JP2564535B2 - Direct spheroidizing method for hot rolled steel wire rod - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] This method relates to a direct spheroidizing method for hot-rolled steel wire rods.

[Conventional technology]

The hot-rolled steel wire rod made of carbon steel or alloy steel containing C: 0.10 to 1.00 wt.% Has a structure such as pearlite, bainite, reticulated cementite, etc. It is scarce and cracks occur when cold working to a high strength. Therefore, in order to improve the cold workability of such a steel wire rod, conventionally, a spheroidizing annealing treatment for making cementite into a spherical shape has been performed.

As a typical example of a conventional spheroidizing annealing method, a hot-rolled steel wire rod is held at a temperature immediately above A ₁ point (transformation point) for about 6 hours, and then immediately below A ₁ point. Examples include a method of gradually cooling to a temperature, holding at that temperature for about 8 hours, and then gradually cooling. However, the spheroidizing annealing method described above requires a long time of about 20 hours for processing one coil as a whole.

In order to directly soften the hot-rolled steel wire rod while omitting the spheroidizing annealing treatment, the following methods (1) and (2) have been conventionally performed.

(1) A method in which a material to be rolled during hot rolling is water-cooled before finish rolling to finish-roll at a temperature near A ₁ point, and then to slowly cool the steel.

(2) A method of softening the steel by rapidly cooling the hot-rolled steel wire rod to a temperature directly above the A ₁ point and then slowly cooling it.

[Problems to be solved by the invention]

However, the steel structures obtained by the above methods (1) and (2) are mainly ferrite and pearlite structures. Therefore, spherical carbides cannot be obtained by these methods. That is, the obtained steel wire rod cannot reach the level of the steel wire rod subjected to the spheroidizing annealing treatment in hardness and cold workability.

Therefore, an object of the present invention is to eliminate the spheroidizing annealing treatment in the secondary working step, which has been conventionally indispensable, by controlling the cooling process of the hot-rolled steel wire rod. It is an object of the present invention to provide a direct spheroidizing treatment method for a hot-rolled steel wire rod, which can produce a steel wire rod having a spheroidization ratio and hardness equivalent to those of a spheroidizing annealing treatment material in a short time.

[Means for solving problems]

This invention hot-rolls a steel slab containing C: 0.10 to 1.00 wt.%, Sets the temperature of the material to be rolled on the inlet side of the finishing rolling mill group to 650 to 850 ° C, and outputs the finishing rolling mill group. To prepare a steel wire rod having a final finishing temperature of 750 to 900 ° C. on the side, and then cooling the steel wire rod to a temperature of 650 ° C. or less at a cooling rate of 2 ° C./second or more, and then The cooled steel wire rod is heated at a heating rate of 2 ° C./sec or more from A _{c1 to} A _c1 + 160 ° C.
Heating up to the temperature range of
Hold the time for less than a minute, then heat and hold the steel wire rod in the temperature range, _{Ar r1} ~ _Ar1-60 at any cooling rate
Cool to a temperature range of ℃, and in the temperature range 5
Characterized by holding for a time of ~ 60 minutes, or
The steel wire rod heated and held in the temperature range of A _{c1 to} A _c1 + 160 ° C. is cooled to a temperature of _{Ar 1} at an arbitrary cooling rate, and then the steel wire rod cooled to the temperature is cooled to 2 ° C. / sec following cooling rates are those characterized by a cooling to a temperature of a _r1 -80 ° C..

 Next, the method of the present invention will be described in detail.

In order to rapidly make a steel wire into a spherical shape, it is important to obtain a uniform and fine pre-structure suitable for the spherical shape.

Therefore, in the present invention, in order to generate a fine austenite structure by hot rolling, the material to be rolled is formed by water cooling zones arranged at the inlet side and the outlet side of the finishing rolling mill group (finishing block mill). The temperature of the material to be rolled on the inlet side of the finishing rolling mill group is kept within the range of 650 to 850 ° C, while the final finishing of the material to be rolled just before winding by the lining head on the outlet side of the finishing rolling mill group Temperature 750-900
It was limited to within the range of ° C.

The temperature of the material to be rolled on the entry side of the finishing mill group is 650 to 8
The reason for limiting the temperature within the range of 50 ° C is that if the temperature exceeds 850 ° C, it becomes difficult to generate fine austenite grains due to the temperature rise due to heat generation during rolling in the finishing rolling mill group, while the temperature is This is because if the temperature is lower than 650 ° C, the load on the finishing rolling mill group becomes large and there is a risk of roll breakage.

The reason for limiting the final finishing temperature of the material to be rolled within the range of 750 to 900 ° C is that when the temperature exceeds 900 ° C, growth of austenite grains easily occurs, while the temperature is 750 ° C.
If it is less than the above range, it becomes difficult to wind the steel wire rod into a coil by the raying head.

The steel wire rod wound into a coil by the laying head through the steps described above should be rapidly cooled to a temperature of 650 ° C or lower at a cooling rate of 2 ° C / sec or more immediately after the start of transportation. This is because it is possible to obtain fine ferrite and pearlite structures (depending on the chemical composition of steel, bainite, cementite, or martensite may be mixed therein).

The reason for limiting the cooling rate to 2 ° C./second or more is that such a rapid cooling makes it possible to obtain a fine pre-structure suitable for spheroidization in a short time.

If the cooling rate is less than 2 ° C./sec, pearlite becomes coarse and it takes time to transform, which is not suitable. The upper limit of the cooling rate is usually about 100 ° C./second, which is usually water cooling which can be easily carried out industrially.

The reason why the cooling temperature is limited to 650 ° C. or lower is that if it exceeds 650 ° C., it takes a long time to transform austenite to pearlite and the resulting structure becomes a coarse structure not suitable for spheroidization.

The lower limit of the cooling temperature is preferably about 300 ° C. When the cooling temperature becomes lower than about 300 ℃,
The rapid heating in the next process causes energy loss, which is economically disadvantageous.

In this way, the steel wire rod cooled to a temperature of 650 ° C. or lower is then heated at a heating rate of 2 ° C./sec or higher to A _{c1 to} A _c1 +160.
Heat to a temperature range of ° C and hold at heated temperature for up to 5 minutes.

The purpose of this step is to dissolve a part of the cementite in the fine structure such as pearlite obtained in the previous step, and leave a part as a nucleus for spheroidization.

The reason for limiting the heating rate to 2 ° C./second or more is that it takes a long time to heat to the target temperature at a heating rate of less than 2 ° C./second.

Set the heating target temperature and heating holding temperature to A _{c1 to} A _c1 + 160 ° C.
The reason for limiting the temperature range is that if the previous period temperature is less than A _c1 , dissolution of cementite does not occur, while the above temperature is A _c1 +1.
This is because if it exceeds 60 ° C, cementite melts rapidly and it becomes difficult to leave nuclei for spheroidization.

The reason why the heating and holding time is within 5 minutes is that the purpose can be sufficiently achieved within a short time within 5 minutes.

Thus, the steel wire rod heated and held in the temperature range of A _{c1 to} A _c1 + 160 ° C. is cooled in order to precipitate carbon in austenite as spheroidized carbide by either of the following two methods.

The first method is cooled in any cooling rate to a temperature range of A _r1 ~A _r1 -60 ℃, and, performed by retention time of the cooling temperature 5-6 minutes.

The second method is cooled in any cooling rate to a temperature of A _r1, it is then carried out by cooling to a temperature of A _r1 -80 ° C. The following cooling rate 2 ° C. / sec.

In the first and second methods, the cooling rate is arbitrarily selected depending on the heating and holding temperature (A _{c1 to} A _c1 + 160 ° C.) in the previous step.

For example, when the heating and holding temperature in the previous step is a temperature close to A _c1 , it is preferable to cool as slowly as possible in order to promote the dissolution of cementite. On the other hand, when the heating and holding temperature is close to A _c1 + 160 ° C., it is preferable to quench as much as possible in order to prevent the cementite from rapidly melting.

The reason for limiting the cooling temperature in the first method to the temperature range of A _{r1 to} A _r1 −60 ° C. is that it is necessary to cool to a minimum of A _r1 in order to precipitate solid solution carbon as spherical cementite. Because.

After cooling, the temperature range for maintaining the same temperature is A _r1
~ _Ar1 -60 ° C, holding time should be 5-60 minutes.
Capped, when kept isothermally at a low temperature of less than A _r1 -60 ° C.,
This is because the precipitation of cementite proceeds rapidly and the carbide does not become spherical but becomes layered pearlite. Further, if the isothermal holding time is less than 5 minutes, the spheroidization of the carbide is not completed sufficiently, while if it exceeds 60 minutes, the improvement of the spheroidization of the carbide is not observed, and the purpose can be sufficiently achieved in 60 minutes or less. Is.

In the second method, A _r1 to A at a rate of 2 ° C / sec or less
The reason for cooling to _r1 -80 ° C. is because spheroidizing of carbides in annealing to A _r1 -80 ° C. to complete.

The reason for limiting the cooling rate to 2 ° C./sec or less is 2
This is because the carbide is not spherical and becomes layered pearlite when cooled at a high cooling rate exceeding ° C / sec.

The non-concentric ring-shaped steel wire rod that has undergone the above-mentioned treatment is
It is formed into a coil with a refforming tab and then air-cooled.

The chemical composition of the steel piece in the present invention is not particularly limited except that the carbon content is within the range of 0.10 to 1.00 wt.%, And the carbon steel for cold heading containing the above carbon, the structure Applicable to alloy steel, bearing steel, carbon tool steel, etc.

〔Example〕

Next, the method of the present invention will be described with reference to Examples and Comparative Examples.

S4 as a test material having the chemical composition shown in Table 1
5C (A _c1 = 725 ° C, A _r1 = 715 ° C) 114φ steel slab was hot-rolled into 8φ steel wire rod, and the equipment shown in Fig. 2 was used to perform the spheroidizing process shown below. Specimen No. within the scope of the invention
Samples Nos. 5 to 10 were prepared for Comparative Samples 1 and 2 and outside the scope of the present invention.

Similarly, a 114 φ steel piece of SC _r 440 (A _c1 = 740 ° C, A _r1 = 720 ° C) as a test material having the chemical composition shown in Table 1 was used.
98φ steel wire rod is hot rolled and the equipment shown in Fig. 2 is used.
By the spheroidizing treatment step shown below, the specimen No. 3, 4 within the scope of the present invention, and the comparative specimen No. outside the scope of the present invention.
Prepared to 11-16.

In FIG. 2, 3 is a water cooling zone on the entry side of the finishing rolling mill group,
4 is a finishing rolling mill group, 5 is a finishing rolling mill group outlet side water cooling zone,
6 is a raining head, 7 is a quenching device, 8 is a reheating device,
Reference numeral 9 is an isothermal holding (or gradual cooling) device. Rolled material 1
Is a finishing rolling mill group entry side water cooling zone 3, a finishing rolling mill group 4,
After finishing rolling is performed by passing through the finishing-rolling machine group water cooling zone 5 in this order, it is wound into a coil by a lining head 6, and then the coiled steel wire 2 is cooled by a quenching device 7 and a reheating device 8. And isothermal holding (or slow cooling) device 9
In this order, rapid cooling, reheating, cooling, isothermal holding or slow cooling are performed in sequence.

Next, the spheroidizing process will be described with reference to the graph showing the first heat pattern.

In FIG. 1, T ₁ is the inlet temperature of the finishing rolling mill group 4, T ₂
Is the final finishing temperature at the exit side of the finishing rolling mill group 4, T ₃ is the maximum heating start temperature after being rapidly cooled by the quenching device 7, T ₄ is the isothermal holding temperature after being heated by the maximum heating device 8, and T ₅ Is the isothermal holding temperature after cooling, and T ₆ is the slow cooling stop temperature.

After the hot-rolled sample material is cooled to the temperature of T ₁ by the water-cooling zone 3 on the entry side of the finishing rolling mill group, the finishing rolling mill group 4
Therefore, finish rolling was performed. After recuperating heat by finishing rolling of the finishing rolling mill group 4, by using the water cooling zone 5 on the delivery side of the finishing rolling mill group.
After cooling to a temperature of T ₂ , it was wound into a coil by a laying head 6. Next, the coiled sample material is cooled to a temperature of T ₃ at a rate of 2 to 15 ° C./sec by a wind blow in the quenching device 7 while being conveyed on a Stelmore line (not shown), and then, The cooled coil-shaped test material was rapidly heated to the temperature of T ₄ by the reheating device 8 and kept at the temperature of T ₄ for 30 to 60 seconds.

Then, the test material held at the temperature of T ₄ for 30 to 60 seconds is put into T ₅
To the temperature of T ₅
The temperature was kept isothermal for ₅ minutes, and then air cooled.

On the other hand, the test material held 30-60 seconds at a temperature of T ₄ A _r1
To the temperature of T ₆ by means of the slow cooling device 9.
After gradually cooling at a rate of 0.05 ° C / sec, it was cooled by air.

By changing the temperatures of T _{1 to} T ₆ described above, several kinds of specimens within and outside the scope of the present invention were prepared. Then, the spheroidization rate (SA rate) and hardness (H _v ) of the structure of each test piece were examined, and the results are shown in Table 2 together with the processing temperatures T _{1 to} T ₆ .

As shown in Table 1, Comparative Specimens No. 5 and No. 11 in which the finishing rolling mill group inlet side temperature T ₁ and the final finishing temperature T ₂ were too higher than the range of the present invention had a pre-spheroidizing structure. It was coarse and inferior in spheroidization rate.

Reheatable start temperature T ₃ after quenching was comparatively higher than the range of the present invention for comparative specimens No. 6 and No. 12, as a nucleus for spheroidization due to insufficient cooling to A ₁ or less. The carbide was insufficient, the spheroidization rate was poor, and the hardness was high.

The comparative specimens No. 7 and No. 13, whose isothermal holding temperature T ₄ was higher than A _c1 + 160 ° C, had a shortage of carbide as a nucleus for spheroidization as a result of rapid dissolution of cementite. The spheroidization rate was poor and the hardness was high.

The isothermal holding temperature T ₅ was higher than A _r1 .
Nos. 8 and 14 were air-cooled before solid solution carbon was sufficiently precipitated, and as a result, coarse regenerated pearlite appeared and the spheroidization rate was poor, and the hardness was high.

On the contrary, the comparative specimens No. 9 and No. 15, whose isothermal holding temperature T ₅ was lower than _{Ar 1} −60 ° C., did not sufficiently spheroidize as a result of rapid precipitation of carbides. Furthermore, the hardness was also high.

For the comparative test samples No. 10 and No. 16 in which the slow cooling stop temperature T ₆ was higher than _{Ar 1} −80 ° C., the regenerated pearlite was generated in the subsequent air cooling process because the slow cooling was insufficient. As a result, the spheroidization rate was poor and the hardness was high.

On the other hand, Sample Nos. 1 to 4 of the present invention had good spheroidization rate and low hardness.

FIG. 3 is a 400 times micrograph showing the metal structure of the sample 4 of the present invention.

As shown in FIG. 3, the sample 4 of the present invention has a good spheroidization, and the spheroidization condition is quite comparable to that of the steel wire rod subjected to the usual spheroidization annealing treatment. It was

〔The invention's effect〕

As described above, according to the method of the present invention, a wire having a spheroidization rate and hardness equivalent to that of the wire processed by the conventional spheroidizing annealing method is used, Since it can be manufactured by
The industrially useful effect that the production efficiency is significantly improved can be obtained.

[Brief description of drawings]

FIG. 1 is a graph showing a heat pattern of a spheroidizing treatment method of the present invention, FIG. 2 is a process diagram showing an outline of a spheroidizing treatment equipment of a hot-rolled steel wire rod, and FIG. 3 is manufactured by the method of the present invention. is a photomicrograph showing the metal structure of the steel wire rod SC _r 440.
In the drawings, 1 ... Rolled material, 2 ... Steel wire rod, 3 ... Finishing rolling mill group inlet side water cooling zone, 4 ... Finishing rolling mill group, 5 ... Finishing rolling mill outlet side water cooling zone, 6 ... ... Raining head, 7 ... quick cooling device, 8 ... reheating device, 9 ... isothermal holding (or slow cooling) device.

Claims (2)

(57) [Claims] 1. A steel slab containing C: 0.10 to 1.00 wt.% Is hot-rolled, and the temperature of the material to be rolled on the inlet side of the finishing rolling mill group is adjusted.
650 to 850 ° C., the final finishing temperature of the rolled material on the exit side of the finishing rolling mill group is set to 750 to 900 ° C. to prepare a steel wire rod, and then the steel wire rod is cooled at 2 ° C./sec or more. At speed
It is cooled to a temperature of 650 ° C or lower, then the cooled steel wire is heated to a temperature range of A _{c1 to} A _c1 + 160 ° C at a heating rate of 2 ° C / sec or more, and within 5 minutes in the temperature range. and the retention time, then, heated to the temperature range, the steel wire is held, cooled in any cooling rate to a temperature range of a _r1 ~A _r1 -60 ℃, and, in the temperature range 5 to 60
A method for directly spheroidizing a hot-rolled steel wire rod, characterized by holding for a time of 3 minutes. 2. A steel slab containing C: 0.10 to 1.00 wt.% Is hot-rolled, and the temperature of the material to be rolled at the entry side of the finishing rolling mill group is adjusted.
650 to 850 ° C., a steel wire rod is prepared by setting the final finishing temperature of the rolled material on the exit side of the finishing rolling mill group to 750 to 900 ° C., and then cooling the steel wire rod at 2 ° C./second or more. At speed
It is cooled to a temperature of 650 ° C. or lower, then the cooled steel wire is heated to a temperature range of A _{c1 to} A _c1 + 160 ° C. at a heating rate of 2 ° C./sec or more, and within 5 minutes in the temperature range. The steel wire rod, which has been held for a time of, and heated and held in the temperature range, is cooled to a temperature of _Ar1 at an arbitrary cooling rate, and then the steel wire rod cooled to the temperature is 2 ° C / sec or less. characterized by cooling at a cooling rate to a temperature of a _r1 -80 ° C., directly spheroidizing treatment method of the hot rolled steel wire rod.