JP5534492B2 - Carbon tool steel strip manufacturing method - Google Patents

Description

  The present invention relates to a method for producing a carbon tool steel strip.

For example, a steel strip made of carbon tool steel suitable for use in springs, valves, etc. (hereinafter referred to as “carbon tool steel strip”) is generally rolled to a predetermined plate thickness, and then pre-tropical and heated zone. By using a continuous heating facility in which a quenching furnace, a spraying device, a water-cooled surface plate and a tempering furnace are continuously arranged in this order, a method of continuously quenching and tempering while unwinding a carbon tool steel strip It is manufactured.
A specific manufacturing method for quenching and tempering the above-described carbon tool steel strip using continuous heating equipment is described in, for example, Japanese Patent Application Laid-Open No. 56-139627 (Patent Document 1) by the present applicant. There is a manufacturing method. This manufacturing method prevents the occurrence of flaws and distortion in the carbon tool steel strip when quenching by quenching the carbon tool steel strip exiting the quenching furnace, and reduces the variation in hardness after quenching. Is something that can be done.
Such continuous heating equipment includes a heat source that uses combustion heat obtained by burning natural gas, light oil, heavy oil or the like with a burner, or a heat source that uses heat generated by energization heating such as an electric heater. In any case, the carbon tool steel strip is heated by radiant heat from these heat sources, and heat transfer to the carbon tool steel strip is mainly due to radiant heating.
Moreover, among the continuous heating equipment which concerns on a prior art, the hardening furnace becomes a structure which provided the some soaking zone continuously in the part through which a carbon tool steel strip is generally passed. As a general configuration of the soaking zone, for example, from a pre-tropical zone in which the carbon tool steel strip is preheated to a certain temperature, and one or more heating zones in which the carbon tool steel strip is maintained at a predetermined quenching temperature. It has become. With such a configuration, the carbon tool steel strip first unwound at normal temperature and passed through a quenching furnace can be heated to the quenching temperature.

JP-A-56-139627

The present inventors have studied a method for improving the production capacity by improving the heat treatment ability of the carbon tool steel strip using the continuous heating facility described in Patent Document 1 described above.
In order to improve the heat treatment capacity per unit time, for example, a method of extending the length of the quenching furnace and increasing the sheet passing speed can be considered.
However, the conventional radiant heating has a sufficient effect for improving the heat treatment capacity because the heating rate of the carbon tool steel strip is slow in the pretropical temperature range among the multiple soaking zones that constitute the quenching furnace. There is a problem that can not be. Further, in the case of radiant heating, there is a problem that it is not easy to control the temperature increase rate of the carbon tool steel strip in the above temperature range and other temperature conditions to an arbitrary condition.
An object of the present invention is to provide a method for producing a carbon tool steel strip that can facilitate temperature control and improve heat treatment capacity in producing the carbon tool steel strip.

The present invention has been made in view of the above-described problems.
That is, the present invention
(1) an unwinding step of unwinding the carbon tool steel strip by an unwinding machine;
(2) a preheating step of preheating by induction heating while passing the carbon tool steel strip unwound by the unwinding step;
(3) A quenching step in which the carbon tool steel strip preheated by the preheating step is passed through an inert gas atmosphere heated to more than 850 ° C. and not higher than 1100 ° C., and then quenched and quenched.
(4) A tempering step in which the carbon tool steel strip quenched in the quenching step is tempered by passing it through an inert gas atmosphere heated to 300 to 450 ° C.
(5) a winding step of winding the carbon tool steel strip tempered by the tempering step;
Is a method for producing a carbon tool steel strip.

Preferably, it is a manufacturing method of the carbon tool steel strip whose temperature of the said preheating process is 600-750 degreeC.
Moreover, in this invention, the rapid cooling of the said hardening process is the 2nd which cools below to Ms point, restraining a carbon tool steel strip within a water-cooled surface plate after the 1st cooling process cooled to 200-350 degreeC. Preferably, a cooling step is provided, and more preferably, in the second cooling step, a plurality of the water-cooled surface plates that restrain the carbon tool steel strip are provided.
Moreover, in this invention, it is preferable to provide the grinding | polishing process which grind | polishes the tempered carbon tool steel strip surface between the said tempering process and the said winding-up process.
In the preheating step, the induction heating is preferably performed by an induction heating zone in the same atmosphere as the quenching step in which the ceramic tube is disposed inside the induction heating coil.

  According to the present invention, since the carbon tool steel strip can be rapidly heated by a preheating furnace using induction heating, temperature control is facilitated and excellent heat treatment ability is obtained, and further, the heating time is shortened and heat treatment ability is improved. By doing so, productivity can be greatly improved. Moreover, decarburization can be reliably prevented by setting the preheating furnace and the heating furnace to an inert gas atmosphere, and a desired hardness can be obtained more reliably.

It is a schematic diagram which shows an example of a structure of the continuous heating equipment of this invention. It is a schematic diagram which shows an example of a structure of the preheating furnace of this invention. It is a figure which shows the relationship between the heating time of a carbon tool steel strip, and the temperature of the carbon tool steel strip which raised temperature. It is a cross-sectional microscope picture of the carbon tool steel strip which applied the manufacturing method of the present invention. It is a cross-sectional microscope picture of the carbon tool steel strip which applied the manufacturing method of the comparative example.

The feature of the manufacturing method of the present invention is that when the temperature of the carbon tool steel strip is raised to the quenching temperature, the temperature is not raised only by the pre-tropical zone of the quenching furnace, but induction heating is used before the quenching furnace. The carbon tool steel strip can be rapidly heated by installing a preheating furnace and raising the temperature in the preheating furnace. Moreover, desired hardness can be obtained, without generating decarburization by making the atmosphere at the time of hardening and tempering into an inert gas atmosphere.
The present invention will be described below.
First, the carbon tool steel strip used in the present invention only needs to have a carbon tool steel composition defined by JIS-G-3311 (polished special strip steel). Preferable compositions are C: 0.8-1.2%, Si: 0.1-0.35%, Mn: 0.1-0.5%, Cr: 0.05-0.3% by mass%. The balance is Fe and impurities.

FIG. 1 shows an example of the layout of the continuous heating equipment used in the present invention. The present invention will be described with reference to FIG.
A carbon tool steel strip 2 for continuously performing quenching and tempering is unwound by an unwinding machine 1 (unwinding step). Subsequently, the unrolled carbon tool steel strip 2 is passed through a preheating furnace 3 using induction heating (preheating step).
In the present invention, the preheating is performed by the preheating furnace using induction heating because the carbon tool steel strip 2 generates heat by the Joule heat of the eddy current induced in the carbon tool steel strip 2 by the alternating magnetic flux. This is because it becomes possible to heat up more rapidly than in the case of heating. Examples of the induction heating method include a solenoid method and a transverse method. In the present invention, the solenoid method is preferable. This is because, in the case of the solenoid system, temperature control of the carbon tool steel strip is facilitated using the Ac1 point of the carbon tool steel strip 2, and the carbon tool steel strip is easily heated uniformly.
The lower limit of the preheating temperature in this preheating step is preferably in the range of 600 ° C or higher. This is because when the preheating temperature of the carbon tool steel strip is less than 600 ° C., it may be difficult to heat the carbon tool steel strip to the quenching temperature performed after the preheating step. If so, sufficient quenching hardness cannot be obtained. The upper limit of the preheating temperature is preferably 750 ° C. This is because there is a risk of deformation of the carbon tool steel strip that is rapidly heated to a temperature exceeding 750 ° C. by high-frequency heating. Moreover, when the temperature of Ac1 point of the carbon tool steel strip near 750 ° C. is exceeded, the metal structure is transformed from ferrite to austenite. Austenite is non-magnetic, and solenoid-type high-frequency heating is convenient for controlling the temperature of the carbon tool steel strip in the preheating process because the heating efficiency is reduced and the overheating is prevented at a temperature higher than the Ac1 point. .
Further, in the preheating furnace, the same inert gas atmosphere as the quenching process described later is used to prevent decarburization of the surface of the carbon tool steel strip, and the ceramic tube 11 is provided inside the induction heating coil 10 to maintain the atmosphere. It is preferable to dispose the carbon tool steel strip 2 through the ceramic tube 11. Since the ceramic tube 11 is a nonmagnetic material and induction heating does not occur, the carbon tool steel strip can be induction heated efficiently without losing power. The ceramic tube is preferably covered with a heat insulating material in order to reduce heat loss due to heat dissipation.
In addition, if the plate passing speed in the preheating process is excessively high, the above-described preheating effect cannot be obtained. Therefore, it is preferable to adjust the plate passing speed and pass the plate so that the time required for the portion having the carbon tool steel strip to pass through the preheating furnace is in the range of 0.8 to 3 seconds.

After the above-mentioned preheating step, the carbon tool steel strip 2 is passed through a quenching heating furnace 4 having an inert gas atmosphere in order to prevent decarburization, and the temperature is kept above 850 ° C. and below 1100 ° C. (quenching step). . Suitable inert gases include nitrogen, argon, ammonia decomposition gas, etc. Among them, nitrogen is preferably used because it is easily available and inexpensive.
In the present invention, when the quenching heating temperature is 850 ° C. or lower, the solid solution of the carbide is insufficient and the fatigue characteristics are reduced. Further, when the quenching heating temperature exceeds 1100 ° C., the amount of carbides dissolved increases, and not only the hardness becomes difficult to increase during tempering, but also the residual carbides decrease and the punchability deteriorates. Therefore, the range of the quenching heating temperature is set to more than 850 ° C. and not more than 1100 ° C.
If the plate passing speed in the quenching process is excessively high, the above-described effect of the heating temperature during quenching cannot be obtained. Therefore, it is preferable to adjust the plate passing speed and pass the plate so that the time required for adding a quenching furnace to a part having the carbon tool steel strip is in the range of 35 to 90 seconds.
Subsequently, the carbon tool steel strip 2 is quenched and quenched. As a method of rapid cooling, there is a method using a salt bath or molten metal, but the method of spraying water is the simplest method, and a thin oxide film can be formed on the surface of the carbon tool steel strip. it can. This thin oxide film is hard and can suppress the occurrence of flaws on the surface of the carbon tool steel strip when the water-cooled surface plate 6 described later is passed through. Therefore, it is preferable to use a method of injecting water for rapid cooling to the carbon tool steel strip 2 used in the present invention.

In order to rapidly cool by jetting the water described above in the present invention, it is preferable to provide a spraying device 5 installed on the exit side of the quenching furnace 4.
Moreover, rapid cooling of the quenching process is performed by water cooling so that the carbon tool steel strip is sandwiched after the first cooling step in which the carbon tool steel strip 2 is cooled to 200 to 350 ° C. by the spray device 5 using compressed air and purified water. It is preferable that a martensite structure is formed by performing a second cooling step of restraining the inner surface of the platen 6 and cooling to the Ms point or lower while correcting the shape.
The two stages of cooling are to avoid the pearlite nose in the first cooling process, reduce the strain that occurs when quenching the carbon tool steel strip 2, and perform martensitic transformation in the next second cooling process. This is because the shape of the carbon tool steel strip 2 can be adjusted.
The reason why the lower limit of the quenching of the carbon tool steel strip 2 is set to a temperature exceeding the Ms point in the first cooling step described above is to reduce strain generated when the carbon tool steel strip 2 is quenched. This is because if the Ms point or less is set at a stroke in the first cooling step, the distortion at the time of quenching becomes too large and the shape may become unstable. The Ms point varies slightly depending on the composition of the carbon tool steel strip, but is approximately 180 to 190 ° C. The lower limit of the preferable cooling temperature in the first cooling step is preferably 10-20 ° C. higher than the Ms point. In addition, the upper limit of the cooling temperature in the first cooling step is set to 350 ° C., because when the temperature exceeds 350 ° C., it may be difficult to cool to the Ms point or less in a water-cooled surface plate described later. is there. There are three forms of heat transfer: heat conduction, heat convection, and radiant heat. If cooling is performed below the Ms point while restraining the carbon tool steel strip 2 in the water-cooled surface plate 6, water cooling is performed in the water-cooled surface plate. It is rapidly cooled by heat conduction by physical contact between the surface plate 6 and the carbon tool steel strip 2. And since it becomes possible to restrain the carbon tool steel strip 2 with the water-cooled surface plate 6, it is because the deformation | transformation of the carbon tool steel strip 2 can be prevented or a deformation | transformation can be corrected.
It is preferable that a plurality of water-cooled surface plates 6 used in the present invention are continuously arranged while being cooled with water. This is because the time for restraining in the water-cooled surface plate can be lengthened, so that it can be more reliably cooled to the Ms point or less, and therefore, the deformation and correction of the carbon tool steel strip 2 are more reliably performed. Because it can.

Following the above-described quenching step, the carbon tool steel strip 2 is heated to 300 to 450 ° C. and passed through a tempering furnace 7 in an inert gas atmosphere to perform tempering. The inert gas atmosphere is used to prevent decarburization of the carbon tool steel strip.
In the present invention, if the tempering temperature is less than 300 ° C., the hardness becomes too high. On the other hand, when the tempering temperature is higher than 450 ° C., the hardness is lowered.
Therefore, even if the tempering temperature is too high or too low, an appropriate hardness cannot be obtained, and punchability is hindered. If the hardness adjusted by tempering is Vickers hardness of 500 to 650 (Hv), good punchability can be obtained.
In addition, if the plate passing speed in the tempering process is excessively high, the above-described effect of the heating temperature during tempering cannot be obtained. Therefore, it is preferable to adjust the threading speed so that the time required for the carbon tool steel strip to pass through the tempering furnace is in the range of 50 to 125 seconds.

The carbon tool steel strip 2 tempered by the tempering step described above is preferably polished on the surface of the carbon tool steel strip by a polishing device 8 disposed on the downstream side of the tempering furnace 7.
As described above, in addition to removing the oxide film formed at the time of quenching, it is possible to prevent fatigue failure due to surface flaws by removing or reducing flaws inevitably attached to the surface. .
Then, the carbon tool steel strip which has desired hardness can be obtained, without generating decarburization by winding up with the winder 9 (winding process).
In the present invention, as described above, each process from the unwinding process to the winding process can be continuously performed until the carbon tool steel strip unwound from the coil is wound around the coil again. It has sex. In addition to this, the carbon tool steel strip can be rapidly heated by a preheating furnace using induction heating, giving it excellent heat treatment capacity, reducing the heating time, and improving the processing capacity. It can be greatly improved.

First, in order to confirm the effect of the preheating furnace, the carbon tool steel strip having a thickness of 0.3 mm shown in Table 1 was used for preheating (invention example) and when preheating was not performed ( About the comparative example), the temperature rise curve of the carbon tool steel strip when the holding temperature of the quenching furnace was 900 ° C. was measured. The result is shown in FIG. The induction heating method used in the preheating furnace is based on a solenoid system.
From the result of FIG. 3, it can be seen that the temperature can be raised to 600 ° C. or more within 2 seconds when the temperature rise curve of the present invention which has been preheated is seen. In addition, when preheating is performed, the time for the carbon tool steel strip to reach 900 ° C. is 75 seconds, whereas when preheating is not performed, it takes 100 seconds. According to the results, it was found that the heating time can be greatly shortened.
Next, the carbon tool steel strip was actually quenched and tempered using the continuous heating equipment shown in FIG. At this time, the preheating furnace 3 was operated in the present invention example, and no preheating was performed in the comparative example (conventional example). Further, based on the result of the preliminary experiment, the plate passing speed was set to 7 m / min in the present invention example and 5 m / min in the comparative example. The quenching temperature, tempering temperature, and cooling rate were the same for both the present invention and the comparative example.

A carbon tool steel strip shown in Table 1 having a thickness of 0.3 mm and a length of about 1500 m was prepared. The continuous quenching and tempering furnace for quenching and tempering the prepared carbon tool steel strip has the structure shown in FIG. 1, and the preheating furnace 3 has the structure shown in FIG. In particular,
Unwinding the carbon tool steel strip 2 for continuous quenching and tempering by the unwinder 1,
The unrolled carbon tool steel strip 2 has the same nitrogen gas atmosphere as that of the quenching heating furnace 4, and a solenoid system in which a ceramic tube 11 is disposed inside the induction heating coil 10 to keep the atmosphere is easily controlled. The plate was passed through a preheating furnace 3 preheated by induction heating using an induction heating device, and the carbon tool steel strip of the present invention was heated to 650 to 700 ° C. continue,
The carbon tool steel strip 2 was heated to the quenching temperature shown in Table 2 and passed through a quenching furnace 4 having a nitrogen gas atmosphere.
Water was sprayed onto the carbon tool steel strip 2 by the spraying device 5 installed on the exit side of the quenching furnace 4 to perform the first cooling. The temperature of the carbon tool steel strip was about 250 ° C. Furthermore, the carbon tool steel strip 2 is restrained by four cooling surface plates 6 installed so as to sandwich the carbon tool steel strip 2, and the carbon tool steel strip 2 is quenched and quenched to about 140 ° C.,
In order to prevent decarburization of the carbon tool steel strip 2, the steel is passed through a tempering furnace 7 in a nitrogen gas atmosphere and tempered at 400 ° C., and the polishing tool 8 disposed downstream of the tempering furnace 7 is used for carbon tool steel. The steel strip surface is polished and the carbon tool steel strip 2 is wound up by a winder 9.
Table 2 shows the preheating temperature and quenching temperature performed this time. In addition, the time of the preheating process at the time of applying the manufacturing method of this invention, a quenching process, and a tempering process was 2.5 second, 70 second, and 95 second, respectively.

In order to investigate the presence / absence of decarburization, hardness, and metal structure from the carbon tool steel strip subjected to quenching and tempering, each test piece was collected. As representative metal structures, micrographs of Example 1 of the present invention and Comparative Example are shown in FIGS.
In the metal structures of FIG. 4 (Invention Example 1) and FIG. 5 (Comparative Example), white fine carbides were observed at the base of martensite, and both were inferior. Table 3 shows the results of the presence or absence of decarburization, hardness, and crystal grain size. The crystal grain size is obtained by measuring prior austenite grains according to JIS0551.

From the above results, it can be seen that in the present invention, various characteristics possessed by the carbon tool steel strip subjected to continuous quenching and tempering conventionally performed are maintained. Further, in this example, the required time when the present invention was applied was about 214 minutes, whereas in the comparative example, it was about 300 minutes.
Therefore, the carbon tool steel strip can be rapidly heated by a preheating furnace using induction heating, so that excellent heat treatment capability is imparted, and further, the heating time is shortened by increasing the plate passing speed, thereby improving the processing capability. Thus, productivity can be greatly improved.

DESCRIPTION OF SYMBOLS 1 Unwinder 2 Carbon tool steel strip 3 Preheating furnace 4 Quenching furnace 5 Spraying device 6 Water-cooled surface plate 7 Tempering furnace 8 Polishing device 9 Winding machine 10 Induction heating coil 11 Ceramic tube

Claims (4)

(1) an unwinding step of unwinding the carbon tool steel strip by an unwinding machine;
(2) a preheating step of preheating by induction heating while passing the carbon tool steel strip unwound by the unwinding step;
(3) A quenching step in which the carbon tool steel strip preheated by the preheating step is passed through an inert gas atmosphere heated to more than 850 ° C. and not higher than 1100 ° C., and then quenched and quenched.
(4) A tempering step in which the carbon tool steel strip quenched in the quenching step is tempered by passing it through an inert gas atmosphere heated to 300 to 450 ° C.
(5) a winding step of winding the carbon tool steel strip tempered by the tempering step;
In the manufacturing method of carbon tool steel strip, which is continuously performed ,
The rapid cooling in the quenching step is a second cooling in which the carbon tool steel strip is constrained by a plurality of water-cooled surface plates and cooled to the Ms point or lower after the first cooling step in which the Ms point is cooled to 350 ° C. or lower. The manufacturing method of the carbon tool steel strip characterized by providing a process.   The temperature of the said preheating process is 600-750 degreeC, The manufacturing method of the carbon tool steel strip of Claim 1 characterized by the above-mentioned. The carbon tool steel strip according to claim 1 or 2 , further comprising a polishing step for polishing a surface of the tempered carbon tool steel strip between the tempering step and the winding step. Production method. In the preheating step, the induction heating is induction heating zone of the quenching process and in the same atmosphere of arranging the ceramic tube inside the induction heating coil, claim 1, which comprises carrying out up to claim 3 1 The manufacturing method of the carbon tool steel strip as described in a term.

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