JP2979604B2 - Manufacturing method of martensitic stainless steel seamless tube - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) The present invention has excellent mechanical properties and sulfide stress cracking resistance,
In addition, the present invention relates to a method for producing a martensitic stainless steel seamless tube having a small anisotropy of these properties.

Oil (prior art) martensitic stainless steel seamless pipe, comprising strength, OCTG toughness and corrosion resistance is required, is used as a transport pipe, a CO ₂ for particularly excellent in resistance to CO ₂ corrosion, It is awarded for oil well pipes for gas. However, environments containing CO ₂ often also contain trace amounts of H ₂ S, and martensitic stainless steels are generally susceptible to sulfide stress cracking (SSC), which limits their use at present.

Seamless pipes used for oil country tubular goods are usually manufactured by inclined rolling method such as Mannesman mandrel mill method, Mannesmann plug mill method or extrusion method of Yugene method, and then subjected to quenching and tempering (QT) treatment. Certain mechanical properties are provided. In the past, it was common practice to reheat and quench the pipe once cooled to room temperature after pipe making, but recently, in order to streamline the process and save energy, immediately cool and quench the pipe following the pipe making process. Direct quenching (DQ) methods are being adopted.

The inventor of the present invention has also previously filed a patent application for a method of manufacturing a martensitic stainless steel seamless pipe used for oil country tubular goods using this direct quenching method (Japanese Patent Application Laid-Open No. 63-293111). The invention of the prior application is characterized in that, after piercing and rolling a billet of martensitic stainless steel, final rolling is performed in a non-recrystallization temperature range, and then direct quenching treatment is performed.

(Problem to be Solved by the Invention) The seamless pipe manufactured by the method of the prior application is
Although all of the strength, toughness and stress corrosion cracking resistance are superior to ordinary QT materials, anisotropy appears in these characteristics due to the elongated grain structure rolled in the unrecrystallized region. Since the anisotropy increases as the workability of the final rolling increases,
In the manufacturing method of the prior application, the final rolling degree cannot be made too large, and it is not suitable when the final rolling degree becomes large depending on the product size.

It is an object of the present invention to provide strength and toughness required for a seamless pipe used for oil country tubular goods, etc., as well as excellent corrosion resistance even in an environment containing CO ₂ and H ₂ S, and anisotropy of these properties. An object of the present invention is to provide a method capable of manufacturing a small seamless steel pipe.

(Means for Solving the Problems) The present inventors have studied in detail the direct quenching method of martensitic stainless steel and the appearance of anisotropy, and have obtained the following knowledge. That is, (1) The structure of the material processed in the unrecrystallized region, cooled and quenched as it is, has a (100) crystal plane and a (111) crystal plane accumulated on the rolled surface, and due to its plastic anisotropy, The rolling surface becomes brittle.

(2) When the austenite has a work strain, the textures generated during quenching have a choice of orientation-related variants when transforming from austenite to martensite, and the (100) crystal plane and the (111) crystal plane Collect.

(3) To improve the anisotropy, it is effective to transform the martensite after completely recovering the strain after the final rolling.

(4) It is necessary to completely recover the processing strain by static recrystallization, not dynamic recrystallization.

(5) If the final processing is performed under the condition that static recrystallization occurs, and it is cooled by air cooling after the final processing, a martensitic stainless steel seam with small anisotropy can be obtained only by tempering without re-quenching. Tubeless can be manufactured.

The present invention based on the above-mentioned findings provides a method of manufacturing a seamless pipe by hot piercing and rolling from a billet made of martensitic stainless steel, wherein after piercing and rolling, the final finish rolling step has a working ratio of 5% or more. Under the conditions satisfying the following formula, and then cooled to at least 500 ° C. at a cooling rate of 10 ° C./min or more to form a structure in which 95% or more by volume is martensite and temper at a point of Ac ₁ point or less. Characteristic method of producing martensitic stainless steel seamless steel ".

Tε-990ε-1000 ≧ 0, where T: final rolling start temperature (° C.) ε: workability (%) As described above, a method of manufacturing a seamless steel pipe by piercing and rolling a billet is a Mannes mandrel mill as described above. There are an inclined rolling method such as a method, a Mannesmann plug mill method, and an extrusion method of a Eugene method, and the method of the present invention can be applied to all seamless pipe production methods.

In the present invention, the final finish rolling step after piercing rolling means a final hot rolling step for producing the seamless steel pipe. For example, in the Mannes mandrel mill method, a stretch reducer step is performed, and in the plug mill method, a sizer step is a final finish rolling step. Alternatively, in the Eugene system in which the process includes one extrusion step, the extrusion step is the final rolling step.

The method of the present invention can be applied to the production of seamless pipes made of martensitic stainless steel in general. That is, there is no particular limitation on the chemical composition of the material network. However, a typical chemical composition is as follows.

By weight%, C: 0.001 to 1.2%, Si: 1% or less, Mn: 2% or less, Cr: 8 to 17%, Sol. Al: 0.005 to 0.1%, P, S: 0.05% or less for each, balance Fe And unavoidable impurities, or one or more of the following components, if necessary.

Mo: 3% or less, Ni: 8% or less, N: 0.01 to 0.15%, B: 0.01%
Below, Ti: 0.5% or less, Nb: 0.5% or less, V: 0.5% or less, C
a, Mg, La, Ce: 0.01% or less each.

(Operation) Hereinafter, in the present invention, the reason for limiting the final finish rolling, the cooling rate, the martensite ratio, and the tempering condition as described above will be described.

[Final finish rolling]

In the final finish rolling step, the material after piercing and rolling has a workability of 5%.
% Or more is performed under conditions that satisfy the following formula. The degree of processing is calculated by the cross-sectional reduction rate and is expressed by the following equation.

Tε-990ε-1000 ≧ 0, where T: Final rolling start temperature (° C) ε: Workability (%) The lower limit of the working ratio is set to 5% because if it is higher than this, a substantially effective plastic deformation can be obtained.

Further, the reason for performing the final rolling under the condition that satisfies the above-described formula defined by the final rolling start temperature and the working ratio is that recrystallization does not occur after working under conditions that deviate from the formula, and the anisotropy is not improved. is there.

FIG. 1 shows the results obtained when a billet of a martensitic stainless steel shown in Table 1 described below was subjected to final rolling by changing the final rolling start temperature (T) and the area reduction rate (ε) as shown in Table 2. 3 is a graph showing a region where recrystallization of the GaN occurs.

From FIG. 1, it can be seen that recrystallization occurs in a region satisfying Tε-990ε-1000 ≧ 0.

〔cooling〕

Cooling after the final rolling is performed at a cooling rate of 10 ° C./min or more up to 500 ° C. where carbide is easily precipitated, and a structure in which 95% or more by volume is martensite is obtained. If the cooling rate to 550 ° C. is lower than 10 ° C./min, carbides precipitate and the toughness decreases. When the amount of martensite is less than 95%, ferrite or retained austenite remains in a band, so that anisotropy appears and toughness and corrosion resistance decrease.

[Tempering]

Tempering needs to be performed in a ferrite region of not more than _one point of Ac. Austenite is formed when Ac exceeds ₁ point,
This transforms into martensite by cooling after tempering, and lowers corrosion resistance and toughness. The cooling after tempering may be air cooling, but it is preferable to cool at a rate as fast as possible in order to avoid tempering embrittlement peculiar to high Cr steel.

(Example) Six kinds of martensitic stainless steels shown in Table 1 were melted and made into a billet of 100 mmφ × 300 mml.
Tubes were produced under the respective conditions shown in the table. The size of the product steel pipe was unified to an outer diameter of 150 mmφ, and the wall thickness was changed according to the rolling reduction.

A test specimen was taken from each product steel pipe and subjected to a tensile test, an impact test, and a corrosion resistance test.

In the tensile test, 0.2% proof stress and tensile strength of a test piece having a diameter of 4 mm in the parallel portion were measured. Impact test 5m from L direction and C direction
A 2V notch Charpy impact test specimen of mx 10 mm was collected and
The absorbed energy in ° C. was measured. The specimen in the C direction was prepared by adding electron beam welding. Corrosion performs four-point bending notched test, 1 [sigma in notch _y: adds stress _{(sigma y strength), 30atmCO 2 + 0.003atmH 2 S} + 5% NaC
The sample was immersed in 1 l of the solution kept at 20 ° C. and evaluated by the presence or absence of cracks after 336 hours. Also in this case, the test piece in the C direction was prepared by adding a beam by electron beam welding, and the welded portion was sealed and tested.

In Table 1, No. 1 to No. 18 correspond to Examples of the present invention,
After hot working, recrystallized, quenched and tempered. Each of them has good toughness and sulfide stress cracking resistance and has no anisotropy.

In contrast, the comparative examples of Nos. 22 to 30 were processed in the non-recrystallized region, cooled, and then tempered.
It is excellent in toughness in the L direction and sulfide stress cracking resistance, but is inferior in the C direction toughness and sulfide stress cracking resistance, and the difference between the properties in the L direction and the C direction is large. Nos. 19 to 21 are conventional examples adopting ordinary quenching and tempering processes, which have small anisotropy, but do not have a high toughness level and have sulfide stress cracking.

(Effects of the Invention) As is clear from the results of the above examples, according to the method of the present invention, quenching treatment for reheating after pipe production is not required, and mechanical properties and sulfide stress cracking resistance are obtained only by tempering treatment. A martensitic stainless steel seamless tube with excellent anisotropy and small anisotropy can be manufactured, and the effect of saving process and energy is great.

[Brief description of the drawings]

Fig. 1 shows the final rolling start temperature (T) and the area reduction rate (ε).
7 is a graph showing a region where recrystallization occurs when final rolling is performed while changing the temperature.

Claims (1)

(57) [Claims] 1. A method for producing a seamless pipe by hot piercing and rolling from a billet of martensitic stainless steel, wherein after piercing and rolling, a final finishing rolling step is performed with a working ratio of 5% or more by the following formula. And then at least
Cool to 500 ° C at a cooling rate of 10 ° C / min or more, and
A process for producing a martensitic stainless steel seamless tube, characterized in that the structure is at least 5% martensite and tempered at an Ac of ₁ point or less. Tε-990ε-1000 ≧ 0, where T: Final rolling start temperature (° C) ε: Workability (%)