JP4358807B2 - Method for preventing cracks in continuous cast pieces of high-strength steel - Google Patents

Description

  The present invention relates to a method for preventing cracks in continuously cast pieces of high-tensile steel (high-tensile steel).

In recent years, in the field of automobiles, efforts have been actively made to improve fuel efficiency by reducing weight as part of a campaign to reduce carbon dioxide emissions. One example of this is the introduction of high-tensile steel (high-tensile steel) that enables thinning of the vehicle body and parts.
In addition, the demand for high-tensile steel is increasing in order to prevent the deformation of the vehicle body at the time of a vehicle collision and increase the strength of the vehicle body / parts from the need to ensure the safety of passengers.

  The high-tensile steel is added with C, Si, or Mn (hereinafter also referred to as a high tensile additive) for the purpose of increasing the strength.

  However, a new problem has occurred with the increasing trend of high-tensile additives (high-tensification). That is, the cooling rate of continuous cast pieces (hereinafter also referred to as continuous cast pieces) of high-strength steel from immediately after continuous casting (hereinafter also referred to as continuous cast pieces) to room temperature is specially controlled in the same manner as mild steel. If it is not cooled, it will cause cracks as shown in FIG. 3 and FIGS. This type of cracking is thought to be due to thermal stress generated during cooling, and it occurred very rarely even in mild steel, but in the field of high-strength steel, it frequently occurs especially with the recent increase in high tensile strength. It was.

As this type of technology, Patent Literature 1 and Patent Literature 2 disclose a method for preventing cracking of a continuous cast piece of bearing steel. According to the descriptions in Patent Document 1 and Patent Document 2, if the bearing steel (continuous cast piece) after continuous casting is charged into a heating furnace before the surface temperature is cooled to 600 ° C. or less, the continuous cast piece is obtained. It is said that thermal stress during cooling can be reduced.
Japanese Patent No. 3149663 (Claim 1, 0013) Japanese Patent No. 3149964 (Claim 1, 0013)

According to Patent Document 1 and Patent Document 2 described above, the continuous cast piece is not cooled to a room temperature of 600 ° C. or lower, for example, around 100 ° C., between the processes of continuous casting and the heating furnace. On the other hand, surface defects (small cracks, impurity precipitates, etc.) of continuous cast pieces can be detected only after cooling to room temperature.
Therefore, in the above method, the thermal stress during cooling may be reduced as described in Patent Documents 1 and 2, but the surface defect of the continuous cast piece cannot be detected, and naturally it cannot be repaired.

  The present invention has been made in view of such various points, and its main purpose is to provide a crack prevention method that does not cause crack even when a continuous cast piece of high strength steel cast in a continuous casting facility is cooled to room temperature. It is to provide.

Means and effects for solving the problems

  The problems to be solved by the present invention are as described above. Next, means for solving the problems and the effects thereof will be described.

The carbon (C) content is 0.12 w% or more and 0.25 w% or less, the silicon (Si) content is 1.2 w% or more and 2.0 w% or less, and manganese (Mn) is 1.2 w% or more. In a method for preventing cracking of continuous cast pieces of high-strength steel of 0 w% or less, the cooling rate of continuous cast pieces (V: ° C./hour) according to the surface temperature (T: ° C.) at the center of the wide surface of the continuous cast pieces. ) Is controlled to satisfy the following.
・ At T ≧ 500, V ≦ 70
・ When 500> T ≧ 300, V ≦ 55
・ In 300> T ≧ 200, V ≦ 25
・ When 200> T ≧ 150, V ≦ 15
Note that the “cooling rate of continuous cast slab” here is obtained based on the surface temperature.

  Thereby, the continuous cast piece of high-tensile steel can be cooled to room temperature without generating a set crack. Further, when cooled to room temperature, it becomes possible to detect and repair surface defects of the continuous cast slab, so that it is possible to provide high-strength steel having no surface defects.

  The cooling rate of the continuous cast piece is preferably controlled by sandwiching the continuous cast piece between the other two continuous cast pieces. According to this, no special device for controlling the cooling rate is required, and it is only necessary to sandwich the continuous cast piece with another continuous cast piece, so that the present invention is introduced very inexpensively and easily. it can.

  It is preferable that a plurality of the continuous cast pieces are sandwiched simultaneously by the other two continuous cast pieces. Thereby, the cooling rate of the continuous cast piece can be further suppressed. Moreover, production efficiency can be improved.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

First, the steel types targeted by the method for preventing cracks in a continuous cast piece of high-strength steel according to the present embodiment (hereinafter, also simply referred to as “placement crack preventing method”) will be described. The target steel type of the present embodiment is classified as high-tensile steel among continuous cast pieces continuously cast in a continuous casting facility, and the chemical composition thereof is within the following range.
Carbon content: 0.12 w% or more and 0.25 w% or less Silicon content: 1.2 w% or more and 2.0 w% or less Manganese content: 1.2 w% or more and 4.0 w% or less

Table 1 shows an example of the steel types targeted in this embodiment. Among the steel types (high-tensile steels A to C) in this table, high-tensile steels A and B are the target steel types. On the other hand, the high-tensile steel C having a relatively low carbon content is not subject to the present embodiment because it does not cause cracking without special consideration during cooling. In other words, those having a carbon content of 0.11 w% or less do not cause cracking in the first place during cooling, and therefore no special measures need to be taken.
The other upper limit values (0.25 w%) of the above-mentioned range of carbon content and the definition of the range for silicon / manganese are general values for high-tensile steel.

Next, based on FIG.6 and FIG.7, the characteristic of the said high strength steel is demonstrated. FIG. 6 is a graph showing the results of a tensile test of high-tensile steel B around 800 ° C., and FIG. 7 is a graph showing the results of a Charpy impact test of high-ten steel B around 150 ° C.
As shown in FIG. 6, the high-strength steel usually has remarkable brittleness in a temperature range around 800 ° C. (embrittlement III region).
Further, as shown in FIG. 7, high-strength steel has a property that brittleness changes abruptly in a temperature range of about 150.degree. Specifically, as soon as the temperature of the test piece becomes 150 ° C. or less, the specimen rapidly becomes brittle. This is due to the fact that the silicon content has remarkably increased with the recent high tempering.

Therefore, the crack prevention method according to the present embodiment appropriately controls the cooling rate when passing through the temperature zone for the embrittlement III region, while the rapid brittleness generated around 150 ° C. For the increase, the cooling rate in the temperature range higher than 150 ° C., specifically in the range of 500 ° C. to 150 ° C., is appropriately controlled.
Specifically, in the above-described method of preventing cracking, the cooling rate (V: ° C./hour) of the continuous cast piece is set as follows based on the surface temperature (T: ° C.) at the center of the wide surface of the continuous cast piece.
・ At T ≧ 500, V ≦ 70
・ When 500> T ≧ 300, V ≦ 55
・ In 300> T ≧ 200, V ≦ 25
・ When 200> T ≧ 150, V ≦ 15
Note that the “cooling rate of continuous cast slab” here is obtained based on the surface temperature.

Next, a specific method for controlling the cooling rate of the continuous cast slab will be described. FIG. 1 is a diagram showing a method for cooling high-tensile steel according to an embodiment of the present invention.
As shown in FIG. 1A, in the present embodiment, the continuous cast pieces immediately after continuous casting are stacked together with the other continuous cast pieces immediately after continuous casting. And as shown in FIG.1 (b), it anneals by air cooling in the state pinched | interposed by the other two continuous cast pieces from the upper and lower sides. The other two continuous cast pieces may be at room temperature (appropriate temperature lower than 200 ° C., for example, 100 ° C.) or 500 ° C. at the time when the sandwiched five continuous cast pieces are started. You may preheat to the extent. Further, the other two continuous cast pieces may be used only for slow cooling with the continuous cast pieces sandwiched therebetween, or may be used as another semi-finished product that has been cooled to room temperature once. It may be a slab.
In this way, the continuous cast pieces immediately after continuous casting are stacked together with other continuous cast pieces, and cooled in a state sandwiched between the other two continuous cast pieces from the upper and lower sides, thereby reducing the cooling rate of the continuous cast pieces. It can be controlled as described above.

Thereafter, the surface of the continuous cast piece that has been gradually cooled to room temperature is visually checked for small flaws and impurity precipitates. If there is such a surface defect, the surface defect is removed by a polishing machine or the like.
And if all the surface defects are eliminated, it will move to the heating furnace which is the next process, will be heated to a suitable temperature, and will be rolled with a rolling device.

  Next, tests (tests A to C) for confirming the effect of the present invention will be described with reference to FIG. FIG. 2 is a diagram showing the measurement results of the surface temperature of the continuous cast slab. A broken line indicated by a reference sign CR in the drawing indicates the fastest cooling rate among the cooling patterns satisfying the conditions defined in the above-described crack prevention method according to the present embodiment. In addition, the surface temperature in the wide surface center part of a continuous cast piece was measured using the thermocouple.

[Test A]
This test A is a test carried out so as to satisfy the conditions defined in the laying crack prevention method according to the embodiment. That is, the continuous cast slab was gradually cooled so that the cooling rate was always slower than the cooling pattern CR in all temperature bands (surface temperature: 1000 ° C. to 100 ° C.).
Specifically, the continuous cast pieces immediately after continuous casting were stacked together with the other three continuous cast pieces immediately after continuous casting, and gradually cooled in a state of being sandwiched by other continuous cast pieces from the upper and lower sides.
The measurement result of the surface temperature at the center of the wide surface of the continuously cast slab that has been gradually cooled under the conditions of this test A is indicated by symbol A in the figure.
[Test B]
This test B is a test that was performed so as not to satisfy the conditions defined in the method for preventing cracks according to the embodiment. That is, the continuous cast slab was cooled so that the cooling rate was always higher than that of the cooling pattern CR in all temperature zones.
Specifically, continuous cast pieces immediately after continuous casting are stacked together with other continuous cast pieces immediately after continuous casting, or are not stacked between other continuous cast pieces from the upper and lower sides. Air cooling was performed from just after continuous casting to room temperature.
The measurement result of the surface temperature in the center part of the wide surface of the continuous cast piece cooled under the conditions of the test B is indicated by a symbol B in the figure.
[Test C]
This test C is a test that was performed so as not to completely satisfy the conditions defined in the method for preventing cracks according to the embodiment. That is, the continuous cast slab was cooled under the same conditions as in Test A above in the temperature range until the surface temperature reached 500 ° C. In other words, only a part of the cracking prevention method according to the above embodiment is applied to the continuous cast piece in the temperature range.
On the other hand, in the temperature range from 500 ° C. to room temperature, the continuously cast slab was cooled under the same conditions as in Test B above. In other words, the continuous cast pieces were individually cooled in the temperature range without being stacked or sandwiched between other continuous cast pieces from the upper and lower sides.
In short, in this test C, the above-mentioned crack prevention method according to this embodiment is not applied.
The measurement result of the surface temperature in the center part of the wide surface of the continuous cast piece cooled under the conditions of the test C is indicated by a symbol C in the figure.

And the continuous cast piece cooled to room temperature was observed, and it was investigated whether the setting crack as shown to FIGS.
[Test A (with prevention method applied)]
In the continuous cast slab which was gradually cooled as indicated by reference symbol A in FIG. In other words, the continuous cast slab could be cooled to room temperature without generating cracks.
[Test B (no prevention method applied)]
In the continuously cast slab cooled as indicated by reference symbol B, a crack as shown in FIGS. 3 to 5 occurred.
[Test C (only some of the prevention methods apply)]
As shown in FIG. 3C, even in the case where the above-mentioned crack prevention method is partially applied only to a part of the temperature range from immediately after continuous casting to room temperature, as in the case of the test B, FIG. Placement cracks as shown in FIG. 5 occurred.

  From these results, the above-described method for preventing cracking is required to prevent cracking evenly in all temperature ranges from immediately after continuous casting to room temperature (at least less than 200 ° C.). I can say that.

As described above, in the present embodiment, the carbon (C) content is 0.12 w% or more and 0.25 w% or less, the silicon (Si) content is 1.2 w% or more and 2.0 w% or less, and manganese ( The cooling rate (V: ° C./hour) of the continuous cast slab of high-strength steel having Mn) of 1.2 w% or more and 4.0 w% or less is determined according to the surface temperature (T: ° C.) at the center of the wide surface. The following effects can be achieved by controlling and cooling as described above.
・ At T ≧ 500, V ≦ 70
・ When 500> T ≧ 300, V ≦ 55
・ In 300> T ≧ 200, V ≦ 25
・ When 200> T ≧ 150, V ≦ 15
As described above, the “cooling rate of the continuous cast slab” is obtained based on the surface temperature.

(effect)
The continuous cast slab of high-strength steel can be cooled to room temperature without generating cracks. In other words, it is possible to avoid the influence due to the brittleness specific to high-strength steel as shown in FIGS.
Moreover, since the surface defect of a continuous cast piece can be detected and repaired when it cools to room temperature, the high strength steel without a surface defect can be provided.

  Further, as described above, in this embodiment, the cooling rate of the continuous cast piece is preferably controlled by sandwiching the continuous cast piece between the other two continuous cast pieces. According to this, no special device for controlling the cooling rate is required, and the continuous cast piece is simply sandwiched between other continuous cast pieces, so that the present invention can be introduced very inexpensively and easily. Can do.

  Further, as described above, in the present embodiment, it is preferable that a plurality of continuous cast pieces are sandwiched simultaneously by the other two continuous cast pieces. Thereby, the cooling rate of the continuous cast piece can be further suppressed. Moreover, production efficiency can be improved.

  Note that the above tests (test conditions and test results) do not limit the present invention or the operational effects thereof.

  The preferred embodiments and examples of the present invention have been described above, but the above-described embodiments can be modified as follows.

  For example, in the above embodiment, a plurality of continuous cast pieces immediately after continuous casting are stacked at the time of cooling. However, it is not always necessary to stack them, and it is sufficient if the above conditions for the cooling rate are satisfied. is there. Accordingly, there may be a case in which the continuous cast pieces are not stacked and only one piece is cooled, and the upper and lower sides are cooled in a state of being sandwiched between the other two continuous cast pieces.

Moreover, in the said embodiment, although the continuous cast piece was cooled in the state pinched | interposed by the other two continuous cast pieces from the upper and lower sides, it is not restricted to this, The continuous cast piece just after another casting And may be cooled in a stacked state.
In addition, as a method for controlling the cooling rate of the continuous cast slab, for example, a cooling suppression device provided with a heater may be used.

  As mentioned above, the cooling rate of the continuous cast slab is preferably moderately low from the viewpoint of generation of thermal stress, but for operational reasons, it is cooled within 5 days to a temperature of about room temperature (for example, 100 ° C.). I decided to.

The figure which shows the cooling method of the high strength steel which concerns on one Embodiment of this invention. The figure which shows the measurement result of the surface temperature of a continuous cast piece. The schematic diagram which shows the placement crack of a continuous cast piece. The figure which shows the actual photograph of the placement crack of a continuous cast piece. The figure which shows the actual photograph of the placement crack of a continuous cast piece. The figure which shows the result of the tension test in about 800 degreeC of high tensile steel with a graph. The figure which shows the result of the Charpy impact test in about 150 degreeC of high-tensile steel with a graph.

Claims (3)

The carbon (C) content is 0.12 w% or more and 0.25 w% or less, the silicon (Si) content is 1.2 w% or more and 2.0 w% or less, and manganese (Mn) is 1.2 w% or more. In the method of preventing cracks in continuous cast pieces of high-strength steel with 0 w% or less,
A high-strength steel characterized by controlling the cooling rate (V: ° C./hour) of the continuous cast piece so as to satisfy the following in accordance with the surface temperature (T: ° C.) at the center of the wide surface of the continuous cast piece. To prevent cracks in continuous cast pieces.
・ At T ≧ 500, V ≦ 70
・ When 500> T ≧ 300, V ≦ 55
・ In 300> T ≧ 200, V ≦ 25
・ When 200> T ≧ 150, V ≦ 15   The cooling rate of the continuous cast slab is controlled by sandwiching the continuous cast slab between the other two continuous cast slabs. Prevention method. 3. The method for preventing cracking of a high-strength steel continuous cast piece according to claim 2, wherein a plurality of the continuous cast pieces are sandwiched simultaneously by the other two continuous cast pieces.