JP2000239795A - Rolled wire rod excellent in softening characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the coarsening of ferrite grain size in the surface layer of a rolled wire rod and to obtain a wire rod excellent in softening characteristic by controlling the difference between the ferrite grain size in the surface layer and that in the inner part to a specific value or below. SOLUTION: The difference between the ferrite grain size in the surface layer and that in the inner part is controlled to 1.0 or less. It is preferable that the rolled wire rod has a chemical composition consisting of, by mass, 0.01-0.25% C, <=0.5% (not including 0%) Si, <=2.0% (not including 0%) Mn, and the balance iron with inevitable impurities and further containing at least either of <=0.05% (not including 0%) Al and <=0.01% (not including 0%) B. In this method of manufacturing the rolled wire rod, the difference between finish rolling temperature and coiling temperature is controlled to <=200 deg.C at the time of hot rolling and also the finish rolling temperature is controlled to <=900 deg.C. By this method, the rolled wire rod can be efficiently manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rolled wire having excellent softening and annealing properties and a method for efficiently producing the rolled wire. INDUSTRIAL APPLICABILITY The rolled wire rod of the present invention is suitably used for manufacturing parts such as bolts and nuts manufactured by a cold header, and is extremely useful in that cracking during a cold header can be effectively prevented.

[0002]

2. Description of the Related Art Parts manufactured by cold headers, such as bolts and nuts, are usually made into wires by hot rolling carbon steel or alloy steel, and then pickled or mechanically descaled. After the annealing treatment, the wire is drawn and formed into a predetermined shape by a cold header. Further, if necessary, a skin pass may be performed after a softening treatment such as spheroidizing annealing is performed before the cold header.

[0003] It is essential that the parts manufactured in this way do not crack due to the cold header. However, if the rolled wire is annealed before drawing, the ferrite grain size of the surface layer of the rolled wire may become coarse after annealing. This ferrite coarsening phenomenon is particularly noticeable when annealing a rolled wire having a C content of 0.20% by mass or less and a ferrite fraction of 70% or more. In order to cause problems such as induction, coarsening of the surface ferrite should be avoided as much as possible.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to prevent the ferrite grain size in the surface layer of a rolled wire from becoming coarse and to provide excellent softening and annealing characteristics. An object of the present invention is to provide a rolled wire and a method capable of efficiently producing such a rolled wire.

[0005]

A rolled wire rod according to the present invention which has solved the above-mentioned problems has a structure in which the difference between the ferrite grain number in the surface layer and the ferrite grain number in the inside is controlled to 1.0 or less. Have a gist. In the present invention, the chemical component in steel is C: 0.01 to 0.25% (% by mass,
The same shall apply hereinafter), Si: 0.5% or less (excluding 0%), Mn: 2.0% or less (excluding 0%), balance: iron and inevitable impurities are preferred embodiments, Further, it is more preferable to contain at least one selected from the group consisting of Al: 0.05% or less (excluding 0%) and B: 0.01% or less (excluding 0%).

Further, a method of manufacturing a rolled wire rod according to the present invention which has solved the above problems has a gist in controlling a difference between a finish rolling temperature and a winding temperature during hot rolling to 200 ° C. or less. It is. Here, the finish rolling temperature is 900
It is a preferred embodiment of the present invention to control the temperature to not more than ° C.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have made intensive studies to prevent the ferrite on the surface layer of a rolled wire from becoming coarse after annealing when the rolled wire is annealed before drawing. As a result, it was found that in the structure after rolling, when the ferrite grain size in the surface layer portion is excessively small compared to the ferrite grain size in the inside, the ferrite in the surface layer tends to become coarse. Based on this finding, as a result of further study, in order to prevent ferrite coarsening of the surface layer of the rolled wire rod, the difference between the ferrite particle number of the surface layer and the internal ferrite particle number should be controlled to 1.0 or less. Thus, the present invention has been completed.

[0008] Thus, the rolled wire rod according to the present invention having excellent softening and annealing properties is characterized in that the difference between the ferrite grain number in the surface layer and the ferrite grain number in the inner layer is controlled to 1 or less.

Here, “surface layer” means a range of 0.5 mm depth from the surface in the circumferential direction, and “inside” means a part excluding the above “surface layer”. The ferrite grain size number is measured based on a ferrite grain size test method described in JIS G 0552.

In the rolled wire rod of the present invention, the difference between the ferrite grain number of the surface layer and the ferrite grain number of the inner layer may be controlled to 1.0 or less. Is not particularly limited. However, in order to more effectively prevent the coarseness of the surface ferrite and obtain more excellent softening annealing characteristics, as long as the above range is satisfied, the ferrite particle number of the surface layer should be 6 to 11, and the internal ferrite particle number should be 6-11
It is recommended to control within the range.

In order to further improve the pickling property, it is recommended that the ratio of FeO to the entire Fe-based oxide in the scale be controlled to a thickness ratio of 70% or more. When the proportion of FeO in the Fe-based oxide is less than 70%, Fe-based oxides (Fe ₃ O ₄ , Fe ₂ O _3, etc.) that are hardly soluble in acids
Is high, and even if the scale thickness is reduced to 10 μm or less, the pickling property deteriorates. In order to more stably exhibit good pickling properties, the ratio of FeO in the Fe-based oxide is preferably set to 80% or more. Further, in order to obtain more excellent pickling properties, it is preferable to control the scale thickness to 10 μm or less. If the scale thickness is reduced to 10 μm or less, it is expected that there is no need to perform a pickling treatment before performing a heat treatment such as annealing. More preferably, it is 5 μm or less.

Further, the chemical composition of the steel in the rolled wire of the present invention is not particularly limited, and the steel composition of a commonly used rolled wire can be applied. For example, C: 0.01 to 0.25%,
Si: 0.5% or less (excluding 0%), Mn: 2.0
% Or less (excluding 0%), balance: Iron and the use of rolled wire which is an inevitable impurity are recommended. Further, Al:
0.05% or less (excluding 0%) and / or B: 0.
01% or less (excluding 0%) may be positively added.

Next, a method for producing the rolled wire rod of the present invention will be described. In order to control the difference between the surface ferrite grain size number and the inner ferrite grain size number to 1.0 or less, the difference between the finish rolling temperature and the winding temperature during hot rolling must be adjusted to 200 ° C or less. . The present inventors have studied, coarsening of the ferrite grain size of the surface layer portion, compared to the finish rolling temperature, occurs significantly when the winding temperature is extremely low, due to supercooling only the surface layer portion. Was found to occur. Therefore, in order to prevent the surface ferrite from coarsening, it is necessary to reduce the difference between the rolling finish temperature and the subsequent winding temperature (equal to the cooling start temperature) when cooled on a conveyor. From such a viewpoint, it is up to those differences to be specified at 200 ° C. or less. The smaller the difference between the rolling finishing temperature and the winding temperature, the better.

Further, in order to control the ratio of FeO to the total Fe-based oxide in the scale to be 70% or more in thickness in order to impart excellent pickling properties, it is particularly preferable that the thickness be in the range of 600 to 4%.
It is recommended to cool the temperature range of 00 ° C at an average cooling rate of 0.5 ° C / sec or more. If the cooling is performed at an average cooling rate of less than 0.5 ° C./sec, the desired scale composition cannot be obtained. Preferably, the average cooling rate at 600 to 400 ° C is 1 ° C / sec or more, more preferably 1.5 ° C / sec.
It is recommended that the temperature be set to at least ° C / sec. The upper limit is not particularly limited, but may be 1 in consideration of prevention of increase in strength.
It is recommended to control the temperature to 0 ° C./sec or less, more preferably 5 ° C./sec or less.

In order to further improve the pickling property, in order to adjust the scale thickness to 10 μm or less, the finish rolling temperature is 900 ° C. or less (more preferably 850 ° C. or less) and the winding temperature is 800 ° C. It is preferable to control the temperature to below (more preferably 775 ° C. or lower). Finish rolling temperature: over 900 ° C,
This is because if the winding temperature is higher than 800 ° C., the scale thickness becomes large and exceeds the preferable upper limit of 10 μm. Therefore, in order to produce a rolled wire rod whose scale composition and scale thickness satisfy the requirements of the present invention, 90%
After finish rolling at 0 ° C. or lower, winding is performed at a temperature of 800 ° C. or lower, and a cooling speed (about 0.5 to 10 ° C. /
After cooling in (sec), it is recommended to cool in a temperature range of 600 to 400 ° C. at an average cooling rate of 0.5 ° C./sec or more. Here, adjusting the difference between the finish rolling temperature and the winding temperature when cooling on the conveyor (equal to the cooling start temperature) to 200 ° C. or less (more preferably 100 ° C. or less) means that the pickling property is reduced. This is extremely useful because it can be further increased.

Hereinafter, the present invention will be described in detail based on examples. However, the following examples do not limit the present invention, and all modifications and implementations without departing from the spirit of the preceding and the following are included in the technical scope of the present invention.

[0017]

[Example] JIS standard SWRCH12K and SWRC
Using H20A, hot rolling conditions (winding temperature, average cooling rate from winding temperature to 600 ° C., 600 to
Each rolled wire was manufactured by varying the average cooling rate at 400 ° C. (wire diameter: 6 mm). In order to confirm the tendency of ferrite crystal grains to become coarse after annealing, 650 ° C. × 2
An hr annealing experiment was performed to measure the ferrite grain size number in the surface layer portion and the inside based on the ferrite crystal grain size test method described in JIS G 0552, and to observe the tendency of the surface layer portion to have a ferrite crystal grain coarsening tendency.

These results are summarized in Table 1.

[0019]

[Table 1]

In the table, Nos. 6, 7, 16 and 17 all have a ferrite grain size number of the surface layer and an internal ferrite grain size because the difference between the finish rolling temperature and the winding temperature during hot rolling exceeds 200 ° C. In these examples, the difference in the number exceeded No. 1.0, and the ferrite coarsening of the surface layer was observed in each case.

On the other hand, No. satisfying the requirements of the present invention.
Nos. 1 to 5, 8 to 10; Nos. 11 to 15; and Nos. 18 to 2
0, no ferrite coarsening of the surface layer was observed in any case.

Further, the average cooling rate in the temperature range of 600 to 400 ° C. is 0.5 ° C./sec or more.
Nos. 5, 12 to 15 have no residual scale and are extremely excellent in pickling properties.

On the other hand, Nos. 1 and 11 are 600 to 4
Average cooling rate in the temperature range of 00 ° C is 0.5 ° C / s
Since it is less than ec, the ratio of FeO in the scale is an example that does not satisfy the requirements of the present invention, and it can be seen that in all cases, residual scale is generated and the pickling property is poor. Also, No.
Nos. 6 to 7, 18 to 17 are examples in which the finishing temperature exceeds 900 ° C. and the scale thickness exceeds 10 μm / the proportion of FeO in the scale is low;
No. 9 is an example in which the winding temperature exceeds 800 ° C. and the scale thickness exceeds 10 μm;
Because the temperature exceeds 000 ° C and the winding temperature exceeds 800 ° C,
This is an example in which the scale thickness exceeds 10 μm / the ratio of FeO in the scale is as low as less than 70%. In each case, residual scale is generated and the pickling property is poor.

[0024]

Since the rolled wire of the present invention is constituted as described above, it is possible to prevent the ferrite grain size in the surface layer of the rolled wire from becoming coarse and to provide a rolled wire having excellent softening and annealing properties. it can.

Claims (5)

[Claims] 1. A rolled wire excellent in soft annealing characteristics, wherein a difference between a ferrite grain size number in a surface layer and an inner ferrite grain size number is controlled to 1.0 or less. 2. The chemical composition in steel is as follows: C: 0.01 to 0.25% (mass%, hereinafter the same), S
i: 0.5% or less (excluding 0%), Mn: 2.0
% Or less (not including 0%), balance: iron and inevitable impurities, the rolled wire according to claim 1. 3. Al: 0.05% or less (excluding 0%) and B:
The rolled wire rod according to claim 2, comprising at least one selected from the group consisting of 0.01% or less (excluding 0%). 4. The method for producing a rolled wire according to claim 1, wherein a difference between a finish rolling temperature and a winding temperature during hot rolling is controlled to 200 ° C. or less. 5. The method for producing a rolled wire according to claim 4, wherein the finish rolling temperature is controlled to 900 ° C. or lower.