JP2009242916A - Wire steel or bar steel in which spheroidizing is omissible - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wire steel or a bar steel that has excellent cold workability even when heat treatment such as spheroidizing is omitted and the steel is in a hot-rolled state and that can be manufactured by actual equipment, and particularly, to provide a wire steel or a bar steel can reduce dynamic strain ageing and reduce deformation resistance of steel in a process heat generation area even when the steel has a high deformation resistance at room temperature. <P>SOLUTION: The steel includes N, B and Ti satisfying an expression (1): -0.0060≤[N]-1.3×[B]-0.29×[Ti]≤0.0020, wherein [ ] represents the content of each element, and the balance iron and unavoidable impurities. The steel has a mixture structure of ferrite and pearlite, in which grain numbers of the ferrite present in a range from the center of the steel to a position of D/8, wherein D is the diameter of the steel, are from 6 to 12. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a linear steel or rod-shaped steel having good cold workability even when spheroidizing annealing is omitted, and a machine part obtained using the steel.

  Machine parts and electrical parts such as bolts, nuts and screws are pickled and descaled by rolling the linear steel (wire material) and bar steel (bar steel) obtained by rolling. Generally, cold drawing is performed after drawing at a processing rate of about 10 to 40% by cold drawing. Cold working is widely used because it is more productive than hot working or cutting, and the yield of steel is good. However, the above-described method has a problem that the life of a tool used for processing is shortened when the processing rate during cold processing is increased to increase the processing load. Moreover, when the ductility of the said linear steel and rod-shaped steel was low, the crack might generate | occur | produce at the time of cold work.

  Therefore, when the working load during cold working is large or when cracks occur during cold working, heat treatment such as low temperature annealing, annealing, spheroidizing annealing, etc. is performed before cold working. By performing the heat treatment, it is possible to cold work in a state where the steel material is softened and the ductility is increased.

  However, since the above heat treatment needs to be performed for a long time of several hours to several tens of hours, a linear steel that can omit heat treatment such as spheroidizing annealing for the purpose of improving productivity, energy saving measures, and cost reduction. Or the development of bar steel is eagerly desired.

  Moreover, the steel material used for cold work is required to be excellent in cold workability. Specifically, it is required that the deformation resistance during cold working is small (that is, the working load is small) and the ductility (elongation and drawing) is high. When the deformation resistance of steel increases, the life of the tool used during cold working decreases, whereas when the ductility decreases, cracks are likely to occur during cold working, causing defective products. In particular, when cold working is performed, processing heat is generated, so that the deformation resistance of steel usually increases.

  The present applicant proposes a technique for providing a linear steel or a rod-like steel that is excellent in cold workability even in a hot-rolled state while omitting heat treatment such as spheroidizing annealing in Patent Document 1 and Patent Document 2. Yes.

Patent Literature 1 and Patent Literature 2 are based on the idea of improving the cold workability of steel by suppressing an increase in both the deformation resistance of steel at room temperature and the deformation resistance of steel in the processing heat generation region. Instead of adding an element that refines ferrite crystal grains such as Ti, Nb, V, etc., the amount of solid solution N is increased by adding B or Zr that does not refine (rather coarsen) the ferrite crystal grains. Discloses a technique for improving the cold workability of steel by controlling the temperature.
JP 2001-303189 A JP 2001-342544 A

  In the said patent document 1 and patent document 2, the cold workability of steel in the case where heat processing, such as spheroidizing annealing, was abbreviate | omitted by controlling appropriately the relationship between Zr, B, and N was able to be improved. However, in Patent Document 1 and Patent Document 2 described above, since Zr was added as a main element, good results were obtained at the laboratory level. However, when the production scale was expanded to the actual machine level, the expected value was stably obtained. Some characteristics could not be obtained.

  By the way, the deformation resistance of steel in the processing heat generation region (for example, about 300 ° C.) depends to some extent on the deformation resistance of steel at room temperature. There is also a tendency for the deformation resistance to increase. The deformation resistance of steel at room temperature is greatly influenced by the composition of steel (particularly, C, Si, Mn). Therefore, when the deformation resistance of steel at room temperature is large, cold workability is deteriorated. Therefore, even when the deformation resistance of steel at room temperature is large, it is desired to improve the cold workability by reducing the deformation resistance of steel in the processing heat generation region.

  The present invention has been made in view of such a situation, and the purpose thereof is to omit heat treatment such as spheroidizing annealing, and is excellent in cold workability even in a state of hot rolling. It is another object of the present invention to provide a linear steel or a bar steel that can be manufactured using an actual machine. In particular, the present invention provides a linear steel or a rod-like steel that can reduce dynamic strain aging even when the deformation resistance of the steel at room temperature is large and can reduce the deformation resistance of the steel in the processing heat generation region. There is. Another object of the present invention is to provide mechanical parts such as bolts and nuts obtained by using the linear steel or bar steel.

The linear steel or rod-shaped steel according to the present invention that has solved the above problems is C: 0.05 to 0.35% (meaning mass%, the same shall apply hereinafter), Si: 0.3% or less (0 %), Mn: 0.15 to 1.8%, P: 0.015% or less (not including 0%), S: 0.02% or less (not including 0%), Cr: 0 .01-0.5%, sol. Al: 0.01-0.06%, N: 0.0005-0.006%, B: 0.0003-0.0015%, Ti: 0.003-0.030% are satisfied, and N B and Ti satisfy the following formula (1), and the balance is steel composed of iron and inevitable impurities. The steel has a mixed structure of ferrite and pearlite, and when the diameter of the steel is D, the steel The gist of the present invention is that the grain size number of the ferrite present in the range from the center of the steel to the D / 8 position is 6-12.
−0.0060 ≦ [N] −1.3 × [B] −0.29 × [Ti] ≦ 0.0020
(In the formula, [] represents the content of each element.)
The present invention also includes a machine part obtained by using the linear steel or rod-shaped steel.

  According to the present invention, by using Ti instead of Zr added in Patent Document 1 and Patent Document 2 described above, heat treatment such as spheroidizing annealing is stably eliminated even when the production scale is expanded to the actual machine level. It is possible to provide a linear steel or a bar steel excellent in possible cold workability. In addition, according to the present invention, by appropriately controlling the balance of N, B, and Ti, even when the deformation resistance of steel at room temperature is large, the dynamic strain aging is reduced and the processing heat generation region The deformation resistance of steel at can be reduced.

  When the present inventors manufactured the steel proposed in Patent Document 1 and Patent Document 2 at the actual machine level, it was found that the desired characteristics could not be obtained stably. Further examination of this reason revealed that the cause is that Zr easily forms an oxide. That is, in molten steel at around 1500 ° C., the free energy when Zr forms an oxide is smaller than the free energy when forming a nitride, and Zr combines with O (oxygen) to form a nitride. It was easier to form oxides. Further, Zr may deprive oxygen from other oxides (for example, Al oxide) to form Zr oxide. At the laboratory level, since the amount of O contained in the molten steel is small, the amount of Zr that forms oxide is also reduced, and it is considered that Zr that effectively contributes to the control of solute N can be secured. However, when the production scale is expanded to the actual machine level, the amount of O contained in the molten steel increases, so that Zr is consumed as an oxide, and it is considered difficult to effectively contribute to the control of the solute N.

  Therefore, the present inventors suppressed dynamic strain aging during cold working even when heat treatment such as spheroidizing annealing was omitted, and reduced the deformation resistance of the steel in the work heat generation region, resulting in good cold workability. In order to produce and provide a linear steel or a bar-shaped steel at the actual machine level, intensive studies have been made. As a result, it was found that if Ti is added instead of Zr and the content balance of N, B, and Ti is appropriately controlled, the desired linear steel or rod-like steel can be produced even with an actual machine, and the present invention has been completed. .

  Ti added in the present invention is an element that hardly forms an oxide as compared with Zr, and does not reduce other oxides to become a Ti oxide. Therefore, when Ti is added, Ti nitride can be efficiently formed, the amount of dissolved N can be stably adjusted, and the cold workability of steel can be improved even at the actual machine level.

  First, the content balance of N, B, and Ti characterizing the present invention will be described.

In the steel of the present invention, it is important that the amounts of N, B and Ti satisfy the relationship of the following formula (1). In the formula, [] means the content (% by mass) of each element. In the following, for convenience of explanation, the middle side of the equation (1) is referred to as “Z value” [see the following equation (2)].
−0.0060 ≦ [N] −1.3 × [B] −0.29 × [Ti] ≦ 0.0020 (1)
Z value = [N] −1.3 × [B] −0.29 × [Ti] (2)

  The upper limit of the above formula (1) was set so that the amount of solute N was not excessive in consideration of the amounts of N, B and Ti added. That is, when the Z value exceeds 0.0020, the amount of N is excessive with respect to the amount of B and Ti, so excess N exists as solid solution N in the steel, and dynamic strain aging occurs during cold working. Occurs, the deformation resistance of the steel in the heat generation region increases, and the cold workability deteriorates. In addition, cracks are likely to occur during cold working.

  Therefore, the steel of the present invention controls the contents of N, B and Ti so that the Z value is 0.0020 or less. Z value becomes like this. Preferably it is 0.0015 or less, More preferably, it is 0.001 or less.

  On the other hand, the lower limit value of the above formula (1) is set in order to effectively exhibit the solid solution N fixing action by B and Ti in consideration of the amount of added N, B and Ti. That is, when the Z value is less than -0.0060, B and Ti become excessive, so that solute B and TiC are generated in the steel, and cracks are likely to occur during cold working. In particular, TiC causes precipitation strengthening, which increases the deformation resistance of steel in the heat generation region during cold working.

  Therefore, in the steel of the present invention, the contents of N, B, and Ti are controlled so that the Z value is −0.0060 or more. The Z value is preferably −0.0055 or more, more preferably −0.0050 or more.

  In the steel of the present invention, the relationship between the amount of N, B and Ti satisfies the relationship of the above formula (1), but the amount of N, B and Ti must satisfy the following range. is there.

[N: 0.0005 to 0.006%]
N is preferably dissolved as little as possible because it dissolves in ferrite and causes dynamic strain aging during cold working. Therefore, in the present invention, the upper limit is set to 0.006% in consideration of the separately added B amount and Ti amount. Preferably it is 0.0055% or less, More preferably, it is 0.005% or less. The lower limit of N is not particularly limited, but in order to reduce it to less than 0.0005%, it becomes difficult to produce industrially suitable for cost. Preferably it is 0.001% or more, More preferably, it is 0.002% or more.

[B: 0.0003 to 0.0015%]
B combines with N to form BN to reduce solute N in the steel, suppress dynamic strain aging during cold working and reduce the deformation resistance of the steel in the heat generation region, It is an element that effectively acts to suppress solid solution strengthening by solid solution N and to prevent cracking during cold working. Further, when Ti is bonded to N and B exists in a solid solution state, it effectively acts as a hardenable element, and is an element that acts to adjust the strength by quenching and tempering in the final process of the part. Therefore, B is made 0.0003% or more. Preferably it is 0.0004% or more, More preferably, it is 0.0005% or more. However, when it contains excessively, the amount of solid solution B will increase and it will cause a crack at the time of cold work on the contrary. Therefore, B is 0.0015% or less. Preferably it is 0.0013% or less, More preferably, it is 0.001% or less.

[Ti: 0.003-0.030%]
Ti, like B, forms TiN by combining with N to reduce solid solution N in the steel, suppresses dynamic strain aging during cold working, and suppresses deformation resistance of steel in the heat generation region. It is an element that effectively acts to reduce cracking and prevent cracking during cold working by suppressing solid solution strengthening by solid solution N. Moreover, as mentioned above, if it is added simultaneously with B, the hardenability improving effect by the solid solution B can be fully exhibited. In order to exert these effects, 0.003% or more of Ti is contained. Preferably it is 0.004% or more, More preferably, it is 0.005% or more. However, if Ti is contained excessively, TiC is formed to cause precipitation strengthening, which causes an increase in the deformation resistance of the steel in the heat generation region during cold working. Therefore, Ti is made 0.030% or less. Preferably it is 0.028% or less, More preferably, it is 0.025% or less.

  The steel of the present invention contains N, B, and Ti in the above range, as well as C, Si, Mn, P, S, Cr, sol. Al is contained in the following range.

[C: 0.05 to 0.35%]
C is an element necessary for ensuring the strength of the steel itself. Therefore, C is made 0.05% or more. Preferably it is 0.10% or more, more preferably 0.15% or more. However, when C is excessive, the strength of the steel itself becomes too high and the deformation resistance increases. Therefore, as described above, even when the amount of dissolved N is appropriately controlled, the deformation resistance of steel in the heat generation region during cold working cannot be sufficiently reduced, and the tool life used for cold working is extremely reduced. Let Therefore, C is set to 0.35% or less. Preferably it is 0.33% or less, More preferably, it is 0.30% or less.

[Si: 0.3% or less (excluding 0%)]
Si is an element useful as a deoxidizing agent, but if contained excessively, it causes the deformation resistance of steel to increase due to solid solution strengthening. Therefore, Si is made 0.3% or less. Preferably it is 0.25% or less, more preferably 0.2% or less. Although the minimum of Si is not specifically limited, In order to make it act effectively as a deoxidizer, it is preferable to make it contain 0.005% or more. More preferably, it is 0.01% or more, More preferably, it is 0.05% or more, Most preferably, it is 0.1% or more.

[Mn: 0.15 to 1.8%]
Mn is an element useful as a deoxidizer and a desulfurizer. In order to exert such an effect, it is necessary to contain 0.15% or more. Preferably it is 0.2% or more, More preferably, it is 0.25% or more. However, if Mn is contained excessively, the strength of the steel itself becomes too high and the deformation resistance increases. Therefore, Mn is 1.8% or less. Preferably it is 1.5% or less, More preferably, it is 1% or less, More preferably, it is 0.8% or less.

[P: 0.015% or less (excluding 0%)]
P is an element that microsegregates during solidification and segregates at the grain boundaries during hot rolling and embrittles the grain boundaries, and cracks are likely to occur during cold working due to the embrittlement of the grain boundaries. Therefore, in the present invention, it is recommended to reduce P as much as possible, and it should be 0.015% or less. Preferably it is 0.014% or less, More preferably, it is 0.013% or less. In addition, P is usually contained about 0.001%.

[S: 0.02% or less (excluding 0%)]
S is an element that forms sulfide inclusions such as MnS, segregates at the grain boundaries during hot rolling and embrittles the grain boundaries, and easily cracks during cold working due to the embrittlement of the grain boundaries. Become. Therefore, in the present invention, S is preferably reduced as much as possible, and is 0.02% or less. Preferably it is 0.018% or less, More preferably, it is 0.015% or less. In addition, S is usually contained about 0.001%.

[Cr: 0.01 to 0.5%]
Cr promotes ferrite + pearlite transformation during hot rolling, precipitates carbide without increasing the strength of the steel more than necessary, reduces solid solution C, and contributes to reduction of dynamic strain aging due to solid solution C Element. In order to exhibit such an effect, Cr is contained by 0.01% or more. Preferably it is 0.03% or more, More preferably, it is 0.05% or more. However, when Cr becomes excessive, the strength of the steel itself becomes too large and the deformation resistance increases. Therefore, Cr is 0.5% or less. Preferably it is 0.4% or less, More preferably, it is 0.3% or less.

[Sol. Al: 0.01 to 0.06%]
sol. Al is an element usefully acting as a deoxidizer, and is contained in an amount of 0.01% or more. Preferably it is 0.015% or more, More preferably, it is 0.02% or more.

  However, Al is an element that easily produces nitride (AlN) next to Ti and B. When the amount of AlN increases, the grain size becomes finer, so the ferrite grain size number is in the range of 6-12. From the viewpoint of control, it is necessary to suppress the addition of Al as much as possible. Therefore, Al is made 0.06% or less. Preferably it is 0.055% or less, More preferably, it is 0.05% or less.

  The contained elements defined in the present invention are as described above, and the remaining component is iron. However, inevitable impurities brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc. are allowed.

  Next, the metal structure of the steel of the present invention will be described.

  The metal structure of the linear steel or rod-shaped steel of the present invention is a mixed structure of ferrite and pearlite, and when the diameter of this steel is D, the range from the center of the steel (D / 2 position) to the D / 8 position. The grain size number of ferrite present in No. 6 is No. 6-12. Bainitic ferrite is not included in the ferrite because it has poor ductility.

  In the steel of the present invention, the grain size number of ferrite existing in the range of D / 2 to D / 8 positions is 6-12. If the ferrite grain size number exceeds 12, the ferrite crystal grains become too fine, the strength of the steel itself increases, and the deformation resistance increases. Therefore, the grain size number of ferrite is 12 or less. Preferably it is 11 or less. However, if the ferrite crystal grains become too coarse and the ferrite grain size number is less than 6, cracks are likely to occur during cold working. Therefore, the grain size number of ferrite is 6 or more. Preferably it is 7 or more. The ferrite grain size number is measured and defined by the method shown in the examples described later.

  Next, a method for producing the linear steel or the bar steel according to the present invention will be described.

  As described above, the feature of the wire steel or rod steel of the present invention is that the balance of the content of N, B, and Ti is appropriately controlled, and Zr used in Patent Document 1 and Patent Document 2 described above. By using Ti instead of, solid solution N can be stably fixed.

  In melting the steel of the present invention, after deoxidizing using Si and Al, B and Ti may be added to fix N in the molten steel. The conditions disclosed in Patent Document 1 and Patent Document 2 can be applied as they are as conditions for producing the linear steel or rod-shaped steel of the present invention from the steel pieces obtained by melting. That is, in order to obtain the desired microstructure of the present invention, the steel slab is heated to a range of 800 to 1100 ° C., rolled to a predetermined wire diameter in the range of 750 to 1100 ° C., and then mainly by water flow. It is necessary to cool to 500 to 800 ° C. at a cooling rate of 600 to 6000 ° C./min and control the temperature recovered by recuperation (adjusted cooling start temperature) to 750 to 875 ° C.

  Hereinafter, each requirement will be described.

[Heating temperature of steel slab: 800 to 1100 ° C]
The heating temperature was set to suppress the refinement of ferrite crystal grains and to reduce the deformation resistance of the steel. The “heating temperature of the steel slab” is measured by a radiation thermometer, and strictly speaking, means “the surface temperature of the steel slab”.

  When heated above 1100 ° C., the ferrite crystal grain size becomes too large and the ductility decreases. Preferably it is 1050 degrees C or less, More preferably, it is 1000 degrees C or less. On the other hand, if the heating temperature is less than 800 ° C., the deformation resistance of the steel in the processing heat generation area during cold working becomes too high, which may cause wear of the rolling roll, trip of the rolling motor, etc., resulting in production hindrance. There is. Preferably it is 850 degreeC or more.

[Rolling temperature: 750-1100 ° C.]
The rolling temperature is set in order to suppress the refinement of the ferrite crystal grain size and reduce the deformation resistance of the steel during rolling, as in the case of heating the steel slab. “Rolling temperature” is also measured by a radiation thermometer, and strictly speaking, it means “surface temperature of a steel slab”.

  When it rolls over 1100 degreeC, a ferrite crystal grain size will become large too much and ductility will fall. Preferably it is 1050 degrees C or less, More preferably, it is 1000 degrees C or less. On the other hand, when the rolling temperature is less than 750 ° C., not only the ferrite crystal grain size becomes small, but also the dislocations are not easily recrystallized and unnecessary dislocations are easily left in the ferrite grains, and the strength after rolling increases. Preferably it is 850 degreeC or more.

[Adjusted cooling start temperature: 750-875 ° C.]
The adjusted cooling start temperature refers to a cooling zone (cooling conveyor) after the final rolling, after cooling the outermost surface temperature to a minimum of about 500 to 800 ° C. at a cooling rate of 600 to 6000 ° C./min mainly using water as a medium. ) Means the temperature recovered by the heat (recovery) of the billet.

  When the temperature is higher than 875 ° C., the scale becomes thick, and not only troubles are likely to occur in the subsequent descaling process, but also the cooling time becomes long, which hinders productivity. Preferably it is 850 degrees C or less. On the other hand, when the temperature is lower than 750 ° C., martensite in the surface layer portion generated during cooling is not recovered by recuperation, and tempered martensite is generated and becomes hard and brittle steel, which is not suitable for cold working. A preferable operating level is 775 ° C or higher.

[Cooling rate to 600 ° C .: 0.05 to 5 ° C./second]
In the present invention, the cooling rate at the time of cooling to 600 ° C. after reaching the adjusted cooling start temperature is specified. As in the present invention, in order to control the structure to be a mixed structure of ferrite and pearlite, it is preferable to reduce the cooling rate. However, if it is too small, the lamellar spacing in pearlite (layered structure of ferrite and cementite) becomes wide. The lower limit was set to 0.05 ° C./second because there is a risk of forming a structure with poor ductility. The upper limit is 5 ° C./second. In consideration of quality stability and the like in industrial production, it is recommended that the temperature be 0.2 ° C./second or more and 3 ° C./second or less.

  In addition, according to the present invention, excellent cold workability can be obtained even with a hot-rolled wire or steel bar, but this wire or steel bar is immersed in a bath of acid (hydrochloric acid, sulfuric acid, etc.) or mechanically. After removing the scale by applying strain, etc., pre-drawing of zinc phosphate coating, calcium phosphate coating, lime, etc., and drawing, cold rolling, etc. using metal soap as a lubricant The same excellent cold workability can be obtained also in a steel wire.

  The wire steel or rod steel of the present invention is not limited in its application. For example, as steel parts for machine structures, bolts, screws, nuts, sockets, ball joints, torsion bars, clutch cases, cages, housings, Hub, cover, case, washer, tappet, saddle, bulg, inner case, clutch, sleeve, outer race, sprocket, core, stator, anvil, spider, rocker arm, body, flange, drum, fitting, connector, pulley, In addition to automobile parts such as metal fittings, yokes, caps, valve lifters, spark plugs, bracket nuts, bracket bolts, universal joints, the present invention can be applied to the manufacture of machine parts, electrical parts, and the like.

  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 is implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all included in the technical scope of the present invention.

  A steel piece obtained by vacuum-melting a test steel containing the chemical components shown in Table 1 below (the balance is iron and inevitable impurities) on a laboratory scale is hot-worked, then adjusted and cooled to a diameter of 20 mm. A linear steel was obtained. In melting the test steel, the amount of O contained in the finally obtained wire steel is approximately the same as the amount of O contained in the wire steel obtained when manufactured with an actual machine. The amount of oxygen was adjusted.

  Hot rolling is performed at a rolling start temperature of 890 ° C. and a rolling temperature of 870 ° C., the adjusted cooling start temperature after final rolling is 810 ° C., and the cooling rate when cooling from the adjusted cooling start temperature to 600 ° C. is 0.00. The temperature was 3 ° C / second.

Table 1 below also shows Z values calculated using the following formula (2) from the amounts of N, B, and Ti contained in the test steel. [] Shows content of each element.
Z value = [N] −1.3 × [B] −0.29 × [Ti] (2)
Note that the examples containing Zr (No.1 in Table 1), was calculated Z ₁ value from the following equation (3) in consideration of the influence of the amount of Zr. The calculation results are shown in Table 1 below.
Z ₁ value = [N] −1.3 × [B] −0.29 × [Ti] −0.15 × [Zr] (3)

  Next, the metal structure of the obtained rolled material was observed as follows.

  When the cross-sectional diameter of the rolled material was D, the metal structure at the D / 8 position was observed at a magnification of 400 times using an optical microscope. As a result of observation, no. 1 to 11, 15 and 16 were mixed structures of ferrite (F) and pearlite (P). No. Nos. 12 to 14 were mixed structures of bainitic ferrite ferrite (BF) and pearlite.

  Moreover, the particle size number of the ferrite which exists in the range from the center (D / 2 position) of a rolling material to D / 8 position was calculated | required by the method prescribed | regulated to JISG0552. The number of measurement points of the ferrite particle size number was three, and the average value was calculated. The calculation results are shown in Table 2 below.

  Next, the obtained rolled material was pickled and scale-removed without spheroidizing annealing, and then a lubricating film was formed, and this was drawn to produce a cold work test wire. The diameter of the cold work test wire was adjusted to 10 mm or 15 mm.

  Using the wire for cold work test, (A) the measurement of deformation resistance of the steel by the compression test, and (B) the crack projected area when compressed, the cold workability of the wire was evaluated.

(A) The deformation resistance of steel is a cylindrical shape obtained by cutting a cold work test wire having a diameter of 10 mm or a diameter of 15 mm so that the length is “1.5 × D” mm when the diameter is D. Using the test piece, an upset compression test was performed using a constrained pressure plate, and measurement was performed. The strain rate during compression was 10 s ⁻¹ , the compression rate was 80%, and the deformation resistance value of the steel at 70% was measured. The test temperatures are room temperature (23 ° C.) and 300 ° C., and the results measured at room temperature are shown as σ _RT (MPa), and the results measured at 300 ° C. are shown as σ ₃₀₀ (MPa) in Table 2 below. The reason why the compression test was performed at 300 ° C. is to simulate the deformation resistance of steel in the processing heat generation region.

Moreover, the reduction rate of the deformation resistance of the steel at 300 ° C. with respect to the deformation resistance of the steel at room temperature was calculated from the following equation (4), and the calculation results are shown in Table 2 below. In addition, in this invention, the case where the reduction rate of the deformation resistance of steel was 12.0% or more was evaluated as the pass, and the case of less than 12.0% was evaluated as the failure.
Reduction rate of deformation resistance of steel (%) = [(σ _RT −σ ₃₀₀ ) / σ _RT ] × 100 (4)
FIG. 1 shows the relationship between the Z value (or Z ₁ value) and the reduction rate of deformation resistance of steel. In FIG. No. 1 in Table 1, ○ is No. in Table 2 below. Examples 2 to 10 and ▲ are Nos. In Table 2 below. The results of the examples 11 to 16 are shown.

Further, as a reference example, the Z value or the Z ₁ value was calculated for some examples disclosed in Patent Document 1, and the reduction rate of the deformation resistance of the steel was calculated. Table 3 shows the chemical composition of steel (the balance is iron and inevitable impurities), and Table 4 shows σ _RT , σ ₃₀₀ , and the deformation resistance reduction rate of steel.

Further, Z value (or Z ₁ value) are also shown a relationship between reduction rate of deformation resistance of the steel in FIG. In FIG. It is a result of the example of 21-24.

  (B) The crack projection area is obtained by taking a photograph of the side surface of the test piece compressed at a compression ratio of 80%, and analyzing the image to binarize the cracked part (cracking part) and the rest, and determine the area ratio of the cracked part. Calculated.

  Photography was on one side of the compressed specimen. The photograph which image | photographed the side surface of the test piece is shown to Fig.2 (a)-FIG.7 (a). FIG. 4 side, FIG. No. 6 side, FIG. 7 side surface, FIG. No. 8 side, FIG. No. 15 side, FIG. It is a drawing substitute photograph which each image | photographed 16 sides.

  For image analysis, “Photoshop (software name)” manufactured by Adobe was used, and the threshold value for binarization was set to 20/255 to 60/255, and the cracked portion and the others were binarized.

  No. in Table 2 below. FIGS. 2 to 7B show the results obtained by binarizing the photographs obtained by photographing the side surfaces of 4, 6, 7, 8, 15, and 16 by image analysis.

  For the area ratio of the cracked portion, the ratio of the area of the cracked portion to the area of the side surface of the photographed test piece (the crack projected area) was calculated. The calculation results are shown in Table 2 below.

  In the present invention, when the crack projected area (area ratio of the crack portion) is 5% or more, the cold workability was evaluated as bad, and when it was less than 5%, the cold workability was evaluated as good. In addition, the crack projection area was measured only about the case where the wire for cold work tests whose diameter is 15 mm was used.

  From Tables 1 and 2, it can be considered as follows. No. 1 is an example of simulating the wire steel disclosed in Patent Documents 1 and 2 described above. Since the amount of solute N is controlled using Zr and B, when manufactured assuming an actual machine, The reduction rate of the deformation resistance of the steel in the processing heat generation region with respect to the deformation resistance of the steel at room temperature is small.

  No. 2 to 10 are examples that satisfy the requirements defined in the present invention. The amount of solute N is controlled using Ti and B, and the balance of N, B, and Ti content is appropriately adjusted. Therefore, even when manufactured assuming an actual machine, It is possible to greatly reduce the deformation resistance of the steel in the heat generation region during cold working for the deformation resistance. In addition, the occurrence of cracks during cold working can be prevented. Therefore, steel excellent in cold workability is obtained.

  No. 11 to 16 are examples that do not satisfy any of the requirements defined in the present invention, and the reduction rate of the deformation resistance of the steel in the processing heat generation region with respect to the deformation resistance of the steel at room temperature is small. Moreover, the crack has generate | occur | produced at the time of cold processing. In particular, no. Nos. 11 to 14 and 16 have an excessive amount of N in comparison with the amounts of B and Ti, resulting in an increase in the amount of dissolved N, dynamic strain aging occurs during cold working, and the deformation resistance of steel in the heat generation region is low. It is getting bigger.

  No. 21 to 24 are steels previously proposed by the present applicant. When these examples are compared with the present invention examples (Nos. 2 to 10), the present invention examples are more resistant to deformation of steel at room temperature. It is possible to increase the reduction rate of deformation resistance of steel in the heat generation region during cold working.

FIG. 1 is a graph showing the relationship between the Z value (Z ₁ value) and the reduction rate of deformation resistance of steel. FIG. 4 is a diagram showing a state when a crack projection area is measured for No. 4. FIG. FIG. 6 is a diagram showing a state when a crack projection area is measured for No. 6. FIG. FIG. FIG. 7 is a diagram showing a state when a crack projection area is measured for 7; FIG. 8 is a diagram showing a state when a crack projection area is measured for No. 8. FIG. FIG. It is the figure which showed the mode when the crack projection area was measured about 15. FIG. FIG. It is the figure which showed the mode when the crack projection area was measured about 16. FIG.

Claims (2)

C: 0.05 to 0.35% (meaning mass%, the same shall apply hereinafter),
Si: 0.3% or less (excluding 0%),
Mn: 0.15 to 1.8%,
P: 0.015% or less (excluding 0%),
S: 0.02% or less (excluding 0%),
Cr: 0.01 to 0.5%
sol. Al: 0.01 to 0.06%,
N: 0.0005 to 0.006%,
B: 0.0003 to 0.0015%,
Ti: 0.003 to 0.030% is satisfied, N, B and Ti satisfy the following formula (1),
The balance is steel consisting of iron and inevitable impurities,
The steel has a mixed structure of ferrite and pearlite,
When the diameter of this steel is D, the grain size number of ferrite existing in the range from the center of the steel to the D / 8 position is No. 6-12, and a linear steel capable of omitting spheroidizing annealing Or bar steel.
−0.0060 ≦ [N] −1.3 × [B] −0.29 × [Ti] ≦ 0.0020
(In the formula, [] represents the content of each element.) A machine part obtained by using the linear steel or rod-shaped steel according to claim 1.