JP2000256741A - Manufacture of hot rolled bar or wire - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method by which a bar and a wire which are as hot- rolled, free from any abnormal structure comprising a decarburization phase on a surface layer part, and of spheroidizing structure in surface layer part and of finely soft structure inside are efficiently manufactured. SOLUTION: A stock steel slab is heated to 900-1250 deg.C, and rolled at a final stand of an intermediate rolling line of a rolling mill comprising a multi-stand rough rolling line, the intermediate rolling line, and a finish rolling line at 700-900 deg.C of the surface temperature of a material to be rolled, and with the reduction of area of >=20%. The forced cooling to cool the material at the cooling speed of >=5 deg.C/second is achieved at least once before the final finish rolling so that the surface temperature of the material is not higher than Ms point. The finish rolling is achieved with the reduction of area of >=5% immediately after the surface temperature of the material after the forced cooling is restored to 650-750 deg.C, or the material is heated at the heating speed of >=5 deg.C/ second to keep its surface temperature at 650-750 deg.C, the material after the finish rolling is kept at least for five minutes in the temperature range of 650-750 deg.C in surface temperature, and cooled in the atmosphere.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the manufacture of steel bars and wires (hereinafter also referred to as bars and wires) by hot rolling using a rolling mill comprising a multi-stand rough rolling row, an intermediate rolling row, and a finishing rolling row. In relation to the method, more specifically, a hot rolled rod having a spherical structure in which the surface layer portion is uniformly dispersed with a spherical carbide and a fine soft structure in other portions, and having no abnormal structure in which the surface layer portion has a decarburized phase. The present invention relates to a method capable of manufacturing a wire rod with high efficiency.

[0002]

2. Description of the Related Art Piano wires specified by JIS G 3502 manufactured by hot rolling, carbon steel wires for cold heading specified by G 3507, and G 4051, G 4104, and G 4105 Carbon steels and alloy steels for machine structural use, bearing steels specified in G 4805, spring steels specified in G 4801, etc.
Rods and wires based on tool steel etc. specified in G 4401, G 4401 etc.
Through cold working processes such as drawing, cutting, forging, and cutting, the product can be finished to the required product.

[0003] However, the hot-rolled bar
Many of the wires usually have a hard phase such as a pearlite phase, a bainite phase or a martensite phase in their structure, and are inferior in cold workability. Therefore, in the production of these rods and wires, a method has been adopted in which softening heat treatment such as annealing or spheroidization is performed after hot rolling to reduce the strength, thereby increasing ductility and improving cold workability.

However, the above softening heat treatment involves 10 to 10
It often requires as long as 20 hours. For this reason, from the viewpoint of productivity improvement and energy saving, it has been strongly desired to develop a method capable of shortening the heat treatment time and omitting steps.

[0005] Therefore, it is possible to change the rolling structure by variously adjusting the rolling conditions and cooling conditions during continuous hot rolling of a rod or a wire rod, to obtain a softened structure while hot rolling and to shorten the heat treatment time. Various methods have been proposed (for example, JP-A-56-41325 and JP-A-60-2559).
No. 22, No. 62-139817, No. 64-5
No. 5330, JP-A-4-173921).

That is, of the above publications,
The method disclosed in Japanese Patent No. 41325 discloses a process of rapidly cooling a wire to 700 to 850 ° C. in a controlled cooling stage after hot rolling, and completes progress of recrystallization by preventing supercooling and reheating during the rapid cooling process. And cooling at a cooling rate of 5 ° C./sec or less to increase the lamellar spacing of the pearlite phase and shift the supersaturated carbon in the ferrite phase to an equilibrium state.
After cooling at a cooling rate of 0.025 to 0.25 ° C./sec to a cooling rate of 0.025 to 0.25 ° C., a rapid cooling step is continuously performed at an optional cooling rate. Is a good method. According to this method, a wire having a softened structure can be obtained as it is during hot rolling. However, in this method, just before the end of the regulated cooling, 6
The temperature range of 00 to 675 ° C is set to 0.025 to 0.25 ° C /
Since cooling is performed at a very slow cooling rate of 2 seconds, the decarburization phase cannot be avoided on the surface layer of the material, and there is a disadvantage that only products having an abnormal structure on the surface layer can be obtained. I have.

[0007] In the method disclosed in Japanese Patent Application Laid-Open No. 60-255922, finish rolling is performed in two parts, the wire after the first finish rolling is rapidly cooled to more than 500 ° C and 850 ° C or less, and then the second finish is rolled. Rolling is performed to give a plastic strain of 20 to 80%, and a cooling rate of 1 to 10 is maintained while maintaining the plastic strain.
This is a method of cooling at a rate of ° C./sec. According to this method, a wire rod having a structure in which a fine pearlite phase or a bainite phase is mixed in a finely dispersed proeutectoid ferrite phase can be obtained, and the time required for spheroidizing heat treatment after hot rolling can be reduced. Is possible. However, this method requires that the steel
In the case of a highly hardenable alloy steel containing a large amount of r, Mo, Ni, Mn, etc. (for example, SCM430, SCM440, etc.), since there are many bainite phases which are hard phases, the tensile strength as rolled is very high. High and difficult to soften,
There is a drawback that only products that are difficult to wire-draw while being rolled can be obtained.

In the method disclosed in Japanese Patent Application Laid-Open No. Sho 62-139817, the finish rolling is started at 650 to 850 ° C., finished at less than 950 ° C., and immediately after the completion of rolling, 650 to 850 ° C.
This is a method of rapidly cooling to 750 ° C., winding up non-concentrically, and then cooling to 500 ° C. at a cooling rate of 0.5 ° C./sec or more. According to this method, a wire having a mixed structure of a fine ferrite phase and a pearlite phase can be obtained, and the time required for spheroidizing heat treatment after hot rolling can be shortened. However, in this method, when the steel is the same high hardenability alloy steel as described above, when the cooling rate is 0.5 ° C./sec or more, a bainite phase or a martensite phase, which is a hard phase, is generated. It does not form a mixed structure of fine ferrite phase and pearlite phase, and as in the above case, the tensile strength as rolled is extremely high, making it difficult to soften, making it difficult to wire-draw as rolled It has the disadvantage that only suitable products can be obtained.

In the method disclosed in Japanese Patent Application Laid-Open No. 64-55330, a raw steel slab is heated to a temperature of ₃ or more Ac or higher than an Accm point and subjected to hot rolling, while at least the austenite unrecrystallized region is obtained at the finish rolling stage. With a processing rate of 10 to 80
%, And then cooling to a temperature range just above the Ms point at a cooling rate of 1.5 to 100 ° C./sec. According to this method, it is possible to shorten the time required for the spheroidizing heat treatment after the hot rolling. However, in this method, when the steel is an alloy steel having high hardenability, cooling after finish rolling is extremely fast, so that bainite or martensite structure, which is a hard phase, is generated, and a fine ferrite phase and a pearlite phase are formed. It has a drawback that it does not form a mixed structure, has a very high tensile strength as rolled, is difficult to soften, and can only obtain a product that is difficult to wire-draw as rolled.

[0010] The methods disclosed in JP-A-4-173921, the first heating and holding less than one minute to 20 hours material steel pieces Ac ₁ point ~Acm point or Ac below ₃ points, Acm point or Ac ₃ The heating is performed in two steps of a second heating to form a mixed structure of an undissolved carbide and an austenite phase by heating to a temperature below 1050 ° C., while the finish rolling is performed at an Ar ₃ point or an Arcm point
After finishing by adding 30% or more processing in the temperature range of Ac ₃ point or Acm point, hold at 660 to 740 ° C for 30 minutes or more, or cool from rolling end temperature to 650 ° C at a cooling rate of 100 ° C / h. It is a method of cooling in less than. According to this method, a wire having a spherical structure can be obtained as it is during hot rolling, and the time required for heat treatment after hot rolling can be shortened or the heat treatment itself can be omitted. However, according to this method, the wire rod after the finish rolling is 660 to 7
Since it is kept in the temperature range of 40 ° C. for a long time, it is unavoidable that a decarburized phase is generated on the surface layer of the material, and only a product having an abnormal structure on the surface layer is obtained. . In addition, when subjected to secondary processing while hot rolling,
In order to obtain a spherical structure finally required in the secondary processing, there is also a disadvantage that a time longer than that required for normal heat treatment may be required.

[0011]

SUMMARY OF THE INVENTION An object of the present invention is to provide a spherical structure in which the surface layer has a uniform dispersion of spherical carbides, and a fine soft structure in other parts, and an abnormal structure having a decarburized phase in the surface layer. It is an object of the present invention to provide a method capable of producing a hot-rolled steel bar or wire rod without any problem at a high efficiency, and to reduce the time required for heat treatment after hot rolling to a specific value.

[0012]

The gist of the present invention lies in the following method for producing a hot-rolled steel bar or wire.

[0013] A method for producing a hot-rolled steel bar or wire rod, which is sequentially processed under the following conditions:

The raw steel slab is heated to 900-1250 ° C.

The rolling at the final stand of the intermediate rolling row of the rolling mill consisting of a multi-stand rough rolling row, an intermediate rolling row, and a finishing rolling row is performed at a surface temperature of 700 to 90 of the material to be rolled.
It is carried out at 0 ° C. and a surface reduction rate of 20% or more.

Before the finish rolling, at least one forced cooling treatment for cooling at a cooling rate of 5 ° C./sec or more is performed so that the surface temperature of the material to be rolled becomes Ms point or less.

The surface temperature of the material to be rolled after the forced cooling treatment is increased by 650 to 75 due to the heat retained inside the material to be rolled.
After the temperature reaches 0 ° C. or after heating at a heating rate of 5 ° C./sec or more to raise the surface temperature to 650 to 750 ° C., finish rolling is immediately performed at a surface reduction rate of 5% or more.

The rolled material after the finish rolling is kept in a temperature range of 650 to 750 ° C. for at least 5 minutes, and then cooled to the atmosphere.

In the above-mentioned present invention, the raw steel slab contains 0.01 to 1.2% by weight of C and 0.1% by weight of Si.
01-2%, Mn: 0.01-2%, Al: 0.001
0.1%, Cr: 0 to 2%, Mo: 0 to 0.6%, C
u: 0 to 2%, Ni: 0 to 4%, Ti: 0 to 0.1%,
N: 0 to 0.03%, V: 0 to 0.4%, Nb: 0 to 0
0.15%, B: 0 to 0.005%, S: 0 to 0.1
%, Pb: 0 to 0.35%, Ca: 0 to 0.01%, the balance being Fe and unavoidable impurities, and P having a chemical composition of 0.05% or less in impurities. It is preferably used.

The present invention has been completed based on the following findings. That is, the present inventors can shorten the time required for heat treatment after hot rolling than the above-mentioned conventional method, and furthermore, the surface layer has a spherical structure in which spherical carbides are uniformly dispersed, and the other portions have a fine soft structure. As a result of studying the manufacturing conditions for obtaining a hot-rolled rod or wire rod having no abnormal structure composed of a decarburized phase in the surface layer, the following was found.

The material to be rolled before the finish rolling is subjected to at least one forced cooling treatment in which the material is cooled at a cooling rate of 5 ° C./sec or more until the surface temperature becomes Ms or less. After the surface temperature reaches 650 to 750 ° C., finish rolling is performed at a reduction in area of 5%, and the material to be rolled after the finish rolling is kept in a temperature range of 650 to 750 ° C. for 5 minutes or more. In that case, despite the fact that the structure inside the material to be rolled is a mixed structure of a ferrite phase and a pearlite phase, the formation of spherical carbides is promoted, and even if the heat treatment time after hot rolling is shortened, the surface layer portion is formed. It is possible to obtain a desired product having a spherical structure in which spherical carbides are uniformly dispersed and a fine soft structure in other portions and having no abnormal structure composed of a decarburized phase in a surface layer.

However, if the raw steel slab is heated above 1250 ° C., decarburization occurs at the surface layer of the product. Also,
If the surface temperature of the material to be rolled at the final stand in the middle row of the rolling mill during rolling is less than 700 ° C. or more than 900 ° C. and the area reduction rate is less than 20%, then the forced cooling to the above-mentioned Ms point or less is performed. The internal structure of the product becomes a super-cooled structure composed of a hard low-temperature transformation phase such as a martensite phase or a bainite phase even if the treatment or reheating to 650 to 750 ° C. is performed, and a uniform softened structure is obtained. In some cases, it cannot be obtained stably.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the reasons why the processing conditions in each step of the present invention are determined as described above will be described in detail.

Heating temperature of raw slab: 900 ° C
If less, the load on the rolling mill increases. In addition, it takes a long time to uniformly heat the temperature in the cross section of the slab, which not only reduces the productivity but also causes cracks during rolling. On the other hand, when heating is performed at a temperature exceeding 1250 ° C., the amount of decarburization during heating increases rapidly, and it may not be possible to prevent decarburization from occurring at the surface layer of the product wire. Therefore, the heating temperature of the raw steel slab was 900 to 1250 ° C.

Rolling at the last stand of the intermediate rolling train:
If the surface temperature of the material to be rolled at the time of rolling at the final stand in the intermediate rolling row is lower than 700 ° C., not only the load on the rolling mill is increased, but also cracks may occur during rolling. On the other hand, if the surface temperature exceeds 900 ° C., the crystal grain size on the entry side of the finish rolling becomes too coarse, and it is difficult to obtain a desired fine soft structure. Further, when the area reduction rate is less than 20%, it is difficult to obtain a desired fine soft tissue. Therefore, rolling at the final stand in the middle row of the rolling mill is performed at a surface temperature of the rolled material of 700 to 900.
° C and the area reduction rate was 20% or more.

It is to be noted that the higher the area reduction rate, the better, and the upper limit does not need to be particularly defined.
Since it becomes difficult to finish the product wire rod with the dimensional accuracy required in the finish rolling, the upper limit is preferably set to 40%.

Cooling and reheat treatment before finish rolling: In the present invention, the material to be rolled after being rolled by the final stand in the middle row of the rolling mill is cooled at a cooling rate of 5 ° C./sec or more, and its surface temperature is reduced. The forced cooling treatment for lowering the temperature to the Ms point or lower is performed at least once, and the surface temperature of the material to be rolled after this forced cooling treatment is 650 to 750 due to the heat retained therein.
After the temperature has returned to ℃, finish rolling is performed immediately.

As described above, when the rolled material whose surface temperature is restored to 650 to 750 ° C. after the forced cooling treatment to the Ms point or lower is subjected to finish rolling under the conditions described later, the rolled material after the finish rolling is applied. A product having a spherical structure can be obtained while hot-rolled only by keeping the rolled material in a temperature range described below for a relatively short time. The reason is as follows.

That is, the structure of a steel material during general hot rolling is usually an austenite structure. In order to obtain a spherical structure in which spherical carbides are uniformly dispersed by changing the heat history from the austenite structure, it is necessary to hold the steel material in the vicinity of the Ac ₁ transformation point for several hours, specifically, at least 2 hours. There is. In order to realize this at an actual production site, it is necessary to introduce heat treatment equipment capable of maintaining the temperature for a long time at an isothermal temperature, which increases the equipment cost, lowers productivity, and increases product manufacturing cost.

However, after the steel material being hot-rolled is cooled to the Ms point or lower to once produce a martensite structure, the surface temperature is raised to 650 to 650 mm by the retained heat inside the steel material.
When reheated to 750 ° C., spherical θ carbides are generated in a relatively short time with ε carbides in the generated martensite structure as nuclei. That is why a spherical structure can be obtained with hot rolling.

Here, the reason why the cooling rate in the forced cooling treatment is set to 5 ° C./sec or more is that if the cooling rate is less than 5 ° C./sec, martensite transformation does not occur even if the cooling rate is below the Ms point. This is because the structure may not be a martensite structure.

The above-mentioned cooling rate is determined by using a suitable cooling means such as a water-cooling device capable of adjusting the amount of supplied water or a salt bath tub capable of adjusting the bath temperature with the final stand of the intermediate rolling train and the first stand of the finishing rolling train. It is controllable by installing between.

The surface temperature after reheating is set to 650 to 750.
The reason why the temperature is set to ° C is that if the surface temperature after reheating is lower than 650 ° C, the formation of spherical carbides is not sufficient, and if the surface temperature exceeds 750 ° C, the generated martensite structure is liable to reversely transform into an austenite structure.

In the reheating treatment after the forced cooling treatment, the heating may be positively performed by using a heating means such as an induction heating coil, instead of waiting for the temperature increase due to the heat retained in the steel material. Good. In this case, the transition time to finish rolling can be shortened, and the productivity is improved.

However, the heating rate during heating by the heating means must be 5 ° C./sec or more. This is because if the heating rate is less than 5 ° C./sec, the heating time required to reheat to the above surface temperature becomes too long, and the transition time to finish rolling cannot be shortened. This is because it is necessary to make the equipment layout in which the distance between the final stand and the first stand of the finish rolling train is long, which increases the equipment cost and increases the production cost of the product.

For the above reasons, in the present invention, the material to be rolled by the final stand in the intermediate rolling row of the rolling mill is cooled at least at a cooling rate of 5 ° C./sec or more until the surface temperature falls below the Ms point. The material is forcibly cooled once, and the surface temperature of the material to be rolled after the forcible cooling is 650 to 75 due to internal heat retention.
Reheat to 0 ° C. or use heating means to reach
The finish rolling was performed immediately after heating to 50 ° C.

The higher the cooling rate in the forced cooling treatment, the better. The upper limit does not need to be specified. However, if the cooling rate exceeds 300 ° C./sec, a cooling means having a higher capacity is required. Therefore, since the equipment cost increases and the production cost of the product increases, the upper limit is preferably set to 300 ° C./sec.

The heating rate at the time of heating by the heating means is preferably as fast as possible. The upper limit does not need to be particularly defined. However, if the heating rate exceeds 300 ° C./sec, as in the case of the cooling means described above, Since a heater having a higher capacity is required as the heating means, which increases the equipment cost and raises the production cost of the product, the upper limit is preferably set to 300 ° C./sec.

Finish Rolling: In the finish rolling, as described above, the surface temperature of the material to be rolled after forced cooling is 650 to 750.
° C or 650-75 with heating means.
Heat to 0 ° C. or immediately. However, if the reduction in area at that time is less than 5%, the distortion imparted by the finish rolling is too small and the grain refinement is insufficient, so that spherical carbides are not sufficiently generated, and the heat treatment after the rolling is completed. Time cannot be shortened. Therefore, the finish rolling is performed at a surface reduction rate of 5% or more.

The higher the area reduction rate at the time of finish rolling, the better. The upper limit is not particularly required.
%, The material temperature rises due to the heat generated during processing, and the fine grains obtained by processing tend to grow and become coarse, so the upper limit is preferably set to 70%.

Processing after finish rolling: The material to be rolled after finish rolling needs to be held for at least 5 minutes while maintaining the surface temperature in the temperature range of 650 to 750 ° C.
This is because, similarly to the case of the above reheat treatment, when the surface temperature is lower than 650 ° C., the generation of spherical carbides is not sufficient, and when the surface temperature is higher than 750 ° C., the generated martensite structure is liable to reversely transform into an austenite structure. That's why.

If the holding time is less than 5 minutes, the formation of spherical carbides is insufficient, and the untransformed austenite in the austenite phase or the mixed structure of the austenite phase, the ferrite phase and the pearlite phase is not sufficient. This is because the phase is not completely transformed and a subsequent cooling may result in a supercooled structure composed of a hard low-temperature transformed phase such as a martensite phase or a bainite phase. Therefore, the material to be rolled after finish rolling has a surface temperature of 650.
The temperature range was maintained at a temperature of ７750 ° C. for at least 5 minutes.

It is to be noted that the longer the above-mentioned holding time is, the better, and the upper limit does not need to be particularly defined. However, if the holding time exceeds 30 minutes, a cooling facility composed of an extremely long stermore type conveyor or the like is required. In addition to this, a heating device is required to maintain the surface temperature at 650 to 750 ° C., which increases the equipment cost and raises the production cost of the product, so the upper limit is preferably set to 30 minutes.

Next, preferred steels to be used as the raw slab in the method of the present invention will be described. In the following, “%” means “% by weight”.

Preferred steels for use in the method of the present invention are C:
0.01 to 1.2%, Si: 0.01 to 2%, Mn:
It is a carbon steel or low alloy steel containing 0.01 to 2% and Al: 0.001 to 0.1%, and P in the impurities is 0.05% or less.

Here, the above-mentioned low-alloy steel means, as an optional additive element other than the above-mentioned components, 2% or less of Cr, 0.6%
Mo, 2% or less Cu, 4% or less Ni, 0.1%
% Ti or less, 0.03% or less N, 0.4% or less V, 0.15% or less Nb, 0.005% or less B,
S up to 0.1%, Pb up to 0.35% and 0.0
It means a Fe-based alloy containing one or more selected from Ca of 1% or less.

The reason why the content of each component is determined as described above will be described below.

C is an effective element for securing the strength of the final product, but if it is less than 0.01%, the desired strength cannot be secured, and if it exceeds 1.2%, the toughness is significantly reduced.
The content was set to 0.01 to 1.2%.

Si is added for the purpose of securing the strength of the final product by deoxidizing molten steel and strengthening the solid solution. However, if the content is less than 0.01%, these effects are not sufficient, and if it exceeds 2%, these effects are saturated. However, on the contrary, the toughness is reduced, so the content is set to 0.01 to 2%.

Mn is an effective element for improving the hardenability of steel and ensuring the strength of the final product, but if its content is less than 0.01%, its effect is not sufficient, and if it exceeds 2%, its effect is obtained. Saturates and conversely causes a decrease in toughness.
01 to 2%.

Al is added for the purpose of deoxidizing molten steel and refining crystal grains. If the content is less than 0.001%, these effects are not sufficient, and if it exceeds 0.1%, these effects are saturated. Conversely, it causes a decrease in toughness.
01-0.1%.

P segregates at the grain boundaries and the center of the steel and lowers the toughness. Particularly, if the content exceeds 0.05%, the toughness is remarkably reduced.

Cr, Ni, Mo and B are all
It is an element that has the effect of increasing the strength of the final product through the improvement of hardenability. Therefore, when it is desired to obtain the effect, one or more selected from these can be added. The effect is remarkable when Cr is 0.3% or more, Ni is 0.1% or more, Mo is 0.05% or more, and B is 0.0002% or more. However, Cr is 2
%, Ni is 4%, Mo is more than 0.6%, and B is more than 0.005%, the bainite and martensite structure is generated in the hot-rolled state, and it becomes impossible to secure a uniform soft structure. Instead, a decarburized phase is likely to be generated in the surface layer of the product wire. Therefore, the content of these elements when added is 0.3 to 2% for Cr, 0.1 to 4% for Ni, 0.05 to 0.6% for Mo,
B is preferably set to 0.0002 to 0.005%.

Ti, Nb, V and N are all effective elements for adjusting the crystal grain size. Therefore, when it is desired to obtain the effect, one or more selected from these can be added. The effect is
Ti becomes remarkable at 0.002% or more, Nb and V become 0.005% or more, and N becomes 0.001% or more. However, Ti
When the content exceeds 0.1%, the content of Nb is 0.15%, the content of V is 0.4%, and the content of N exceeds 0.03%, the effect is saturated and conversely, the toughness is reduced. Therefore, the content of these elements when added is 0.002 to 0.1 for Ti.
%, Nb 0.005 to 0.15%, V 0.005 to 0.4%, N 0.001 to
It is good to make it 0.03%.

Cu is an element effective for improving toughness and hardenability. Therefore, if it is desired to obtain the effect, it can be added, and the effect becomes remarkable at 0.05% or more. However, when the content exceeds 2%, the effect is saturated, and on the contrary, toughness is reduced. Therefore, the content of Cu when added is preferably 0.05 to 2%.

S, Pb and Ca are all effective elements for improving machinability. Therefore, when it is desired to obtain the effect, one or more selected from these can be added. The effect becomes remarkable when S and Pb are 0.05% or more and Ca is 0.001% or more. However, S is 0.1%, Pb is 0.35%, C
If the content of a exceeds 0.01%, the effect is saturated and conversely, the toughness is reduced. Therefore, the content of these elements when added is 0.05 to 0.1 for S.
%, Pb for 0.05 to 0.35%, and Ca for 0.001 to 0.01%.

[0057]

EXAMPLE A square billet (140 mm square, length 10 mm) made of three types of steel having the chemical compositions shown in Table 1 was used.
m) was prepared.

[0058]

[Table 1]

Each of the prepared square billets was hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled wire having an outer diameter of 10 mm.

A test piece was taken from each of the obtained wires and subjected to a tensile test and microscopic observation to check the tensile strength. On the other hand, the surface portion was a spherical structure, and the other portion was a fine soft structure. It was investigated whether or not there was any abnormal tissue consisting of the decarburized phase. The results are shown in Table 2. The symbol F in the microstructure column of Table 2 indicates a ferrite phase, P indicates a pearlite phase, and B indicates a bainite phase.

[0061]

[Table 2]

As can be seen from the results shown in Table 2, when the method of the present invention (Test Nos. 1 to 10) was used, the surface layer had a spherical structure and the other portions had a fine soft structure. No abnormal structure consisting of the decarburized phase was present in the surface layer, and a soft wire having a tensile strength of 725 MPa or less was obtained.

On the other hand, the surface temperature of the material to be rolled at the final stand of the intermediate rolling row of the rolling mill, the presence or absence of forced cooling before finish rolling, the surface temperature and cooling rate of the material to be cooled at the time of cooling, the temperature immediately before finish rolling, Any one or more of the surface temperature of the material to be rolled, the reduction in area in the finish rolling, and the holding time at the surface temperature of the material to be rolled after the finish rolling at 650 to 750 ° C. are out of the range specified in the present invention. In the case of using the method of the comparative example (Test Nos. 11 to 20), not only the surface layer portion is not a spherical structure, but also the surface layer portion has an abnormal structure including a hard phase bainite structure and a decarburized phase, Mixed structure of ferrite phase and pearlite phase with coarse inside and ferrite phase,
It was a mixed structure of a pearlite phase and a bainite phase.
In addition, trial numbers 12, 13, 15, 16, 18, and 19
In Example 1, the tensile strength was abnormally high because a bainite phase was formed in the surface layer, and a soft wire was not obtained.

[0064]

According to the method of the present invention, while being rolled,
An abnormal structure consisting of a decarburized phase does not exist in the surface layer portion, and it is possible to efficiently manufacture a steel bar or a wire having a soft structure with a spherical surface structure and a fine internal portion.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kenichi Kawazoe F-term in Kokura Works, Kokura Works, Kitakyushu-ku, Kitakyushu-shi, Fukuoka Prefecture F-term (reference) 4E002 AA07 AC12 AC14 BC05 BC07 BD07 BD08 CB01 CB03 4K032 AA01 AA02 AA04 AA05 AA06 AA07 AA08 AA11 AA12 AA14 AA15 AA16 AA17 AA19 AA21 AA22 AA23 AA24 AA28 AA29 AA31 AA32 AA35 AA36 BA02 CA01 CA02 CA03 CC02 CC03 CC04 CD02 CD03 CD05

Claims (2)

[Claims] 1. A method for producing a hot-rolled steel bar or a wire rod, which is sequentially performed under the following conditions: The raw steel slab is heated to 900-1250 ° C. Rolling in the final stand of the intermediate rolling row of the rolling mill including the rough rolling row, the intermediate rolling row, and the finishing rolling row of multiple stands is performed at a surface temperature of the rolled material of 700 to 900 ° C. and a reduction in area of 20% or more. Before finish rolling, at least one forced cooling process of cooling at a cooling rate of 5 ° C./sec or more is performed so that the surface temperature of the material to be rolled becomes equal to or lower than the Ms point. After the surface temperature of the material to be rolled after the forced cooling treatment rises to 650 to 750 ° C. due to the retained heat inside the material to be rolled, or is heated at a heating rate of 5 ° C./sec or more using a heating means. Immediately after the surface temperature is set to 650 to 750 ° C., finish rolling is performed at a surface reduction rate of 5% or more. The material to be rolled after finish rolling is subjected to a surface temperature of 650 to 7
After the temperature is kept within a temperature range of 50 ° C. for at least 5 minutes, the atmosphere is cooled. (2) C: 0.01 to 100 wt.
1.2%, Si: 0.01 to 2%, Mn: 0.01 to 2
%, Al: 0.001 to 0.1%, Cr: 0 to 2%, M
o: 0 to 0.6%, Cu: 0 to 2%, Ni: 0 to 4%,
Ti: 0 to 0.1%, N: 0 to 0.03%, V: 0 to 0%
0.4%, Nb: 0 to 0.15%, B: 0 to 0.005
%, S: 0 to 0.1%, Pb: 0 to 0.35%, Ca:
2. The heat according to claim 1, wherein the steel contains 0 to 0.01%, the balance being Fe and inevitable impurities, and P in the impurities having a chemical composition of 0.05% or less. A method for producing cold-rolled steel bars or wires.