JP2001323340A - Martensitic stainless steel seamless steel tube excellent in machinability and hot workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a martensitic stainless steel seamless steel tube excellent in machinability and also having excellent hot workability in a Mannesman's rolling method. SOLUTION: This martensitic stainless steel seamless steel tube excellent in machinability and hot workability has a composition containing, by mass, 0.001 to 0.05% C, <=0.5% Si, 0.1 to 1.5% Mn, <=0.03% P, 0.01 to 0.04% S, 10 to 14% Cr, 2 to 8% Ni, 0.5 to 3% Mo, 0.5 to 3% Cu, <=0.2% Al, 0.001 to 0.05% N, 0.0005 to 0.02% B and <=0.005% O and further containing, if required, one or more kinds selected from 0.001 to 0.01% Ca, 0.0005 to 0.01% Mg, 0.001 to 0.05% rare earth metals, 0.01 to 0.05% Zr, 0.005 to 0.05% Ti, 0.05 to 0.5% Nb, 0.05 to 0.5% V and 0.5 to 3% W, and the balance Fe with inevitable impurities.

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 excellent in machinability and hot workability.

[0002]

2. Description of the Related Art Martensitic stainless steel has been developed using AI.
As represented by SI420 steel, it has been used as an oil country tubular goods since about 1980 because it is excellent in strength and CO ₂ corrosion resistance and relatively inexpensive. In recent years, Japanese Patent Publication No. 59-15978 and Japanese Patent Publication No. 3-22, which have higher corrosion resistance than AISI 420 steel, in order to be adaptable to an oil well environment containing high temperature and a large amount of CO ₂ and H ₂ S in addition thereto.
No. 27, etc., a steel type such as a low C-Ni-Mo-added steel or a low C-Cu-Ni-Mo-added steel, such as that disclosed in JP-A-2-217444. Steel type) has been developed.

In general, when the amount of alloy is increased, corrosion resistance is improved, but workability is deteriorated. The above steel oil country tubular goods are usually produced seamlessly by the Mannesmann rolling method. Conventionally, Mannesmann rolling is 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 a Mannesmann rolling method. When piped, rolling flaws were sometimes generated. For this reason, at present, it is manufactured by minimizing the content of impurities harmful to hot workability represented by S.

[0004] On the other hand, since oil country tubular goods are usually connected by threaded joints, it is customary to cut male threads at both ends of the steel pipe. However, martensitic stainless steel is more machinable than ordinary steel. Due to the poor quality, the problem is that the service life of the screw cutting chaser is short. As a martensitic stainless steel with improved machinability, S is added in a large amount (0.15% or more) to improve machinability.
US 416 steel and SUS420F steel are Jls G43
However, the martensitic stainless steel to which such a large amount of S is added has extremely poor hot workability, and it is difficult to produce a seamless pipe by the Mannesmann hot rolling method. there were.

[0005] As described above, according to the techniques presented heretofore, a seamless hot-workable martensitic stainless steel pipe having excellent machinability and capable of being manufactured by the Mannesmann rolling method is manufactured. It was difficult.

[0006]

SUMMARY OF THE INVENTION The present invention is intended to solve the above-mentioned problems, and has an excellent machinability and an excellent hot working which can be manufactured by the Mannesmann rolling method. It is an object of the present invention to provide a martensitic stainless steel seamless steel pipe having a property and a method for producing the same.

[0007]

The present inventors have repeatedly studied machinability and hot workability of various materials having different components. As a result, in the present steel type, S was replaced with ordinary free-cutting stainless steel. By making it in the range of 0.01 to 0.04%, which is much smaller than that of steel, and by adding 0.0005 to 0.02% of B to this, the Mannesmann system can be used without causing cracking during hot rolling. It has been found that it is possible to produce a seamless pipe by hot rolling and that the machinability can sufficiently satisfy the required level.

The present invention has been made based on such knowledge, and the gist 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 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: A martensitic stainless steel seamless with excellent machinability and hot workability, characterized by containing 0.0005 to 0.02, O: 0.005 or less, and the balance being Fe and inevitable impurities. Steel pipe. (2) The composition according to the above (1), further containing mass%
Ca: 0.001 to 0.01; Mg: 0.0005 to 0.01; REM: 0.001 to 0.05; Zr: 0.01 to 0.05; A seamless martensitic stainless steel pipe excellent in machinability and hot workability, characterized in that it contains one or more kinds of steels of No. 05 and the balance consists of Fe and inevitable impurities. (3) The composition according to (1) or (2), further containing Nb: 0.05 to 0.5 and V: 0.0 by mass%.
5 to 0.5, W: 0.5 to 3, containing one or more kinds, the balance being Fe and unavoidable impurities, characterized by having excellent machinability and hot workability. Sight-based stainless steel seamless steel pipe.

[0009]

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

On the other hand, Modified 13Cr steel (0.02C-0.02N-
FIG. 1 shows the effect of S and B on the hot workability of 1.5Cu-12.2Cr-5.8Ni-2.0Mo). Vertical and horizontal axes in FIG. 1 respectively and aperture deformation temperature T _1. The results shown in FIG. 1 are obtained by performing a hot tensile test under the conditions shown in FIG. That is, after heating to 1250 ° C and holding for 1 minute,
After cooling at a rate of 10 ° C./sec to the deformation temperature (T ₁ ° C.), holding at that temperature for 1 minute, a tensile test was performed at a strain rate of 3 / sec.

The value obtained by dividing the sectional area of the fractured portion after the test by the sectional area before the test is defined as the aperture value. The higher the aperture value, the better the hot workability. From the findings so far, an aperture value of 7
It is known that if the content is 5% or more, good hot workability is exhibited at that temperature. From FIG. 1, the hot workability of the Modified 13Cr steel decreases as the S content increases, but the S content is 0.1%.
Even when it is as high as 01% or more, B significantly improves 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 described. The component content is% by mass. C: C is an element that forms a Cr carbide or the like and deteriorates corrosion resistance. On the other hand, it is also a strong austenite-forming element and has the effect of suppressing the formation of a δ ferrite phase during high-temperature heating. However, if the content is less than 0.001%, the effect is not exhibited. If the content exceeds 0.05%, a large amount of Cr carbide precipitates at the grain boundaries and a Cr-deficient layer is formed.
O ₂ Corrosion characteristics are reduced, and grain boundary strength is reduced, so that sulfide stress cracking resistance is deteriorated. Further, the weldability also deteriorates. Therefore, the C content is set to 0.001 to 0.05%.

Si: Si is added as a deoxidizing agent in the steel making process and remains. If the content exceeds 0.5%, 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 and improves machinability, and is an element that prevents deterioration of hot workability due to S. Further, Mn is an austenite stabilizing element and has an effect of suppressing the formation of a δ ferrite phase when heated at a high temperature. However, if the content is less than 0.1%, these effects are not exhibited, and if the content exceeds 1.5%, the grain boundary strength is reduced, 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 boundary to lower the grain boundary strength and deteriorates the resistance to sulfide stress cracking and toughness, and is desirably as low as possible. Considering possible level and cost, the upper limit is 0.03
%.

S: S combines with Mn to form MnS,
It is an effective element for improving machinability. If the content 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 by B cannot be sufficiently obtained, and surface flaws occur during pipe production. 0.04%
And

Cr: Cr is an element for improving corrosion resistance, and 10% is required 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 when it is contained in excess of 14%, a δ ferrite phase is formed at the time of high temperature heating, and the hot workability is deteriorated. Therefore, the Cr content was 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 a δ ferrite phase during high-temperature heating. However, if the content is less than 2%, these effects are not exhibited, and if the content exceeds 8%, the Ac1 transformation point is greatly reduced, and the tempering becomes difficult. Therefore, the Ni content was set to 2 to 8%.

Mo: Mo is an element effective for improving corrosion resistance. However, if the content is less than 0.5%, the effect is not exhibited, so the lower limit is set to 0.5%. On the other hand, Mo is also a strong ferrite-stabilizing element, and when contained in an amount exceeding 3%, a δ-ferrite phase is formed at the time of high-temperature heating, and the hot workability deteriorates. Therefore, the Mo content is set to 0.5 to 3%.

Cu: Cu is an element effective for improving 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 a δ ferrite phase during high-temperature heating. However, these effects are not exhibited at 0.5% or less, and when added over 3%, the grain boundary strength is reduced due to the grain boundary segregation of Cu and the hot workability is significantly deteriorated. And it is difficult to prevent rolling flaws even if an appropriate amount of B is added. Therefore, the Cu content is set to more than 0.5% to 3%.

Al: Al is added as a deoxidizing agent in the steel making process and remains. If the content exceeds 0.2%, a large amount of AlN is formed, and the hot workability and toughness decrease. Therefore, the upper limit is set to 0.2%.

N: is a strong austenite-forming element and has the effect of suppressing the formation of a δ-ferrite phase during high-temperature heating. 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 the content exceeds 0.05%, coarse nitrides precipitate and 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 at the grain boundaries by itself to improve the grain boundary bonding force, suppresses the grain boundary segregation of S and Cu, increases the grain boundary strength, and improves hot workability. It is an effective element. However, if the content is less than 0.0005%, the effect is not exhibited. If the content exceeds 0.02%, the melt embrittlement temperature is significantly reduced, the hot working temperature range is significantly restricted, and coarse boride is used. Forming hot workability,
The toughness and weldability deteriorate. Therefore, the B content is 0.00
05 to 0.02%.

O: O remaining after deoxidation in the steelmaking process is
It is an impurity element that remains in steel as a nonmetallic inclusion and impairs cleanliness and degrades hot workability, corrosion resistance and toughness. It is desirable that the level be as low as possible. In consideration of the above, the upper limit is made 0.005%.

The above are the basic components of the steel of the present invention. Other than the above, the steel consists of Fe and unavoidable impurities. However, depending on other necessary characteristics such as corrosion resistance and hot workability, Ca, Mg, R
One or more of EM, Zr, Ti, Nb, V and W may be contained.

Ca, Mg, REM, Zr, Ti: These elements suppress the deterioration of hot workability due to S, and are added as necessary. However, if the content is too small, the effect is not exhibited. If too much, coarse oxides and nitrides are formed and sulfide stress cracking resistance and toughness deteriorate, so
Ca is 0.001 to 0.01%, Mg is 0.0005 to
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 that improve corrosion resistance and are added as necessary. However, if the content is too small, the effect is not exhibited, and if the content is too large, the toughness is deteriorated. , Nb is 0.05 to 0.5%, and V is 0.1 to 0.5%.
05 to 0.5%, and W is added to 0.5 to 3%.

The steel of the present invention is prepared by subjecting steel melted in a usual converter or electric furnace to degassing as required, and then using a steel slab or the like manufactured by ingot casting or continuous casting. The pipe is seamlessly formed by the Mannesmann hot rolling method. The hot rolling method of the Mannesmann method referred to here is a hot rolling method for producing a normal seamless steel pipe, using a material for pipe making having a rectangular cross section or a round cross section, and piercing with a press roll drill or a Mannesmann drill. After that, if necessary, the thickness is reduced by an inclined rolling mill (Elongator Mill), and elongation rolling is performed to increase the length. Further, if necessary, the outer diameter is reduced by a reduction rolling mill (hollow shell reducer). The thickness is adjusted with a mandrel mill or plug mill, the tube is adjusted with a reeler mill if necessary, and heated to a predetermined temperature by a reheating furnace. The final finishing mill (stretch reducer or sizer mill)
This is a series of processes for forming a tube by molding to a predetermined outer diameter.

[0029]

EXAMPLES Continuous cast slabs having a composition shown in Table 1 and having a cross section of 217 mm × 217 mm were formed into a seamless pipe by a Mannesmann hot rolling method as cast. After the completion of the rolling, the occurrence of surface flaws on the pipe was examined. Table 1 shows the results.
Also, a cutting test of the pipe was performed using an NC lathe,
The tool life was evaluated. The results are shown in Table 1. In addition,
The cutting test was performed at a cutting speed of 100 m / min and a depth of cut of 0.1 m / min.
The cutting was performed using a super steel tool under cutting conditions of 75 mm and a feed of 0.15 mm / rev. The tool life was evaluated as a relative value assuming that the tool life when cutting carbon steel (STKM13A steel) under the same conditions was 1, and it was determined that the machinability was good if the tool life was 0.8 or more.

From Table 1, it can be seen that in the examples of the present invention (Nos. 1 to 4), the incidence of surface flaws is low and good machinability is obtained. On the other hand, in the comparative example (No. 5) in which the S content is less than 0.01%, the surface flaw occurrence rate is low but the machinability is poor. Although the S content is within the scope of the present invention,
In Comparative Examples (Nos. 6 and 7) in which the amounts deviate from the component ranges shown in the present invention, it can be seen that the machinability is good, but the incidence of surface flaws is high. Further, although the B content is within the range of the present invention, the comparative example (No.
In 8), it can be seen that the machinability is good but the incidence of surface flaws is high.

As described above, by specifying the amounts of S and B in the component ranges shown in the present invention, a seamless steel pipe for machine structures having a low incidence of surface flaws and good machinability can be obtained. It is clear that.

[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 of manufacturing the same. Can be provided.

[Brief description of the drawings]

Fig. 1 Modified 13Cr steel (0.02C-0.02N-1.5Cu-12.2C
4 is a table showing the influence of S and B on hot workability of (r-5.8Ni-2.0Mo-0.0018S).

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

Claims (3)

[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 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, the balance being Fe and unavoidable impurities, the martensitic stainless steel having excellent machinability and hot workability. Seamless steel pipe. 2. The composition according to claim 1, further comprising: Ca: 0.001 to 0.01; Mg: 0.0005 to 0.01; REM: 0.001 to 0.05; One or more of Zr: 0.01 to 0.05 and Ti: 0.005 to 0.05, with the balance being Fe and unavoidable impurities, the machinability and hot work Martensitic stainless steel seamless steel pipe with excellent workability. 3. The composition according to claim 1, which further comprises the following components: Nb: 0.05 to 0.5; V: 0.05 to 0.5; 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 consists of Fe and inevitable impurities.