JP2010065262A - Soaking annealing treatment method for high carbon steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a soaking annealing treatment method for high carbon steel by which huge carbide such as eutectic carbide is rendered harmless substantially, occurrence of flaws such as the surface crack can be prevented or reduced, and the treatmentprocessing time can be shortened. <P>SOLUTION: When a cast billet of the high carbon steel is heated to a predetermined soaking temperature in a heating furnace, the cast billet is heated at heating rate of &le;50&deg;C/h in the transforming temperature zone from the starting point of the transformation to the finishing point of the transformation, and is heated at higher heating rate than the heating rate in the transformation temperature zone in the temperature zone other than the above temperature zone, and then is held in a predetermined soaking temperature. Further, it is preferable that the heating rate in the temperature zone of lower than the starting point and/or the temperature zone exceeding the finishing point is &le;100&deg;C/h. Furthermore, an isothermal conservation may be performed at just below the starting point of the transformation and/or just above the finishing point of transformation. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a soaking annealing method for a high carbon content steel material, and more particularly to shortening of the processing time in a soaking annealing process for a Cr containing high carbon steel material.

In high carbon steel materials, especially high carbon steel materials with high Cr content such as bearing steel, for example, when cast by the continuous casting method, huge eutectic carbides crystallize in the central part where the final solidification occurs. Since it does not disappear even by hot rolling, there is a problem that the mechanical properties are deteriorated.
In order to solve such a problem, for example, Non-Patent Document 1 requires a soaking process at a temperature of about 1200 ° C. for 10 hours or more for a long time to eliminate the giant carbides in the bearing steel. It has been pointed out. In order to eliminate such giant carbides and to prevent cracking of the slab, in the actual operation, as the soaking process, the heating time to the soaking temperature is about 50 hours or more, and the soaking time is about 20 hours. The above-described long-time treatment was performed.

  Further, in Patent Document 1, a slab having a circular cross section is cast by a vertical continuous casting method, and a flat roll is used while the central solidification rate is 0.3 to 0.6 with respect to a slab drawn from a water-cooled mold. It is characterized by eliminating the central segregation by applying a light reduction with a reduction ratio of 1 to 3%, and subjecting the obtained slab to soaking that is heated at a temperature of 1150 to 1260 ° C. for 2 to 5 hours and performing batch rolling. A method for producing bearing steel is described. The technique described in Patent Document 1 intends to reduce or eliminate the center segregation by applying a light reduction during casting to shorten the subsequent soaking time. According to the technique described in Patent Document 1, the soaking time can be reduced to about half of the conventional time.

Patent Document 2 describes a method for producing a high carbon chromium bearing steel. The technique described in Patent Document 2 uses a slab for high carbon chromium bearing steel having a P concentration of 0.002 to 0.009 mass%, at least a soaking temperature of 1150 to 1260 ° C., and a holding time of less than 2 hours It is characterized by performing soaking and hot rolling, which makes it possible to highly suppress giant carbides.
JP-A-7-299550 JP 2006-016683 A Takami Ota et al .: Iron and Steel, 52 (1966) No. 13, p.1851-1859

  According to the technique described in Patent Document 1, it is possible to shorten the annealing time when soaking the slab, but it requires equipment for light reduction of the slab at the time of casting. There was a problem that the capital investment of became indispensable. Further, according to the technique described in Patent Document 2, it is possible to shorten the soaking holding time, but there is a problem that it is not possible to achieve further shortening of the total time of soaking.

  The present invention solves such problems of the prior art, makes a large amount of carbides such as eutectic carbides substantially harmless, without making a large capital investment and without requiring a long time treatment. An object of the present invention is to provide a soaking annealing method for a high carbon steel material, which can prevent or reduce the occurrence of defects such as slab cracking and surface cracking and has excellent productivity.

  In order to achieve the above-mentioned object, the present inventors diligently studied factors affecting the occurrence of defects such as slab cracking and surface cracking during soaking annealing of high carbon steel slabs. As a result, especially in the case of high carbon steel slabs, in order to shorten the soaking annealing time, it was newly found that when the transformation temperature range is heated at a large heating rate exceeding a specific range, cracks occur in the slab. It was. This is because the temperature difference between the inner and outer layers of the slab increases as the heating rate increases. Especially in the transformation temperature range, the thermal expansion changes due to transformation differ between the inner and outer layers, and a large tensile thermal stress is generated on the surface. In order to prevent the occurrence of such cracking of the cast slab, it has been thought that it is necessary to heat the transformation temperature range at a heating rate within a specific range.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) In the soaking annealing method of a high carbon steel material, the slab of high carbon steel material is heated to a predetermined soaking temperature using a heating furnace, and maintained at the predetermined soaking temperature for a predetermined time. Next to the high carbon steel (a)
Starting point (° C): Temperature at which the thermal expansion of a high carbon steel material changes from expansion to contraction (a)
From the start point defined by (b)
End point (° C): Temperature at which the thermal expansion of high carbon steel changes from shrinkage to expansion (b)
The temperature range up to the end point defined by is heating performed at a heating rate of 50 ° C./h or less by furnace temperature control, and the temperature range below the start point and / or the temperature range above the end point The soaking is performed at a heating rate faster than the heating rate in the temperature range from the start point to the end point.
(2) In the method (1), the high-carbon steel material is a Cr-containing high-carbon steel material.
(3) In (1) or (2), the heating rate in the temperature range below the starting point and / or in the temperature range exceeding the end point is 100 ° C./h or less by furnace temperature control. A soaking annealing method for high carbon steel.
(4) In any one of (1) to (3), at a temperature in the temperature range not higher than the start point (start point −30 ° C.) and / or higher and / or higher than the end point (end point + 30 ° C.). A soaking annealing method for high carbon steel, characterized by performing isothermal holding.
(5) In any one of (1) to (3), heating in the temperature range below the start point (start point −30 ° C.) and / or above and / or the end point (end point + 30 ° C.) and below Is a heating treatment at a heating rate of 10 ° C./h or less by controlling the furnace temperature.

  According to the present invention, it is possible to shorten the soaking annealing time without making a large capital investment, to substantially detoxify giant carbides such as eutectic carbides, and to further prevent or reduce the occurrence of flaws such as cracks. In addition, there is a remarkable industrial effect that the maintenance of the slab can be reduced and the productivity can be remarkably improved.

  In the present invention, a high carbon steel slab is charged into a furnace, heated to a predetermined soaking temperature, and held at the soaking temperature for a predetermined time. As used herein, “high carbon steel” includes C: 0.95% by mass or more, preferably 1.1% by mass or less, has a giant carbide such as a eutectic carbide, and preferably contains at least 0.9% by mass of Cr. It shall mean steel material to be used. More preferably, the “high carbon steel material” is a high carbon chromium bearing steel material defined in JIS G 4805, SUJ2 to SUJ5.

  In the present invention, the soaking temperature is preferably 1200 to 1250 ° C. By adjusting the soaking temperature to the above temperature range, the disappearance and detoxification of the giant carbide can be easily achieved. If the soaking temperature is less than 1200 ° C., the diffusion of C and alloy elements is insufficient, and the disappearance of giant carbides cannot be sufficiently achieved within an industrially possible time. On the other hand, when the temperature exceeds 1250 ° C., the segregation part melts. For this reason, the soaking temperature is preferably 1200 to 1250 ° C. With the soaking temperature described above, a holding time of about 1 to 20 hours is sufficient.

In the present invention, the heating to the soaking temperature described above is preferably performed after the high carbon steel material (a) to be heated.
Starting point (° C): Temperature at which the thermal expansion of a high carbon steel material changes from expansion to contraction (a)
From the start point defined by (b)
End point (° C): Temperature at which the thermal expansion of high carbon steel changes from shrinkage to expansion (b)
The temperature range up to the end point defined by is defined as heating performed at a heating rate of 50 ° C./h or less by furnace temperature control. The start point and end point are the transformation start point and the transformation end point during heating of the high carbon steel material that is the heat-treated material. The starting point and the ending point of transformation may be values obtained in advance by performing thermal expansion measurement by classifying each steel type. Moreover, what is necessary is just to use the value, when already known by literature etc.

  By heating the temperature range from the start point to the end point (so-called transformation temperature range) at a heating rate of 50 ° C./h or less, cracks occurring in the slab can be prevented or significantly reduced. Thereby, subsequent care of the slab can be omitted or reduced, and the production efficiency is remarkably improved. The heating rate in the transformation region is preferably 40 ° C./h or less, more preferably 30 ° C./h or less.

  In the present invention, in a temperature range other than the transformation temperature range described above, a heating rate higher than the heating rate in the transformation temperature range can be achieved. This is because, by setting the heating rate in the transformation temperature range as described above, even if the temperature range other than the transformation temperature range is heated at a heating rate faster than the heating rate in the transformation temperature range, defects such as cracks are generated. It is not allowed. As a result, the soaking annealing treatment time can be significantly shortened.

  In the present invention, heating in the temperature range immediately below the transformation temperature range and / or heating in the temperature range immediately above the transformation temperature range is preferably gradual heating at a heating rate of 10 ° C./h or less. As a result, the temperature difference between the inner and outer layers of the slab can be made smaller or substantially uniform, and the probability of cracking can be further reduced. If the heating rate exceeds 10 ° C./h, the desired uniform temperature in the cross section cannot be achieved sufficiently. Note that the temperature range immediately below the transformation temperature range should be not more than the start point (start point −30 ° C.), and the temperature range just above the transformation temperature range should be not less than the end point (end point + 30 ° C.) or less. It is preferable that The temperature range immediately below the transformation temperature range may be gradually heated from a temperature range lower than (start point -30 ° C) or the temperature range just above the transformation temperature range to a temperature range higher than (end point + 30 ° C). Are almost the same, and only the annealing time is long.

  In place of the above-described slow heating, isothermal holding at a temperature in the above-described temperature range may be performed. The isothermal holding temperature is more preferably set to the above-described start point or end point. In addition, the preferable isothermal holding time is 1 to 5 hours. More preferably, it is 2 h or less. The isothermal holding pattern involves considerable difficulty in accurately performing isothermal holding in actual operation. Usually, a temperature fluctuation of about 10 ° C. per 1 h, desirably a temperature rise, often occurs inevitably. For this reason, even if it is called isothermal holding, a temperature change of about 10 ° C./h is inevitable in many cases, and the isothermal holding includes a temperature change (temperature increase) of about 10 ° C./h. It is acceptable.

High carbon steel (SUJ2: 0.95 to 1.10 mass% C-0.15 to 0.35 mass% Si-0.50 mass% or less Mn-0.025 mass% or less P-0.025 mass% or less S-1.30 to 1.60 mass% Cr-balance Fe and inevitable Impregnated slabs (size: 350 mm thickness x 450 mm width) were subjected to a soaking annealing treatment under the conditions shown in Table 1. In addition, the pattern of each soaking annealing process is typically shown in FIGS. In addition, the temperature at the time of starting soaking annealing was 200 degreeC. Moreover, the above-mentioned transformation start point and end point of the high-carbon steel material were obtained in advance by measurement of thermal expansion during heating. The transformation start point was 740 ° C and the end point was 810 ° C. The soaking condition was 1220 ° C. × 10 h.

FIG. 2 shows an example of a soaking annealing pattern in which isothermal holding is performed below the start point and at the end point. 3 is a conventional example in which heating is performed at a constant heating rate to a soaking temperature, and FIG. 4 is a conventional example in which heating is performed at a constant heating rate to a soaking temperature including isothermal holding in the middle.
The slab was cooled in a furnace after soaking, and cooled to around 30 ° C., and the number of surface defects was measured by a magnetic damage method. The surface flaws were measured after removing the decarburized layer by peeling. The same measurement was performed on 50 slabs subjected to soaking annealing under the same conditions, and the average number of soots was obtained, which was used as the average number of soots in each soaking annealing treatment.

After the soaking annealing process, the slab center is cut and sampled, the cross section is polished, then corroded with a corrosive liquid (Nital liquid), and there is a giant carbide (size: 10μm or more) near the center. Presence / absence was observed using an optical microscope (400 times), and the case where a giant carbide was present was evaluated as x, and the case where it was not present was evaluated as ◯. Giant carbides can be easily distinguished as white structures.
The obtained results are shown in Table 1.

  In all of the examples of the present invention, the soaking annealing time (total) can be shortened as compared with the conventional example, the generation of soot is remarkably reduced, and the giant carbides disappear.

It is explanatory drawing which shows typically an example of the soaking annealing process pattern of the example of this invention used in the Example. It is explanatory drawing which shows typically an example of the soaking annealing process pattern of the example of this invention used in the Example. It is explanatory drawing which shows typically the soaking annealing process pattern of the prior art example used in the Example. It is explanatory drawing which shows typically the soaking annealing process pattern of the prior art example used in the Example.

Claims (5)

In the soaking annealing treatment method for a high carbon steel material in which a slab of high carbon steel material is heated to a predetermined soaking temperature using a heating furnace and held at the predetermined soaking temperature for a predetermined time, the heating is performed by the high carbon steel material. The temperature range from the starting point defined by (a) below to the ending point defined by (b) below is the heating performed at a heating rate of 50 ° C./h or less by furnace temperature control, and less than the starting point. The heating in the temperature range and / or the temperature range exceeding the end point is performed at a heating rate faster than the heating rate in the temperature range from the start point to the end point. A soaking annealing method for high carbon steel.
Starting point (° C): Temperature at which the thermal expansion of a high carbon steel material changes from expansion to contraction (a)
End point (° C): Temperature at which the thermal expansion of high carbon steel changes from shrinkage to expansion (b)   The high-carbon steel material according to claim 1, wherein the high-carbon steel material is a Cr-containing high-carbon steel material.   The heating rate in a temperature range below the starting point and / or in a temperature range exceeding the end point is 100 ° C./h or less by furnace temperature control. A soaking annealing method for carbon steel.   The isothermal holding is performed at a temperature in a temperature range of not more than the starting point (starting point -30 ° C) and / or not less than the ending point (ending point + 30 ° C). The soaking annealing method for the high carbon steel material according to any one of the above.   Heating in the temperature range below the start point (start point −30 ° C.) and / or above the end point (end point + 30 ° C.) and below at a heating rate of 10 ° C./h or less by furnace temperature control The soaking annealing method for a high carbon steel material according to any one of claims 1 to 3, wherein the heating is performed.

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