JP6176314B2 - Case bar or wire rod - Google Patents

Description

  The present invention relates to a case bar or a wire rod. Specifically, the component cost is low, the machinability is excellent, and the surface fatigue treatment or carburizing and nitrocarburizing quenching is excellent in bending fatigue strength and pitching strength, and variations in these strengths are small. The present invention relates to a case steel bar or wire suitable as a material for carburizing or carbonitriding parts.

  Many machine parts such as automobile parts are usually formed into a predetermined rough shape by hot forging or cold forging using steel bar or wire as a raw material, and then machined into machine parts such as gears by cutting, It is manufactured by tempering after surface hardening treatment such as carburizing and quenching and carbonitriding and quenching. For this reason, good machinability is required for the material of carburized or carbonitrided parts.

“Carburizing and quenching” generally uses low-carbon “skin-hardened steel” as the material steel (dough steel), and the surface to be quenched after intruding and diffusing C in the high-temperature austenite region of ₃ points or higher. It is a hardening process, and “carbonitriding and quenching” is a surface hardening process in which carburizing and nitriding are simultaneously performed and then quenched.

  In recent years, automobiles are required to be lighter and have higher torque. For this reason, carburizing or carbonitriding parts for automobiles are required to have higher bending fatigue strength and higher pitching strength than before. In particular, gears used for transmissions and the like are required to have high bending fatigue strength at the root and high pitching strength at the tooth surface.

  Further, in carburized or carbonitrided parts, the lower limit of bending fatigue strength and pitching strength variation may be one of the strength standards in design, and it is also required that these strength variations be small.

  In the present specification, “carburized or carbonitrided parts” will be described below as “carburized parts”.

  For example, Patent Literature 1 and Patent Literature 2 propose techniques relating to strength improvement of carburized parts.

  In Patent Document 1, in mass percent, C: 0.10 to 0.35%, Si: 0.02 to 0.5%, Mn: 0.3 to 1.8%, S: 0.005 to 0.00. 15%, Al: 0.02-0.05%, N: 0.005-0.014%, Cr: 0.4-1.8%, Mo: 0.02-1.0 %, Ni: 0.1 to 3.5%, V: 0.03 to 0.5%, or one or more, P: 0.025% or less, Ti: 0.01% or less, O: Each is limited to 0.0025% or less, the balance is composed of iron and inevitable impurities, the precipitation amount of AlN is limited to 0.005% or less, the bainite structure fraction is limited to 30% or less, hot Disclosed is a “skin-hardened steel with excellent coarsening prevention characteristics” characterized in that the ferrite band of the cross-sectional structure parallel to the rolling direction has a score of 1 to 3

In Patent Document 2, in mass ratio, C: 0.10 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.30 to 1.50%, Cr: 0.30 to 2. 00%, Al: 0.005 to 0.050%, Nb: 0.01 to 0.1%, N: 0.0080 to 0.0250%, V: 0.01% or less, if necessary Furthermore, after hot-rolling steel material containing Mo: 0.80% or less and comprising the remainder Fe and impurity elements to 1150 to 1350 ° C., hot forging is performed under the condition that the forging finish temperature is 1100 to 1300 ° C. The Nb contained in the steel was in a substantially solid solution state (the number of precipitates was 0.3 / μm ² or less), and after cooling to 620 ° C. to 700 ° C. after forging. There is disclosed a “manufacturing method for hot forged parts for high-temperature carburizing” characterized by holding at that temperature for 30 minutes to 5 hours and cooling to room temperature followed by carburizing.

JP-A-11-106866 JP 2004-300519 A

  In Patent Document 1, by restricting the content of P, which is an impurity, in steel, by suppressing the decrease in fatigue strength, and by limiting the content of Ti, O, which is an impurity, in steel, There is a disclosure that the precipitation amount of AlN is limited in the state of the steel material after hot rolling, and the effect of suppressing the coarsening of crystal grains by the pinning action of AlN during carburizing heating is stably exhibited. However, it is not considered to reduce the difference in the content of impurities in the steel material cross section (hereinafter, the content may be referred to as “concentration”). Therefore, high bending fatigue strength and pitching strength may not be secured stably.

  Patent Document 2 discloses that the content of V, which is an impurity, in the steel material is limited to 0.01% or less to suppress abnormal grain growth during high-temperature carburization. However, it is not considered to reduce the difference in impurity concentration in the cross section of the steel material. Therefore, high bending fatigue strength and pitching strength may not be secured stably.

  The present invention has been made in view of the above situation, and its purpose is low component cost, excellent machinability, and bending fatigue strength and pitching in the case of carburizing quenching or carbonitriding quenching, which are surface hardening treatments. An object of the present invention is to provide a case bar or wire rod that is excellent in strength and has a small variation in strength and is suitable as a material for carburized parts.

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

  (A) In order to secure high bending fatigue strength and high pitching strength stably, and to further reduce variations in these strengths, it is effective to define the component composition of steel and limit the impurity concentration.

  (B) In order to suppress variation in strength, it is effective to limit variation in impurity concentration in the cross section of the steel material, particularly in the cross section perpendicular to the longitudinal direction of the steel material.

This invention is completed based on said knowledge, The summary exists in the steel bar or wire rod for case hardening shown to following (1)-( 3 ).

(1) In mass%,
C: 0.13-0.25%
Si: 0.02 to 0.35%,
Mn: 0.50 to 0.90%,
S: 0.005-0.030%,
Cr: 0.50 to 1.60%,
Al: 0.010 to 0.060% ,
Nb: 0.050% or less and N: 0.0100 to 0.0250%,
And the balance consists of Fe and impurities,
P, Ti, V and O (oxygen) in the impurity are respectively
P: 0.030% or less,
Ti: 0.005% or less,
V: 0.005% or less and O: 0.0015% or less, and
The total content of Ti and V is 0.006% or less, and the concentration difference of each impurity element in the cross section perpendicular to the longitudinal direction is 0.005% or less for P and 0.003% for Ti. Hereinafter, V is 0.003% or less, and O is 0.0005% or less,
Steel bar or wire rod for case hardening.

(2) Instead of a part of Fe, in mass%,
Cu: 0.20% or less,
Ni: 0.20% or less and Mo: 0.50% or less,
Characterized in that it contains one or more of
The steel bar or wire rod for case hardening described in (1) above.

( 3 ) Instead of part of Fe, in mass%,
Ca: 0.0050% or less,
Containing,
Steel bar or wire rod for case hardening as set forth in (1) or (2) above.

  The steel bar or wire rod for skin hardening of the present invention has a low component cost, is excellent in machinability, and has good processing characteristics. Furthermore, the carburized parts obtained by using the steel bar or wire rod for skin hardening according to the present invention are excellent in bending fatigue strength and pitching strength, and there is little variation in these strengths. For this reason, the case-hardening steel bar or wire of the present invention is used as a material for carburizing or carbonitriding parts such as automobile gears that require high bending fatigue strength and high pitching strength to reduce weight and increase torque. Is preferred.

When the cross section perpendicular to the longitudinal direction of the steel bar or wire is a “circular” having a diameter of 40 mm or more, in order to increase the analytical accuracy in determining the “concentration difference in the cross section perpendicular to the longitudinal direction” It is a figure which illustrates typically an example of the method of extract | collecting and O analysis sample. In FIG. 1, (a) is based on the geometric center [5] of the steel bar or wire, and is located at a position of 3 mm from both outer peripheral sides on the line passing through the cross-sectional center [5] [1] and [9] , And [1] and [9] are divided into eight equal parts, [2] to [8], and a total of nine positions, respectively, indicate that chips and O analysis samples are collected, and ( b) In any cross section 1, chips are collected from a total of 9 positions corresponding to the above [1] to [9], and in the cross section 2 within 100 mm in the longitudinal direction from the cross section 1 It shows that O analysis samples are collected respectively from a total of nine positions corresponding to [1] to [9]. In order to increase analytical accuracy when determining the “concentration difference in the cross section perpendicular to the longitudinal direction” when the cross section perpendicular to the longitudinal direction of the steel bar or wire is a “circular” having a diameter of 30 mm or more and less than 40 mm FIG. 2 is a diagram schematically illustrating an example of a method of collecting chips and an O analysis sample, and in any cross section 1 and cross section 2 within 100 mm in the longitudinal direction from cross section 1, the above [1] to [[ [9] The chips are collected from a total of nine positions corresponding to [9], and in the section 3 within 100 mm in the longitudinal direction from the section 2 and in the section 4 within 100 mm in the longitudinal direction from the section 3, the above [1 ] To [9] indicate that O analysis samples are collected from a total of 9 positions. In order to increase the analytical accuracy in determining the “concentration difference in the cross section perpendicular to the longitudinal direction” when the cross section perpendicular to the longitudinal direction of the steel bar or wire is “circular” having a diameter of 20 mm or more and less than 30 mm FIG. 2 is a diagram schematically illustrating an example of a method of collecting chips and an O analysis sample, and in the sections 1 to 4 within 100 mm in the longitudinal direction from adjacent sections, the above [1] to [9 In each of the cross-sections 5 to 8 that are within 100 mm in the longitudinal direction from the cross-section 4 and within 100 mm in the longitudinal direction from the adjacent cross-sections, respectively, It shows that O analysis samples are collected respectively from a total of nine positions corresponding to [1] to [9]. In order to increase the analytical accuracy in determining the “concentration difference in the cross section perpendicular to the longitudinal direction” when the cross section perpendicular to the longitudinal direction of the steel bar or wire is “circular” having a diameter of 10 mm or more and less than 20 mm FIG. 5 is a diagram schematically illustrating an example of a method for collecting chips and an O analysis sample, and in the sections 1 to 9 within 100 mm in the longitudinal direction from adjacent sections, the above [1] to [9 In each of the cross sections 10 to 18 which are within 100 mm in the longitudinal direction from the cross section 9 and within 100 mm in the longitudinal direction from the adjacent cross sections, respectively, It shows that O analysis samples are collected respectively from a total of nine positions corresponding to [1] to [9]. 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. The unit of the dimension in the figure is “mm”. It is a figure which shows the rough shape as cut out from the steel bar of the small roller test piece for a roller pitching test used in the Example. The unit of the dimension in the figure is “mm”. It is a figure which shows the rough shape as cut out from the steel bar of the large roller for roller pitching tests used in the Example. In FIG. 7, (a) is a front view when a rough large roller is halved by a center line, and (b) is a cross-sectional view at the center line. The unit of the dimension in the figure is “mm”. In an Example, it is a figure which shows the heat pattern of the "carburization hardening-tempering" performed to the test piece shown in FIG. 5 and FIG. Cp in the figure represents the carbon potential. In an Example, it is a figure which shows the heat pattern of the "carburization hardening-tempering" performed to the test piece shown in FIG. Cp in the figure represents the carbon potential. 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. The unit of the dimension in the figure is “mm”. It is a figure which shows the finishing shape of the small roller test piece for a roller pitching test used in the Example. The unit of the dimension in the figure is “mm”. It is a figure which shows the finishing shape of the large roller for a roller pitching test used in the Example. In FIG. 12, (a) is a front view when the large roller is divided in half by the center line, and (b) is a cross-sectional view at the center line. The unit of the dimension in the figure is “mm”.

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

C: 0.13-0.25%
C is an essential element for securing the strength of the carburized component. In order to obtain this effect sufficiently, a C content of 0.13% or more is necessary. However, when there is too much content of C, machinability will fall by the increase in hardness. Therefore, the content of C is set to 0.13 to 0.25%. The lower limit of the preferred C content is 0.15%. Moreover, the upper limit of preferable C content is 0.23%.

Si: 0.02-0.35%
Si has an action of improving hardenability and a deoxidizing action. In order to sufficiently obtain these effects, a Si content of 0.02% or more is necessary. However, Si is an oxidizing element, and if its content is too large, 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. For this reason, the depths of the grain boundary oxide layer and the incompletely quenched layer, which are carburized abnormal layers, increase. When the depth of the carburized abnormal layer is increased, the bending fatigue strength and the pitching strength may be reduced. Therefore, the Si content is set to 0.02 to 0.35%. The lower limit of the preferred Si content is 0.05%. Moreover, the upper limit of preferable Si content is 0.30%.

Mn: 0.50 to 0.90%
Mn has an effect of improving hardenability and a deoxidizing action. In order to sufficiently obtain these effects, a Mn content of 0.50% or more is necessary. However, when there is too much content of Mn, there exists a possibility of causing the fall of machinability by the increase in hardness. Also, like Si, Mn is an oxidizing element, so if its content is too high, Mn oxide is generated on the steel surface, so the grain boundary oxidation layer and incomplete quenching that are carburizing abnormal layers The depth of the layer increases. When the depth of the carburized abnormal layer is increased, the bending fatigue strength and the pitching strength may be reduced. Therefore, the Mn content is set to 0.50 to 0.90%. The minimum of preferable Mn content is 0.55%. Moreover, the upper limit of preferable Mn content is 0.80%.

S: 0.005-0.030%
S combines with Mn to form MnS and has the effect of improving machinability. In order to sufficiently obtain this effect, an S content of 0.005% or more is necessary. However, if the S content is too large, the formation of coarse MnS leads to a decrease in hot workability, cold forgeability, bending fatigue strength, and pitching strength. Therefore, the content of S is set to 0.005 to 0.030%. The lower limit of the preferable S content is 0.010%. Moreover, the upper limit of preferable S content is 0.025%.

Cr: 0.50 to 1.60%
Cr has the effect of improving hardenability. In order to sufficiently obtain this effect, a Cr content of 0.50% or more is necessary. However, when there is too much content of Cr, there exists a possibility of causing the fall of machinability by the increase in hardness. Also, like Si and Mn, Cr is an oxidizing element, so if its content is too large, Cr oxide is generated on the steel surface, so the grain boundary oxidation layer that is an abnormal carburizing layer and incomplete The depth of the hardened layer is increased. When the depth of the carburized abnormal layer is increased, the bending fatigue strength and the pitching strength may be reduced. Therefore, the content of Cr is set to 0.50 to 1.60%. The lower limit of the preferable Cr content is 0.80%. Moreover, the upper limit of preferable Cr content is 1.50%.

Al: 0.010 to 0.060%
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 0.060%, the machinability, bending fatigue strength, and pitching strength are significantly reduced. Therefore, the content of Al is set to 0.010 to 0.060%. The minimum of preferable Al content is 0.020%. Moreover, the upper limit of preferable Al content is 0.050%.

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 of preferable N content is 0.0130%. Moreover, the upper limit of preferable N content is 0.0230%.

  One of the steel bar or wire rod for skin hardening of the present invention contains the above-described elements from C to N, and the balance consists of Fe and impurities.

  Here, “impurities” refer to those mixed from ore as a raw material, scrap, or the production environment when industrially producing steel materials.

  In the present invention, the contents of P, Ti, V, and O (oxygen) in the impurities are P: 0.030% or less, Ti: 0.005% or less, V: 0.005% or less, and O, respectively. : 0.0015% or less, and the total content of Ti and V (hereinafter referred to as “Ti + V amount”) is 0.006% or less. Further, the difference in concentration of each impurity element in the cross section perpendicular to the longitudinal direction is 0.005% or less for P, 0.003% or less for Ti, 0.003% or less for V, and 0.0005 for O. It is necessary to limit it to less than%.

  This will be described below.

P: 0.030% 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 0.030%, the degree of embrittlement becomes significant. Therefore, in the present invention, the content of P in the impurities is set to 0.030% or less. A more preferable content of P is 0.020% or less.

Ti: 0.005% or less Ti has a high affinity with N, so it binds with N in steel to form hard and coarse non-metallic inclusions TiN, which reduces bending fatigue strength and pitching strength Furthermore, machinability is also reduced. Therefore, in the present invention, the content of Ti in the impurities is set to 0.005% or less. A more preferable Ti content is 0.004% or less.

V: 0.005% or less V combines with C and N to form a carbonitride. When the V content in the impurities is large, coarse V carbonitrides are formed and the crystal grains are coarsened by high-temperature carburization. Therefore, in the present invention, the content of V in the impurities is set to 0.005% or less. A more preferable V content is 0.004% or less.

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.

Ti + V content: 0.006% or less If the Ti + V content exceeds 0.006% even if the individual Ti and V contents in the impurities are limited to the above ranges, high bending fatigue strength, pitching strength and stability The effect of suppressing the coarsening of crystal grains cannot be obtained. Therefore, in the present invention, the amount of Ti + V is set to 0.006% or less. A more preferable amount of Ti + V is 0.005% or less.

Difference in concentration of P, Ti, V and O in the cross section perpendicular to the longitudinal direction: P: 0.005% or less, Ti: 0.003% or less, V: 0.003% or less, and O: 0.0005% The case-hardening steel bar or wire is formed into a predetermined rough shape by hot forging or cold forging, and then finished into parts such as gears by cutting, followed by surface hardening treatment such as carburizing and quenching and carbonitriding. After doing, it is tempered.

  Depending on the method for forming the rough shape, the inside of the case bar or wire is located on the surface of the component, and it is inevitable that the impurity content on the surface of the component becomes high. The content, in particular, the content of P, Ti, V, and O may increase, and the bending fatigue strength and the pitching strength may decrease. However, the content of P, Ti, V and O and the amount of Ti + V are limited to the ranges already described, and the content of P, Ti, V and O in the cross section perpendicular to the longitudinal direction of the case steel bar or wire rod If the concentration difference is reduced, specifically, if P is 0.005% or less, Ti is 0.003% or less, V is 0.003% or less, and O is 0.0005% or less, even if the skin Even if the inside of the bar or wire rod is located on the surface of the part, it is possible to suppress a decrease in bending fatigue strength and pitching strength and to reduce variations in these strengths. More preferable concentration differences of P, Ti, V and O in the cross section perpendicular to the longitudinal direction are 0.004% or less for P, 0.002% or less for Ti, 0.002% or less for V, and O And 0.0004% or less.

  The “concentration difference in the cross section perpendicular to the longitudinal direction” of the impurity elements P, Ti, V, and O is the value obtained by subtracting the minimum value from the maximum value for the concentration of each element investigated by appropriate analysis means such as EPMA. Point to.

  In order to improve the analysis accuracy, chips are collected, and P, Ti, and V are analyzed by ICP emission spectroscopic analysis using a solution obtained by dissolving the chips in acid. Take a sample of 3 mm and a length of 50 mm, remove the oxidized scale on the surface by sanding, cut the sample from the cross-section side so that the sample mass is 0.5 to 1 g, and then remove O with an oxygen analyzer. It is preferable to analyze.

  Here, in the case where the cross section perpendicular to the longitudinal direction of the steel bar or wire according to the present invention is “circular”, the above-described chips and O analysis sample for obtaining “concentration difference in the cross section perpendicular to the longitudinal direction” An example of the method of collecting the bar is shown by dividing the dimensions of the steel bar or wire into the following <a> to <d> (see FIGS. 1 to 4). It should be noted that the steel bar or wire has a small diameter, and in the same cross section as shown in FIG. 1 (a), the geometric center [5] of the steel bar or wire is used as a reference, and both the lines on the line passing through the cross-sectional center [5] Chips from 9 positions in total, [1] and [9], which are 3 mm from the outer peripheral side, and [2] to [8], which are positions that equally divide between [1] and [9]. Similarly, when it is not possible to collect the O analysis sample from a total of nine positions [1] to [9] in the same cross section, in the same cross section, the above [1] Chips and O analysis samples may be collected from a total of nine positions corresponding to ~ [9].

<a> When the diameter is 40 mm or more As shown in FIG. 1A, chips and O analysis samples are collected from a total of nine positions corresponding to the above [1] to [9] in the same cross section. Specifically, as shown in FIG. 1 (b), in any cross section 1, chips are collected from a total of 9 positions corresponding to the above [1] to [9] with a drill having a diameter of 3 mm, and In the cross section 2 within 100 mm in the longitudinal direction from the cross section 1, O analysis samples having a diameter of 3 mm and a length of 50 mm are collected from a total of nine positions corresponding to the above [1] to [9].

<B> In the case where the diameter is 30 mm or more and less than 40 mm As shown in FIG. Chips were collected from 9 positions with a drill having a diameter of 3 mm, and in section 3 within 100 mm in the longitudinal direction from section 2 and section 4 within 100 mm in the longitudinal direction from section 3, the above [1] Samples of O analysis with a diameter of 3 mm and a length of 50 mm are collected from a total of 9 positions corresponding to [9].

<C> When the diameter is 20 mm or more and less than 30 mm As shown in FIG. 3, a total of 9 corresponding to the above [1] to [9] in the cross sections 1 to 4 within 100 mm in the longitudinal direction from the adjacent cross sections. Chips were collected from each position with a drill having a diameter of 3 mm, and in the cross sections 5 to 8 within 100 mm in the longitudinal direction from the cross section 4 and within 100 mm in the longitudinal direction from the adjacent cross sections, the above [ O analysis samples having a diameter of 3 mm and a length of 50 mm are collected from a total of nine positions corresponding to 1] to [9].

<D> When the diameter is 10 mm or more and less than 20 mm As shown in FIG. 4, a total of 9 corresponding to the above [1] to [9] in cross sections 1 to 9 within 100 mm in the longitudinal direction from adjacent cross sections. From each position, chips are collected with a drill having a diameter of 3 mm, and in the sections 10 to 18 within 100 mm in the longitudinal direction from the section 9 and within 100 mm in the longitudinal direction from the adjacent sections, the above [ O analysis samples having a diameter of 3 mm and a length of 50 mm are collected from a total of nine positions corresponding to 1] to [9].

  Another one of the steel bar or wire rod for skin hardening according to the present invention contains one or more elements selected from Cu, Ni, Mo, Nb and Ca instead of a part of the above-mentioned Fe. .

  Hereinafter, the effect of the said Cu, Ni, Mo, Nb, and Ca which are arbitrary elements, and the reason for limitation of content are demonstrated.

  Cu, Ni, and Mo all have an effect of improving hardenability. For this reason, in order to acquire said effect, you may contain these elements. Hereinafter, the above Cu, Ni, and Mo will be described.

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 reduced. Therefore, an upper limit is set for the amount of Cu in the case of inclusion, and it is set to 0.20% or less. When Cu is contained, the amount of Cu is preferably 0.15% or less.

  On the other hand, in order to stably obtain the effect of Cu described above, the amount of Cu in the case of inclusion is preferably 0.05% or more.

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. In order to obtain these effects, Ni may be included. However, Ni is an expensive element, and excessive addition leads to an increase in component cost. Therefore, an upper limit is set for the amount of Ni in the case of inclusion, and it is set to 0.20% or less. When Ni is contained, the amount of Ni is preferably 0.20% or less.

  On the other hand, in order to stably obtain the above-described effect of improving the characteristics of Ni, the amount of Ni when contained is preferably 0.05% or more.

Mo: 0.50% or less Mo has an effect of improving hardenability. 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. For this reason, you may contain Mo. However, Mo is an expensive element, and excessive addition leads to an increase in the component cost, and the effect of improving the strength is saturated. Therefore, an upper limit is set for the amount of Mo in the case of inclusion, and is set to 0.50% or less. When Mo is contained, the amount of Mo is preferably 0.45% or less.

  On the other hand, in order to stably obtain the above-described effect of improving the properties of Mo, the amount of Mo when contained is preferably 0.05% or more.

  Said Cu, Ni, and Mo can be contained only in any one of them, or 2 or more types of composites. The total content of these elements may be 0.90%, but is preferably 0.75% or less.

Nb: 0.080% or less Nb combines with C and N to form fine carbides, nitrides and carbonitrides to refine austenite crystal grains, and has the effect of improving bending fatigue strength and pitching strength . For this reason, you may contain Nb for the further improvement of bending fatigue strength and the improvement of pitching strength. However, when the Nb content is excessive, hot ductility is reduced, and surface flaws are likely to occur during hot rolling or hot forging. Therefore, an upper limit is set for the amount of Nb in the case of inclusion, and it is set to 0.080% or less. When Nb is included, the amount of Nb is preferably 0.060% or less.

  On the other hand, in order to stably obtain the above-described effect of improving the characteristics of Nb, the amount of Nb when contained is preferably 0.005% or more, and more preferably 0.020% or more.

Ca: 0.0050% or less Ca has an effect of improving machinability. For this reason, you may contain Ca for machinability improvement. 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, when Ca is included, the amount of Ca is set to 0.0050% or less. When Ca is contained, the amount of Ca is preferably 0.0030% or less.

  On the other hand, in order to stably obtain the above-described effect of improving the machinability of Ca, the Ca content in the case of inclusion is preferably 0.0003% or more.

  Hereinafter, an example of the manufacturing method for obtaining the steel bar or wire rod for case hardening of the present invention will be described. It goes without saying that the method for producing a case bar or wire rod according to the present invention is not limited to this.

(i) A steel having the chemical composition defined in the present invention is melted by a well-known method to produce a slab.
When steel is smelted using a converter, it is sufficiently subjected to de-P and deoxidation treatments, and the components are adjusted by performing secondary refining twice. Next, using a vertical bending die continuous casting machine, the molten steel is cooled in a mold and cast to produce a slab. At this time, by performing electromagnetic stirring in the mold, the unsolidified molten steel is stirred, and the structure inside the slab becomes equiaxed and the segregation of impurities can be dispersed. The slab out of the mold is pulled out while spray-cooling with high-speed water, etc., and rolling down and straightening with a roll. By optimizing the arrangement pitch and alignment of the rolls and the pressing force by the rolls, a slab with less segregation of impurities can be obtained. Further, by performing spray cooling so that the slab is uniformly cooled, it is possible to obtain a slab having a uniform solidified structure with less segregation of impurities. If the casting speed, that is, the speed at which the slab is pulled out is too fast, the final solidification position in the axial direction of the slab tends to fluctuate and segregation of impurities inside the slab increases, so the casting speed is preferably 1 m / min or less.

(ii) The produced cast slab is subjected to ingot rolling to produce a steel slab.
The slab is charged into a heating furnace, heated at a temperature of about 1200 to 1300 ° C. and heated for a heating time of 1 hour or more, then rolled in pieces, and then cooled until the surface temperature becomes 50 ° C. or less. Get a billet. Here, “heating temperature” means the average temperature in the furnace, and “heating time” means the in-furnace time.

(iii) The obtained steel slab is hot-rolled to produce a bar or wire.
After removing the surface flaws with a grinder or the like, the steel slab is charged into a heating furnace, heated at a heating temperature of about 1100 to 1300 ° C. and heated for 30 minutes or more, and then hot-rolled to obtain a steel bar or wire. obtain. The “heating temperature” here means the average temperature in the furnace, and the “heating time” also means the in-furnace time.

  The hot rolling is performed so that the finishing temperature is 800 to 1200 ° C. and the total area reduction ratio RA from the cast slab defined by the following formula <1> is 90% or more. The finishing temperature here means the average value of the surface temperatures of the steel bars or the wire rods on the exit side of the final stand of the multi-stand rolling mill.

RA = {1− (cross-sectional area of steel bar or wire / cross-sectional area of slab)} × 100 (1)
The cross-sectional area in the formula <1> means a cross-sectional area (so-called “cross-sectional area”) perpendicular to the longitudinal direction (that is, the casting direction or the rolling direction).

  For example, when manufactured by the above process, segregation of impurity elements, particularly segregation of P, Ti, V and O, is reduced, so that the content of each element and the amount of Ti + V, as well as in the cross section perpendicular to the longitudinal direction. It is possible to obtain a steel bar or wire rod for case hardening in which the difference in the concentration of P, Ti, V and O is controlled in the range already described.

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

  Steel bars 1 to 10 having chemical compositions shown in Table 1 were used to produce steel bars. The chemical composition shown in Table 1 is an analysis value of a ladle sample collected from molten steel except for O, and O is an analysis value of a sample collected from R / 2 part of a 70 mm steel bar described later.

Steel 5 in Table 1 is a steel whose chemical composition is within the range defined by the present invention. Steels 1 to 4 and steels 6 to 10 are steels whose chemical compositions deviate from the conditions specified in the present invention.

  Hereinafter, the conditions under which the steel bar was produced will be described in detail.

  Steels 1 to 5 and Steel 8 were melted using a converter, subjected to de-P and deoxidation treatments, and subjected to secondary refining twice to adjust the components. Next, the molten steel was cooled in a mold and cast using a vertical bending type continuous casting machine to produce a slab. At this time, electromagnetic agitation was performed in the mold to agitate the unsolidified molten steel, thereby making the structure inside the slab an equiaxed crystal and dispersing the segregation of impurities. The slab out of the mold was drawn out at 0.6 m / min while spray-cooled with high-speed water and reduced and corrected with a roll. At this time, based on the results of previous experiments, the roll arrangement pitch and alignment and the pressing force by the rolls are optimized so that a slab with less segregation of impurities is obtained, and the solidified structure is uniform and has less segregation of impurities. The slab was uniformly cooled by spray cooling to obtain a slab.

  The obtained slab is charged into a heating furnace, heated at a heating temperature of 1250 ° C. and for a heating time of 2 hours, then rolled in pieces, and once cooled until the surface temperature becomes 50 ° C. or less. A 180 mm square steel piece was produced.

  After removing the surface flaws with a grinder, the steel slab thus obtained was charged into a heating furnace, heated at a heating temperature of 1150 ° C. for a heating time of 1 hour, and then hot rolled to a diameter of 70 mm. A steel bar was produced. The finishing temperature of the hot rolling was 900 to 1000 ° C., and the total area reduction ratio RA from the slab was 97%.

Steel 9 was melted in a 180 kg vacuum melting furnace and then ingot-formed to produce an ingot. Next, the obtained ingot was charged into a heating furnace, heated at a heating temperature of 1250 ° C. and a heating time of 5 hours, and then hot forged to produce a steel bar having a diameter of 70 mm. Here, RA ′ = {1− (cross-sectional area of bar steel / cross-sectional area of ingot)} × 100... <1 ′>
The total area reduction ratio RA ′ obtained by the hot forging obtained from the formula (1) was 91%.

  Steel 6, steel 7 and steel 10 were melted in a 50 kg vacuum melting furnace and then ingoted to produce ingots. Next, the obtained ingot was charged into a heating furnace, heated at a heating temperature of 1150 ° C. and for a heating time of 1 hour, and then hot forged to produce a steel bar having a diameter of 70 mm. Here, the total area reduction ratio RA ′ obtained by the hot forging obtained from the formula <1 ′> was 40%.

  For each of the steels 1 to 10, the 70 mm diameter steel bars obtained as described above were used to investigate the difference in the concentration of P, Ti, V and O in the cross section perpendicular to the longitudinal direction.

  That is, chips are collected from a total of nine positions corresponding to [1] to [9] of the cross section 1 shown in FIG. 1B with a drill having a diameter of 3 mm, and a solution obtained by acid-dissolving the chips is used. P, Ti and V were analyzed by ICP emission spectroscopy. Further, in the cross section 2 which is 100 mm in the longitudinal direction from the cross section 1, O analysis samples having a diameter of 3 mm and a length of 50 mm are collected from a total of 9 positions corresponding to the above [1] to [9], After removing the oxide scale on the surface and cutting the sample from the cross-sectional side so that the sample mass was 0.5 to 1 g, O was analyzed by an oxygen analyzer. Next, a value obtained by subtracting the minimum value from the maximum value among the nine concentrations of each element was taken as the concentration difference of each impurity element in the cross section perpendicular to the longitudinal direction. Since the bar is 70 mm in diameter, the interval between adjacent positions such as [1] and [2] shown in FIG. 1A is 8 mm.

  Next, normalization was performed on each steel bar having a diameter of 70 mm of the steels 1 to 10 obtained as described above, which was kept at 925 ° C. for 1 hour and then allowed to cool.

  In addition, in order to produce a large roller for roller pitching test described later, a JIS-SCM420H slab that was melted using a converter was rolled into pieces to produce a steel bar having a diameter of 140 mm at 925 ° C. After holding for 2 hours, normalization was allowed to cool.

  Various test pieces were produced by the method described below using each of the above normalized steel bars.

[1] Machining (roughing or finishing):
From the R / 2 part of each steel bar having a diameter of 70 mm ("R" represents the radius of the steel bar) and the central part, the Ono rotary bending with a rough notch shown in Fig. 5 parallel to the rolling direction or the forging axis A fatigue test piece and a coarse roller pitching small roller test piece shown in FIG. 6 were cut out.

  Further, a coarse roller pitching test large roller shown in FIG. 7 was cut out from the center of the steel bar having a diameter of 140 mm in parallel with the forging axis. In FIG. 7, (a) is a front view when a coarse roller 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 each of the above cut-out test pieces shown in FIGS. 5 to 7 is “mm”, and the finish 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).

  From each steel bar having a diameter of 70 mm, a test piece having a diameter of 65 mm and a length of 450 mm was also collected for machinability evaluation.

[2] Carburizing and quenching-tempering:
The coarse-shaped rotating bending fatigue test piece and the small roller test piece cut out in the above [1] were subjected to “carburizing and tempering” by the heat pattern shown in FIG. Further, the “large carburizing-tempering” by the heat pattern shown in FIG. 9 was performed on the large large roller cut out in the above [1]. Note that “Cp” in FIGS. 8 and 9 represents a carbon potential. In addition, “150 ° C oil quenching” was quenched in oil at an oil temperature of 150 ° C, “50 ° C oil quenching” was quenched in oil at an oil temperature of 50 ° C, and “AC” was air-cooled. Represents.

  The rotating bending fatigue test piece and the 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 large roller was subjected to the above 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 quenching oil so that it might be uniformly quenched.

[3] Machining (carburizing and quenching-finishing of tempered material):
The above-mentioned test pieces and large rollers subjected to carburizing quenching-tempering treatment are finished, and the Ono rotary bending fatigue test piece with notches shown in FIG. 10, the small roller test piece for roller pitching test shown in FIG. A large roller for roller pitching test shown in FIG. In FIG. 12, (a) is a front view when the large roller is halved by the 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. 10-12 and a large roller is all "mm", and the finishing symbols "▽" and "▽▽▽" in each said figure are the previous FIG. 7 are “triangular symbols” indicating the surface roughness in 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 and quenched and tempered surface of [2].

  Using the various test pieces produced as described above, investigation of fatigue characteristics by the Ono-type rotary bending fatigue test, investigation of pitting resistance characteristics by the roller pitching test, and machinability evaluation were performed.

  Hereinafter, the contents of the fatigue characteristics, the pitting resistance characteristics, and the machinability evaluation will be described in detail.

<< 1 >> Investigation of fatigue characteristics by Ono type rotating bending fatigue test:
Using the Ono rotary bending fatigue test piece finished in [3] 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.

  When the bending fatigue strength of the R / 2 part is 400 MPa or more, the bending fatigue strength of the central part is 370 MPa or more, and the difference between the bending fatigue strengths of the R / 2 part and the central part is 50 MPa or less Furthermore, this was the target because it was excellent in bending fatigue characteristics.

<< 2 >> Investigation of anti-pitching characteristics by roller pitting test:
A roller pitching test (two-cylinder rolling fatigue test) was carried out under the following test conditions using the finished small roller test piece and the large roller of [3], and the above-mentioned small roller was repeated 10 ⁷ times. The pitching strength was evaluated by the maximum surface pressure at which no pitting with a length of 1 mm or more on the long side occurred on the test piece.

・ Slip rate: 80%
・ Rotation speed: 1000 rpm,
・ Lubrication: Lubricating oil for automatic transmission at an oil temperature of 100 ° C. was sprayed at a rate of 2.0 liters / minute onto the contact portion between the small roller test piece and the large roller.

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

  In addition, when all of the three conditions of the pitching strength of the R / 2 part being 1800 MPa or more, the pitching strength of the center part being 1600 MPa or more, and the difference in the pitching strength between the R / 2 part and the center part being 200 MPa or less are satisfied, This was the target because it was excellent in pitching characteristics.

<< 3 >> Machinability test:
The outer peripheral part of the test piece having a diameter of 65 mm and a length of 450 mm produced in [1] was turned using an NC lathe to evaluate the machinability.

  Turning was performed with a cutting speed of 180 m / min, a cutting depth of 1.5 mm, a feed of 0.3 mm / rev, and no lubricant. Using a dynamometer, the machinability was evaluated by the cutting resistance during turning.

Cutting resistance is the sum of main component force, feed component force and back component force,
Cutting resistance = (Main component force ² + Feed component force ² + Back component force ² ) ^0.5
It was obtained and evaluated by the following formula. If the cutting resistance was 900 N or less, the cutting resistance was assumed to be small, and this was targeted.

  Table 2 summarizes the results of each of the above investigations.

From Table 2, in the case of Test No. 1-4 of the present invention of a test numbers 5 and Reference Examples using satisfies bars specified in the present invention is excellent in flexural fatigue properties and pitting resistance properties, cutting resistance It is clear that it has a small and good machinability.

  On the other hand, among the comparative examples deviating from the conditions defined in the present invention, in the case of test numbers 6 to 8 and test number 10, at least one of the target bending fatigue characteristics and pitting resistance characteristics is In the case of test number 9, the cutting resistance is large and the machinability is inferior.

  That is, in the case of test number 6, the difference in V concentration in the cross section perpendicular to the longitudinal direction of the steel bar was 0.006%, exceeding the range specified in the present invention. For this reason, the bending fatigue strength of the central portion is as low as 360 MPa, the difference in bending fatigue strength between the R / 2 portion and the central portion is as large as 60 MPa, and the bending fatigue characteristics do not reach the target. Further, the difference in pitching strength between the R / 2 part and the central part is as large as 250 MPa, and the anti-pitting characteristic does not reach the target.

  In the case of test number 7, the difference in Ti concentration in the cross section perpendicular to the longitudinal direction of the steel bar was 0.005%, exceeding the range specified in the present invention. For this reason, the bending fatigue strength at the central portion is as low as 350 MPa, the difference in bending fatigue strength between the R / 2 portion and the central portion is as large as 70 MPa, and the bending fatigue characteristics do not reach the target. Further, the difference in pitching strength between the R / 2 part and the central part is as large as 250 MPa, and the anti-pitting characteristic does not reach the target.

  In the case of test number 8, the amount of Ti + V of steel 8 was 0.009%, which exceeded the range specified in the present invention. For this reason, the bending fatigue strength at the R / 2 part is as low as 390 MPa and the bending fatigue strength at the center part is as low as 360 MPa, and the bending fatigue characteristics do not reach the target.

  In the case of test number 9, the C content of steel 9 was 0.28%, exceeding the range specified in the present invention. For this reason, cutting resistance is as high as 930 N, and the target machinability is not obtained.

  In the case of test number 10, the concentration differences of P, Ti, and O in the cross section perpendicular to the longitudinal direction of the steel bar were 0.009%, 0.004%, and 0.0007%, respectively, exceeding the ranges specified in the present invention. It was. For this reason, the bending fatigue strength is as low as 380 MPa in the R / 2 part and 320 MPa in the central part, and the difference in bending fatigue strength between the R / 2 part and the central part is as large as 60 MPa, so that the bending fatigue characteristics do not reach the target. Further, the pitching strength is as low as 1700 MPa at the R / 2 part and 1400 MPa at the central part, and the difference in the pitching strength between the R / 2 part and the central part is as large as 300 MPa, and the anti-pitting characteristic does not reach the target.

  The steel bar or wire rod for skin hardening of the present invention has a low component cost, is excellent in machinability, and has good processing characteristics. Furthermore, the carburized parts obtained by using the steel bar or wire rod for skin hardening according to the present invention are excellent in bending fatigue strength and pitching strength, and there is little variation in these strengths. For this reason, the case-hardening steel bar or wire of the present invention is used as a material for carburizing or carbonitriding parts such as automobile gears that require high bending fatigue strength and high pitching strength to reduce weight and increase torque. Is preferred.

Claims (3)

% By mass
C: 0.13-0.25%
Si: 0.02 to 0.35%,
Mn: 0.50 to 0.90%,
S: 0.005-0.030%,
Cr: 0.50 to 1.60%,
Al: 0.010 to 0.060%,
Nb: 0.050% or less and N: 0.0100 to 0.0250%,
And the balance consists of Fe and impurities,
P, Ti, V and O (oxygen) in the impurity are respectively
P: 0.030% or less,
Ti: 0.005% or less,
V: 0.005% or less and O: 0.0015% or less, and
The total content of Ti and V is 0.005 % or less, and the concentration difference of each impurity element in the cross section perpendicular to the longitudinal direction is 0.005 % or less for P and 0.003% for Ti. Hereinafter, V is 0.003% or less, and O is 0.0005% or less,
Steel bar or wire rod for case hardening. Instead of part of Fe, in mass%,
Cu: 0.20% or less,
Ni: 0.20% or less and Mo: 0.50% or less,
Characterized in that it contains one or more of
The steel bar or wire rod for case hardening according to claim 1. Instead of part of Fe, in mass%,
Ca: 0.0050% or less,
Containing,
The steel bar or wire rod for case hardening according to claim 1 or 2.