JPH0360904B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD This invention relates to martensitic stainless steel used for seamless steel pipes such as oil country tubular goods and line pipes. Conventional technology Martensitic stainless steels, represented by SUS410 steel and SUS420 steel, exhibit excellent corrosion resistance in corrosive environments containing _CO2 , and are therefore attracting attention as materials for oil country tubular goods, geothermal country tubular goods, line pipes, etc. Bathing. By the way, seamless stainless steel pipes are generally manufactured by inclined rolling methods such as plug mill method and mandrel mill method, or hot extrusion methods such as Eugene Segyurne method and Erhardt push bench method, but martensitic stainless steel Some types of steel, that is, steel types with a small amount of ferrite during hot working, have poor hot workability, so when producing seamless steel pipes using inclined rolling methods such as the plug mill method or mandrel mill method, piercing mills are used. When drilling with a piercer, defects may occur on the outer or inner surface or cracks may occur at the end of the tube. In other words, when manufacturing seamless martensitic stainless steel pipes using the plug mill method or mandrel mill method, if the amount of ferrite is large at the normal hot working temperature, specifically, the amount of ferrite at 1200℃ is amount is 40
%, it has traditionally been possible to manufacture pipes without defects during manufacturing, but
In the case of steel types with a low ferrite content of less than 40% at 1200°C, defects on the inner and outer surfaces and cracks at the pipe ends frequently occur during pipe manufacturing, making it difficult to apply in actual processes. Therefore, such steel types with a ferrite content of 40% or less at 1200°C have conventionally been manufactured by a hot extrusion method typified by the Eugene-Ségiurne process. Problems to be Solved by the Invention However, when applying the hot extrusion method to directly perforate a billet (direct perforation method), if the length of the billet is 5 to 7 times the diameter, the uneven thickness will become large. , it is difficult to manufacture long tubes. In order to solve this problem, a method has traditionally been adopted in which a guide hole is formed in the center of the billet by machining in advance, and then a long tube is manufactured using the so-called expansion method. has been done. However, even in this expansion method, the length of the billet is limited to about 15 times the diameter. Furthermore, the Eugene Sejourne method, a typical hot extrusion method, uses a glass lubricant.
Although it is necessary to peel off the glass lubricant after rolling, there is a drawback that this peeling process requires time and cost. As mentioned above, in the hot extrusion method represented by the Eugene Sejourne method, the billet length is restricted, so productivity cannot be increased beyond a certain level, and the yield may be low due to the use of short billets. Unavoidably, there is a problem in that it is disadvantageous in terms of cost. On the other hand, both the plug mill method and the mandrel mill method use a piercing mill that utilizes the Mannesmann effect, and these methods make long tubes more difficult to manufacture than hot extrusion methods such as the Eugene Sejourne method. It is known that martensitic stainless steel has various advantages such as high productivity and cost advantages, but as mentioned above, due to the problem of defects during pipe manufacturing, martensitic stainless steel The reality is that it has been difficult to apply this method to some steel types, that is, steel types with a ferrite content of 40% or less at 1200°C. This invention was made in view of the above circumstances, and it has been difficult to manufacture seamless steel pipes using the conventional plug mill method or mandrel mill method due to the problem of defects occurring during pipe production. The plug mill method and mandrel mill method can be practically applied even to martensitic stainless steel with a ferrite content of 40% or less at 1200℃, and this makes it possible to produce seamless pipes of this type of martensitic stainless steel with high productivity. The purpose is to enable manufacturing at low cost. Means for Solving the Problems The present inventors have discovered that the amount of ferrite at 1200°C is 40
As a result of repeated investigation and research into the causes of the above-mentioned defects in martensitic stainless steel of less than , Further research revealed that S was 0.003.
% or less, and by regulating P to 0.02% or less, seamless steel pipes can actually be manufactured by the plug mill method or the mandrel mill method without causing the above-mentioned defects. It was reached. Specifically, the martensitic stainless steel for seamless steel pipes of the first invention contains C0.025 to 0.28%,
Cr11.5~14%, Si0.16~1.0%, Mn0.2~1.0%,
Contains Al 0.04% or less, N 0.076% or less, and δ ₁₂₀₀ (%) = -195 x (%C) -185 x (%N) -5 x (%Mn) + 5 x (%Si) + 12 ×(%Cr) +50×(%Al)−120 The amount of ferrite at 1200℃ δ ₁₂₀₀ is 40
% or less, P content is 0.02% or less, and S content is 0.02% or less.
It is characterized by being regulated to 0.003% or less, with the remainder consisting of iron and unavoidable impurities. In addition, the martensitic stainless steel for seamless steel pipes of the second invention has C0.025 to 0.28% and Cr11.5 to 14%.
%, Si0.16~1.0%, Mn0.2~1.0%, Al0.04% or less, N0.076% or less, and Ni3.1%
Contains one or more selected from Mo2.2% or less and Cu0.7% or less, and δ ₁₂₀₀ (%) = -195 x (%C) -185 x (%N) - Calculated as 18×(%Ni)−6×(%Cu)−5×(%Mn) +5×(%Si)+12×(%Cr)+5×(%Mo) +50×(%Al)−120 Ferrite amount δ ₁₂₀₀ at 1200℃ is 40
% or less, P content is 0.02% or less, and S content is 0.02% or less.
It is characterized by being regulated to 0.003% or less, with the remainder consisting of iron and unavoidable impurities. Furthermore, the martensitic stainless steel for seamless steel pipes of the third invention has C0.025 to 0.28% and Cr11.5 to 14%.
%, Si0.16~1.0%, Mn0.2~1.0%, Al0.04% or less, N0.076% or less, and Nb0.05
% or less, V 0.2% or less, Ti 0.05% or less, and δ ₁₂₀₀ (%) = -195 x (% C) - 185 x (% N ) −5×(%Mn)+5×(%Si)+12×(%Cr) +16(%V)+6×(%Nb)+5×(%Ti)+5×(%Al)−120 1200 calculated as Ferrite amount δ ₁₂₀₀ at °C is 40
% or less, P content is 0.02% or less, and S content is 0.02% or less.
It is characterized by being regulated to 0.003% or less, with the remainder consisting of iron and unavoidable impurities. And the martensitic stainless steel for seamless steel pipes of the fourth invention has C0.025 to 0.28% and Cr11.5 to 14%.
%, Si0.16~1.0%, Mn0.2~1.0%, Al0.04% or less, N0.076% or less, and Ni3.1%
Below, one or more selected from Mo2.2% or less, Cu0.7% or less, Nb0.05% or less,
1 selected from V0.2% or less and Ti0.05% or less
species or two or more species, and δ ₁₂₀₀ (%) = −195×(%C) −185×(%N) −18(%Ni) −6×(%Cu) −5×(%Mn ) +5×(%Si)+12×(%Cr)+16×(%V) +5×(%Mo)+6×(%Nb)+5×(%Ti) +50×(%Al)−120 1200 calculated as Ferrite δ ₁₂₀₀ at ℃ 40%
In addition, the amount of P is regulated to 0.02% or less, the amount of S is regulated to 0.003% or less, and the remainder consists of iron and inevitable impurities. Furthermore, the martensitic stainless steels for seamless steel pipes of the fifth invention, sixth invention, seventh invention, and eighth invention each have the components specified in the first invention, second invention, third invention, and fourth invention, respectively. In addition, Ca is added in a range of 1x (%S) to 10x (%S) to further improve hot perforation properties. Function The martensitic stainless steel of the present invention will be explained in more detail below. First, the reason for limiting each component in the steel of this invention will be explained. C: 0.025% or more of C is required to ensure strength. On the other hand, if C exceeds 0.28%, corrosion resistance deteriorates. Therefore, C was limited to 0.025% or more and 0.28% or less. Cr: Cr is an element that significantly improves corrosion resistance in a _CO2 environment, and is 11.5% to prevent pitting and crevice corrosion.
The above addition is necessary. On the other hand, Cr itself is a ferrite-forming element, and if the amount added exceeds 14%, the amount of ferrite increases, and it is difficult to obtain the specified strength under heat treatment conditions (tempering temperature) that do not impair stress corrosion cracking resistance. It becomes difficult. Therefore
The range of Cr was 11.5 to 14%. Si: Si is an element necessary for steelmaking as a deoxidizing agent, reducing oxygen in steel that degrades hot workability.
If the Si content is less than 0.16%, the deoxidizing effect is insufficient and hot workability deteriorates, so the lower limit was set at 0.16%. Also, if a large amount of Si is added, the toughness will deteriorate, so the upper limit of Si was set at 1.0%. Mn: Mn is also an element necessary for steelmaking as a deoxidizing agent.
Reduces oxygen in steel, which deteriorates hot workability. If the Mb content is less than 0.2%, the deoxidizing effect is insufficient and hot workability deteriorates, so the lower limit is set at 0.2%.
And so. On the other hand, if a large amount of Mn is added, the toughness will deteriorate, so the upper limit was set at 1.0%. Al: Al is effective as a deoxidizing agent, but if added in large amounts it degrades cleanliness, so the upper limit was set at 0.04%. N: N is effective for improving strength through solid solution strengthening, but if added in a large amount, toughness deteriorates, so the upper limit was set at 0.076%. S: S is contained in steel as an unavoidable impurity, but it significantly deteriorates hot workability in the steel types targeted by this invention. The negative effect is particularly noticeable when making pipes using the plug mill method or mandrel mill method, or when making holes with a piercing mill. must be regulated to 0.003% or less. P: P is also contained in steel as an unavoidable impurity, but it significantly deteriorates hot workability at high temperatures of 1200° C. or higher. In particular, P causes scratches that occur on the inner surface when making holes with a piercing mill.
If P exceeds 0.02%, it becomes difficult to drill holes without causing scratches, so P must be regulated to 0.02% or less. In addition to the above components, the martensitic stainless steels for seamless steel pipes of the second, fourth, sixth, and eighth inventions further contain one or more of Ni, Mo, and Cu. In addition, in the martensitic stainless steel for seamless steel pipes of the third, fourth, seventh, and eighth inventions,
The martensitic stainless steel for seamless steel pipes of the fifth to eighth inventions contains one or more of Nb, V, and Ti, and Ca is added. The reasons for these additions and limitations are as follows. Ni, Mo, Cu: These are all elements that are effective in improving corrosion resistance, but if they are contained in excess, their effects will be saturated and they will actually impede hot workability, so the upper limit for Ni should be 3.1 %, Mo has an upper limit of 2.2%, Cu has an upper limit of 0.7
%. Nb, V, Ti: All of these prevent coarsening of austenite crystal grains and promote improvement of toughness by refining the crystal grains, but even if they are added in large quantities, the effect is saturated, and on the contrary, the toughness is reduced. In order to cause deterioration, the upper limit of Nb is set to 0.05.
%, V has an upper limit of 0.02%, and Ti has an upper limit of 0.05%. Ca: Ca is a strong sulfide element, and is effective in reducing the amount of solid solution S in steel and improving hot workability by forming Ca sulfides.
However, if the amount of Ca is less than the equivalent amount of S, the effect will be small, while if it exceeds 10 times the amount of S, the effect will be saturated and there is a risk of increasing surface defects due to these oxides or sulfides. Therefore, the amount of Ca added is 1×(%S) to 10×(%S)
limited to the range of Further, the steel targeted by this invention has a ferrite content of 40% or less at 1200°C. This is because for steels with a ferrite content of more than 40% at 1200°C, seamless steel pipes can be produced without any problems using the plug mill or mandrel mill method, as described above, without any particular defects. Here, the amount of ferrite δ ₁₂₀₀ (%) at 1200°C is determined by the following (1) depending on the type of component element contained in each invention of the present application.
or (2), or (3) or (4). In other words, in the case of the first invention and the fifth invention, the following formula (1) is used, and in the case of the second invention and the sixth invention, the following formula
The formula (2), the third invention, and the seventh invention are defined by the following formula (3), and the fourth invention and the eighth invention are defined by the following formula (4). δ ₁₂₀₀ (%) = −195×(%C) −185×(%N) −5×(%Mn)+5×(%Si)+12×(%Cr) +50×(%Al)−120 …(1 ) δ ₁₂₀₀ (%) = -195 x (%C) -185 (%N) -18 x (%Ni) -6 x (%Cu) -5 x (%Mn) + 5 x (%Si) + 12 x ( %Cr)+5(%Mo)+50×(%Al)−120…(2) δ ₁₂₀₀ (%)=−195×(%C)−185×(%N) −5×(%Mn)+5(% Si) +12×(%Cr) +16×(%V)+6×(%Nb)+5×(%Ti) +50×(%Al)−120…(3) δ ₁₂₀₀ (%)=−195×(%C ) −185×(%N) −5×(%Mi) −6×(%Cu) −5×(%Mn) +5×(%Si)+12×(%Cr)+16×(%V) +5×( %Mo) + 6 x (%Nb) + 5 x (%Ti) + 50 x (%Al) - 120...(4) Example Table 1 shows the chemical composition and defect occurrence status of the examples and comparative examples of this invention. Shown below. However, in Table 1, the defect occurrence situation is as follows: A billet with an outer diameter of 175 mm made of steel of each component was heated to 1230°C, and a billet with an outer diameter of 185 mm and a wall thickness of 185 mm was heated with a piercing mill (piercer).
The inner and outer surfaces of the holes drilled to a diameter of 19.76 mm were observed, and if defects such as scratches or cracks occurred, they were marked with an x mark, and if there were no defects, they were marked with an ○ mark.
Further, in Table 1, the amount of ferrite δ ₁₂₀₀ (%) at 1200° C. was determined by each of the above formulas, and when the value was negative, it was written as 0.

【table】

【table】

[Table] In Table 1, sample materials No. 35 to 46 are comparative examples.
In each case, the amount of P and/or the amount of S exceeds the upper limit of the scope of the present invention,
Defects occurred in all of these. Effects of the Invention As is clear from the above explanation, the martensitic stainless steel for seamless steel pipes of the present invention has a high ferrite content at 1200°C, which has traditionally been difficult to manufacture seamless steel pipes using the plug mill method or mandrel mill method. Although it is a steel type with a carbon content of 40% or less, it was possible to improve hot workability, especially hot perforation, by regulating the amounts of P and S, and therefore it is suitable for hot extrusion. Seamless steel pipes can be manufactured in the actual process using the plug mill method or mandrel mill method without any pipe manufacturing defects, and therefore, the plug mill method or mandrel mill method is applied to this type of steel in the actual process. As a result, it is possible to improve productivity and lower yield than in the past, thereby reducing costs. In particular, the martensitic stainless steels for seamless steel pipes of the fifth to eighth inventions have further improved hot perforability by regulating the amounts of P and S and at the same time adding Ca. Therefore, a healthier seamless steel pipe can be manufactured using a plug mill method or a mandrel mill method.

Claims (1)

[Claims] 1 C0.025 to 0.28% (weight%, same hereinafter),
Cr11.5~14%, Si0.16~1.0%, Mn0.2~1.0%,
Contains Al 0.04% or less, N 0.076% or less, and δ ₁₂₀₀ (%) = -195 x (%C) -185 x (%N) -5 x (%Mn) + 5 x (%Si) + 12 ×(%Cr) +50×(%Al)−120 The amount of ferrite at 1200℃ δ ₁₂₀₀ is 40
% or less, P content is 0.02% or less, and S content is 0.02% or less.
Martensitic stainless steel for seamless steel pipes, which is regulated to 0.003% or less, with the remainder consisting of iron and unavoidable impurities. 2 C0.025~0.28%, Cr11.5~14%, Si0.16~1.0
%, Mn0.2~1.0%, Al0.04% or less, N0.076% or less, Ni3.1% or less, Mo2.2%
The following contains one or more selected from Cu0.7% or less, and δ ₁₂₀₀ (%) = -195 x (%C) -185 x (%N) -18 x (%Ni ) −6×(%Cu)−5×(%Mn) +5×(%Si)+12×(%Cr)+5×(%Mo) +50×(%Al)−120 Ferrite at 1200℃ quantity δ ₁₂₀₀ is 40
% or less, P content is 0.02% or less, and S content is 0.02% or less.
Martensitic stainless steel for seamless steel pipes, which is regulated to 0.003% or less, with the remainder consisting of iron and unavoidable impurities. 3 C0.025~0.28%, Cr11.5~14%, Si0.16~1.0
%, Mn0.2~1.0%, Al0.04% or less, N0.076% or less, Nb0.05% or less, V0.2%
The following contains one or more selected from Ti0.05% or less, and δ ₁₂₀₀ (%) = -195 x (%C) -185 x (%N) -5 x (%Mn )+5(%Si)+12×(%Cr)+16×(%V)+6×(%Nb)+5×(%Ti)+50×(%Al)−120 Ferrite amount δ at 1200℃ calculated as ₁₂₀₀ is 40
% or less, P content is 0.02% or less, and S content is 0.02% or less.
Martensitic stainless steel for seamless steel pipes, which is regulated to 0.003% or less, with the remainder consisting of iron and unavoidable impurities. 4 C0.025~0.28%, Cr11.5~14%, Si0.16~1.0
%, Mn0.2~1.0%, Al0.04% or less, N0.076% or less, Ni3.1% or less, Mo2.2%
Below, one or more selected from Cu0.7% or less, Nb0.05%, V0.2% or less, Ti0.05
% or less, and δ ₁₂₀₀ (%) = -195 x (% C) - 185 x (% N) -18 (% Ni) - 6 x ( %Cu) −5×(%Mn) +5×(%Si)+12×(%Cr)+16×(%V) +5×(%Mo)+6×(%Nb)+5×(%Ti) +50×(% The amount of ferrite δ ₁₂₀₀ at 1200℃ calculated from Al) −120 is 40
% or less, P amount is 0.02% or less, S
A martensitic stainless steel for seamless steel pipes whose content is regulated to 0.003% or less, with the remainder consisting of iron and unavoidable impurities. 5 C0.025~0.28%, Cr11.5~14%, Si0.16~1.0
%, Mn 0.2 to 1.0%, Al 0.04% or less, N 0.076% or less, and δ ₁₂₀₀ (%) = −195×(%C) −185×(%N) −5×(% Ferrite amount δ at 1200℃ calculated as Mn) + 5 x (%Si) + 12 x (%Cr) + 50 x (%Al) - 120 ₁₂₀₀ is 40
% or less, P amount is 0.02% or less, S
The amount of Ca is regulated to 0.003% or less, and furthermore, Ca is regulated to 1× (%
A martensitic stainless steel for seamless steel pipes, characterized in that the content is in the range of S) to 10×(%S), with the remainder consisting of iron and inevitable impurities. 6 C0.025~0.28%, Cr11.5~14%, Si0.16~1.0
%, Mn0.2~1.0%, Al0.04% or less, N0.076% or less, Ni3.1% or less, Mo2.2%
The following contains one or more selected from Cu0.7% or less, and δ ₁₂₀₀ (%) = -195 x (%C) -185 x (%N) -18 x (%Ni )−6×(%Cu)−5×(%Mn) +5×(%Si)+12×(%Cr)+5×(%Mo)+50×(%Al)−120 at 1200℃ Ferrite amount δ ₁₂₀₀ is 40
% or less, P amount is 0.02% or less, S
The amount of Ca is regulated to 0.003% or less, and furthermore, Ca is regulated to 1× (%
A martensitic stainless steel for seamless steel pipes, characterized in that the content is in the range of S) to 10×(%S), with the remainder consisting of iron and inevitable impurities. 7 C0.025~0.28%, Cr11.5~14%, Si0.16~1.0
%, Mn0.2~1.0%, Al0.04% or less, N0.076% or less, Nb0.05% or less, V0.2%
The following contains one or more selected from Ti0.05% or less, and δ ₁₂₀₀ (%) = -195 x (%C) -185 x (%N) -5 x (%Mn ) ₊₅ 40
In addition, the amount of P is regulated to 0.02% or less, the amount of S is regulated to 0.003% or less, and Ca is regulated to 1 × (%
A martensitic stainless steel for seamless steel pipes, characterized in that the content is in the range of S) to 10×(%S), with the remainder consisting of iron and inevitable impurities. 8 C0.025~0.28%, Cr11.5~14%, Si0.16~1.0
%, Mn0.2~1.0%, Al0.04% or less, N0.076% or less, Ni3.1% or less, Mo2.2%
Below, one or more selected from Cu0.7% or less, Nb0.05% or less, V0.2% or less,
Contains one or more selected from Ti0.05% or less, and δ ₁₂₀₀ (%) = -195 x (%C) - 185 x (%N) -18 (%Ni) - 6×(%Cu)−5×(%Mn) +5×(%Si)+12×(%Cr)+16×(%V) +5×(%Mo)+6×(%Nb)+5×(%Ti) +50 The amount of ferrite δ ₁₂₀₀ at 1200℃ calculated as × (%Al)−120 is 40
% or less, P amount is 0.02% or less, S
The amount of Ca is regulated to 0.003% or less, and furthermore, Ca is regulated to 1× (%
A martensitic stainless steel for seamless steel pipes, characterized in that the content is in the range of S) to 10×(%S), with the remainder consisting of iron and inevitable impurities.