JPH04305350A - Manufacture of billet of stainless steel for seamless steel tube - Google Patents

Abstract

PURPOSE:To prevent the inner flaw from generating at the time of making tube from billet. CONSTITUTION:When cast billet for high chrome stainless steel tube stock is continuously cast and the obtained cast billet is hot rolled and the billet is manufactured, the central part of the cast billet is made to the negative segregation by executing with roll reduction an unsolidified thickness 50-100% at the position that the solid phase rate shows 0.5-0.9 at the preceding stage before completing the solidification of the inside of case billet in the continuous casting of the cast billet.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention deals with materials such as 13Cr stainless steel, which significantly reduce workability when forming seamless steel pipes by hot drilling due to δ ferrite that occurs in the segregated part at the center of continuously cast slabs. The present invention relates to a method for producing stainless steel billets for seamless steel pipes, which can improve the hot workability of stainless steel billets.

0002

BACKGROUND OF THE INVENTION In high chromium steel or high chromium alloy steel such as martensitic stainless steel, an α phase coexisting with a γ phase is generated in a high temperature range (hereinafter referred to as δ ferrite). When two phases coexist in this way, the hot workability of both phases differs, so when drilling with a mandrel mill, the plastic deformation of the material cannot follow the intense processing, resulting in frequent internal defects of the pipe. I will do it.

Segregation formed in a continuously cast slab varies depending on the cross-sectional shape of the slab 1. For example, as shown in FIG. 3(a), when the slab 10 has a square cross section, a central segregation point 11 is formed. It is known that a center segregation band 12 is formed in the case of a rectangular cross section as shown in FIG. 3(b). In view of the fact that the cross-sectional shapes of slabs are different, Japanese Patent Application Laid-Open No. 1996-
As disclosed in Japanese Patent Application No. 293909, the mold aspect ratio (=long side length L1/short side length L2), the center roughness occurrence index of the slab 10, and the wrinkling flaw occurrence index during rolling are shown in FIG. From the relationship, the appropriate range of mold aspect ratio is 1.6 to 3.0.
It has been proposed to make the diameter relatively large in order to reduce the degree of segregation in the center of the slab and to prevent the occurrence of center dents. When rolling the slab into a round billet, a method is adopted in which long-term segregation diffusion heating is performed, or two-heat rolling is performed in which the heating-rolling-cooling cycle is repeated twice.

0004

SUMMARY OF THE INVENTION According to the above-mentioned conventional technology, the manufacturing process is complicated and costs are high. In addition, when slabs with a large aspect ratio are used to reduce segregation, the fishtail at the leading and trailing ends becomes large during billet rolling, increasing crop loss, resulting in a lower yield.

By the way, in high chromium alloy steel, if the heating temperature exceeds 1250 to 1260°C, δ ferrite crystallizes and causes flaws when drilling with a mandrel mill, so the maximum heating temperature falls within this range. However, if the C content is 0.2%, 12
At temperatures below 50°C, there is no crystallization of δ ferrite on the phase diagram. However, in the center of actual slabs, there is a region where the constituent elements contained are concentrated and segregated, and elements such as P that promote crystallization of ferrite are particularly concentrated, so it is very difficult to control the heating temperature mentioned above. Even if the process was carried out strictly and slabs with a large aspect ratio were used, it was not possible to completely prevent the formation of δ ferrite. In order to reduce the center segregation of slabs, in addition to the above management, we perform segregation diffusion annealing,
As mentioned above, in the process of rolling a slab into a round billet, so-called two-heat rolling, in which heating → rolling → cooling → heating → rolling → cooling, has been performed to diffuse segregation and prevent the formation of δ ferrite. .

On the other hand, during casting, as shown in FIG. 5, in order to reduce the center segregation rate and reduce non-metallic inclusions, it was necessary to lower the degree of superheating of the molten steel in the tundish and perform low-temperature casting. However, the amount of nonmetallic inclusions tends to increase by performing low-temperature casting. For this reason, conventionally, continuous casting has been carried out with the degree of superheating of molten steel set at 15 to 30° C., which is a temperature range that satisfies the tolerance range for both center segregation and nonmetallic inclusions.

[0007] The present invention guarantees the cleanliness of the steel by raising the casting temperature, uses a slab with a relatively small aspect ratio of 1.0 or more, which has a good rolling yield, and does not require conventional one-heat rolling. It is an object of the present invention to provide a method for producing a billet for stainless steel for seamless steel pipes that causes fewer defects on the inner surface of the pipe.

[0008]

[Means for Solving the Problems] To achieve the above object, the present invention provides a seamless steel pipe material of high chromium stainless steel or martensitic stainless steel containing 8 to 18% Cr and 2.5% or less Ni. When producing a billet by continuously casting a slab and hot rolling the obtained slab, it is a stage before the solidification of the inside of the slab in the continuous casting of the slab is completed, and the center of the slab is The solid phase ratio is 0.5
50-100% of the unsolidified thickness at a position showing ~0.9
The center of the slab is rolled down to have negative segregation, and this slab is heated in a heating furnace, then hot rolled into a billet, and the resulting billet is perforated to prevent the formation of δ ferrite. This is a method for producing a billet for stainless steel for seamless steel pipes, which is characterized in that it is possible to suppress and prevent the occurrence of internal flaws in pipes.

In the present invention, the length ratio (aspect ratio) between the long side and the short side of the slab is in the range of 1.0 to 3.0, and the superheating temperature of the molten steel in the tundish during continuous casting is set to be within the range of 1.0 to 3.0. Preferably, the temperature is in the range of 20 to 50°C, and furthermore, during billet rolling, the surface of the slab is heated to 900°C in the heating furnace.
It is more preferable to hold the temperature at the higher temperature for 6 hours or less.

0010

[Operation] The reason for setting the above conditions will be explained. (1) Position and reduction conditions of the unsolidified area of the slab When the solid fraction of the slab exceeds 0.9, it is essentially the same as if it had all solidified, and the effect of reducing segregation cannot be obtained.
It shall be 0.9 or less. In addition, if the solid phase ratio is too small and the liquid phase ratio is high, a large amount of liquid phase will remain under the reduction that does not cause operational instability, and the molten steel will thicken even after reduction, reducing the segregation reduction effect. Therefore, set the lower limit to 0.
5. Further, the reason why 50% or more of the unsolidified thickness is reduced is because if the reduction is less than this, a liquid phase will remain even after reduction, and the concentration of the component elements will proceed. On the other hand, if the reduction exceeds 100%, internal cracks will occur between the newly solidified columnar crystals, so the upper limit of the reduction is set at 100%.

[0011] Here, the above-mentioned solid phase ratio at the center of the slab specifically refers to the position at which the temperature at the center of the slab lies between the liquidus temperature and the solidus temperature determined by the steel type. For example, a solid fraction of 1.0 indicates that the temperature is the solidus temperature, and 0.5 indicates that the temperature is between the liquidus temperature and the solidus temperature. Note that a solid phase ratio of 100% means that there is no liquid phase at this position and the entire solid phase is present. Normally, the solidification interface does not change in a stepwise manner from the solid phase to the liquid phase, but there is a coexistence region of solid and liquid phases, and the solid phase is usually 100% at the solidus temperature, and the liquidus temperature is 100%. At the position, the liquid phase becomes 100%.

[0012] Further, for rolling down the slab, it is possible to use, for example, an intermittent forging device proposed by the present applicant and disclosed in Japanese Patent Application Laid-Open No. 127148/1983. That is, as shown in FIG. 1, the reduction lever 3 is moved around the fulcrum 9 using the hydraulic cylinder 4, and the slab 10 is reduced using the mold 5 attached to this lever. During slab reduction, this forging device is moved downward at the same speed as the slab, and is returned to its original position using the hydraulic cylinder 8 for return after surface pressure. Since the slab is intermittently compressed by the forging die, the solidified shell 2 is crushed and the pool of unsolidified molten steel 1 is blocked. Therefore, center segregation due to shrinkage of this pool does not occur. 7 indicates a pinch roll.

(2) Conditions for the aspect ratio of the slab If the aspect ratio is less than 1.0, the vertical and horizontal directions are essentially reversed, so the lower limit is set at 1.0. The upper limit is set at 3.0 because if the ratio of the long sides becomes too large, wrinkles are likely to appear on the long sides when rolled into a round bar, resulting in a large number of wrinkles or folding scratches on the surface of the round bar. It is better to keep this aspect ratio as close to 1.0 as possible so that the resinous crystals from the outer peripheral surface of the slab grow symmetrically within the cross section, thereby maintaining uniformity in later workability and material quality.

(3) Molten steel superheating condition in the tundish If the temperature is less than 20°C, inclusions will be difficult to float and separate in the molten steel in the tundish, making it impossible to maintain the cleanliness of the steel material, so the superheating temperature should be 20°C or higher. The upper limit is set at 50°C because if the temperature is too high, a solidified shell will not be formed properly in the mold, increasing the occurrence of operational troubles such as breakouts.

(4) Billet Heating Conditions If the material is kept in a high temperature range for a long time during heating, decarburization and dechromium will occur on the surface of the material due to oxygen contained in the atmosphere in the furnace. This reaction does not occur much at low temperatures, but becomes active at high temperatures above 900°C. In addition, the holding time at this high temperature range is 5
If the time is exceeded, the depth of surface decarburization and the depth of the de-Cr phase will increase, and if it exceeds 6 hours, it may reach a range that is unacceptable for normal pipe products, so the upper limit of the high temperature holding time should be set. It will be 6 hours. In addition, in order to reduce heating fuel consumption as much as possible and reduce costs, it is preferable to use one-heat rolling.

[0016]

[Example] Table 1 shows representative chemical compositions of molten steel in a ladle after vacuum degassing treatment of molten steel blown in a top-bottom blowing converter in which inert gas is blown from the bottom using an RH degassing device.

[0017]

[Table 1]

[0018] Molten steel having the chemical composition shown in Table 1 was repeatedly subjected to intermittent forging using a continuous bloom caster to form a slab with a size of 400 x 560 mm (aspect ratio 1.47) using the above-mentioned intermittent forging device shown in Fig. 1. . In this case, the pressing position of the slab was set at a position where the solid phase ratio at the center of the slab was approximately 0.7, and about 80% of the unsolidified thickness was pressed down to make the center of the slab negative segregation. The obtained slab with a center segregation rate of 0.75 was heated for one heat in a continuous heating furnace, and then hot rolled into a billet of 175 mmφ. At this time, the continuous heating furnace maintained the slab at a high temperature of 900°C or higher for 90 to 130 minutes, and the slab temperature at the time of removal from the furnace was 1180 to 1200°C. The rolling yield when billet rolling was performed under heating in this manner was 0.7% higher than when billet rolling was performed from a conventional slab with an aspect ratio of 2.8.

When this billet was made into a pipe by perforating it with the mandrel of a mandrel mill, the pipe inner surface flaw occurrence index of the present invention was compared with the conventional billet 1 heat rolling and billet 2 heat rolling.
Fig. 2 shows a comparison with a conventional heat-rolled example. As shown in FIG. 2, according to the present invention, it was possible to reduce the pipe inner surface flaw occurrence index to 1/4 compared to the conventional example, especially the 2-heat method, and to 1/22 compared to the 1-heat method.

[0020]

[Effects of the Invention] As explained above, according to the present invention,
Creates a slab shape with good yield and eliminates center segregation,
In addition, billet rolling can be performed with the minimum amount of heating required. This improves the yield of billet rolling and reduces rolling costs, and since δ ferrite, which has poor hot workability, does not form in the center, the occurrence of internal defects when drilling with a mandrel mill or Mannesmann plug mill is greatly reduced. can be reduced to

[Brief explanation of drawings]

FIG. 1 is a side view showing an intermittent forging device used in an example of the present invention.

FIG. 2 is a bar graph showing a comparison of the pipe inner surface flaw occurrence index between the method of the present invention and conventional one-heat rolling and two-heat rolling.

FIG. 3 is an explanatory diagram showing the state of center segregation formed in a slab with a square cross section and a slab with a rectangular cross section according to a conventional example.

FIG. 4 is a line graph showing the relationship between the flatness ratio of a slab, a central roughness occurrence index, and a wrinkling occurrence index according to a conventional example.

FIG. 5 is a line graph showing the relationship between the degree of superheating of molten steel in a tundish, a nonmetallic inclusion index, and a center segregation index according to a conventional example.

[Explanation of symbols]

1 Unsolidified molten steel 2 Solidified shell 3 Press down lever 4 Hydraulic cylinder 5 Mold 6 Main body guide roll 7 Pinch roll 8 Hydraulic cylinder for return under surface pressure 9 Fulcrum 10 Slab 11 Central segregation point 12 Central segregation zone

Claims (3)

[Claims] [Claim 1] Cr 8-18%, Ni 2.5%
When continuous casting of high chromium stainless steel or martensitic stainless steel steel slabs containing the following is performed, and hot rolling of the obtained slabs to produce billets, continuous casting of the slabs is performed. This is the stage in casting before the solidification inside the slab is completed, and the solid phase ratio at the center of the slab is 0.
．． 50 to 100 of the unsolidified thickness at the position showing 5 to 0.9
% to make the center of the slab negative segregation, and after heating this slab in a heating furnace, hot rolling it into a billet,
A method for producing a stainless steel billet for seamless steel pipes, characterized in that generation of δ ferrite is suppressed during pipe forming by perforating the obtained billet, thereby preventing the occurrence of internal flaws in the pipe. [Claim 2] The length ratio (aspect ratio) between the long side and the short side of the slab is set in the range of 1.0 to 3.0, and the superheating temperature of the molten steel in the tundish during continuous casting is set in the range of 20 to 50.
2. The method for producing a stainless steel billet for seamless steel pipes according to claim 1, wherein the temperature is within the range of .degree. 3. The method for producing a stainless steel billet for seamless steel pipes according to claim 1, wherein during billet rolling, the time period during which the surface of the slab is maintained at a high temperature of 900° C. or higher in a heating furnace is 6 hours or less.