JP5379511B2 - Machine structural steel and cold work steel parts with excellent cold workability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for machine structures which not only has excellent cold workability but also can secure prescribed hardness and strength after working, and to provide a steel component for cold working obtained by using the steel for machine structures. <P>SOLUTION: In this steel, a chemical component composition in which, while controlling the content of N as a solid-soluted state to 0.007 to 0.018%, the relations in equalities (1) and (2) are satisfied is suitably regulated, and the steel has a structure where the area ratio of pearlite and cementite is &le;3%: (1) 0.5&ge;(10[C]+[N]), where [C] and [N] denote the contents (mass%) of C and N, respectively; and(2) 0&ge;126[C]+3[Mn]+84[S]-10 where [C], [Mn] and [S] denote the contents (mass%) of C, Mn and S, respectively. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

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 secure a predetermined strength, and to 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 producing the same for a low carbon steel by using solute C to suppress the progress of normal temperature aging and predetermined by aging heat treatment. 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 normal temperature 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: 0.045% or less (not including 0%), Si: 0.05% or less (not including 0%), Mn: 0.30 -1.5%, P: 0.05% or less (not including 0%), S: more than 0.05 to 0.12%, Al: 0.06% or less (not including 0%), and N: Each containing 0.008 to 0.025%, satisfying the relationship of the following formulas (1) and (2), the balance being composed of iron and unavoidable impurities, and N in the solid solution state: 0.007 It has a gist in that it is a steel structure in which the area ratio of pearlite and cementite is 3% or less.
0.5 ≧ (10 [C] + [N]) (1)
However, [C] and [N] indicate the contents (% by mass) of C and N, respectively.
0 ≧ 126 [C] +3 [Mn] +84 [S] −10 (2)
However, [C], [Mn], and “S” indicate the contents (mass%) of C, Mn, and S, respectively.

  In the steel for machine structure of the present invention, if necessary, (a) Ca: 0.05% or less (not including 0%), REM: 0.05% or less (not including 0%), Mg: 0 0.05% or less (excluding 0%) and Te: at least one selected from the group consisting of 0.1% or less (excluding 0%), (b) B: 0.005% or less (0% (C) 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 from the group consisting of: (d) Cr: 2% or less (not including 0%) and / or Mo: 2% or less (not including 0%), (e) Cu: 5% or less (Not including 0%), Ni: not more than 5% (not including 0%) and Co: not more than 5% (not including 0%) It is also useful to contain at least one selected from the group consisting of (f) Pb: 0.5% or less (excluding 0%) and / or Bi: 0.5% or less (excluding 0%), etc. Depending on the type of component contained, the properties of the steel for machine structural use 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 expression (3). .
H ≧ (DR + 200) /2.5 (3)
[In formula (3), 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.

It is a graph which shows the relationship between the number of cracks (n / 10) and the value of the right side of (2) Formula. It is a graph which shows the influence which the amount of solid solution N has on the maximum value DR of deformation resistance, and the hardness H after a process. It is a graph which shows the relationship between the component hardness H after cold processing, and the maximum value DR of a deformation resistance. It is a graph which shows the relationship between the value of the right side of (2) Formula, and a MnS area.

  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 steel which optimized the relationship between C content and N content, secured a predetermined amount of solute N, appropriately controlled chemical composition, and appropriately controlled the area ratio of pearlite and cementite. The present inventors have found that a mechanical structural steel material that can achieve the above-described object can be realized if the structure is used. First, in the steel for machine structure of the present invention, the reason why chemical components are specified is as follows.

[C: 0.045% or less (excluding 0%)]
C is an element useful as a deoxidizing element during melting. When the C content is 0.045%, the structure is substantially a ferrite single phase structure with a slight amount of fine cementite at the grain boundaries. However, when the C content is excessive, fine cementite forms pearlite, and the pearlite area ratio increases. Pearlite increases the deformation resistance by work hardening of steel, and there is a possibility that the workability may deteriorate. For these reasons, the C content needs to be 0.045% or less (not including 0%), preferably 0.04% or less, more preferably 0.035% or less. Moreover, in order to exhibit said effect effectively, it is preferable to contain C 0.0005% or more, More preferably, it is 0.01% or more.

[Si: 0.05% or less (excluding 0%)]
Si is effective as a deoxidizing element during melting. However, if the Si content is excessive and exceeds 0.05%, the deformation resistance is increased due to the solid solution strengthening of Si, which is not preferable. Note that the Si content is preferably 0.0005% or more, and more preferably 0.001% or more.

[Mn: 0.30 to 1.5%]
When the amount of solute N 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. Mn is also an element useful as a deoxidizing element during melting. In order to exhibit these effects effectively, it is necessary to contain 0.30% or more, preferably 0.32% or more, more preferably 0.35% or more. On the other hand, if Mn is excessively contained, deformation resistance becomes excessive and nonuniformity of the structure due to segregation occurs. Therefore, it is necessary to make it 1.5% or less, preferably 1.2% or less, more preferably 1. 0% or less.

[P: 0.05% or less (excluding 0%)]
Phosphorus (P) is an unavoidable impurity, but if it is contained in ferrite, it segregates at the ferrite grain boundaries, and the grain boundaries become brittle, thereby deteriorating cold workability. Therefore, from the viewpoint of improving cold workability, the P content needs to be 0.05% or less. Although it is preferably 0.03% or less, it is industrially difficult to make the P content 0%.

[S: more than 0.05 to 0.12%]
Since sulfur (S) has an effect of improving machinability, it is necessary to contain more than 0.05% from the viewpoint of improving machinability. Preferably it is 0.055% or more. However, S is basically an unavoidable impurity like P, and is an element that precipitates as FeS at the grain boundaries and degrades workability. From the viewpoint of securing the deformability, the S content needs to be 0.12% or less, preferably 0.1% or less (more preferably 0.08% or less).

[Al: 0.06% or less (excluding 0%)]
Al is effective as a deoxidizing element during melting. However, if the Al content becomes excessive and exceeds 0.06%, it becomes difficult to secure the amount of solute N in the steel, and a predetermined component strength cannot be obtained. The Al content is preferably 0.055% or less, more preferably 0.05% or less. Moreover, in order to exhibit the said effect, it is preferable to make it contain 0.005% or more, More preferably, it is 0.01% or more.

[N: 0.008 to 0.025%]
Nitrogen (N) is an important element for securing 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 structure of the present invention is characterized by promoting static strain aging without increasing the deformation resistance by securing a predetermined amount of N in the solid solution state (solid solution N). In order to ensure a predetermined strength (hardness) after cold working, the amount of solute N needs to be 0.007% or more. However, if the amount of solute N becomes excessive, cold workability deteriorates, so it is necessary to make it 0.018% 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 N compound amount 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] need to satisfy the relationship of the following formula (1). When the value on the right side of the formula (1) (= 10 [C] + [N]) exceeds 0.5 (mass%), the contents of C and N become excessive and the deformation resistance becomes excessive. . Note that the value of (10 [C] + [N]) is preferably 0.42 or less, and more preferably 0.36 or less.
0.5 ≧ (10 [C] + [N]) (1)
However, [C] and [N] indicate the contents (% by mass) of C and N, respectively.

Further, in the steel material of the present invention, the contents [C], [Mn] and [S] of C, Mn and S must satisfy the relationship of the following formula (2). This equation (2) is obtained from the relationship between the above-mentioned elements that affect cracking, and the value (126 [C] +3 [Mn] +84 [S] −10) on the right side is set to 0 or less. As a result, MnS in the steel material is finely dispersed (see FIG. 4 described later), and coarse MnS that becomes a starting point of cracking during cold forging can be reduced.
0 ≧ 126 [C] +3 [Mn] +84 [S] −10 (2)
However, [C], [Mn], and “S” indicate the contents (mass%) of C, Mn, and S, respectively.

  The process of obtaining the above equation (2) is as follows. First, steel materials (steel types 2A to 2Q: those whose chemical components deviate from the range specified in the present invention) whose component compositions are shown in Table 5 below are prepared, and various test pieces are cut out therefrom (for the production conditions, the examples described later) The number of cracks (n / 10) during cold working of the obtained steel material was measured. Based on this result, the relationship between [C], [Mn] and [S] and the number of cracks was subjected to regression analysis, and the result of the above equation (2) was obtained. For the regression analysis, the item “regression analysis” in the software “analysis tool” added to the product name “Microsoft (registered trademark) Office Excel 2003” (manufactured by Microsoft Corporation) was executed. Tables 1 to 3 below show the results of regression analysis.

  The reason why C, Mn, and S are selected as elements in the regression analysis is that these elements affect the growth of MnS in the solidification process of the steel material. The higher the C content, the lower the temperature of the two-phase region of δ + L in the phase diagram, and the longer the liquid phase state, the easier MnS grows. Moreover, since MnS precipitates easily in a liquid phase state as Mn content and S content increase, MnS becomes easy to grow. Therefore, it is presumed that MnS can be made finer and cracks after cold forging can be prevented as the respective contents [C], [Mn] and [S] of C, Mn and S are reduced. (2) The fact that the signs of the coefficients [C], [Mn], and [S] are all positive indicates that [C], [Mn], and [S] should be reduced. (2) Consistent with the derivation basis of the equation. Further, as shown in FIG. 4 to be described later, there is a correlation between the calculation result of the equation (2) and the magnitude of MnS, which also indicates the correctness of the basis for deriving the equation (2).

  The contained elements specified in the present invention are as described above, and the balance is iron and inevitable impurities. As this unavoidable impurity, the mixing of elements (for example, O, Sn, As, etc.) brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. can be allowed. Moreover, you may further contain the following elements (selective element) as needed.

[Ca: 0.05% or less (not including 0%), REM: 0.05% or less (not including 0%), Mg: 0.05% or less (not including 0%), Te: 0.0. At least one selected from the group consisting of 1% or less (excluding 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, 0.0005% or more, more preferably 0.001% or more of REM is preferably contained. 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 0.0005% or more, more preferably 0.001% or more of Mg. However, even if the Mg content 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, and further Preferably it is 0.01% or less.

  Te, 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, it is preferable to contain 0.0001% or more, more preferably 0.001% or more of Te. However, even if the Te content is excessive, the effect is saturated and an effect commensurate with the content cannot be expected. Therefore, the content is preferably 0.1% or less, more preferably 0.05% or less, and further Preferably it is 0.03% or less.

[B: 0.005% or less (excluding 0%)]
B, like Ti, Nb, and V below, has a strong affinity with N, forms an N compound in coexistence with N, refines the crystal grains of steel, and enhances the toughness of the processed product obtained after cold working. It is an effective element for improving the crack resistance. The B content is preferably 0.005% or less, more preferably 0.0035% or less, and still 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.

[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. In the case where these elements are contained, it is preferable that both be 0.2% or less (excluding 0%). More preferably, both are 0.15% or less (more preferably 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.

[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.

  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.

[Cu: 5% or less (not including 0%), Ni: 5% or less (not including 0%) and Co: 5% or less (not including 0%)]
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 hardening the steel material by strain aging, and are effective elements for improving the strength after processing. In order to exert such an effect, 0.1% or more (more preferably 0.3% or more) of Cu and 0.01% or more (more preferably 0.05% or more) of Ni and Co are contained. preferable. However, if these contents are excessive, there is a risk of inducing cracking. Therefore, it is preferable that both be 5% or less (not including 0%), more preferably 4% or less (more preferably 3%). The following is recommended.

[Pb: 0.5% or less (not including 0%) and / or Bi: 0.5% or less (not including 0%)]
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 0.5% 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 basically composed of a structure of pearlite and cementite + ferrite, but pearlite and cementite have different work hardening rates compared to ferrite, so that voids and the like are formed at the interface between ferrite and cementite. It tends to be a starting point of defects, and if pearlite and cementite increase, workability may be deteriorated. In the steel material of the present invention, the pearlite and cementite area ratio in the structure needs to be 3% or less from the viewpoint of ensuring crack resistance. Preferably it is 2.5% or less, More preferably, it is 2% 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 (3).
H ≧ (DR + 200) /2.5 (3)
[In formula (3), 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 2X) having the chemical composition shown in Tables 4 and 5 below were prepared, each of which was billet-melted, heated to 1150 to 1250 ° C., and hot-forged to obtain a 155 mm square. It was made of steel pieces. The steel slab was heated to 1000 to 1200 ° C. and rolled into a round bar of φ80 mm (then cooled to 200 ° C. at a cooling rate of 1 ° C./second). Next, a test piece of φ10 mm × length 15 mm was cut out from the D / 4 position (D is a diameter) of the round bar. In Tables 4 and 5, “solid solution N” is a value indicating the N content in a solid solution state measured by a method based on the above-mentioned JIS G 1228. In addition, about the round bar manufactured using the test material of steel type 1P, the heat treatment shown in Table 4 (additional heat treatment: heated to each temperature and cooled to room temperature RT (20 ° C) at a cooling rate of 3 ° C / second). By applying, after adjusting the amount of solute N, a test piece was cut out from the D / 4 position.

Each test piece is compressed from a state in which the end face of the test piece is constrained by cold forging using a press machine having a capacity of 1600 tons, and the maximum value DR (hereinafter referred to as “deformation resistance DR”) of deformation resistance during cold working. May be called). The cold forging conditions at this time were as follows: processing strain rate: 10 / second, processing temperature: 20 ° C., compression rate: 80%. The processing strain rate was an average value of strain rates during processing (plastic deformation). The compression ratio is obtained by [(1-L / L ₀ ) × 100 (%)] (L ₀ : length of the test piece before processing, L: length of the test piece after processing). It is what

  The values (and determinations) of the formula (2) for each steel material are shown in Tables 6 and 7 below together with the cold working temperature (working temperature). Tables 6 and 7 also show the area ratio of cementite (pearlite), which was measured by the following method.

[Measurement method of cementite area ratio]
(I) The test piece was cut at the center in the transverse direction.
(Ii) The cross section (observation surface) was embedded in a resin so that it could be observed, and polishing with emery paper, polishing with a diamond buff and electrolytic polishing were sequentially performed to finish the observation surface as a mirror surface.
(Iii) Corroded with nital (3% nitric acid ethanol solution).
(Iv) The D / 4 position was observed at a magnification of 100 with an optical microscope, and five photographs were taken.
(V) Using image analysis software (Media Cybernetics: Image-Pro Plus), the ferrite phase is white and cementite (pearlite) is black, and the area ratio of each is obtained. The area ratio was calculated.

  For each processed product obtained, the surface was observed with a stereomicroscope at a magnification of 20 times to confirm the number of cracks (number of cracks n for 10 test pieces). 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. As a result, [deformation resistance (DR), hardness H after processing, the number of cracks (when cracking occurs, x, when cracking does not occur), the value on the right side of (3) (( DR + 200) /2.5), deformation resistance evaluation [relationship of equation (3)] and hardness after processing are shown in Tables 8 and 9 below.

  In these measurement results, the obtained processed product has no cracks and its deformation resistance is low with respect to the Vickers hardness [specifically, when the condition shown in the above formula (3) 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 (3) Formula, comprehensive determination of what a crack generate | occur | produced was displayed by "x".

この結果から、次のように考察できる（尚、下記記号は、表８、９の鋼種記号を示す）。表８に示した１Ａ〜１Ｚ（但し、（３）式の関係を満足しない１Ｐ−１〜３、７、組成および当該組成に基づく条件を充足しない１Ｄ、１Ｑ、１Ｔ、１Ｕ、１Ｖ、１Ｙを除く）は、本発明で規定する要件を満足する実施例であり、部品に割れが無く、且つ加工後硬さＨに対して鋼の変形抵抗ＤＲが低い鋼材が得られている。これに対し、本発明の要件を満たさない比較例は（表９に示した２Ａ〜２Ｘ）、割れが発生しているか、加工後硬さＨに対する加工時の変形抵抗ＤＲが高くなっている。尚、表８に示した１Ｄ、１Ｑ、１Ｔ、１Ｕ、１Ｖ、１Ｙは、本発明で規定する組成および当該組成に基づく条件を充足しないが、良好な特性が発揮されている参考例である。

From this result, it can be considered as follows (note that the following symbols indicate steel type symbols in Tables 8 and 9). 1A to 1Z shown in Table 8 (however, 1P-1 to 3 and 7 which do not satisfy the relationship of the formula (3), 1D, 1Q, 1T, 1U, 1V and 1Y which do not satisfy the composition and the conditions based on the composition) Ex ) is an example that satisfies the requirements defined in the present invention, and a steel material having no cracks in parts and having a low deformation resistance DR of steel with respect to post-working hardness H is obtained. On the other hand, in the comparative examples not satisfying the requirements of the present invention (2A to 2X shown in Table 9), cracks are generated or the deformation resistance DR during processing with respect to the post-processing hardness H is high. In addition, 1D, 1Q, 1T, 1U, 1V, and 1Y shown in Table 8 are reference examples in which good characteristics are exhibited, although the composition defined in the present invention and the conditions based on the composition are not satisfied.

  The samples of 2A to 2Q do not satisfy the relationship of the expression (2) (determination: “x”), and in all cases, the deformation resistance increased and the parts were cracked. Among these, those of 2M and 2N are those in which the Mn content is outside the upper and lower limits specified in the present invention, and those of 2O and 2P are excessive in P content and S content, respectively. Are those in which the N content is excessive. All seem to have an adverse effect on cracking.

  In 2R, since the Si content is excessive, deformation resistance increases and cracks occur in parts. 2S has an excessive amount of C and does not satisfy the relationship of the formulas (1) and (2), and the deformation resistance DR is increased to cause cracks in the parts. 2T does not satisfy the relationship of formula (2) because the Al content is excessive. Therefore, the amount of solute N is insufficient, and a predetermined strength (hardness H after processing) after cold working is not obtained.

  2U lacks the N content, and accordingly, the solid solution N amount is also insufficient, and a predetermined strength after cold working (hardness H after working) is not obtained.

  2V has insufficient Mn content (S content is also insufficient), N content is also insufficient, and accordingly, solute N content is also insufficient. The deformation resistance DR of is high. 2W has an excess of C content and Si content (S content is insufficient), N content is insufficient, and accordingly, solute N content is also insufficient (2) The relationship of the formula is not satisfied, and the deformation resistance DR during processing with respect to the post-processing hardness H is high.

  In 2X, the processing temperature is 200 ° C., and the deformation resistance DR during processing with respect to the post-processing hardness H is high.

  Based on the results of Table 9 (Comparative Example) (steel types 2A to 2Q), the relationship between the number of cracks (n / 10) and the value of the right side of equation (2) is shown in FIG. It can be seen that the value of is correlated with the number of cracks (n / 10). In addition, based on this result, the relationship of said Formula (2) was calculated | required by regression analysis.

  FIG. 2 shows the effect of the solid solution N amount on the deformation resistance DR and the post-working hardness H based on the results of the above 1P-1 to 7 shown in Table 8, but appropriately adjusting the solid solution N amount. Thus, it can be seen that the deformation resistance DR and the post-processing hardness H can be controlled within appropriate ranges.

  Based on the results of 1A to 1F (Example) in Table 8 and 2R to 2X (Comparative Example) in Table 9, the relationship between post-processing hardness H and deformation resistance DR is shown in FIG. It can be seen that the balance between the post-processing hardness H and the deformation resistance DR is improved as compared with FIG.

[Example 2]
Test steels of steel types (3A to 3D) having chemical composition shown in Table 10 below were prepared, and test pieces were cut out in the same manner as in Example 1. Using each test piece, the size (area: μm ² ) of MnS contained in the steel material was measured by the following method. The results are shown in Table 11 below together with the processing temperature, the value on the right side of equation (2), and the area ratio of cementite.

[Measurement method of MnS size]
(I) The test piece was cut at the center in the transverse direction.
(Ii) The cross section (observation surface) was embedded in a resin so that it could be observed, and polishing with emery paper, polishing with a diamond buff and electrolytic polishing were sequentially performed to finish the observation surface as a mirror surface.
(Iii) Corroded with nital (3% nitric acid ethanol solution).
(Iv) The D / 4 position was observed at a magnification of 100 with an optical microscope, and five photographs were taken.
(V) Using an image analysis software (manufactured by Sumitomo Metal Technology Co., Ltd .: “Particle Analysis Ver. 3.0”), the MnS area is obtained and the average value of the five fields of view is defined as the size of MnS (area: μm ² ). Calculated.

Then, the deformation resistance DR, the post-processing hardness H, the number of cracks, and the number of cracks of the obtained steel were determined in the same manner as in Example 1. The results are shown in Table 12 below. Further, based on this result, the relationship between the value of the right side of the equation (2) and the size of MnS (MnS area) is shown in FIG. 4, but by satisfying the relationship of the equation (2), the MnS is fine. It can be seen that they are distributed.

Claims (8)

In mass%, C: 0.045% or less (not including 0%), Si: 0.05% or less (not including 0%), Mn: 0.30 to 1.5%, P: 0.05 % Or less (excluding 0%), S: more than 0.05 to 0.12%, Al: 0.06% or less (not including 0%) and N: 0.008 to 0.025%, respectively. And the following formulas (1) and (2) are satisfied, the balance is made of iron and inevitable impurities, and N as a solid solution state is 0.007 to 0.018%. A steel material for machine structural use having excellent cold workability, characterized by having a steel structure having a cementite area ratio of 3% or less.
0.5 ≧ (10 [C] + [N]) (1)
However, [C] and [N] indicate the contents (% by mass) of C and N, respectively.
0 ≧ 126 [C] +3 [Mn] +84 [S] −10 (2)
However, [C], [Mn], and “S” indicate the contents (mass%) of C, Mn, and S, respectively.   Furthermore, Ca: 0.05% or less (not including 0%), REM: 0.05% or less (not including 0%), Mg: 0.05% or less (not including 0%), and Te: 0 The steel material for machine structure of Claim 1 containing at least 1 sort (s) chosen from the group which consists of 0.1% or less (excluding 0%).   The steel for machine structure according to claim 1 or 2, further comprising B: 0.005% 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 in any one of Claims 1-3 containing the at least 1 sort (s) chosen.   Furthermore, the steel for machine structures in any one of Claims 1-4 containing Cr: 2% or less (excluding 0%) and / or Mo: 2% 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 structures in any one of Claims 1-5 to contain.   Furthermore, Pb: 0.5% or less (0% is not included) and / or Bi: 0.5% or less (0% is not included) Steel material. A cold-worked steel part manufactured by cold-working the steel for machine structural use according to any one of claims 1 to 7 at a working temperature of less than 100 ° C, wherein the hardness of the part after cold work ( H) and the maximum value (DR) of deformation resistance during cold working satisfy the relationship of the following formula (3).
H ≧ (DR + 200) /2.5 (3)
[In formula (3), H: component hardness after cold working (Hv), DR: maximum deformation resistance (MPa) during cold working. ]

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