JPH0338941B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention provides a method for casting, for example, using a synchronous continuous casting machine.
The present invention relates to a method for producing a thin slab by a rapid solidification method in which molten metal is rapidly solidified and directly cast into a thin slab. [Prior Art and its Problems] In recent years, research has been conducted on methods of directly casting thin slabs by ultra-rapidly solidifying molten metal. For example, JP-A No. 58-210150 discloses a method for casting a thin strip of amorphous alloy by ultra-quenching a molten amorphous alloy consisting of iron, boron, and silicon to rapidly solidify it. There is. However, the method described above has a quenching rate of
Since the temperature is 10 ⁵ °C/sec or higher, the thickness of the thin ribbon to be cast must be approximately 200 μm or less, and the width must be several hundred mm or less. Therefore, its use was limited to special things. On the other hand, to simplify the casting process, e.g.
at least one moving in the same direction and at the same speed
A pair of endless bands are arranged facing each other at a predetermined distance to form a horizontal mold, and the molten metal supplied into the mold is rapidly solidified by contact with the mold, and is synchronized with the endless band. A synchronous continuous casting method consists of continuously casting a thin plate-shaped slab by pulling it out, or a method in which molten metal supplied onto the surfaces of a pair of rotary cooling bodies is mixed with the rotary cooling bodies. Many various methods have been proposed, such as a twin-roll continuous casting method, which involves continuous casting of thin plate-shaped slabs by rapid cooling and solidification through contact with the steel. According to the above-mentioned casting method, the cooling rate of the molten metal is not ultra-quick as in the case of casting an amorphous alloy ribbon, so it is possible to continuously cast thick and wide plate-shaped slabs. can. However, because the cooling rate of slabs cast by rapidly solidifying molten metal using the method described above is slow, it is thought that only a structure similar to that of conventional ingot materials can be obtained, and the structure and material properties of the slabs are difficult to obtain. Almost no consideration was given. [Object of the Invention] Therefore, an object of the present invention is to continuously cast thin slabs by rapidly cooling and solidifying molten metal, and to control changes in the metal structure caused by the rapid solidification during casting and subsequent steps. It is an object of the present invention to provide a method for producing a thin plate-like slab having excellent properties by optimizing the properties of the steel. The present inventors have conducted extensive research in order to develop a method for homogenizing the composition and structure of the obtained slab, or for refining the structure, when rapidly cooling molten metal and continuously casting thin slabs. layered. As a result, although continuous casting methods such as the synchronous or twin roll method described above cause smaller changes in the structure than the ultra-quenching method, changes in the structure do occur due to rapid cooling, and this change in structure can be controlled by the casting method. In other words, if the quenching rate of the molten metal is set within the range of 10°C/sec or more and less than 10 ⁵ °C/sec, and the slab is treated under specific conditions after being quenched at a rate within the above range, the problem becomes noticeable. It has been found that a slab with a homogenized composition and structure, or a slab with a fine-grained structure, can be obtained. [Summary of the Invention] This invention was made based on the above findings, and provides carbon steel containing 0.05 wt.% or more of carbon,
Manganese-containing steel containing 0.3wt.% or more of manganese, phosphorus-containing steel containing 0.02wt.% or more of phosphorus, or boron-containing steel containing 0.002wt.% or more of boron is rapidly cooled to produce thin sheets. In the method for manufacturing a thin plate slab, which involves continuously casting slabs in the form of a steel plate, the molten steel is rapidly cooled at a cooling rate in the range of 10°C/second or more and less than 10 ⁵ °C/second, and then By holding the rapidly solidified slab at a temperature of 0.3 T _n (where T _n is the melting point expressed in absolute temperature) or higher for 30 seconds or more, a slab with a homogenized composition and structure is manufactured. or, in the case of low carbon, ultra-low carbon aluminum killed steel, by reheating the rapidly solidified slab above to pass through the transformation point at a temperature within the range of 0.3 to 0.9T _n . This method is characterized by producing slabs having a fine-grained structure. [Structure of the invention] In this invention, molten metal is rapidly cooled at 10°C/
The reason why the cooling is performed at a cooling rate within the range of 2 seconds or more and less than 10 ⁵ ° C./second is as follows. That is, the above rapid cooling
If the temperature is less than 10° C./sec, the structure becomes the same as that of a slab cast by a conventional method, and it is not possible to homogenize the composition, structure, or refine the structure. On the other hand, when the above-mentioned quenching temperature exceeds 10 ⁵ °C/sec, the thickness of the slab must be reduced to about 200 μm or less.
Therefore, the width of the slab becomes shorter, resulting in the problem that the product is limited to those for special uses. In this invention, after the molten metal is rapidly cooled to a temperature within the above range and solidified, it is not cooled to room temperature,
The reason why cooling is stopped at a temperature of 0.3T _n or higher and held at that temperature for 30 seconds or more is that this reduces microscopic segregation in the structure to a level that does not pose a practical problem and makes the composition and structure homogeneous. This is to make it more effective. If the above-mentioned holding temperature is less than 0.3T _n , it is not practical because a long time is required to reduce segregation, and if the above-mentioned holding time is less than 30 seconds, the desired effect cannot be obtained. Steel types that can benefit from the above-mentioned treatment include carbon steel with a carbon content of 0.05wt.% or more;
Elements that tend to cause segregation, such as high Mo steel with a manganese content of 0.3wt.% or more, phosphorus-added steel with a phosphorus content of 0.02wt.% or more, and boron-containing steel with _a boron content of 0.002wt.% or more. This is a type of steel that includes. In addition, in this invention, the reason why the molten metal is rapidly cooled and solidified and then reheated to pass the transformation point at a temperature within the range of 0.3 to 0.9T _n is that this makes the structure finer. This is for the purpose of achieving this goal. In other words, as mentioned above, simply by rapidly cooling and solidifying the molten metal at a rate within the range of 10 °C/sec or more and less than 10 ⁵ °C/sec, it is not possible to achieve grain refinement equivalent to that of rolling heat-treated material. Furthermore, even if an attempt is made to refine the grain structure of the rapidly solidified slab by rolling, the desired grain refinement effect cannot be achieved because the plate thickness is too thin to achieve a large rolling reduction ratio. I can't do it. However, at temperatures within the range of 0.3 to 0.9T _n , steel types with a solid phase transformation point are rapidly solidified at a rate within the above range, cooled once to a temperature below the transformation point, and then transformed. By performing heat treatment through the transformation point, which consists of reheating to a temperature above the point and cooling, a certain degree of grain refinement can be achieved without rolling. Such reheating and cooling treatment performed during cooling may be repeated any number of times, and if rolling is performed in addition to this treatment, it is possible to obtain a fine grain structure equivalent to that of conventional rolled heat-treated materials. can. In the above, if the reheating temperature is less than 0.3T _n ,
Even if heat treatment is performed to pass the transformation point, the transformation takes a long time and is not practical; on the other hand, the reheating temperature
If it exceeds 0.9T _n , the crystal grains will grow and the grain refining effect will not be obtained. Steel types that can benefit from the above-mentioned treatment include, for example, steel types that have a transformation point in the temperature range of 0.3T _n to 0.9T _n , such as low carbon aluminum killed steel and ultra-low carbon aluminum killed steel. [Examples of the Invention] Next, the present invention will be further described in detail with reference to Examples. Example 1 Molten Si-Mn steel having the composition shown in Table 1 was
Several types of test materials with different cooling rates, ie, slab thicknesses, were prepared using the rapid solidification method, and the component segregation of each material was investigated.

[Table] The component segregation was investigated using an X-ray microanalyzer, and the profile of Mn was obtained by line analysis in the thickness direction at the center of the width of the plate, as shown in (1) below.
The segregation degree (P _c ) of Mn was calculated using the formula. P _c = C(Mn) _nax −C(Mn) _nio /C(Mn) _Ave ...
…(1) However, C(Mn) _nax : Number of counts when the amount of Mn reaches the maximum in the thickness direction C(Mn) _nio : Number of counts when the amount of Mn reaches the minimum in the thickness direction C( Mn) _Ave : Average number of counts Figure 1 is a graph showing the relationship between the Mn segregation degree (P _c ) determined above and the cooling rate and slab thickness. , 10℃/
When held at 900℃ for 30 seconds after being rapidly cooled at a cooling rate of at least 10 seconds and less than ¹⁰⁵ ℃/second
The degree of segregation of Mn is shown, and the mark x indicates the degree of segregation of Mn when the material is continuously air-cooled after rapid solidification as described above. As is clear from Fig. 1, the faster the cooling rate, that is, the thinner the slab thickness, the lower the degree of segregation. In the case of holding, the degree of segregation is lower than in the case of continuous air cooling after rapid solidification. In particular, when the cooling rate is set to 10 ² C/sec or more, the degree of segregation becomes extremely small, and the composition and structure can be made homogeneous. Example 2 Several types of test materials with different cooling rates, that is, slab thicknesses, were prepared by rapid solidification of molten low carbon aluminum killed steel having the composition shown in Table 2, and ferrite grains in the cross section of the slab were prepared. The diameter was measured.

[Table] The ferrite particle size was measured by a cutting method, and the average value of the center part of the plate thickness and the surface layer part of the plate thickness was taken as the ferrite particle size. FIG. 2 is a graph showing the relationship between the average ferrite grain size determined above, the cooling rate, and the thickness of the slab. As is clear from Figure 2,
As the cooling rate increases, the ferrite grain size becomes finer. Figure 3 shows the temperature at about 10 ² °C/sec (slab thickness 10
For the test material that was rapidly solidified at a cooling rate of mm),
This is an example of a heat pattern when reheating is performed by the method of the present invention and when reheating is not performed for comparison. In Figure 3, (a), (b), and (c) are examples of the present invention; (a) is when the temperature is rapidly cooled to 850°C and then reheated to 920°C; (b) is when the temperature is increased to 850°C. If the rapid cooling and subsequent reheating to 920℃ are repeated three times,
(c) is rapidly cooled to 800℃ and then reheated to a temperature of 1150℃.
Furthermore, this is the case where 70% rolling is performed at a finishing temperature of 900°C. In addition, (d) and (e) are comparative examples; (d) is the case where the test material that was rapidly solidified as described above was rolled by 70% at a finishing temperature of 900°C; This is the case where the solidified test material was cooled as it was to room temperature. FIG. 4 is a graph showing the average ferrite particle size of the test material subjected to the treatment shown in FIG. 3 above. As in Comparative Example (e), the average ferrite particle size of the test material cooled to room temperature after rapid solidification is approximately 63 μm, but as in Example (a), the average ferrite particle size when reheated to a temperature of 920°C The particle size is approximately 20 μm, and in the case of (b), where such rapid cooling and reheating is repeated, it is approximately 15 μm.
In the case of (c), which was further subjected to rolling after reheating, the grains were refined to about 10 μm. Such refinement of the ferrite particle size is achieved at 900℃ as in Comparative Example (d).
This is comparable to 70% rolling at the finishing temperature of Note that the average ferrite grain size of slabs cast by conventional continuous casting is 2 to 5 mm. Table 3 shows examples (a), (b) of the present invention described above,
(c) shows the mechanical properties of the slabs according to Comparative Examples (d) and (e) and the no-skin pass material (f) obtained by hot rolling the slabs produced by the conventional manufacturing method, that is, continuous casting. In addition, comparative example (f)
The heating temperature during hot rolling is 1150℃, and the finishing temperature is 921℃.
℃, and the winding temperature is 500℃.

〔Effect of the invention〕

As described above, according to the method of this invention,
When molten metal is rapidly solidified by synchronous continuous casting method, twin roll continuous casting method, etc. to continuously cast thin slabs, the slabs after rapid solidification are held at a predetermined temperature or at a predetermined temperature. By reheating at a high temperature, an industrially excellent effect is brought about in that the composition and structure are homogenized, or the structure is made finer without rolling. Although the above embodiments have been described with respect to steel, the present invention is not limited to steel, and can also be applied to iron, non-ferrous alloys, and the like.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between Mn segregation degree and slab thickness, Figure 2 is a graph showing the relationship between average ferrite grain size, cooling rate, and slab thickness, and Figure 3 is a graph showing the relationship between the average ferrite grain size, cooling rate, and slab thickness, and Figure 3 is a graph showing the relationship between Mn segregation degree and slab thickness. Figure 4 is a graph showing an example of the heat pattern when the test material was rapidly solidified by the method of the present invention and when it was not reheated for comparison. It is a graph showing the average ferrite particle size of the test material.

Claims (1)

[Claims] 1. Carbon steel containing 0.05wt.% or more of carbon;
By rapidly cooling molten steel of manganese-containing steel containing 0.3 wt.% or more of manganese, phosphorus-containing steel containing 0.02 wt.% or more of phosphorus, or boron-containing steel containing 0.002 wt.% or more of boron, In a method for producing a thin plate slab in which a thin plate slab is continuously cast, the molten steel is rapidly cooled at a cooling rate within a range of 10°C/second or more and less than 10 ⁵ °C/second, and then ,
The slab that has been rapidly solidified in this way is heated to a temperature of 0.3Tm or higher (Tm is the melting point expressed in absolute temperature).
A method for manufacturing a thin slab, characterized by holding the slab for 30 seconds or more. 2. A method for producing thin plate slabs in which molten low carbon or ultra-low carbon aluminum killed steel is rapidly cooled to continuously cast thin plate slabs, wherein the molten steel is rapidly cooled at a rate of 10°C/sec or more, 10 ⁵ ℃/sec, and the thus rapidly solidified slab is cooled through the transformation point to a temperature below said transformation point at a temperature in the range of 0.3 to 0.9 Tm. and then reheating to a temperature that passes through the transformation point and exceeds the transformation point. 3. A patent characterized by performing multiple times of rapid cooling at a cooling rate in the range of 10°C/second or more and less than 10 ⁵ °C/second, and a transformation point reheating treatment at a temperature in the range of 0.3 to 0.9Tm. A method for manufacturing a thin plate-shaped slab according to claim 2.