JP2011136363A - CONTINUOUS CASTING METHOD FOR ROUND SLAG MADE OF Cr-CONTAINING ALLOY STEEL - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To continuously cast a round slab with a diameter of &ge;300 mm made of a Cr-containing alloy steel comprising, by mass, &le;0.15% C and 1.0 to 3.0% Cr with internal quality and surface quality secured. <P>SOLUTION: In this method, a round slab with a diameter of &ge;300 mm is continuously cast from a Cr-containing alloy steel comprising, by mass, &le;0.15% C and 1.0 to 3.0% Cr using a continuous casting machine at least having a horizontal part. Casting velocity is controlled to 0.45 to 0.55 m/min, the specific water content in secondary cooling is controlled to 0.8 to 1.4 l/kg, and solidification is completed to the inlet side of the horizontal part at the latest. A round slab with a diameter of &ge;300 mm made of a Cr-containing alloy comprising, by mass, &le;0.15% C and 1.0 to 3.0% Cr can be continuously cast with internal quality and surface quality secured. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention has a C content of 0.15 mass% or less, a Cr content of 1.0 mass% or more and 3.0 mass% or less, for example, a diameter of 300 mm or more made of a Cr-containing alloy steel used as a pipe material for piping. This is a method of continuously casting round cast slabs, which makes it possible to ensure inner surface quality and surface quality.

  When continuously casting round cast slabs made of Cr-containing alloy steel, tensile stress is applied during the final solidification of the slabs, and cracks are likely to occur in the slab axis. This is because the solidification temperature range of ferrite having a low tensile strength shown in FIG. 6 is large as shown in Table 1 below in the case of Cr-containing alloy steel.

  On the other hand, when Cr-containing alloy steel is cast with square slabs and subjected to ingot rolling, cracks in the shaft center are crimped, but there is a problem that ingot rolling costs are incurred. In addition, when the rolling ratio is small, as shown in FIG. 7, the occurrence rate of surface defects increases.

  Therefore, as a solution, Patent Document 1 discloses a steel having a C content of 0.10% by mass or less, or a Cr content of 1.0% by mass or more and a C content of 0.15% by mass or less. A method for reducing axial center cracks in continuous casting of round slabs from molten steel is disclosed. This method applies compressive stress at the time of final solidification of a slab by cooling at the end of solidification.

  However, in the method disclosed in Patent Document 1, it is understood that the heat resistance of the solidified shell increases as the diameter of the slab increases, so that the cooling effect on the shaft center may not be achieved depending on the cooling conditions. Yes.

  In Patent Document 2, when continuously casting round cast pieces having a diameter of 300 mm or more from molten steel having a C content of 0.10% by mass or less, the cooling rate is 10 ° C./min or less in the secondary cooling zone. A method is disclosed in which cooling is performed and an equiaxed crystal structure is within a range of 60 mm or less from the center of the slab cross section. According to this method, it is said that the length of the axial center crack can be reduced.

  Here, the equiaxed crystal structure is a solidified crystal that is generated and grows by dividing and releasing the growing columnar dendrite or by new nucleation in the molten steel, and the columnar crystal extends from the cast wall in the temperature gradient direction. A crystal that grows isotropically.

  However, Patent Document 2 does not discuss the relationship between the casting speed and the cooling speed in the secondary cooling zone. Further, there is no detailed description of the effect of forming an equiaxed crystal structure at a high cooling rate of 50 ° C./min or more.

JP 2004-330252 A JP 2006-95565 A

  The problem to be solved by the present invention is that the C content is 0.15% by mass or less, the Cr content is 1.0% by mass or more and 3.0% by mass or less, and the diameter of the Cr-containing alloy steel is 300 mm. There is no effective method for continuously casting the above round cast slab while ensuring inner surface quality and surface quality.

The continuous casting method of the Cr-containing alloy steel round cast of the present invention,
A round slab made of Cr-containing alloy steel having a C content of 0.15% by mass or less, a Cr content of 1.0% by mass or more and 3.0% by mass or less, and having a diameter of 300 mm or more is obtained. To ensure quality and continuous casting,
A continuous casting machine having at least a horizontal portion of a round slab having a diameter of 300 mm or more from a Cr-containing alloy steel containing C: 0.15% by mass or less and Cr: 1.0% by mass to 3.0% by mass Using a continuous casting method,
The casting speed is set to 0.45 m / min to 0.55 m / min, and the specific water amount of the secondary cooling is set to 0.8 liter / kg to 1.4 liter / kg to complete the solidification by the entry side of the horizontal portion at the latest. This is the main feature.

  According to the present invention, a round slab having a diameter of 300 mm or more made of Cr-containing alloy steel containing C: 0.15 mass% or less, Cr: 1.0 mass% or more, and 3.0 mass% or less, It becomes possible to perform continuous casting while ensuring the inner surface quality and surface quality. Therefore, the conventional square slab-bullet rolling process can be omitted, and the cost can be rationalized.

It is a figure explaining the continuous casting method of this invention, (a) is sectional drawing which shows the example of a continuous casting machine typically, (b) is the slab of the final solidification position when a casting speed is 0.5 m / min. (C) is a figure similar to (b) when the casting speed is 0.8 m / min. (A) is a slab cross-sectional macrostructure photograph when the casting speed is 0.8 m / min, and (b) is a slab cross-section macrostructure photograph when the casting speed is 0.5 m / min. It is the figure which showed the relationship between the final solidification position of the round slab at the time of continuous casting, and the ratio of the equiaxed crystal which gathers to the upper side of a perpendicular direction from the center of a cross section. It is the figure which showed the relationship between the amount of specific water in a secondary cooling zone, and the equiaxed crystal ratio which exists in the center of a round slab. It is the figure which showed the relationship between the casting speed, the specific water quantity of a secondary cooling zone, the property of the manufactured product, etc. among the results shown in Table 3. It is a figure explaining the temperature range (ferrite solidification temperature range) containing a ferrite ((delta) iron) in the process of a structure transformation from a liquid phase-> ferrite ((delta) iron)-> austenite (gamma iron). It is the figure which showed the relationship between the rolling ratio and the depth ratio in a round slab and a square slab.

  In the present invention, a round slab made of Cr-containing alloy steel containing C: 0.15% by mass or less, Cr: 1.0% by mass or more, and 3.0% by mass or less has a diameter of 300 mm or more. The objective of continuous casting with ensuring surface quality was achieved by optimally setting the casting speed and the specific water amount in the secondary cooling zone.

The present invention will be described below.
When equiaxed crystals gather at the center of the cross section of a round slab produced by continuous casting, even if circumferential tensile stress is generated due to thermal stress in the final solidification zone, this tensile stress is dispersed and cracking occurs. Extension is suppressed.

  On the other hand, if there is no equiaxed crystal in the center of the cross section of the round slab produced by continuous casting, the δ phase grains of columnar crystals are generated in the final solidification zone when circumferential tensile stress due to thermal stress occurs. Cracks extend along the border.

  Therefore, the inventor completed the solidification of the round slab at the vertical portion of the continuous casting machine by performing continuous casting at a low casting speed and performing secondary cooling at a high cooling speed, etc. We considered collecting axial crystals in the center of the round slab.

  FIG. 2 shows a macro structure photograph of the cross section of the slab when the casting speed during continuous casting is 0.8 m / min and 0.5 m / min, and the inside surrounded by the broken line in the photograph is an equiaxed crystal. It is a belt.

  From FIG. 2, when the casting speed during continuous casting is 0.5 m / min, equiaxed crystals are gathered at the center of the cross section of the round slab, but when the casting speed is 0.8 m / min. It can be seen that equiaxed crystals are gathered slightly below in the vertical direction from the center of the cross section of the round cast slab (see FIG. 1).

  This is because in the casting using a continuous casting machine (curved type or vertical bending type) having a horizontal part, when the casting speed is fast (for example, 0.8 m / min), the round slab is finally solidified in the horizontal part, This is because the equiaxed crystal does not exist on the upper side in the vertical direction in the cross section as shown in FIG.

  On the other hand, as the casting speed decreases, solidification at the vertical part of the continuous casting machine is completed, so that equiaxed crystals that settle out of the mold cross the round slab as shown in FIG. The rate at the center of the surface has increased.

  FIG. 4 shows the relationship between the specific water amount in the secondary cooling zone and the equiaxed crystal ratio existing in the center of the round cast slab. From FIG. It can be seen that the crystallinity increases.

  Therefore, the inventor changed the casting speed and the specific water amount in the secondary cooling zone by using a steel of the component system shown in Table 2 below with a curved continuous casting machine, and a round slab having a diameter of 360 mm was obtained. Manufactured. The produced round cast slab was rolled by a Mannesmann-plug mill type piercing and rolling machine, and the generated surface defects were investigated. The casting conditions and casting results at that time are shown in Table 3 below.

  Of the results shown in Table 3, FIG. 5 shows the relationship between the casting speed, the specific water content of the secondary cooling zone, and the properties of the manufactured product.

  From Table 3 and FIG. 5, if the cooling in the secondary cooling zone becomes excessive, the surface temperature of the round cast slab is lowered and surface cracks occur, or the produced round cast slab is warped and the inside of the continuous casting machine It has been found that corrections cannot be made at The case where the cooling in the secondary cooling zone is excessive is applicable not only when the casting speed is slow and the specific water amount is large, but also when the specific water amount is too large even when the casting speed is high.

  On the other hand, if the specific water amount in the secondary cooling zone is too small, the final solidification position of the round slab becomes the horizontal part of the continuous casting machine, and is equiaxed upward in the vertical direction in the cross section as shown in FIG. There is a concern that the crystal will no longer exist and axial cracks will occur.

  On the other hand, when the relationship between the casting speed and the specific water amount in the secondary cooling zone is within the optimum range, equiaxed crystals gather at the center of the cross section of the round slab to suppress surface cracks and slab inability to correct the slab. I understood that I could do it.

The present invention has been made based on the above findings,
A continuous casting machine having at least a horizontal portion of a round slab having a diameter of 300 mm or more from a Cr-containing alloy steel containing C: 0.15% by mass or less and Cr: 1.0% by mass to 3.0% by mass Using a continuous casting method,
The casting speed is set to 0.45 m / min to 0.55 m / min, and the specific water amount of the secondary cooling is set to 0.8 liter / kg to 1.4 liter / kg to complete the solidification by the entry side of the horizontal portion at the latest. This is a continuous casting method for Cr-containing alloy steel round cast slabs.

  In the present invention, it is limited to a round slab having a diameter of 300 mm or more manufactured from a Cr-containing alloy steel containing C: 0.15% by mass or less and Cr: 1.0% by mass to 3.0% by mass. The reason is as follows.

C: 0.15 mass% or less
C is an austenite stabilizing element, and the content of C largely controls the amount ratio of ferrite and austenite. This is because the ferrite phase generally has a lower strength than the austenite phase, and the axial crack due to the ferrite phase is likely to occur in a slab having a C content of 0.15% by mass or less.

Cr: 1.0 mass% or more and 3.0 mass% or less
Cr is a ferrite stabilizing element, and when the Cr content is 1.0% by mass or more, the axial center crack is likely to occur even if the C content exceeds 0.15% by mass. It is. In addition, if the Cr content is in the range of 1.0% by mass to 3.0% by mass, the solidification temperature range of ferrite is widely common, so in the present invention, the upper limit of the Cr content is 3%. 0.0 mass%.

Diameter of round slab: 300mm or more As the diameter of the slab to be manufactured increases, cracks occurring in the shaft center tend to increase. When the diameter exceeds 300mm, the effect of secondary cooling decreases and the shaft This is because it leads to the expansion of mind breaking.

Continuous casting machine: Continuous casting machine having at least a horizontal part, for example, a continuous casting machine using a curved continuous casting machine or a vertical continuous casting machine having no horizontal part, is equiaxed without using the method of the present invention. This is because crystals tend to gather in the center of the round slab.

Casting speed: 0.45 m / min to 0.55 m / min, specific water volume for secondary cooling: 0.8 liter / kg to 1.4 liter / kg
According to the above-mentioned experiment by the inventor, the casting speed: 0.45 m / min to 0.55 m / min, the specific water amount of secondary cooling: 0.8 liter / kg to 1.4 liter / kg, the inner surface This is because no flaws or surface flaws were generated, and good pipe making was possible.

  The present invention is not limited to the above example, and it goes without saying that the embodiments may be changed as appropriate within the scope of the technical idea described in the claims.

Claims (1)

A continuous casting machine having at least a horizontal portion of a round slab having a diameter of 300 mm or more from a Cr-containing alloy steel containing C: 0.15% by mass or less and Cr: 1.0% by mass to 3.0% by mass Using a continuous casting method,
The casting speed is set to 0.45 m / min to 0.55 m / min, and the specific water amount of the secondary cooling is set to 0.8 liter / kg to 1.4 liter / kg to complete the solidification by the entry side of the horizontal portion at the latest. A continuous casting method for a Cr-containing alloy steel round cast slab.

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