JPH08260037A - Manufacturing method of martensitic stainless steel with excellent resistance to sulfide stress cracking, toughness and hot workability - Google Patents

Abstract

(57) [Summary] [Purpose] A method for producing martensitic stainless steel with excellent hot workability, sulfide stress cracking resistance and toughness. [Composition] wt.%, C: 0.1-0.3, Si: 0.01-1.0, Mn:
0.1 ~ 1.0, P ≤ 0.02, S ≤ 0.01, Cr: 11 ~ 14, Ni <0.0
5, containing N: 0.015-0.15, or Ca, Mg, REM
1 or more, including 0.001 to 0.3% each, γval. = 13C + 1
Steel that satisfies 1.5N-Cr ≥-10.86 and the balance is Fe and impurities is hot-worked, cooled to room temperature, heated to a temperature of 880 to 1050 ℃, and cooled to a cooling stop temperature of 600 to 350 ℃ at 2 ℃ / s
It is characterized in that after cooling at ec or higher and air cooling to room temperature, tempering treatment is performed at a temperature of Ac ₁ or lower. [Effect] By the method of the present invention, the market needs to be met in which a steel having a toughness and sulfide stress cracking resistance superior to those of conventional steels can be obtained with the chemical composition of conventional steels such as AISI420.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to martens which have sulfide stress cracking (SSC) resistance and toughness, which are extremely important properties particularly when used as an oil country tubular good, and are excellent in hot workability. The present invention relates to a method for manufacturing site stainless steel.

[0002]

2. Description of the Related Art Martensitic stainless steel represented by AISI type 420 steel has excellent corrosion resistance in a CO ₂ environment and is used in oil well pipes for the development of CO ₂ -containing oil and gas wells. However, in the environment where H ₂ S exists, SS
Due to its high sensitivity to C, its application has been limited to CO ₂ environments and has been avoided for oil and gas wells containing H ₂ S at 0.001 atm or higher partial pressure. H like this
Duplex stainless steel, which is more resistant to SSC, has been applied to the environment containing ₂ S.

However, since duplex stainless steel is expensive, there is an increasing need for a material that is as inexpensive as possible and has high SSC resistance while maintaining CO ₂ corrosion resistance. Further, with the recent deepening of wells, the need for higher strength is increasing, and expectations for steel having high strength and SSC resistance are also very high. Furthermore, these steels contain CO
_Since there are many oil wells in cold regions such as the North Sea and Alaska, many cases require toughness assurance.

Conventionally, in order to meet this need, AISI4
Several improved versions of 20 steel have been proposed. For example, in Japanese Examined Patent Publication No. 61-3391, AISI 42
By adding 0.05 to 0.5% (wt%, the same applies below) of Ni to 0 series steel, CO ₂ corrosion resistance and SS resistance are improved.
The aim is to improve the C property and to stabilize the austenite phase (hereinafter, γ phase) to improve the hot workability. On the other hand, in JP-A-60-116719, in order to improve the SSC resistance, Ni <0.
The limit is 1%. Further, in Japanese Patent Laid-Open No. 3-75308, the precipitation of grain boundary carbides is suppressed by accelerated cooling after normalizing to improve SSC resistance and toughness.

[0005]

However, in reality, it is clear that in an environment containing H ₂ S, the higher the Ni content, the more easily micro-cracks called “hair cracks” or “fissures” occur. It becomes, and it is special public Sho6
There is a problem that the environment in which the SSC resistance is improved by the addition of Ni, which is disclosed in Japanese Patent Laid-Open No. 1-3391, is limited only to the condition that the H ₂ S partial pressure is close to 0. Further, when the amount of Ni is reduced, the γ phase stabilizing ability is lowered in the hot rolling temperature range, and the δ-ferrite phase (hereinafter, referred to as the δ phase) is easily generated, but its influence is clear so far. It's not.

Further, JP-A-60-116719 discloses that the pitting resistance (the pitting in this case is considered to be close to what is expressed as hair cracking) is improved by limiting the amount of Ni. However, according to the examples and the like, the effect is confirmed only in the case of Ni ≧ 0.05%, and the effect in the range of Ni <0.05% is not clear. Here, rapid heat treatment to the austenite region is essential, but there is a problem that large-scale equipment such as an induction heating device is required.

Further, in Japanese Patent Laid-Open No. 3-75308, the SSC resistance and toughness are improved by suppressing the precipitation of grain boundary carbides, but in the material with high Ni, microcracks are generated and the SSC resistance is substantially Recently, it has become clear that it is bad. Further, Japanese Patent Laid-Open No. 6-1
In Japanese Patent No. 98611, Ni <0.05% prevents SSC.
Although it is said that the strength is improved, SSC resistance of high strength material
It has been found that the toughness is not sufficient and the low temperature toughness is reduced by reducing Ni.

The present invention is intended to solve the above-mentioned problems and is excellent in resistance to SSC and toughness which are problems when used as an oil country tubular good, and martensite having good hot workability. The purpose is to provide a system stainless steel.

[0009]

[Means for Solving the Problems] As a result of conducting research and development to achieve such an object, the present inventors
It has been found that in order to improve the SSC resistance of steel, it is sufficient to suppress the generation of microcracks in a sour environment. In particular, it has been clarified that limiting the amount of Ni is most effective in suppressing the occurrence of this minute crack.

FIG. 1 shows the effect of Ni content on SSC resistance. At this time, the SSC resistance was 10% at 1 atm of a round bar tensile test piece in a 5% NaCl solution at 25 ° C.
Uniaxial tensile stress is applied in a corrosive environment saturated with H ₂ S + 90% N ₂ gas, and the ratio of the maximum initial stress (σ _th ) and the yield stress (YS) at which no fracture occurs in 720 hours (Rs value = σ _th / Y
It was evaluated by S). As the materials, materials that have been heat-treated to a level of YS to 60 kgf / mm ² by normalization / tempering are used. If Rs ≧ 0.8, it can be said that the characteristics are excellent. From the figure, it can be seen that if the Ni content is less than 0.05%, the SSC resistance is good.

Since the fracture of the test piece is caused by microcracks, it is clear that the frequency of microcracks decreases when the Ni content is 0.05% or less. Furthermore, since all the test materials have been subjected to normal normalization / tempering treatment, Ni <0.05% can improve SSC resistance without performing rapid heating to the austenite region. Turned out to be possible.

Further, in the case of a high strength material, the generation of SSC may be a problem even if the generation of fissures is suppressed. At this time,
It has been clarified that SSC is likely to occur along the grain boundaries, and it has been found that the SSC resistance is lowered particularly when carbide is precipitated in a rod shape along the grain boundaries. Also,
It was clarified that the precipitation of this rod-shaped grain boundary carbide depends on the cooling rate during normalization. Fig. 2 shows type 420 steel with 980
The continuous cooling curve at the time of austenitizing at ℃ is shown.
From this result, it is clear that precipitation of grain boundary carbides can be suppressed if the cooling rate during normalization is 2 ° C./sec or more.

Further, it has been found that suppressing the precipitation of the rod-shaped grain boundary carbide improves the toughness. In particular, it has been found that it is possible to compensate the decrease in toughness due to the reduction of Ni by suppressing the grain boundary carbides.

Further, in martensitic stainless steels including type 420, sufficient hot workability is ensured and defects such as cracks and flaws do not occur even in a severe hot rolling process such as seamless steel pipe rolling. It was found that the structure at the time of hot working needs to be a γ single phase without containing a δ phase in order to be manufacturable. In the chemical composition of type 420 steel, C, N, Cr, and Ni have strong influence as additive elements on phase formation. Among these, since it is necessary to suppress the amount of Ni added from the viewpoint of SSC resistance, a phase diagram is prepared using a material in which the amounts of C, N, and Cr added in the Nifree component system are changed, and the hot working temperature is set. When the phases in the area were examined, they were arranged as shown in FIG. From the figure, it is necessary to satisfy γval. = 13C + 11.5N-Cr ≥ -10.86 (each element symbol is% by weight) in order to suppress the formation of δ phase in the hot working temperature range and make it a γ single phase. It is clear that there is. Therefore, in the present martensitic stainless steel, when selecting a material for rolling a seamless steel pipe, it is necessary to use a component system that satisfies this formula.

The present invention is based on these findings, and the summary thereof is as follows. Ie% by weight
And C: 0.1 to 0.3%, Si: 0.01
~ 1.0%, Mn: 0.1-1.0%, P ≤
0.02%, S ≦ 0.01%, Cr:
11-14%, Ni <0.05%, N
: 0.015 to 0.1%, and if necessary, one, two or more of Ca, Mg and REM, respectively.
001-0.3%, γval. = (13C + 11.5N-
Cr) ≧ −10.86 (each element symbol is% by weight) and the balance is steel with iron and inevitable impurities, hot working, cooling to room temperature, heating to 880 to 1050 ° C. and 2 ° C./sec or more. Sulfide stress cracking resistance, characterized in that it is cooled to a cooling stop temperature of 600 to 350 ° C. at a cooling rate of ₁ , then cooled to room temperature at a rate of air cooling or higher, and then tempered at a temperature of Ac ₁ or lower. , A method of producing martensitic stainless steel having excellent toughness and hot workability.

The present invention will be described in detail below. First, the reasons for limiting the components will be described. C: C is a strong austenite stabilizing element, and
If it is less than 1%, the δ phase is precipitated during hot working and surface defects are likely to occur. On the other hand, if it exceeds 0.3%, a large amount of Cr carbide precipitates during heat treatment, and the amount of solid solution Cr effective for corrosion resistance decreases. From this viewpoint, 0.1 to 0.3 wt. I decided to limit it to%.

Si: Si is a deoxidizing agent remaining during steelmaking, and its effect is insufficient with addition of less than 0.01%. Further, since it is a strong ferrite stabilizing element, and if added in excess of 1%, a δ phase which is harmful to hot workability is generated, so its upper limit was made 1%. Mn: Mn is a residual deoxidizing agent for steelmaking similar to Si and has an effect of fixing S which adversely affects hot workability, and needs to be added in an amount of 0.1% or more. on the other hand,
Addition in excess of 1.0% is detrimental to SSC resistance.
Therefore, the optimum range is 0.1 to 1.0%.

P: P is not an element to be added intentionally, but it has a bad effect on hot workability and a bad effect on SSC resistance, so that a low level is desirable. However,
Since the content of 0.02% or less is a level that causes no practical problem, the upper limit was made 0.02%. S: S is not an element to be added intentionally, and it has a bad effect on hot workability and corrosion resistance, and therefore its low level is desirable. However, at 0.01% or less, this effect is so small that it can be ignored. Therefore, 0.01% is the upper limit for practical use.

Cr: Cr is an essential element for corrosion resistance, but its effect is small if it is added in an amount of 11% or less. Further, it is a strong ferrite stabilizing element, and if added in excess of 14%, a δ phase is formed and hot workability is deteriorated. From this viewpoint, 11 to 14% was set as an appropriate range. Ni: Ni is not an element intentionally added in the present steel. If the content exceeds 0.05%, it becomes easy to form fine cracks in the sour environment as described above, and the SSC resistance decreases. Therefore, the upper limit is set to less than 0.05%.

N: N is an element which, like Ni, suppresses the formation of the δ phase during hot working, reduces the occurrence of flaws during hot working, and is effective in suppressing general corrosion in a CO ₂ environment.
The effect is not sufficient if less than 0.01% is added. Further, if the content exceeds 0.1%, casting defects such as internal bubbles are likely to occur, and the hot workability may be significantly reduced. Therefore, the proper addition range of N is 0.01 to
It was set to 0.1%.

The above-mentioned component conditions are satisfied, and γva
Steel satisfying l. = (13C + 11.5N-Cr) ≧ -10.86 (each element symbol is% by weight) has sufficient hot workability, but if further excellent workability is required, Ca, Mg, and REM may be added to this. Ca, Mg, R
All of EM suppress the deterioration of hot workability due to S. If less than 0.001%, the effect is not exhibited, and if added over 0.3%, the effect is saturated. 001-0.3% was made into the suitable addition range.

Next, the reasons for limiting the heat treatment conditions will be described. First, the above steel is melted, subjected to hot working such as seamless steel pipe rolling, and then subjected to normalizing treatment. The heating temperature at this time is set to a temperature at which the carbide does not completely form a solid solution and the crystal grains do not coarsen in the austenite phase stable temperature range of the Cr-containing stainless steel, and the temperature at which the carbide does not aggregate and coarsen. The lower limit was set. That is, 1050
When heated to a temperature higher than ℃, the carbide completely dissolves into the solid solution, so that a large amount of Cr carbide and the like precipitates at the grain boundaries during cooling,
Corrosion resistance and SSC resistance are significantly deteriorated. Also 88
When heated to a low temperature of less than 0 ° C., a large amount of coarse Cr carbide remains in the crystal grains, which increases the corrosion rate. Therefore, the heating temperature for normalizing treatment is 880 to 10
It was set to 50 ° C.

If the cooling rate during this normalizing treatment is slow, carbides are precipitated in the grain boundaries in the form of plates, and the SSC resistance and toughness are markedly reduced, so it is necessary to increase the cooling rate. As shown in FIG. 2, the condition should be 2 ° C./sec or more, preferably 15 ° C./sec or more, which does not cut the precipitation nose of grain boundary carbide. The cooling stop temperature must be 600 ° C. or lower, which is lower than the grain boundary carbide precipitation temperature, and 350 ° C. or higher, which is the martensitic transformation point. If accelerated cooling is performed to such a temperature range and then cooled at an air-cooling temperature rate, it is possible to prevent quench cracking due to martensitic transformation.

When cooled to room temperature in this way, martensitic transformation occurs to form a martensitic single-phase structure. The residual stress in this martensitic structure is eliminated by recovery, and supersaturated carbon atoms are precipitated as carbides to enhance toughness and ductility, and tempering treatment is performed to obtain desired strength. At this time, in order to withstand SSC characteristics occurs reverse transformation when heated to a temperature above Ac ₁ transformation point martensite to austenite is remarkably reduced, the tempering treatment is carried out at Ac ₁ transformation point or above the temperature. The martensitic stainless steel produced by the method of the present invention as described above has good hot workability and SSC resistance superior to that of conventional steel.
Toughness and toughness.

[0025]

EXAMPLES The present invention will be further described based on examples.
After casting the steel with the chemical composition (wt.%) Shown in Table 1 in the usual melting process, a steel pipe was manufactured by hot rolling, and the heat treatment and tempering treatment were applied to the steel pipe, CO ₂
The corrosion characteristics and SSC resistance characteristics were investigated. Table 2 shows the presence or absence of surface cracking during hot working, heat treatment conditions, cooling rate and cooling stop temperature during normalization, and material properties. The toughness is the absorbed energy of a Charpy impact test (vE _-20 ; at -20 ° C) when a test piece with a 2 mm V notch is used.
J). CO ₂ corrosion resistance is 40 atm of CO
₂ Corrosion rate in artificial seawater at 120 ° C (C.
R. ; Mm / y) If the corrosion rate is 0.1 mm / y or less, it can be said to have corrosion resistance. SSC resistance was measured by applying a round bar tensile test piece to a 1% atmosphere at 25 ° C in a 5% NaCl solution.
Uniaxial tensile stress is applied in a corrosive environment saturated with 0% H ₂ S + 90% N ₂ gas, and the ratio of the maximum initial stress (σ _th ) and the yield stress (YS) (Rs value = σth) at which no fracture occurs in 720 hours.
/ YS) was calculated. If Rs ≧ 0.8, it can be evaluated that the characteristics are excellent.

From the results shown in Table 2, in the steel produced by the method of the present invention, cracks do not occur during hot working, and good CO ₂ corrosion resistance, SSC resistance and toughness are exhibited. It is clear that any of the characteristics is inferior in the comparative method out of the range.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

As described above, according to the present invention, SSC resistance can be improved.
Of the martensitic stainless steel having excellent hot workability and excellent workability and toughness as compared with the conventional type 420 steel.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of Ni content on SSC resistance.

FIG. 2 is a diagram showing a continuous cooling curve in type 420 steel.

FIG. 3 is a diagram showing an influence of C, N, and Cr contents on phases in a hot working temperature range (900 to 1250 ° C.).

Claims (2)

[Claims] 1. By weight%, C: 0.1 to 0.3%, Si: 0.01 to 1.0%, Mn: 0.1 to 1.0%, P ≤ 0.02%, S ≦ 0.01%, Cr: 11 to 14%, Ni <0.05%, N: 0.015 to 0.1% inclusive, γval. = (13C + 11.5N-Cr) ≧ −10.86 (each element symbol Of steel and the balance of iron and inevitable impurities are hot-worked, cooled to room temperature, heated to 880 to 1050 ° C., and cooled to 600 to 350 ° C. at a cooling rate of 2 ° C./sec or more. Sulphide stress cracking resistance, toughness and hot working, characterized by cooling to a cooling stop temperature in between, then cooling to room temperature at a rate of air cooling or higher, and then tempering at a temperature of Ac ₁ transformation point or lower. A method for producing martensitic stainless steel with excellent properties. 2. In% by weight, C: 0.1 to 0.3%, Si: 0.01 to 1.0%, Mn: 0.1 to 1.0%, P ≤ 0.02%, S ≦ 0.01%, Cr: 11 to 14%, Ni <0.05%, N: 0.015 to 0.1% inclusive, and Ca, M
g, REM 1 type or 2 types or more 0.001-
Steel containing 0.3%, satisfying γval. = (13C + 11.5N-Cr) ≧ -10.86 (each element symbol is% by weight) and the balance being iron and inevitable impurities is hot worked to room temperature. It is cooled, heated to 880 to 1050 ° C., cooled to a cooling stop temperature of 600 to 350 ° C. at a cooling rate of 2 ° C./sec or more, and cooled to room temperature at a rate of air cooling or more, and then Ac ₁ transformation A method for producing martensitic stainless steel excellent in sulfide stress cracking resistance, toughness, and hot workability, which is characterized by performing tempering at a temperature below the point.