JP3506033B2 - Method of manufacturing hot-rolled steel bars or wires - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the production of steel bars and wire rods (hereinafter also referred to as "bars / wire rods") by hot rolling using a rolling mill consisting of a multi-stand rough rolling row, intermediate rolling row and finish rolling row. Related to the method, in more detail, the surface layer has a spherical structure in which spherical carbides are uniformly dispersed, the other part has a fine soft structure, and the surface layer part is a hot-rolled bar with no abnormal structure composed of 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 wire rods specified by JIS G 3502 manufactured by hot rolling, carbon steel wire rods for cold heading specified by G 3507, G 4051, G 4104, G 4105. Machine structural carbon steels and alloy steels, G4805 bearing steels, G4801 spring steels, and alloy steels
Bars and wire rods that use G 4401, G 4401, etc. as the base material for tool steel are wire-drawing called secondary processing and tertiary processing,
The required products are finished through cold working processes such as drawing, cutting, forging, and cutting.

However, the above-mentioned hot-rolled bar
Most of the wire rods 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, at the time of manufacturing these rods / wires, there has been adopted a method of enhancing the ductility by lowering the strength by performing a softening heat treatment such as annealing or spheroidizing after hot rolling to improve the cold workability.

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

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

That is, of the above publications, Japanese Patent Laid-Open No.
The method disclosed in JP-A-41325 completes a step of rapidly cooling a wire rod to 700 to 850 ° C. in a controlled cooling stage after hot rolling, prevention of overcooling in this rapid cooling step, and progress of recrystallization by reheat. In order to maintain the temperature at 700 to 775 ° C or to cool at a cooling rate of 5 ° C / sec or less, 600 to 675 in order to coarsen the lamellar spacing of the pearlite phase and shift the supersaturated carbon in the ferrite phase to the equilibrium state.
The process of cooling at a cooling rate of 0.025 to 0.25 ° C./sec to the temperature of 0 ° C. and then rapidly cooling at an arbitrary cooling rate is continuously performed. At that time, the temperature of the wire rod immediately after rolling is set to 900 ° C. or less. Is a good method. According to this method, it is possible to obtain a wire rod having a softened structure as hot rolled. However, in this method, the
The temperature range of 00 to 675 ° C is 0.025 to 0.25 ° C /
Since the cooling is performed at a very slow cooling rate of 2 seconds, it is unavoidable that a decarburization phase occurs in the surface layer of the material, and there is a drawback that only a product with an abnormal structure in the surface layer can be obtained. There is.

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

In the method disclosed in Japanese Patent Laid-Open No. 62-139817, finish rolling is started at 650 to 850 ° C., while finish rolling is completed at less than 950 ° C.
It is a method in which it is rapidly cooled to 750 ° C., wound into a non-concentric shape, and then cooled to 500 ° C. at a cooling rate of 0.5 ° C./sec or more. According to this method, it is possible to obtain a wire having a mixed structure of fine ferrite phase and pearlite phase, and it is possible to shorten the time required for the spheroidizing heat treatment after hot rolling. However, in this method, when the steel is an alloy steel having the same high hardenability as described above, when cooled at a cooling rate of 0.5 ° C./sec or more, a bainite phase and a martensite phase, which are hard phases, are generated, It does not form a fine microstructure of ferrite and pearlite phases, and as in the case above, the tensile strength as rolled is extremely high and it is difficult to soften it, making it difficult to perform wire drawing as rolled. It has the drawback that only good products can be obtained.

In the method disclosed in Japanese Patent Laid-Open No. 64-55330, the material steel slab is heated to an Ac ₃ point or more or an Ac cm point or more and hot-rolled, while at least a finish rolling stage is performed in an austenite unrecrystallized region. With a processing rate of 10-80
%, 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 hot rolling. However, also in this method, when the steel is an alloy steel having high hardenability, the cooling after finish rolling is extremely fast, so that a bainite or martensite structure that 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 only a product that is difficult to wire-draw as rolled is obtained.

In the method disclosed in Japanese Patent Laid-Open No. 4-173921, the first heating for keeping the raw steel billet at the Ac ₁ point to the Acm point or the Ac ₃ point or less for 1 minute to less than 20 hours and the Acm point or the Ac ₃ point. Point to less than 1050 ° C. and heating in two steps of the second heating to form a mixed structure of undissolved carbide and austenite phase, while finish rolling is performed at the Ar ₃ point or the Arcm point
After finishing by adding 30% or more of processing in the temperature range of Ac ₃ points or A cm points, hold at 660 to 740 ° C for 30 minutes or more, or from the rolling end temperature to 650 ° C at a cooling rate of 100 ° C / h. It is a method of cooling at less than. According to this method, it is possible to obtain a wire rod having a spherical structure as it is while hot rolling, and it is possible to shorten the time required for heat treatment after hot rolling or omit the heat treatment itself. However, in this method, the wire rod after finish rolling is 660 to 7
Since it is kept in the temperature range of 40 ° C. for a long time, it is inevitable that a decarburization phase occurs in the surface layer of the material, and only a product having an abnormal structure in the surface layer can be obtained. . Also, when subjected to secondary processing as hot rolling,
There is also a drawback that in order to obtain the spherical structure finally required in the secondary processing, it may take more time than usual heat treatment.

[0011]

The object of the present invention is to provide a spherical structure in which spherical carbides are uniformly dispersed in the surface layer portion, a fine soft structure in the other portion, and an abnormal structure composed of a decarburized phase in the surface layer portion. It is an object of the present invention to provide a method capable of producing a hot-rolled steel bar or a wire rod without a high efficiency with a high efficiency and requiring a short time for heat treatment after the hot-rolling to a specific value.

[0012]

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

A method for producing a hot-rolled steel bar or a wire rod, which is characterized in that the treatments are sequentially performed under the following conditions.

The raw steel billet is heated to 900 to 1250 ° C.

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

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

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

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

Further, in the above-mentioned present invention, in the material steel billet, C: 0.01-1.2%, Si: 0.
01 to 2%, Mn: 0.01 to 2%, Al: 0.001
~ 0.1%, Cr: 0-2%, Mo: 0-0.6%, 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
0.15%, B: 0 to 0.005%, S: 0 to 0.1
%, Pb: 0 to 0.35%, Ca: 0 to 0.01%, the balance Fe and unavoidable impurities, and P in the impurities has a chemical composition of 0.05% or less. It is preferably used.

The present invention described above was 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 the surface layer portion has a spherical structure in which spherical carbides are uniformly dispersed, and other portions have a fine soft structure. In addition, as a result of efforts to investigate the manufacturing conditions for obtaining the hot-rolled bar / wire having an abnormal structure composed of the decarburized phase in the surface layer, the following was found.

The material to be rolled before finish rolling is subjected to at least one forced cooling treatment for cooling it at a cooling rate of 5 ° C./sec or more until the surface temperature thereof becomes equal to or lower than Ms, and the material to be rolled after this forced cooling treatment. After the surface temperature of the steel sheet reaches 650 to 750 ° C, finish rolling is performed at a surface reduction rate of 5%, and the material to be rolled after the finish rolling is held in the temperature range of the surface temperature of 650 to 750 ° C for 5 minutes or more. In that case, even though the structure inside the material to be rolled is a mixed structure of ferrite phase and 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 It is possible to obtain a desired product having a spherical structure in which spherical carbides are uniformly dispersed, a fine soft structure in the other portions, and having no abnormal structure composed of a decarburized phase in the surface layer.

However, when the raw steel billet is heated above 1250 ° C., decarburization occurs at the surface layer of the product. Also,
When the surface temperature of the material to be rolled at the final stand in the middle row of the rolling mill is less than 700 ° C or more than 900 ° C and the surface reduction rate is less than 20%, thereafter, forced cooling to the Ms point or less is performed. Even if heat treatment or reheat to 650 to 750 ° C. or heat treatment is performed, the internal structure of the product becomes a supercooled structure composed of a hard low-temperature transformation phase such as a martensite phase or a bainite phase, and a uniform softened structure is obtained. It may not be possible to obtain stable results.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION The reason why the processing conditions in each step of the present invention are set as above will be described in detail below.

Heating temperature of the raw steel billet: The heating temperature is 900 ° C.
When it is less than 1, the load on the rolling mill becomes large. Further, it takes a long time to uniformly heat the temperature in the cross section of the billet, which not only lowers the productivity but also causes cracks during rolling. On the other hand, if the temperature is higher than 1250 ° C, the amount of decarburization during heating may increase rapidly, and it may be impossible to prevent decarburization from occurring in the surface layer of the product wire. Therefore, the heating temperature of the raw steel billet is set to 900 to 1250 ° C.

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

The higher the surface reduction rate, the better. The upper limit need not be specified, but if it exceeds 40%,
Since it is difficult to finish a product wire rod having the required dimensional accuracy in finish rolling, the upper limit is preferably 40%.

Cooling and reheat treatment before finish rolling: In the present invention, the material to be rolled after rolling 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 The forced cooling treatment for lowering the temperature to below the Ms point 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 internal heat retained therein.
After recuperating to ℃, finish rolling immediately.

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

That is, the structure of the 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 this 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 a heat treatment facility capable of holding an isothermal long period of time, which increases equipment cost and 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 generate a martensitic structure, the surface temperature is kept at 650 to 650 by the heat retained inside the steel material.
When reheated to 750 ° C., spherical ε-carbide is produced in a relatively short time with ε-carbide in the produced martensite structure as a nucleus. This is why the spherical structure can be obtained by hot rolling.

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

For the above cooling rate, a suitable cooling means such as a water cooling device capable of adjusting the amount of supplied water or a salt bath capable of controlling the bath temperature is used for the final stand of the intermediate rolling train and the first stand of the finish rolling train. It can be controlled by installing it in between.

The surface temperature after recuperation is set to 650 to 750.
The reason why the temperature is set to be 0 ° C is that if the surface temperature after recuperation is less than 650 ° C, the formation of spherical carbides is not sufficient, and if it exceeds 750 ° C, the formed martensite structure is likely to undergo reverse transformation to an austenite structure.

In the post-heat treatment after the forced cooling treatment, instead of waiting for the temperature rise due to the heat retained inside the steel material, the heating means such as an induction heating coil may be used to actively heat and raise the temperature. Good. In this case, the transition time to finish rolling can be shortened and productivity is improved.

However, the heating rate at the time of 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 last stand and the first stand of the finish rolling row is long, which increases the equipment cost and increases the manufacturing cost of the product.

For the above reasons, in the present invention, the material to be rolled after being rolled by the final stand of the intermediate rolling row of the rolling mill is at least cooled at a cooling rate of 5 ° C./sec or more until the surface temperature becomes equal to or lower than the Ms point. After forced cooling once, the surface temperature of the material to be rolled after the forced cooling is 650 to 75 due to the heat retained inside.
Reheat to 0 ° C or use heating means to reach 650-7
After heating to 50 ° C., finish rolling was immediately performed.

The higher the cooling rate in the forced cooling treatment, the better, and it is not necessary to set the upper limit. However, if it exceeds 300 ° C./second, a cooling means having a higher capacity is required. Since the equipment cost increases and the manufacturing cost of the product increases, the upper limit is preferably 300 ° C./second.

Further, the higher the heating rate at the time of heating by the heating means, the better, and it is not necessary to set the upper limit in particular, but if it exceeds 300 ° C./sec, as in the case of the above cooling means, Since a heating means having a higher capacity is required and the equipment cost increases and the manufacturing cost of the product increases, the upper limit is preferably set to 300 ° C./second.

Finish rolling: In finish rolling, as described above, the surface temperature of the material to be rolled after forced cooling is 650 to 750.
Reheat to ℃ or using heating means 650-75
Heat to 0 ° C or immediately. However, if the area reduction rate at that time is less than 5%, the strain 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 finished. The time cannot be shortened. Therefore, the finish rolling is performed at a surface reduction rate of 5% or more.

It should be noted that the higher the area reduction rate during finish rolling is, the better it is.
%, The material temperature rises due to heat generation during processing, and the fine particles obtained by processing tend to grow and coarsen, so the upper limit is preferably 70%.

Treatment 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, as in the case of the above-mentioned reheat treatment, when the surface temperature is less than 650 ° C, the formation of spherical carbides is not sufficient, and when it exceeds 750 ° C, the formed martensite structure is likely to undergo reverse transformation into an austenite structure. This is because.

If the holding time is less than 5 minutes, the formation of spherical carbides is insufficient, and untransformed austenite in the internal structure of the austenite phase or the mixed structure of the austenite phase and the ferrite phase and the pearlite phase is obtained. This is because the phase may not be completely transformed and a supercooled structure composed of a hard low-temperature transformation phase such as a martensite phase or a bainite phase may be formed by subsequent cooling. Therefore, the surface temperature of the rolled material after finish rolling is 650
It was decided to maintain the temperature range of ˜750 ° C. for at least 5 minutes.

It should be noted that the longer the holding time is, the better it is, and it is not necessary to set the upper limit in particular, but if it exceeds 30 minutes, a cooling facility including an extremely long Stelmore type conveyor is required. In addition to this, a heating device is required to maintain the surface temperature at 650 to 750 ° C., which increases equipment costs and raises the manufacturing cost of the product. Therefore, the upper limit is preferably set to 30 minutes.

Next, in the above-mentioned method of the present invention, preferable steels to be used as the raw material billet will be explained. In the following, "%" means "% by weight".

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

Here, the above-mentioned low alloy steel means, in addition to the above-mentioned components, 2% or less of Cr and 0.6% as optional addition elements.
Mo below, Cu below 2%, Ni below 4%, 0.1
% Or less Ti, 0.03% or less N, 0.4% or less V, 0.15% or less Nb, 0.005% or less B,
0.1% or less S, 0.35% or less Pb and 0.0
It means an 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 element effective 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 remarkably deteriorated.
The content was 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 solid solution, but if it is less than 0.01%, these effects are not sufficient, and if it exceeds 2%, these effects saturate. On the contrary, since the toughness is lowered, the content is set to 0.01 to 2%.

Mn is an element effective for improving the hardenability of steel and ensuring the strength of the final product. If it is less than 0.01%, its effect is not sufficient, and if it exceeds 2%, its effect is obtained. Is saturated and conversely leads to a decrease in toughness, so the content is set to 0.
It was set to 01 to 2%.

Al is added for the purpose of deoxidizing molten steel and refining crystal grains. If less than 0.001%, these effects are not sufficient, and if more than 0.1%, these effects saturate. On the contrary, since it causes toughness deterioration, its content is 0.0
It was set to 01 to 0.1%.

P is segregated at the grain boundaries and the central part of the steel to lower the toughness, and particularly when it exceeds 0.05%, the toughness is remarkably decreased, so P is set to 0.05% or less.

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, in order to obtain the effect, one kind or two or more kinds selected from these can be added. The effect becomes 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 4%, Mo 0.6%, and B more than 0.005%, bainite and martensite structures are formed in the hot rolled state, and a uniform soft structure cannot be secured. Not only that, the decarburization phase easily occurs on 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 0.0002 to 0.005%.

Ti, Nb, V and N are all effective elements for adjusting the crystal grain size. Therefore, in order to obtain the effect, one kind or two or more kinds selected from these can be added. The effect is
It becomes remarkable when Ti is 0.002% or more, Nb and V are 0.005% or more, and N is 0.001% or more. However, Ti
If 0.1%, Nb is 0.15%, V is 0.4% and N is more than 0.03%, the effect is saturated and conversely the toughness is lowered. 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%, and N 0.001 to 0.15%.
It is good to set it to 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, if the content exceeds 2%, the effect is saturated and, conversely, toughness is reduced. Therefore, the Cu content when added is preferably 0.05 to 2%.

S, Pb and Ca are all effective elements for improving machinability. Therefore, in order to obtain the effect, one kind or two or more kinds 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 lowered. Therefore, the content of these elements when added is 0.05 to 0.1 for S.
%, Pb is preferably 0.05 to 0.35%, and Ca is preferably 0.001 to 0.01%.

[0057]

EXAMPLE A square billet (140 mm square, length 10 mm) of a raw billet made of three kinds of steels having the chemical compositions shown in Table 1
m) was prepared.

[0058]

[Table 1]

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

A test piece was taken from each of the obtained wire rods and subjected to a tensile test and micro observation to examine the tensile strength, while the surface layer had a spherical structure and the other parts had a fine soft structure, and the surface layer had a fine structure. It was investigated whether or not there was an abnormal structure composed of a decarburized phase, and the results are also shown in Table 2. The symbol F in the microstructure column in Table 2 means a ferrite phase, P means a pearlite phase, and B means a bainite phase.

[0061]

[Table 2]

As can be seen from the results shown in Table 2, according to the method of the present invention (trials Nos. 1 to 10), the surface layer has a spherical structure and the other parts have a fine soft structure, and A soft wire having a tensile strength of 725 MPa or less was obtained without an abnormal structure composed of a decarburized phase in the surface layer.

On the other hand, the surface temperature of the material to be rolled in the final stand of the intermediate rolling train 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 rolled at the time of cooling, the temperature immediately before the finish rolling. Any one or more of the surface temperature of the material to be rolled, the surface reduction rate in finish rolling, and the holding time at the surface temperature of the material to be rolled 650 to 750 ° C. after finish rolling deviates from the range specified in the present invention. In the case of using the method of the comparative example (trial numbers 11 to 20), not only the surface layer portion does not have a spherical structure, but the surface layer portion has an abnormal structure composed of a bainite structure of a hard phase and a decarburized phase, Mixed structure of ferrite phase and pearlite phase with coarse inside and ferrite phase,
It had a mixed structure of pearlite phase and bainite phase.
Also, trial numbers 12, 13, 15, 16, 18, and 19
However, since a bainite phase was generated in the surface layer, the tensile strength was abnormally high, and a soft wire could not be obtained.

[0064]

According to the method of the present invention, as-rolled,
It is possible to highly efficiently manufacture a steel bar or a wire rod having no abnormal structure composed of a decarburized phase in the surface layer part and having a spherical structure in the surface layer part and a soft structure having a fine inside.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI C22C 38/58 C22C 38/58 (56) Reference JP-A-4-173921 (JP, A) JP-A-2-213416 (JP , A) JP 55-8484 (JP, A) JP 59-136423 (JP, A) JP 1-205031 (JP, A) JP 2000-256740 (JP, A) (58) Survey Fields (Int.Cl. ⁷ , DB name) C21D 8/06 C21D 9/52 C21D 9/573 B21B 1/16 C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. A method for producing a hot-rolled steel bar or wire rod, which is characterized in that the treatments are sequentially performed under the following conditions. The raw steel billet is heated to 900 to 1250 ° C. Rolling is performed in the final stand of the intermediate rolling train of the rolling mill including the rough rolling train, the intermediate rolling train, and the finish rolling train of multiple stands at a surface temperature of the rolled material of 700 to 900 ° C. and a reduction rate of 20% or more. Before finish rolling, a forced cooling treatment is performed at least once to cool the material to be rolled at a cooling rate of 5 ° C./sec or more so that the surface temperature of the material to be rolled is 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 heat retained inside the material to be rolled, or is heated at a heating rate of 5 ° C./second or more using a heating means. After setting the surface temperature to 650 to 750 ° C., finish rolling is immediately performed with a surface reduction rate of 5% or more. The surface temperature of the rolled material after finish rolling is 650 to 7
Hold in the temperature range of 50 ° C. for at least 5 minutes and then cool to the atmosphere. 2. The weight ratio of C: 0.01 to the raw steel billet.
1.2%, Si: 0.01 to 2%, Mn: 0.01 to 2
%, Al: 0.001 to 0.1%, Cr: 0 to 2%, M
o: 0-0.6%, Cu: 0-2%, Ni: 0-4%,
Ti: 0 to 0.1%, N: 0 to 0.03%, V: 0
0.4%, Nb: 0 to 0.15%, B: 0 to 0.005
%, S: 0 to 0.1%, Pb: 0 to 0.35%, Ca:
The steel according to claim 1, characterized in that a steel containing 0 to 0.01%, the balance being Fe and unavoidable impurities, and having a chemical composition in which P in the impurities is 0.05% or less is used. Method for producing hot rolled steel bar or wire rod.