JP5385574B2 - Machine structural steels and cold forged steel parts with excellent cold forgeability - Google Patents

Description

  The present invention relates to a steel material for machine structure that exhibits good cold workability during processing and also exhibits a predetermined hardness and strength after processing, and a cold-worked steel part obtained from such steel material for machine structure. It is.

In recent years, from the viewpoint of environmental protection, there has been an increasing demand for weight reduction of various parts for automobiles for the purpose of improving the fuel efficiency of vehicles such as automobiles, such as bolts, nuts, pinion gears, steering shafts, valve lifters, common rails, etc. With respect to steel materials for cold working (steel materials for machine structures) for manufacturing steel, there is an increasing demand for weight reduction, that is, higher strength. In order to respond to this kind of weight reduction, a method of ensuring a predetermined strength is generally adopted by adjusting the content of various alloy elements added to the base metal. On the other hand, in order to reduce CO ₂ emissions in the component manufacturing process, there is an increasing demand for cold forging of components such as crankshafts, connecting rods and transmission gears that have been processed by hot forging.

  Cold work (cold forging) is usually work in an atmosphere of 200 ° C. or less, and this cold work is more productive than hot work or warm work, and also has dimensional accuracy and steel material. There is an advantage that both yields are good.

  However, a problem in manufacturing parts by such cold working is that in order to ensure the strength of the cold-worked parts to be higher than the expected value, it is inevitably necessary to have deformation resistance. It is necessary to use high steel materials. However, the higher the deformation resistance of the steel material used, there is a drawback that not only the life of the cold working mold is reduced, but also cracking is likely to occur during cold working.

  Therefore, conventionally, a method of manufacturing a high-strength part having a predetermined strength (or hardness) is performed by performing a heat treatment such as quenching and tempering after cold working a steel material into a predetermined shape. There was also. However, applying heat treatment after cold working inevitably changes the component dimensions, so it is necessary to make secondary corrections by machining such as cutting, so that the heat treatment and subsequent processing can be omitted. It is the actual situation that is desired.

  For this reason, several measures have been proposed in order to reduce the deformation resistance of the steel material during cold working, and at the same time to ensure a predetermined strength, improve productivity, and save energy. For example, Patent Document 1 discloses a cold forging wire rod and bar steel excellent in strain aging characteristics and a method for manufacturing the same, by suppressing the progress of normal temperature aging by using solute C in a low carbon steel, and by predetermined strain aging. It is disclosed that the age-hardening amount of is secured. Patent Document 2 proposes a cementite-free ferrite structure with an average particle size of 500 nm or less for a high-strength steel wire or bar steel excellent in cold workability, a high-strength molded product, and a production method thereof. ing.

However, the technique of Patent Document 1 suppresses strain aging by the amount of solute C, and the technique of Patent Document 2 obtains a high-strength steel wire by using a cementite-free ferrite structure. Therefore, it has been difficult to obtain a steel material that satisfies both the required cold workability and the required properties of securing a predetermined strength after processing.
JP-A-10-306345 Japanese Patent Laying-Open No. 2005-320630

  The present invention has been made in view of such circumstances, and the object thereof is excellent in cold workability (particularly, the cold-worked steel part is not cracked, and at the time of working on the part hardness. This means that the deformation resistance can be kept low, and that the life of the mold can be extended, and at the same time, the machine structural steel that can secure the specified hardness and strength after processing, and such mechanical structural steel It is to provide a cold-worked steel part obtained.

The steel for machine structure according to the present invention is mass%, C: 0.025% or less (excluding 0%), Si: 0.005 to 0.4%, Mn: 0.3 to 1%, P: 0.05% or less (excluding 0%), S: 0.05% or less (not including 0%), N: 0.008 to 0.025%, and satisfying the following formula (1), The balance is made of iron and unavoidable impurities, and N as a solid solution is 0.007% or more, and the content of Al as the unavoidable impurities is 0.005% or less (including 0%). The steel structure is a ferrite single phase structure, and the average crystal grain size of the ferrite is in the range of 10 to 200 μm.
0.3 ≧ (10 [C] + [N]) (1)
However, [C] and [N] indicate the contents (% by mass) of C and N, respectively.

  In the steel for machine structure of the present invention, if necessary, (a) Cr: 2% or less (not including 0%) and / or Mo: 2% or less (not including 0%), (b) Ti: At least 1 selected from the group consisting of 0.2% or less (not including 0%), Nb: 0.2% or less (not including 0%), and V: 0.2% or less (not including 0%) Species, (c) B: 0.005% or less (excluding 0%), (d) Cu: 5% or less (excluding 0%), Ni: 5% or less (excluding 0%), and Co : At least one selected from the group consisting of 5% or less (not including 0%), (e) Ca: 0.05% or less (not including 0%), REM: 0.05% or less (0% Not included), Mg: 0.02% or less (not including 0%), Li: 0.02% or less (not including 0%), Pb: 0.5% It is also useful to contain at least one selected from the group consisting of the following (not including 0%) and Bi: 0.5% or less (not including 0%), etc. Accordingly, the properties of the wire for cold working are further improved.

On the other hand, the cold-worked steel part that has achieved the above object is a cold-worked steel produced by cold-working the steel for machine structural use according to the present invention as described above at a working temperature of less than 100 ° C. It is a component and has a gist in that the component hardness (H) after cold working and the maximum value (DR) of deformation resistance during cold working satisfy the relationship of the following equation (2). .
H ≧ (DR + 200) /2.5 (2)
[In formula (2), H: component hardness after cold working (Hv), DR: maximum deformation resistance (MPa) during cold working. ]

  The steel for machine structural use according to the present invention reduces the deformation resistance of the steel during cold working, so that the life of the cold working mold is prolonged, cracking is less likely to occur, and it is obtained after working. Since the parts can ensure predetermined strength and hardness, they contribute to productivity improvement and energy saving.

  The present inventors have studied from various angles in order to realize a steel material for machine structure that exhibits good workability during cold working and can ensure a predetermined hardness and strength after working. As a result, the relationship between the C content and the N content is optimized, and a predetermined amount of solid solution N is ensured to appropriately control the chemical composition, and the ferrite crystal grain size in the ferrite single phase structure is appropriately controlled. As a result, it was found that a steel material for mechanical structure capable of achieving the above object can be realized by controlling the present invention, and the present invention has been completed. First, in the steel for machine structure of the present invention, the reason why chemical components are specified is as follows.

[C: 0.025% or less (excluding 0%)]
C is an element that greatly affects the formation of the structure of the steel material, and in order to make the structure a ferrite single-phase structure, it is necessary to reduce as much as possible, and if excessively contained, pearlite is generated in the structure of the steel material, Deformation resistance may be excessive due to work hardening of pearlite. For these reasons, the C content needs to be 0.025% or less (excluding 0%), preferably 0.023% or less, more preferably 0.021% or less. However, if the C content is extremely reduced, it becomes difficult to deoxidize the steel material during melting. Therefore, the lower limit is preferably 0.0005%, more preferably 0.0008% or more, and still more preferably 0. 0.001% or more.

[Si: 0.005 to 0.4%]
Si is effective as a deoxidizing element during melting. If the Si content is less than 0.005%, deoxidation is insufficient and blow holes are generated during melting. However, if the Si content is excessive and exceeds 0.4%, the deformation resistance is increased due to the solid solution strengthening of Si, and cracks become remarkable, which is not preferable. The Si content is preferably 0.006% or more (more preferably 0.007% or more), preferably 0.35% or less (more preferably 0.32% or less).

[Mn: 0.3 to 1%]
When the N content in the steel material is increased, cracks are likely to occur due to dynamic strain aging due to heat generation during processing, but Mn has the effect of improving the workability at that time and suppressing cracks. In order to exhibit such an effect effectively, it is necessary to contain 0.3% or more, preferably 0.35% or more, more preferably 0.40% or more. On the other hand, if Mn is contained excessively, deformation resistance becomes excessive and nonuniformity of the structure due to segregation occurs, so it is necessary to make it 1% or less, preferably 0.95% or less, more preferably 0.90%. It is as follows.

[P: 0.05% or less (excluding 0%)]
Phosphorus (P) is an unavoidable impurity, but when it is contained in ferrite, it is an element that segregates at the ferrite grain boundaries and degrades cold workability. It is also an element that causes solid solution strengthening of ferrite to increase deformation resistance. Therefore, from the viewpoint of improving cold workability, the P content is set to 0.05% or less. Although it is preferably 0.03% or less, it is industrially difficult to make the P content 0%.

[S: 0.05% or less (excluding 0%)]
Sulfur (S) is also an unavoidable impurity like P and is an element that precipitates in the form of a film at the grain boundary as FeS and degrades workability. It also has the effect of causing hot brittleness. Therefore, from the viewpoint of improving the deformability, in the present invention, the S content is set to 0.05% or less (preferably 0.03% or less). However, it is industrially difficult to reduce the S content to 0%. In addition, since S has an effect of improving machinability, it is preferable to contain 0.002% or more, more preferably 0.006% or more from the viewpoint of improving machinability. .

[N: 0.008 to 0.025%]
Nitrogen (N) is an important element for obtaining a predetermined strength by static strain aging after processing. In order to exert such effects, the N content needs to be 0.008% or more. However, if the N content becomes excessive and exceeds 0.025%, in addition to static strain aging, the influence of dynamic strain aging during processing becomes significant, and the deformation resistance increases. In addition, the minimum with preferable N content is 0.0085% (more preferably 0.009% or more), and a preferable upper limit is 0.023% (more preferably 0.02% or less).

  The steel for machine structural use according to the present invention is characterized in that static strain aging is promoted without increasing the deformation resistance by setting N in a solid solution state (solid solution N) to a predetermined amount. In order to ensure a predetermined strength after cold working, the amount of solute N needs to be 0.007% or more. However, if the amount of the solute N becomes excessive, the cold workability deteriorates, so the content is preferably 0.025% or less.

  In addition, content of the solid solution N in this invention is a value calculated | required by subtracting N amount in all N compounds from total N amount in steel materials based on JISG1228. A practical method for measuring the content of this solute N is exemplified below.

(A) Inert gas melting method-thermal conductivity method (total N content measurement)
A sample cut from the test material is put in a crucible, extracted in an inert gas stream to extract N, the extract is transported to a thermal conductivity cell, and the change in thermal conductivity is measured to determine the total N amount. Ask.
(B) Ammonia distillation separation indophenol blue spectrophotometry (measurement of total N compound amount)
A sample cut out from the test material is dissolved in a 10% AA-based electrolytic solution, subjected to constant current electrolysis, and the total amount of N compounds in the steel is measured. The 10% AA electrolyte used is a non-aqueous solvent electrolyte consisting of 10% acetone, 10% tetramethylammonium chloride, and the remainder methanol, and does not generate a passive film on the steel surface.

About 0.5 g of a sample of the test material is dissolved in this 10% AA-based electrolytic solution, and the resulting insoluble residue (N compound) is filtered through a polycarbonate filter having a hole size of 0.1 μm. The obtained insoluble residue is decomposed by heating in a chip made of sulfuric acid, potassium sulfate and pure copper, and the decomposition product is combined with the filtrate. After making this solution alkaline with sodium hydroxide, steam distillation is performed, and the distilled ammonia is absorbed in dilute sulfuric acid. Further, phenol, sodium hypochlorite and sodium pentacyanonitrosyl iron (III) are added to form a blue complex, and the absorbance is measured using an absorptiometer to determine the total N compound amount.
The total amount of N compounds determined by the method (b) can be subtracted from the total N amount determined by the method (a) to determine the solid solution N amount.

In the steel material of the present invention, solid solution C greatly increases deformation resistance and does not contribute much to static strain aging, while solid solution N does not increase deformation resistance and can promote static strain aging. Therefore, it has the effect | action which can increase the hardness after a process. Therefore, in the steel material of the present invention, the C content [C] and the N content [N] are as follows in order to increase the hardness after processing without increasing the deformation resistance during processing. 1) It is necessary to satisfy the relationship of the formula. When the value on the right side (= 10 [C] + [N]) in equation (1) exceeds 0.3 (mass%), the contents of C and N become excessive and the deformation resistance becomes excessive. . The value of (10 [C] + [N]) is preferably 0.29 or less, more preferably 0.28 or less.
0.3 ≧ (10 [C] + [N]) (1)
However, [C] and [N] indicate the contents (% by mass) of C and N, respectively.

  The contained elements specified in the present invention are as described above, and the balance is iron and inevitable impurities. As this unavoidable impurity, mixing of the element brought in by conditions, such as a raw material, material, a manufacturing facility, can be accept | permitted. Of these, Al must be suppressed as follows. Moreover, you may further contain the following elements (selective element) as needed.

[Al: 0.005% or less (including 0%)]
Al is an unavoidable impurity element that combines with solid solution N to reduce the amount of solid solution N and lower the hardness after processing. Therefore, the lower the Al content, the better. Further, reducing Al as much as possible can ignore the bonding / decomposition to the N compound by the heat treatment, and can reduce variations in and between parts. The Al content is preferably 0.0045% or less, and more preferably 0.004% or less.

[Cr: 2% or less (not including 0%) and / or Mo: 2% or less (not including 0%)]
Cr is an element that has the effect of improving the deformability of steel by increasing the strength of the grain boundaries, and if necessary, it is preferably 0.1% or more, more preferably 0.2% or more. Can do. However, if Cr is excessively contained, deformation resistance increases and cold workability may be lowered. Therefore, the content is preferably 2% or less (excluding 0%), more preferably. Is 1.5% or less, more preferably 1% or less.

  On the other hand, Mo is an element having an action of increasing the hardness and deformability of the steel material after processing, and can be contained by 0.04% or more, more preferably 0.08% or more, if necessary. However, if Mo is excessively contained, the cold workability may be deteriorated. Therefore, it is preferably 2% or less (excluding 0%), more preferably 1.5% or less, and still more preferably. 1% or less.

[Ti: 0.2% or less (not including 0%), Nb: 0.2% or less (not including 0%) and V: 0.2% or less (not including 0%) At least one selected]
When the steel material of this invention contains at least 1 sort (s) chosen from the group which consists of Ti, Nb, and V, Ti, Nb, and V may contain 1 type individually or 2 types or more simultaneously. These Ti, Nb and V have a strong affinity for N, coexist with N to form an N compound, refine the steel crystal grains, improve the toughness of the processed product obtained after cold working, It is an effective element for improving crack resistance. When these elements are contained, Ti: 0.2% or less (not including 0%), Nb: 0.2% or less (not including 0%), and V: 0.2% or less (0% Is not included). More preferably, Ti is 0.15% or less, Nb is 0.15% or less, and V is 0.1% or less. In order to effectively exhibit these effects, it is preferable to contain 0.001% or more, and it is recommended to contain 0.002% or more.

[B: 0.005% or less (excluding 0%)]
B, like Ti, Nb, and V, has a strong affinity with N, forms an N compound in coexistence with N, refines the crystal grains of steel, and improves the toughness of a processed product obtained after cold working. It is an element effective for improving and improving crack resistance. When B is contained, the required solid solution N amount can be ensured and the strength after cold working can be improved, so the content is preferably 0.005% or less, more preferably Is 0.0035% or less, more preferably 0.002% or less. In order to effectively exhibit these effects, B is preferably contained in an amount of 0.0001% or more, and more preferably 0.0002% or more.

[Cu: 5% or less (not including 0%), Ni: 5% or less (not including 0%) and Co: 5% or less (not including 0%)]
When the steel material of this invention contains at least 1 sort (s) chosen from the group which consists of Cu, Ni, and Co, Cu, Ni, and Co may contain 1 type (s) or 2 or more types simultaneously. These Cu, Ni and Co all have an effect of strain aging and hardening the steel material, and are effective elements for improving the strength after processing. In order to exhibit such an effect, it is preferable to contain 0.1% or more (more preferably 0.3% or more). However, if these contents are excessive, the effects of strain aging and hardening of the steel material, as well as the effect of improving the strength after processing are saturated, and there is a possibility of inducing cracks. Is preferably included), more preferably 4% or less (more preferably 3% or less).

[Ca: 0.05% or less (not including 0%), REM: 0.05% or less (not including 0%), Mg: 0.02% or less (not including 0%), Li: 0.0. 02% or less (excluding 0%), Pb: 0.5% or less (not including 0%), and Bi: 0.5% or less (not including 0%)]
Ca is an element that spheroidizes sulfide inclusions such as MnS, improves the deformability of steel, and contributes to improvement of machinability. In order to effectively exhibit such an effect, it is preferable to contain 0.0005% or more, more preferably 0.001% or more of Ca. However, even if the Ca content is excessive, the effect is saturated. Therefore, the content is preferably 0.05% or less, more preferably 0.03% or less, and still more preferably 0.01% or less. .

  REM (rare earth element) is an element that contributes to improvement of machinability while increasing the deformability of steel by spheroidizing sulfide inclusions such as MnS as in the case of Ca. In order to effectively exhibit such an effect, it is preferable to contain 0.0002% or more, more preferably 0.0005% or more of REM. However, even if the content of REM becomes excessive, the effect is saturated and an effect commensurate with the content cannot be expected. Therefore, the content is preferably 0.05% or less, more preferably 0.03% or less. Preferably it is 0.01% or less.

  Mg, like Ca, is an element that spheroidizes sulfide inclusions such as MnS to enhance the deformability of steel and contribute to the improvement of machinability. In order to effectively exhibit such an effect, it is preferable to contain 0.0002% or more, more preferably 0.0005% or more of Mg. However, even if the Mg content is excessive, the effect is saturated, and an effect commensurate with the content cannot be expected. Therefore, the content is preferably 0.02% or less, more preferably 0.015% or less, and further Preferably it is 0.01% or less.

  Li, like Ca, can spheroidize sulfide inclusions such as MnS to increase the deformability of steel, and lower the melting point of Al-based oxides to make them harmless, contributing to improved machinability. Element. In order to effectively exhibit such an effect, 0.0002% or more, more preferably 0.0005% or more of Li is preferably contained. However, even if the Li content is excessive, the effect is saturated and an effect commensurate with the content cannot be expected. Therefore, the content is preferably 0.02% or less, more preferably 0.015% or less, and further Preferably it is 0.01% or less.

  Pb is an element effective for improving machinability. In order to exhibit such an effect, Pb is preferably contained in an amount of 0.005% or more, more preferably 0.01% or more. However, if the Pb content is excessive, problems in production such as the occurrence of rolling defects occur, so the upper limit is preferably 0.5% or less, more preferably 0.4% or less, Preferably it is 0.3% or less.

  Bi is an element effective for improving machinability like Pb. In order to exert such an effect, Bi is preferably contained in an amount of 0.005% or more, more preferably 0.01% or more. However, even if the Bi content is excessive, the effect is saturated and an effect commensurate with the content cannot be expected. Therefore, the upper limit is preferably 0.2% or less, more preferably 0.4%. Hereinafter, it is more preferably 0.3% or less.

  The steel structure of the steel material of the present invention is composed of a ferrite single-phase structure. The average grain size of ferrite constituting the ferrite single-phase structure is in the range of 10 to 200 μm in order to improve workability. It is necessary. If the average grain size of ferrite is less than 10 μm, the deformation resistance becomes too high, so the lower limit is 10 μm, preferably 12 μm or more, more preferably 15 μm or more. However, if the average crystal grain size of ferrite exceeds 200 μm, toughness, fatigue characteristics and the like deteriorate, so the upper limit is 200 μm, preferably 180 μm or less, more preferably 150 μm or less.

  The steel for machine structural use according to the present invention is then cold-worked and steel parts (cold-worked parts such as bolts, nuts, pinion gears, steering shafts, valve lifters, and common rails, and cranks that have been machined by hot forging so far. Automotive parts such as shafts, connecting rods, transmission gears, and other mechanical parts). The cold working method here includes cold working such as cold forging, cold forging, cold rolling, cold punching and the like. Further, if necessary for the processing of the parts, processing such as wire drawing and rolling may be performed.

By cold working the steel material of the present invention as described above at a processing temperature of less than 100 ° C., the cold worked steel part of the present invention is obtained. Hardness (H) and the maximum value (DR) of deformation resistance during cold working satisfy the relationship of the following formula (2).
H ≧ (DR + 200) /2.5 (2)
[In formula (2), H: component hardness after cold working (Hv), DR: maximum deformation resistance (MPa) during cold working. ]

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and may be implemented with appropriate modifications within a range that can meet the gist of the preceding and following descriptions. Of course, any of these is also included in the technical scope of the present invention.

[Example 1]
Sample steels (1A to 2F) having the chemical composition shown in Table 1 below were prepared, each of these was billet-melted, heated to 1150 to 1250 ° C., and hot forged to produce a 155 mm square steel. It was a piece. This steel slab was heated to 950 to 1100 ° C. and rolled into a round bar of φ12 mm. Next, this round bar was cut to cut out a test piece having a length of φ10 × 15 mm from the center. In Table 1, “Solubility N” is a value indicating the N content in a solid solution state measured by a method based on the above JIS G 1228. Table 1 also shows the crystal grain size of ferrite with a round bar cross section, which was measured by the following method. In addition, about the round bar manufactured using the specimen of steel type 1B, after performing the heat processing (addition heat processing) shown in Table 1, the test piece was cut out.

[Measurement method of average grain size of ferrite]
The average crystal grain size of ferrite was measured for the structure of each test piece in accordance with JIS G 0552. That is, the cross section of the steel piece cut out from each round bar is mirror-polished and then corroded with nital. Next, the D / 4 position (D: diameter) was observed with an optical microscope at a magnification of 100 times and photographed. And the number of crystal grains of an arbitrary cross section was calculated | required with the cutting method, and it converted into the average crystal grain diameter (micrometer) of a ferrite.

Each test piece was compressed from a state in which the end face of the test piece was constrained by cold forging using a press machine having a capacity of 1600 tons, and deformation resistance (DR: maximum value of deformation resistance during cold working) was measured. . The conditions for cold forging at this time were a processing strain rate of 10 / second, a processing temperature of 20 to 80 ° C., and a compression rate of 80%. The processing strain rate was an average value of strain rates during processing (plastic deformation). The compression rate shown in Table 2 below is [(1-L / L ₀ ) × 100 (%)] (L ₀ : length of the test piece before processing, L: length of the test piece after processing. Is required).

  About each obtained processed product, the presence or absence of the crack was confirmed by observing the surface by magnification: 20 times with the stereomicroscope. Also, using a Vickers hardness tester, the load: 1000 g, measurement position: Vickers of each workpiece under the conditions of the center of D / 4 position (D: diameter) of each workpiece cross section and the number of measurements: 5 times Hardness (Hv) was measured. Processing conditions (heating temperature, compressibility), deformation resistance (DR), presence / absence of cracking (when cracking occurs, x, when cracking does not occur), (2) The value on the right side [(DR + 200) /2.5] and the hardness after processing are shown in Table 2 below.

  In these measurement results, the obtained processed product has no cracks and has a low deformation resistance with respect to the Vickers hardness [specifically, when the condition shown in the above expression (2) is satisfied]. Was judged to be excellent in cold workability, and the overall judgment was displayed as “◯”. Moreover, about the processed goods which do not satisfy the conditions of said (2) formula, the comprehensive determination of what a crack generate | occur | produced was displayed by "x".

  From this result, it can be considered as follows (note that the following symbols indicate steel type symbols in Tables 1 and 2). 1A, 1B, 1E, 1F, 1I, 1J, 1M, 1O, 1P, 1R, 1S, 1U, 1X to 2F are examples satisfying the requirements defined in the present invention, and there are no cracks in the parts. A steel material having low deformation resistance of steel with respect to hardness has been obtained. On the other hand, those that do not satisfy the requirements of the present invention are cracked or have high deformation resistance during processing with respect to component hardness.

  In 1C, the amount of C is excessive and does not satisfy the formula (1), and the deformation resistance increases and the part is cracked. Since 1D had insufficient Si content, the crack considered to be attributable to the defect during melting occurred during cold working.

  In 1G, since the Si content was excessive, deformation resistance increased and cracks occurred in the parts. In 1K, since the Mn content was excessive, the deformation resistance increased and the parts were cracked. In 1 L, since the P content was excessive, deformation resistance increased and cracks occurred in the parts.

  In 1N, since the S content is excessive, the deformation resistance increased and the parts were cracked. 1Q, 1T, and 1W all have excessive amounts of solute N, and the deformation resistance during processing with respect to the component hardness is high.

[Example 2]
150 kg of test steel with the chemical composition shown in Table 3 below (2G to 2U) was melted in a vacuum induction furnace, cast into an ingot having an upper surface: φ245 mm × lower surface: φ210 mm × length: 480 mm, and forging (soaking) 1250 ° C. × 3 hr, forging heating: 1000 ° C. × 1 hr) and cutting, forging into a round bar of φ80 mm via a 150 mm side × length: 680 mm square shape, length: about 100 mm Every cut. A test piece of φ10 mm × length: 15 mm was cut out from the D / 4 position (D: diameter) of the obtained round bar (forged material). Furthermore, from the central part, test pieces of φ20 mm × length: 30 mm and φ30 mm × length 45 mm were cut out. Table 3 also shows the values of “Solubility N” and “Crystal grain size (average grain size of ferrite)”, which are measured by the method described above.

  Each test piece was compressed under the conditions shown in Example 1. About each obtained processed product, it carried out similarly to Example 1, and measured deformation resistance DR, the presence or absence of a crack, and the Vickers hardness (Hv) of each processed product. Processing conditions for each processed product (test piece size, heating temperature, compression ratio), deformation resistance DR, presence or absence of cracks (when cracking occurs, x, when cracking does not occur), (2) The value [(DR + 200) /2.5] on the right side of the equation and the hardness after processing are shown in Table 4 below.

  From this result, it can be considered as follows (note that the following symbols indicate steel type symbols in Tables 3 and 4). 2G to 2P are examples that satisfy the requirements defined in the present invention, and a steel material having no cracks in parts and having low deformation resistance of steel with respect to the hardness of the parts is obtained. On the other hand, those that do not satisfy the requirements of the present invention are cracked or have high deformation resistance during processing with respect to component hardness.

  2Q does not satisfy the formula (2), and the deformation resistance is increased. 2R and 2S do not satisfy the formula (2), the deformation resistance increases, and the parts are cracked. In 2T, the amount of C is excessive and does not satisfy the formula (1), the deformation resistance increases, and the part is cracked. 2U does not satisfy the formula (2), and the deformation resistance is increased.

Claims (7)

In mass%, C: 0.025% or less (excluding 0%), Si: 0.005 to 0.4%, Mn: 0.3 to 1%, P: 0.05% or less (0% Not including), S: 0.05% or less (not including 0%), N: 0.008 to 0.025%, and the following formula (1) is satisfied, and the balance is composed of iron and inevitable impurities, and N in the solid solution state is 0.007% or more, and the content of Al as the inevitable impurity is suppressed to 0.005% or less (including 0%). A steel for machine structural use having excellent cold forgeability, characterized by having a phase structure and an average crystal grain size of ferrite in the range of 10 to 200 μm.
0.3 ≧ (10 [C] + [N]) (1)
However, [C] and [N] indicate the contents (% by mass) of C and N, respectively.   The steel for machine structure according to claim 1, further comprising Cr: 2% or less (not including 0%) and / or Mo: 2% or less (not including 0%).   Furthermore, Ti: 0.2% or less (not including 0%), Nb: 0.2% or less (not including 0%) and V: 0.2% or less (not including 0%) The steel material for machine structure of Claim 1 or 2 containing the at least 1 sort (s) chosen.   The steel for machine structure according to any one of claims 1 to 3, further comprising B: 0.005% or less (excluding 0%).   Further, at least one selected from the group consisting of Cu: 5% or less (not including 0%), Ni: 5% or less (not including 0%), and Co: 5% or less (not including 0%) The steel material for machine structure in any one of Claims 1-4 contained.   Furthermore, Ca: 0.05% or less (not including 0%), REM: 0.05% or less (not including 0%), Mg: 0.02% or less (not including 0%), Li: 0 0.02% or less (not including 0%), Pb: 0.5% or less (not including 0%), and Bi: 0.5% or less (not including 0%) The steel material for machine structures in any one of Claims 1-5 containing. Processing machine structural steel according to claim 1 Temperature: a cold forging steel components produced by cold forging at less than 100 ° C., parts hardness after cold forging ( A cold forged steel part characterized in that the maximum value (DR) of deformation resistance during H) and cold forging satisfies the relationship of the following formula (2).
H ≧ (DR + 200) /2.5 (2)
[In formula (2), H: component hardness after cold forging (Hv), DR: maximum deformation resistance (MPa) during cold forging . ]