JP2005307257A5 - - Google Patents

Description

Steel material for carburized parts or carbonitrided parts

The present invention relates to a suitable steel material as a material for carburized parts or carbonitrided parts. More particularly, it relates gears, pulleys, a suitable steel material as a material for carburized parts or carbonitrided parts such as a shaft having a large surface fatigue strength.

  Conventionally, parts such as gears, pulleys, and shafts of automobiles and various industrial machines have been formed from JIS standard SCr420, SCM420, SNCM420 and other alloy steels for machine structures, and subjected to carburizing or carbonitriding. Post-quenching, then tempering at a temperature of 200 ° C. or less, and by performing shot peening treatment as necessary, surface fatigue strength, bending fatigue strength, wear resistance and the like required for each part The characteristics were secured.

  In recent years, the various carburized parts and carbonitrided parts mentioned above, for example, have been demanded to reduce the weight, reduce the size, and increase the stress load in order to meet the demand for improving the fuel efficiency of automobiles and increasing the output of engines. Is extremely large.

  It should be noted that as the parts become lighter and smaller and the engine output increases, the stress repeatedly applied to the parts surface increases dramatically. For this reason, the above-mentioned various carburized parts and carbonitrided parts are required to have particularly high surface fatigue strength. Furthermore, damages to parts such as gears, pulleys, and shafts of automobiles and industrial machines lead to major accidents. Therefore, it is strongly desired that the various parts described above have stable and excellent characteristics.

  Conventionally, it has been known that when a pearlite structure or a bainite structure that is softer than a martensite structure is generated in the surface layer portion of a carburized and quenched carbonitrided material, the surface fatigue strength is greatly reduced. The pearlite structure and the bainite structure are often generated in the vicinity of the grain boundary oxide layer generated in the surface layer part, and this is because the elements such as Si, Mn, and Cr that improve hardenability are more easily oxidized than Fe. For this reason, it is considered that a deficient layer of Si, Mn and Cr is formed in the vicinity of the grain boundary oxide layer by being preferentially oxidized during the carburizing process or the carbonitriding process.

  Therefore, in Patent Document 1, a steel for carburized gears suitable for obtaining a gear excellent in wear resistance and surface fatigue strength in addition to bending fatigue strength by adding Si and V in combination has been proposed. .

  Patent Document 2 defines the precipitation amount of Nb (CN) and AlN after hot rolling, the structure fraction of bainite, and the ferrite band of the structure having a cross section parallel to the hot rolling direction. High temperature carburizing steel and hot forging member for high temperature carburizing ”have been proposed. In the technique proposed in Patent Document 2, since the hardness reduction in the outermost layer region of the carburized part leads to a reduction in surface fatigue strength, the microsegregation is reduced by using the ferrite band score as an index, and nitride and carbonitride By limiting the amount of precipitation, etc., a decrease in hardness in the outermost layer region is suppressed, thereby ensuring good surface fatigue strength.

JP-A-7-126803 JP 2001-279383 A

An object of the present invention is to provide a suitable steel material as a stable gear having excellent surface fatigue strength, pulleys, carburizing parts or carbonitrided parts such as the shaft material in a mass-production.

  According to the techniques disclosed in Patent Document 1 and Patent Document 2 described above, excellent bending fatigue strength and wear resistance can be obtained, but large surface fatigue strength cannot always be ensured.

  That is, since the technique proposed in Patent Document 1 does not consider segregation, the surface fatigue strength becomes unstable in large-scale mass production. Although consideration has been given to reduce the non-metallic inclusions by reducing the oxygen content to 0.0015% or less, the size and form of the inclusions are not taken into consideration. Fatigue strength is unstable.

The technique proposed in Patent Document 2 is not sufficiently considered that the surface fatigue strength is reduced if there is a portion having a low hardness in the outermost surface layer even in a minute region. In mass production, the surface fatigue strength becomes unstable. In addition, although consideration has been given to reduce non-metallic inclusions, such as oxygen and Ti contents of 0.0025% or less and 0.01% or less, respectively, the size and form of inclusions Thus, as in the case of Patent Document 1, the surface fatigue strength of the mass-produced product is unstable.
In order to solve the above-mentioned problems, the present inventors have established conditions that can stably suppress the formation of pearlite structure and bainite structure in the surface layer part, and in particular, conditions related to chemical components and segregation status. Various investigations and researches were conducted. As a result, the following findings (a) to (d) were obtained.

  (A) Even if the size of the pearlite structure or bainite structure present in the martensite structure is a minute one having a diameter of about 10 μm, the surface fatigue strength is greatly reduced.

  (B) In order to reduce the grain boundary oxide layer, the contents of Si, Mn and Cr may be reduced. However, even if the content of Si, Mn and Cr is reduced, the grain boundary oxide layer cannot be completely eliminated, and in combination with the decrease in hardenability due to the reduction of the content of Si, Mn and Cr. Further, the formation of a pearlite structure and a bainite structure cannot be completely suppressed.

  (C) The pearlite structure and the bainite structure are not generated in all parts in the vicinity of the grain boundary oxide layer, but are formed in a part thereof. And the part which the pearlite structure | tissue and the bainite structure | tissue produced | generated in the vicinity of the grain boundary oxide layer has the density | concentration of Mn, Cr, and Mo lower than the part which the martensite structure | tissue produced | generated in the vicinity of the grain boundary oxide layer.

  (D) Therefore, in order to stably and reliably suppress the formation of the pearlite structure and the bainite structure in the surface layer part, it is only necessary to manage the average content in the materials of Si, Mn, Cr and Mo that are hardenability improving elements. Insufficient amount of Si, Mn, Cr and Mo to form martensite even in a region where the content of Si, Mn and Cr is reduced by the grain boundary oxide layer in the negative segregation part. It must be contained.

  In addition, even when the formation of the pearlite structure and the bainite structure in the surface layer portion is suppressed, the present inventors have investigated and studied repeatedly using the test pieces that have been broken because the surface fatigue strength may be low. As a result, the following findings (e) and (f) were obtained.

  (E) Oxide inclusions and TiN, which are hard inclusions, are the starting points for destruction. For this reason, oxide inclusions and TiN, which are hard inclusions, are often present at the fracture starting point. On the other hand, there are sulfides as inclusions, but this is not a starting point of destruction because it is soft.

(F) Inclusions observed at the fracture starting point have various sizes of 5 to 30 μm in major axis. However, in many cases, there are other inclusions in the vicinity of the inclusions observed at the starting point of the destruction, and even if the individual inclusions are fine, they are gathered together to cause the starting point of the destruction. Can be. Thus, not enough to refine the individual inclusions, Ru necessary der not inclusions was crowd.

The present invention has been completed based on the above findings.

Gist of the present invention is a steel material shown in the following (1) and (2).

(1) It is a steel material for carburized parts or carbonitrided parts, and in mass%, C: 0.1 to 0.3%, Si: 0.3 to 1.5%, Mn: 0.2 to 1. 5%, S: 0.003-0.05%, Cr: 0.5-2.0%, Mo: 0.1-0.8%, Al: 0.01-0.05% and N: 0 0.008-0.025%, the balance is Fe and impurities, Ti in the impurities is 0.005% or less, O (oxygen) is 0.002% or less, P is 0.025% or less, And in the steel material cross section, the minimum value of the value A represented by the following formula (1) is 13 or more, and the maximum length of the inclusion group excluding sulfide in a cross-sectional area of 1500 mm ² is 30 μm or less. A steel material characterized by being.

A = (1 + 0.681Si) (1 + 3.066Mn + 0.329Mn ² ) (1 + 2.007Cr) (1 + 3.14Mo) (1),
Here, the element symbol in the formula (1) represents the content in mass% of the element. In addition, the inclusion group refers to an inclusion in which the interval between the inclusions is 5 μm or less as one group.

(2) Steel material for carburized parts or carbonitrided parts, and in mass%, C: 0.1 to 0.3%, Si: 0.3 to 1.5%, Mn: 0.2 to 1. 5%, S: 0.003-0.05%, Cr: 0.5-2.0%, Mo: 0.1-0.8%, Al: 0.01-0.05% and N: 0 In addition to 0.008-0.025%, further contains one or more of Nb: 0.01-0.08% and V: 0.02-0.15%, and the balance consists of Fe and impurities, Ti in the impurity is 0.005% or less, O (oxygen) is 0.002% or less, P is 0.025% or less, and in the cross section of the steel material, the value of A represented by the following formula (1) A steel material having a minimum value of 15 or more and a maximum length of inclusions excluding sulfide in a cross-sectional area of 1500 mm ² is 30 μm or less.

A = (1 + 0.681Si) (1 + 3.066Mn + 0.329Mn ² ) (1 + 2.007Cr) (1 + 3.14Mo) (1),
Here, the element symbol in the formula (1) represents the content in mass% of the element. Also, the inclusion group will have what distance inclusions each other was regarded as one group of the inclusions is 5μm or less.

Also, steel and steel bars and wire rods, etc. the material is further refers to those processed in the part shape by performing as required forging, cutting or the like.

Hereinafter, the inventions according to the steel of the above (1) and (2), respectively, as "the present invention (1)" and "present invention (2)". Also, it may be collectively referred to as “the present invention”.

Parts subjected to carburizing treatment or carbonitriding treatment to the steel materials of the present invention (1) and (2), or parts subjected to further tempering after the quenching, if necessary, have stable and good surface fatigue strength ( hereinafter, because it has a called pitting strength.), the gear is a component for automobiles and industrial machinery, pulleys, Ru can be used, such as the shaft.

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

(A) Chemical composition of steel material C: 0.1 to 0.3%
C is an essential element for securing the core strength of the parts that have been rapidly cooled after carburizing or carbonitriding. However, if the C content is less than 0.1%, the above effects are insufficient. On the other hand, when the content of C exceeds 0.3%, the machinability of the steel material is greatly deteriorated. Therefore, the content of C is set to 0.1 to 0.3%.

Si: 0.3 to 1.5%
Si is an effective element for increasing the pitching strength because the effect of increasing the hardenability and the temper softening resistance is greater than the adverse effect on the increase of the grain boundary oxide layer. However, if the content is less than 0.3%, the above effect is insufficient. On the other hand, when the Si content exceeds 1.5%, not only the effect of increasing the pitching strength is saturated, but also the machinability of the steel material is greatly reduced. Therefore, the Si content is set to 0.3 to 1.5%. When the Si content is 0.5% or more, the improvement of the pitching strength becomes remarkable. For this reason, the Si content is desirably 0.5 to 1.5%.

Mn: 0.2 to 1.5%
Mn is an element effective for increasing the pitching strength because the effect of increasing the hardenability and temper softening resistance is greater than the adverse effect on the increase in the grain boundary oxide layer. However, if the content is less than 0.2%, the above effect is insufficient. On the other hand, when the Mn content exceeds 1.5%, not only the effect of increasing the pitching strength is saturated, but also the machinability of the steel material is greatly reduced. Therefore, the Mn content is set to 0.2 to 1.5%. In addition, when the Mn content is 0.4% or more, the improvement of the pitching strength becomes remarkable. For this reason, it is desirable that the Mn content be 0.4 to 1.5%.

S: 0.003-0.05%
S combines with Mn to form MnS and has an effect of improving the machinability. However, if the content is less than 0.003%, it is difficult to obtain the above effect. On the other hand, when the content of S increases, coarse MnS tends to be generated and the pitching strength tends to decrease. In particular, when the content exceeds 0.05%, other requirements may be satisfied. A desired pitching strength (pitching strength of 2730 MPa or more in Examples described later) cannot be obtained. Therefore, the content of S is set to 0.003 to 0.05%. In addition, it is preferable that content of S shall be 0.01-0.03%.

Cr: 0.5 to 2.0%
Cr is an element effective in increasing the pitching strength because the effect of increasing the hardenability and the temper softening resistance is greater than the adverse effect on the increase in the grain boundary oxide layer. However, if the content is less than 0.5%, the above effect is insufficient. On the other hand, when the content of Cr exceeds 2.0%, not only the effect of increasing the pitching strength is saturated, but also the machinability of the steel material is significantly lowered. Therefore, the Cr content is set to 0.5 to 2.0%. Note that when the Cr content is 1.2% or more, the improvement of the pitching strength becomes remarkable. For this reason, the Cr content is desirably 1.2 to 2.0%.

Mo: 0.1 to 0.8%
Mo has an effect of increasing hardenability and tempering softening resistance, and is an element effective for increasing pitching strength. However, if the content is less than 0.1%, the above effect is insufficient. On the other hand, if the Mo content exceeds 0.8%, not only the effect of increasing the pitching strength is saturated, but also the machinability of the steel material is significantly reduced. Therefore, the Mo content is set to 0.1 to 0.8%. The Mo content is preferably 0.1 to 0.4%.

Al: 0.01 to 0.05%
Al is an element having a deoxidizing action. Further, Al is an element that is easily bonded to N to form AlN. AlN is effective for refining crystal grains and has the effect of increasing the pitching strength. However, when the Al content is less than 0.01%, it is difficult to obtain the effects described above. On the other hand, Al tends to form hard oxide inclusions, and when the Al content exceeds 0.05%, the maximum length of the inclusion group described later may be 30 μm or less in an observation area of 1500 mm ^2. It becomes difficult and the pitching strength is remarkably lowered, and even if other requirements are satisfied, a desired pitching strength (pitching strength of 2730 MPa or more in the examples described later) cannot be obtained. Therefore, the Al content is set to 0.01 to 0.05%. The Al content is preferably 0.02 to 0.04%.

N: 0.008 to 0.025%
N is easily bonded to Al, Nb, V and Ti to form AlN, NbN, VN and TiN. Among them, AlN, NbN and VN are effective for refining crystal grains and have an effect of increasing the pitching strength. is there. However, when the N content is less than 0.008%, it is difficult to obtain the above effect. On the other hand, if the content of N exceeds 0.025%, it becomes difficult to make the maximum length of the inclusion group, which will be described later, 30 μm or less in an observation area of 1500 mm ² , and the pitching strength is significantly reduced. Even if other requirements are satisfied, a desired pitching strength (pitching strength of 2730 MPa or more in Examples described later) cannot be obtained. Therefore, the N content is set to 0.008 to 0.025%. The N content is preferably 0.011 to 0.018%.

  In the present invention, the contents of Ti, O (oxygen) and P as impurity elements are limited as follows.

Ti: 0.005% or less Ti combines with N to form TiN and lowers the pitching strength. In particular, when the Ti content exceeds 0.005%, it becomes difficult to make the maximum length of the inclusion group, which will be described later, 30 μm or less in the observation area of 1500 mm ² , and the pitching strength is significantly reduced. Even if other requirements are satisfied, a desired pitching strength (pitching strength of 2730 MPa or more in Examples described later) cannot be obtained. Therefore, the Ti content is set to 0.005% or less. Note that the content of Ti as an impurity element is desirably as small as possible, and is more preferably 0.002% or less in consideration of costs for raw materials and steelmaking.

O (oxygen): 0.002% or less O is liable to form a hard oxide inclusion by bonding with Al, and lowers the pitching strength. In particular, when the content of O exceeds 0.002%, it becomes difficult to make the maximum length of the inclusion group described later to be 30 μm or less in the observation area of 1500 mm ² , and the pitching strength is significantly reduced. Even if other requirements are satisfied, a desired pitching strength (pitching strength of 2730 MPa or more in Examples described later) cannot be obtained. In addition, it is desirable to reduce the content of O as an impurity element as much as possible, and considering the cost in steelmaking, it is more preferable to make it 0.001% or less.

P: 0.025% or less P tends to segregate at the grain boundaries and embrittle the grain boundaries. In particular, when the content exceeds 0.025%, the grain boundary embrittlement becomes significant and the pitching strength is reduced. Invite. Therefore, the content of P is set to 0.025% or less. The P content is preferably 0.020% or less.

  Therefore, regarding the chemical composition of the steel material according to the present invention (1), it contains elements from C to N in the above-mentioned range, the balance consists of Fe and impurities, Ti in the impurities is 0.005% or less, and O is 0 It was specified that P was 0.025% or less at 0.002% or less.

  In addition to the above components, the steel material according to the present invention may optionally include one or more elements of Nb: 0.01 to 0.08% and V: 0.02 to 0.15% as necessary. May be added and contained.

  Hereinafter, Nb and V as the optional additive elements will be described.

Nb: 0.01 to 0.08%
Nb is an element that easily forms NbC, NbN, and Nb (C, N) by combining with C or / and N. NbC, NbN, and Nb (C, N) are effective in complementing the above-described crystal grain refinement by AlN and have an effect of increasing the pitching strength. In order to reliably obtain this effect, the Nb content is preferably 0.01% or more. However, if the Nb content exceeds 0.08%, coarse Nb (C, N) is likely to be generated in the central segregation portion, and the pitching strength is decreased. Therefore, the content of Nb when added is set to 0.01 to 0.08%. In addition, the range of more preferable Nb content in the case of adding is 0.02 to 0.05%.

V: 0.02-0.15%
V is an element that is easily bonded to C and N to form VC and VN. Among the above, VN is effective in complementing the above-described crystal grain refinement by AlN and has an effect of increasing the pitching strength. Further, when VN is precipitated during carbonitriding, there is an effect of further increasing the pitching strength. In order to reliably obtain these effects, it is preferable that V is a content of 0.02% or more. However, when the content of V exceeds 0.15%, the machinability of the steel material is greatly deteriorated. Therefore, when V is added, the content of V is set to 0.02 to 0.15%. In addition, the range of more preferable content of V in the case of adding is 0.04 to 0.10%.

  For the above reasons, the chemical composition of the steel material according to the present invention (2) is further added to the chemical composition of the steel material according to the present invention (1), with Nb: 0.01 to 0.08% and V: 0.02 to 0. It was defined as containing 15% or more.

(B) Alloy elements in cross section The steel materials according to the present invention (1) and the present invention (2) have a minimum value of A represented by the above formula (1) of 13 or more and 15 or more, respectively, in the steel material cross section. It is necessary to.

  The above rules are based on the following experimental results conducted by the present inventors.

  That is, the present inventors melted steels A to D shown in Table 1 in a 150 kg vacuum melting furnace, and then cast them into ingots using cast iron as a mold. At the time of dissolution, careful attention was paid to the selection of raw materials so that the impurity elements were sufficiently reduced.

  Next, each of the above ingots was heated at 1150 ° C. for 30 minutes, and then hot forged so that the finishing temperature would be 950 ° C. or higher to produce a steel bar having a diameter of 35 mm.

  Each steel bar thus obtained was divided into five pieces, heat-treated under the conditions shown in Table 2, and allowed to cool to room temperature. Thereafter, the mixture was further heated and held at 925 ° C. for 1 hour, and then allowed to cool to room temperature.

  About the test piece cut out from each specimen having a diameter of 35 mm subjected to the above heat treatment, the cross section cut through the center line parallel to the forging axis is mirror-polished, and at the position shown in FIG. In the direction, line analysis was performed for each element of Si, Mn, Cr and Mo using EPMA. In addition, although C is also known as an element that easily segregates, C was not measured because it easily and uniformly diffuses when heated in the austenite region. In the line analysis by EPMA, the beam diameter was 1 μm, and the scanning speed was 200 μm / min.

  From the measurement results with EPMA, the contents of Si, Mn, Cr and Mo were quantified at the positions where the respective contents of Si, Mn, Cr and Mo were the lowest. Here, since the segregation tendency of Si, Mn, Cr, and Mo is the same, the contents of Si, Mn, Cr, and Mo are quantified at positions where the respective contents of Si, Mn, Cr, and Mo are the lowest. If so, it can be evaluated as the minimum value of A represented by the formula (1).

  In addition, hardenability is, for example, in Inoue Kei's 131st and 132rd Nishiyama Memorial Lecture "Present and Future of Material Prediction and Control Technology of Steel Materials", pages 215 to 217 (Japan Steel Association, September 1989). As is shown on the 25th, it can be estimated from the contents of C and other alloy elements and the austenite grain size. In order to improve the pitching strength aimed by the present invention, the hardenability of the surface layer portion subjected to carburizing treatment or carbonitriding treatment is important. And when carburizing or carbonitriding, the general C content of the surface layer is often about 0.8%, and the austenite crystal grain size contains neither Nb nor V In the case of containing a particle size number of about 9, and in the case of containing one or more of Nb and V, it is often about a particle size number of about 11, so steel not containing Nb and V and steel containing one or more of Nb and V And hardenability can be evaluated from the contents of Si, Mn, Cr and Mo. Therefore, based on Inoue's “Present and Future of Material Prediction / Control Technology of Steel Materials”, the present inventors adopt the value of the above equation (1), that is, the value of A as the hardenability evaluation standard. did.

  Table 3 shows the minimum value of A represented by the formula (1) and the contents of Si, Mn, Cr, and Mo corresponding to the value for each test piece.

  Further, from each specimen having a diameter of 35 mm heat-treated under the conditions shown in Table 2, a test piece having a diameter of 26 mm and a thickness of 15 mm was taken as shown in FIG. 2, and the conditions shown in FIG. Gas carburizing treatment was also performed. In addition, “CP” in FIG. 3A means a carbon potential.

  About each test piece subjected to the above gas carburizing treatment, the section cut through the center line parallel to the forging axis is mirror-polished and then corroded with nital. Then, as shown in FIG. Were observed using an SEM (scanning electron microscope), and the presence or absence of a bainite structure and a pearlite structure was investigated.

  Table 3 shows the results of a survey on the presence of the bainite structure and the pearlite structure. FIG. 5 shows the results of the above investigation organized by the minimum value of A expressed by the above equation (1). Here, the numbers in the column “No. of heat treatment” in Table 3 correspond to those in Table 2. In addition, “None” and “Yes” in the “Perlite structure, bainite structure” column indicate that “Baynite structure and pearlite structure do not exist” and “Bainite structure and pearlite structure, respectively, or both exist, respectively. Means "to do". Furthermore, “◯: no pearlite, no bainite” and “×: no pearlite, no bainite” in FIG. 5 are respectively “no both bainite structure and pearlite structure” and “one of bainite structure and pearlite structure”. Or both exist. " In FIG. 5, a case where Nb and V are not included is expressed as “Nb and V are absent”, and a case where one or more of Nb and V are included is expressed as “Nb and V is present”. The minimum value of A represented by the above equation (1) is represented as “value of A represented by equation (1)” in Table 3, and is simply “A value” in FIG. It was written.

  From the above Table 3 and FIG. 5, when Nb and V are not included, if the minimum value of A represented by the above formula (1) is 13 or more, both bainite structure and pearlite structure do not exist, On the other hand, when one or more of Nb and V are included, it is clear that both the bainite structure and the pearlite structure do not exist if the minimum value of A represented by the formula (1) is 15 or more.

  Therefore, in the steel materials according to the present invention (1) and the present invention (2), the minimum value of the value A represented by the above formula (1) is defined as 13 or more and 15 or more, respectively, in the steel material cross section.

  Here, in the case of steel bars and wire rods, the steel material cross section is preferably a cross section cut through a center line parallel to the rolling direction or the forging axis, and when the steel material is a part shape, the surface layer portion to the surface layer is 15 mm. A range of cross-sections is desirable.

  In addition, the average composition of steel, a solidification rate, a solidification form, etc. influence the value of A represented by said Formula (1) in a steel material cross section. It is also affected by steelmaking facilities.

  For this reason, in order to make the minimum value of the value A represented by the above formula (1) in the steel material cross section 13 or more or 15 or more, the following method may be adopted.

  That is, in order to set the minimum value of the A value in the steel material cross section to 13 or more, for example, when producing a large cross section of 400 mm × 300 mm square by continuous casting, first, the average composition of the steel is expressed by the above formula (1). It melts so that the value of A represented by 20 becomes 20 or more. Then, after sufficiently performing electromagnetic stirring of the molten steel, continuous casting is performed, and further, homogenization heat treatment is performed on the bloom at 1200 to 1280 ° C. for 8 hours or more, and the billet is billet having a side of 200 mm or less. What is necessary is just to hot-roll so that a rolling finishing temperature may be 850-1000 degreeC after heating at 1200-1280 degreeC for 2 hours or more.

  Moreover, in order to make the minimum value of the A value in the steel material cross section 15 or more, for example, when producing a large cross-sectional bloom of 400 mm × 300 mm square by continuous casting, first, the average composition of the steel is expressed by the formula (1). It melts so that the value of A represented by can be 23 or more. Then, after sufficiently performing electromagnetic stirring of the molten steel, continuous casting is performed, and further, homogenization heat treatment is performed on the bloom at 1200 to 1280 ° C. for 8 hours or more, and the billet is billet having a side of 200 mm or less. What is necessary is just to hot-roll so that a rolling finishing temperature may be 850-1000 degreeC after heating at 1200-1280 degreeC for 2 hours or more.

(C) Maximum length of inclusion group The steel materials according to the present invention (1) and the present invention (2) have a maximum length of inclusion group excluding sulfide in a cross-sectional area of 1500 mm ² in a steel cross section of 30 μm. It is necessary to do the following.

  The above rules are based on the following experimental results conducted by the present inventors.

  That is, the present inventors melted steels E to H shown in Table 4 in a 150 kg vacuum melting furnace, and cast them into ingots. For steels E to G, cast iron was used as the mold (hereinafter, the cast iron mold was referred to as “normal mold”), and for steel H, a silica mold was used to slow the solidification rate.

  Further, steels I to L shown in Table 4 were melted in a 30 kg vacuum melting furnace and cast into an ingot using a normal mold. Among these, for steel L, a refractory was damaged in the mold, and the refractory was intentionally mixed.

  Furthermore, steel M and steel N shown in Table 4 were melted in a 70 t (ton) converter, and a 400 mm × 300 mm square bloom was produced by continuous casting. For steel M, RH vacuum degassing treatment was performed for a long time by secondary refining, and electromagnetic stirring of the molten steel was sufficiently performed. On the other hand, the steel N was subjected to VAD treatment (vacuum arc degassing treatment) by secondary refining and further weakened electromagnetic stirring of the molten steel.

  For steels E to L, the ingot was heated at 1150 ° C. for 30 minutes, and then hot forged so that the finishing temperature was 950 ° C. or higher to produce a steel bar having a diameter of 35 mm. Each of these steel bars was held at 1250 ° C. for 12 hours and then allowed to cool to room temperature, then further heated and held at 925 ° C. × 1 hour, and then allowed to cool to room temperature.

  For steel M and steel N, a 400 mm × 300 mm square bloom was homogenized and heat treated at 1250 ° C. for 12 hours, and then rolled into a 180 mm × 180 mm square billet, which was heated at 1250 ° C. for 2 hours. Then, it was hot-rolled into a steel bar having a diameter of 35 mm at a rolling finishing temperature of 900 to 950 ° C.

  About the test piece cut out from each steel bar 35 mm in diameter of steels E to N obtained in this way, the cross section cut through the center line parallel to the rolling direction or the forging axis is mirror-polished and interposed using an optical microscope. Things were measured.

In addition, observation with said optical microscope was performed for every range of 10 mm x 10 mm, and the maximum length of the inclusion group and the maximum length of each inclusion were measured. For each sample, 15 fields of view were measured, and the maximum length of inclusions and the maximum length of individual inclusions in a measurement area of 1500 mm ² were determined. In addition, an inclusion group refers to what considered the inclusion whose space | interval of inclusions is 5 micrometers or less as one group.

  In the above survey, sulfide was excluded from the measurement. The reason is that sulfide has not been detected at the starting point of destruction from the investigations so far. In addition, since the inclusions having a maximum length of 2 μm or less are difficult to distinguish by observation with an optical microscope, they were also excluded from the measurement. In addition, since sulfide exhibits gray in optical microscope observation, it can be easily distinguished from other inclusions.

  A roller pitching test was also conducted.

  That is, from each steel bar 35 mm in diameter of steels E to N produced by the method described above, a small roller used in a roller pitching test with a diameter of 26 mm is collected, and carburizing and quenching is performed under the conditions shown in FIG. Next, tempering was performed under the conditions shown in FIG.

  The large roller used in the roller pitching test was made of JIS standard SUJ2 steel, and was produced by a general process, that is, a normal “spheroidizing annealing → test piece processing → quenching → tempering → polishing” process. Here, the large roller had a diameter of 130 mm and a crowning of 100 mmR.

  Using the small roller and the large roller produced as described above, a roller pitching test was performed under the conditions shown in Table 5.

The number of tests is 7 each, and an SN graph is created with the vertical axis representing the surface pressure and the horizontal axis representing the number of repetitions until failure, and the surface pressure at the number of repetitions of 1.0 × 10 ⁷ is pitched. Strength.

  In addition, since the chemical composition of the steel E corresponds to the JIS standard SCr420 steel generally used, the pitching strength of the steel E is 1950 MPa, and the goal was to exceed this by 40% or more.

  Table 6 shows the results of the above investigations for steels E to N, that is, the maximum length of inclusions, the maximum length of individual inclusions, and the pitching strength. In Table 6, the maximum length of each inclusion was expressed as “inclusion maximum length”, and the maximum length of the inclusion group was expressed as “inclusion group maximum length”.

  FIG. 6 shows the relationship between the maximum length of the inclusion group (indicated as “inclusion group maximum length” in FIG. 6) and the pitching strength in the roller pitching test. FIG. 7 shows the relationship between the maximum length of each inclusion (indicated as “inclusion maximum length” in FIG. 7) and the pitching strength in the roller pitching test.

  As can be seen from FIGS. 6 and 7, the maximum length of the inclusion group is more correlated with the pitching strength than the maximum length of each inclusion, and the maximum length of the inclusion group is 30 μm or less. Thus, the pitching strength is greatly improved and the target value is satisfied.

Therefore, in the steel materials according to the present invention (1) and the present invention (2), the maximum length of the inclusion group excluding sulfide in a cross-sectional area of 1500 mm ² is defined as 30 μm or less in the steel material cross section.

  Here, in the case of steel bars and wire rods, the steel material cross section is preferably a cross section cut through a center line parallel to the rolling direction or the forging axis, and when the steel material is a part shape, the surface layer portion to the surface layer is 15 mm. A range of cross-sections is desirable.

  In addition, the composition of inclusions, the solidification rate, and solidification segregation affect the size and distribution of inclusions. It is also affected by steelmaking facilities.

For this reason, in order to make the maximum length of the inclusion group excluding sulfide in a cross section of 1500 mm ² in the steel material cross section, the following method may be employed. For example,
(A) The content in steel shall be 0.05% or less for Al, 0.002% or less for O, 0.003% or less for Ti, and 0.025% or less for N.

  (B) Prevent melting of refractories such as ladle and tundish and entrainment of slag and powder during casting.

  (C) When casting with an ingot, use a small mold and use a material with good thermal conductivity for the mold. In addition, in the case of a 30 kg ingot used in the examples and the like, the maximum length of the target inclusion group can be obtained by satisfying the above (a) and (b) and using a material with good thermal conductivity as the mold material. Is obtained.

(D) On the other hand, for example, when producing a large cross section of 400 mm × 300 mm square by continuous casting, and then producing a steel bar or wire rod, first, the content in the steel is 0.04% or less for Al and O for After 0.001% or less, Ti is 0.002% or less, and N is 0.018% or less, RH vacuum degassing treatment is performed for a long time by secondary refining, and electromagnetic stirring of the molten steel is performed sufficiently. further, the total forging ratio, i.e. "cross-sectional area of the cross-sectional area / steel bar or wire rod bloom" is not good if processed to have 40 or more.

Steels a to p having chemical compositions shown in Table 7 were melted.

  Among the above steels, steels a to c were melted in a 150 kg vacuum melting furnace and cast into an ingot using a normal mold. In addition, about steel c, the refractory was damaged in the mold, and the refractory was intentionally mixed.

  Steels dn were melted in a 30 kg vacuum melting furnace and cast into an ingot. For steels dm, a normal mold was used, and for steel n, a silica mold was used to slow the solidification rate.

  Furthermore, steel o and steel p were melted in a 70 t (ton) converter, and a 400 mm × 300 mm square bloom was produced by continuous casting. For steel o, RH vacuum degassing treatment was performed for a long time by secondary refining, and electromagnetic stirring of the molten steel was sufficiently performed. On the other hand, the steel p was subjected to VAD treatment (vacuum arc degassing treatment) by secondary refining and further weakened electromagnetic stirring of the molten steel.

  The following process was performed on the ingot and bloom obtained as described above.

  First, the ingots of steels a to c were heated at 1150 ° C. for 30 minutes, and then hot forged so that the finishing temperature was 950 ° C. or higher to produce a steel bar having a diameter of 35 mm. Each of these bar steels was divided into five pieces, heat-treated under the conditions shown in Table 2 (Nos. 1 to 5), and allowed to cool to room temperature. Thereafter, the mixture was further heated and held at 925 ° C. for 1 hour, and then allowed to cool to room temperature.

  Moreover, after heating the ingot of steel dn for 30 minutes at 1150 degreeC, it hot-forged so that finishing temperature might be 950 degreeC or more, and produced the bar steel of 35 mm in diameter. For each of these steel bars, the condition No. The heat treatment of 2 was performed and allowed to cool to room temperature. Thereafter, the mixture was further heated and held at 925 ° C. for 1 hour, and then allowed to cool to room temperature.

  The bloom of steel o is the condition no. After the heat treatment of No. 2 was performed and allowed to cool to room temperature, it was rolled into pieces and formed into 180 mm × 180 mm square billets. Next, the billet was heated at 1250 ° C. for 2 hours, and then hot-rolled into a steel bar having a diameter of 35 mm at a rolling finishing temperature of 900 to 950 ° C.

  Furthermore, the bloom of steel p is the condition no. After the heat treatment of No. 4 was performed and the mixture was allowed to cool to room temperature, it was subjected to ingot rolling to form a 180 mm × 180 mm square billet. Next, this billet was heated at 1150 ° C. for 1 hour, and then hot-rolled into a steel bar having a diameter of 35 mm at a rolling finishing temperature of 900 to 950 ° C.

  About the test piece cut out from each steel bar 35 mm in diameter of steel a to p obtained in this way, the cross section cut through the center line parallel to the rolling direction or the forging axis is mirror-polished and interposed using an optical microscope. Things were measured.

In addition, observation with said optical microscope was performed for every range of 10 mm x 10 mm, and the maximum length of the inclusion group in the range was measured. For each sample, 15 fields of view were measured, and the maximum length of the inclusion group in the measurement area of 1500 mm ² was determined. In the above survey, sulfides were excluded from the measurement.

  Moreover, the line analysis using EPMA was performed about each element of Si, Mn, Cr, and Mo in the said mirror-polished surface. The line analysis by EPMA was performed with a beam diameter of 1 μm and a scanning speed of 200 μm / min.

  Furthermore, a roller pitching test was also conducted.

In other words, from the steel bar with a diameter of 35mm steel a~p prepared by the above method, were taken small roller used in the roller pitting test 26mm in diameter, carburizing or under the conditions shown below Symbol Table 8, and Figure 8 After performing the carbonitriding treatment, it was rapidly cooled, kept at 160 ° C. for 2 hours, and then tempered under the condition of cooling.

  The large roller used in the roller pitching test was made of JIS standard SUJ2 steel, and was produced by a general process, that is, a normal “spheroidizing annealing → test piece processing → quenching → tempering → polishing” process. Here, the large roller had a diameter of 130 mm and a crowning of 100 mmR.

  Using the small roller and the large roller produced as described above, a roller pitching test was performed under the conditions shown in Table 5 above.

In addition, the number of tests was set to 7 each, and an SN diagram was prepared in which the vertical axis represents the surface pressure and the horizontal axis represents the number of repetitions until failure, and the surface pressure at the number of repetitions of 1.0 × 10 ⁷ times. Was defined as the pitching strength.

  The target for the pitching strength was to exceed 1950 MPa, which is the pitching strength of the steel E described above corresponding to the JIS standard SCr420 steel, by 40% or more, that is, to exceed 2730 MPa.

Tables 9 and 10 show the results of the above tests. The numbers in the “homogenization treatment conditions” column of Tables 9 and 10 are the treatment condition numbers in Table 2. It corresponds to. The numbers in the “Carburizing (nitriding) treatment conditions” column correspond to the treatment numbers in Table 8 . The minimum value of A represented by the above formula (1) is expressed as “value of A represented by formula (1)” in Tables 9 and 10.

  From Tables 9 and 10, it is clear that in the case of a test number that deviates from the conditions specified in the present invention, the pitching strength in the roller pitching test is a low one that does not reach 2730 MPa.

In contrast, when the conditions are met Test No. defined in the present invention, pitting strength in the roller pitching test Ru apparent der to be a large value exceeding 2730MPa to target.

Parts that have been quenched after the carburizing or carbonitriding treatment of the steel materials of the present invention (1) and (2), or parts that have been further tempered after the quenching, have stable and good pitching strength. since the gear is a component for automobiles and industrial machinery, pulleys, Ru can be used, such as the shaft.

It is a figure which shows the position which performed the line analysis using EPMA about each element of Si, Mn, Cr, and Mo. It is a figure explaining the condition which extract | collects the test piece whose diameter is 26 mm and thickness is 15 mm from the test material of 35 mm in diameter. (A) is a figure explaining the conditions of a gas carburizing process and subsequent cooling, and (b) is a figure explaining the conditions of tempering. It is a figure explaining the structure | tissue investigation of the area | region of 200 micrometers from the surface layer using SEM. Presence of bainite structure and pearlite structure when SEM observation is performed on a 200 μm region from the surface layer where the minimum value of A (expressed as “A value” in the figure) represented by the above formula (1) and the content of Nb and V are 200 μm from the surface layer. It is a figure shown about the influence which it has on the presence or absence of. It is a figure which shows the relationship between the maximum length of an inclusion group (it describes with "inclusion group maximum length" in a figure), and the pitching intensity | strength in a roller pitching test. It is a figure which shows the relationship between the maximum length of each inclusion (it describes with "inclusion maximum length" in a figure), and the pitching strength in a roller pitching test. It is a figure explaining the method of performing carburizing process or carbonitriding process, and subsequent quenching twice.

Claims (2)

It is a steel material for carburized parts or carbonitrided parts, and in mass%, C: 0.1 to 0.3%, Si: 0.3 to 1.5%, Mn: 0.2 to 1.5%, S: 0.003-0.05%, Cr: 0.5-2.0%, Mo: 0.1-0.8%, Al: 0.01-0.05% and N: 0.008- 0.025% is contained, the balance consists of Fe and impurities, Ti in impurities is 0.005% or less, O (oxygen) is 0.002% or less, P is 0.025% or less, and steel material In the cross section, the minimum value of A represented by the following formula (1) is 13 or more, and the maximum length of the inclusion group excluding sulfide in the cross sectional area of 1500 mm ² is 30 μm or less. Features steel.
A = (1 + 0.681Si) (1 + 3.066Mn + 0.329Mn ² ) (1 + 2.007Cr) (1 + 3.14Mo) (1)
Here, the element symbol in the formula (1) represents the content in mass% of the element. In addition, the inclusion group refers to an inclusion in which the interval between the inclusions is 5 μm or less as one group. It is a steel material for carburized parts or carbonitrided parts, and in mass%, C: 0.1 to 0.3%, Si: 0.3 to 1.5%, Mn: 0.2 to 1.5%, S: 0.003-0.05%, Cr: 0.5-2.0%, Mo: 0.1-0.8%, Al: 0.01-0.05% and N: 0.008- In addition to 0.025%, it further contains at least one of Nb: 0.01 to 0.08% and V: 0.02 to 0.15%, and the balance is made of Fe and impurities. Ti is 0.005% or less, O (oxygen) is 0.002% or less, P is 0.025% or less, and in the steel cross section, the minimum value of A represented by the following formula (1) is 15 or more, and the maximum length of the inclusion group excluding sulfide in a sectional area of 1500 mm ² is 30 μm or less.
A = (1 + 0.681Si) (1 + 3.066Mn + 0.329Mn ² ) (1 + 2.007Cr) (1 + 3.14Mo) (1)
Here, the element symbol in the formula (1) represents the content in mass% of the element. Also, the inclusion group will have what distance inclusions each other was regarded as one group of the inclusions is 5μm or less.