JPH07150251A - Production of seamless martensitic stainless steel tube excellent in hot workability and corrosion resistance and having high toughness - Google Patents

Abstract

PURPOSE:To produce a martensitic seamless steel tube excellent in hot workability and corrosion resistance and having high toughness. CONSTITUTION:A steel, which has a composition containing, by weight, <=0.05% C, <=0.50% Si, <=1.0% Mn, <=0.03% P, <=0.01% S, 11-17% Cr, 0.2-<7% Cu, 1.5-5% Ni, <=0.05% Al, and 0.02-1% Nor further containing 0.5-2& Mo and satisfying C+0.8N>=0.06%, is hot worked and then heat to a temp. between Ac3+10 deg.C and Ac3+200 deg.C and cooled down to room temp., or further heated to a temp. between Ac1 and Ac3 and cooled down to room temp., followed by tempering treatment at <=Ac1. By this method, corrosion resistance can be improved if C is regulated to <=0.05% and Cr, Ni, and Cu are added or further Mo is added, and particularly, Cu is effective when added by >=0.2%. Moreover, hot workability can be improved if C+0.8N>=0.06% is satisfied, and, if Cu is limited to <1%, the occurrence of surface flaw at the time of hot working can be prevented.

Description

Detailed Description of the Invention

[0001]

The present invention relates to hot workability and C resistance.
The present invention relates to a method for producing a high toughness martensitic stainless steel seamless steel pipe having excellent O ₂ corrosion resistance and sulfide stress cracking resistance.

[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 excellent in corrosion resistance such as CO ₂ corrosion resistance and sulfide stress cracking resistance is required in many cases. Among them, as martensitic stainless steel excellent in corrosion resistance and hot workability, for example, C and N are 0.015% or less, Ni:
0.1-3.0%, Cr: 11-17% and Nb: 0.0
1 to 0.05%, and if necessary, Mo: 0.05 to 1.
Japanese Patent Publication No. 59-15977 and Japanese Patent Publication No. 59-15, which present a seamless steel containing 0% and having excellent corrosion resistance.
No. 978, etc. are mentioned. These steels are conventional S
Compared to US410, SUS420J1, J2 steel, etc., C
Since the content is limited to a low level, the amount of precipitation of Cr carbide is small, and the amount of solid solution Cr effective for corrosion resistance can be secured, resulting in excellent corrosion resistance.

[0003]

However, in such a martensitic stainless steel, when the contents of C and N are remarkably reduced, δ ferrite which deteriorates the hot workability in the austenite matrix during the heating of the ingot. There is a drawback that phases are easily generated. Therefore, under severe working conditions such as seamless steel tube rolling, many hot working cracks and flaws are generated, and an increase in cost due to a decrease in yield is unavoidable. Further, in order to further improve the corrosion resistance, a steel containing a small amount of Cu as described in the above publication has been developed, but if the Cu content is high, a surface defect occurs during hot working. It was necessary to increase the cost because it required. Therefore, it has hitherto been considered difficult to produce a martensitic stainless steel seamless steel pipe having high corrosion resistance and less surface defects generated during hot working.

The present invention is a method for producing a high toughness martensitic stainless seamless steel pipe excellent in hot workability and corrosion resistance based on the findings as will be described later, which have been produced from various examination results from the present state of the art. Is provided.

[0005]

In order to solve the problems of the conventional martensitic stainless steel seamless steel pipes, the present inventors have found from many experimental results that CO ₂ corrosion resistance and sulfide stress corrosion resistance are high. Corrosion resistance such as cracking property is maintained by reducing C and adding necessary amount of Cr, Ni, Cu or further Mo, and hot workability reduces inclusion of P, S, etc. by reducing inclusions. It has been found that it can be obtained by performing a metallurgical operation such as controlling the phase fraction and shape of different phases having different deformation resistances by suppressing Ni and adding Ni.
Furthermore, it was also found that the generation of surface defects during hot working can be suppressed by limiting the addition amount of Cu.

In particular, the present inventors have paid attention to the effects of C and N and obtained the following findings. Figure 1 shows the base component
₂ shows the CO ₂ corrosion resistance property and the reduction value during hot working when the C and N contents were changed as a% Cu-4% Ni-15% Cr steel. In FIG. 1, C.I. R. Is 40 atm
The annual corrosion rate (CR) in artificial seawater at 180 ° C. in equilibrium with CO _{2 of} C. R. If it is <0.1 mm / y, it can be evaluated as having sufficient corrosion resistance. In addition, R.
A. Is a sample heated to 1250 ° C and strain rate is 3 at 900 ° C.
The draw ratio (R.S.) when uniaxially tensile deformed under the condition of sec ^-1 .
A.) and R.A. A. If it is> 70%, it can be said that the hot deformability is good. In addition, in the CO ₂ corrosion test, after hot working, quenching and tempering treatment was performed, and the yield strength was 720 MPa.
The one indicating the degree was used. From FIG. 1, it is necessary to set C <0.05% to satisfy the CO ₂ corrosion resistance,
Further, in order to have good hot workability, C + 0.8
It can be read that it is necessary to make N> 0.06 (the unit of the content of each element symbol is wt.%).

Next, as shown in FIG. 2, 18% of 4% Ni-15% Cr steel was used.
C. in a CO ₂ environment at 0 ° C. R. R. and 900 ° C. A. The influence of the amount of Cu on the is shown. When the amount of Cu added exceeds 0.2%, C.I. R. <0.1 mm / y, indicating that the corrosion resistance is sufficient. When the Cu content exceeds 1%, R. A. <70%. This is because the occurrence of surface defects such as baldness increases as the amount of Cu increases.

The present invention is constructed by combining the above-mentioned findings, and its gist is as follows. That is, in weight%, C ≦ 0.05%, Si
≦ 0.50%, Mn ≦ 1.0%, P ≦
0.03%, S ≦ 0.01%, Cr: 11
-17%, Ni: 1.5-5%, Cu: 0.2
~ 1%, Al ≤ 0.05%, N: 0.02
0.1% and satisfying C + 0.8N> 0.06, or further containing Mo: 0.5 to 2%, the balance of which is Fe and unavoidable impurities, hot working into a seamless steel pipe After cooling to a low temperature, Ac ₃ transformation point + 10 ° C to Ac
_It is heated to a temperature of ₃ transformation points + 200 ° C. and cooled to room temperature at a cooling rate of air cooling or higher, and subsequently tempered at a temperature of Ac ₁ transformation point or lower, or the above Ac ₃ transformation point +10
° C. to Ac ₃ After cooling transformation point + 200 ° C. temperature of the heating and air cooling or more, is heated to a temperature of Ac ₁ transformation point to Ac ₃ transformation point is cooled at least air-cooled to a low temperature rate, then, Ac ₁
This is a method for producing a martensitic stainless steel seamless steel pipe with excellent corrosion resistance, which is tempered at a temperature below the transformation point.

The present invention will be described in detail below. First, the reasons for limiting the steel components will be described. C is an element that forms Cr carbide and deteriorates the corrosion resistance, but as a typical austenite forming element, it is 90 in the hot working temperature range.
At 0 to 1250 ° C., it has an effect of suppressing the generation of δ ferrite phase. However, if the content exceeds 0.05%, a Cr-deficient layer is generated due to the precipitation of Cr carbide, etc.
O ₂ Corrosion is deteriorated. Further, grain boundary precipitation of many carbides significantly deteriorates sulfide stress corrosion cracking resistance. Therefore, in the present invention, in order to obtain excellent corrosion resistance such as CO ₂ corrosion resistance and sulfide stress corrosion cracking resistance, the C content should be 0.
It was set to 05% or less.

Si is added as a deoxidizing agent in steelmaking, and if it is contained in the steel in an amount of more than 0.50%, toughness and sulfide stress corrosion cracking resistance are deteriorated.
It was set to 0.50% or less. Mn is an element that forms inclusions and impairs resistance to sulfide stress corrosion cracking in a corrosive environment, but is added as a useful component for forming an austenite single phase. However, if added in excess of 1.0%, a large amount of inclusions are formed, and the sulfide stress corrosion cracking resistance and toughness in a corrosive environment are reduced. Therefore, the Mn content is set to 0.1% or less.

Since P segregates at the grain boundaries to weaken the grain boundary strength and deteriorates the hot workability and the resistance to sulfide stress corrosion cracking, P is set to 0.03% or less. Since S forms inclusions as sulfides and deteriorates hot workability and the like, it is preferably as small as possible like S element, and its upper limit was made 0.01%. Cr is required to be contained in an amount of 11% or more in order to impart the target CO ₂ corrosion resistance of the present invention and the required corrosion resistance as stainless steel. But 1
If added in excess of 7%, a ferrite phase is likely to be generated. Therefore, the Cr content range is set to 11 to 17%.

Ni has the effect of improving the corrosion resistance of Cr-containing steel. Further, Ni is a strong austenite forming element, and has the effects of suppressing the formation of the δ ferrite phase during high temperature heating, suppressing the growth of cracks formed inside the δ ferrite phase, and improving the hot workability. However, when N: 0.02%, Ni: 1.
Addition of less than 5% does not provide these effects, and 5
If added in excess of%, the Ac ₃ transformation point is lowered, so that a retained austenite phase is formed and the strength and toughness are impaired. Therefore, the range is set to 1.5 to 5%.

Cu has the effect of improving the CO ₂ corrosion resistance. Further, it is an austenite stabilizing element, and Ac
_{1 It} also has the advantage of not lowering the transformation point, but if its content is less than 0.2%, the effect of improving corrosion resistance is not sufficient, and if it exceeds 1%, it is concentrated by oxidation during heating and surface defects such as bald marks are formed. Since it is generated, the addition amount is 0.2 to
The range was limited to 1%. Moreover, since the above effect is small when Cu alone is added, it is necessary to add Cu in combination with Ni.

Similar to Si, Al is added as a deoxidizer and is contained. If it exceeds 0.05%, A
A large number of 1N are formed and the toughness is significantly reduced. Therefore, the upper limit of the amount added is set to 0.05%.

N is harmless to the corrosion resistance and is a typical austenite forming element like C, and has the effect of suppressing the formation of the ferrite phase in the hot working temperature range of 900 to 1250 ° C. The effect is Ni-C as described above.
r steel as a base component, C + 0.8N <0.06 (C and N are
wt. %) Is effective in the range of the content satisfying%). Therefore, in the case of C <0.05%, it is necessary to add 0.02% or more of N in order not to generate a ferrite phase in the hot working temperature range and to obtain good hot workability. In addition, since it is difficult to add 0.1% or more of N in a usual melting process, the range is set to 0.02 to 0.1%.

Mo is an element effective for improving the pitting corrosion resistance, and is added if necessary. However, the effect is small with the addition of 0.5% or less. Further, since it is a strong ferrite stabilizing element, if it is added in an amount exceeding 2%, the δ phase is likely to be generated.
%.

Next, the reasons for limiting the heat treatment conditions will be described. Steel pieces produced in various shapes such as square shape and round shape through the ordinary steel piece manufacturing process with the above-mentioned composition are the ordinary seamless steel pipes such as the Pramigul method, the Mannesmann plug mill method or the hot extrusion method. In the manufacturing process, the steel pipe is hot worked into a pipe shape and then cooled to a low temperature. The seamless steel pipe produced in this manner is coarse and exhibits a martensite structure in which a ferrite structure is partially formed, and satisfactory toughness cannot be obtained. Further, during cooling from a high temperature, Cr carbides and intermetallic compounds are precipitated at grain boundaries, and there is a problem that corrosion resistance, sulfide stress corrosion cracking and toughness are impaired. The present invention performs post-processing to solve these problems.

First, the seamless steel pipe produced by hot working is heated to a temperature range of Ac ₃ transformation point + 10 ° C. to Ac ₃ transformation point + 200 ° C., and then cooled at a speed higher than air cooling. The process by solution of Cr carbides and other precipitates precipitated in the grain boundaries, there is to recover the precipitated deterioration such as corrosion resistance and toughness due to such as Cr carbides, Ac ₃ transformation point + 1
At a low temperature of less than 0 ° C., a ferrite structure, a martensite structure, or a Cr carbide partially remains and a stable single austenite phase cannot be obtained, and the Ac ₃ transformation point +2.
At a high temperature exceeding 00 ° C., the crystal grains become coarse and satisfactory toughness cannot be obtained. Further, when cooling from that temperature, in order to prevent deterioration of corrosion resistance and toughness due to reprecipitation of Cr carbide and the like, cooling is performed to room temperature at a speed equal to or faster than air cooling.

The seamless steel pipe that has undergone such heat treatment exhibits a single-phase structure of martensite and has high strength.
Low toughness due to generation of large residual stress in martensite structure. In order to precipitate supersaturated solid solution carbon in the martensite structure, and to remove residual stress and recover toughness, heating and cooling to a temperature below the Ac ₁ transformation point that does not reverse transform to the austenite phase. A tempering process is performed. In particular, when a seamless steel pipe containing Cu in the composition as in the present invention is tempered, precipitation strengthening of Cu is suppressed, and toughness and sulfide stress corrosion cracking resistance are significantly improved.

Further, in order to improve the corrosion resistance and the toughness, which are the objectives of the present invention, the present invention further comprises a solution treatment of heating to a temperature range of Ac ₃ transformation point + 10 ° C. to Ac ₃ transformation point + 200 ° C., and Ac
₁ during the heating to a temperature below the transformation point cooling to tempering treatment, a heat treatment cooling with Ac ₁ transformation point to Ac ₃ 2-phase region temperature to the heated air faster than the transformation point. This intermediate treatment softens the high-strength martensite structure hardened by the solution treatment, and produces a seamless steel pipe having uniform toughness after the tempering treatment. In such a treatment method at a temperature deviating from the conditions of the present invention or at a slow cooling rate, a seamless steel pipe exhibiting excellent corrosion resistance and uniform toughness due to a locally existing high-strength martensite structure is produced. I can't.

According to the method of the present invention as described above, it is possible to produce a seamless steel pipe which has few surface defects due to hot working and is excellent in CO ₂ corrosion resistance, sulfide stress cracking resistance and toughness. it can.

[0022]

EXAMPLE Steels having the chemical composition shown in Table 1 were cast in a usual melting process and then hot rolled to produce steel pipes. First, the surface of the as-rolled steel pipe was observed to investigate the occurrence of flaws. Next, strength, toughness, CO ₂ corrosion resistance, and sulfide stress cracking resistance were investigated using a material that had been subjected to heat treatment and tempering treatment. Table 2 shows the 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 180 ° C. equilibrated with CO _{2 at} 40 atm. If the corrosion rate is 0.1 mm / y or less, it can be considered to have corrosion resistance. Resistance to sulfide stress cracking was measured by applying a round bar tensile test piece to a 5% NaCl solution at 25 ° C. under 1 atm of 99% CO ₂ + 1% H _2.
Applying uniaxial tensile stress in a corrosive environment saturated with S gas,
The ratio (Rs value) of the maximum initial stress and the yield stress at which breakage did not occur in 20 hours was determined. If Rs ≧ 0.8, it can be said that the characteristics are excellent. From the results shown in Table 2, the steel pipe produced by the method of the present invention has no surface defects during hot working and shows good CO ₂ corrosion resistance, sulfide stress cracking resistance and high toughness. It is clear that any of the properties is inferior in the comparative method outside the scope of the present invention.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

EFFECTS OF THE INVENTION According to the present invention, it is possible to produce a martensitic stainless steel seamless steel pipe having no surface defects during hot working and having excellent CO ₂ corrosion resistance, sulfide stress cracking resistance and high toughness. Became.

[Brief description of drawings]

FIG. 1 shows the effect of C and N on CO ₂ corrosion resistance.

FIG. 2 shows the influence of Cu-containing steel on the corrosion resistance and the draw ratio in a CO ₂ atmosphere.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location C22C 38/44

Claims (4)

[Claims] 1. By weight%, C ≦ 0.05%, Si ≦ 0.50%, Mn ≦ 1.0%, P ≦ 0.03%, S ≦ 0.01%, Cr: 11 to 17%, Cu: 0.2 to less than 1%, Ni: 1.5 to 5%, Al ≦ 0.05%, N: 0.02 to 0.1%, and a component satisfying C + 0.8N> 0.06 After the steel containing Al and the balance Fe and unavoidable impurities is hot worked into a seamless steel pipe and cooled to a low temperature, Ac ₃ transformation point + 10 ° C. to Ac ₃ transformation point +2
Excellent in hot workability and corrosion resistance, characterized by heating to a temperature of 00 ° C., then cooling to a low temperature at a cooling rate of air cooling or higher, and then tempering at a temperature of Ac ₁ transformation point or lower. High toughness martensitic stainless steel seamless steel pipe manufacturing method. 2. By weight%, C ≦ 0.05%, Si ≦ 0.50%, Mn ≦ 1.0%, P ≦ 0.03%, S ≦ 0.01%, Cr: 11 to 17%, Cu: 0.2 to less than 1%, Ni: 1.5 to 5%, Mo: 0.5 to 2%, Al ≦ 0.05%, N: 0.02 to 0.1%, and C + 0. Steel containing a component satisfying 8N> 0.06 and the balance being Fe and unavoidable impurities is hot worked into a seamless steel pipe and cooled to a low temperature, and then Ac ₃ transformation point + 10 ° C to Ac ₃ transformation Point +2
Excellent in hot workability and corrosion resistance, characterized by heating to a temperature of 00 ° C., then cooling to a low temperature at a cooling rate of air cooling or higher, and then tempering at a temperature of Ac ₁ transformation point or lower. High toughness martensitic stainless steel seamless steel pipe manufacturing method. 3. By weight%, C ≦ 0.05%, Si ≦ 0.50%, Mn ≦ 1.0%, P ≦ 0.03%, S ≦ 0.01%, Cr: 11 to 17%, Cu: 0.2 to less than 1%, Ni: 1.5 to 5%, Al ≦ 0.05%, N: 0.02 to 0.1%, and a component satisfying C + 0.8N> 0.06 After the steel containing Al and the balance Fe and unavoidable impurities is hot worked into a seamless steel pipe and cooled to a low temperature, Ac ₃ transformation point + 10 ° C. to Ac ₃ transformation point +2
After heating to a temperature of 00 ° C., cooling to a low temperature at a cooling rate of air cooling or higher, then heating to a temperature of Ac ₁ transformation point to Ac ₃ transformation point, and then cooling to a low temperature at a cooling rate of air cooling or higher. Then, a tempering process is then carried out at a temperature not higher than the Ac ₁ transformation point, which is a method for producing a high toughness martensitic stainless steel seamless steel pipe having excellent hot workability and corrosion resistance. 4. By weight%, C ≦ 0.05%, Si ≦ 0.50%, Mn ≦ 1.0%, P ≦ 0.03%, S ≦ 0.01%, Cr: 11 to 17%, Cu: 0.2 to less than 1%, Ni: 1.5 to 5%, Mo: 0.5 to 2%, Al ≦ 0.05%, N: 0.02 to 0.1%, and C + 0. Steel containing a component satisfying 8N> 0.06 and the balance being Fe and unavoidable impurities is hot worked into a seamless steel pipe and cooled to a low temperature, and then Ac ₃ transformation point + 10 ° C to Ac ₃ transformation Point +2
After heating to a temperature of 00 ° C., cooling to a low temperature at a cooling rate of air cooling or higher, heating to a temperature of Ac ₁ transformation point to Ac ₃ transformation point, and then cooling to a low temperature at a cooling rate of air cooling or higher. Then, a method of producing a high toughness martensitic stainless steel seamless steel pipe having excellent hot workability and corrosion resistance, which is characterized by performing tempering at a temperature not higher than the Ac ₁ transformation point.