JP5163241B2 - Case-hardened steel - Google Patents

Description

  The present invention relates to case-hardened steel. Specifically, the present invention relates to a case-hardened steel having a low component cost, excellent bending fatigue strength and pitting resistance, and suitable for use as a material for carburized parts such as automobile gears.

  Automotive gears used in automobile parts, especially transmissions, are generally tempered after surface hardening treatment such as carburizing and quenching from the viewpoint of improving bending fatigue strength at the tooth root and improving pitching strength of the tooth surface. It is manufactured by applying.

In addition, the above-mentioned “carburizing and quenching” generally uses low-carbon “skin-hardened steel” as material steel (dough steel), and invades and diffuses C in a high temperature austenite region of Ac ₃ points or more. After that, it is a quenching process.

  In recent years, automobiles are required to be lightweight and high torque. For this reason, carburized parts such as the above-mentioned automobile gears require higher bending fatigue strength and higher pitching strength than ever before. In the present specification, “carburized parts” will be described below by using “gears” as representatives.

  Increasing the amount of alloying elements such as Ni, Cr, and Mo in case-hardened steel can ensure high bending fatigue strength and high pitching strength in the gear, but increases the component cost due to the increase in alloying elements. .

  However, on the other hand, both Ni and Mo are important elements that increase the depth of the carburized layer and the hardness of the core, and both Ni and Mo are non-oxidizing elements. In addition, it has an effect of improving the hardenability of the carburized layer without increasing the depth of the grain boundary oxide layer formed on the surface.

  For this reason, “nickel chrome molybdenum steel” such as SNCM220H defined in JIS G 4052 (2003) and “chromium molybdenum steel” such as SCM420H are used as “skin-hardened steel” as a material of gears. In particular, in light of recent increases in the prices of Ni and Mo, it is possible to suppress the content of Ni and Mo as much as possible, to reduce the component cost, and to provide the gear with high bending fatigue strength and high pitching strength. There is a great demand for case-hardened steel.

  Therefore, in order to meet the above-mentioned demand, for example, Patent Document 1 and Patent Document 2 propose “high chromium steel for carburizing and carbonitriding” and “manufacturing method of high fatigue strength case-baked product”, respectively.

  Specifically, in Patent Document 1, in mass percent, C = 0.10 to 0.30%, Si = 0.15% or less, Mn = 0.90 to 1.40%, P = 0.015% Hereinafter, Cr = 1.25 to 1.70%, Al = 0.010 to 0.050%, Nb = 0.001 to 0.050%, O = 0.0015% or less, N = 0.0100 to 0 0.0200%, if necessary, (a) Ni = 0.15% or less, Mo = 0.10% or less, or (b) Ti = 0.005-0 0.015%, (c) S = 0.005 to 0.035%, Pb = 0.01 to 0.09%, Bi = 0.04 to 0.20%, Te = 0.002 to 0.050% , Zr = 0.01-0.20%, Ca = 0.0001-0.0100%, one or two or more of three groups of elements Or a combination of two or more, the remaining Fe and inevitable impurity element steel is heated to 1200 ° C or higher, and after finishing hot forming such as hot rolling at a finishing temperature of 800 ° C or higher, 30 ° C / min or higher. “Chromium steel for carburizing and carbonitriding”, which is obtained by cooling to 600 ° C. or less at an average cooling rate of 5%, is disclosed.

In Patent Document 2, as weight ratios, C: 0.10 to 0.30%, Mn: 0.50 to 2.0%, S: 0.01 to 0.20%, Cr: 0.50 1.50%, Al: 0.02 to 0.10%, N: 0.010 to 0.025%, if necessary, (a) Nb: 0.020 to 0.120%, Ti: One or two of 0.005 to 0.10%, (b) Ni: 4.0% or less, Mo: 1.0% or less, V: 1.0% or less, Cu: 3. Contains one or two elements in combination of 1 or 2% of 0% or less, Si: 0.10% or less, P: 0.010% or less, O: 0.005% or less The steel material consisting of the remaining Fe and inevitable impurities is processed into the required product shape, and the amount of retained austenite at the surface layer of 0.02 mm is reduced to the area ratio. After performing the carburizing becomes such conditions 20 to 60 percent range, the stress concentration portion, the repeated bending stress in the range of 70~120kgf / mm ² at maximum stress the net at the outermost surface, 10 ³ times A “method for producing a high fatigue strength case-baked product” characterized by the following is disclosed.

JP 2001-152284 A Japanese Patent Laid-Open No. 2-259012

  Although the technique disclosed in Patent Document 1 described above has a technical idea of suppressing the grain boundary oxidation by suppressing the Si content to a low level, the grain boundary oxide layer and the incompleteness that cause a decrease in bending fatigue strength and pitching strength are included. No consideration has been given to suppressing the depth of the hardened layer (hereinafter, sometimes referred to collectively as the “carburized abnormal layer”). For this reason, it has not necessarily been able to ensure high bending fatigue strength and high pitching strength in the gear.

  Although the technique disclosed in Patent Document 2 also has the technical idea of reducing grain boundary oxidation by limiting the Si content to 0.1% or less, the carburizing abnormal layer that reduces bending fatigue strength and pitting strength. No consideration has been given to controlling depth. For this reason, it has not necessarily been able to ensure high bending fatigue strength and high pitching strength in the gear. Moreover, in the case of the technique disclosed in Patent Document 2, the generation of coarse MnS that becomes a starting point of cracking when hot rolling the raw steel or hot forging or cold forging into a desired product shape is suppressed. There is no consideration for this. For this reason, hot workability such as hot rolling and hot forging and cold forgeability are reduced, and cracks frequently occur in production on an industrial scale, leading to a significant decrease in product yield. Furthermore, since the above-mentioned coarse MnS itself decreases the bending fatigue strength and the pitching strength, the desired high bending fatigue strength and high pitching strength may not be ensured.

  Accordingly, an object of the present invention is to provide SNCM220H of “nickel chrome molybdenum steel” defined in JIS G 4052 (2003) and gears even when the expensive elements Ni and Mo are not contained as much as possible. Bending fatigue strength and pitching strength can be ensured to the same extent or higher than when SCM420H of “chromium molybdenum steel” is used as raw material steel, and the component cost is low, and hot and cold rolling and forging are also possible. It is to provide a case-hardened steel that also has good workability.

  The present inventors have made various studies in order to solve the above-described problems. As a result, first, the following findings (a) to (d) were obtained.

  (A) In order to ensure high bending fatigue strength and high pitching strength without containing Ni and Mo as much as possible, the composition of the steel is reduced in the hardenability caused by reducing the Ni and Mo contents. It needs to be able to be deterred.

  (B) Since the generation of coarse MnS causes a decrease in bending fatigue strength and pitching strength, it is necessary to suppress the formation of coarse MnS in order to ensure high bending fatigue strength and high pitching strength. .

  (C) Coarse MnS becomes a starting point of cracks during hot working such as hot rolling and hot forging and cracks during cold forging. For this reason, it is necessary to reduce coarse MnS as much as possible in order to suppress cracking during hot working and cracking during cold forging.

  (D) In order to reduce coarse MnS as much as possible, it is necessary not only to control the individual contents of Mn and S but also to optimize the balance of the contents of Mn and S. Specifically, by controlling the value of fn1 represented by the formula of [fn1 = Mn / S] to be 30 or more and 150 or less, with the element symbol in the formula being the content in mass% of the element, Generation of coarse MnS can be suppressed. Therefore, in order to ensure good hot workability and cold forgeability to suppress cracking during hot working and cold forging, and to ensure high bending fatigue strength and high pitching strength, Mn It is necessary to control the individual contents of S and S so that they satisfy the above relational expression.

  Therefore, various studies have been made on steel that secures hardenability corresponding to the reduction in Ni and Mo contents, and suppresses the formation of coarse MnS by optimizing the Mn and S contents and their balance. Went. As a result, the following findings (e) to (h) were obtained.

  (E) High bending fatigue strength and high pitting strength cannot be ensured only by guaranteeing the decrease in hardenability caused by reducing the Ni and Mo contents and suppressing the formation of coarse MnS, and hardenability. It is also necessary to reduce the depth of the carburized abnormal layer, that is, the depth of the grain boundary oxide layer and the incompletely hardened layer, in addition to securing the formation of coarse MnS.

  (F) By optimizing the content balance of oxidizing elements, especially Cr, Si, and Mn, the depth of the grain boundary oxide layer and the incompletely hardened layer, which are carburized abnormal layers, can be reduced. . Specifically, the value of fn2 represented by the formula [fn2 = Cr / (Si + 2Mn)] is 0.7 or more and 1.1 or less, where the element symbol in the formula is the content in mass% of the element. By doing so, it becomes possible to reduce the depth of the carburized abnormal layer, and it is possible to ensure high bending fatigue strength and high pitching strength.

(G) In order to ensure high bending fatigue strength and high pitching strength, type B and type D large hard inclusions measured according to ASTM-E45-05 method A, that is, mainly Al ₂ Of type B inclusions that are O ₃ inclusions and type D inclusions that are mainly TiN inclusions, it is necessary to suppress those having a large thickness. This is because the large hard inclusions of type B and type D described above serve as starting points for fatigue failure.

  (H) In order to suppress the large hard inclusions of type B and type D described above, the content of Ti and O (oxygen) among impurities is particularly less than 0.005% and 0.0015%, respectively. It is necessary to control the following. In order to suppress large hard inclusions of type B and type D, when ingots are melted in a vacuum melting furnace, or in a converter, secondary refining is repeated or continuous casting. In this case, it is desirable to perform electromagnetic stirring.

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

(1) By mass%, C: 0.15 to 0.30%, Si: more than 0.10% and 1.0% or less, Mn: 0.30 to 1.0%, S: 0.030% Hereinafter, Cr: more than 1.25% and 3.0% or less, Mo: 0.04 to 0.10%, Al: 0.010 to 0.050% and N: 0.0100 to 0.0250% In addition, the contents of Si, Mn, Cr and S are 30 ≦ fn1 ≦ 150 and 0.7 ≦ fn2 in the values of fn1 and fn2 represented by the following formulas (1) and (2), respectively. ≦ 1.1, the balance is made of Fe and impurities, and P, Ti and O (oxygen) in the impurities are P: 0.020% or less, Ti: less than 0.005% and O: 0.0015, respectively. % Case hardened steel.
fn1 = Mn / S (1),
fn2 = Cr / (Si + 2Mn) (2).
However, the element symbols in the formulas (1) and (2) represent the content in mass% of the element.

  (2) The above (1) characterized by containing one or two of Cu: 0.20% or less and Ni: 0.20% or less in mass% instead of a part of Fe The case-hardened steel described in 1.

  (3) The above (1), characterized in that, instead of a part of Fe, one or two of V: 0.20% or less and Nb: 0.050% or less are contained in mass% Or the case hardening steel as described in (2).

  (4) The case-hardened steel according to any one of (1) to (3) above, which contains Ca: 0.0050% or less in mass% instead of part of Fe.

  Hereinafter, the inventions related to the case-hardened steels (1) to (4) are referred to as “present invention (1)” to “present invention (4)”, respectively. Also, it may be collectively referred to as “the present invention”.

  The case-hardened steel of the present invention has a low component cost, has good workability during hot and cold rolling and forging, and carburized parts made from this case-hardened steel are JIS G 4052. The bending fatigue strength and the pitching strength are the same as or higher than those of carburized parts made of SNCM220H of “nickel chromium molybdenum steel” and SCM420H of “chromium molybdenum steel” defined in (2003). For this reason, the case-hardened steel of the present invention is suitable for use as a material for carburized parts such as automobile gears that require high bending fatigue strength and high pitching strength in order to reduce weight and torque.

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

C: 0.15-0.30%
C is an essential element for securing the strength of the gear, and a content of 0.15% or more is necessary. However, if the C content is too large, the hardness increases and the machinability is lowered. In particular, if the content exceeds 0.30%, the machinability is significantly lowered as the hardness increases. Become. Therefore, the content of C is set to 0.15 to 0.30%.

  In addition, when much better machinability is required, the C content is preferably 0.15 to 0.20%.

Si: more than 0.10% and 1.0% or less Si has an effect of improving hardenability and a deoxidizing action. Moreover, Si has a temper softening resistance and has an effect of preventing the softening of steel under high temperature conditions. In order to obtain these effects, it is necessary to contain Si in an amount exceeding 0.10%. However, since Si is an oxidizing element, when its content increases, Si is selectively oxidized by a small amount of H ₂ O or CO ₂ contained in the carburizing gas, and Si oxide is generated on the steel surface. Therefore, the depths of the grain boundary oxide layer and the incompletely hardened layer, which are carburized abnormal layers, are increased. And if the depth of a carburizing abnormal layer becomes large, it will cause the fall of bending fatigue strength and pitching strength. Further, when the Si content increases, the effect of temper softening resistance is saturated, and the machinability also decreases. In particular, when the Si content exceeds 1.0%, the bending fatigue strength and the pitching strength are significantly decreased due to the increased depth of the carburized abnormal layer, and the machinability is also significantly decreased. Therefore, the Si content is more than 0.10% and 1.0% or less.

  When better machinability is required, the Si content is preferably more than 0.10% and 0.70% or less.

  In addition, Si content needs to satisfy | fill 0.7 <= fn2 <= 1.1 also in the value of fn2 represented by said Formula (2) in said range.

Mn: 0.30 to 1.0%
Mn has an action of improving hardenability and a deoxidizing action. In order to obtain these effects, a content of 0.30% or more is necessary. However, when the Mn content increases, the hardness increases and machinability is lowered. In particular, when the content exceeds 1.0%, the machinability is significantly lowered as the hardness is increased. Become. Moreover, since Mn is an oxidizing element like Si, when its content increases, Mn oxide is generated on the steel surface. Therefore, the grain boundary oxide layer and the incompletely hardened layer, which are carburized abnormal layers, are formed. The depth of will increase. And when the depth of the carburizing abnormal layer is increased, the bending fatigue strength and the pitching strength are reduced. In particular, when the Mn content exceeds 1.0%, the bending fatigue strength due to the increase in the depth of the carburizing abnormal layer and The decrease in pitching strength becomes significant. Therefore, the content of Mn is set to 0.30 to 1.0%. In addition, the minimum with preferable Mn content is 0.60%. Moreover, a preferable upper limit is 0.90%.

  The Mn content is within the above range, and the values of fn1 and fn2 represented by the above formulas (1) and (2) are 30 ≦ fn1 ≦ 150 and 0.7 ≦ fn2 ≦ 1.1, respectively. It is necessary to satisfy.

S: 0.030% or less S is an element contained as an impurity. In addition, although S has the effect | action which couple | bonds with Mn and forms MnS and improves machinability, when content of S exceeds 0.030%, coarse MnS will be formed and it will be hot. Workability, cold forgeability, bending fatigue strength and pitting strength are reduced. Therefore, the content of S is set to 0.030% or less.

  In order to reliably obtain the effect of improving the machinability of S described above, the S content is preferably set to 0.010% or more. For this reason, the desirable S content is 0.010 to 0.030%.

  When even better hot workability, cold forgeability, bending fatigue strength and pitching strength are required, the S content is preferably set to 0.010 to 0.020%.

  In addition, S content needs to satisfy | fill 30 <= fn1 <= 150 also in the value of fn1 represented by said Formula (1) in said range.

Cr: more than 1.25% and 3.0% or less Cr has an effect of improving hardenability. In order to obtain this effect, a content exceeding 1.25% is required. However, as the Cr content increases, the hardness increases and machinability is reduced. In particular, when the content exceeds 3.0%, the machinability is significantly reduced as the hardness increases. Become. Moreover, since Cr is an oxidizing element like Si and Mn, when its content increases, Cr oxide is generated on the steel surface. The depth of the layer increases. And when the depth of the carburizing abnormal layer is increased, the bending fatigue strength and the pitching strength are reduced. In particular, when the Cr content exceeds 3.0%, the bending fatigue strength due to the increase in the depth of the carburizing abnormal layer and The decrease in pitching strength becomes significant. Therefore, the Cr content is more than 1.25% and 3.0% or less.

  When even better machinability is required, the Cr content is preferably more than 1.25% and not more than 2.0%.

  In addition, the content of Cr is within the above range, and the value of fn2 represented by the above formula (2) needs to satisfy 0.7 ≦ fn2 ≦ 1.1.

Mo: 0.04 to 0.10%
Mo has the effect | action which improves hardenability, has the effect of ensuring the strength of carburized components by improving the surface hardness, the hardened layer depth, and the core hardness after carburizing and quenching. Moreover, since Mo is a non-oxidizing element, the steel surface can be toughened without increasing the depth of the grain boundary oxide layer during carburizing. In order to obtain these effects, the Mo content needs to be 0.04% or more. However, Mo is an expensive element, and excessive addition leads to an increase in component cost. In particular, when the Mo content exceeds 0.10%, the cost increase becomes large. Therefore, the Mo content is set to 0.04 to 0.10%. In addition, preferable Mo content is 0.07 to 0.10%.

Al: 0.010 to 0.050%
Al has a deoxidizing action. Moreover, Al also has the effect | action which combines with N, forms AlN, refines | miniaturizes a crystal grain, and strengthens steel. However, when the Al content is less than 0.010%, it is difficult to obtain the above effect. On the other hand, when the Al content is excessive, the machinability is lowered due to the formation of hard and coarse Al ₂ O ₃ , and the bending fatigue strength and the pitching strength are also lowered. In particular, when the Al content exceeds 0.050%, the machinability, bending fatigue strength, and pitching strength are significantly reduced. Therefore, the content of Al is set to 0.010 to 0.050%. In addition, the minimum with preferable Al content is 0.020%. Moreover, a preferable upper limit is 0.040%.

N: 0.0100 to 0.0250%
N has the effect of making the crystal grains finer by forming nitrides and improving the bending fatigue strength. In order to acquire this effect, it is necessary to contain N 0.0100% or more. However, when the N content is excessive, coarse nitrides are formed, leading to a decrease in toughness. In particular, when the content exceeds 0.0250%, the toughness is significantly decreased. Therefore, the N content is set to 0.0100 to 0.0250%. In addition, the minimum with preferable N content is 0.0130%. Moreover, a preferable upper limit is 0.0200%.

fn1 value: 30 to 150 Since the generation of coarse MnS causes a decrease in bending fatigue strength and pitching strength, the production of coarse MnS is suppressed in order to ensure high bending fatigue strength and high pitching strength. It is necessary. Moreover, the above coarse MnS is also a starting point for cracking during hot working such as hot rolling and hot forging and cracking during cold forging, so cracking during hot working and cracking during cold forging. In order to suppress this, it is necessary to reduce coarse MnS as much as possible.

  When the value of fn1 represented by the above formula (1) is smaller than 30, the content of S is excessive and the generation of coarse MnS is unavoidable, whereas the value of fn1 is larger than 150 In this case, the Mn content becomes excessive, and coarse MnS is generated in the central segregation part. Therefore, in any case, the bending fatigue strength and the pitching strength are lowered, and it is difficult to avoid cracks during hot working and cold forging. Therefore, the value of fn1 represented by the above formula (1), that is, [fn1 = Mn / S] is set to satisfy 30 ≦ fn1 ≦ 150. The preferred lower limit of the value of fn1 is 50. The preferred upper limit is 100.

fn2 value: 0.7 or more and 1.1 or less In order to provide high bending fatigue strength and high pitching strength without containing Ni and Mo as much as possible, it is a carburizing abnormal layer while ensuring hardenability. It is necessary to reduce the depth of the grain boundary oxide layer and the incompletely hardened layer. For that purpose, the content of Cr, Si, and Mn among the oxidizing elements is set in the above range, and the content balance of these elements is expressed by the formula (2). The value of fn2 needs to be 0.7 or more and 1.1 or less. That is, when the value of fn2 expressed by the above equation (2) is smaller than 0.7 and larger than 1.1, the depth of the carburized abnormal layer increases, so that the bending fatigue strength and the pitching are increased. Strength will fall. Therefore, the value of fn2 represented by the above equation (2), that is, [fn2 = Cr / (Si + 2Mn)] is set to satisfy 0.7 ≦ fn2 ≦ 1.1. A preferable range of the value of fn2 is 0.8 ≦ fn2 ≦ 1.1.

  In the present invention, the contents of P, Ti and O (oxygen) in the impurities are limited to P: 0.020% or less, Ti: less than 0.005% and O: 0.0015% or less, respectively. There is a need.

  This will be described below.

P: 0.020% or less P is an impurity contained in the steel and segregates at the grain boundaries to embrittle the steel. In particular, when the content exceeds 0.020%, the degree of embrittlement becomes significant. Therefore, in the present invention, the content of P in the impurities is set to 0.020% or less. In addition, it is preferable that content of P in an impurity shall be 0.010% or less.

Ti: Less than 0.005% Ti has a high affinity with N, so it combines with N in steel to form TiN, which is a hard, coarse non-metallic inclusion, reducing bending fatigue strength and pitting strength. Furthermore, machinability is also reduced. Therefore, in the present invention, the content of Ti in the impurities is less than 0.005%.

O (oxygen): 0.0015% or less O (oxygen) combines with Si or Al in steel to generate an oxide. Among the oxides, in particular, Al ₂ O ₃ is hard, so that machinability is reduced, and further, bending fatigue strength and pitching strength are reduced. Therefore, in the present invention, the content of O in the impurities is set to 0.0015% or less.

  For the above reasons, the case-hardened steel according to the present invention (1) has C: 0.15 to 0.30%, Si: more than 0.10% and 1.0% or less, Mn: 0.30 to 1 0.0%, S: 0.030% or less, Cr: more than 1.25% to 3.0% or less, Mo: 0.04 to 0.10%, Al: 0.010 to 0.050%, and N : 0.0100 to 0.0250%, and the contents of Si, Mn, Cr and S are 30 in terms of the values of fn1 and fn2 represented by the above formulas (1) and (2), respectively. ≦ fn1 ≦ 150 and 0.7 ≦ fn2 ≦ 1.1 are satisfied, the balance is made of Fe and impurities, and P, Ti and O (oxygen) in the impurities are respectively P: 0.020% or less, Ti: 0 Less than 0.005% and O: 0.0015% or less.

In addition, the case-hardened steel according to the present invention (1) is further replaced with a part of Fe, if necessary,
First group: one or two of Cu: 0.20% or less and Ni: 0.20% or less,
2nd group: V: 0.20% or less and Nb: 1 type or 2 types of 0.050% or less,
Group 3: Ca: 0.0050% or less,
One or more elements of each group can be selectively contained.

  That is, in order to obtain even better characteristics, instead of a part of Fe in the case-hardened steel of the present invention (1), one or more elements of any one of the groups from the first group to the third group are used. You may make it contain as an arbitrary element.

  Hereinafter, the above optional elements will be described.

First group: one or two of Cu: 0.20% or less and Ni: 0.20% or less Both Cu and Ni have an effect of improving hardenability. For this reason, when it is desired to obtain greater hardenability, it may be contained in the following range.

Cu: 0.20% or less Cu has an effect of improving hardenability, and may be contained for further improving hardenability. However, Cu is an expensive element, and as the content increases, hot workability is deteriorated. Particularly, when it exceeds 0.20%, the hot workability is remarkably deteriorated. Therefore, the Cu content is set to 0.20% or less.

  In order to reliably obtain the effect of improving the hardenability of Cu described above, the Cu content is preferably set to 0.05% or more. For this reason, the more desirable amount of Cu in the case of containing is 0.05 to 0.20%.

Ni: 0.20% or less Ni has an effect of improving hardenability. Ni has the effect of improving toughness and is a non-oxidizing element, so that the steel surface can be toughened without increasing the depth of the grain boundary oxide layer during carburizing. For this reason, in order to acquire these effects, you may contain Ni. However, Ni is an expensive element, and excessive addition leads to an increase in component cost. In particular, when the Ni content exceeds 0.20%, the cost increase becomes large. Therefore, the Ni content is set to 0.20% or less.

  In order to surely obtain the above-described effect of improving Ni characteristics, the Ni content is preferably 0.05% or more. For this reason, the more desirable Ni content is 0.05 to 0.20%.

  In addition, said Cu and Ni can be contained only in any 1 type in them, or 2 types of composites.

Group 2: V: 0.20% or less and Nb: 1 or 2 types of 0.050% or less V and Nb are all bonded to C and N to form fine carbides, nitrides and charcoal. It has the effect of forming nitrides to refine crystal grains and improving bending fatigue strength and pitching strength. For this reason, V and Nb may be contained in the following ranges in order to further improve the bending fatigue strength and the pitching strength.

V: 0.20% or less V has the effect of combining with C and N to form fine carbides, nitrides, and carbonitrides to refine crystal grains and improve bending fatigue strength and pitching strength. However, when the V content is excessive, the hot ductility is reduced. In particular, when the content exceeds 0.20%, the hot ductility is significantly reduced during hot rolling or hot forging. Surface scratches are likely to occur. Therefore, the content of V is set to 0.20% or less.

  In order to surely obtain the effect of improving the characteristics of V described above, the content is preferably 0.05% or more. For this reason, the more desirable V content is 0.05 to 0.20%. A more desirable V content is 0.05 to 0.10%.

Nb: 0.050% or less Nb combines with C and N to form fine carbides, nitrides, and carbonitrides to refine crystal grains, and has an effect of improving bending fatigue strength and pitching strength. However, when the content of Nb is excessive, the hot ductility is reduced. Particularly, when the content exceeds 0.050%, the hot ductility is significantly reduced, and during hot rolling or hot forging. Surface scratches are likely to occur. Therefore, the Nb content is set to 0.050% or less.

  In order to reliably obtain the above-described Nb characteristic improvement effect, the content is preferably set to 0.005% or more. For this reason, the more desirable Nb content is 0.005 to 0.050%. Note that a more preferable lower limit of the Nb content is 0.020%. A more preferred upper limit is 0.040%.

  In addition, said V and Nb can be contained only in any 1 type of them, or 2 types of composites.

Third group: Ca: 0.0050% or less Ca has an effect of improving machinability. For this reason, you may contain Ca in order to improve machinability. However, excessive addition leads to an increase in the component cost. In particular, when the Ca content exceeds 0.0050%, the machinability improving effect is saturated, so the cost is increased and the economic efficiency is impaired. In addition, when the Ca content exceeds 0.0050%, a coarse oxide is formed, leading to a decrease in bending fatigue strength and pitching strength. Therefore, the Ca content is set to 0.0050% or less.

  In order to reliably obtain the above-described Ca machinability improving effect, the Ca content is preferably set to 0.0003% or more. For this reason, content of more desirable Ca is 0.0003 to 0.0050%. Note that the more desirable Ca content is 0.0003 to 0.0030%.

  For the above reasons, the case-hardened steel according to the present invention (2) is replaced with a part of Fe of the case-hardened steel according to the present invention (1), Cu: 0.20% or less and Ni: 0.20%. It was specified to contain one or two of the following.

  Similarly, the case-hardened steel according to the present invention (3) is replaced with a part of Fe of the case-hardened steel according to the present invention (1) or the present invention (2), and V: 0.20% or less and Nb: It was specified to contain one or two of 0.050% or less.

  Furthermore, the case-hardened steel according to the present invention (4) is Ca: 0.0050% or less in place of a part of Fe of the case-hardened steel according to any of the present invention (1) to the present invention (3). It was prescribed that it contains.

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

  Steels 1 to 13 having the chemical composition shown in Table 1 were melted by a converter or a vacuum melting furnace to produce a slab or an ingot.

  Specifically, after melting by a 70-ton converter, the steel 1 was subjected to secondary refining twice to adjust the components and then continuously cast to produce a slab. During continuous casting, electromagnetic stirring was controlled to float up inclusions and remove them sufficiently.

  Steels 2 to 12 were melted in a 150 kg vacuum melting furnace and then ingoted to produce ingots.

  Steel 13 was melted by a 70-ton converter, then subjected to secondary refining once to adjust the components, and then continuously cast to produce a slab. In the continuous casting, electromagnetic stirring was not performed.

  Steels 1 to 6 in Table 1 are steels of the present invention examples whose chemical compositions are within the range specified by the present invention, while Steels 7 to 13 are from the conditions specified by the chemical composition of the present invention. It is steel of the comparative example which has come off.

  Among the steels of the above comparative examples, Steel 8, Steel 9 and Steel 10 are steels corresponding to SNCM220H, SCM420H and SCr420H defined in JIS G 4052 (2003), respectively.

  From each slab and ingot, steel bars having diameters of 20 mm, 30 mm, and 55 mm were produced by the steps shown in [1] and [2] below.

[1] Split rolling:
Each slab was held at 1250 ° C. for 2 hours and then rolled into a 180 mm square billet.

[2] Hot rolling or hot forging:
The surface flaws of the 180 mm square billet produced by the above-mentioned block rolling were removed with a grinder, held at 1250 ° C. for 50 minutes, and then hot rolled to produce steel bars having diameters of 20 mm, 30 mm, and 55 mm, respectively.

  Each ingot was held at 1250 ° C. for 8 hours, and then hot forged to produce steel bars having diameters of 20 mm, 30 mm, and 55 mm, respectively.

  Further, steel 14 shown in Table 2, that is, SCM420H having a chemical composition defined in JIS G 4052 (2003) was melted in a 70-ton converter and continuously cast to produce a slab.

  The steel 14 slab was held at 1250 ° C. for 2 hours, and then rolled into a 180 mm square billet. Next, the surface flaws of the billet were removed with a grinder, held at 1250 ° C. for 1 hour, and then hot forged to produce a steel bar having a diameter of 140 mm.

  Various test pieces were produced from the steel bars having diameters of 20 mm, 30 mm, 55 mm, and 140 mm obtained as described above by the steps [3] to [6] below.

[3] Normalization:
Each steel bar having a diameter of 20 mm and 30 mm was kept at 900 ° C. for 1 hour, and then allowed to cool in the atmosphere to normalize. Further, each steel bar having a diameter of 55 mm was kept at 900 ° C. for 2 hours and then allowed to cool in the atmosphere and normalized. Further, the steel bar having a diameter of 140 mm was kept at 900 ° C. for 4 hours and then allowed to cool in the atmosphere and normalized.

[4] Machining (roughing or finishing):
From the center part of each steel bar having a diameter of 20 mm after the normalization, an Ono-type rotating bending fatigue test piece having a rough shape shown in FIG. 1 was cut out in parallel with the rolling direction or the forging axis. Each of the steel bars having a diameter of 30 mm after the normalization cut out a coarse roller pitching small roller test piece shown in FIG. 2 in parallel with the rolling direction or the forging axis from a part of the central part. Further, a coarse roller pitching large roller test piece shown in FIG. 3 was cut out from the central portion of the steel bar having a diameter of 140 mm after normalization in parallel with the forging axis.

  In FIG. 3, (a) is a front view when a rough roller pitching large roller test piece is halved by a center line, and (b) is a cross-sectional view at the center line.

  1 to 3 are all “mm”, and the finishing symbols “▽”, “▽▽” and “▽▽▽” in the figures are JIS B 0601 (1982) is a “triangle symbol” indicating the surface roughness in Table 1.

  Further, “G” added to “▽▽▽” means an abbreviation of a processing method indicating “grinding” defined in JIS B 0122 (1978).

  Similarly, “E (paper finish)” means an abbreviation of a processing method indicating “polishing” with “paper file”.

  Further, from the central part of each steel bar having a diameter of 55 mm after the normalization, a test piece for hot compression having a finished shape having a diameter of 50 mm and a length of 100 mm parallel to the rolling direction or the forging axis was produced.

  The remaining part of each steel bar having a diameter of 30 mm after normalization was subjected to water quenching and then subjected to a nonmetallic inclusion investigation. Details of the survey method will be described later.

[5] Carburizing and quenching-tempering:
“Carburization quenching and tempering” by the heat pattern shown in FIG. 4 for all of the Ono-type rotating bending fatigue test pieces with notches, roller pitching small roller test pieces, and roller pitching large roller test pieces cut out in [4] above. Was given. Note that “Cp” in FIG. 4 represents a carbon potential. “120 ° C. oil quenching” indicates quenching in oil at an oil temperature of 120 ° C., and “AC” indicates air cooling.

  Note that the Ono-type rotating bending fatigue test piece with notches and the roller pitching small roller test piece were subjected to the above-described treatment in a state of being suspended by passing a wire through a hole processed for suspension. On the other hand, the roller pitching large roller test piece was subjected to the above-described treatment in a state where it was laid flat on a jig on a wire mesh.

  About oil quenching, the test piece was thrown into the quenching oil stirred so that it might be uniformly quenched.

[6] Machining (carburizing and quenching-finishing of tempered material):
Each of the above-mentioned test pieces subjected to carburizing quenching and tempering are finished and processed, and an Ono type rotating bending fatigue test piece with a notch shown in FIG. 5, a roller pitching small roller test piece shown in FIG. 6, and a roller shown in FIG. A pitching large roller specimen was prepared.

  In FIG. 7, (a) is a front view when a roller pitching large roller test piece is halved by a center line, and (b) is a cross-sectional view at the center line.

  In addition, the unit of the dimension in the above-mentioned each test piece shown in FIGS. 5-7 is all “mm”, and the finishing symbols “▽” and “▽▽▽” in each figure are the same as those in FIGS. Similarly, each is a “triangular symbol” indicating the surface roughness in the explanatory table 1 of JIS B 0601 (1982).

  Further, “G” added to “▽▽▽” means an abbreviation of a processing method indicating “grinding” defined in JIS B 0122 (1978).

  Furthermore, “˜” is a “waveform symbol”, which means that it is a dough, that is, it remains the carburized quenching-tempered surface of [5].

  For each of steels 1 to 13, investigation of hot workability by hot compression test, investigation of non-metallic inclusions, investigation of surface hardness, investigation of core hardness, investigation of effective hardened layer depth, grain boundary oxide layer Investigation of depth, investigation of fatigue characteristics by Ono-type rotary bending fatigue test, and investigation of anti-pitting characteristics by roller pitching test were conducted.

  Hereinafter, the contents of each of the above surveys will be described in detail.

<< 1 >> Investigation of hot workability:
The test piece for hot compression having a diameter of 50 mm and a length of 100 mm produced as described in [4] above is held at 1200 ° C. for 30 minutes, and then the length direction is set to a height as shown in FIG. And compressed by a crank press to a height of 20 mm.

  FIGS. 8A and 8B are diagrams schematically showing dimensions and shapes of test pieces before and after a hot compression test, respectively.

  In addition, when five compression tests using the above crank press are performed for each steel, and cracks on the outer peripheral surface are visually observed, and no cracks with an opening width of 2 mm or more are found in all five test pieces. It was evaluated that it was excellent in hot workability.

<< 2 >> Investigation of non-metallic inclusions:
About the steel bar having a diameter of 30 mm, which has been subjected to the normalizing treatment as described in [3] above, the remainder obtained by cutting the coarse roller pitching small roller test piece shown in FIG. 2 is held at 900 ° C. for 30 minutes, and then water quenched. did.

  After water quenching, the surface cut along the center line in parallel with the rolling direction of the steel bar or the forging axis (hereinafter referred to as “longitudinal section”) is embedded in the resin so that it becomes the test surface, The surface was polished so as to have a mirror finish.

  Next, in accordance with ASTM-E45-05 method A, non-metallic inclusions of type B and type D having a large thickness, specifically, the thickness is more than 4 μm and not more than 12 μm. , And more than 8 μm and 13 μm or less were measured, and each grade was determined.

  In the following description, the non-metallic inclusions of type B and type D having a large thickness are referred to as “BH” and “DH”, respectively.

<3> Investigation of surface hardness and core hardness:
Using the Ono-type rotating bending fatigue test piece with a notch that has been carburized and quenched and tempered as described in [5] above, the resin is placed so that the cut surface becomes the test surface across the notch with a diameter of 8 mm. After embedding, the surface was polished so as to have a mirror finish, and the surface hardness and core hardness were examined using a micro Vickers hardness tester.

  Specifically, in accordance with “Vickers hardness test-test method” described in JIS Z 2244 (2003), Vickers hardness at any 10 points at a depth of 0.03 mm from the surface of the test piece. (Hereinafter referred to as “Hv hardness”) was measured with a micro Vickers hardness tester with a test force of 0.98 N, and the surface hardness was evaluated by arithmetically averaging the values.

  Similarly, in accordance with the above JIS regulations, the Hv hardness at any 10 points in the core, which is the portion of the fabric that is not affected by carburization, is measured with a micro Vickers hardness tester at a test force of 2.94N. The core hardness was evaluated by arithmetically averaging the values.

<4> Investigation of effective hardened layer depth:
The effective hardened layer depth was investigated using the resin-embedded test piece used in the surface hardness survey of the above << 3 >>.

  Specifically, as in the case of the surface hardness investigation in the above << 3 >>, according to “Vickers hardness test-test method” described in JIS Z 2244 (2003), In the direction from the surface to the center, the test force is 2.94N, measured with a micro Vickers hardness tester, the depth from the surface when the Hv hardness is 550, and the minimum value measured at any 10 points Was the effective hardened layer depth.

<< 5 >> Investigation of grain boundary oxide layer depth and incompletely quenched layer depth:
Using the resin-filled test pieces used in the above << 3 >> and << 4 >>, the grain boundary oxide layer depth and the incompletely hardened layer depth were investigated.

  Specifically, the above-mentioned resin-filled test piece is polished again, and the surface portion of the test piece is arbitrarily observed with an optical microscope at a magnification of 1000 times in a state where it is not corroded while being mirror-finished. The oxide layer observed along the grain boundary in the part was defined as the grain boundary oxide layer, and the depth of the grain boundary oxide layer was evaluated by arithmetically averaging the depths.

  Furthermore, the same test piece was corroded for 0.2 to 2 seconds with nital, and the surface part of the test piece was arbitrarily observed in 10 visual fields with an optical microscope at a magnification of 1000 times. The incomplete hardened layer was used as an incompletely hardened layer, and the depth of the incompletely hardened layer was evaluated by arithmetically averaging the depths.

<< 6 >> Investigation of fatigue characteristics by Ono type rotating bending fatigue test:
Using the Ono rotary bending fatigue test piece finished in [6] above, an Ono rotary bending fatigue test was carried out under the following test conditions, and bending fatigue strength with the maximum strength that did not break at 10 ⁷ cycles. Evaluated.

・ Temperature: Room temperature,
・ Atmosphere: In air
-Number of rotations: 3000 rpm.

  In addition, when the bending fatigue strength is equal to or higher than steel 8 and steel 9, which are steels corresponding to SNCM220H and SCM420H specified in JIS G 4052 (2003), the bending fatigue characteristics are excellent.

<< 7 >> Investigation of anti-pitching characteristics by roller pitting test:
A roller pitching test (two-cylinder rolling fatigue test) was performed under the following test conditions using the finished roller pitching small roller test piece and roller pitching large roller test piece of [6], and the number of repetitions was 10 ⁷ times. The pitching strength was evaluated at the maximum surface pressure at which no pitting with a long side of 1 mm or more occurred.

・ Slip rate: 40%
・ Rotation speed: 1000 rpm,
Lubrication: Manual transmission lubricating oil with an oil temperature of 100 ° C. was sprayed at a rate of 2.0 liters / minute onto the contact portion between the roller pitching small roller test piece and the roller pitching large roller test piece.

However, the above-mentioned “slip ratio” is a value calculated by the following formula, where “V1” is the tangential speed of the roller pitching small roller test piece surface and “V2” is the tangential speed of the roller pitching large roller test piece surface. Point to.
{(V2-V1) / V1} × 100.

  In addition, when the pitching strength is the same as or higher than steel 8 and steel 9 corresponding to SNCM220H and SCM420H defined in JIS G 4052 (2003), the pitching strength is excellent.

  Table 3 summarizes the results of the above investigations.

  FIG. 9 shows the relationship between the value of fn2, that is, the value of the formula of [Cr / (Si + 2Mn)] and the depth of the grain boundary oxide layer. Further, FIG. 10 shows the relationship between the value of fn2, that is, the value of the formula of [Cr / (Si + 2Mn)] and the depth of the incompletely hardened layer.

  From Table 3, in the case of test numbers 1 to 6 using steels 1 to 6 that satisfy the conditions specified in the present invention as materials, it has good hot workability, and the Mo content in steels 1 to 6 Is 0.04 to 0.10%, and even though Ni is not included or extremely low, the nickel-molybdenum molybdenum steel defined in JIS G 4052 (2003) Bending fatigue strength and pitching strength equal to or higher than those of Test No. 8 and Test No. 9 using Steel 8 and Steel 9 corresponding to SCM420H of SNCM220H and “Chromium Molybdenum Steel” are high, and high bending It is clear that it is possible to ensure fatigue strength and high pitting strength.

  On the other hand, in the case of test number 7 and test numbers 10 to 13 of comparative examples that deviate from the conditions defined in the present invention, both the bending fatigue strength and the pitching strength were tested using the steel 8 and steel 9 described above. It is inferior to the cases of No. 8 and Test No. 9. Furthermore, in the case of test number 11 and test number 12, hot workability is also low.

  That is, in the case of test number 7, the value of fn2 of steel 7, that is, the value of [Cr / (Si + 2Mn)] is below the range defined in the present invention, so the bending fatigue strength and the pitching strength are as low as 525 MPa and 1650 MPa, respectively. These are inferior to the bending fatigue strength and the pitching strength in the case of test number 8 and test number 9 using the steel 8 and steel 9.

  In the case of test number 10, steel 10 is a steel corresponding to SCr420H specified in JIS G 4052 (2003), does not contain Mo, the content of O is higher than the value specified in the present invention, and fn2 Value, that is, the value of [Cr / (Si + 2Mn)] is below the range specified in the present invention, the bending fatigue strength and the pitting strength are as low as 515 MPa and 1750 MPa, respectively. 8 and test number 9 are inferior to the bending fatigue strength and the pitting strength. Also, type B non-metallic inclusions of grade 1.0 were observed.

  In the case of test number 11, the Mn content of steel 11 is higher than the value specified in the present invention, and the value of fn1, that is, the value of [Mn / S] exceeds the range specified in the present invention. Since the value, that is, the value of [Cr / (Si + 2Mn)] is below the range defined in the present invention, the bending fatigue strength and the pitching strength are as low as 540 MPa and 1650 MPa, respectively. And it is inferior to the bending fatigue strength and pitching strength in the case of test number 9. Furthermore, a crack having an opening width of 2 mm or more is generated by a compression test using a crank press, and the hot workability is inferior.

  In the case of test number 12, the S, Ti and O contents of steel 12 are higher than the values specified in the present invention, and the value of fn1, that is, the value of [Mn / S] is below the range specified in the present invention. Moreover, since the value of fn2, that is, the value of [Cr / (Si + 2Mn)] exceeds the range defined in the present invention, the bending fatigue strength and the pitching strength are as low as 525 MPa and 1600 MPa, respectively. It is inferior to the bending fatigue strength and pitching strength in the case of the test numbers 8 and 9 used. Also, grade 2.0 type B non-metallic inclusions and grade 1.5 type D non-metallic inclusions were observed. Furthermore, a crack having an opening width of 2 mm or more is generated by a compression test using a crank press, and the hot workability is inferior.

  In the case of test number 13, the contents of Mn, Ti and O of steel 13 are higher than the values specified in the present invention, and the value of fn2, that is, the value of [Cr / (Si + 2Mn)] is also specified in the present invention. Therefore, the bending fatigue strength and the pitching strength are as low as 515 MPa and 1650 MPa, respectively, which is inferior to the bending fatigue strength and the pitching strength in the case of test number 8 and test number 9 using the steel 8 and steel 9. . Grade 2.5 non-metallic inclusions and grade 2.0 non-metallic inclusions were also observed.

  In addition, the steel 1-6 which satisfy | fills the conditions prescribed | regulated by this invention has not produced | generated coarse MnS, and has favorable cold forgeability.

  The case-hardened steel of the present invention has a low component cost, has good workability during hot and cold rolling and forging, and carburized parts made from this case-hardened steel are JIS G 4052. The bending fatigue strength and the pitching strength are the same as or higher than those of carburized parts made of SNCM220H of “nickel chromium molybdenum steel” and SCM420H of “chromium molybdenum steel” defined in (2003). For this reason, the case-hardened steel of the present invention is suitable for use as a material for carburized parts such as automobile gears that require high bending fatigue strength and high pitching strength in order to reduce weight and torque.

It is a figure which shows the rough shape as cut out from the steel bar of the Ono-type rotary bending fatigue test piece with a notch used in the Example. It is a figure which shows the rough shape as cut out from the bar steel of the roller pitching small roller test piece used in the Example. It is a figure which shows the rough shape as cut out from the steel bar of the roller pitching large roller test piece used in the Example. In FIG. 3, (a) is a front view when a coarse roller pitching large roller test piece is halved by a center line, and (b) is a cross-sectional view at the center line. It is a figure which shows the heat pattern of the carburizing quenching-tempering in an Example. It is a figure which shows the finishing shape of the Ono type | formula rotation bending fatigue test piece with a notch used in the Example. It is a figure which shows the finishing shape of the roller pitching small roller test piece used by the roller pitching test of an Example. It is a figure which shows the finishing shape of the roller pitching large roller test piece used by the roller pitching test of an Example. In FIG. 7, (a) is a front view when the roller pitching large roller test piece is halved by the center line, and (b) is a cross-sectional view at the center line. It is a figure explaining the hot compression test done in the Example, (a) and (b) in a figure typically show the size and shape of the test piece before the compression test in the hot and after the compression test, respectively. FIG. It is a figure which rearranges and shows the relationship between the grain boundary oxide layer depth investigated in the Example, and the value of [fn2 = Cr / (Si + 2Mn)]. It is a figure which rearranges and shows the relationship between the incomplete quenching layer depth investigated in the Example, and the value of [fn2 = Cr / (Si + 2Mn)].

Claims (4)

In mass%, C: 0.15 to 0.30%, Si: more than 0.10% and 1.0% or less, Mn: 0.30 to 1.0%, S: 0.030% or less, Cr : More than 1.25% and 3.0% or less, Mo: 0.04 to 0.10%, Al: 0.010 to 0.050% and N: 0.0100 to 0.0250% , Si, Mn, Cr and S are the values of fn1 and fn2 represented by the following formulas (1) and (2), respectively, 30 ≦ fn1 ≦ 150 and 0.7 ≦ fn2 ≦ 1. 1 and the balance consists of Fe and impurities, and P, Ti and O (oxygen) in the impurities are P: 0.020% or less, Ti: less than 0.005% and O: 0.0015% or less, respectively. Case-hardened steel characterized by being.
fn1 = Mn / S (1)
fn2 = Cr / (Si + 2Mn) (2)
However, the element symbols in the formulas (1) and (2) represent the content in mass% of the element.   The skin according to claim 1, comprising one or two of Cu: 0.20% or less and Ni: 0.20% or less in mass% instead of a part of Fe. Burnt steel.   3. Instead of a part of Fe, it contains one or two of V: 0.20% or less and Nb: 0.050% or less in mass%. Hardened steel.   The case-hardened steel according to any one of claims 1 to 3, which contains Ca: 0.0050% or less in mass% instead of part of Fe.

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