JP3328967B2 - Manufacturing method of martensitic stainless steel seamless steel pipe excellent in toughness and stress corrosion cracking resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a high resistance to CO ₂ corrosion properties, the present invention relates to the preparation of high tenacity martensitic stainless steel seamless pipe having a resistance to sulfide stress cracking.

[0002]

2. Description of the Related Art In recent years, development of gas wells for producing gas containing a large amount of CO ₂ and CO ₂ injection for secondary recovery have been widely performed. In such an environment, the corrosion of the steel pipe is severe, so that a martensitic stainless steel pipe having excellent resistance to CO ₂ corrosion is often used. In particular, Japanese Patent Publication No. 59-15977 is a martensitic stainless steel excellent in corrosion resistance and hot workability. However, in this martensitic stainless steel, the contents of C and N are remarkably reduced in order to improve the corrosion resistance, and a δ ferrite phase which deteriorates hot workability is formed in an austenite matrix when the ingot is heated. Has disadvantages. Therefore, cracks and flaws are generated under severe processing conditions such as seamless rolling, and increase in cost due to reduced yield is unavoidable. Production of seamless steel pipes with such components and having high corrosion resistance has been extremely difficult. Was difficult.

In the production of such a martensitic stainless steel, Japanese Patent Publication No. Sho 63-60808 discloses that "low C martensitic stainless steel is 900 to
"A heat treatment method in which the temperature is kept at a temperature in the range of 1000 ° C. and then gradually cooled, or the temperature is kept in a temperature range of 350 ° C. or lower and the temperature is gradually cooled”, and column 6 of JP-B-1-25810 “Generally adopted. The heat treatment is a normal normalizing / tempering process. The forged steel is tempered at 950 ° C. or higher after forging and rolling, and then tempered at a temperature of 700 ° C. or higher and Ac ₁ or lower. ” In addition, since rapid cooling such as water cooling after cooling from the heating temperature after rolling tends to cause cracks, it is manufactured by performing slow cooling such as air cooling. However, a martensitic stainless steel heat-treated by such a method can be obtained as an excellent product without residual stress or cracking, but has a problem on the other hand that the toughness and stress corrosion cracking resistance are not sufficient.

[0004]

SUMMARY OF THE INVENTION In view of such circumstances, an object of the present invention is to provide a method of manufacturing a martensitic stainless steel seamless steel pipe having excellent toughness and stress corrosion cracking resistance.

[0005]

Means for Solving the Problems The present inventors have resistance CO ₂ corrosion results many experiments is maintained if the addition of Cr Contact <br/> by beauty C u of the required amount to reduce the C It has been found that the sulfide stress cracking resistance can be improved by controlling the structure showing crack resistance. In addition, the hot workability is reduced by reducing P, S, etc. to suppress the formation of inclusions, and by controlling the addition amount of C, N and further adding Ni, different phases having different deformation resistances are obtained. It has been found that it is maintained by performing metallurgical operations such as controlling the fraction and shape.

In particular, the present inventors have focused on the effects of C and N and have obtained the following findings. FIG. 1 shows the CO ₂ corrosion resistance and the reduction in hot working when the base component is 1.5% Ni-12.5% Cr steel and the C and N contents are changed. In FIG. R. (Corrosion rate) is the annual corrosion rate in 180 ° C. artificial seawater equilibrated with 40 atm of CO ₂ , and C.I. R. If it is <0.1 mm / y, it can be evaluated that it has sufficient corrosion resistance. In addition, R.
A. Is a sample heated to 1250 ° C. and a strain rate of 3 at 900 ° C.
It is the drawing ratio when uniaxial tensile deformation is performed under the condition of sec ⁻¹ , and when it is 70% or more, it can be said that the hot deformability is good. In the CO ₂ corrosion test, quenching and tempering treatment was performed after hot working, and a material having a yield strength of about 720 MPa was used. From the figure, it is necessary to make C <0.05% in order to satisfy the CO ₂ corrosion resistance, and to have sufficient hot workability, C + 0.8N> 0.06.
(It is understood that the unit of the content of each element symbol is% by weight.)

Further, when gradually or air-cooled during normalizing, coarse and plate-like thin Cr carbides precipitate along the old austenite grain boundaries, and a Cr-deficient layer is formed around the carbides to substantially reduce the Cr content in the portion. And the steel is selectively corroded, so that the stress corrosion cracking of steel is reduced. Further, since the coarse plate-like thin carbide serves as a starting point of cracking, the toughness of the steel deteriorates. When the temperature at which this coarse carbide was deposited was investigated, it was found to be 750-550 ° C. Therefore, in order to improve the toughness and stress corrosion cracking resistance of the above martensitic stainless steel, such coarse C
It is necessary to employ rapid cooling as a method for suppressing the formation of r carbides.

The present invention is constructed by combining the above-mentioned findings. That is, the gist of the present invention is as follows. C ≦ 0.05%, Si ≦ 0.50%, M
n ≦ 1.0%, P ≦ 0.03%, S ≦ 0.01%,
Cr: 11 to 17%, Ni: 1.5 to 5%, Cu: 1
% To 4%, Al ≦ 0.05%, N: 0.02 to 0.
1% and C + 0.8N> 0.06% (% of component
Is satisfied by weight), the remainder is substantially hot-worked from steel and substantially inevitable impurities, and naturally cooled to room temperature, and then heated to a temperature of Ac3 transformation point + 10 ° C to Ac3 transformation point + 200 ° C, from the cooling starting temperature of the heating temperature to 750 ° C. to a cooling stop temperature of five hundred fifty to three hundred and fifty ° C. allowed to cool at 5 ° C. / sec or more, then, 2 ° C. or more air-cooled to room temperature / sec
Cooled at a rate, followed by either processed tempering at Ac1 transformation point temperature, C ≦ 0.05%, Si ≦
0.50%, Mn ≦ 1.0%, P ≦ 0.03%, S ≦
0.01%, Cr: 11-17%, Ni: 1.5-5
%, Cu: more than 1% to 4%, Al ≦ 0.05%, N:
0.02-0.1%, and C + 0.8N> 0.0
A steel that satisfies 6% (% by weight of component) and further contains Mo: 0.5 to 2% as required (% by weight of component), and the balance substantially consists of Fe and unavoidable impurities Is hot-worked and allowed to cool to room temperature, then the Ac3 transformation point + 10 ° C
ＡAc3 transformation point + 200 ° C, and the temperature from the heating start temperature of 750 ° C to the cooling stop temperature of 550-350 ° C is cooled at 5 ° C / sec or more.
It is cooled to room temperature by air cooling or more at a rate of 2 ° C./sec or less , further heated to the temperature of the Ac1 transformation point to Ac3 transformation point, cooled to room temperature by the air cooling rate or more, and then cooled to the Ac1 transformation point or less. This is a method for producing a martensitic stainless steel seamless steel pipe with excellent corrosion resistance, which is tempered.

Hereinafter, the present invention will be described in detail. First, the reasons for limiting the steel components will be described. C is an element that forms a Cr carbide or the like and deteriorates corrosion resistance, but is a typical austenite-forming element and has an effect of suppressing the generation of a δ ferrite phase at a hot working temperature range of 900 to 1250 ° C. To be contained. However, if the content exceeds 0.05%, a large amount of carbides such as Cr carbides precipitate to form a Cr-deficient layer, so that the CO ₂ corrosion resistance decreases, and carbides are likely to precipitate at grain boundaries. Therefore, the sulfide stress cracking resistance is significantly reduced. Therefore C
The content was 0.05% or less.

[0010] Si is added and contained as a deoxidizing material on steel making. If contained in steel in an amount exceeding 0.50%, toughness and sulfide stress cracking resistance are reduced.
50% or less. Mn is an element that forms inclusions and impairs crack resistance in a corrosive environment, but is included because it is a useful component for forming a single phase of austenite. However, if added in excess of 1.0%, a large amount of inclusions are formed, so that crack resistance and toughness in a corrosive environment are reduced. Therefore, the content of Mn is set to 1.0% or less.

P segregates at the grain boundaries to weaken the strength of the grain boundaries and reduces hot workability and sulfide stress cracking resistance.
03% or less. Since S forms inclusions as sulfides and lowers hot workability, the upper limit is set to 0.01%.

[0012] Cr must be contained in an amount of 11% or more in order to impart the CO ₂ corrosion resistance aimed at by the present invention and to have the corrosion properties as stainless steel. However, if added in excess of 17%, a ferrite phase is likely to be formed, so the range was set to 11 to 17%.

Ni has the effect of improving the corrosion resistance of Cr-containing steel. In addition, it is a powerful austenite-forming element and has the effect of suppressing the formation of the δ-ferrite phase during high-temperature heating, and has the effect of suppressing the growth of cracks formed inside the δ-ferrite phase during hot working in addition to making the shape thinner and shorter. Therefore, it also has the effect of improving hot workability.
However, when N: 0.02%, Ni: 1.5% or less does not show these effects, and when added over 5%, the Ac ₁ point becomes extremely low, and refining is difficult. And the remaining austenite phase is formed to impair the strength and toughness.

Cu has the effect of improving the CO2 corrosion resistance. Also, it is an austenite stabilizing element,
It also has the advantage of not lowering the c ₁ transformation point. However, if the content is 1% or less, the effect of improving corrosion resistance is not sufficient, and if the content exceeds 4%, it is susceptible to hot cracking and the hot workability deteriorates, so that the content is more than 1 % to 4 %.
%. In addition, since the effect described above is small when Cu alone is added, it is always necessary to include Cu in combination with Ni.

Al is added and contained as a deoxidizing agent in the same manner as Si. If more than 0.05% is contained, A
Many 1N are formed, and the toughness is significantly reduced. Therefore, the upper limit of the content is set to 0.05%.

N is not harmful to corrosion resistance and is a typical austenite-forming element like C, and has an effect of suppressing the formation of a ferrite phase at a hot working temperature range of 900 to 1250 ° C. The effect is 1.
When 5% Ni-12.5% Cr steel is used as a base component, it is effective in the range of the content satisfying C + 0.8N <0.06 (C and N are weight%). Therefore, C <
In the case of 0.05%, a ferrite phase is not generated in the hot working temperature range, and N is set to 0.1 to obtain good hot workability.
It is necessary to add 02% or more. In addition, it is difficult to make the content exceed 0.1% in the usual melting process, so the content range is set to 0.02 to 0.1%.

Mo is an element effective for improving the pitting corrosion resistance, and is added as necessary. However, 0.
The effect is small when the addition is less than 5%. In addition, it is a strong ferrite stabilizing element.
Since the phase is easily formed, the limited range is 0.5
２2%.

Next, the reasons for limiting the heat treatment conditions will be described. The upper limit of the austenitizing heating temperature is the temperature at which carbides are not completely dissolved in the γ loop of the Cr-containing stainless steel and the crystal grains do not become coarse, and the lowest temperature at which the austenite phase becomes stable is the lower limit. And That is, the steel pipe cooled by hot working is converted to the Ac ₃ transformation point +2.
When heated to a temperature of 00 ° C. or more, the carbide completely dissolves in solution, so that during cooling, a large amount of Cr carbide precipitates at the grain boundaries, significantly reduces corrosion resistance, and further causes coarsening of crystal grains. descend. In addition, Ac ₃ transformation point + 10 ° C
When heated to the following low temperature, the austenite phase is not stabilized, and it is difficult to obtain stable strength.
Therefore, the heat treatment temperature is the Ac ₃ transformation point + 10 ° C. to A
c ₃ transformation point + 200 ° C.

The martensitic stainless steel thus heated is heated to a heating temperature of 7 to 7 during the cooling process.
The cooling is performed at a rate of 2 ° C./sec or more from a cooling start temperature of 50 ° C. to a cooling stop temperature of 550 to 350 ° C. The reason for setting the controlled cooling conditions is to suppress the precipitation of carbides by passing through a temperature range of 750 to 550 ° C., which is a region where plate-shaped Cr-based carbides are precipitated, in a short time. However, 350
Rapid quenching to 3 ℃ or less is likely to cause cracking.
Must stop above 50 ° C. On the other hand, 550
At 750 ° C., nucleation and growth of carbides are fast, and at a cooling rate lower than 2 ° C./sec, plate-like carbides precipitate at crystal grain boundaries.

The steel cooled to a temperature of 550 to 350 ° C. is further cooled at a speed higher than air cooling to cause a martensitic transformation and a martensite single phase structure. The residual stress in the martensite structure is eliminated by recovery, and the supersaturated carbon atoms are precipitated as carbides, thereby increasing the toughness and ductility and performing a tempering treatment to obtain a desired strength. At this time, if heating is performed at a temperature equal to or higher than the Ac ₁ transformation point, reverse transformation occurs and the toughness is significantly reduced. Therefore, the tempering treatment is performed at a temperature equal to or lower than the Ac ₁ transformation point.

Before the tempering treatment after the austenitizing treatment, the Ac ₁ transformation point to Ac ₃
A two-phase region heating process is performed by heating to a temperature range of the transformation point. The purpose of this is to temper the steel to a low strength that cannot be obtained by a single tempering process, and by using this process to temper the steel to a low strength, the steel has sufficient sulfide stress cracking resistance. Property can be imparted.

The steel pipe manufactured by the method of the present invention as described above is excellent in toughness, CO ₂ corrosion resistance and sulfide stress cracking resistance.

[0023]

EXAMPLES First, steel having the chemical composition shown in Table 1 was cast in a normal melting process, and then a steel tube was manufactured by hot rolling, and then subjected to heat treatment and tempering treatment.
The strength, toughness, resistance to CO ₂ corrosion, and resistance to sulfide stress cracking were investigated. Table 2 shows materials such as heat treatment temperature and strength at that time. The CO ₂ corrosion resistance was evaluated by the corrosion rate in artificial seawater at 150 ° C. equilibrated with 40 atm of CO ₂ . If the corrosion rate is 0.1 mm / y or less, it can be regarded as having corrosion resistance. Sulfide stress cracking resistance was measured using two round bar tensile test pieces.
One atmosphere of 99% CO ₂ +1 in a 5% NaCl solution at 5 ° C.
A uniaxial tensile stress was applied in a corrosive environment saturated with% H ₂ S gas, and the ratio (Rs value) between the maximum initial stress and the yield stress at which no fracture occurred in 720 hours was determined. If Rs ≧ 0.8, it can be said that the characteristics are excellent.

According to the results shown in Table 2, the steel pipe manufactured by the method of the present invention shows good resistance to CO ₂ corrosion, resistance to sulfide stress corrosion cracking, and high toughness, whereas the steel pipe produced by the method of the present invention is out of the scope of the present invention. It is clear that the method is inferior in either property.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

As described above, according to the present invention, the components are specified and the processing conditions are regulated to produce CO ₂ -resistant products.
A seamless martensitic stainless steel pipe having excellent corrosion resistance and sulfide stress corrosion cracking resistance and high toughness can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the C and N contents on the CO ₂ corrosion resistance (corrosion rate) and the drawability during hot working.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-2-217444 (JP, A) JP-A-6-88130 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/00-8/10 C21D 9/08 C22C 38/00-38/60

Claims (3)

(57) [Claims] (1) In terms of% by weight, C ≦ 0.05%, Si ≦ 0.5
0%, Mn ≦ 1.0%, P ≦ 0.03%, S ≦ 0.01
%, Cr: 11 to 17%, Cu: more than 1% to 4%, Ni:
1.5-5%, Al ≦ 0.05%, N: 0.02-
0.1% and a steel containing a component satisfying C + 0.8N> 0.06%, and the balance substantially consisting of Fe and unavoidable impurities is hot-worked and naturally cooled to room temperature. Ac ₃ transformation point +1
It is heated to a temperature of 0 ° C. to Ac ₃ transformation point + 200 ° C., and from this heating temperature to a cooling start temperature of 750 ° C. to 550 to 350 ° C.
Cool down to 5 ° C / sec or more until the cooling stop temperature of, then cool to room temperature with air cooling or more at a rate of 2 ° C / sec or less ,
Subsequently, a method for producing a martensitic stainless steel seamless steel pipe excellent in toughness and stress corrosion cracking resistance, characterized by tempering at a temperature not higher than the Ac ₁ transformation point. (2) C ≦ 0.05%, Si ≦ 0.5% by weight
0%, Mn ≦ 1.0%, P ≦ 0.03%, S ≦ 0.01
%, Cr: 11 to 17%, Cu: more than 1% to 4%, Ni:
1.5-5%, Al ≦ 0.05%, N: 0.02-
0.1% and a component containing C + 0.8N> 0.06%, with the balance being substantially composed of Fe and unavoidable impurities, hot-worked and naturally cooled to room temperature, then Ac ₃ transformation points +1
It is heated to a temperature of 0 ° C. to Ac ₃ transformation point + 200 ° C., and from this heating temperature to a cooling start temperature of 750 ° C. to 550 to 350 ° C.
Cool down to 5 ° C / sec or more until the cooling stop temperature of, then cool to room temperature with air cooling or more at a rate of 2 ° C / sec or less ,
Further, it is heated to a temperature of the Ac ₁ transformation point to the Ac ₃ transformation point, cooled to room temperature at a speed of air cooling or higher, and subsequently tempered at a temperature of the Ac ₁ transformation point or less. Manufacturing method of martensitic stainless steel seamless steel pipe with excellent stress corrosion cracking. 3. A seamless martensitic stainless steel pipe having excellent toughness and stress corrosion cracking resistance according to claim 2, further comprising Mo: 0.5 to 2% by weight. Manufacturing method.