JPH10298641A - Production of steel excellent in spheroidize-annealing treatability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce energy cost and to execute sufficient spheroidize-annealing treatment by an inexpensive heat treatment by applying a steel cong. a specified ratio of C with the magnetic field in a specified temp. range in succession to hot rolling and furthermore executing cooling at a specified cooling rate. SOLUTION: In the steel to be produced, the content of C is regulated to 0.01 to 0.8 mass%. This steel is applied with the magnetic field in the temp. range of 800 to 500 deg.C at the time of cooling in succession to hot rolling, and the cooling rate in the temp. range is regulated to <=10 deg.C/sec. In this way, before austenite is transformed into a ferritic/pearlitic structure, the precipitation of cementite in pearlite is suppressed, and the lamellar intervals in cementite is shortened, by which a fine ferritic/pearlitic structure an be obtd. Preferably, the intensity of the magnetic field to be applied is regulated 2 to 30 T, and the size of the magnetic field gradient is regulated to 0.1 to 10 T/cm by absolute value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a steel material and, in a steel material subjected to a spheroidizing annealing process for facilitating plastic working and cutting, to shorten the spheroidizing annealing time. Related to technologies that can be used.

[0002]

2. Description of the Related Art Conventionally, carbon steel or alloy steel has been widely used as a material for mechanical parts used in automobiles and industrial machines. These mechanical parts are usually manufactured by spheroidizing and annealing bar steel wires produced by hot rolling, cutting, cold forging to a predetermined shape, and finally performing finishing such as cutting. I have. In the manufacturing process of such mechanical parts, cold forging is widely used because it is excellent in terms of product processing accuracy, mass productivity and cost,
It is desired to improve the cold forgeability from the viewpoint of processing accuracy and mold life, and for that purpose, spheroidizing annealing is performed in advance for spheroidizing carbides in steel and reducing deformation resistance of steel materials. .

[0003] Such spheroidizing annealing is described in FIG.
As shown in the example of the heat pattern, a method of heating and holding immediately above the transformation point of Ac ₁ for a long time followed by slow cooling is generally used.During this heating and holding, cementite in steel is dissolved in austenite to form a solid solution. And reprecipitated and grown spherically with the remaining cementite as nuclei. But,
This heat treatment had a problem that the heat treatment cost was increased due to high temperature and long-time heating.

As a solution to these problems, for example,
The 2898 discloses, discloses a short-time spheroidization heat treatment method of the high carbon chromium bearing steel, until the steel to 780-820 less heated and held after Ar ₁ transformation point to ℃ a heat pattern specifically shown in FIG. 2 50 Primary spheroidizing treatment at ~ 200 ° C / h, Ac
A proposal has been made to perform a heat treatment consisting of a combination of three or more secondary spheroidizing treatments, which are heated from ₁ transformation point to Ac ₁ transformation point + 30 ° C and then cooled to 50 ° C to 200 ° C / h below the Ar ₁ transformation point. is there. JP-A-4-362123, as shown in FIG. 3 as a method for producing a material for bearings, quenched after heating hold _three or more points A, then reheated to A ₃ + (5~30) ℃ was held a ₁ - holding (5-30) in ° C., then a ₁ + (5-30) ° C.
It has been proposed to adopt a heat pattern heat treatment method in which holding at a temperature of A _1- (5 to 30) ° C. is performed at least once.

[0005] However, the method disclosed in Japanese Patent Publication No. Hei 6-2898 cited above is a method which took 20 hours conventionally.
Although the heat treatment time was shortened to 10 hours, the heat treatment still required a long time of 10 hours and repeated thermal cycles were repeated several times.Therefore, problems remained in terms of energy cost and temperature control. It has been done. Further, the method described in the above-mentioned Japanese Patent Application Laid-Open No. 4-362123 is improved in terms of energy cost, but is insufficient in terms of the degree of spheroidization of carbides which should be noticed in spheroidizing annealing. .

[0006]

SUMMARY OF THE INVENTION In view of the above circumstances, the present invention provides a method of manufacturing a steel material having excellent spheroidizing annealing property, which can reduce the energy cost and sufficiently spheroidize at low heat treatment cost. The aim is to propose a method.

[0007]

Means for Solving the Problems In order to achieve the above object, the present inventors have proposed a hypoeutectoid steel containing 0.01 to 0.8 mass% of carbon and a hypereutectoid steel containing 0.8 to 2.0 mass% of carbon. As a result of intensive investigation and research on shortening the spheroidizing annealing process, it was found that the maximum temperature holding time and the degree of spheroidization of carbides were greatly affected by the structure before spheroidizing annealing in each case. We have found and further studied and found the following findings.

[0008] In the case of hypoeutectoid steel containing 0.01 to 0.8 mass% of carbon, in order to shorten the heating time in spheroidizing annealing, it is preferable to make the ferrite + pearlite structure as fine as possible. Then, since all of cementite does not form a solid solution and becomes a nucleus of spheroidization, appropriate refinement is required. Such appropriate miniaturization can be easily achieved by applying a magnetic field during cooling subsequent to rolling.

In the case of a hypereutectoid steel containing 0.8 to 2.0 mass% of carbon, the structure before spheroidizing annealing is as follows:
As the amount of proeutectoid cementite is smaller, the degree of spheroidization of the steel material that has undergone spheroidizing annealing in the next step is improved, and the time required for annealing is reduced. The pro-eutectoid cementite is generated during cooling after hot rolling, but the generation is suppressed by applying a magnetic field during cooling.

The gist of the present invention based on the above findings is as follows. In a method of producing a steel material containing C: 0.01 to 0.8 mass% through hot rolling in which spheroidizing annealing treatment is performed in the next step, a magnetic field is applied in a temperature range of 800 to 500 ° C during cooling after hot rolling. And a cooling rate in the temperature range of 10 ° C./s or less. A method for producing a steel material having excellent spheroidizing annealing property (first invention). In a method of producing a steel material containing 0.8 to 2.0 mass% of C through hot rolling, which is subjected to spheroidizing annealing in the next step, a magnetic field is applied in a temperature range of 800 to 500 ° C during cooling after hot rolling. And a cooling rate in the temperature range of 10 ° C./s or less, the method for producing a steel material excellent in spheroidizing annealing treatment (second invention). The method for producing a steel material excellent in spheroidizing annealing treatment according to the first invention or the second invention, wherein a cooling rate during application of a magnetic field is 50 ° C./h or less (third invention). In the first invention or the second invention, the strength of the magnetic field is set to 2 to 30 T, and the magnetic field gradient is set to an absolute value of 0.1.
A method for producing a steel material having excellent spheroidizing annealing property, characterized in that the pressure is 1 to 10 T / cm (fourth invention). By adopting the present invention when manufacturing a steel material, the time during which the steel material is kept at a high temperature during the annealing process can be significantly reduced as compared with the conventional case, and the cost energy required for spheroidization can be reduced.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the first invention, in order to obtain a fine ferrite pearlite structure, a magnetic field is applied in a temperature range of 800 to 500 ° C. during cooling after hot rolling, and a cooling rate in the temperature range is applied. Is set to 10 ° C./s or less for the following reason.

By applying a magnetic field before the transformation of austenite into a ferrite-pearlite structure during the cooling process following the rolling, the precipitation of cementite in pearlite is suppressed, the lamella spacing of cementite is shortened, and fine It comes to have a ferrite-pearlite structure. In order to obtain a ferrite-pearlite structure, the cooling rate must be 10 ° C./s or less, and a magnetic field must be applied during the cooling process at 800 to 500 ° C. where the ferrite-pearlite transformation occurs. The cooling rate at 500 ° C. or lower can be either rapid cooling or slow cooling. A more preferred cooling rate is 50 ° C./h or less.

The strength of the applied magnetic field is preferably 2 to 30 T, and more preferably 2 to 15 T. If it is less than 2T, the effect of refining pearlite cannot be obtained. Spherical carbide cannot be obtained. Next, the reason for setting the magnitude of the magnetic field gradient to 0.1 T / cm or more and 10 T / cm or less in absolute value is as follows. First, since the effect of the magnetic field gradient is determined by its absolute value, it can be either positive or negative. 0.1 T / c
If less than 10 m / m, the effect of applying a magnetic field is not recognized, while 10 T / cm
Therefore, the magnitude of the magnetic field gradient is limited to 0.1 to 10 T / cm, since the effect reaches saturation even if it exceeds.

Next, in the second invention, during cooling subsequent to hot rolling, a magnetic field is applied in a temperature range of 800 to 500 ° C., and the cooling rate in that temperature range is set to 10 ° C./s or less. ,It is as follows.

800-500 ° C. for cooling after hot rolling
The application of a magnetic field during the cooling of cement mainly causes the formation of cementite from austenite during cooling at 800 to 500 ° C. By applying a magnetic field in this temperature range, the formation of proeutectoid cementite is suppressed. This is because you can do it. Further, the lamella of the pearlite structure is also changed to a divided form by the application of the magnetic field, and spheroidization is more likely to occur in the subsequent spheroidizing treatment. The cooling rate in this temperature range needs to be 10 ° C./s or less, and when the cooling rate exceeds 10 ° C./s, it is difficult to suppress the formation of proeutectoid cementite by applying a magnetic field. Therefore, a preferable cooling rate is 50 ° C./h or less.

The strength of the applied magnetic field is preferably from 2 to 30T. Below 2T, the effect of suppressing the generation of proeutectoid cementite by the magnetic field is not clear. On the other hand, the application of a magnetic field exceeding 30T requires 2-30T because the apparatus becomes large and the cost increases. Next, the magnitude of the magnetic field gradient is
The reason for setting the absolute value to 0.1 T / cm or more and 10 T / cm or less is as follows. First, since the effect of the magnetic field gradient is determined by its absolute value, it can be either positive or negative. 0.
At less than 1 T / cm, the effect of applying a magnetic field was not observed.
Since the effect reaches saturation even when the temperature exceeds T / cm 2, the magnitude of the magnetic field gradient is limited to 0.1 to 10 T / cm 2.

The steel material to be used in the present invention may be any steel material having a carbon content of 0.01 to 0.8 mass% and a steel material having a carbon content of 0.8 to 2.0. Therefore, the present invention
It can be applied to a wide range from low carbon steel to medium carbon steel, high carbon steel, low alloy steel (for example, high carbon chromium bearing steel, tool steel), and high alloy steel (stainless steel).

Such a material is subjected to hot working, typically hot rolling, after heating, and the heating condition and the hot rolling condition may be performed according to the conditions usually applied to each material. It is important that the cooling following the hot rolling be performed under the conditions according to the present invention. Thus, when the steel material of the present invention is subjected to spheroidizing annealing, it takes less time than before.

[0019]

【Example】

(Example 1) Steels having various chemical compositions shown in Table 1 were melted in a converter, turned into billets by a continuous casting method, and then rolled into 55 mmφ steel bars. A magnetic field was applied under various conditions shown in Table 2 during the cooling process after the rolling of the bar. The magnetic field application temperature range is 80
0-500 ° C. Then, these steel bars were subjected to spheroidizing annealing according to the heat pattern shown in FIG. The steel shape after spheroidizing annealing was examined for carbide shapes at 5000 times in 10 fields of view, and the degree of spheroidization of the carbide was observed. The degree of spheroidization was calculated from the ratio (%) of the amount of carbide having a ratio of major axis / minor axis of 2 or less. Table 2 shows the results.

[0020]

[Table 1]

[0021]

[Table 2]

Nos. 1 to 8 are examples of the present invention, and in all cases, a good spheroidized material having a spheroidization ratio of more than 90% was obtained by spheroidization treatment for a short time. No. 9 is a result of the conventional steel, in which the spheroidization of carbide is not sufficiently performed despite the spheroidization treatment for a long time. No. 1
0, No. 11, No. 13, and No. 16 are cases where the magnetic field or the magnetic field gradient is insufficient, and sufficient spheroidization was not performed in a short time. Nos. 14 and 17 are cases where the magnetic field was excessive, and sufficient spheroidization was not performed. No. 12, No. 15, No. 18
Was unsuitable because the cooling rate was too fast and the structure became bainite.

As is clear from Table 2, the production method according to the present invention provided a good spheroidizing property in which the degree of spheroidization of the carbide was close to 100% in a very short time as compared with the conventional method. . Therefore, it was found that, when the spheroidizing annealing was performed on the steel material manufactured according to the present invention, the spheroidizing could be sufficiently achieved even if the holding time at the highest temperature was short. That is, the energy cost can be reduced as compared with the related art, and the inexpensive spheroidizing treatment can be performed.

(Example 2) Steels having various component compositions shown in Table 3 were melted in a converter, made into a billet by a continuous casting method, and then rolled into a 55 mmφ steel bar. 800 to
The sample was cooled at a predetermined cooling rate in a range of 500 ° C. while applying a predetermined magnetic field shown in Table 4. From the thus obtained sample, a micro sample was cut out, and the structure was observed at 10 times visual field at 400 times to examine the fraction of proeutectoid cementite. Next, as shown in FIG. 5, the sample was heated and maintained at 755 ° C. for a predetermined time shown in Table 4, then cooled at a cooling rate of 15 ° C./h to 650 ° C., and then cooled to room temperature under spherical conditions. Annealed by annealing. A micro sample was cut out from the material after the spheroidizing annealing, and the carbide shape was examined at 5,000 times in ten visual fields, and the degree of spheroidization of the carbide was examined. The results are shown in Table 4.

[0025]

[Table 3]

[0026]

[Table 4]

As is clear from Table 4, in the case of the steel material manufactured by the method according to the present invention, the amount of carbide having a ratio of major axis / minor axis of 2 or less when the spheroidizing annealing heat holding time is as short as 1 hour. Is close to 100%, and a spheroidized structure showing good workability is obtained. In contrast, Comparative Example No.
In Nos. 6 and 7, even when the spheroidizing annealing heat holding time was increased to 4 hours, the ratio of carbide having a ratio of major axis / minor axis of 2 or less was small, resulting in an insufficient spheroidized structure.

[0028]

As described above, according to the method for manufacturing a steel material of the present invention, 800 to 800 mm
By applying a magnetic field in the temperature range of 500 ° C and setting the cooling rate in that temperature range to 10 ° C / s or less,
A good structure having a spherical carbide before spheroidizing annealing is obtained, and it is possible to produce a steel material excellent in spheroidizing annealing processability that can sufficiently achieve spheroidization at a low heat treatment cost, which is extremely useful in industry. The effect is brought.

[Brief description of the drawings]

FIG. 1 is a diagram showing a heat pattern of general spheroidizing annealing.

FIG. 2 is a diagram showing an example of a heat pattern of a conventional spheroidizing annealing.

FIG. 3 is a diagram showing an example of a heat pattern of a conventional spheroidizing annealing.

FIG. 4 is a view showing a heat pattern of spheroidizing annealing in Example 1.

FIG. 5 is a view showing a heat pattern of spheroidizing annealing in Example 2.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Takuya Atsumi 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. 1-chome (without address) Mizushima Works, Kawasaki Steel Corporation (72) Inventor Kenichi Amano 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Prefecture (without address) Mizushima Works, Kawasaki Steel Corporation (72) 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Pref.

Claims (4)

[Claims] 1. A spheroidizing annealing treatment is performed in the next step.
C: In a method for producing a steel material containing 0.01 to 0.8 mass% through hot rolling, in the cooling following the hot rolling,
A method for producing a steel material having excellent spheroidizing annealing property, wherein a magnetic field is applied in a temperature range of 800 to 500 ° C and a cooling rate in the temperature range is 10 ° C / s or less. 2. A spheroidizing annealing treatment is performed in the next step.
C: In a method for producing a steel material containing 0.8 to 2.0 mass% through hot rolling, in the case of cooling subsequent to hot rolling,
A method for producing a steel material having excellent spheroidizing annealing property, wherein a magnetic field is applied in a temperature range of 800 to 500 ° C and a cooling rate in the temperature range is 10 ° C / s or less. 3. The method for producing a steel material excellent in spheroidizing annealing treatment according to claim 1, wherein the cooling rate during application of the magnetic field is 50 ° C./h or less. 4. The steel material excellent in spheroidizing annealing treatment according to claim 1 or 2, wherein the magnetic field strength is 2 to 30 T and the magnetic field gradient is 0.1 to 10 T / cm in absolute value. Production method.