JP3456468B2 - Martensitic stainless steel seamless steel pipe with excellent machinability and hot workability - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a martensitic stainless steel seamless steel pipe having excellent machinability and hot workability.

[0002]

2. Description of the Related Art Martensitic stainless steel is AI
As represented by SI420 steel, it has been used as an oil country tubular good since around 1980 because it has excellent strength and CO ₂ corrosion resistance and is relatively inexpensive. In recent years, JP-B-59-15978 and JP-B-3-22 have corrosion resistance superior to that of AISI 420 steel because they can be applied to an oil well environment containing high temperature and a large amount of CO ₂ and H ₂ S in addition thereto.
Steel types such as low C-Ni-Mo added steel as seen in Japanese Patent No. 27 or the like, or low C-Cu-Ni-Mo added steel as seen in Japanese Patent Laid-Open No. 2-217444 (so-called Modified 13Cr steel). Steel type) is being developed.

Generally, as the amount of alloy increases, the corrosion resistance improves, but the workability deteriorates. The oil well pipe of the above-mentioned steel is usually manufactured into a seamless pipe by a Mannesmann rolling method. Mannesmann rolling has hitherto been known as the most severe working method among hot working methods. Since these steels contain a large amount of alloying elements such as Cr, Ni, Mo, and Cu, they are manufactured by the Mannesmann rolling method. When piped, rolling flaws sometimes occurred. Therefore, the present situation is that the content of impurities, such as S, which are harmful to hot workability is reduced as much as possible.

On the other hand, since the oil country tubular goods are usually joined by screw joints, it is customary to machine male threads on both ends of the steel pipe, but martensitic stainless steel has a machinability higher than that of ordinary steel. Since it is inferior, there is a problem such as a short life of the checker for screw cutting. As the martensitic stainless steel with improved machinability, S was added in a large amount (0.15% or more) to improve machinability.
US416 steel and SUS420F steel are JLS G43
Although the martensitic stainless steel containing a large amount of S as described above has extremely poor hot workability, it is difficult to manufacture a seamless pipe by the Mannesmann hot rolling method. there were.

As described above, according to the conventionally proposed technique, a martensitic stainless steel seamless steel pipe having excellent machinability and capable of being manufactured by the Mannesmann rolling method and having excellent hot workability is manufactured. It was difficult.

[0006]

DISCLOSURE OF THE INVENTION The present invention is intended to solve the above problems and is excellent in machinability and excellent in hot working capable of being manufactured by the Mannesmann rolling method. An object of the present invention is to provide a martensitic stainless steel seamless steel pipe having properties and a manufacturing method thereof.

[0007]

As a result of repeated studies on machinability and hot workability with respect to various materials having different components, the inventors of the present invention have found that in this steel type, S is an ordinary free-cutting stainless steel. The range of 0.01 to 0.04%, which is significantly smaller than that of steel, and the addition of 0.0005 to 0.02% of B to the range of the Mannesmann system without causing the problem of cracking during hot rolling. It was found that it is possible to manufacture seamless pipes by the hot rolling method, and the machinability can sufficiently satisfy the required level.

The present invention is constructed on the basis of such knowledge, and the gist thereof is as follows. (1) In mass%, C: 0.001 to 0.05, Si: 0.5 or less, Mn: 0.1 to 1.5, P: 0.03 or less, S: 0.01 to 0.04. , Cr: 10-14, Ni: 2-8, Mo: 0.5-3, Cu: more than 0.5-3, Al: 0.2 or less, N: 0.001-0.05, B: 0.0005 to 0.02, O: 0.005 or less, the balance being Fe and inevitable impurities, the martensitic stainless steel having excellent machinability and hot workability Steel pipe. (2) Containing the component described in (1) above, and further, mass%
Then, Ca: 0.001-0.01, Mg: 0.0005-0.01, REM: 0.001-0.05, Zr: 0.01-0.05, Ti: 0.005-0. A martensitic stainless steel seamless steel pipe having excellent machinability and hot workability, characterized in that it contains one or two or more of No. 05 and the balance is Fe and inevitable impurities. (3) The composition according to (1) or (2) above is contained, and further in mass%, Nb: 0.05 to 0.5, V: 0.0.
Martens excellent in machinability and hot workability, characterized by containing one or more kinds of 5 to 0.5 and W: 0.5 to 3 and the balance being Fe and inevitable impurities. Site-based stainless steel seamless steel pipe.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. Modified 13Cr steel (0.02C-0.02N-1.5Cu-12.2Cr
-5.8Ni-2.0Mo), the result of the investigation of the effect of S content on the machinability is that 0.01% or more of S is required to have the same level of machinability as ordinary steel in thread cutting. Must be included.

On the other hand, Modified 13Cr steel (0.02C-0.02N-
The effect of S and B on the hot workability of (1.5Cu-12.2Cr-5.8Ni-2.0Mo) is shown in Fig. 1. The vertical axis and the horizontal axis in FIG. 1 indicate the aperture value and the deformation temperature T ₁ , respectively. The results shown in FIG. 1 are obtained by performing a hot tensile test under the conditions shown in FIG. That is, after heating at 1250 ° C and holding for 1 minute,
The sample was cooled to a deformation temperature (T ₁ ° C) at 10 ° C / sec, held at that temperature for 1 minute, and then subjected to a tensile test at a strain rate of 3 / sec.

A value obtained by dividing the cross-sectional area of the fractured part after the test by the cross-sectional area before the test is defined as an aperture value. The higher the drawing value, the better the hot workability. From the findings so far, the aperture value is 7
It is known that if it is 5% or more, good hot workability is exhibited at that temperature. From FIG. 1, the hot workability of Modified 13Cr steel decreases as the S content increases, but the S content is 0.
Even if it is as high as 01% or more, it is significantly improved by adding B,
It can be seen that if the S amount is 0.04% or less, an aperture value of 75% or more can be obtained.

Next, the reasons for limiting the components of the martensitic stainless steel seamless steel pipe of the present invention will be explained. The content of components is% by mass. C: C is an element that forms Cr carbide and deteriorates the corrosion resistance. On the other hand, it is also a strong austenite forming element, and has an effect of suppressing the formation of the δ ferrite phase when heated at high temperature. However, if it is less than 0.001%, the effect is not exhibited, and if it exceeds 0.05%, a large amount of Cr carbide is precipitated at the grain boundaries and a Cr deficient layer is formed, so that the C resistance is low.
0 ₂ corrosion properties is decreased, also the grain boundary strength sulfide stress cracking resistance is deteriorated to decrease. Further, the weldability also deteriorates. Therefore, the C content is set to 0.001 to 0.05%.

Si: Si is added and remains as a deoxidizer in the steelmaking process. If the content exceeds 0.5%, the toughness and sulfide stress cracking resistance deteriorate, so the upper limit was made 0.5%.

Mn: Mn is an element that combines with S to form MnS to improve machinability, and also to prevent deterioration of hot workability due to S. Further, Mn is an austenite stabilizing element and has an effect of suppressing the formation of the δ ferrite phase at the time of heating at high temperature. However, if it is less than 0.1%, these effects are not exhibited, and if it exceeds 1.5%, the grain boundary strength is lowered, so that the sulfide stress cracking resistance and toughness are deteriorated. Therefore, the Mn content is 0.1 to 1.5%
And

P: P is an impurity element that segregates at the grain boundaries to lower the grain boundary strength and deteriorates the sulfide stress cracking resistance and toughness. It is desirable that the level is as low as possible, but the present refining technology has arrived. The upper limit is 0.03 in consideration of possible level and cost
%.

S: S combines with Mn to form MnS,
It is an effective element that improves machinability. If it is less than 0.01%, this effect is not exhibited, and if it is added in excess of 0.04%, the effect of improving hot workability due to B cannot be sufficiently obtained, and surface defects occur during pipe manufacturing. 0.04%
And

Cr: Cr is an element that improves the corrosion resistance, and is 10% to obtain sufficient corrosion resistance as stainless steel.
It is necessary to contain the above. On the other hand, it is also a ferrite stabilizing element, and if it is contained in excess of 14%, a δ ferrite phase is generated during high temperature heating and hot workability deteriorates. Therefore, the Cr content is set to 10 to 14%.

Ni: Ni is an element that improves the corrosion resistance of Cr-containing steel. Further, it is a strong austenite forming element, and has an effect of suppressing the formation of the δ ferrite phase during high temperature heating. However, if it is less than 2%, these effects are not exhibited, and if it exceeds 8%, the Ac1 transformation point is significantly lowered, and it becomes difficult to adjust the strength. Therefore, the Ni content is set to 2 to 8%.

Mo: Mo is an element effective for improving the corrosion resistance. However, if less than 0.5%, the effect is not exhibited, so the lower limit was made 0.5%. On the other hand, Mo is also a strong ferrite stabilizing element, and if it is contained in excess of 3%, a δ ferrite phase is generated during high temperature heating and hot workability deteriorates. Therefore, the Mo content is set to 0.5 to 3%.

Cu: Cu is an element effective for improving the corrosion resistance. In particular, to improve the stability of the corrosion coating by combined addition with Ni, the resistance to C0 ₂ corrosion properties can be remarkably improved. Further, it is an austenite stabilizing element and has an effect of suppressing the formation of the δ ferrite phase during high temperature heating. However, if 0.5% or less, those effects are not exhibited, and if added in excess of 3%, the grain boundary strength is lowered due to the grain boundary segregation of Cu and the hot workability is significantly deteriorated. It is difficult to prevent rolling defects even if the amount of B is limited and an appropriate amount of B is added. Therefore, the Cu content is set to more than 0.5% to 3%.

Al: Al is added and remains as a deoxidizer in the steelmaking process. If the content exceeds 0.2%, a large amount of AlN is formed and the hot workability and toughness deteriorate. Therefore, the upper limit is set to 0.2%.

N: is a strong austenite forming element, and has an effect of suppressing the formation of the δ ferrite phase when heated at high temperature. Further, fine nitrides have the effect of suppressing crystal grain growth during high temperature heating and improving hot workability. But,
If less than 0.001%, those effects are not exhibited, and
If it is contained in excess of 05%, coarse nitrides are precipitated and the hot workability and toughness deteriorate. Further, the weldability also deteriorates.
Therefore, the N content is set to 0.001 to 0.05%.

B: B segregates itself at the grain boundaries to improve the grain boundary binding force, suppresses the grain boundary segregation of S and Cu, enhances the grain boundary strength, and improves hot workability. It is an effective element. However, if it is less than 0.0005%, its effect is not exhibited, and if it exceeds 0.02%, the melt embrittlement temperature is significantly lowered, the hot working temperature range is significantly limited, and coarse boride is used. Forming hot workability,
Toughness and weldability deteriorate. Therefore, the B content is 0.00
It was set to 05 to 0.02%.

O: O remaining after deoxidation in the steelmaking process is
It is an impurity element that remains in steel as non-metallic inclusions and impairs cleanliness, and deteriorates hot workability, corrosion resistance, and toughness. It is desirable that the level be as low as possible. Taking the above into consideration, the upper limit was made 0.005%.

The above are the basic components of the steel of the present invention, and other than the above, Fe and unavoidable impurities are included, but Ca, Mg, R are added depending on other necessary characteristics such as corrosion resistance and hot workability.
You may contain 1 type, or 2 or more types of EM, Zr, Ti, Nb, V, and W.

Ca, Mg, REM, Zr, Ti: These elements suppress the deterioration of hot workability due to S, and are added as necessary, but if the content is too small, the effect is exhibited. However, if too much, coarse oxides and nitrides are formed and sulfide stress cracking resistance and toughness are deteriorated.
Ca is 0.001-0.01%, Mg is 0.0005-
0.01%, REM 0.001-0.05%, Zr 0.01-0.05%, Ti 0.005-0.05%
And

Nb, V, W: These elements are elements for improving the corrosion resistance, and are added as necessary. However, if the content is too small, the effect will not be exhibited, and if it is too large, the toughness will be deteriorated. , Nb is 0.05 to 0.5%, V is 0.
05-0.5% and W 0.5-3% are added.

The steel of the present invention is obtained by subjecting a steel produced by melting in an ordinary converter or an electric furnace to degassing treatment if necessary, and then using a billet produced by an ingot making method or a continuous casting method. It is made into a seamless pipe by the Mannesmann hot rolling method. The Mannesmann hot-rolling method here is a normal hot-rolling method for producing seamless steel pipes, which uses a pipe-making material with a rectangular cross section or a round cross section, and punches with a press roll punching machine or a Mannesmann punching machine. After that, if necessary, the thickness is reduced by an inclined rolling mill (Elongator mill) and stretch rolling is performed to extend the length, and if necessary, the outer diameter is reduced by a reduction rolling mill (hollow shell reducer). Furthermore, the wall thickness is adjusted with a mandrel mill or a plug mill, and if necessary, a reeler mill is used to perform tube grinding, and the reheating furnace heats it to a specified temperature, and the final finishing rolling machine (stretch reducer or sizer mill).
It is a series of processes for forming a pipe by molding to a predetermined outer diameter.

[0029]

Example Continuously cast slabs having a composition of 217 mm × 217 mm in cross section shown in Table 1 were formed into a seamless pipe by the Mannesmann hot rolling method as cast. After completion of rolling, the state of occurrence of surface defects on the pipe was investigated. The results are shown in Table 1.
In addition, a cutting test of the above pipe was performed using an NC lathe,
Tool life was evaluated. The results are also shown in Table 1. In addition,
The cutting test was performed at a cutting speed of 100 m / min and a cut of 0.
It carried out using the super-steel tool on the cutting conditions of 75 mm and feed 0.15 mm / rev. The tool life was evaluated as a relative value with the tool life when carbon steel (STKM13A steel) was cut under the same conditions as 1, and if it was 0.8 or more, the machinability was considered good.

From Table 1, it is understood that the invention examples (Nos. 1 to 4) have a low surface flaw generation rate and good machinability. On the other hand, in the comparative example (No. 5) in which the amount of S is less than 0.01%, the surface defect occurrence rate is low, but the machinability is poor. In addition, the amount of S is within the range of the present invention,
In Comparative Examples (Nos. 6 and 7) in which the amount deviates from the component range shown in the present invention, it is found that the machinability is good but the surface flaw occurrence rate is high. Further, the B content is within the range of the present invention, but the S content exceeds 0.04% (No.
In 8), it is found that the machinability is good but the surface flaw occurrence rate is high.

From the above, by specifying the amounts of S and B within the component ranges shown in the present invention, it is possible to obtain a seamless steel pipe for machine structure having a low occurrence rate of surface defects and good machinability. It is clear.

[0032]

[Table 1]

[0033]

As described above, according to the present invention, a martensitic stainless steel seamless steel pipe having excellent machinability and excellent hot workability that can be manufactured by the Mannesmann rolling method, and a method for manufacturing the same. Can be provided.

[Brief description of drawings]

[Figure 1] Modified 13Cr steel (0.02C-0.02N-1.5Cu-12.2C
(r-5.8Ni-2.0Mo-0.0018S) is a chart showing the effect of S and B on the hot workability.

FIG. 2 is a chart showing conditions of a hot tensile test.

Claims (3)

(57) [Claims] 1. In mass%, C: 0.001 to 0.05, Si: 0.5 or less, Mn: 0.1 to 1.5, P: 0.03 or less, S: 0.01 to 0. .04, Cr: 10 to 14, Ni: 2 to 8, Mo: 0.5 to 3, Cu: more than 0.5 to 3 or less, Al: 0.2 or less, N: 0.001 to 0.05, B: 0.0005 to 0.02, O: 0.005 or less, and the balance of Fe and inevitable impurities, the martensitic stainless steel having excellent machinability and hot workability. Seamless steel pipe. 2. The composition according to claim 1, further comprising, in mass%, Ca: 0.001-0.01, Mg: 0.0005-0.01, REM: 0.001-0.05, Zr: 0.01 to 0.05, Ti: 0.005 to 0.05, containing one or more kinds, and the balance consisting of Fe and unavoidable impurities. Martensitic stainless steel seamless steel pipe with excellent workability. 3. The composition according to claim 1 or 2, further comprising, in mass%, Nb: 0.05 to 0.5, V: 0.05 to 0.5, and W: 0.5 to 3. A martensitic stainless steel seamless steel pipe excellent in machinability and hot workability, characterized in that it contains one or more kinds and the balance is Fe and inevitable impurities.