JPH0860238A - Manufacturing method of martensitic stainless steel with excellent hot workability and sulfide stress cracking resistance - Google Patents

Abstract

(57) [Abstract] [Purpose] Manufacturing method of martensitic stainless steel with excellent hot workability and sulfide stress cracking resistance [Constitution] 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 <0.05%, N: 0.01
It contains 5 to 0.1%, or 0.001 to 0.3% each of Ca, Mg and REM, and A = 13C (%) + 11.5N (%)-
Steel with Cr (%) ≥-10.86 and balance Fe and impurities is hot-worked, cooled to room temperature, heated to 880 to 1050 ℃ and cooled to room temperature at a higher speed than air-cooling.
Tempering is performed at a temperature of ₁ or less. That is, the present invention is SSC resistant
Ni ≦ 0.05% is required to improve the workability, and the formula A is required to ensure good hot workability. [Effect] According to the present invention, the market needs that a sour martensitic stainless steel that does not contain Ni or Mo and is inexpensive can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a martensite system having excellent sulfide stress cracking (SSC) resistance, which is a very important characteristic especially when used as an oil country tubular good, and excellent in hot workability. The present invention relates to a method for manufacturing stainless steel.

[0002]

2. Description of the Related Art Martensitic stainless steel represented by AlSl type 420 steel is used for oil well pipes for developing CO ₂ -containing oil and gas wells because it has excellent corrosion resistance in a CO ₂ environment. 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
_For duplex environments containing ₂ S, high duplex duplex stainless steel with better SSC resistance has been applied. 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.

Conventionally, to meet this need, AlS14
Several improved versions of 20 steel have been proposed. For example, in JP-B-61-3391, AlSl
For 420 series steel, 0.05 to 0.5 wt. % Ni to improve the CO ₂ corrosion resistance and SSC resistance, and further to stabilize the austenite phase (hereinafter referred to as γ phase) to improve the hot workability.
On the other hand, in JP-A-60-116719, S resistance
In order to improve the SC property, Ni ≦ 0.1 wt. %, And the necessity of microstructure refinement by rapid heating to the austenite region is stated.

[0004]

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. Has become
By adding Ni as shown in Japanese Examined Patent Publication No. 61-3391, S-resistant
There is a problem that the environment in which the SC property is improved is limited only to the condition that the H ₂ S partial pressure is close to zero. 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. It has not been revealed.

Further, Japanese Patent Application Laid-Open No. 60-116719 describes that the pitting property (pitting in this case: considered to be close to what is expressed as hair cracking) is improved by limiting the amount of Ni. However, according to Examples and the like, the effect is confirmed only when Ni ≧ 0.05, and the effect in the range of Ni <0.05% is not clear. Further, although rapid heat treatment to the austenite region is essential here, there is a problem that large-scale equipment such as an induction heating device is required. The present invention is intended to solve the above problems, and provides a martensitic stainless steel having excellent resistance to SSC which is a problem when used as an oil country tubular good and having good hot workability. With the goal.

[0006]

As a result of research and development for achieving the above object, the present inventors have found that in order to improve the SSC resistance of type 420 steel, it is necessary to suppress the generation of microcracks in a sour environment. I found it good. In particular, it has been found that limiting the amount of Ni is effective in suppressing the occurrence of this minute crack.

FIG. 1 shows the effect of Ni content on SSC resistance. The SSC resistance at this time is 25 for the round bar tensile test piece.
10% H ₂ S + 90 at 1 atm in 5% NaCl solution at ℃
Maximum initial stress (σ _th ) at which uniaxial tensile stress is applied in a corrosive environment saturated with% N ₂ gas and no fracture occurs in 720 hours
And the yield stress (YS) ratio (Rs value = σ _th / YS). If Rs ≧ 0.8, it can be said that the characteristics are excellent. From the figure, if the Ni content is less than 0.05%, SS resistance
It can be seen that the C property is good. Most of the fractured materials were caused by microcracks, which revealed that the frequency of microcracks decreased when the Ni content was 0.05% or less. Further, it has been found that by setting Ni <0.05%, it becomes possible to improve the SSC resistance without performing rapid heating to the austenite region.

Further, in order to ensure sufficient hot workability and to enable production without causing defects such as cracks and flaws even in a severe hot rolling process such as seamless steel pipe rolling, hot working is required. It was found that the structure at the time of processing needs to be a γ single phase without including a δ phase.

In the AlSl420 type steel composition, C, N, Cr and Ni have a strong influence as additive elements on the phase formation. Of these, SS resistant
Since it is necessary to suppress the amount of Ni added from the viewpoint of C property, N
A phase diagram was prepared using materials in which the addition amounts of C, N, and Cr were changed in the i free component system, and the phases in the hot working temperature range were examined. The results were arranged as shown in FIG. Figure 2
Therefore, in order to suppress the formation of the δ phase in the hot working temperature range and make it the γ single phase, (13 × Cwt%) + (11.5 × Nwt%) −
It is clear that it is necessary to satisfy Crwt% ≧ -10.86. Therefore, in the present martensitic stainless steel, when selecting a material for a seamless steel pipe, it is necessary to use a component system that satisfies the above formula.

The present invention is based on these findings, and the summary thereof is as follows. Ie% by weight
As C: 0.1 to 0.3%, Si: 0.0
1 to 1.0%, Mn: 0.1 to 1.0%, P ≤
0.02%, S ≦ 0.01%, Cr: 1
1 to 14%, Ni <0.05%, N:
0.015 to 0.1%, and if necessary, C
a, Mg, REM, one or more of 0.001 to 0.3% are contained, and 13 · C% +11.5 · N% -Cr% ≥ -10.86 is satisfied, and the balance Of steel consisting of iron and unavoidable impurities is hot-worked, cooled to room temperature, heated to 880 to 1050 ° C., cooled to room temperature at a rate of air cooling or higher, and then tempered at a temperature of Ac ₁ or lower. This is a method for producing martensitic stainless steel having excellent hot workability and sulfide stress cracking resistance.

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
1 wt. If it is less than%, the δ phase is likely to precipitate during hot working and surface defects are likely to occur. On the other hand, 0.3 wt. If it exceeds 0.1%, 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. %.

Si: Si is a deoxidizing agent remaining during steelmaking, and its effect is insufficient if added in an amount of 0.01% or less. 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%.

[0013] P: P is not an element to be added intentionally, but it has an adverse effect on hot workability and also has an adverse effect on SSC resistance, so 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, if it is less than 0.01%, this effect is so small that it can be ignored.
Will be the upper limit.

Cr: Cr is an element essential 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 it is contained in an amount of 0.05% or more, it becomes easy to form minute cracks in the sour environment as described above, and SS resistance
C property is lowered. Therefore, its content is 0.05%
Less than

N: N is an element effective in suppressing the formation of the δ phase during hot working, reducing the occurrence of flaws during hot working, and suppressing general corrosion in a CO ₂ environment. The effect is not sufficient if less than 0.015% is added. Also, 0.
If the content exceeds 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 0.015 to 0.
It was set to 1%.

The above-mentioned component conditions are satisfied, and 13
・ C (%) + 11.5 ・ N (%)-Cr (%) ≧ -1
Steel satisfying 0.86 (each element is contained by weight%) has sufficient hot workability, but if further workability is required, one of Ca, Mg and REM is added to this. Alternatively, two or more kinds may be added. Ca, Mg, and REM all suppress deterioration of hot workability due to S, and if each is less than 0.001%, the effect is not exhibited, and if added in excess of 0.3%, the effect is saturated. Therefore, 0.001 to 0.3% is set as an appropriate addition range. The C, N, and Cr content conditions have been described above, but as is clear from FIG. 2, the formation of the δ phase is suppressed in the hot working temperature range,
13 · C% +11.5 · N% -Cr% ≧ for single phase
It is necessary to satisfy -10.86.

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 the upper limit of the 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, when heated to a temperature of 1050 ° C. or higher, the carbide completely dissolves,
During cooling, a large amount of Cr carbide and the like precipitates at the grain boundaries, and the corrosion resistance and SSC resistance are significantly deteriorated. Further, when heated to a low temperature of 880 ° C. or less, a large amount of coarse Cr carbide remains in the crystal grains, so that the corrosion rate increases.
Therefore, the heating temperature for normalizing treatment is 880 to 1050 ° C.
And If the cooling temperature after this heating is lower than the air cooling rate, carbides precipitate in the grain boundary in a plate shape, and the SSC resistance and toughness are remarkably reduced. Therefore, the cooling rate is limited to the air cooling or higher.

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, if heating is performed at a temperature exceeding the Ac ₁ transformation point, reverse transformation from martensite to austenite occurs and the SSC resistance is significantly deteriorated. Therefore, tempering is performed at a temperature not higher than the Ac ₁ transformation point. The martensitic stainless steel produced by the method of the present invention as described above has good hot workability and SS resistance superior to that of conventional steel.
It has C property.

[0019]

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 a normal melting process, a steel pipe was manufactured by hot rolling, and the heat treatment and tempering treatment were used to obtain strength and resistance. CO ₂
The corrosion characteristics and SSC resistance characteristics were investigated. Table 2 shows the occurrence of surface cracks during hot working, heat treatment conditions and material properties.
Shown in

The CO ₂ corrosion resistance was evaluated by the corrosion rate in artificial seawater at 120 ° C. equilibrated with CO _{2 at} 40 atm. If the corrosion rate is 0.1 mm / y or less, it can be evaluated as having corrosion resistance. SSC resistance of the round bar tensile test piece is 5% at 25 ° C.
A uniaxial tensile stress was applied in a NaCl solution in a corrosive environment saturated with 10% H ₂ S + 90% N ₂ gas at 1 atm to obtain 720
The ratio (Rs value = σ _th / YS) between the maximum initial stress (σ _th ) and the yield stress (YS) at which breakage does not occur with time was determined. Rs ≧
It can be said that 0.8 is an excellent characteristic.

[0021]

[Table 1]

[0022]

[Table 2]

From the results shown in Table 2, the steel produced by the method of the present invention does not cause cracks during hot working and exhibits good CO ₂ corrosion resistance and SSC resistance, whereas the range of the present invention It is clear that any of the comparative examples deviated from each other had inferior properties.

[0024]

As described above, according to the present invention, SSC resistance can be improved.
The martensitic stainless steel is superior in properties to conventional AlSl420 type steels and has good hot workability.

[Brief description of drawings]

FIG. 1 shows the effect of Ni content on SSC resistance.

FIG. 2 shows the influence of C, N, and Cr contents on the phases in the 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% are included, and 13 · C% + 11.5 · N% −Cr% ≧ −10. .86 and a balance of iron and inevitable impurities is hot-worked, cooled to room temperature, heated to 880 to 1050 ° C., cooled to room temperature at a rate of air cooling or higher, and then a temperature of Ac ₁ or lower. A method for producing a martensitic stainless steel excellent in hot workability and sulfide stress cracking resistance, which is characterized by performing a tempering treatment at. 2. C: 0.1 to 0.3% by weight, 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%, and one or more of Ca, Mg and REM, respectively. Steel containing 001 to 0.3% and satisfying 13 · C% + 11.5 · N% -Cr% ≧ -10.86 with the balance consisting of iron and unavoidable impurities is hot-worked to room temperature. Excellent in hot workability and sulfide stress cracking resistance, which is characterized by cooling to 80 ° C. to 1050 ° C., cooling to room temperature at a rate of air cooling or more, and then tempering at a temperature of Ac ₁ or less. Martensitic stainless steel manufacturing method.