JPS637328A - Production of steel having excellent sulfide corrosion cracking resistance - Google Patents

Abstract

PURPOSE:To produce a steel having excellent sulfide corrosion cracking resistance by heating a low-carbon steel specified in the contents of Mn, Nb, Al, N, Ca, etc. to a specific temp. region, then hardening the steel. CONSTITUTION:The steel consisting of 0.005-0.06wt% C, 0.05-0.8% Si, 0.5-2.2% Mn, <=0.020% P, <=0.004% S, 0.005-0.08% Nb, 0.01-0.085 Al, 0.002-0.01% N, 0.0005-0.01% Ca, and the balance Fe and unavoidable impurities is heated to (Ac3-30) deg.C-1,100 deg.C and is then hardened. A tempering treatment stage is omitted. >=1 Kinds among <=1.0% Cu, <=1.0% Ni, <=1.0% Cr, <=0.5% Mo, <=0.1% V, <=0.1% Ti, and <=0.003% B are properly incorporated into the steel consisting of the above-mentioned compsn. at need. The steel which is improved in both the hydrogen induced cracking resistance and the sulfide stress corrosion cracking resistance by the above-mentioned compsn. and heat treatment is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention The present invention relates to a method for producing steel with low ptl (3.4 to 3.8) containing H2S, which is resistant to sulfide corrosion cracking.
Hydrogen-induced cracking resistance of base metal and welded joints in the environment (
The present invention aims to provide a method for producing steel that can improve both the HIC resistance (hereinafter referred to as HIC resistance) and the sulfide stress corrosion cracking resistance (hereinafter referred to as SSC resistance).

Industrial applications Sour gas line pipes and their accessories (fittings, flanges,
Manufacturing technology for steel materials and pipes that require sulfide corrosion cracking resistance, such as bent pipes.

Conventional technology Line pipes and their auxiliary equipment or H for transporting so-called sour oil and sour gas containing hydrogen sulfide () [28]
Steel materials such as chemical plant piping that handle fluids containing 2S are required to have both HIC resistance and SSC resistance. In that case, the HIC resistance is usually NACE 5tanda.
H2S in artificial seawater specified in rd TM-02-84
A gas-saturated solution (pl+-5.0 to 5.2),
5% NaCA specified in NACE TMOI-77
A solution saturated with I-(2S gas) at +0.5% CHZCOOIL (pH-3, 4 to 3.8) is used for the evaluation.Especially for HIC resistance in low pl+ environments such as the latter.
As for the i-material, countermeasures have been taken such as (1) controlling the form of inclusions by reducing S content to an extremely low level and adding Ca, and (2) controlling the hardness of segregated parts by reducing Mn and P.

However, when the strength of the steel material is increased, the component nitrification in the segregated portion increases, and it may not necessarily satisfy the aged C resistance in a low PL+ environment. Also, SS resistance when stress is applied
C property, for example, in a constant load type SSC test according to the NACE-TM01-77 standard (a 6.35 mmφ round bar test piece was tested at 5%
NaCj! +0.5%C) 13cOO11+Saturation■1□
When evaluated by a test in which tensile stress is applied under a certain load in S liquid and the rupture time is determined at various stresses, the above conventional steel material has a critical stress of rupture (maximum stress without rupture for 720 hours).
is approximately 50 to 70%×yield stress (σy).

Low carbon bainitic steels have improved these properties. This lowers the C content of the steel material and reduces the resulting decrease in strength due to Mn.
, Nb, B, etc., to make the microstructure a uniform low-carbon henite structure, and it has excellent HIC resistance even though it has relatively high strength compared to normal C-low Mn type steel materials. can be obtained even in low pH environments. Recently, not only controlled rolling but also controlled cooling has been developed.
It has been published that excellent HIC resistance and SSC resistance of the base material can be obtained by combining also ng).

Problems that the invention aims to solve
The critical stress (σ) of TMOI-77) is 50 to 65
%σ, which is only equivalent to the SSC resistance level of conventional steel.

On the other hand, by making the microstructure into a uniform tempered martensitic structure through quenching and tempering (QT) treatment, HIC resistance is achieved.
It is known that QT treatment after welding improves the SSC resistance of joints, but industrially Q treatment and T treatment must be performed, and the treatment process It is difficult to say that it is necessarily suitable for mass production due to issues such as efficiency.

"Structure of the invention" Means for solving the problem 1, C: 0.005 to 0.06w or less, Si: 0
．． 05 to 0.8 h or less, Mn: 0.5 to 2.2 w or less, P: 0.020 w or less, S: 0.
004w or less, Nb: 0.005~0.08w
Below, Af: 0.01-0.08 L%, N: 0.0
02~0.01w or less, Ca: 0.0005~0.01
A method for producing steel with excellent sulfide corrosion cracking resistance, characterized by heating and quenching a steel containing: .

2. C2 0.005-0.06wt%,
Si: 0.05 ~ 0.8 or less! 1n: 0.5~
2.2w or less, P: 0.020w or less, S: 0.004w or less, Nb: 0.005
~0.08w or less, A E: 0.01~0.08i
n or less, N: 0.002 to 0.01rL910°Ca
:O,0O05~0.01w or less, Cu: 1.0wt% or less, Ni: 1.0wt% or less. Any one of O-1% or less, Cr: 1.0wt% or less, Mo: 0.5w or less, V: 0.1w or less, Ti: O, 1w or less, B: 0.003wt% or less (Ac330) ℃~110
A method for producing steel with excellent sulfide corrosion cracking resistance, which comprises heating to a temperature of 0°C and then quenching.

The effect is less than w (hereinafter simply referred to as %), C: 0.005% or more, Mn: 0.5% or more, and Ni s as appropriate.
Strength is ensured by containing Cr, Mo, and VSB.

C: 0.06% or less, Mn: 2.2% or less, P: 0.0
2% or less, S: 0.004% or less, Nb: 0.0
The HI resistance of the base metal and welded parts can be improved by setting the content to 8% or less.
Achieves both C properties as-quenched.

By setting C: 0.06% or less, S: 0.004% or less, and appropriately adding Ni to 1.0% or less and Mo to 0.5% or less, the SSC resistance of the base metal and welded part can be improved. It retains its properties even after being quenched.

Si: Q, 3Q% or less, Mn: 2.2% at 0.5% or more
Below, Nb: 0.08% or less, Aj! : 0.08% or less, N: 1001] pnl or less, and Cr added as appropriate is 1.0% or less, ■ is 0.1% or less, and Ti is 0.1%.
% or less and B is 0.003% or less to ensure toughness.

By containing Nb of o, oos% or more, the structure before quenching is refined, A1 is 0.01% or more, and N is 20pp.
By containing more than m, the effect of pinning crystal grain coarsening during quenching and heating can be obtained.

By setting AN to 0.08% or less and N to looppm or less, the occurrence of surface defects is suppressed, Ca to 5 ppm or more to control the shape of inclusions, and 1100 pp or less to suppress staining of the steel and prevent deterioration of sour resistance properties. To avoid.

Cu as an optional element is set to 1.0% or less to avoid deterioration of hot workability, and M is also an optional element.

Good weldability is achieved by setting 0.5% or less and 0.1% or less.

By setting the quenching temperature to (Ac330)"C or higher, the SSC resistance deterioration region that would occur if a quenching temperature below this temperature is adopted is avoided, and by setting the quenching temperature to 1100°C or lower, HIC resistance and SSC resistance are improved. Avoid deterioration of both.

As a result, favorable SSC and HIC resistance can be obtained in the as-quenched state.

Examples To further explain the present invention as described above, the present inventors have completed the present invention as a result of various intensive studies in view of the conventional circumstances as described above. To explain, below w (hereinafter simply referred to as %), C:
0.005-0.06%, Si: 0.05-0.
8%, Mn: 0.5-2.2%, P: 0.02
0% or less, S: 0.004% or less, Nb: o, o
o s ~ 0.08%, Aj! :0.01~0.08%
, N: 0.002-0.01%, Ca:O,0O0
5 to 0.01%, and if necessary Cu: 1.
0% or less, Ni: 1.0% or less, Cr: 1.0% or less,
Mo: 0.5% or less, V: 0.1% or less, Ti: 0
．． 1% or less, B: 0.003% or less, and the remainder is Fe and unavoidable impurities. Steel and steel joints (Ac330) are heated to a temperature of 1100°C to 1100°C and then quenched. This is a method for manufacturing steel and m-joints with excellent sulfide corrosion cracking resistance that eliminates tempering treatment.

That is, the main points of the present invention are as follows:
It's like ■.

(2) Select a low carbon steel, avoid forming martensite and lower bainite that are highly susceptible to hydrogen embrittlement even when quenched, and create a low carbon bainite structure or a mixed structure with ferrite.

■ (Act 30) “C (approximately 850 for this component)
Quenching (quenching) from a temperature range of 1100℃ (equivalent to "C") to 1100℃.

■ Omit the tempering process.

The explanation regarding (2) above is as follows.

That is, FIG. 1(A) shows xC-0.21~0.25%5i
-1,43~1.56%MnO,011~0.01
2%P-≦0.001%S-0.036~0.039%
Nb-0,028~0.039%AA-0,0028~
0゜0051%N O,0020~0.0028%C
A-type steel material (all plate T'J, 25 dragon) 950-10
After holding for 00"c I minutes and water cooling (average cooling rate of 25°C/S from 800 to 400°C), 5% NaC without tempering.
1+0.5%CH:l COOH→-kaibaowa II 2
Si content 2&, HIC test l- in the so-called NACE liquid.
The figure shows the relationship between the crack length ratio (CLR) and Cl. Regarding the crack length ratio (CL R), the crack length ratio (CL
1 (where Σ7!i: total length of cracks in 3 cross sections). Furthermore, after applying the same heat treatment to the same steel material as in Fig. 1, when a constant load SSC test based on NACETMOI-77 was conducted at 80% of the yield stress, the time to rupture was as shown in Fig. 2. It is. 1st
From FIG. 2, it can be seen that if the Cl is kept at 0.06% or less, the HIC resistance and, of course, the SSC resistance are thicker (improved) than in the conventional case even after quenching.

- When the amount of C increases, the structure shifts to a hardened structure, that is, bainite-martensite, and becomes a so-called hardened structure. These structures can be improved by tempering, but if left unquenched, the steel is highly susceptible to hydrogen embrittlement and deteriorates both HIC resistance and SSC resistance. Particularly in the segregated areas, the components are concentrated, so the degree of deterioration is large.

Further, as the C-deafness decreases, it becomes a so-called uniform low carbon bainite structure or a low carbon bainite + ferrite mixed structure, and the hardness decreases even as it is quenched, so it exhibits good HIC1 SSC resistance. Furthermore, by lowering C in the segregation area, not only the segregation area of Mn, P, etc. is reduced, but also the transformation diffusion during heating prevents the formation of a hardened structure, and the sulfide corrosion cracking resistance is improved.

These phenomena also apply to the heat shadow area of the weld. For example, the SSC limit stress of a joint after welding the inner and outer surfaces of low C steel using the submerged arc welding method is approximately 50 to 65%σ (NA
CE TMOI-77), but it breaks at HAZ by quenching without tempering.
The cyclostructure becomes a uniform low-carbon bainite (+ferrite) structure, improving SSC resistance (however, it is still important to use a low-C weld metal to prevent the weld metal from hardening during quenching. ).

Incidentally, Fig. 3 (A) shows the steel material used in Fig. 1.2 as SA.
After welding one layer of inner and outer surfaces with W (inner surface 53 KJ/cm, outer surface 56 KJ/c+++ welding wire and melting type flux such that the amount of C in the weld metal is about 0.05 W or less), welded at 950 to 1000 ° C. Hold for a minute, then cool with water (
NACE for joints with an average cooling rate of approximately 25°C/S)
Based on TMOI-77 <SSC test results are shown. Incidentally, the test piece sampling position of this joint is as shown in FIG. 3(B). The yield stress is unified as 80%σ, but the third
As shown in Figure (A), it can be seen that if the Cl content of the base material is 0.06 W or less, no breakage occurs even after 720 hours have elapsed. Also, if 1 is less than 0.062%, 5% NaCj!+
0.5%CHz COOH ten saturated H, S liquid (25℃X9
HIC did not occur at all even in the HIC test (6 hours).

From the above, if the Cl content of the base metal is reduced to 0.06% or less, good sulfide corrosion cracking resistance (H
It has been found that a base material or joint having IC and SSC properties can be obtained. Of course, it is essential to control the shape of inclusions by adding extremely low 5-Ca.

Furthermore, the above-mentioned item (2) will be explained as follows. That is, in FIG. 4, 0.037%C-0.20%Si
-,1,48%Mn-0,008%P-0.001%S
-0.29%Cu-0,11%NiO,041%
Nb-0.032%/'7! -0,0043%N steel (
The relationship between the heat treatment temperature and HIC and SSC resistance when quenched after rolling (t=20 mm) is shown. Heat treatment holding time is 1
cooling is water cooling (average cooling rate of about 28 °C/s from 800 to 400 °C). The test solution was also 5% Na
It is a low pl+ so-called N A CE liquid of Cl + 0.5% C) [3 COOH and 10 saturated H2S. In addition, the welding conditions for the joint include SAW welding of the inner and outer surfaces (46 K
J/cm, outer surface 49 K J 70m, ? The welding wire and molten basic flux were such that the C content of the weld metal was 0.07% and the Mn content was 1.52%.

From this figure 4, the HfC resistance is 11% due to the low C content.
It shows excellent performance with no cracks in a very wide range up to 00℃, while based on NACET MOI 77
In the SSC test (stress 80% σy), it can be seen that there is a region where the SSC resistance deteriorates when quenching is performed at a temperature below 860°C.

In addition, if quenching is performed at a high temperature exceeding 1100℃, H-resistant
Both IC properties and SSC resistance deteriorate. That is, the appropriate quenching temperature from the viewpoint of sulfide corrosion cracking resistance is 860 to 1100°C, considering the HIC and SSC resistance of the base material and joint.
It can be evaluated as follows. Around 860℃, this type of low C steel Ac,
It is also about 30°C lower than the point.

Furthermore, the finer the grains in the structure before quenching, the more the same grain size will be carried over into the structure after quenching, and the SSC resistance tends to improve. It is desirable to perform controlled rolling at a relatively low temperature (1000-1250°C), below the non-recrystallization temperature, with a reduction of 30% or more, and at a finishing temperature of 650-800°C.
It is not something that is intentionally forced. Of course, there is no problem even if controlled cooling is applied to this.

By the way, the quenching temperature is (Acz 30)'C to 1
Although the temperature was set at 100° C., the SSC characteristics deteriorate due to the coarsening of crystal grains at high temperatures, which are hardened by hardening. As a rough guideline, holding time at 1100°C - within 2 minutes; holding time at 1000°C - within 5 minutes; holding time at 900°C - within 20 minutes.

On the other hand, the cooling rate is not necessarily limited as long as a low carbon bainite or a mixed structure of bainite and ferrite can be obtained by quenching (depending on the I!μ component meter, quenching temperature, and holding time), but it is 800~ The average cooling rate at 400°C is 5-60°C/S.

Next, the limited range of essential components in the present invention will be described as follows.

C2 As already explained, if O exceeds 6%, the HIC resistance and SSC resistance deteriorate even though the steel is quenched.

- At least 0.005% is required from the viewpoint of ensuring strength.

Si: 0.05% or more is required even without deoxidizing, but if it exceeds 0.80%, the toughness will deteriorate rapidly.

Mn: If it is less than 0.5%, it will be difficult to obtain the low carbon heinite structure, and if Mn is less than 0.5%, it will be difficult to obtain the low carbon heinite structure.
The lower limit is 5%. If the content exceeds 2.2%, there is concern that toughness and HIC resistance will deteriorate.

P: In order to ensure HIC resistance and toughness in as-quenched condition, the upper limit is 0.020%.

S: O to ensure HIGH resistance, SSC resistance and ductility.
, OO4% is the upper limit.

Nb: Suppresses the coarsening of crystal grains as Nb (CN) during heating for quenching, and makes the quenched surface structure finer (effect of CR)
In order to achieve this, at least 5% of O, OO is required. Conversely, if it exceeds 0.08%, HIC resistance, toughness, etc. will deteriorate, so this is set as the upper limit.

A1:A7! N has a pinning effect on coarsening of crystal grains during heating during quenching, and at least 0.01% is required as a deoxidizing agent. However, taking into account the occurrence of surface flaws, deterioration of toughness, etc., the upper limit is set at 0.08%.

N: At least 20 ppm of N b (CN), A It N is required to achieve a pinning effect during heating, but if it exceeds 1100 ppm, surface flaws may occur or the toughness of the as-quenched joint may deteriorate. This gives rise to a tendency to

It is preferable to add Ca/S at a ratio of 1 to 5 depending on Ca:Si.If it is not added at least 5 ppm or more, it will not be effective in controlling the shape of inclusions, and if it exceeds 100 pp+w, it will cause calcium oxysulfide and calcium aluminum. The steel becomes contaminated with esters, etc., and its sour resistance properties deteriorate.

Furthermore, the limited range of optionally added elements in the steel of the present invention is as follows.

Cu: May be necessary to ensure HIC resistance when pH > 5, and has a small effect on as-quenched strength, but if it exceeds 1.0%, hot workability deteriorates. So 1
．． The upper limit is 0%.

Ni: Effective for strength and toughness, but tends to produce low-temperature transformation products and is harmful to SSC resistance, so the upper limit is set at 1.0%.

Cr: May be added to ensure strength, but care must be taken as exceeding 1.0% deteriorates toughness.

Gate o: Although it has a great effect on increasing the strength of as-quenched steel, it becomes hard, which is bad for SSC resistance and weldability.
The upper limit is set at 6.5%.

Although it may be added to ensure V2 strength, excessive addition causes deterioration of toughness and weldability, so the upper limit is set to 0.1%.

Ti: Similar to Nb, it has the effect of refining the structure before quenching, and when B is added, it plays the role of protecting B as TiN, but when added in excess, it deteriorates the toughness of the HAZ and the base material. Since it impairs toughness, the upper limit is set at 0.1%.

B: May be added to compensate for the decrease in strength in the extremely low C region. The amount added is also related to the amount of Ti, Al, and N, but
Roughly 0. OO is up to 3%, and if it exceeds that, the toughness will actually deteriorate.

When welding steel made of these components, it is important to select welding materials and welding conditions so that the Vickers hardness of the weld metal does not exceed 260, especially during post-quenching.

Specific manufacturing examples of products according to the present invention are shown below together with conventional methods and comparative examples.

Table 1 below shows the composition of steels produced by the method of the present invention and by the conventional method.

The following Table 2 summarizes the rolling conditions, welding conditions, and heat treatment conditions performed for each province as described above, and also shows the results of the base metals and joints. The HIC test and SSC test results for each are shown in Table 3. Note that the HIC resistance in Table 3 is 5% NaCA + 0.5% CH3CO0H +
25°C in a so-called NACE solution with a low pH of saturated Hz S.
CLR (crack length ratio) after X96hr test, -
The SSC resistance was evaluated based on the rupture time in a test based on the NACE TMOI-77 standard.

In other words, based on these results, it is clear that the manufactured example of the present invention has superior HIC1 SSC resistance compared to the conventional example. In the case of pipes and bent pipes, as shown in Table 3, not only the base metal parts exhibit excellent sulfide corrosion cracking resistance, but also the welded parts exhibit excellent sulfide corrosion cracking resistance. It is clear that it has

"Effects of the Invention" According to the present invention as explained above, both hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance in a low pHlIQ environment such as p113.4 to 3.8 containing H2S are improved. In particular, it is possible to appropriately improve the properties not only of the base material but also of the welded part and effectively improve the properties of steel pipes for sour gas lines and other parts,
This invention has great industrial effects.

[Brief explanation of the drawing]

The drawings show the technical content of the present invention, and Figure 1 is a diagram showing the influence of the amount of C on the H I C crack length ratio in as-quenched materials and how to determine the crack length ratio. Fig. 2 is a chart showing the influence of the amount of C on the rupture time of the SSC test on as-quenched joints, and Fig. 3 is the effect of the amount of C on the rupture time of the SSC test on the as-quenched joint after welding. Figure 4 is a diagram showing the influence of quenching temperature on sulfide corrosion cracking resistance. Note that in FIG. 4, . indicates a joint portion, and O indicates a base material. Patent applicant Nippon Kokan Co., Ltd. Inventor Kita 1) Yutaka
Car distance Kazu Ume
Boma Heito Tomo Ishihara Toshibe 1st / En C weighing Fushi'i1m L-C4 (%) 4th Entai fishing, )jL (Shun X/Heinu cold

Claims (1)

[Claims] (1) C: 0.005-0.06wt%, Si: 0.0
5 to 0.8 wt%, Mn: 0.5 to 2.2 wt%, P:
0.020wt% or less, S: 0.004wt% or less, N
b: 0.005-0.08wt%, Al: 0.01-0
．． 08wt%, N: 0.002-0.01wt%, Ca
:0.0005~0.01wt%, with the balance consisting of Fe and inevitable impurities (
Ac_3-30) A method for producing steel with excellent sulfide corrosion cracking resistance, which comprises heating to a temperature of 1100°C and then quenching. 2, C: 0.005-0.06wt%, Si: 0.05
~0.8wt%, Mn: 0.5-2.2wt%, P: 0
．． 020wt% or less, S: 0.004wt% or less, Nb
:0.005~0.08wt%, Al:0.01~0.
08wt%, N: 0.002-0.01wt%, Ca:
Contains 0.0005 to 0.01 wt%, Cu: 1.0 wt% or less, Ni: 1.0 wt% or less, C
r: 1.0wt% or less, Mo: 0.5wt% or less, V:
0.1wt% or less, Ti: 0.1wt% or less, B: 0.
Steel containing any one or more of 0.003wt% or less, with the remainder consisting of Fe and unavoidable impurities, is heated to (Ac_3-30)℃~11
A method for producing steel with excellent sulfide corrosion cracking resistance, which comprises heating to a temperature of 0.000C and then quenching.