quenched and tempered shaped material, the back surface of the surface that has been induction-hardened is ground, and then the surface of the surface that has been induction-quenched is lapped to provide a thickness of 5.0 mm. A dynamic fatigue test piece was prepared and subjected to a rolling fatigue test.

  The rolling fatigue test was performed using a thrust type rolling fatigue tester under conditions of a maximum contact surface pressure of 5490 MPa and a repetition rate of 1800 cpm.

  The detailed conditions of the rolling fatigue test are as shown in Table 6 above.

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

  Tables 9 and 10 show the rolling fatigue life determined as described above.

From Table 9 and Table 10, it manufactures by the method of this invention (7), and the chemical composition and nonmetallic inclusion (namely, average composition of an oxide) of steel satisfy | fill the prescription | regulation of this invention (4)-(6). In the case of Test No. 28, Test No. 29, Test No. 34, Test No. 35, Test No. 40 and Test No. 41 in which the slab was squeezed by the method of the present invention (4) to (6), 9.73 × It can be seen that a long rolling fatigue life of 10 ⁶ or more is obtained.

On the other hand, a slab manufactured by the method of the present invention (7), in which the chemical components and non-metallic inclusions (that is, the average composition of the oxide) of the steel satisfy the provisions of the present invention (4) to (6) Even when a reduction in which the total reduction ratio is 15 or more is added to the above, the reduction ratio in the temperature range of 1000 ° C. or less is out of the conditions of the present invention (4) to (6). In the case of Test No. 27, Test No. 32, Test No. 33, Test No. 38, Test No. 39, and Test No. 44 squeezed with the maximum thickness of the sulfide, the present invention (1) to (3) Deviates from the conditions specified in. Therefore, the rolling fatigue life is 4.27 × 10 ⁶ , 3.98 × 10 ⁶ , 3.93 × 10 ⁶ , 4.08 × 10 ⁶ , 4.12 × 10 ⁶ and 3.68 × 10 ^6. And short.

In addition, a slab manufactured by the method of the present invention (7) was used, in which the chemical components and non-metallic inclusions (that is, the average composition of the oxide) of the steel satisfy the requirements of the present invention (4) to (6). Even in this case, the test number 30, the test number 31, the test number 36, the test number 37, the test number 42, and the test number 43 were reduced by the method in which the total reduction ratio was outside the conditions of the present invention (4) to (6). In this case, since the maximum thickness of the oxide and the maximum thickness of the sulfide are increased and deviate from the conditions defined in the present invention (1) to (3), the rolling fatigue life is 2.32 × 10 2 respectively. ⁶ and 3.45 × 10 ⁶ , 3.24 × 10 ⁶ , 3.73 × 10 ⁶ , 3.86 × 10 ⁶ and 2.69 × 10 ⁶ .

Furthermore, even if the nonmetallic inclusions (that is, the average composition of the oxide) satisfy the requirements of the present inventions (4) to (6), the S content as a chemical component of the steel is the present invention (4) to (6). In the case of test numbers 45 to 50 using slabs that deviate from the provisions of the above, the maximum thickness of the sulfide is increased regardless of the rolling conditions and deviates from the conditions prescribed in the present invention (1) to (3), In addition, depending on the rolling conditions, the maximum thickness of the oxide is increased as shown in Test No. 48 and Test No. 49 and is not within the conditions defined in the present invention (1) to (3). .54 × 10 ⁶ , 3.97 × 10 ⁶ , 4.27 × 10 ⁶ , 2.78 × 10 ⁶ , 2.89 × 10 ⁶ and 3.32 × 10 ⁶ .

A slab in which the non-metallic inclusions (that is, the average composition of oxides) deviate from the provisions of the present inventions (4) to (6) even if the chemical composition of the steel satisfies the provisions of the present inventions (4) to (6). In the case of the test numbers 51 to 56 used, the maximum thickness of the oxide is increased regardless of the rolling conditions and deviates from the conditions defined in the present invention (1) to (3). It is as short as 3.21 × 10 ⁶ , 4.32 × 10 ⁶ , 4.06 × 10 ⁶ , 2.98 × 10 ⁶ , 2.52 × 10 ⁶ and 3.11 × 10 ⁶ .

Test numbers 57 to 62 using slabs in which the chemical components and non-metallic inclusions (that is, the average composition of oxides) corresponding to conventional Al killed steel deviate from the provisions of the present invention (4) to (6) In this case, the maximum thickness of the oxide becomes large regardless of the rolling condition and deviates from the conditions specified in the present invention (1) to (3). Therefore, the rolling fatigue life is 3.98 × 10 ⁶ , respectively. It is as short as 3.35 × 10 ⁶ , 4.21 × 10 ⁶ , 2.67 × 10 ⁶ , 2.97 × 10 ⁶ and 3.15 × 10 ⁶ .

  The steel material for induction hardening according to the present invention has good durability against damage due to rolling fatigue and has a long rolling fatigue life even under severe use environment of rolling members in recent years. It can be used as a material for rolling members that perform induction hardening, such as “constant velocity joints” and “hub units”. This steel for induction hardening can be manufactured by the method of the present invention.

Claims (7)

In mass%, C: 0.35 to 0.7%, Si: 0.1 to 0.8%, Mn: 0.1 to 1.5%, P: 0.03% or less, S: 0.010 %: Cr: 0.01% or more and less than 0.50%, Al: 0.005% or less, Ca: 0.0005% or less, O: 0.0020% or less, N: 0.02% or less The balance consists of chemical components of Fe and impurities. For non-metallic inclusions, the average composition of the oxide is mass%, CaO: 10 to 60%, Al ₂ O ₃ : 20% or less, MnO: 50% or less, and MgO: 15% or less of the remaining SiO ₂ and impurities, and the arithmetic average value of the maximum oxide thickness and the maximum thickness of sulfide existing in 10 areas of 100 mm ^{2 in} the longitudinal longitudinal section of the steel material Inductive quenching characterized in that the arithmetic average value of each is 8.5 μm or less Wood. In mass%, C: 0.35 to 0.7%, Si: 0.1 to 0.8%, Mn: 0.1 to 1.5%, P: 0.03% or less, S: 0.010 % Or less, Cr: 0.01% or more and less than 0.50%, Al: 0.005% or less, Ca: 0.0005% or less, O: 0.0020% or less, N: 0.02% or less V: 0.3% or less and Nb: 0.1% or less, and the balance consists of chemical components of Fe and impurities, and the non-metallic inclusions have an average oxide composition of mass. % _{in, CaO: 10~60%, Al 2} O 3: 20% or less, MnO: 50% or less and MgO: with the balance consisting of SiO ₂ and impurities 15% or less, of the 10 positions in the longitudinal direction longitudinal section of the steel material Arithmetic mean value of maximum thickness of oxide existing in 100 mm ² area and arithmetic mean value of maximum thickness of sulfide Are steel materials for induction hardening, each being 8.5 μm or less. In mass%, C: 0.35 to 0.7%, Si: 0.1 to 0.8%, Mn: 0.1 to 1.5%, P: 0.03% or less, S: 0.010 % Or less, Cr: 0.01% or more and less than 0.50%, Al: 0.005% or less, Ca: 0.0005% or less, O: 0.0020% or less, N: 0.02% or less B: 0.005% or less and Ti: 0.05% or less, with the balance being composed of chemical components of Fe and impurities, with respect to non-metallic inclusions, the average composition of oxides is mass%, and CaO: 10 _{~60%, Al 2 O 3:} 20% or less, MnO: 50% or less and MgO: with the balance consisting of SiO ₂ and impurities 15% or less, in the area of 100 mm ² of 10 positions in the longitudinal vertical section of the steel material The arithmetic average value of the maximum thickness of oxide present and the arithmetic average value of the maximum thickness of sulfide are A steel material for induction hardening, which is 8.5 μm or less. To the slab or steel ingot having the average composition of the chemical components and oxides according to claim 1, a reduction in which the total reduction ratio is 15 or more is applied, and in that reduction, in a temperature range of 1000 ° C. or less. A method for producing a steel material for induction hardening, wherein the reduction is performed with a reduction ratio of 4 or more.
However, the total reduction ratio refers to the value obtained by dividing the cross-sectional area of the slab or steel ingot by the cross-sectional area of the steel for induction hardening obtained by the final reduction, and the reduction ratio in the temperature range of 1000 ° C or lower. The term refers to a value obtained by dividing the cross-sectional area of the intermediate steel material before the reduction in the temperature range by the cross-sectional area of the steel material for induction hardening obtained by the final reduction. To the slab or steel ingot having the average composition of the chemical component and oxide according to claim 2, a reduction in which the total reduction ratio is 15 or more is added, and in that reduction, in a temperature range of 1000 ° C. or less. A method for producing a steel material for induction hardening, wherein the reduction is performed with a reduction ratio of 4 or more.
However, the total reduction ratio refers to the value obtained by dividing the cross-sectional area of the slab or steel ingot by the cross-sectional area of the steel for induction hardening obtained by the final reduction, and the reduction ratio in the temperature range of 1000 ° C or lower. The term refers to a value obtained by dividing the cross-sectional area of the intermediate steel material before the reduction in the temperature range by the cross-sectional area of the steel material for induction hardening obtained by the final reduction. To the slab or steel ingot having the average composition of the chemical components and oxides according to claim 3, a reduction in which the total reduction ratio is 15 or more is added, and in that reduction, in a temperature range of 1000 ° C. or less. A method for producing a steel material for induction hardening, wherein the reduction is performed with a reduction ratio of 4 or more.
However, the total reduction ratio refers to the value obtained by dividing the cross-sectional area of the slab or steel ingot by the cross-sectional area of the steel for induction hardening obtained by the final reduction, and the reduction ratio in the temperature range of 1000 ° C or lower. The term refers to a value obtained by dividing the cross-sectional area of the intermediate steel material before the reduction in the temperature range by the cross-sectional area of the steel material for induction hardening obtained by the final reduction. The slab or steel ingot is subjected to secondary refining using a flux that does not substantially contain Al after oxidative refining without performing Al deoxidation treatment, and the basicity of the final slag after completion of secondary refining The value of CaO / SiO ₂ is 0.8 to 2.0, and the slag composition is mass%, MgO: 15% or less, F: 10% or less, Al ₂ O ₃ : 15% or less. The method for producing a steel material for induction hardening according to any one of claims 4 to 6, wherein the steel material is subsequently cast.