JPH08269545A - Production of steel plate for mo-added ultrahigh strength steel tube excellent in toughness in weld zone - Google Patents

Abstract

PURPOSE: To produce a steel plate for Mo-added ultrahigh strength steel tube excellent in toughness in weld zone by applying treatments of reheating, rolling, etc., to a steel slab of specific composition under respectively specified conditions. CONSTITUTION: A steel slab, having a composition which consists of, by weight, 0.05-0.10% C, <=0.6% Si, 1.8-2.5% Mn, <=0.015% P, <=0.001% S, 0.1-1.0% Ni, 0.35-0.60% Mo, 0.01-0.10% Nb, 0.005-0.030% Ti, <=0.004% Al, 0.001-0.006% N, <=0.003% O, and the balance iron with inevitable impurities and in which the value of P, defined by equation, is regulated to 1.9-2.8, is reheated to 950-1200 deg.C. Subsequently, rolling is done so that cumulative rolling reduction at <=900 deg.C, cumulative rolling reduction in the ferrite-austenite two-phase region between the Ar3 and the Ar1 point, and rolling finishing temp. are >=70%, 15-35%, and 680-820 deg.C, respectively. Further, the resulting rolled plate is cooled at >=10 deg.C/sec cooling rate down to an arbitrary temp. of <=400 deg.C and then tempered at 400-650 deg.C.

Description

Detailed Description of the Invention

[0001]

This invention relates to the American Petroleum Institute (API)
Ultra high strength of X120 or more (tensile strength of about 950 N / mm ² or more) as standard and excellent low temperature toughness of base material and welded part,
The present invention relates to a method for manufacturing a steel sheet having on-site weldability.

[0002]

2. Description of the Related Art Line pipes used in pipelines for long-distance transportation of crude oil and natural gas are (1) to improve transportation efficiency by increasing pressure and (2) to improve welding efficiency in the field by reducing wall thickness. There is a tendency for the tensile strength to become higher and higher. Until now, line pipes up to X80 according to the API standard have been put into practical use, but there has recently been a need for line pipes with higher strength.

Currently, ultrahigh strength line pipes of X100 or more are manufactured by the method of manufacturing X80 class line pipe (NKK Technical Report No. 1).
38 (1992), pp24-31 and The 7t.
hOffshore Mechanics and A
rctic Engineering (1988), V
Olume V, pp179-185), but this is at most X100 (yield strength 9
89 N / mm ² or more, tensile strength 760 N / mm ² or more) The production of line pipes is considered to be the limit. The ultra-high-strength pipeline has many problems such as the balance between strength and low temperature toughness, weld heat affected zone (HAZ) toughness, on-site weldability, and softening of joints. There is a demand for early development of high strength line pipes (X100 or more).

[0004]

SUMMARY OF THE INVENTION According to the present invention, tensile strength of 950 N / mm ² or more (API standard X10
It provides the manufacturing technology of ultra high strength steel pipe for steel pipe.

[0005]

SUMMARY OF THE INVENTION The gist of the present invention is the weight%
C: 0.05 to 0.10%, Si: 0.6% or less,
Mn: 1.8 to 2.5%, P: 0.015% or less, S:
0.001% or less, Ni: 0.1 to 1.0%, Mo:
0.35 to 0.60%, Nb: 0.01 to 0.10%,
Ti: 0.005-0.030%, Al: 0.004%
Below, N: 0.001 to 0.006%, O: 0.003
% Or less, and if necessary, V: 0.01 to 0.1
0%, Cu: 0.1 to 0.7%, Cr: 0.1 to 0.8
%, Ca: 0.001 to 0.005% of one kind or two or more kinds, and the balance consisting of iron and inevitable impurities, and P = 2.7C + 0.4Si + Mn + 0.8Cr.
+0.45 (Ni + Cu) + Mo + V-1 is 1.9 ≦ P
After reheating a steel piece satisfying ≦ 2.8 to a temperature of 950 to 1200 ° C., a cumulative reduction amount of 900 ° C. or less of 70% or more and Ar ₃ point to Ar ₁ point of ferrite austenite 2
The cumulative rolling reduction in the phase region is 15 to 35% and the rolling end temperature is 68.
Rolling is performed at 0 to 820 ° C, and then 10 ° C /
It is to cool to an arbitrary temperature of 400 ° C. or lower at a cooling rate of 2 seconds or more, and to perform tempering treatment at a temperature of 400 to 650 ° C.

The method of manufacturing the steel sheet for ultra high strength steel pipe of the present invention will be described in detail below. The features of the present invention are (1)
Low C-Ni-Nb-Mo-T containing substantially no Al
(2) Strictly controlled rolling of the i-type steel in the austenite-ferrite two-phase region is followed by accelerated cooling, which results in ultra-high strength and excellent base material and low-temperature toughness of the weld zone, and local weldability. Achieved at the same time.

Conventionally, low carbon-high Mn-Nb-Mo steel is well known as a steel for line pipes having a fine acicular ferrite structure, but the upper limit of its tensile strength is 750 MPa at the most. It was In order to further increase the strength, it is necessary to (1) increase the amount of C and the amount of alloying elements, and (2) quench-temper from a high temperature of 900 ° C. or higher. The low temperature toughness is insufficient.

First, regarding the low temperature toughness of the base material, in the pipeline, the propagation stopping characteristics as well as the brittle fracture occurrence characteristics are extremely important. In order to improve the propagation stopping property of brittle fracture, (1) miniaturize the microstructure, (2) use the separation generated on the fracture surface of the test piece such as the so-called Charpy impact test (separation is the impact test It is considered that the propagation deteriorating property of the brittle crack is improved by reducing the triaxial stress level at the brittle crack tip by a delamination phenomenon that occurs at times parallel to the plate surface).

Next, regarding the low temperature toughness of the weld zone, the weld heat affected zone (HAZ) toughness of the low alloy steel is as follows: (1) grain size, (2) high carbon island martensite (M ^* ), It is governed by various metallurgical factors such as the dispersed state of a hardened phase such as upper bainite (Bu), (3) presence or absence of grain boundary embrittlement, and (4) microsegregation of elements. Especially, it is known that the size of the crystal grains and the M ^* of HAZ have a great influence on the low temperature toughness. In the present invention, by reducing the amount of Al in the steel, the amount of M ^* produced in the HAZ is suppressed and the HAZ toughness is improved by finely dispersing it.

In particular, the higher the strength, the more the amount of alloying elements added, and it becomes difficult to completely suppress the production of M ^{* in} the HAZ. However, even in this case, M ^* is finely dispersed if it does not substantially contain Al, and HA
Z toughness is improved. When Al is added, since Al does not form a solid solution with carbides, γ in untransformed austenite
Is stabilized and the production of M ^* becomes remarkable.

First, the reasons for limiting the manufacturing conditions of the present invention will be described. In the present invention, after reheating the billet to a temperature range of 950 to 1200 ° C., the cumulative rolling reduction of 900 ° C. or less is 70%.
Above, rolling is carried out so that the cumulative rolling reduction in the ferrite-austenite two-phase region of Ar ₃ point to Ar ₁ point is 15 to 35% and the rolling end temperature is 680 to 820 ° C., and thereafter 10 ° C./sec or more. It is cooled to an arbitrary temperature of 400 ° C. or lower at a cooling rate, and is aged at 400 to 650 ° C.

The reason why the reheating temperature of the steel slab (slab) is set to 950 ° C. or higher is that the steel slab having a coarse casting structure is sufficiently solution-treated in the austenite region to secure the rolling end temperature. However, if the reheating temperature exceeds 1200 ° C., austenite grains grow during reheating and the crystal grains after rolling also increase, resulting in deterioration of low temperature toughness and sour resistance. Therefore, the upper limit of the reheating temperature is 1200 ° C.

The reheated steel slab has a cumulative reduction of 900% or less of 70% or more, a cumulative reduction of 15 to 35% in the ferrite-austenite two-phase region of Ar ₃ point to Ar ₁ point, and a rolling end temperature. It must be rolled to 680-820 ° C. The reason why the cumulative reduction amount at 900 ° C. or less is 70% or more is to strengthen rolling in the austenite unrecrystallized region and to refine the austenite structure before transformation. Since the X100 line pipe requires higher toughness than ever before, especially for safety, its cumulative reduction amount must be 70% (the larger the cumulative reduction amount is, the more preferable it is, and the upper limit is not limited).

Further, in the present invention, the cumulative reduction amount in the ferrite-austenite two-phase region is set to 15 to 35%, and the rolling end temperature is set to 680 to 820 ° C. This is because the austenite structure finely grained in the unrecrystallized austenite region is further refined, and the ferrite is processed to strengthen the ferrite and facilitate the occurrence of separation during the impact test. When the cumulative rolling reduction in the two-phase region is 15% or less, the separation is not sufficiently generated and the propagation stopping property of the brittle crack cannot be improved.

If the cumulative reduction is 35% or more, the embrittlement of ferrite due to working becomes remarkable and the low temperature toughness deteriorates. Therefore, the range of the cumulative reduction amount in the two-phase region is set to 15 to 35%. On the other hand, even if the cumulative reduction amount is appropriate, excellent low temperature toughness cannot be achieved if the rolling temperature is inappropriate. When the rolling end temperature is 680 ° C. or lower, not only the effect of accelerated cooling due to the progress of ferrite transformation disappears but also the embrittlement of ferrite due to working becomes remarkable,
The lower limit of the rolling end temperature was 680 ° C. However, if the rolling end temperature is 820 ° C. or higher, refinement of the austenite structure and occurrence of separation are not sufficient, so the upper limit of the rolling end temperature is limited to 820 ° C.

After the completion of rolling, the steel sheet needs to be cooled at a cooling rate of 10 ° C./sec or more to an arbitrary temperature of 400 ° C. or less. This is transformation strengthening due to the formation of bainite structure,
This is for achieving a finer structure. When the cooling rate is 10 ° C./sec or less and the water cooling stop temperature is 400 ° C. or more, improvement in strength / low temperature toughness balance due to transformation strengthening cannot be expected sufficiently. The higher the cooling rate, the more effective the transformation strengthening is, and the upper limit is not particularly limited, but the practically feasible cooling rate is about 40 ° C./sec although it depends on the plate thickness.

Further, the steel sheet after rolling and cooling is 400 to 65
It is necessary to temper at a temperature of 0 ° C. When the tempering treatment temperature is 400 ° C. or lower, the low temperature toughness becomes insufficient, and when the tempering treatment temperature is 650 ° C. or higher, the strength decreases.

Next, the reasons for limiting the constituent elements will be described. The lower limit of 0.05% of C is the minimum amount for ensuring the strength and low temperature toughness of the base material and the welded portion, and exerting the precipitation hardening effect by the addition of Nb and V and the refinement of crystal grains. However, if the C content is too large, the low temperature toughness, the field weldability and the sour resistance are significantly deteriorated, so the upper limit was made 0.10%.
Si is an element to be added for deoxidation and strength improvement, but if added in a large amount, it deteriorates the field weldability and HAZ toughness, so the upper limit was made 0.6%. Deoxidation of steel is Ti or A
Only 1 is sufficient, and Si does not necessarily have to be added.

Mn is an essential element for securing strength and low temperature toughness, and its lower limit is 1.8%. But M
If n is too large, the hardenability of steel will increase and the local weldability, H
The upper limit was set to 2.5% because it not only deteriorates the AZ toughness but also promotes the center segregation of the continuously cast steel pieces, and also deteriorates the sour resistance and the low temperature toughness.

The purpose of adding Ni is to improve the strength of the low carbon steel of the present invention without deteriorating the low temperature toughness and field weldability. Compared with Mn, Cr, and Mo additions, it was found that Ni addition causes less formation of a hardened structure detrimental to low temperature toughness and sour resistance in the rolling structure (especially the central segregation zone of the slab) and increases strength. did. However, if the amount of addition is too large, not only economy but also local weldability and H
Since the AZ toughness is deteriorated, the lower limit is set to 0.1% and the upper limit is set to 1.0%. Ni is also effective in preventing Cu cracks during continuous casting and hot rolling. In this case, it is necessary to add Ni to 1/3 or more of the Cu amount.

The reason for adding Mo is to improve the hardenability of steel. Further, Mo coexists with Nb to strongly suppress recrystallization of austenite during controlled rolling, and is also effective for refining the austenite structure. To obtain such an effect, Mo must be at least 0.35%. However, excessive addition of Mo deteriorates HAZ toughness and field weldability, so the upper limit was made 0.60%.

In the steel of the present invention, N is an essential element.
b: 0.01 to 0.10%, Ti: 0.005 to 0.0
Contains 30%. Nb contributes to refinement of crystal grains and precipitation hardening in controlled rolling, and has an action of strengthening steel. However, if Nb is added in an amount of 0.10% or more, on-site weldability and HAZ toughness are adversely affected, so the upper limit was made 0.06% and the lower limit was made 0.01%. When Ti is added, fine TiN is formed, and when the slab is reheated and when welding HA
It suppresses coarsening of the austenite grains of Z to make the microstructure finer and improves the low temperature toughness of the base material and HAZ. In order to bring out such an effect of TiN, at least 0.
005% Ti addition is required. However, if the amount of Ti is too large, coarsening of TiN and precipitation hardening due to TiC occur and the low temperature toughness deteriorates, so the upper limit is 0.030%.
Must be limited to.

Although Al is usually contained in steel as a deoxidizer,
It is an element that is not preferred in the present invention. Al amount is 0.00
If it exceeds 4%, the amount of M ^* produced in HAZ becomes remarkable,
Since the HAZ toughness is deteriorated, the upper limit was made 0.004%. Deoxidation is also possible with Ti or Si, and it is not always necessary to add it.

Further, in the present invention, P, which is an impurity element,
S and O contents are 0.015% or less and 0.001%, respectively
Hereinafter, it will be 0.003% or less. The main reason for this is to further improve the low temperature toughness of the base material and HAZ toughness. Reduction of the amount of P reduces center segregation of the continuously cast slab, prevents intergranular fracture, and improves low temperature toughness. Further, the reduction of the amount of S has the effect of reducing the stretched MnS and improving the sour resistance and ductility. The reduction of the amount of O reduces the oxides in the steel and is effective in improving the sour resistance and the low temperature toughness. Therefore, the lower the amount of P, S, O, the more preferable.

N forms TiN and suppresses coarsening of austenite grains in the slab reheating and in the welded HAZ and improves the low temperature toughness of the base metal and HAZ. The minimum amount required for this is 0.001%. However, if it is too large, it may cause deterioration of the HAZ toughness due to slab surface defects and solid solution N, so the upper limit must be suppressed to 0.006%.

Next, the reason for adding V, Cu, Cr and Ca will be explained. The main purpose of adding these elements to the basic composition is to increase the manufacturable plate thickness and improve the properties such as strength and toughness of the base metal without impairing the excellent characteristics of the steel of the present invention. is there. Therefore, the amount added is of a nature that should be limited by itself.

V has almost the same effect as Nb, but its effect is much weaker than that of Nb. The lower limit is 0.0
1%, the upper limit is 0.10 from the viewpoint of local weldability and HAZ toughness.
Permissible up to%. Cu has almost the same effect as Ni, and also has an effect of improving the corrosion resistance and the hydrogen-induced cracking resistance. Also, the strength is significantly increased by Cu precipitation hardening. However, if added excessively, the base metal, H
Since the toughness of AZ decreases and Cu cracks occur during hot rolling, the upper limit was set to 0.7%.

[0028] Cr increases the strength of the base material and the welded portion, but if it is too much, it deteriorates the field weldability and HAZ toughness remarkably. Therefore, the upper limit of the amount of Cr is 0.8%, preferably 0.6%. The lower limit of 0.1% of the amount of Cu and Cr is the minimum amount at which the effect on the material due to the addition of each element becomes remarkable. Ca controls the morphology of sulfide (MnS) and improves low temperature toughness (such as increase of absorbed energy in Charpy test). In particular, in the steel of the present invention utilizing the separation in the impact test, the absorbed energy in the Charpy test and the like tends to decrease, so that the addition of Ca is essential. However, if the amount of Ca is less than 0.001%, there is no practical effect, and if it exceeds 0.005%, CaO-C is added.
A large amount of aS is generated to form clusters and large inclusions,
Not only does it impair the cleanliness of steel, but it also adversely affects the field weldability. Therefore, the amount of Ca added is 0.001 to 0.0.
Limited to 05%. In the case of ultra-high strength steel, the S and O contents were reduced to 0.001% and 0.002% or less, respectively.
It is particularly effective to set P = (Ca) [1-124 (O)] / 125 (S) to 0.5 ≦ ESSP ≦ 10.0.

In addition to the above limitation of the individual additive elements, in the present invention, P = 2.7C + 0.4Si + Mn + 0.
8Cr + 0.45 (Ni + Cu) + Mo + V-1
It is limited to 9 ≦ P ≦ 2.8. This is to achieve the desired balance between strength and low temperature toughness without impairing HAZ toughness and field weldability. The lower limit of the P value is set to 1.9 in order to obtain a strength of 950 N / mm ² or more and excellent low temperature toughness. The upper limit of the P value is set to 2.8 in order to maintain excellent HAZ toughness and field weldability.

[0030]

EXAMPLES Steel sheets were manufactured by various manufacturing methods from billets having various steel components by a converter-continuous casting method, and various properties were investigated. The mechanical properties were investigated in the direction perpendicular to rolling. HAZ toughness was investigated by applying a simulated heat cycle equivalent to a heat input of 5 kJ / mm. Examples are shown in Table 1. The steel sheet produced according to the present invention has excellent strength and low temperature toughness. On the other hand, the comparative steel is not suitable in terms of chemical composition or steel plate manufacturing conditions, and either characteristic is inferior.

Steel 9 has an excessively large amount of C, so that it has poor low temperature toughness (Charpy absorbed energy, transition temperature) and HAZ toughness, and has a high preheating temperature during welding. Since the amount of Mn added and the P value of Steel 10 are too high, the base metal and HAZ toughness are poor, and the preheating temperature during welding is also extremely high. Steel 11 is Nb
Since it is not added, the strength is slightly lower than that of Nb-added steel, the Charpy transition temperature is high (the strength / low temperature toughness balance is poor), and the HAZ toughness is also poor.

Since Steel 12 contains no Ti, it has a high Charpy transition temperature and is inferior in HAZ toughness. Steel 13 requires preheating at the time of welding because it contains too much Mo. Steel 14 does not achieve the target strength because the Mo content is too small. Steel 15 has too much Al, so HA
Z toughness is poor. Steel 16 has an appropriate chemical composition, but has a high Charpy transition temperature because the reheating start temperature of the billet in the manufacturing conditions is too high. Since the reheating temperature of the steel slab is too low, steel 17 is insufficient in solution treatment and has low strength.

Steel 18 has too little cumulative reduction below 900 ° C., so low temperature toughness is a step ahead. Steel 19 has a high Charpy transition temperature because the cumulative rolling reduction in the austenite-ferrite two-phase region is too small. Steel 20 has an excessively large amount of rolling reduction in the two-phase region, so that the low temperature toughness is rather deteriorated. Steel 21 is not rolled in the two-phase region and the rolling end temperature is too high, so the low temperature toughness is poor. Steel 22 has an inferior low temperature toughness because the rolling end temperature is too low. Steel 23 has low strength because the water cooling stop temperature is too high. Steel 24 has a low strength because the tempering temperature is too high. Steel 25 has a low tempering temperature, so the low temperature toughness is poor.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

Industrial Applicability According to the present invention, it becomes possible to stably manufacture a steel plate of an ultra high strength line pipe (API standard X100 or more) excellent in low temperature toughness and field weldability. As a result, the safety of the pipeline was significantly improved, and the construction efficiency and transportation efficiency of the pipeline were dramatically improved.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication location C22C 38/58 C22C 38/58 (72) Inventor Takuya Hara 20-1 Shintomi, Futtsu-shi Nippon Steel Technology Development Co., Ltd.

Claims (2)

[Claims] 1. By weight%, C: 0.05 to 0.10%, Si: 0.6% or less, Mn: 1.8 to 2.5%, P: 0.015% or less, S: 0 0.001% or less, Ni: 0.1 to 1.0%, Mo: 0.35 to 0.60%, Nb: 0.01 to 0.10%, Ti: 0.005 to 0.030%, Al : 0.004% or less, N: 0.001 to 0.006%, O: 0.003% or less, the balance consisting of iron and unavoidable impurities, and the P value defined by the following formula is 1 ．
After reheating a steel slab in the range of 9 to 2.8 to a temperature of 950 to 1200 ° C, a ferrite / austenite two-phase region in which the cumulative reduction amount of 900 ° C or less is 70% or more and Ar ₃ point to Ar ₁ point Rolling is performed so that the cumulative reduction amount of 15 to 35% and the rolling end temperature become 680 to 820 ° C., and then 1
Of a steel plate for Mo-added ultra-high strength steel pipe having excellent weld toughness, characterized by cooling to an arbitrary temperature of 400 ° C. or lower at a cooling rate of 0 ° C./second or more and tempering at a temperature of 400 to 650 ° C. Production method. P = 2.7C + 0.4Si + Mn + 0.8Cr + 0.45 (Ni +
Cu) + Mo + V-1 2. By weight%, C: 0.05 to 0.10%, Si: 0.6% or less, Mn: 1.8 to 2.5%, P: 0.015% or less, S: 0 0.001% or less, Ni: 0.1 to 1.0%, Mo: 0.35 to 0.60%, Nb: 0.01 to 0.10%, Ti: 0.005 to 0.030%, Al : 0.004% or less, N: 0.001 to 0.006%, O: 0.003% or less, if necessary, V: 0.01 to 0.10%, Cu: 0.1 to 0. 7%, Cr: 0.1 to 0.8%, Ca: 0.001 to 0.005%, containing one or more kinds, and the balance consisting of iron and inevitable impurities, defined by the following formula. After reheating a steel slab having a P value in the range of 1.9 to 2.8 to a temperature of 950 to 1200 ° C, 900 ° C
The following cumulative reduction amount is 70% or more, and Ar ₃ points to Ar ₁
Rolling is performed so that the cumulative reduction amount of the ferrite-austenite two-phase region at the point is 15 to 35% and the rolling end temperature is 680 to 820 ° C, and then at any cooling rate of 400 ° C or less at a cooling rate of 10 ° C / sec or more. Cool to temperature, 400-650
A method for producing a steel sheet for ultrahigh-strength Mo-added steel pipe with excellent weld toughness, characterized by performing tempering at a temperature of ° C. P = 2.7C + 0.4Si + Mn + 0.8Cr + 0.45 (Ni +
Cu) + Mo + V-1