JP2014024087A - Cooling method of cast piece - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cooling method of cast pieces, by which the cast pieces obtained after continuous casting can be cooled to a normal temperature without occurrence of a surface crack due to thermal stress.SOLUTION: In the cooling method of cast pieces, the cast pieces are made of steel having 1 mass% or more of Si percentage content and are cut after continuous casting, and when a plurality of the cast pieces are stacked, the Si percentage content of the cast pieces and the length of the cast pieces satisfy: [Si]×L<A, wherein [Si] is the Si percentage content (mass%) of the cast pieces, L is the length (m) of the cast pieces, and A is a value (mass%×m) which is preliminarily obtained by an examination and at which the surface crack of the cast pieces according to a cooling rate of the surface of a width direction center of the cast piece positioned on the uppermost position of the cast pieces does not occur.

Description

  The present invention relates to a slab cooling method, and more particularly, to a slab cooling method capable of preventing thermal stress cracking even for a slab made of steel having a high Si content.

  A slab made of steel having a high Si content (for example, a steel having a Si content of 1.0% by mass or more, hereinafter also referred to as “high Si steel”) has high thermal cracking susceptibility, and thus cracks are generated on the surface. Cheap.

  Patent Documents 1 and 2 have proposed methods for preventing such surface cracks of a slab.

  In Patent Document 1, a slab having a Si content of 1.50 to 6.50 wt% is targeted, and the slab is heated in a heating furnace without cooling from an upper limit temperature specified by the Si content and indicating a decrease in ductility. It is described that hot rolling is performed after charging in a furnace and soaking.

  However, in the method described in Patent Document 1, since the furnace must be charged without cooling from the temperature specified by the Si content, the distance from the casting equipment to the furnace is short, or It is necessary to prepare a dedicated cart that can be transported while preventing the temperature drop of the slab.

  A problem similar to this is that the slab, which has been conventionally used as a method for preventing surface cracking of a slab made of high-Si steel, is taken out of a continuous casting machine at a high temperature and before being heated again for rolling. In addition, a method of rolling down the end portion of the slab also occurs.

  In addition, if an abnormality occurs during casting or the timing of charging the heating furnace is missed, the slab cannot be charged into the heating furnace at an appropriate time. (For example, slow cooling or natural cooling described later), and surface cracks are inevitably generated.

  In Patent Document 2, as a method of preventing thermal cracking on the surface of a slab of high carbon martensitic stainless steel, the outside of the slab is surrounded by a heat insulating cover, and ultra-slow cooling with a cooling rate of less than 10 ° C./h is performed. How to do is described.

  However, when the method described in Patent Document 2 is applied to a slab made of high-Si steel, if the Si content is relatively high, or if the slab is long, surface cracks of the slab may occur. there were.

Japanese Patent Laid-Open No. 2-38526 Japanese Unexamined Patent Publication No. 60-9568

  As mentioned above, suppressing the occurrence of surface cracks in slabs made of high-Si steel, especially suppressing the occurrence of surface cracks from continuously cast slabs made of high-Si steel to high temperatures after casting. However, it was difficult to cool down. In the following description, “slab in the state after the continuous casting and before the temperature drops to room temperature” is referred to as “hot slab”, and “slab after the continuous casting the temperature is reduced to room temperature” as “cold slab” "

  The occurrence of this surface crack is because the heat piece has a temperature distribution in the width direction and the longitudinal direction, and when it is left to cool or water-cooled as it is, the temperature is lowered at the lower temperature part, and local thermal stress is generated. Occur. Due to this, surface cracks occur in the slab. When the surface crack is large, the crack propagates to the inside of the slab, and the slab may be completely divided.

  The present invention has been made in view of this problem. After continuously casting a slab, it is cut into hot pieces of a predetermined length, and when cooled to cold pieces, surface cracks are generated due to thermal stress. It aims at providing the cooling method of the slab which can prevent.

  The present inventor paid attention to the relationship between the cooling rate of the surface of the slab (hot piece), the Si content, and the length of the slab, and studied the relaxation of local thermal stress in the slab. As a result, according to the cooling rate of the slab surface, it was found that if the product of the Si content and the length of the slab is less than a predetermined value, the thermal stress is relaxed and surface cracks of the slab can be suppressed. . Details of this examination will be described later.

  This invention is made | formed based on this knowledge, The summary exists in the continuous casting method of the slab of following (1)-(4).

(1) A cooling method for a slab made of steel having a Si content of 1% by mass or more and cut after continuous casting, wherein a plurality of the slabs are stacked, and the Si content of the slab The method for cooling a slab, wherein the length of the slab satisfies the following formula (1):
[Si] × L <A (1)
Here, [Si]: Si content (% by mass) of the slab, L: length (m) of the slab, A: the top of the slab obtained in advance by testing. It is a value (mass% x m) at which the surface crack of the slab does not occur according to the cooling rate of the surface in the center in the width direction.

(2) The cooling rate of the surface in the center in the width direction of the uppermost one of the slabs is set to 10 ° C./h or less, and A = 25. Cooling method.

(3) The casting according to (1), wherein the cooling rate of the surface in the center in the width direction of the slab positioned at the top is a rate exceeding 10 ° C./h, and A = 10. How to cool a piece.

(4) The slab cooling method according to any one of (1) to (3), wherein L ≦ 10.

  In the following description, stacking slabs is also referred to as “piling”.

  According to the method for cooling a slab of the present invention, a slab (hot piece) after continuous casting can be cooled to room temperature without causing surface cracks to form a cold piece. Therefore, it is possible to manufacture a cold piece without surface cracks in accordance with the hot rolling schedule, and to manufacture a coil using this cold piece as a raw material, thereby preventing an increase in coil inventory due to unnecessary preparation. be able to. In addition, it is possible to perform rolling according to the order of small lot materials.

  Also, conventionally, in steel grades that had surface cracks when cooled to care for the slab, the cast slab was rolled without cooling and the defective part was trimmed away after forming the coil. The yield was low. However, according to the method for cooling a slab of the present invention, it is possible to obtain a cold slab having no surface cracks, so that it is possible to care at the slab stage, and after the slab is rolled into a coil, a defective part is obtained. Therefore, it is not necessary to remove the trimming, and a decrease in yield can be suppressed.

It is the schematic explaining the state of the cooling method of the slab which uses a slow cooling cover. It is a figure which shows the relationship between the elapsed time after providing a slow cooling cover, and the surface temperature of a slab, The figure (a) shows the case where eight slabs are piled, The figure (b) shows 12 pilings. FIG. It is a figure which shows the relationship between the surface cooling rate of the uppermost stage slab, and the number of piles of a slab. It is the schematic of another cooling method of the slab using a slow cooling cover.

  Hereinafter, the contents of the study for completing the present invention and the mode for carrying out the present invention will be described.

1. Contents of examination Paying attention to the relationship between the cooling rate of the slab, the Si content and length, and the occurrence of surface cracks, the following three tests were conducted.

1-1. First test (relationship between the number of piles and the cooling rate of the slab)
FIG. 1 is a schematic diagram illustrating a state of a slab cooling method using a slow cooling cover. As shown in the figure, after continuous casting, a slab (heated piece) 1 cut to a predetermined length with a torch is piled on the ground 2 and gradually cooled to cover the entire slab 1 that has been piling. 3 was provided.

  The slab was a slab having a width of 1240 mm, a thickness of 250 mm, and a length of 10000 mm (10 m). The slow cooling cover was formed in a rectangular parallelepiped shape having a width of 6.5 m, a height of 4 m, and a depth of 10 m constituted by a ceiling surface made of a steel plate having a thickness of 25 mm and outer walls of four side wall surfaces.

  FIG. 2 is a diagram showing the relationship between the elapsed time since the provision of the slow cooling cover and the surface temperature of the slab, and FIG. 2A shows the case where eight slabs are piled. Is a figure when 12 pieces are piled. In either case, the surface temperature at the center in the width direction of the slab was measured with a contact-type thermocouple. As a result, no difference was observed in the transition of the surface temperature of the slab after the slow cooling cover was started. In any case, the surface crack of the slab did not occur.

  Based on the temperature transition shown in FIG. 2, when the cooling rate of the slab was calculated from the time until the surface temperature of the slab decreased to 400 ° C. after starting the slow cooling cover and the temperature decrease range, 5 3 ° C./h. In addition, in the case where eight pieces were piled, the cooling rate of the slab immediately after the start of the slow cooling cover was obtained from FIG.

1-2. Second test (relationship between slab cooling rate and surface cracking)
Next, the relationship between the cooling rate of the slab (hot piece) and surface cracking was investigated. In order to change the cooling rate of the slab, the number of piles of the slab was 1 to 8, and a test was performed with and without the slow cooling cover. The slab and the slow cooling cover used the same thing as the 1st test. The cooling of the slab, which is performed without leaving the slow cooling cover and left in a room temperature atmosphere with open air, is also referred to as “cooling”.

  FIG. 3 is a diagram showing the relationship between the surface cooling rate of the uppermost slab and the number of piles of the slab. When only one slab was allowed to cool, the cooling rate of the slab surface was as high as 33 ° C./h, and a large crack of 10 mm or more occurred in the depth direction from the slab surface.

  On the other hand, when a slow cooling cover is provided, the cooling rate of the slab surface is 20 ° C./h, the cracks on the slab surface are less than 10 mm in the depth direction, and both are compared with the case where only one is allowed to cool. And the crack was small.

  As described above, from FIG. 3, it was found that the smaller the cooling rate, the less the occurrence of surface cracks, and no surface cracks when the cooling rate is 10 ° C./h or less.

1-3. Third test (Si content of slab and relationship between slab length and surface crack)
Furthermore, as a result of investigations by the present inventor, it was found that surface cracks may occur depending on the Si content of the slab or the length of the slab even when a slow cooling cover is provided.

  The following two can be considered as major factors that cause surface cracks in the slab due to thermal stress. First, the composition of the slab (especially the Si content) has a large effect on the thermal cracking susceptibility of the slab, and second, the local thermal stress due to thermal contraction in the longitudinal direction of the slab It depends on the length.

  The inventor paid attention to this phenomenon and conducted a test in which the Si content and length of the slab were changed. The number of pilings was four, and the slab (hot piece) was cooled both when it was slowly cooled by providing a slow cooling cover and when cooled. The Si content of the slab was 0.4 to 3.0% by mass, and the length of the slab was 4 to 10 m. Conditions other than the Si content and length of the slab were the same as in the first test. The surface temperature at the center in the width direction of the uppermost slab was 16.4 ° C./h when allowed to cool and 5 to 10 ° C./h when allowed to cool slowly. The test results are shown in Tables 1 and 2.

  In Table 1, □ indicates that cracking did not occur after cooling, ○ indicates that cracking occurred after cooling, but no cracking occurred when cooling slowly, and cracking occurred when cooling or cooling slowly. Indicated by ▲. However, the depth of the crack in the case of ▲ was less than 10 mm, and it was such that it could be removed by care. From the table, it can be seen that the lower the Si content of the slab, the less likely the surface cracks to occur even if the slab is longer. It can also be seen that surface cracks are less likely to occur when the cooling rate of the slab is lower than when the cooling rate is higher.

In Table 2, the product of the Si content of the slab and the length of the slab was used as an index. By comparing Table 2 and Table 1 above, the product of the Si content [Si] (mass%) of the slab and the length L (m) of the slab is determined according to the cooling rate of the slab. If it is less than the value A (mass% × m), it can be seen that the surface crack of the slab does not occur. That is, in order to prevent the surface crack of the slab, the following equation (1) may be satisfied.
[Si] × L <A (1)

  Specifically, in the case of cooling (cooling rate 16.4 ° C./h), A = 25, and when the cooling cover is provided and cooled slowly (cooling rate 5 to 10 ° C./h), A = 10.

  As a result of further examination by the inventor, when the cooling rate of the surface in the center in the width direction of the uppermost one of the cast slabs is 10 ° C./h or less, A = 25 and 10 ° C. It became clear that A = 10 when the speed was higher than / h.

2. Slow cooling method of slab From the results of the above three tests, the lower the cooling rate of the slab, the wider the range of Si content or the length of the slab that can prevent surface cracking of the slab. all right.

  As described above, as a method for reducing the cooling rate of the slab, there is a method of providing the slow cooling cover shown in FIG. In order to further reduce the cooling rate, a heat insulating material such as a refractory may be provided on the inner surface or the outer surface of the steel plate constituting the outer wall of the slow cooling cover for the purpose of improving the heat retaining property in the slow cooling cover. You may provide in duplicate. The shape of the slow cooling cover is not limited to a rectangular parallelepiped as long as it can provide a space for slow cooling by covering the heat pieces that have been piped.

  FIG. 4 is a schematic view of another cooling method for a slab using a slow cooling cover. This figure has the same configuration as that of FIG. 1 except that hot pieces are arranged as heat insulating materials above and below the slab to be prevented from thermal cracking, and substantially the same parts are denoted by the same reference numerals. doing. As shown in the figure, it is preferable to arrange other heat pieces as the heat insulating material 4 on the upper and lower sides of the slab 1 to be prevented from thermal cracking. Thereby, it is possible to make the cooling rate of the slab 1 made into the object of thermal crack prevention smaller. As the heat insulating material 4, for example, a hot piece of ordinary steel can be used.

  According to the method for cooling a slab of the present invention, a slab (hot piece) after continuous casting can be cooled to room temperature without causing surface cracks to form a cold piece. Therefore, it is possible to manufacture a cold piece without surface cracks in accordance with the hot rolling schedule, and to manufacture a coil using this cold piece as a raw material, thereby preventing an increase in coil inventory due to unnecessary preparation. be able to. In addition, it is possible to perform rolling according to the order of small lot materials.

1: slab, 2: ground, 3: slow cooling cover, 4: heat insulating material

Claims (4)

A method for cooling a slab made of steel having a Si content of 1% by mass or more and cut after continuous casting,
A state where a plurality of the slabs are stacked,
The method for cooling a slab, wherein the Si content of the slab and the length of the slab satisfy the following formula (1):
[Si] × L <A (1)
Here, [Si]: Si content (% by mass) of the slab, L: length (m) of the slab, A: the top of the slab obtained in advance by testing. It is a value (mass% x m) at which the surface crack of the slab does not occur according to the cooling rate of the surface in the center in the width direction. The cooling rate of the surface in the center in the width direction of the uppermost one of the slabs is 10 ° C./h or less,
The slab cooling method according to claim 1, wherein A = 25. The cooling rate of the surface in the center in the width direction of the uppermost one of the slabs is set to a rate exceeding 10 ° C./h,
The slab cooling method according to claim 1, wherein A = 10.   The slab cooling method according to claim 1, wherein L ≦ 10.

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