JPH0569603B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> This invention efficiently and economically produces high-quality seamless duplex stainless steel pipes with good inner and outer surface properties by piercing and rolling round billets cast with a continuous casting machine. It is about the method. <Prior art and its challenges> Duplex stainless steel, which has a two-phase structure of ferrite and austenite phases, has attracted attention for its excellent corrosion resistance, and is in high demand as a material for various chemical industry plants and line pipes in corrosive environments. It's on. Duplex stainless steel is often used as pipe members, and seamless steel pipes, which have stable properties in the circumferential direction, are the mainstream. By the way, seamless steel pipes have conventionally been produced by making a round billet by blooming an ingot obtained by the steel ingot method or a slab cast by a bloom continuous casting machine, and then rolling this by a piercing mill. It was commonly manufactured. However, with recent advances in continuous casting technology, there has been a trend to use round billets cast with round billet continuous casting machines as they are in the production of seamless steel pipes, which has brought significant benefits in terms of production efficiency and production costs. I am getting . However, duplex stainless steel generally has poor hot workability, and if a round billet cast using a continuous round billet casting machine is punch-rolled as it is without blooming, flaws often occur on the inner and outer surfaces of the raw pipe. It has been considered extremely difficult to stably produce the desired seamless steel pipe products. In other words, when producing seamless duplex stainless steel pipes, as in the past, when "ingots obtained by the steel ingot method" are used as starting materials, blooming is performed, so the crystal grains on the surface of the steel billet are fine. However, when a round billet slab cast in a round billet continuous casting machine is used as is, crystal grains on the surface of the slab are Because of the roughness of the tube, many scratches occurred on the inner and outer surfaces of the tube after piercing and rolling, making it difficult to commercialize the tube. Of course, several methods aimed at improving the hot workability of duplex stainless steel have been proposed so far. For example, in Japanese Patent Publication No. 62-6616, the amount of S and O in duplex stainless steel is reduced, and the casting temperature is limited to produce round billet continuous cast slabs, and this is used as a raw material to produce sound slabs. A proposal to manufacture seamless steel pipes has been disclosed, and JP-A-59-
No. 4953 proposes a method for producing seamless duplex stainless steel pipes using round billet continuous cast slabs that are continuously cast while applying forced flow to the molten metal. However, all of these proposed methods for manufacturing seamless steel pipes are based on "hot extrusion processing," and as will be explained later, they require a "piercing mill," which involves complex processing deformation and harsh rolling. Even if these conditions were applied as they were to the ``method for manufacturing seamless steel pipes'', the above-mentioned problems still could not be solved. <Means for Solving the Problems> From the above-mentioned viewpoints, the present inventor has made it possible to use a round billet slab cast by a continuous round billet casting machine as a raw material without subjecting it to blooming treatment, and to improve manufacturing efficiency. In order to stably manufacture high-quality seamless duplex stainless steel pipes without internal and external surface flaws using a good piercing and rolling method, we conducted research from a comprehensive perspective, including the steel composition. Especially the O content of
Below the value of 0.0050%, and the S content is especially 0.0015
duplex stainless steel, which is made by adding Ca within a strictly regulated range, and then combining other ingredients to give the general characteristics required for duplex stainless steel. If steel is used as the raw material and the heating temperature, perforation ratio, and draft rate are appropriately selected during hole-rolling, round billet slabs obtained by a round billet continuous casting machine can be hole-rolled as they are. We were able to obtain the knowledge that it is possible to produce high-quality seamless duplex stainless steel pipes with high efficiency, with almost no defects found on the inner or outer surfaces of the pipes.'' The present invention has been made based on the above knowledge, and is based on the above findings.
is weight%), Si: 0.01-2.00%, Mn: 0.01-3.00%, P: 0.030% or less, S: 0.0015% or less, Cu: 0.01-2.00%, Cr: 20.00-35.00%, Ni: 3.00 ~15.00%, Mo: 0.5~8.00%, sol.Al: 0.001~0.20%, Ca: 0.0015~0.0070%, N: 0.03~0.35%, O: 0.0050% or less, or further W: 0.01~ A duplex stainless steel containing 1.00% Fe with the remainder being substantially Fe is melted and made into a round billet using a round billet continuous casting machine, which is then heated to 1200-1310℃ and then punch-rolled. By rolling the pipe under the conditions of perforation ratio: 1.40 or less and draft ratio: 5.0 or less to produce seamless steel pipes, we can efficiently and stably manufacture high-quality duplex stainless steel seamless steel pipes with good internal and external surface properties. It is characterized by the following points: Here, the reason for numerically limiting the composition and tube manufacturing conditions of the duplex stainless steel as the raw material in the present invention as described above will be explained in detail. (A) Composition of Duplex Stainless Steel (a) C Under current steelmaking technology, C is an element that is unavoidably entrained in steel. Although this C is effective in ensuring the strength of steel, it is undesirable to contain more than 0.08% in terms of corrosion resistance, so the C content is
It was set at 0.08% or less. (b) Si Si is a desirable element because it has a deoxidizing effect on steel, but if its content is less than 0.01%, the deoxidizing effect is low, and current steelmaking technology keeps the Si content below 0.01%. On the other hand, if the Si content exceeds 2.00, the strength of the steel becomes too high, leading to a decrease in workability including hot working, and is also unfavorable in terms of toughness. Therefore, the Si content is 0.01~
It was set at 2.00%. (c) Mn Mn has a favorable effect of improving the strength and toughness of steel, but its content should be reduced to 0.01%.
On the other hand, if the content exceeds 3.00%, the effect of the above action is saturated, and since Mn is an austenite stabilizing element, it is extremely difficult to adjust the content to less than 3.00%. As a result, the original properties of duplex stainless steel are no longer utilized. Therefore, the Mn content was determined to be 0.10 to 3.00. (d) P P is an impurity element that inevitably accompanies steel, and from the viewpoint of toughness and workability, it is preferable to have a small amount, but from the viewpoint of economic efficiency, the upper limit of the P content is set at 0.030%, which is acceptable. Established. (e) SS S is an important element that greatly affects the hot workability of duplex stainless steel, and the lower the content, the better. Further, from the viewpoint of toughness, it is an element that should be reduced as much as possible. In other words, S is an element that should be reduced as much as possible because it precipitates at austenite grain boundaries as sulfides such as NiS and MnS, becomes a starting point for cracks during rolling in a piercing mill, and causes defects on internal and external surfaces. However, by reducing the S content to 0.0015% or less, the above disadvantages can be suppressed to an acceptable level, so the S content was limited to 0.0015% or less. There are currently various methods for reducing the S content, and methods such as an AOD furnace and an RH vacuum degassing tank may be used. (f) Cu Cu has the effect of improving corrosion resistance against non-oxidizing acids, but if it is contained in excess of 2.00% it will have a negative effect on hot workability. Since up to about 0.01% of Cu is usually included in steel even without active addition, the active addition amount of Cu was determined to be 0.01 to 2.00%. (g) Cr Cr is a ferrite stabilizing element and contributes to the formation of the ferrite phase in duplex stainless steel, so it is added from the viewpoint of corrosion resistance and strength, but if its content is less than 20.0%, it will not reach the desired level. On the other hand, it is not possible to ensure the effectiveness of
Even if the content exceeds 35.00%, the effect obtained will be saturated, and moreover, it will be necessary to add a large amount of expensive Ni, which is necessary for austenite phase formation. Therefore, Cr content is 20.00
~35.00%. (h) Ni Ni is an austenite stabilizing element and contributes to the formation of the austenite phase in duplex stainless steel, so it is added from the viewpoint of corrosion resistance and strength, but if its content is less than 3.00%, the desired On the other hand, even if the content exceeds 15.00%, the effect obtained will be saturated, and Ni is an expensive element, so the Ni content is 3.00 to 15.00%.
limited to. (i) Mo Mo has the effect of suppressing local corrosion such as pitting corrosion, but if the content is less than 0.5%, the desired effect of the above effect cannot be obtained, whereas if the content exceeds 8.00%, the desired effect cannot be obtained. The Mo content was set at 0.5 to 8.00% because even if the Mo content was increased, the effect would reach saturation, which would not only be economically disadvantageous but also have a negative effect on hot workability. (j) sol.Al Al is an element added as a deoxidizing agent for steel, but if the content as sol.Al is less than 0.001%, the deoxidizing effect is small, which is an important requirement of the present invention. Oxygen in the steel is insufficiently reduced.
On the other hand, when the sol.Al content exceeds 0.20%
The number of Al ₂ O ₃ inclusions increases, which is unfavorable in terms of toughness. Therefore, the content of sol.Al is 0.001~0.20
%. (k) Ca Ca is a steel component that plays an important role in the present invention. That is, Ca is O and S in steel.
Although it is an element that combines with and forms inclusions,
Under well-regulated amounts of O and S, O and S are fixed as Ca-O-S inclusions and rendered harmless, thereby suppressing the occurrence of defects on the inner and outer surfaces during rolling of seamless steel pipes. Demonstrate. If the Ca content is less than 0.0015%, the above effect will not be sufficient, while if it is more than 0.0070%, Ca-O-S inclusions will increase and the cleanliness of the steel will deteriorate. From, Ca
was set at 0.0015-0.0070%. (l) N N has the effect of improving the corrosion resistance of steel, but if its content is less than 0.03%, the effect of improving corrosion resistance is small; on the other hand, if it is contained more than 0.35%, blowholes will occur due to solubility. Therefore, the N content is 0.03 ~
It was set at 0.35%. By the way, N is generally considered to be a cause of internal and external surface flaws during rolling of seamless steel pipes, but in "steel types that do not contain nitride-forming elements such as B and Ti," which is the subject of this invention, N is a cause of austenite grain boundaries. Since nitrides are rarely precipitated in the steel, it is not considered to be a cause of cracking. (m) O O is an impurity element that is inevitably entrained in steel during the steelmaking process, but it is not possible because it significantly impairs hot workability and also deteriorates the cleanliness of steel as oxide inclusions. It is necessary to reduce it as much as possible. However, when the O content decreases to 0.0050% or less, the defects that occur due to poor hot workability during seamless steel pipe production, especially the defects that occur on the inner surface of the steel pipe, decrease dramatically and become almost unnoticeable. Therefore, in consideration of economic efficiency, this value was set as the upper limit of the O content, and the O content was set at 0.0050% or less. By the way, when making seamless steel pipes using a piercing rolling mill, extremely complicated processing deformation occurs in the material during the drilling process, so generally used evaluation criteria such as "reduction of area and elongation value due to high temperature tension" are not suitable for manufacturing. Difficult to assess gender.
Therefore, a method of evaluating manufacturability (hot deformability) by conducting a torsion test at high temperature and using the number of twists until breakage as the "twist value" has recently been widely used. Figure 1 summarizes the results of evaluating the hot deformability of duplex stainless steel over a wide range of O contents using this method, and shows that the torsion value of the material decreases as the O content decreases. It can be seen that there is a tendency to improve with Comparing this with the results of an investigation into the occurrence of defects on the inner surface of steel pipes during seamless steel pipe manufacturing using a piercing rolling mill,
As is clear from Figure 1, there is a good correspondence between the torsion value of the material and the occurrence of internal defects on the steel pipe, and when the O content is 0.0050% or less, the torsion value shows a value of 30 or more. It was found that the occurrence of internal defects was no longer observed. The above-mentioned "high temperature torsion test" was conducted on the second-phase stainless steel from duplex stainless steel with the chemical composition shown below.
Take a test piece with the shape and dimensions shown in the figure, heat it to 1250℃ for 15 minutes, and then
The experiment was conducted under the conditions of determining the twisting value until it twists at 300 rpm and breaks. Chemical composition of duplex stainless steel C: 0.012-0.020%, Si: 0.32-0.49%, Mn: 1.45-1.60%, P: 0.015-0.022%, S: 0.0006-0.0014%, Cu: 0.04-0.10%, Cr :22.12~26.10%, Ni: 6.01~6.92%, Mo: 3.18~3.96%, sol.Al: 0.015~0.028%, Ca: 0.0018~0.0045%, N: 0.1351~0.1984%, O: 0.0026~0.0058%, W: 0.01-0.43%, Fe and other unavoidable impurities: remainder. There are currently various methods for reducing O in steel, including the use of an AOD furnace, RH
Vacuum degassing in a tank, sealing with Ar gas or N gas during casting, etc. may be used. (n) W W is a ferrite-forming element and has the effect of increasing the strength of steel through solid solution hardening, etc., so it is an ingredient that is added as necessary when higher strength is desired. be. Also,
On the other hand, since it is an element that improves pitting corrosion resistance at high temperatures, it is also a component added when the usage environment is high temperature and corrosion is severe. However, if the W content is less than 0.01%, the above-mentioned effects of improving strength and pitting corrosion resistance are not sufficient. On the other hand, when W is added, the deformation resistance at high temperatures increases, and a structure called a "sigma phase" that extremely deteriorates ductility and toughness tends to precipitate, making the material extremely difficult to handle in actual production. It is also disadvantageous in terms of economy. Therefore, the amount of W added should be kept to the minimum necessary. In most environments where seamless steel pipes are actually used as line pipes and oil country tubular goods, the temperature is below 100°C.
In this temperature range, sufficient pitting corrosion resistance can be ensured even with a W content of about 1.00% or less. Therefore, the W content was determined to be 0.01 to 1.00% in consideration of the effect of improving strength and pitting corrosion resistance, economic efficiency, and ease of handling in actual production. (B) Pipe-making conditions After melting steel with the above components, it can be cast into a round billet using a continuous round billet casting machine. If the casting speed is too fast, cracks may occur, so it is better not to make the casting speed too fast. After casting into a round billet, pipe production may be performed without blooming, but if the round billet is treated by cutting etc., the number of defects that occur on the inner and outer surfaces during rolling of seamless steel pipes will be reduced. However, if you want to skip the process, you can leave it without any maintenance and use the so-called "black leather pipe".
Needless to say, it is okay to do so. The reason why the heating temperature during piercing and rolling with a piercing rolling machine was limited to 1200 to 1310℃ is that the heating temperature is
If the temperature exceeds 1310℃, the austenite grains will become coarser, which will cause scratches to occur on the inner and outer surfaces of the tube during piercing rolling.On the other hand, if the temperature is lower than 1200℃, the rolling temperature will become too low. This is because the workability of the duplex stainless steel deteriorates, which also causes flaws to occur on the inner and outer surfaces of the raw tube. In piercing rolling during seamless steel pipe manufacturing, the piercing is normally performed at a piercing ratio (ratio of the length of the raw pipe after piercing to the length of the round billet) of 1.5 to 5.0, but in the present invention, this piercing ratio is particularly set to 1.40 or less. The reason for this is as follows. In other words, duplex stainless steel is originally a material with poor hot workability, and when such material is subjected to severe rolling with a perforation ratio exceeding 1.40, it is necessary to reduce O and S as much as possible to improve deformability. Even if the material is designed to be improved, it will be deformed beyond its deformability.
In addition, the material is likely to break at the tip of the plug of the punching machine, and when the material breaks, the broken portion is oxidized and is not crimped during subsequent rolling, resulting in internal flaws. Moreover, since the present invention uses a continuously cast round billet, porosity may remain in the shaft center of the round billet, although it may be slight. If porosity remains on the shaft core in this way, if the material breaks at the tip of the plug, the porosity itself will be oxidized and there is a high possibility that it will develop into a large internal surface flaw. However, if the perforation ratio is particularly limited to 1.40 or less, these inconveniences can be prevented, and concerns about the occurrence of internal flaws can be drastically reduced. Also, the formula draft rate = (material diameter) - (gorge diameter) / (material diameter) x
Shown in 100 [%]. The "draft ratio during piercing rolling" is normally set at a value of over 5.0% to around 8.0% to form holes. When drilling a round billet with residual porosities in the shaft core, as in the case of the drilling ratio, if severe rolling with a draft ratio exceeding 5.0% is performed, cracks may occur in the shaft core of the round billet during drilling, or the tip of the plug may be damaged. If a break occurs, the inside of the material will be oxidized and will not be crimped during subsequent rolling, resulting in internal flaws. Furthermore, since duplex stainless steel is a material with high deformation resistance in hot conditions, if the draft rate is increased, the material may not penetrate into the perforation roll or the head may become clogged, which may result in impossibility of perforation. Therefore, especially when drilling uncast duplex stainless steel round billets, it is necessary to perform smooth drilling without causing cracks in the shaft core of the round billet or breakage of the material at the tip of the plug. Keeping the draft rate below 5.0% is an essential condition. Note that the "gorge diameter" in the above formula is the interval between the maximum diameter parts of the perforation rolls, and is also called "gorge interval." On the other hand, the drilling temperature is not particularly limited, but is preferably in the range of 1100 to 1150°C. This is because in order to achieve a drilling temperature exceeding 1150°C, the heating temperature must be increased, which causes the austenite grains to become coarser and cause flaws to occur on the inner and outer surfaces of the raw tube. If it is less than this, the deformability of the round billet will be small and flaws will occur on the inner and outer surfaces of the raw tube. After the rolling by the piercing rolling mill is completed under the above conditions, for example, if a Mannesmann piercing rolling mill is used as the piercing rolling mill, pipe manufacturing is performed using a mandrel mill, plug mill, reeler, etc., and seamless steel pipes are produced. It is considered a product. Note that the applicable piercing rolling mill is not limited to the Mannesmann piercing rolling mill, but it goes without saying that it may be a piercing rolling mill such as an Atssel mill. That is, the major characteristics of the pipe manufacturing conditions of the present invention are the use of a duplex stainless steel material with a specific composition that has been devised, and the temperature at which the deformability of the duplex stainless steel is as large as possible, as well as the permissible temperature. The objective is to reduce defects on the inner and outer surfaces by rolling under the strictest possible rolling conditions. Next, the present invention will be specifically explained with reference to Examples. <Example> Example 1 First, duplex stainless steels A to D having the compositions shown in Table 1 were melted in an AOD furnace and subjected to RH vacuum chamber treatment.

【table】

【table】

【table】

[Table] After that, it was cast into a round billet continuous casting machine to obtain a round billet (slab). On the other hand, for comparison, as a conventional example, duplex stainless steel E shown in Table 1 is melted in a converter furnace, treated in an RH vacuum chamber, and then cast into a square bloom continuous casting machine to form a square bloom slab. It was rolled into a round billet. Next, the obtained round billets A, B, and E were punch-rolled using a Mannesmann-type piercing rolling mill (first piercer, second piercer) (heating temperature: 1305°C, piercing temperature: 1135°C, piercing ratio: 1.30, Draft rate:
4.5) After finishing to the dimensions shown in Table 2 using a plug mill, reeler and sizer, the obtained seamless steel pipes were inspected for blemishes on the inner and outer surfaces. These test results are also shown in Table 2. The test results shown in Table 2 show that, according to the present invention, even if round billet continuous casting slabs are directly pierced and rolled without being subjected to bloom rolling, the inner and outer surface properties of square blooms are different from those of square blooms that are pierced after blooming. It can be seen that a seamless steel pipe similar to that obtained by the conventional rolling method can be obtained. Furthermore, in order to confirm the influence of pipe manufacturing conditions on the quality of seamless steel pipes, the round billets (slabs) of steel types C and D were perforated and rolled using a Mannesmann type perforation mill under different conditions. We investigated the occurrence of defects on the inner and outer surfaces of seamless steel pipes finished using a reeler and sizer. Table 3 shows the tube manufacturing conditions and the occurrence of flaws on the inner and outer surfaces at this time. As can be seen from Table 3, the pipes manufactured according to the conditions specified in the present invention have good internal and external surface properties, but the perforation ratio is 1.40 and the draft rate is 5.0%. In the comparative example above, it is clear that the occurrence of flaws on the inner and outer surfaces of the steel pipe, especially on the outer surface, is noticeable and causes product defects. Furthermore, from the results shown in Table 3, it is possible to use as-cast round billets by selecting pipe-making conditions and taking care such as pickling after pipe-making, even for black-skinned pipes without external billet milling. It can also be confirmed that piercing-rolling pipe production is possible. Example 2 Duplex stainless steels F to I having the compositions shown in Table 4 were melted in a converter-AOD furnace and then cast into a round billet continuous casting machine to form round billets (slabs) with a diameter of 206 mm. I got it. Next, each round billet was heated to 1290°C and then perforated and rolled using a Mannesmann perforation rolling machine (dimensions after perforation: 156 mmφ x 12 mm ^t , perforation ratio: 1.30, number of perforations:
1), the occurrence of flaws on the outer surface of seamless steel pipes finished using a plug mill, reeler, and sizer was investigated. The results are shown in Table 5. The results shown in Table 5 show that if duplex stainless steel with the composition specified by the present invention is used as a raw material and pipe-manufactured under the conditions specified by the present invention, seamless steel pipes of excellent quality can be obtained. On the other hand, it can be seen that if the chemical composition of the raw material steel does not satisfy the conditions specified in the present invention, a high-quality product cannot be obtained from continuously cast round billets even under the same pipe manufacturing conditions.

【table】

【table】

[Table] Indicates that
<Summary of Effects> As explained above, according to the present invention, a high-quality seamless duplex stainless steel pipe with excellent inner and outer surface properties can be produced with high efficiency by piercing and rolling round billets obtained by round billet continuous casting. This brings about extremely useful effects industrially, such as making stable production possible.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between oxygen content, twist value, and inner surface flaws of steel pipes. FIG. 2 is an explanation of the shape and dimensions of the high temperature torsion test piece.

Claims (1)

[Claims] 1. C: 0.08% or less, Si: 0.01-2.00%, Mn: 0.01-3.00%, P: 0.030% or less, S: 0.0015% or less, Cu: 0.01-2.00%, Cr: 20.00-35.00%, Ni: 3.00-15.00%, Mo: 0.5-8.00%, sol.Al: 0.001-0.20%, Ca: 0.0015-0.0070%, N: 0.03-0.35%, O: 0.0050% or less, Fe and unavoidable impurities: After melting the remaining duplex stainless steel and making it into a round billet using a continuous round billet casting machine,
A seamless duplex stainless steel pipe, which is heated to 1200 to 1310°C and then rolled in a piercing rolling machine under the conditions of a perforation ratio of 1.40 or less and a draft ratio of 5.0% or less to form a seamless steel pipe. manufacturing method. 2 In weight percentage C: 0.08% or less, Si: 0.01-2.00%, Mn: 0.01-3.00%, P: 0.030% or less, S: 0.0015% or less, Cu: 0.01-2.00%, Cr: 20.00-35.00% , Ni: 3.00-15.00%, Mo: 0.5-8.00%, sol.Al: 0.001-0.20%, Ca: 0.0015-0.0070%, N: 0.03-0.35%, O: 0.0050% or less, W: 0.01-1.00% , Fe and unavoidable impurities: The remaining duplex stainless steel is melted and made into a round billet using a continuous round billet casting machine.
A seamless duplex stainless steel pipe, which is heated to 1200 to 1310°C and then rolled in a piercing rolling machine under the conditions of a perforation ratio of 1.40 or less and a draft ratio of 5.0 or less to obtain a seamless steel pipe. Production method.