JP5114189B2 - Cold-working steel, its manufacturing method and cold-worked steel parts - Google Patents

Description

  The present invention relates to cold work steel, a method for producing the same, and a cold work steel part.

  Cold processing (for example, processing of steel in an atmosphere of 200 ° C. or lower) has advantages such as higher productivity, better dimensional accuracy, and better steel yield than hot processing and warm processing. Therefore, it is widely used for manufacturing various parts.

  Under such a background, steel used for cold working is required to have low deformation resistance during cold working. This is because when the deformation resistance of steel is high, the life of the mold used for cold working is reduced.

  In order to reduce the deformation resistance of steel, it is known that additive elements such as C (carbon), Si, and Mn may be reduced. However, if the additive element is simply reduced and the deformation resistance is lowered, the life of the mold can be improved, but there is a problem that the required component strength cannot be obtained after processing. Therefore, conventionally, after cold-working steel into a predetermined shape, quenching and tempering has been performed to ensure a predetermined hardness. However, when quenching and tempering is performed after the parts are processed, the dimensions of the parts are likely to change, and there is a problem that the parts must be further processed.

  Patent Document 1 discloses a method for producing a steel bar for cold forging by holding cement in the range of Ac1 to (Ac1-50 ° C.) for 5 hours or more to agglomerate cementite and lower deformation resistance. ing. In addition, cracks during forging are prevented by using inclusions to suppress the coarsening of ferrite grains. In this method, since the cementite is stably present by spheroidizing, the component strength is a strength level corresponding to the deformation resistance, but in order to achieve a higher component strength compared to the deformation resistance, Further study is necessary. Moreover, although productivity becomes better than normal spheroidizing annealing, since a holding time of 5 hours or more is required, it is considered necessary to improve from the viewpoint of productivity improvement and energy saving.

Patent Document 2 shows that an average of 25 or more / 25 μm ² exists in a ferrite structure in a state where nitrides and carbides are mixed or compounded. In this technology, solid solution N and solid solution C that adversely affect deformation resistance are fixed, and a predetermined number or more of nitrides and carbides are precipitated in the ferrite structure, thereby suppressing dynamic strain aging and reducing deformation resistance. It is to reduce. In this technique, since solid solution C and solid solution N exist stably, the component strength is at a strength level corresponding to the deformation resistance, but in order to achieve higher component strength than the deformation resistance. Further consideration is necessary.
JP-A-57-63635 JP 2000-8139 A

  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. The deformation resistance is kept low, and the life of the mold can be extended), and the steel for cold working capable of ensuring a predetermined hardness and strength after processing, the manufacturing method thereof, and the An object of the present invention is to provide a cold-worked steel part obtained by using cold-working steel.

With the steel for cold working according to the present invention,
C: 0.20 to 0.40% (mass%, the same shall apply hereinafter)
Si: 0.01-0.30%,
Mn: 0.2 to 1.0%,
P: 0.05% or less (excluding 0%),
S: 0.05% or less (excluding 0%),
Al: 0.010 to 0.1%, and N: 0.0070% or less (not including 0%) are satisfied, and the balance is composed of iron and inevitable impurities,
When the steel structure was observed at a magnification of 150,000 times using a transmission electron microscope,
It is characterized in that the density of cementite having a particle size of 50 nm or less is 5-25 pieces / 0.25 μm ² and the density of cementite having a particle size of more than 50 nm is 1 piece / 0.25 μm ² .

  The above “cementite having a particle size of 50 nm or less” and “cementite having a particle size of more than 50 nm” are those measured by the method shown in Examples described later (the same applies hereinafter).

The cold work steel is still another element,
(A) Cr: 2% or less (not including 0%) and / or Mo: 2% or less (not including 0%)
(B) 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 (c) B: 0.005% or less (excluding 0%)
(D) at least 1 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%) Species (e) 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 : Selected from the group consisting of 0.02% or less (not including 0%), Pb: 0.5% or less (not including 0%), and Bi: 0.2% or less (not including 0%) At least one kind may be included.

  The present invention also defines a method for producing the steel for cold working, and the method uses a steel having the above chemical composition and is in a temperature range of (Ac1-50 ° C.) to (Ac1 + 20 ° C.). At 60 to 60 minutes, and then cooled to room temperature at a cooling rate of 8 ° C./sec or more.

Further, the present invention is a cold-worked steel part manufactured by cold-working the cold-working steel at a working temperature of 200 ° C. or less, and the cold-worked part strength (H) and cold-working It also includes cold-worked steel parts that are characterized in that the maximum value (DR) of deformation resistance therein satisfies the following formula (1).
H ≧ (DR + 1000) / 6 (1)
[In formula (1), H indicates the strength of the part after cold working (Hv), and DR indicates the maximum value (MPa) of deformation resistance during cold working]

  According to the present invention, the hardness of a cold-worked steel part can be improved as compared with the prior art without performing quenching and tempering or complicated heat treatment after cold working. Further, cold working with a large strain can be performed satisfactorily without causing cracks (for example, cold working with a true strain of 150% can be performed well). Furthermore, it is possible to extend the life of a mold used for cold working.

  The inventors of the present invention show good workability during cold working, can perform processing with large strain, and can extend the life of the mold used for the processing. Intensive research was conducted to obtain a steel for cold working that can ensure the prescribed hardness and strength. As a result, it has been found that fine cementite having a specified size should be present in steel with a specified density. In addition, the chemical composition and production conditions of the steel material for causing the cementite of the above size and amount to exist were examined, and the optimum range was found. Hereinafter, the present invention will be described in detail.

  In the present invention, cementite having a particle size of 50 nm or less is present in the steel. The fine cementite having a particle size of 50 nm or less is decomposed during cold working (deforming), C is diffused in the material (solid solution C is supplied into the steel), and the solid solution C is converted into steel. It is considered that a predetermined part strength can be achieved by being fixed to the movable dislocation inside.

  When the particle size of the cementite exceeds 50 nm, it is not preferable because the stability becomes high and the decomposition becomes difficult. The particle size of the cementite is preferably 45 nm or less, more preferably 40 nm or less.

  In addition, during cold working, the steel part to which solute C is supplied has high deformation resistance, but in the present invention, by defining the upper limit of the cementite amount, the structure other than the cementite bears deformation, Processing can be performed without significantly increasing deformation resistance.

In order to sufficiently exhibit such an effect, it is necessary that five or more fine cementites having a particle diameter of 50 nm or less are present per 0.25 μm ² . This is because when the number is less than 5 per 0.25 μm ^{2, a} large amount of solid solution C exists and deformation resistance increases. The number of cementite of the above size is preferably 8 or more, more preferably 10 or more per 0.25 μm ² . On the other hand, if there is more than 25 cementites of the above size per 0.25 μm ² , an increase in deformation resistance due to precipitation strengthening becomes remarkable, which is not preferable. Therefore, in the present invention, the density of the cementite of the above size is 25 or less per 0.25 μm ² . Preferably it is 23 or less / 0.25 μm ² , more preferably 20 or less / 0.25 μm ² .

Further, the density of cementite having a particle diameter of more than 50 nm needs to be suppressed to 1 or less per 0.25 μm ² . When the density of cementite having a particle size of more than 50 nm is more than 1 / 0.25 μm ^2, it is difficult for fine cementite to precipitate, and it becomes difficult to obtain a predetermined component strength (the expression (1) described later is not satisfied). It is. The density of cementite having a particle diameter of more than 50 nm is preferably 0.8 or less / 0.25 μm ² , more preferably 0.5 or less / 0.25 μm ² .

  In the present invention, such an action mechanism can improve the component strength without increasing the deformation resistance. Therefore, compared with the techniques of Patent Document 1 and Patent Document 2, the component strength can be further increased with the same amount of deformation resistance. Can be increased. In addition, when a part having the same strength as the conventional one is manufactured, it is possible to extend the life of a die used for cold working. In the example of Patent Document 2, there is an example in which the average particle size of carbide is small and the deformation resistance is high, but this is because the number of precipitated carbide is small and a large amount of solute C that contributes to dynamic strain aging remains. Therefore, it seems that deformation resistance increased during processing.

  As described above, the present invention is particularly characterized in that the size and amount of the cementite in the steel is controlled. In this way, the precipitation form of the cementite is controlled to easily ensure good cold workability. At the same time, in order to ensure the strength and other necessary characteristics of the steel part, the chemical composition must be within the following range.

[C: 0.20 to 0.40%]
C is an element necessary for increasing the strength and securing the prescribed cementite. Therefore, the C amount is 0.20% or more. Preferably it is 0.21% or more, more preferably 0.22% or more. However, if the amount of C exceeds 0.40%, deformation resistance increases, which is not preferable. Therefore, in the present invention, the upper limit of the C amount is set to 0.40%. Preferably it is 0.39% or less, More preferably, it is 0.38% or less.

[Si: 0.01-0.30%]
Si is an element used as a deoxidizer in the steelmaking process. If the amount of Si is too small, gas is generated during the solidification process, which tends to act as defects, and thus cracks may occur during cold working. Therefore, the lower limit of Si content is set to 0.01%. Preferably it is 0.015% or more, More preferably, it is 0.02% or more. On the other hand, when the amount of Si is excessive, deformation resistance is increased, and decarburization during rolling is advanced to deteriorate the surface quality of the product. Therefore, the upper limit of Si content is set to 0.30%. Preferably it is 0.28% or less, More preferably, it is 0.25% or less.

[Mn: 0.2 to 1.0%]
Mn is an element that is effective as a deoxidizer or desulfurizer in the steelmaking process, and is also an element that is effective for improving hardenability. Furthermore, it is an element effective for fixing S as MnS and suppressing the adverse effects of S. Therefore, Mn is contained by 0.2% or more. Preferably it is 0.22% or more, more preferably 0.25% or more. On the other hand, when the amount of Mn is excessive, deformation resistance increases, so the amount of Mn is 1.0% or less, preferably 0.98% or less, more preferably 0.95% or less.

[P: 0.05% or less (excluding 0%)]
Phosphorus (P) is an unavoidable impurity and is an element that segregates at the ferrite grain boundaries and degrades the 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 reduce the P content to zero.

[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 amount is 0.05% or less (preferably 0.03% or less). However, it is industrially difficult to reduce the amount of S 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. .

[Al: 0.010 to 0.1%]
Al is an element that is effective as a deoxidizing element in the steelmaking process, and is also an element that is effective in suppressing dynamic strain aging due to solute N by forming solute N and nitride (AlN). is there. In order to fully exhibit the effect, Al is contained in an amount of 0.010% or more, preferably 0.015% or more, more preferably 0.02% or more. However, if the Al content exceeds 0.1%, the toughness is lowered and cracking tends to occur, which is not preferable. Therefore, the Al content is set to 0.1% or less. Preferably it is 0.09% or less, More preferably, it is 0.08% or less.

[N: 0.0070% or less (excluding 0%)]
Nitrogen (N) is an unavoidable impurity and is an element that increases deformation resistance. Therefore, the N amount is 0.0070% or less. Preferably it is 0.006% or less, More preferably, it is 0.005% or less.

  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. In addition, you may further contain the following elements as needed.

[Cr: 2% or less (not including 0%) and / or Mo: 2% or less (not including 0%)]
Cr and Mo are elements having an effect of improving the hardness and toughness of the parts after cold working. In order to effectively exhibit such an effect, when Cr is contained, it is effective to contain 0.1% or more, more preferably 0.2% or more. Moreover, when Mo is contained, it is recommended that the content be 0.04% or more, more preferably 0.1% or more.

  However, excessive addition of Cr increases deformation resistance and decreases cold workability, so the Cr content is preferably 2% or less, and more preferably 1.5% or less. Moreover, since cold workability will deteriorate when Mo is contained excessively, it is preferable to make Mo amount into 2% or less, More preferably, it is 1.5% or less, More preferably, it is 1% or less.

[From the group consisting of 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]
Ti, Nb, and V are effective elements for forming nitrides together with nitrogen to refine crystal grains and increasing the toughness of parts obtained after cold working. In order to effectively express such an effect, when Ti is contained, 0.001% or more, more preferably 0.002% or more of Ti is preferably contained. When Nb is contained, The Nb content is preferably 0.001% or more, more preferably 0.002% or more. In addition, when V is contained, it is recommended that 0.001% or more, more preferably 0.002% or more of V be contained.

  On the other hand, these elements have a strong affinity with nitrogen and form nitrides, reducing the amount of solid solution nitrogen effective for increasing the hardening after cold working. Therefore, the upper limit amount is determined as follows: . The Ti amount is preferably 0.2% or less, more preferably 0.1% or less, the Nb amount is preferably 0.2% or less, more preferably 0.1% or less, and the V amount is Preferably it is 0.2% or less, More preferably, it is 0.1% or less.

[B: 0.005% or less (excluding 0%)]
B is an element that improves the deformability of steel by increasing the strength of grain boundaries. Therefore, it is recommended that the amount of B is preferably 0.0001% or more, more preferably 0.0002% or more as required. However, B has a strong affinity for nitrogen and forms B nitride. However, when this B nitride is excessive, cold workability tends to decrease. Therefore, the B amount is preferably 0.005% or less, more preferably 0.003% 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 are all elements that have the effect of hardening and hardening a steel material, and are effective elements for improving the strength after processing. In order to exert these effects, it is preferable to contain 0.1% or more, and more preferably 0.3% or more, when any element is contained. However, in the case of containing any element, if it exceeds 5%, the effect is saturated, and on the contrary, cracking is promoted, which is not preferable. Therefore, the upper limit in the case of inclusion is 5% (preferably 4%, more preferably 3%).

[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. At least one selected from the group consisting of 02% or less (excluding 0%), Pb: 0.5% or less (not including 0%), and Bi: 0.2% or less (not including 0%) ]
Ca, REM (rare earth element), Mg, Li, Pb and Bi are elements that contribute to improving the machinability of steel. Ca, REM, Mg, and Li also have the effect of increasing the toughness of steel by spheroidizing sulfide inclusions such as MnS. In order to effectively express such an effect, when Ca is contained, 0.0005% or more, more preferably 0.001% or more Ca is preferably contained, and when REM is contained, preferably Is preferably 0.0005% or more, more preferably 0.001% or more of REM. Further, when Mg is contained, it is preferable to contain 0.0005% or more, more preferably 0.0008% or more of Mg, and when Li is contained, preferably 0.0001% or more. More preferably, 0.0005% or more of Li is contained. When Pb is contained, 0.005% or more, more preferably 0.01% or more of Pb is preferably contained. . Further, when Bi is contained, it is recommended that 0.005% or more, more preferably 0.01% or more of Bi is contained.

  However, even if the amount of these elements is excessive, the effect is saturated. Then, the upper limit amount in the case of making it contain was defined as follows, respectively. The Ca amount is preferably 0.05% or less, more preferably 0.01% or less, the REM amount is preferably 0.05% or less, more preferably 0.01% or less, and the Mg amount is preferably 0.00. 02% or less, more preferably 0.01% or less, the Li amount is preferably 0.02% or less, more preferably 0.01% or less, and the Pb amount is preferably 0.5% or less, more preferably 0. .3% or less, and the Bi amount is preferably 0.2% or less, more preferably 0.1% or less.

  Next, the manufacturing method of this invention is demonstrated. As described above, the steel of the present invention is particularly characterized in that the size and amount of cementite are defined. In this way, in order to control the precipitation form of cementite, it is melted and rolled by a general method, and after hot working (hot rolling, hot forging, etc.), the normalizing temperature: (Ac1) It was found that it was very effective to hold at a temperature range of −50 ° C. to (Ac1 + 20 ° C.) for 10 to 60 minutes and then cool to room temperature at a cooling rate of 8 ° C./sec or more.

  If the normalizing temperature is lower than (Ac1-50 ° C.), fine cementite does not precipitate, which is not preferable. Preferably it is (Ac1-40 ° C) or higher, more preferably (Ac1-30 ° C) or higher.

  On the other hand, when the normalizing temperature exceeds (Ac1 + 20 ° C.), the transformation from cementite to austenite is promoted, and the prescribed cementite cannot be obtained.

  Therefore, the normalizing temperature is (Ac1 + 20 ° C.) or less, preferably (Ac1 + 15 ° C.) or less, more preferably (Ac1 + 10 ° C.) or less. In addition, the normalizing temperature may fluctuate within the temperature range as well as being held at a constant temperature within the temperature range of (Ac1-50 ° C) to (Ac1 + 20 ° C).

  Moreover, when the holding time in the said temperature range is short, the number of the cementite of the said size will become inadequate. Accordingly, the holding time is 10 minutes or longer (preferably 15 minutes or longer, more preferably 20 minutes or longer). On the other hand, if the holding time is too long, fine cementite aggregates and begins to coarsen, such being undesirable. Therefore, the holding time is 60 minutes or less. Preferably it is 55 minutes or less, More preferably, it is 50 minutes or less.

  After holding at the above temperature and time, it is cooled to room temperature at a cooling rate of 8 ° C./sec or more. This is because when the cooling rate is slow, the cementite aggregates to increase the size, and a sufficient size of cementite cannot be secured. It is preferable to cool to room temperature at a cooling rate of 8.5 ° C./sec or more, more preferably 9 ° C./sec or more. Examples of the cooling means include natural cooling and air cooling.

  In addition, when manufacturing a wire as steel for cold work, as one means for achieving the cooling rate, the steel is thinned by the hot work (for example, 10 to 12 mm in diameter) and cooled by the above means. Can be mentioned.

  The steel for cold work (for example, wire rods and steel bars) manufactured as described above is then cold worked to become steel parts (parts such as bolts and nuts, parts for automobiles, and other machine 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.

  Since the temperature at the time of processing also affects the cold workability, it is recommended that the upper limit value of the processing temperature is preferably set to 200 ° C, more preferably 180 ° C, and even more preferably 160 ° C. This is because if the processing temperature is too high, dynamic strain aging occurs during deformation and the deformation resistance increases. On the other hand, cold working is usually performed at room temperature. However, if the temperature is lower than 0 ° C., the deformation resistance becomes higher due to temperature dependence. The processing temperature refers to the atmospheric temperature during processing.

The steel part of the present invention obtained under such conditions is characterized in that its strength satisfies the following formula (1) in relation to the maximum value of deformation resistance during cold working.
H ≧ (DR + 1000) / 6 (1)
[In formula (1), H indicates the strength of the part after cold working (Hv), and DR indicates the maximum value (MPa) of deformation resistance 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.

  First, it manufactures sequentially on condition of the following (I) (II), and then it heats on condition (normalization temperature T, holding time t) shown in Table 2 as shown in (III) below, It cooled to room temperature with the cooling rate shown, and obtained the wire of the chemical component composition shown in Table 1.

In Table 1, Ac1 is the following K.I. W. The calculation was performed according to the Andrews equation (“Leslie Steel Material Science” Maruzen (1985) p. 273).
Ac1 (° C.) = 723-10.7 (% Mn) −16.9 (% Ni) +29.1 (% Si) +16.9 (% Cr) +290 (% As) +6.38 (% W)
(The above% indicates mass% of each component in steel)
(I) Melting / Rolling: Billet melting → Heating the melted material to 1150 to 1250 ° C. → Rolling to 155 mm square (II) Bar rolling: Heating to 1100 to 1250 ° C. → Rolling to a diameter of 12 mm → If allowed to cool (III) No: Normalizing temperature T × holding time t shown in Table 2 → cooling to room temperature at the cooling rate shown in Table 2 (cooling at a cooling rate higher than air cooling)
Observation of steel structure (measurement of cementite size and quantity), processing test and measurement of deformation resistance, and measurement of Vickers hardness (Hv) of steel parts using the wire rod having a diameter of 12 mm obtained as described above. Each was performed as follows.

[Observation of steel structure]
The density of cementite having a particle size of 50 nm or less and the density of cementite having a particle size of more than 50 nm were determined by the following procedures (i) to (v).
(I) From the D / 4 position of the wire, slices are collected so that a plane parallel to the rolling direction can be observed.
(Ii) A sample is prepared by an electrolytic thin film method (twin jet method) using Tenupol-2 manufactured by Struers.
(Iii) Using an H-800 transmission electron microscope manufactured by Hitachi, Ltd., observation and photography are performed at an acceleration voltage of 200 kV and an observation magnification of 150,000 times.
(Iv) The above photograph is captured by a scanner, and black and white binarization processing is executed at an arbitrary position using Image Pro PLUS manufactured by Media Cybernetic.
(V) For each area of the black part excluding lamellar cementite, the average value obtained by connecting the two points on the outer periphery of the object and measuring the diameter passing through the center of gravity in increments of 2 ° was defined as the particle size of cementite (particle size Is 1 nm). In this way, the particle size and number of the cementite present in a visual field size of 0.5 μm × 0.5 μm were measured, and the number of cementite particles having a particle size of 50 nm or less per 0.25 μm ^{2 and} cementite having a particle size exceeding 50 nm The number per 0.25 μm ² was determined. These measurement results are shown in Table 2.

  Next, a test piece of φ6 mm × length 9 mm was cut out from the center of the steel material. Using the test piece, a processing test was performed to measure deformation resistance, check for cracks, and measure the Vickers hardness (Hv) after processing (steel part). In general, when the deformation resistance is lowered, the mold life is improved, and when the deformation resistance is lowered by 20%, the mold life is extended by 5 to 10 times. Therefore, the evaluation of the mold life is performed with the value of the deformation resistance.

[Processing test and measurement of deformation resistance]
The above test piece is forged using a processing for master test apparatus having a capacity of 200 kN under processing conditions of strain rate: 1 / second, processing temperature: 20 to 200 ° C., and compression ratio: 20 to 80%, and processed into a steel part. did. As the strain rate, an average value of strain rates during processing (plastic deformation) was used. About the obtained component, the surface was observed with the actual condition microscope with an observation magnification of 20 times, and the presence or absence of the crack was confirmed. The cold working conditions of each part (working temperature, compressibility), the presence or absence of cracks in each part (when cracking occurs, ×, when cracking does not occur), and the maximum deformation resistance Table 3 shows.

The compression rate in Table 3 refers to [(1-L / L ₀ ) × 100 (%)] (L ₀ : test piece length before processing, L: test piece length after processing).

[Measurement of Vickers hardness (Hv) of steel parts]
Using the test piece after the processing test, using a Vickers hardness tester under the conditions of load: 1000 g, measurement position: D / 4 central part (D: part diameter) of the cross section of the test piece, and number of measurements: 5 times The Vickers hardness (Hv) of the steel parts was measured. The measurement results are also shown in Table 3.

[Judgment by Formula (1)]
Table 3 shows whether or not each test piece satisfies the following formula (1): “◯” when the formula (1) is satisfied, and “×” when the formula (1) is not satisfied. Is attached.
H ≧ (DR + 1000) / 6 (1)
However, H: Strength of parts after cold working (Hv), DR: Maximum deformation resistance during cold working (MPa)

  In this example, it was determined that a steel [particularly satisfying the above formula (1)] having excellent crack workability without cracking in the component and having low deformation resistance of the steel with respect to the component hardness. .

It can be considered as follows from Tables 1 to 3 (in addition, the following symbols indicate steel symbols in Tables 1 to 3).
1B, 1C, 1H, 1L, 1M, 1O, 1R, 1S, 1U, 1W, 1Y, 2A, and 2D to 2N are examples that satisfy the requirements defined in the present invention, and there are no cracks in the parts. A steel material having low deformation resistance during processing 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 the component hardness (that is, the equation (1) is not satisfied). Longer life cannot be expected.

  Specifically, 1A does not satisfy the formula (1) because the amount of C is insufficient and the prescribed cementite is not sufficiently secured. In 1D, since the amount of C was excessive, cementite precipitated more than necessary, and the parts were cracked.

  In 1E, since the amount of Si was insufficient, cracks occurred during cold working.

  In 1F, since the amount of Mn was excessive, cracks occurred in the parts. In 1G and 1K, the amount of Mn was insufficient (the amount of Al was insufficient in 1K), so S was not fixed by Mn, but was deposited as a film at the grain boundary as FeS, resulting in cracks in the parts.

  Since 1I excessively contains P that segregates at the ferrite grain boundaries and degrades the cold workability, cracks occurred in the parts. In 1J, since the amount of S was excessive, embrittlement of the crystal grain boundary was promoted, and the part was cracked.

  In 1N, since the Al amount was insufficient and the N amount was excessive, dynamic strain aging due to solute N occurred, and the parts were cracked.

  Since 1P has a short holding time t during normalization, the prescribed cementite cannot be sufficiently secured, and the formula (1) is not satisfied.

  In 1Q and 2B, since the normalizing temperature T exceeds (Ac1 + 20 ° C.), the prescribed cementite could not be precipitated, and the solid solution C contributing to dynamic strain aging could not be sufficiently reduced. The resistance increased and the workability decreased (cracked).

  1T was cooled slowly after being held at the normalizing temperature T, so that sufficient cementite of the prescribed size could not be secured, and the formula (1) was not satisfied.

  In 1V and 1Z, the normalizing temperature was lower than (Ac1-50 ° C.), so that sufficient cementite of the prescribed size could not be secured, and the formula (1) was not satisfied.

  In 1X, since the holding time t during normalization was too long, coarse carbides were formed and the formula (1) was not satisfied.

  In 2C, since normalization was not performed, cementite having a specified size could not be secured, and cracks were generated in the parts and the formula (1) was not satisfied.

Claims (8)

C: 0.20 to 0.40% (mass%, the same shall apply hereinafter)
Si: 0.01-0.30%,
Mn: 0.2 to 1.0%,
P: 0.05% or less (excluding 0%),
S: 0.05% or less (excluding 0%),
Al: 0.010 to 0.1%, and N: 0.0070% or less (not including 0%) are satisfied, and the balance is composed of iron and inevitable impurities,
When the steel structure was observed at a magnification of 150,000 times using a transmission electron microscope,
Cold-working steel characterized in that the density of cementite having a particle size of 50 nm or less is 5 to 25 pieces / 0.25 μm ² and the density of cementite having a particle size of more than 50 nm is 1 piece or less / 0.25 μm ² . As other elements,
Cr: 2% or less (not including 0%) and / or Mo: 2% or less (not including 0%)
The steel for cold work according to claim 1, comprising: As other elements,
Ti: 0.2% or less (excluding 0%),
The Nb: at least one selected from the group consisting of 0.2% or less (not including 0%) and V: 0.2% or less (not including 0%). Steel for cold working.   The steel for cold work according to any one of claims 1 to 3, further comprising B: 0.005% or less (not including 0%) as another element. As other elements,
Cu: 5% or less (excluding 0%),
The cold according to any one of claims 1 to 4, comprising at least one selected from the group consisting of Ni: 5% or less (not including 0%) and Co: 5% or less (not including 0%). Inter-working steel. As other elements,
Ca: 0.05% or less (excluding 0%),
REM: 0.05% or less (excluding 0%),
Mg: 0.02% or less (excluding 0%),
Li: 0.02% or less (excluding 0%),
6. The composition according to claim 1, comprising at least one selected from the group consisting of Pb: 0.5% or less (not including 0%) and Bi: 0.2% or less (not including 0%). Steel for cold work as described in 1. A method for producing the cold work steel according to any one of claims 1 to 6,
Using the steel having the chemical composition according to claim 1, the steel is held for 10 to 60 minutes in a temperature range of (Ac1-50 ° C.) to (Ac1 + 20 ° C.), and then at least 8 ° C./sec. A method for producing cold-working steel, characterized by cooling to room temperature at a cooling rate.
[However, Ac1 is a value calculated by the following equation.
Ac1 (° C.) = 723-10.7 (% Mn) −16.9 (% Ni) +29.1 (% Si) +16.9 (% Cr) +290 (% As) +6.38 (% W)
(The above% indicates the mass% of each component in the steel)] A cold-worked steel part manufactured by cold-working the cold-working steel according to any one of claims 1 to 6 at a working temperature of 200 ° C or less, wherein the strength of the part after cold working (H ) And the maximum value (DR) of deformation resistance during cold working satisfy the following formula (1).
H ≧ (DR + 1000) / 6 (1)
[In formula (1), H indicates the strength of the part after cold working (Hv), and DR indicates the maximum value (MPa) of deformation resistance during cold working]