JP4669300B2 - Steel wire rod excellent in cold forgeability after spheroidizing treatment and method for producing the same - Google Patents

Description

  The present invention reduces the spheroidizing annealing time applied to the material before cold working when manufacturing bolts, nuts, screws, gears, and other mechanical parts by cold working, and after spheroidizing treatment. The present invention relates to a steel wire rod having excellent cold workability and a manufacturing method thereof. It should be noted that the concept of the steel wire that is the subject of the present invention includes so-called “burn-in coils”.

Cold working is used in a wide range of fields because of its high productivity, excellent dimensional accuracy of products, and good yield of steel materials. The steel material used for such cold working is required to have low tensile strength and high ductility because it undergoes large deformation locally. This is because, when the cold workability of the steel material is poor, generation of defective products due to material cracking, breakage of the tool die, and the like occur, and the economic efficiency deteriorates. For these reasons, spheroidizing annealing is performed as one of the measures for improving the cold workability.
The purpose of spheroidizing annealing is to spheroidize and uniformly disperse carbides in the steel, thereby improving cold workability, machinability, and wear resistance of the final product. However, spheroidizing annealing generally requires a processing time of 10 to 20 hours, and a reduction in time is required from the viewpoint of improving productivity and reducing energy costs. However, in order to shorten the time for spheroidizing annealing, it is a necessary condition to have excellent cold workability, which is a basic characteristic after spheroidizing treatment.

Various techniques for shortening the spheroidizing annealing time of steel materials have been developed so far. For example, Patent Document 1 described below proposes a method for converting the structure before spheroidizing treatment into a bainite single phase. Yes. This technique is intended to promote the spheroidization of cementite and shorten the time by making the structure before the spheroidizing treatment into a bainite single phase.
On the other hand, Patent Document 2 below discloses a means in which the pro-eutectoid ferrite fraction is 5 to 30% by area, the remaining structure is mainly bainite, and the average value of lath spacing of bainite is 0.3 μm or more. Thereby, the processing performance after spheroidizing treatment can be improved and the deformation resistance can be reduced.
JP 60-9832 A JP 2001-89830 A

Although the technique described in Patent Document 1 shortens the time for spheroidizing annealing, the bainite single-phase structure has a high deformation resistance during cold working after spheroidizing treatment, resulting in a tool die. The problem of reduced life is still not solved.
The technique described in the above-mentioned Patent Document 2 realizes the structure by performing gradual cooling in the heat treatment after hot rolling, which leads to a decrease in productivity and an increase in cost. In addition, when the bainite fraction becomes excessive, there is a problem that deformation resistance at the time of cold working after spheroidizing treatment becomes high.

  The present invention has been made under such circumstances. The purpose of the present invention is to shorten the spheroidizing annealing time, improve the processing performance after spheroidizing treatment and reduce deformation resistance, and such steel. It is to provide a useful method for producing a wire.

The present invention achieves the above object, and the gist thereof is as follows.
(1) The steel wire rod of the present invention that has achieved the above-mentioned object is: C: 0.005 to 0.6% (mass%, the same shall apply hereinafter ) , Si: 0.50% or less, Mn: 0.20 1.00%, Al: 0.01 to 0.06%, P: 0.02% or less, S: 0.02% or less, N: 0.01% or less, the remainder being Fe and inevitable impurities The pseudo pearlite is 10 area% or more, the bainite is 75 area% or less, the ferrite is 60 area% or less, and the relationship of (pseudo pearlite area + bainite area% + ferrite area%) ≧ 90 area% is satisfied. Features.
(2) Further, in the steel wire of the present invention, Cr: 1.50% or less, Mo: 0.50% or less, Ni: 1.00% or less, V: 0.50% or less, B: 0.005 % Or less, Ti: containing one or more elements selected from the group consisting of 0.05% or less, and the remainder containing Fe and inevitable impurities can be applied.

(4) In manufacturing the steel wire rod of the present invention, after hot rolling, the temperature is set to 750 to 1000 ° C., and the temperature from 750 to 1000 ° C. to 400 to 550 ° C. is increased at a rate of 20 ° C./s or more. It is preferable to cool, hold at 400 to 550 ° C. for 20 seconds or longer, complete the isothermal transformation, and then cool.

The present invention is configured as described above, and can shorten the spheroidizing annealing time, achieve improved processing performance and reduced deformation resistance after spheroidizing treatment, and exhibits excellent cold forgeability. Steel wire rod can be realized.
In the present invention, the area percentage of pseudo pearlite, bainite and ferrite is defined as a desirable range, so that it is possible to balance processing performance and deformation resistance, and obtain a steel wire rod exhibiting excellent cold forgeability. be able to.

  Further, in the present invention, it is possible to shorten the spheroidizing annealing time by processing at a desired temperature and conditions, and cooling after completing the isothermal transformation, and improving the processing performance and deformation after the spheroidizing process. There is an effect that a reduction in resistance can be achieved and a steel wire rod exhibiting excellent cold forgeability can be manufactured.

The present invention will be described in detail below based on the best mode.
As a result of studying the optimum front structure for satisfying both the reduction of the spheroidizing annealing time of the steel material and the improvement of the processing performance and the reduction of the deformation resistance after the spheroidization, the present inventors have found that the pseudo pearlite is 10 Area% or more, bainite is 75 area% or less, ferrite is 60 area% or less, and (pseudo pearlite area% + bainite area% + ferrite area%) ≧ 90 area% for shortening the spheroidizing annealing time Proved effective.

In the steel wire of the present invention, the pseudo pearlite fraction needs to be 10 area% or more. This is because the cementite contained in the pseudo pearlite structure is rapidly spheroidized, so that the processing performance is improved in cold working after a short spheroidizing treatment.
Pseudo pearlite is transformed at low temperature and carbon diffusion is not enough, so cementite is granular or shows a discontinuous form, ferrite and cementite are layered, and the general pearlite structure formed by transformation at high temperature Different. The form of cementite in this pseudo pearlite is as follows.
The ratio of the major axis / minor axis is in the range of 1 to 20, the average value of the total number of measured cementites is 2.0, and the ratio of the major axis / minor axis ratio is 3.0 or less is the total measured cementite. The number percentage of the number is 90% or more. These cementites are adjacent to each other at an interval of a maximum of 1 μm and an average of 0.05 μm. Such a region is defined as a pseudo pearlite structure.
This observation is performed with a scanning electron microscope at a magnification of 5000 times to measure the major axis, minor axis, and cementite interval of 100 or more cementites.

As described above, the pseudo pearlite contains a large amount of granular cementite. Therefore, when pseudo pearlite is contained in an area of 10 area% or more, spheroidization of cementite is promoted during annealing, and the processing performance is improved in the subsequent cold working even as a short spheroidization treatment.
The steel wire rod of the present invention may contain bainite and ferrite. In the bainite structure, cementite is uniformly dispersed, and the processing performance is improved in the cold working after the spheroidizing treatment. However, since bainite is a hard phase, if it exists in a large amount, the deformation resistance becomes high. Therefore, bainite is 75 area% or less.
On the other hand, ferrite is 60 area% or less. When the ferrite fraction is high, the deformation resistance during cold working is reduced, but the dispersibility of spherical cementite is deteriorated and the working performance is lowered. For this reason, a ferrite fraction shall be 60 area% or less.

Furthermore, in the steel wire rod of the present invention, it is necessary to satisfy (pseudo pearlite area% + bainite area% + ferrite area%) ≧ 90 area%.
This is because when there is a lot of layered pearlite structure generated by transformation at high temperature, the cementite in the form of a plate is not rapidly spheroidized, and the processing performance is reduced in cold working after spheroidizing treatment. Because it ends up. Further, since the isothermal transformation is not completed and a large amount of hard martensite is generated, the deformation resistance becomes high. Therefore, it is necessary to satisfy (pseudo pearlite area% + bainite area% + ferrite area%) ≧ 90 area%. is there.

Each condition in the production method of the present invention will be described below.
In the production method of the present invention, after hot rolling, it is scraped at 750 to 1000 ° C. Here, if the winding temperature is less than 750 ° C., it is difficult to scrape in a ring shape, so 750 ° C. is set as the lower limit. In addition, when the temperature exceeds 1000 ° C, the oxide scale increases, and yield loss is caused at a supplier who uses the steel wire rod according to the present invention after processing it into a specific product. Is preferred.
After rolling the rolled material, the temperature from 750 to 1000 ° C. to 400 to 550 ° C. is cooled at a rate of 20 ° C./s or more, and held at 400 to 550 ° C. for 20 seconds or more to complete the isothermal transformation. The constant temperature holding temperature is set to 400 to 550 ° C. If the temperature is lower than 400 ° C., it takes a long time for transformation, and a large amount of bainite and martensite may be generated in the cooled structure. And If the temperature exceeds 550 ° C., the layered pearlite structure increases rapidly, requiring a long time for the spheroidizing annealing time, and lowering the processing performance, so 550 ° C. is set as the upper limit temperature. The holding time is set to 20 seconds or longer in order to complete the constant temperature transformation. The upper limit is not particularly specified, but is preferably within 150 seconds from the viewpoint of improving productivity.

The steel wire rod according to the present invention contains 0.005 to 0.60% of C. When other elements are added in addition to C, Si: 0.50% or less, Mn: 0.20 to 1.00%, Al: 0.01 to 0.06%, P: 0.02% or less , S: 0.02% or less, N: 0.01% or less. The reasons for limiting the ranges of these elements are as follows.
C: 0.005 to 0.60%
C is prescribed | regulated by the intensity | strength calculated | required by the goods formed from the steel wire rod of this invention, such as a volt | bolt and a screw | thread. If less than 0.005%, the required strength cannot be obtained, so the lower limit is made 0.005%. On the other hand, if it exceeds 0.60%, the cold workability and toughness are reduced, so 0.60% is made the upper limit.
Si: 0.50% or less Si is added as a deoxidizing material during steelmaking and for the purpose of increasing the strength of the final product by solid solution hardening. However, if added in a large amount, the strength is significantly increased and the toughness is deteriorated. The upper limit is 0.50%.
Mn: 0.20 to 1.00%
Mn is added as an element that enhances the hardenability of the steel material and improves the strength, but in order to exert its effect, it is necessary to contain 0.20% or more. On the other hand, if the addition amount is excessive, cold forgeability and toughness are reduced, so the upper limit is made 1.00%.

Al: 0.01% to 0.06%
Al has the effect of fixing N and suppressing dynamic strain aging during cold forging and reducing deformation resistance. In order to obtain this effect, it is necessary to contain at least 0.01%. However, the upper limit is set to 0.06% because the toughness is reduced if excessively contained.
P: 0.02% or less, S: 0.02% or less
P and S are components inevitably contained. P causes grain boundary segregation and center segregation in steel and deteriorates toughness, so it is necessary to make it 0.02% or less. Since S is a harmful element in cold working, it needs to be made 0.02% or less.
N: 0.01% or less
N causes dynamic strain aging during cold forging, leading to an increase in deformation resistance and a reduction in processing performance. Therefore, N is set to 0.01% or less.

The basic chemical composition of the steel wire rod of the present invention is as described above. In addition to the above composition, Cr: 1.50% or less, Mo: 0.50% or less, Ni: 1.00% or less V: 0.50% or less, B: 0.005% or less, Ti: containing one or more elements selected from the group consisting of 0.05% or less, improving hardenability and strength of cold forging Advantages such as improvement are obtained.
Cr: 1.50% or less, Mo: 0.50% or less, Ni: 1.00% or less Cr, Mo and Ni are effective elements for improving the hardenability. However, if it is contained excessively, ductility is deteriorated, so it is suppressed within the above range.
V: 0.50% or less
V may be added for the purpose of precipitation strengthening. However, if added in a large amount, it causes deterioration of ductility, so it is suppressed within the above range.

B: 0.0050% or less, Ti: 0.05% or less B is an element that improves hardenability, and may be added if necessary. However, if excessively contained, the toughness is degraded, so the upper limit is made 0.005%. Ti is an element effective in reducing deformation resistance during cold forging due to the effect of suppressing dynamic aging by fixing solute N, and may be added as necessary. However, if it is contained excessively, coarse TiN precipitates and cracks starting from this coarse TiN are likely to occur, so the upper limit is made 0.05%.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and any design changes may be made as appropriate in accordance with the above-described gist or the gist described below. It is included in the technical scope of the invention.

The steel having the components shown in Table 1 was rolled into a wire structure as shown in Table 2 by applying the cooling rate and the constant temperature holding time shown in Table 2.
The wire was rapidly spheroidized and annealed immediately below the AC1 point (heated to 150 ° C./Hr immediately below the AC1 point, held at 1 Hr immediately below the AC1 point, and then air-cooled). In Table 1, Ceq = C% + (1/3) Si% + (1/6) Mn%.

Using a test piece cut out from the wire after spheroidizing annealing (ratio of diameter d to height of cylinder: h / d = 1.5), a cold upsetting test was carried out, cracking, deformation resistance Was measured. The cold uptake test was conducted in accordance with JCFCS-1980 standards. Deformation resistance was measured under the conditions of a compression rate of 70% and a strain rate = 10S- ¹ . In Table 2, Ceq = C% + (1/3) Si% + (1/6) Mn%.
Table 2 shows the results of determining the cold forgeability after rapid spheroidizing annealing from those results. In Table 1, steels A to E are within the scope of the present invention, but steel F is a sample in which Cr and Ni are higher than the scope of the present application, and steel G is a sample in which Cr is high.

In Sample Nos. 2, 8, 14, 20, and 26 shown in Table 2, the constant temperature holding time is insufficient and the transformation is not completed, and a large amount of hard structure such as martensite is generated. For this reason, the deformation resistance at the time of cold forging after the spheroidizing treatment is increased, and is outside the range specified in the present invention.
Sample Nos. 5 and 11 shown in Table 2 have a retention temperature lower than 400 ° C., so a large amount of martensite is generated. When compared with the same Ceq, the deformation resistance during cold forging after spheroidization is high. . Therefore, it is outside the range defined by the present invention.
Sample Nos. 17, 23, and 29 shown in Table 2 have a holding temperature of less than 400 ° C., so that when bainite exceeds 75 area% and compared with the same Ceq, deformation resistance during cold forging after spheroidizing treatment Is expensive. Therefore, it is outside the range defined by the present invention.
Samples Nos. 6, 12, 18, 24, and 30 shown in Table 2 have a two-phase structure of ferrite and pearlite because the cold speed is 20 ° C./s or less, and spheroidization is insufficient in a short time. For this reason, the processing performance at the time of cold forging after the spheroidizing treatment is low, and is outside the range specified in the present invention.

In Sample Nos. 31 to 36, since Ni exceeds the range defined in the present invention, (pseudo pearlite area% + bainite area% + ferrite area%) ≧ 90 area% is not satisfied. This is because a lot of martensite is generated. When compared with equivalent Ceq (steel D), the deformation resistance during cold forging after spheroidization is high, and is outside the range specified in the present invention.
In Sample Nos. 37 to 41, Cr exceeds the range specified in the present invention (pseudo pearlite area% + bainite area% + ferrite area%) ≧ 90 area%. This is because a lot of martensite is generated. When compared with equivalent Ceq (steel D), the deformation resistance during cold forging after spheroidization is high, and is outside the range specified in the present invention.
Sample No. 42 shown in Table 2 contains Cr exceeding the range specified in the present invention, and therefore the bainite ratio exceeds 75 area%. When compared with the equivalent Ceq (C steel), the deformation resistance at the time of cold forging is high, which is outside the range specified in the present invention.
Sample Nos. 1, 3, 4, 7, 9, 10, 13, 15, 16, 19, 21, 22, 25, 27, and 28 shown in Table 2 satisfy the conditions defined in the present invention. . Even after a rapid spheroidizing annealing time, it has the characteristics of high processing performance and low deformation resistance.
Samples corresponding to the examples other than these satisfy the conditions defined in the present invention. Even after a rapid spheroidizing annealing time, the processing performance is high and the deformation resistance is low.

Claims (4)

C: 0.005-0.6% (mass%, the same shall apply hereinafter ) , Si: 0.50% or less, Mn: 0.20-1.00%, Al: 0.01-0.06%, P: 0.02% or less, S: 0.02% or less, N: 0.01% or less, the remainder contains Fe and inevitable impurities , pseudo pearlite is 10 area% or more, bainite is 75 area% or less, A steel wire material excellent in cold forgeability after spheroidizing treatment, wherein the ferrite satisfies the relationship of 60 area% or less and (pseudo pearlite area% + bainite area% + ferrite area%) ≧ 90 area%.   In addition to the steel component regulation, the steel component is further divided into Cr: 1.5% or less, Mo: 0.50% or less, Ni: 1.00% or less, V: 0.50% or less, B: 0.00. The steel wire rod excellent in cold forgeability after spheroidizing treatment according to claim 1, wherein one or more elements selected from the group consisting of 0050% or less and Ti: 0.05% or less are regulated. . C: 0.005-0.6% (mass%, the same shall apply hereinafter ) , Si: 0.50% or less, Mn: 0.20-1.00%, Al: 0.01-0.06%, P: 0.02% or less, S: 0.02% or less, N: 0.01% or less, the remainder contains Fe and inevitable impurities , pseudo pearlite is 10 area% or more, bainite is 75 area% or less, A steel wire rod excellent in cold forgeability after spheroidizing treatment, wherein the ferrite satisfies the relationship of 60 area% or less and (pseudo pearlite area% + bainite area% + ferrite area%) ≧ 90 area% In production, after hot rolling, after cutting at 750 to 1000 ° C., 750 to 1000 ° C. to 400 to 550 ° C. are cooled at a rate of 20 ° C./s or more and held at 400 to 550 ° C. for 20 seconds or more. , Cool after completing the isothermal transformation By this, the manufacturing method of the steel wire material excellent in the cold forgeability after the spheroidization process characterized by shortening the annealing time after spheronization. In addition to the steel component regulation, the steel component is further divided into Cr: 1.5% or less, Mo: 0.50% or less, Ni: 1.00% or less, V: 0.50% or less, B: 0.00. The steel wire rod excellent in cold forgeability after spheroidizing treatment according to claim 3, wherein one or more elements selected from the group consisting of 0050% or less and Ti: 0.05% or less are regulated. Manufacturing method.