JPH07116703A - Hot crack preventing method for thin cast slab - Google Patents

Abstract

PURPOSE:To provide a hot crack preventing method for a continuous cast slab which can obtain a thin cast slab free from defectives such as an end face crack and so forth even when the cast slab is worked directly after it has been cast continuously. CONSTITUTION:Steel is cast by continuous casting into the cast slab of <=80mm, then, light draft is produced so that it is <=5% by one pass in a temperature range of <=1100 deg.C and the total rolling reduction is <=10%, then, continuously rolled further in the temperature range of <=1100 deg.C to be rolled down into the thin cast slab. Consequently, the thin cast slab can be obtained without generating an internal crack and the end face crack.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for preventing hot cracking of continuously cast slabs, which enables thin slabs to be obtained without causing defects such as end face cracks even if the casting is processed immediately after casting.

[0002]

2. Description of the Related Art In recent years, a process using a thin cast piece as a starting material has been researched and developed for the purpose of significantly reducing costs and omitting steps, and a possibility as a new steel sheet manufacturing process has been found. However, in order to achieve the above object, it is necessary to make the thickness of the cast slab as thin as possible, but casting of the cast slab has difficulty in castability, unlike conventional continuous casting. Therefore, it is essential to obtain defect-free thin slabs to enable this process.

Regarding the hot workability of a slab during continuous casting of steel, it is known that when the slab is directly fed and rolled, hot cracking occurs depending on the composition or the thermal history after casting. As a method for preventing hot cracking in such a direct rolling process, there are techniques disclosed in (1) JP-A-59-97701 and (2) JP-A-4-33706. In (1), the thermal history is controlled by cooling and heat retention after casting to prevent cracking, and its application to a process based on thin cast pieces is also clearly stated. Also,
(2) is premised on the production of thin cast pieces, and is intended to avoid cracking by regulating the Mn and S amounts or the rolling temperature after casting.

[0004]

However, in the method of controlling the heat history after casting (Japanese Patent Laid-Open No. 59-97701), the application to thin cast pieces cast by belt casters is also specified. Basically, it regulates the heat history of a cast slab after casting and before rolling. Further, a method for controlling the amount of Mn and S or the rolling temperature after casting with a thin cast piece (Japanese Patent Laid-Open No. 4-33706).
Is intended to avoid the embrittlement temperature range during processing after casting, and both are methods for avoiding processing in the embrittlement temperature range. Therefore, the present invention provides a method for obtaining a thin cast piece having a thickness of 50 mm or less without causing hot cracking even if rolling is performed in an embrittlement temperature range when the thickness is reduced to a predetermined thickness before hot rolling for finishing. The purpose is to do.

[0005]

Means for Solving the Problems As a result of studies for solving the above-mentioned problems, the inventors of the present invention have found that the rolling temperature in the rolling for reducing the thickness which is subsequently performed by light reduction in a high temperature region after casting. I found that I was not regulated. That is, although it depends on the composition of the steel, it has been found that a thin cast piece of 50 mm or less can be obtained without causing hot cracking even when rolling is carried out in a temperature range which has been conventionally regarded as an embrittlement temperature range. .

The present inventors have found that C: 0.02 mass%,
Mn: 0.1 mass% and S: 0.003 to 0.0
Steel containing 2 mass% was melted by vacuum melting. These steels were cast into 75 mm slabs, followed by 1 at various temperature ranges.
After performing 5 passes of light reduction with a reduction amount of 1% per pass, the temperature range of 1000 to 1100 ° C. was reduced by 3 passes to form a 40 mm thin slab, and the presence or absence of cracks on the end face portion at that time was investigated. . The results are shown in Fig. 1 (in the figure: no crack,
X: There is a crack). When the amount of S is 0.003 mass%, cracking does not occur on the end face portion at any temperature when light reduction is performed, but when the amount of S exceeds 0.005 mass%, a difference occurs depending on the temperature range in which light reduction is performed. That is, 1100 ° C
When the temperature is lower than the above value, the end face is cracked, but in the higher temperature range, the end face is not cracked.

Next, C: 0.05 mass%, Mn:
Steel containing 0.25 mass% and S: 0.021 mass% was melted by vacuum melting. After casting this steel into a slab of 60 mm, the steel was rolled in a temperature range of 1000 to 1400 ° C. for 5 passes with various reduction amounts, and the presence or absence of internal cracks was investigated. The results are shown in FIG. 2 (in the figure :: no crack, x: crack). In the region where the total amount of rolling reduction exceeds 10% and in the temperature region of 1100 ° C. or less, internal cracking or cracking occurs in the end face portion.

From these experimental results, in order to obtain a thin slab of 50 mm or less without restricting the components, especially Mn and S amounts, and without causing cracks in the end face portion and the inside of the slab, high temperature after casting It was found that it is necessary to carry out light reduction in the area. As a result of examining the structural factors leading to avoiding the above-mentioned end face cracking, it was found that 5 passes per pass in the high temperature region.
% Re-crystallization also occurs under a light pressure of not more than%, and the segregation of S into austenite grain boundaries is reduced through the refinement of the crystal grains, and the appearance of a liquid phase due to the segregation of S grain boundaries, which is the cause of cracking, appears. It is presumed that it was suppressed. That is, it became clear that it is important to reduce the segregation of S at the austenite grain boundaries.

From the above new knowledge, it is possible to manufacture a thin cast piece without causing cracks in the end face portion in the subsequent rolling for reducing the thickness by performing light reduction in the high temperature region after casting. You can The present invention has been completed based on the above findings. The gist of the present invention is to perform continuous casting of steel into a slab of 80 mm or less, and
In a temperature range higher than 100 ° C., after rolling such that the reduction amount is 5% or less per pass and the total reduction amount is 10% or less, subsequently, further rolling is performed in a temperature range of 1100 ° C. or less, A method of preventing hot cracking of a thin cast piece is characterized in that a thin cast piece of 50 mm or less is used.

[0010]

The present invention will be described in detail below. First, the casting thickness must be 80 mm or less. If the casting thickness is larger than this, the load in reducing the thickness after casting and before finishing hot rolling is increased, and the advantage of casting into a thin cast piece is lost.

Next, the processing conditions for reducing the thickness after casting are the most important in the present invention. That is, in order to prevent internal cracks and at the same time avoid cracks in the end face portion, the processing temperature must be 5% or less per pass as a temperature range higher than 1100 ° C. after casting, and the total reduction amount is 10% or less. . If the processing conditions after casting exceed the reduction amount described above, not only internal cracking occurs, but also the scale of the rolling mill becomes large and the equipment cost increases significantly. On the other hand, if the processing temperature at that time is set to 1100 ° C. or lower, cracking may occur at the end face depending on the S content in the steel, which is not preferable. It should be noted that the lower the S content, the more advantageous it is to suppress the above-mentioned cracking of the end face portion, and it is also known that cracking can be avoided by setting it to 0.005 mass% or less, but the desulfurization cost in steelmaking It is not preferable because it will be greatly increased.

Further, 110 is subsequently performed.
The rolling schedule for processing at 0 ° C. or lower is not particularly limited, but it is necessary to reduce the thickness to 50 mm or less in order to reduce the load in the subsequent hot rolling for finishing. In the present invention, ordinary steel,
In particular, although application of thin plate components to steel is considered, the components are not essentially limited, and steels other than ordinary steel such as stainless steel or electromagnetic steel may be applied. Further, steel containing Cu and Sn inevitably by using scrap may be used.

[0013]

EXAMPLES Steels having the components shown in Table 1 were tapped from a converter and continuously cast into thin slabs having the casting thickness shown in the table.
Immediately after casting, after rolling under the conditions shown in Table 2,
The presence or absence of cracks inside the slab and on the end face was examined. In the table, the results are shown as ◯: no cracks, x: cracks. The one according to the present invention has neither internal cracking nor cracking at the end face portion, but those outside the scope of the present invention have internal cracking or cracking at the end face portion.

[0014]

[Table 1]

[0015]

[Table 2]

[0016]

EFFECTS OF THE INVENTION In order to realize a process using a thin cast piece of 80 mm or less as a starting material, the present invention clarifies a method of avoiding the occurrence of internal cracks and cracks on end faces even if processing is performed immediately after casting. It was done. According to the present invention, even if a cast slab is processed, cracks do not occur,
The slab of m or less can be subjected to the finish hot rolling step without rough rolling, and the hot rolling step can be greatly simplified.

[Brief description of drawings]

FIG. 1 is a view showing a change in a crack generation state of an end face portion due to a change in conditions of an S amount and a light reduction temperature (◯: no crack,
X: There is a crack).

FIG. 2 is a diagram showing changes in the occurrence status of internal cracks or end surface cracks accompanying changes in processing temperature and reduction rate (◯: no cracks, ×: cracks).

Claims (1)

[Claims] 1. In a temperature range higher than 1100 ° C. during cooling, after casting steel into a slab of 80 mm or less by continuous casting,
The reduction amount is 5% or less per pass and the total reduction amount is 1
After rolling to 0% or less, then 1100
A method for preventing hot cracking of a thin cast piece, which comprises further rolling in a temperature range of ℃ or less to obtain a thin cast piece of 50 mm or less.