JP2006241572A - Steel for machine structure use - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for machine structure use having excellent hot forgeability, cold forgeability and rolling fatigue properties. <P>SOLUTION: The steel for machine structure use has a composition comprising 0.35 to 0.55% C, 0.05 to 1.0% Si, 0.4 to 1.5% Mn, ≤0.030% P, 0.005 to 0.050% S, 0 to 0.20% Cu, 0 to 0.20% Ni, 0 to 2.0% Cr, 0 to 1.0% Mo, 0 to 0.20% V, 0 to 0.10% Ti, 0 to 0.0050% B, 0.005 to 0.050% Al, 0.0005 to <0.0050% rare earth metals and ≤0.0020% O (oxygen), and the balance Fe with impurities, and in which Nd accounts for ≥60% of the rare earth metals. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to steel for machine structural use, and more particularly, a machine excellent in hot forgeability, cold forgeability and rolling fatigue characteristics, which is suitable as a material for shafts, bearings and bolts of automobiles and various industrial machines. It relates to structural steel.

  Parts such as shafts, bearings, and bolts of automobiles and various industrial machines are subjected to hot forging using various mechanical structural steels as raw materials, or are subjected to near net shape processing by cold forging, or thereafter In addition, cutting is often performed to finish the final part shape.

  Among the above, when cutting is performed after hot forging to finish the final part shape, the material steel may contain a large amount of S so as to facilitate cutting. However, when a large amount of S is contained, coarse MnS is often generated, and cracks are likely to occur during hot forging starting from the coarse MnS.

  On the other hand, when cold forging is performed, it is necessary to perform long-time heat treatment to soften the material, such as spheroidizing annealing, and when it is desired to avoid long-time heat treatment for softening, cold forging However, in this case, the hardenability is insufficient and a desired strength may not be ensured after quenching and tempering.

  Therefore, in Patent Document 1, the weight ratio of C: 0.40 to 0.65%, Si: 0.01, in which the forgeability is increased by utilizing the reduction effect of MnS inclusions by reducing the amount of S, etc. To 0.60%, Mn: 0.30 to 1.20%, P: 0.018% or less, S: 0.010% or less, Al: 0.005 to 0.018%, O: 0.0020% Hereinafter, Ti: 0.030% or less, Cr: 0.30 to 0.60%, Ni: 0.30 to 0.60%, Mo: 0.03 to 0.13% or one or two of them "High-strength induction hardening steel" that contains more than seeds and consists of the remaining Fe and impurity elements has been proposed.

  Next, in Patent Document 2, in mass%, C: 0.40 to 0.60%, Si: more than 0.05% and 0.10% or less, Mn: 0.20 to 0.65%, Cr: 0.30% or less, Ti: 0.005-0.05%, B: 0.0003-0.0030%, Al: 0.01-0.07% and Mo: 0.05-0.20% And the impurity elements S, O, and N are limited to 0.02% or less, 0.0015% or less, and 0.0070% or less, respectively, and the balance is substantially made of Fe and inevitable impurities. "Carbon steel for machine structure excellent in cold forgeability, hardenability and scale peelability" has been proposed.

Patent Document 3 discloses a so-called Pb-free “machine structural steel” that does not substantially contain Pb as a machinability improving element, that is, a rare earth metal compound having improved machinability and mechanical properties. The rare earth metal compound observed per 1 mm ² in cross section of mechanical structural steel dispersed in the steel is 5 or less having a major axis of more than 50 μm, and 10 to 10 having a major axis of 1 to 50 μm. 500 pieces of “mechanical steel” have been proposed. And in order to produce the above-mentioned size and number of rare earth metal compounds, particularly sulfides, the specific steel composition is rare earth metal: 0.001 to 0.22 mass% and S: 0.01 to 0.22 mass. "Mechanical structural steel" is proposed.

  Furthermore, Patent Document 4 discloses a so-called “Pb-free material that does not substantially contain Pb as a machinability improving component, and has excellent chip controllability and mechanical properties at the time of cutting. In the steel for machine structure in which sulfide inclusions exist, “machine structure steel” having a sulfide particle distribution index F1 defined by a specific formula of 0.5 or less has been proposed. And from the viewpoint of satisfying the required characteristics as mechanical structural steel, the specific steel composition is C: 0.01 to 0.7%, Si: 0.01 to 2.5%, Mn: 0.00. 1-3%, S: 0.01-0.2%, P: 0.05% or less (including 0%), Al: 0.1% or less (including 0%), and N: 0.002 "Mechanical structural steel" containing 0.02% respectively, and further, one selected from the group consisting of Ti: 0.002-0.2% and rare earth elements: 0.0002-0.2% in total "Mechanical steel" containing the above has been proposed.

JP-A-6-81077 JP 2002-241889 A JP-A-11-222645 JP 2000-282171 A

  The technique disclosed in Patent Document 1 described above improves forgeability by utilizing the effect of reducing MnS inclusions by reducing the amount of S. However, simply reducing the amount of S to reduce MnS inclusions inevitably reduces machinability, and cutting for finishing to the final part shape is often difficult.

  In the case of the technique disclosed in Patent Document 2 described above, since the form of sulfide is not taken into consideration, a stable and excellent rolling fatigue life cannot often be obtained.

  In the case of the technique disclosed in Patent Document 3 described above, in order to generate a specified size and number of rare earth metal compounds, particularly sulfides, as described above, a specific steel composition includes rare earth metals: 0.001 to 0.001. 0.22 mass% and S: 0.01-0.22 mass% are contained. Moreover, it is a preferable technique to set the mass ratio of “rare earth metal / S” to 0.05 to 3.5. This is because rare earth metals have a strong bonding force with S and are likely to form sulfides, but when the mass ratio of “rare earth metals / S” is less than 0.05, the amount of rare earth metal sulfides produced is insufficient. Not only is the machinability improving effect difficult to appear, but excess S mainly forms sulfides such as MnS and FeS and degrades the mechanical properties of the steel, while the mass ratio of “rare earth metal / S” is 3 If it exceeds .5, excess rare earth metals are considered to form coarse oxysulfides and oxides having a lower machinability improving effect than sulfides. However, the technique disclosed in Patent Document 3 merely defines the total amount of “rare earth metal” and does not control the ratio of each element in the rare earth metal. For this reason, the mechanical characteristics may vary depending on the type of rare earth metal contained as the total amount, and it has not always been possible to ensure the desired characteristics stably.

  In the case of the technique disclosed in Patent Document 4 described above, the specific steel composition from the viewpoint of satisfying the required characteristics as mechanical structural steel is, as described above, C: 0.01 to 0.7%, Si: 0.01-2.5%, Mn: 0.1-3%, S: 0.01-0.2%, P: 0.05% or less (including 0%), Al: 0.1 % Or less (including 0%) and N: 0.002 to 0.02%, and if necessary, Ti: 0.002 to 0.2% and rare earth elements: 0.0002 to It contains one or more selected from the group consisting of 0.2%. The reason why rare earth elements such as Ce, La, Pr, and Nd are added is because the form such as the distribution state of sulfides changes and excellent characteristics are obtained. However, the technique disclosed in Patent Document 4 also merely defines the total amount of “rare earth metal” and does not control the ratio of each element in the rare earth metal. For this reason, the mechanical characteristics may vary depending on the type of rare earth metal contained as the total amount, and it has not always been possible to ensure the desired characteristics stably.

  The object of the present invention is to avoid cracking in hot forging and cold forging, and to omit or simplify long-time heat treatment for softening the material such as spheroidizing annealing before cold forging. It is possible to provide steel for machine structural use that is excellent in hot forgeability, cold forgeability, and rolling fatigue properties, which is suitable as a material for shafts, bearings and bolts of automobiles and various industrial machines.

  In order to solve the above-mentioned problems, the present inventors can obtain stable and excellent mechanical characteristics, particularly excellent rolling fatigue characteristics, as well as excellent machinability and heat resistance. Machine structural steel that can avoid cracking in cold forging and cold forging, and can omit or simplify long-time heat treatment to soften the material such as spheroidizing annealing before cold forging Various investigations and researches were conducted on the chemical composition of. As a result, first, the following findings (a) to (f) were obtained.

  (A) The affinity for S is higher for rare earth metals than for Mn. For this reason, in steel to which rare earth metals are added, S is combined with rare earth metals preferentially over Mn to form sulfides of rare earth metals, resulting in a decrease in the number of MnS produced.

  (B) The shape of the sulfide of the rare earth metal is not elongated in the rolling direction like MnS but is elliptical, and the sulfide of the rare earth metal is uniformly and finely dispersed. Good mechanical properties with low properties can be obtained.

  (C) Rare earth metal sulfides exist alone or in a state of being combined with other sulfides or oxides.

  (D) Ferrite transformation occurs with sulfides of rare earth metals as nuclei. Ferrite has a low hardness and is therefore a suitable structure for cold forging.

  (E) The sulfide of rare earth metal has the same machinability improving effect as MnS.

  (F) When the content of the rare earth metal is too large, a coarse rare earth oxide is formed, and rolling fatigue characteristics are deteriorated.

  Therefore, we paid attention to the action of sulfides of each element in the rare earth metal, and investigated the influence on various properties by changing the ratio of each element in the rare earth metal. As a result, the following matters became clear.

  (G) If the ratio of each element in the rare earth metal changes, the mechanical characteristics vary, but if the rare earth metal contains Nd in a specific ratio or more, the mechanical characteristics vary little and a stable machine. Characteristics are obtained.

  (H) The ferrite transformation promoting action in the above knowledge (d) is remarkable particularly in the case of sulfides of Nd among sulfides of rare earth metals.

  (I) Even when an inexpensive misch metal that is a mixture of rare earth metals is used, if the rare earth metal contains Nd at a specific ratio, the effects (g) and (h) can be obtained with certainty. It is done.

  And the following knowledge (j) was obtained from each said knowledge.

  (J) In the case of hot forging or cold forging, if a rare earth metal containing Nd is contained in a specific ratio or more and the sulfide is replaced with coarsely dispersed MnS by Nd-based sulfides that are finely and uniformly dispersed, Can be avoided. In addition, since ferrite transformation is promoted with Nd-based sulfides as the core and the proportion of ferrite structure with low hardness increases, the cold forgeability of the steel for machine structure can be improved.

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

  The gist of the present invention resides in the steel for machine structure shown in the following (1) to (4).

  (1) By mass%, C: 0.35 to 0.55%, Si: 0.05 to 1.0%, Mn: 0.4 to 1.5%, P: 0.030% or less, S: 0.005 to 0.050%, Cu: 0 to 0.20%, Ni: 0 to 0.20%, Cr: 0 to 2.0%, Mo: 0 to 1.0%, V: 0 to 0 20%, Ti: 0 to 0.10%, B: 0 to 0.0050%, Al: 0.005 to 0.050%, Rare earth metal: 0.0005% or more and less than 0.0050% and O (oxygen) ): 0.0020% or less, the balance is Fe and impurities, and Nd accounts for 60% or more of the rare earth metal.

  (2) Steel for machine structure as described in said (1) with which content of Cr and Mo satisfy | fills at least any one of Cr: 0.4-1.5% and Mo: 0.1-0.5%.

  (3) The machine according to (1) or (2), wherein the contents of Ti and V satisfy at least one of Ti: 0.02 to 0.05% and V: 0.05 to 0.10%. Structural steel.

  (4) Steel for machine structure as described in any one of said (1) to (3) with which content of B satisfies B: 0.0010 to 0.0030%.

  The “rare earth metal” as used in the present invention means 15 element lanthanoids from Sc of atomic number 21 belonging to group 3A of the periodic table, Y of atomic number 39 and La of atomic number 57 to Lu of atomic number 71 In total, the content of rare earth metals refers to the total content of the 17 elements.

  Hereinafter, the inventions relating to the steel for machine structures of (1) to (4) above are referred to as “present invention (1)” to “present invention (4)”, respectively. Also, it may be collectively referred to as “the present invention”.

  Since the steel for machine structure of the present invention is excellent in hot forgeability and cold forgeability, it is possible to avoid cracks in hot forging and cold forging, and spheroidizing annealing before cold forging, etc. It is possible to omit or simplify the long-time heat treatment for softening the material, and it is also excellent in rolling fatigue characteristics, so it can be used as a material for shafts, bearings and bolts of automobiles and various industrial machines. it can.

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

C: 0.35-0.55%
C is an essential element for securing the strength of steel, and requires a content of 0.35% or more. However, if the C content is too large, it becomes too hard and the cold forgeability is reduced. In particular, if the C content exceeds 0.55%, the cold forgeability is significantly reduced. Therefore, the content of C is set to 0.35 to 0.55%.

  In addition, when the cold forgeability is more important, the C content is preferably 0.35 to 0.45%.

Si: 0.05-1.0%
Si has a deoxidizing action. In order to acquire this effect, it is necessary to make Si content 0.05% or more. However, if the Si content is too large, it becomes too hard and causes a decrease in cold forgeability. In particular, if the Si content exceeds 1.0%, the cold forgeability is significantly reduced. Therefore, the Si content is set to 0.05 to 1.0%.

  When the cold forgeability is more important, the Si content is preferably 0.05 to 0.5%.

Mn: 0.4 to 1.5%
Mn has the effect | action which ensures the intensity | strength of steel, and a deoxidation effect. In order to obtain these effects, the Mn content needs to be 0.4% or more. However, if the Mn content exceeds 1.5%, coarse MnS may be generated and cause cracking during hot forging or cold forging. In addition, coarse MnS causes a reduction in rolling fatigue life. Therefore, the Mn content is set to 0.4 to 1.5%.

  In addition, when hot forgeability, cold forgeability, and rolling fatigue life are more important, the Mn content is preferably 0.4 to 1.2%.

P: 0.030% or less P, which is an inevitable impurity contained in the steel, is a ferrite strengthening element. If the content is too large, the ferrite becomes too hard and the cold forgeability deteriorates. If the P content exceeds 0.030%, the cold forgeability will be significantly reduced. On the other hand, if the P content exceeds 0.030%, P segregated at the grain boundaries may embrittle the steel. Therefore, the content of P is set to 0.030% or less. A more preferable P content is 0.020% or less.

S: 0.005 to 0.050%
S has the effect of combining rare earth metals to form sulfides of rare earth metals to improve machinability, and also has the effect of promoting ferrite transformation using the sulfides of rare earth metals as nuclei. However, if the content is less than 0.005%, it is difficult to obtain the above effect. On the other hand, if the S content increases and exceeds 0.050%, coarse rare earth sulfides and coarse MnS are formed, and hot forgeability, cold forgeability, and rolling fatigue life are reduced. . Therefore, the content of S is set to 0.005 to 0.050%. A more preferable content of S is 0.005 to 0.040%.

Cu: 0 to 0.20%
Addition of Cu is optional. If added, it has the effect of strengthening ferrite. In order to reliably obtain this effect, the Cu content is preferably 0.10% or more. However, if the Cu content exceeds 0.20%, surface flaws are likely to occur during hot rolling or hot forging. Therefore, the Cu content is set to 0 to 0.20%.

  When the cold forgeability is more important, the Cu content is preferably 0 to 0.10%.

Ni: 0 to 0.20%
The addition of Ni is optional. If added, it has the effect of increasing toughness. In order to reliably obtain this effect, the Ni content is preferably 0.10% or more. However, if the Ni content exceeds 0.20%, it becomes hard and cold forgeability deteriorates. Therefore, the content of Ni is set to 0 to 0.20%.

  When the cold forgeability is more important, the Ni content is preferably 0 to 0.10%.

Cr: 0 to 2.0%
The addition of Cr is optional. If added, it has the effect of increasing the strength of the steel and the effect of improving the hardenability. In order to reliably obtain this effect, the Cr content is preferably 0.4% or more. However, if the content of Cr is too large, it becomes too hard and causes a decrease in cold forgeability. In particular, if the content of Cr exceeds 2.0%, the decrease in cold forgeability becomes significant. Therefore, the content of Cr is set to 0 to 2.0%.

  When the cold forgeability is more important, the upper limit of the Cr content is preferably 1.5%.

Mo: 0 to 1.0%
The addition of Mo is optional. If added, it has the effect of increasing the strength of the steel and the effect of improving the hardenability. In order to reliably obtain this effect, the Mo content is preferably 0.1% or more. However, if the Mo content is too large, it becomes too hard and causes a decrease in cold forgeability. In particular, if the Mo content exceeds 1.0%, the cold forgeability decreases significantly. Therefore, the content of Mo is set to 0 to 1.0%.

  When the cold forgeability is more important, the upper limit of the Mo content is preferably 0.5%.

V: 0 to 0.20%
The addition of V is optional. If added, carbonitrides are formed, the crystal grains are made fine, and the strength is increased. To obtain this effect with certainty, it is preferable that V is 0.05% or more. However, when the V content exceeds 0.20%, the hot forgeability and the cold forgeability deteriorate. Therefore, the content of V is set to 0 to 0.20%.

  In addition, when the cold forgeability is more important, the upper limit of the V content is preferably 0.10%.

Ti: 0 to 0.10%
The addition of Ti is optional. If added, carbonitrides are formed, the crystal grains are made fine, and the strength is increased. In order to reliably obtain this effect, it is preferable that the Ti content is 0.02% or more. However, when the Ti content exceeds 0.10%, a large nitride is formed, and hot forgeability, cold forgeability, and rolling fatigue life are reduced. Therefore, the content of Ti is set to 0 to 0.10%.

  In addition, when hot forgeability, cold forgeability, and rolling fatigue life are more important, the upper limit of the Ti content is preferably 0.05%.

B: 0 to 0.0050%
The addition of B is optional. If added, it has the effect of improving hardenability. In order to reliably obtain this effect, it is preferable that B has a content of 0.0010% or more. However, even if the content of B exceeds 0.0050%, the above effect is saturated and the cost is increased. Therefore, the content of B is set to 0 to 0.0050%. The upper limit of the B content is preferably 0.0030%.

Al: 0.005 to 0.050%
Al has a deoxidizing action. In order to acquire this effect, it is necessary to make Al content 0.005% or more. However, if the Al content is too large, a large amount of hard Al ₂ O ₃ is formed, resulting in deterioration of cold forgeability, machinability, and rolling fatigue characteristics. In particular, the Al content is 0.050. If it exceeds 50%, the cold forgeability, machinability, and rolling fatigue characteristics deteriorate significantly. Therefore, the content of Al is set to 0.005 to 0.050%. The Al content is preferably 0.005 to 0.040%.

Rare earth metal: 0.0005% or more and less than 0.0050% Rare earth metal is an element having an important meaning in the present invention. That is, since the rare earth metal has a greater affinity for S than Mn, it preferentially binds to S to form a sulfide. As a result, the number of MnS produced decreases.

  The rare earth metal sulfide does not extend in the rolling direction like MnS, has an elliptical shape, and is uniformly and finely dispersed. Therefore, the hot forgeability and the cold forgeability are good with little anisotropy. Become. Further, the cold forgeability is improved due to the fact that the rare earth metal sulfide becomes the nucleus of the ferrite transformation and the formation of ferrite is promoted. Furthermore, rare earth metal sulfides have the same machinability improving effect as MnS.

  However, if the rare earth metal content is less than 0.0005%, the above-described effects cannot be obtained. On the other hand, if the content of the rare earth metal is too large, a coarse rare earth oxide is formed, resulting in a decrease in rolling fatigue characteristics. In particular, if the content of the rare earth metal is 0.0050% or more, rolling fatigue The deterioration of the characteristics becomes remarkable. Therefore, the rare earth metal content is set to 0.0005% or more and less than 0.0050%. The rare earth metal content is preferably 0.0020% or more and less than 0.0050%.

  As already mentioned, the “rare earth metal” as used in the present invention refers to Sc of atomic number 21 belonging to group 3A of the periodic table, Y of atomic number 39 and La of atomic number 57 to Lu of atomic number 71. A total of 17 elements including the lanthanoids of 15 elements described above are included, and the rare earth metal content indicates the total content of the 17 elements.

Ratio of Nd in rare earth metal: 60% or more Even if the rare earth metal is contained in an amount of 0.0005% or more and less than 0.0050%, mechanical characteristics may vary if the ratio of each element in the rare earth metal is changed. is there. However, if Nd is contained in the rare earth metal by 60% or more, the variation in mechanical characteristics is small, and a particularly stable rolling fatigue life can be ensured. In addition, when the rare earth metal sulfide mainly composed of Nd is used as a nucleus, the effect of promoting the ferrite transformation is increased, and the cold forgeability is also improved. Therefore, the ratio of Nd in the rare earth metal is set to 60% or more. The ratio of Nd in the rare earth metal is preferably 75% or more, and more preferably 90% or more.

O (oxygen): 0.0020% or less O (oxygen) forms oxide inclusions and deteriorates cold forgeability, hot forgeability, machinability, and rolling fatigue life. Was 0.0020% or less. The O content is preferably 0.0015% or less.

  From the above, the chemical composition of the steel for machine structural use according to the present invention (1) contains the elements of C to O (oxygen) in the amounts already described, with the balance being Fe and impurities, and Nd being the aforementioned It was defined as occupying 60% or more of rare earth metals.

  In addition, in order to increase the chemical strength of the steel for machine structural use of the present invention (2) and improve the hardenability, the steel for machine structural use of the present invention (1) has Cr and Mo contents. And Cr: 0.4 to 1.5% and Mo: 0.1 to 0.5%. In addition, Cr and Mo may be contained independently and may be contained in combination.

  Similarly, the mechanical composition steel of the present invention (3) has the same chemical composition as that of the present invention (1) or the present invention (2) in order to form carbonitrides to refine the crystal grains and increase the strength. In the structural steel, it was defined that the contents of Ti and V satisfy at least one of Ti: 0.02 to 0.05% and V: 0.05 to 0.10%. Note that Ti and V may be included singly or in combination.

  Further, in order to improve the hardenability of the chemical composition of the steel for machine structure of the present invention (4), the B content in any one of the steel for machine structure from the present invention (1) to the present invention (3) However, it is defined that B: 0.0010 to 0.0030% is satisfied.

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

  Steels 1 to 27 having chemical compositions shown in Tables 1 and 2 were melted by a 180 kg vacuum melting furnace and cast into ingots.

  Note that rare earth metals are likely to be combined with O (oxygen) to form oxides. Therefore, in the case of steels 1 to 15, steels 18 to 24, steels 26, and steels 27, after sufficient deoxidation treatment with Al or Si to uniformly disperse fine rare earth metal sulfides Rare earth metal was added.

  In addition, in the case of steel 1-4, steel 6, steel 11, steel 12, steel 14, steel 15, and steel 19, Nd was added alone as a rare earth metal. On the other hand, in the case of steel 5, steel 7-10, steel 13, steel 18, steel 20-24, steel 26 and steel 27, the rare earth metal was added in combination with misch metal and single Nd. The misch metal used had a composition of La 28.9%, Ce 50.6%, Pr 4.7%, Nd 15.6%, and others 0.2%.

  Next, each ingot was subjected to solution heat treatment that was maintained at 1250 ° C. for 15 hours for homogenization, and hot forging produced steel bars having diameters of 65 mm, 35 mm, and 20 mm, respectively, and a length of 1000 mm.

  A cylindrical test piece having a diameter of 30 mm and a length of 45 mm was cut out from each steel bar having a diameter of 35 mm, and the hot forgeability was investigated.

  That is, a cylindrical test piece having a diameter of 30 mm and a length of 45 mm was heated at 1100 ° C. for 30 minutes, and then a hot forging test was performed at 1050 ° C. using a crank-type forging press. In addition, a hot forging test is performed on each of six test pieces at various compression ratios, and the presence or absence of cracks on the side of the test piece is visually inspected, and the maximum compression ratio at which no cracks occur on four or more of the six pieces is limited. The compression ratio was used.

The critical compression ratio is calculated from the equation {(L ₀ −L) / L ₀ } × 100, where L ₀ is the height of the test piece before hot forging and L is the height of the test piece after hot forging. did. Note that the target was that the critical compression ratio obtained as described above was 85% or more, and it was judged that the hot forgeability was good when the target was reached.

  Each steel bar having a diameter of 20 mm was subjected to a normalizing treatment in which the steel bar was heated to 870 ° C. and held for 1 hour and allowed to cool, and then the notched specimen shown in FIG. A test was conducted to investigate the cold forgeability.

  That is, both ends of the notched test piece shown in FIG. 1 were restrained using a hydraulic forging press, and an upsetting test was performed in a cold (room temperature) without lubrication. In addition, a cold upsetting test is performed with each of six test pieces at various compression ratios, and the presence or absence of cracks in the test piece notch is visually inspected. Was defined as the critical compression ratio.

Limit compression ratio, as in the previous hot forging, L ₀ the cold upsetting test piece before the height, the height of the test piece after inclusive cold据the L, {(L ₀ -L) / L ₀ } × 100. Note that the target was that the critical compression ratio obtained as described above was 45% or more, and it was judged that the cold forgeability was good when the target was reached.

  Moreover, about the bar material of diameter 20mm, it cut | disconnected in the longitudinal cross-section and performed the structure | tissue investigation using the optical microscope.

  That is, after the longitudinal section was mirror-polished, it was corroded with nital and observed with an optical microscope at magnifications of 100 and 400.

  Each steel bar having a diameter of 65 mm was subjected to a normalization treatment in which the steel bar was heated to 870 ° C. for 2 hours and then allowed to cool, and then a flat plate test piece having a diameter of 60 mm and a thickness of 5 mm was cut out.

  The plate test piece is subjected to induction quenching at a heating temperature of 1000 to 1050 ° C. and tempering treatment at 160 to 180 ° C. for 1 hour so that the “effective hardened layer depth” becomes 2.0 ± 0.3 mm. Adjusted. “Effective hardened layer depth” is the distance from the hardened layer surface at which the Vickers hardness (HV) is 420.

  Next, a plate test piece having a diameter of 60 mm and a thickness of 5 mm subjected to the above quenching and tempering treatment was mirror-polished, and then a rolling fatigue test was performed using a thrust type rolling fatigue tester.

  The conditions of the rolling fatigue test are as follows.

-Maximum contact surface pressure: 5.6 GPa,
・ Rotation speed: 1200rpm,
・ Lubricating oil: Spindle oil # 60,
・ Counterball: three SUJ2 quenching and tempering balls with a diameter of 9.525 mm,
-Number of test pieces: 15 pieces.

For each steel, plot the rolling fatigue test results of 15 specimens on Weibull probability paper with the cumulative failure probability on the vertical axis and the rolling fatigue life on the horizontal axis, and draw a linear approximation line for it. The rolling fatigue life (hereinafter referred to as L10 life) at which the cumulative frequency failure probability was 10% was determined. The L10 life determined as described above was set to 2.0 × 10 ⁷ times or more, and when the target was reached, it was determined that the rolling fatigue characteristics were good.

  Table 3 summarizes the above test results.

  From Table 3, in the case of test numbers that deviate from the conditions specified in the present invention, that is, test numbers 16 to 25, at least one characteristic of hot forgeability, cold forgeability, and rolling fatigue characteristics is the target. It is clear that this value has not been reached.

  That is, in the case of test number 16, since the steel 16 does not contain a rare earth metal, the critical compression ratio in hot forging is as low as 83%, and the critical compression ratio in cold forging is as low as 38%.

  In the case of test number 17, the C content of steel 17 is 0.57%, which is out of the range specified in the present invention, and since rare earth metal is not included, the critical compression ratio in hot forging is as low as 80%. Also, the critical compression ratio in cold forging is as low as 39%.

  In the case of test number 18, the Cr content of the steel 18 is 2.30%, which is out of the range defined in the present invention, so the critical compression ratio in cold forging is as low as 37%.

  In the case of test number 19, the amount of Mo in steel 19 is 1.10%, which is out of the range defined in the present invention, and therefore the critical compression ratio in cold forging is as low as 38%.

In the case of test number 20, since the Ti amount of steel 20 is 0.165%, which is outside the range specified in the present invention, the critical compression ratio in hot forging is as low as 81%, and in cold forging, The critical compression ratio is as low as 39%. Furthermore, the L10 life is as short as 1.7 × 10 ⁷ times.

In the case of test number 21, the Mn content of steel 21 is 1.70%, which is out of the range defined in the present invention, and the Al content is 0.102%, which is out of the range defined in the present invention. The critical compression ratio in cold forging is as low as 80%, and the critical compression ratio in cold forging is as low as 36%. Furthermore, the L10 life is as short as 1.5 × 10 ⁷ times.

  In the case of test number 22, the V amount of steel 22 is 0.340%, which is out of the range specified in the present invention. Therefore, the critical compression ratio in hot forging is as low as 81%, and in cold forging, Is also as low as 35%.

In the case of test number 23, the C content, Cu content and rare earth metal content of the steel 23 are out of the ranges specified in the present invention, so the critical compression ratio in hot forging is as low as 79%, The critical compression ratio in forging is as low as 30%. Furthermore, the L10 life is as short as 0.7 × 10 ⁷ times.

In the case of test number 24, the S content of steel 24 and the ratio of Nd in the rare earth metal are 25%, which is out of the range specified in the present invention, so that the critical compression ratio in hot forging is as low as 80%. The critical compression ratio in cold forging is as low as 39%. Furthermore, the L10 life is as short as 1.2 × 10 ⁷ times.

  In the case of test number 25, the amount of Si and Ni in the steel 25 deviates from the range specified in the present invention, and since no rare earth metal is contained, the critical compression ratio in hot forging is as low as 83%. Further, the critical compression ratio in cold forging is as low as 37%.

In the case of the test number 26, the O (oxygen) amount of the steel 26 is 0.0032%, which is out of the range specified in the present invention. Therefore, the critical compression ratio in the hot forging is as low as 83%, The critical compression ratio in the forging process is as low as 33%. Further, the L10 life is as short as 1.0 × 10 ⁷ times.

  In the case of the test number 27, the ratio of Nd in the rare earth metal of the steel 27 is 22%, which is outside the range specified in the present invention, so that the critical compression ratio in cold forging is as low as 40%.

On the other hand, in the case of test numbers satisfying the conditions defined in the present invention, that is, test numbers 1 to 15, the critical compressibility during hot forging is 85% or more and excellent in hot forgeability, The critical compression ratio during cold forging is 45% or more, and the cold forgeability is excellent. Furthermore, the L10 life in the rolling fatigue test is also 2.0 × 10 ⁷ times or more, and it is clear that the rolling fatigue characteristics are also excellent.

  Since the steel for machine structure of the present invention is excellent in hot forgeability and cold forgeability, it is possible to avoid cracks in hot forging and cold forging, and spheroidizing annealing before cold forging, etc. It is possible to omit or simplify the long-time heat treatment for softening the material, and it is also excellent in rolling fatigue characteristics, so it can be used as a material for shafts, bearings and bolts of automobiles and various industrial machines. it can.

It is a figure which shows the shape of the test piece with a notch for an end surface restraint upsetting test for investigating the cold forgeability used in the Example.

Claims (4)

  In mass%, C: 0.35-0.55%, Si: 0.05-1.0%, Mn: 0.4-1.5%, P: 0.030% or less, S: 0.005 ~ 0.050%, Cu: 0 to 0.20%, Ni: 0 to 0.20%, Cr: 0 to 2.0%, Mo: 0 to 1.0%, V: 0 to 0.20% Ti: 0 to 0.10%, B: 0 to 0.0050%, Al: 0.005 to 0.050%, Rare earth metal: 0.0005% or more and less than 0.0050%, and O (oxygen): 0 A steel for machine structural use, comprising: 0020% or less, the balance being Fe and impurities, and Nd occupying 60% or more of the rare earth metal.   The steel for machine structure of Claim 1 with which content of Cr and Mo satisfy | fills at least any one of Cr: 0.4-1.5% and Mo: 0.1-0.5%.   The steel for machine structure according to claim 1 or 2, wherein the contents of Ti and V satisfy at least one of Ti: 0.02 to 0.05% and V: 0.05 to 0.10%.   The steel for machine structure according to any one of claims 1 to 3, wherein a content of B satisfies B: 0.0010 to 0.0030%.

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