JPH0741865A - Method for spheroidizing high carbon steel sheet by continuous annealing - Google Patents

Abstract

PURPOSE:To execute the carbide spheroidization of a high carbon hot-rolled steel sheet by continuous annealing. CONSTITUTION:At the time of spheroidizing of the high carbon hot-rolled steel sheet containing 0.35-1.40wt.% C, the steel sheet temp. is raised to 730-770 deg.C at 3-20 deg.C/sec heating speed, and after keeping for 10sec-3min, the steel sheet is cooled at least to <=650 deg.C at <=5 deg.C/sec cooling speed. By this method, the spheroidization of the carbide needing the long time can be executed for short time in few min, and uneven quality in a coil becomes small.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of continuously spheroidizing a high carbon steel sheet by annealing.

[0002]

2. Description of the Related Art High-carbon steel sheets are divided into those that are used as hot-rolled and those that are produced by combining hot-rolled sheets with cold rolling and annealing. Chain parts, gear parts, clutch parts, saws, Used for cutlery, etc. These are manufactured by hardening into a target product shape by heat treatment such as quenching and tempering after forming. In most cases, a steel plate is a product with the product plate thickness as it is, and the accuracy of the plate thickness is important. On the other hand, since the hardness of the product is adjusted by quenching and tempering, the ease of solution treatment of the carbide during quenching is also important. Since the high carbon hot rolled steel sheet has a pearlite structure and is hard, cold rolling property and strip thickness accuracy are poor, and lamellar carbides are often spheroidized and cold rolled. The spheroidization of the carbide is also necessary to facilitate the solution treatment of the carbide during quenching.

The spheroidizing of the high carbon steel sheet is performed by box annealing with the coil as it is. In box annealing, the temperature difference depending on the coil position is large, and the spheroidized carbide has a large variation depending on the coil position, which causes the sheet thickness accuracy in the subsequent cold rolling and the material variation after quenching. Further, since it is annealed by a coil, it takes several hours or more at 700 ° C. to substantially spheroidize the carbides, and if heating and cooling are added, it takes several days, resulting in extremely poor productivity.

Japanese Patent Publication No. 62-52011 discloses a technique for performing continuous annealing for spheroidizing annealing of a high carbon steel sheet. This is because medium-high carbon steel is heated in the austenite region and then rapidly cooled, after which 780 to 800 ° C and 100
Rapid heating and rapid cooling are repeated 2 to 6 times below the temperature of ℃. According to this method, it is possible to shorten the spheroidizing annealing by the box annealing, which conventionally takes several hours or more, to several minutes, but since heating and cooling is performed a plurality of times, expensive equipment is required.

[0005]

SUMMARY OF THE INVENTION The object of the present invention is to overcome the above-mentioned drawbacks and to provide a technique for performing carbide spheroidizing annealing of a high carbon steel sheet by continuous annealing and inexpensive equipment, not by conventional box annealing. Especially.

[0006]

The present invention has been made to achieve the above-mentioned object, and the gist of the present invention is to provide a high carbon heat containing C: 0.35 to 1.4% by weight. During spheroidizing annealing of the rolled steel sheet, the heating rate was raised to 730 to 770 ° C at a heating rate of 3 to 20 ° C / sec, and the holding time was kept for 10 seconds to 3 minutes, and then at 650 ° C or less at a cooling rate of 5 ° C / second or less. It is a spheroidizing annealing method by continuous annealing of a high carbon steel sheet characterized by cooling to a temperature.

The constituent features of the present invention will be described in detail below.
First, the experimental facts found to enable the spheroidization of carbides after hot rolling even by continuous annealing will be described. C: 0.8
0%, Si: 0.18%, Mn: 0.80%, Al:
After descaling the 0.035% 4.0 mm thick hot-rolled steel sheet, after heating and holding at a heating temperature of 680 to 800 ° C. for 1 minute,
The structure was investigated by cooling at a cooling rate of 0.1 to 30 ° C./sec. The results are shown in Fig. 1. The line in the figure shows the boundary between the lamellar carbide + martensite structure, where the spheroidal carbide is 95% or more (hatched portion), the lamellar carbide + spheroidal carbide is 95% or less. As is clear from this result, the carbide is spheroidized even if the annealing holding time is about 1 minute. That is, it was found that if the heating temperature is 730 to 770 ° C. and the cooling rate is 5 ° C./sec or less, the carbide can be spheroidized even by the short-time continuous annealing. Similar experiments were conducted for other steel types,
It was confirmed that if the above conditions were selected, the carbides would become spherical.
Based on the above experimental facts, an annealing temperature of 730 to 770 ° C. and a cooling rate of 5 ° C./second or less were specified as the conditions under which the carbide was spheroidized.

Next, using the same steel sheet as in FIG. 1, the heating rate was kept constant at 10 ° C./sec and the cooling rate was kept at 1 ° C./sec, the heating temperature and the heating time were changed, and the influence on the spheroidization of carbide was examined. The results are shown in FIG. The display in the figure is the same as that in FIG. As can be seen from the figure, as the annealing time increases, the carbides do not spheroidize on the contrary, and it is essential to retain for a short time. From this experimental fact, the retention time of 10 seconds to 3 minutes was specified.

Further, the heating rate was examined. The steel sheet used was the same as that in FIG. 1, and the heating temperature was 750 ° C., the holding time was 1 minute, and the cooling rate was 1 ° C./sec. The heating rate was shown in FIG. As can be seen from the figure, if the heating rate is too fast or too slow, no spheroidization occurs,
It is necessary to heat at a heating rate of 2 seconds.

From the above experimental facts, the heating rate: 3 to 2 is a condition under which the carbide is spheroidized in the continuous annealing of the high carbon steel sheet.
0 ° C / sec, retention temperature: 730-770 ° C, retention time: 1
A cooling rate of 0 second to 3 minutes and a cooling rate thereafter of 5 ° C./second or less were specified. The temperature range that requires cooling at 5 ° C / sec after retention is 6
Even if the cooling rate is increased at 50 ° C or higher and thereafter, the spheroidization of the carbide is not affected.

Next, the steel composition constituting the present invention will be described. C is an important element that controls the quenching hardness when used as a final product. Therefore, at least 0.35
%is necessary. On the other hand, if the amount of C is too large, cracking occurs during quenching, so 1.40% was made the upper limit.

Elements other than C are appropriately added depending on the purpose of use. If the Si content exceeds 0.80%, ferrite tends to be solid solution strengthened and the steel sheet tends to become brittle, so the upper limit is preferably made 0.80%.
Mn is an element that enhances hardenability. However, if Mn is too high, it is an element that promotes segregation of P that deteriorates impact properties after quenching and tempering and hydrogen embrittlement. Is preferred. The lower limit is preferably 0.2% to ensure hardenability. It has been conventionally known that Cr is an element that improves hardenability and at the same time improves wear resistance due to carbide precipitation. If it exceeds 1.5%, this effect will be saturated.
Add up to.

It is known that Mo raises not only the austenite coarsening temperature at the time of quenching but also the hardenability and the softening resistance by tempering. However, the effect is saturated when 0.50% or more is added. In the present invention, it may be added in a range of 0.50% or less, if necessary.

B is conventionally known as an element that enhances hardenability. In the present invention as well, in order to enhance the hardenability, it is added in a range of 0.0030% or less, if necessary. Al
Is known to be added as a deoxidizer. Also in the method of the present invention, 0.080% or less is added as required. It is known that Ni is an element effective in strengthening in a solid solution and at the same time, an element enhancing toughness. Also in the present invention, when toughness is particularly required, it is added within the range of 1.5% or less.

The steel having such a composition is melted in a converter or an ordinary melting furnace such as an electric furnace, and a slab is produced by a continuous casting machine or a steel ingot-segmenting rolling. The slab will be surface-cleaned as needed.

Next, hot rolling is performed to manufacture a hot rolled steel strip. In the present invention, spheroidizing is performed by continuous annealing regardless of hot rolling conditions, but it is preferable to employ the following hot rolling conditions in order to uniformly and efficiently spheroidize carbides. That is, when the finishing temperature is equal to or lower than the Ar ₃ point temperature, the workability of hot rolling and the plate thickness accuracy are deteriorated and the material variation is increased. Therefore, the hot rolling finish temperature is preferably set to Ar ₃ or higher. The cooling rate from the finishing temperature to the winding temperature becomes slower, the spheroidizing speed of the carbide becomes slower, or the area where the spheroidizing does not occur partially increases the material variation, so the average from the hot rolling finishing temperature to the winding It is preferable to set the cooling rate to 15 ° C./second or more.

The hot rolled sheet is subjected to spheroidizing treatment annealing of the carbide after descaling by pickling or the like. In the present invention, this spheroidizing treatment of carbide is not a conventional box annealing, but continuous production with good productivity and little material variation. The annealing method is used. In this carbide spheroidizing annealing, the heating rate was raised to a range of 730 to 770 ° C. at a heating rate of 3 to 20 ° C./second, and the temperature was kept for 10 seconds to 3 minutes, and then 5 ° C. /
It is necessary to cool to a temperature of at least 650 ° C. or less at a cooling rate of less than a second.

The spheroidized and annealed steel sheet is supplied to the customer as it is as a conventional product to be a product, or cold rolled if necessary, manufactured by annealing, pressure regulation, or a combination thereof, and supplied to the customer. It The cold rolling condition and the annealing condition are the same as the conventional method.

[0019]

EXAMPLES Steels having the compositions shown in Table 1-1 were manufactured under the manufacturing conditions shown in Table 1-2 and spheroidized and annealed. The spheroidized and annealed coil was sampled from the top, intermediate position and bottom position in the longitudinal direction in the width direction at 10 mm from both ends and from the center to examine the degree of spheroidization of the carbide. For spheroidization of carbides, a perfect spheroidization is rated 5, 90%
The above-mentioned spheroidization is scored 4, spheroidization of 70 to 89% is scoring 3, spheroidization of 50 to 69% is scoring 2, 50%
The following was evaluated with a score of 1. From the knowledge of the prior art, it is known that if the spheroidization is 90% or more, the spheroidization is sufficient. Therefore, a score of 4 or more was used as the spheroidization standard.

[0020]

[Table 1]

[0021]

[Table 2]

A-1, A-2 to I- of Examples within the scope of the present invention
No. 1 and No. 2 have a good spheroidizing score of 4 or 5 even if the steel type is changed. Specifically, the manufacturing conditions up to hot rolling are almost the same A-1, A
-3 and A-4 are compared, A- produced in the range of the present invention
No. 1 is spheroidized, but A-3 whose annealing heating temperature is out of the range of the present invention is hardly spheroidized, and A-4 whose annealing retention time is too long has insufficient spheroidization. Also, C: 0.
B-1 and B-2 in the present invention range of 53% have a good spheroidization score, but the cooling rate is B-3 outside the present invention range, and the hot rolling winding temperature is B-4 outside the present invention. B-5, which has a high annealing temperature, has insufficient spheroidization. In the example of C: 0.62%, both C-1 and C-2 in the range of the present invention are spheroidized, but C-3 having an excessively slow annealing heating rate is insufficient in spheroidization. As described above, it is understood that the carbides are not spheroidized by the continuous annealing for a short time unless the heating rate, the holding temperature / time and the cooling conditions are all satisfied.

Next, the components shown in Table 2 and the hot-rolled sheet under the hot-rolling conditions are within the conditions of the present invention: heating rate: 10 ° C./sec, heating retention condition: 750 ° C. × 60 sec, cooling rate: 1 ° C. / Spheroidized by continuous annealing for 2 seconds and spheroidized by conventional box annealing (6
90 ℃ × 12hr) 850 ℃ × 6 by high frequency heating
After holding for 0 second, it was quenched in oil at 60 ° C. and the hardness was measured. The obtained hardness is shown in the table. The method material of the present invention is H
While v: 812 is well quenched, the conventional condition material is Hv: 300, and the hardenability is almost insufficient.
As described above, it is understood that the present invention also has an effect of improving hardenability for a short time.

[0024]

[Table 3]

[0025]

As described in the above embodiments, by specifying the heating rate, the heating temperature, the holding time, and the cooling rate in an inseparable relationship, it is possible to obtain an expensive facility for spheroidizing annealing that conventionally required several days. It can be done in several minutes by continuous annealing without using it. Further, the steel sheet spheroidized by this method can obtain a sufficient quenching hardness by a short-time solution treatment such as induction quenching.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between spheroidization, annealing temperature, and cooling rate.

FIG. 2 is a diagram showing the relationship between spheroidization, annealing temperature, and holding time.

FIG. 3 is a drawing for explaining the relationship between the temperature rising rate during annealing and spheroidization.

Claims (1)

[Claims] 1. A heating rate for spheroidizing annealing of a high carbon hot rolled steel sheet containing C: 0.35 to 1.4% by weight:
The temperature is raised to 730 to 770 ° C. at 3 to 20 ° C./sec, the holding time is 10 seconds to 3 minutes, and then 65 at a cooling rate of 5 ° C./second or less.
A spheroidizing annealing method by continuous annealing of a high carbon steel sheet, characterized by cooling to a temperature of 0 ° C or less.