JPH08209287A - Steel for high strength line pipe having low yield ratio and excellent in low temperature toughness - Google Patents

Abstract

PURPOSE: To produce a steel for a high strength line pipe having high tensile strength and low yield ratio and excellent in low temp. toughness and field weldability by forming the microstructure of a steel having a specified componental compsn. contg. Ni, Cu, Mo, Nb, Ti or the like into a specified one. CONSTITUTION: The compsn. of a steel for a line pipe is constituted of the one contg., by weight, 0.05 to 0.10% C, <=0.6% Si, 1.7 to 2.5% Mn, <=0.015% P, <=0.003% S, 0.1 to 1.0% Ni, 0.8 to 1.2% Cu, 0.35 to 0.50% Mo, 0.01 to 0.10% Nb, 0.005 to 0.030% Ti, <=0.06% Al and 0.001 to 0.006% N, furthermore contg., at need, prescribed amounts of V, Cr and Ca, and the balance iron with inevitable impurities, and in which the value of P shown by the formula is regulated to 2.0 to 3.5. Moreover, its microstructure is formed of martensite, bainite and ferrite, and in which the ferrite fractional rate is regulated to 20 to 90%, the rate of the worked ferrite in the ferrite is regulated to 50 to 100%, and the average grain size of the ferrite is regulated to <=5μm.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to an ultra-high strength steel having a tensile strength (TS) of 950 MPa or more and excellent low temperature toughness and weldability, including various line pipes for transporting natural gas and crude oil. It can be widely used as a welding steel material for pressure vessels and industrial machinery.

[0002]

2. Description of the Related Art In recent years, line pipes used in pipelines for transporting crude oil and natural gas over long distances have been developed in order to improve transport efficiency by increasing the pressure and reduce the outer diameter and weight of the line pipes, thereby improving local construction efficiency. The strength tends to be higher and higher. Up to now, a line ha pipe up to X80 (tensile strength of 620 MPa or more) has been put into practical use according to the American Petroleum Institute (API) standard, but there is a growing need for a line pipe with higher strength.

At present, research on a method for manufacturing an ultra-high-strength line pipe is conducted by a conventional X80 line pipe manufacturing technique (for example, N
KK Technical Report No.138 (1992), pp24-31, and The 7th Offs
horeMechanics and Arctic Engineering (1988), Volum
e V, pp179-185), but the production of X100 (tensile strength 760 MPa or more) line pipe is considered to be the limit. The ultra-high strength of pipelines has many problems such as balance between strength and low temperature toughness, weld heat affected zone (HAZ) toughness, field weldability, and softening of joints. There is a demand for early development of strength line pipes (X100 or more).

[0004]

[Problems to be Solved by the Invention] The present invention has an excellent balance between strength and low temperature toughness, and is suitable for ultrahigh strength and low yield ratio line pipes with a tensile strength of 950 MPa or more (API standard X100 or more) that facilitates field welding. It provides steel.

[0005]

[Means for Solving the Problems] The present inventors have identified the chemical composition (composition) of a steel material for obtaining an ultra-high strength steel having a tensile strength of 950 MPa or more and excellent low temperature toughness and field weldability. Through intensive research on the microstructure, a new ultra-high-strength welding steel was invented.

That is, the gist of the present invention is as follows: (1) wt%, C: 0.05 to 0.10%, S
i: 0.6% or less, Mn: 1.7 to 2.5%,
P: 0.015% or less, S: 0.003% or less,
Ni: 0.1-1.0%, Cu: 0.8-1.2
%, Mo: 0.35 to 0.50%, Nb: 0.
01-0.10%, Ti: 0.005-0.030
%, Al: 0.06% or less, N: 0.00
1 to 0.006%, and if necessary, V: 0.1
0% or less, Cr: 0.6% or less, Ca: 0.
001-0.006% of 1 type or 2 or more types,
The balance consists of iron and unavoidable impurities, and the P value defined by the following formula is in the range of 2.0 or more and 3.5 or less, and its microstructure consists of martensite, bainite and ferrite, and the ferrite fraction Is 20-90%
In addition, 50 to 100 processed ferrite in ferrite
%, And an average grain size of ferrite is 5 μm or less, a high strength line pipe steel excellent in low temperature toughness having a low yield ratio, and P = 2.7C + 0.4Si + Mn + 0.8Cr + 0.4
5 (Ni + Cu) + Mo + V-1 (2) A steel for high strength line pipe having a low yield ratio and excellent in low temperature toughness, which is characterized by being tempered at a temperature of Ac ₁ or lower.

The contents of the present invention will be described in detail below. The feature of the present invention is that it is a low carbon / high Mn system to which Ni—Cu—Mo—Nb—a trace amount of Ti is added in combination, and the ferrite whose microstructure is fine (the average grain size is 5 μm or less, a certain amount or more). (Including processed ferrite of), martensite, and bainite.

Conventionally, low carbon-high Mn-Nb-Cu steel has been known as a steel for line pipes having a fine acicular ferrite structure, but the upper limit of its tensile strength is at most 750MP. . Fine ferrite including processed ferrite and martensite in this basic component system,
There is no high-strength line pief steel with bainite mixed hard-soft microstructure. This is because it was thought that the tensile strength of 950 MPa or more was not possible at all in the martensite / bainite two-phase mixed structure, and the low temperature toughness and the field weldability were insufficient.

However, the present inventors have found that even in Nb-Cu steel, ultrahigh strength and excellent low temperature toughness can be achieved by strictly controlling the chemical composition and microstructure. In other words, the features of the steel of the present invention are that it has excellent ultra-high strength and low temperature toughness without tempering, has a lower yield ratio compared to quenching and tempering, and has excellent steel pipe formability and low temperature toughness (Charpy transition temperature). Remarkably excellent, etc. (In the steel of the present invention, even if the yield strength is low in the state of the steel sheet, the yield strength is increased by the steel pipe forming, and the target yield strength can be obtained) .

First, the microstructure of the steel of the present invention will be described. In order to achieve an ultrahigh strength of 950 MPa or more in tensile strength, it is necessary to make the microstructure of steel a certain amount or more of martensite bainite, and for that purpose, the ferrite fraction is 20 to 90% (martensite. The bainite fraction should be 10-80%). When the ferrite fraction exceeds 90%, the martensite / bainite fraction becomes too small and the desired strength cannot be achieved (the ferrite fraction also depends on the C content, and the C content is 0.05% or more. Then, it is difficult to substantially set the ferrite content to 90% or more). In the steel of the present invention, the most desirable ferrite fraction in terms of strength and low temperature toughness is 30 to 80%. However, ferrite is originally soft, and even if the ferrite fraction is 20 to 90%, if the proportion of worked ferrite is too small, the desired strength (especially yield strength) and low temperature toughness cannot be achieved. Therefore, the proportion of processed ferrite is set to 50 to 100%. The processing (rolling) of ferrite enhances the yield strength of ferrite by strengthening dislocations and subgrains, and at the same time, is extremely effective in improving the Charpy transition temperature as described later.

However, even if the kind of microstructure is limited as described above, it is not sufficient to achieve excellent low temperature toughness.
For this purpose, the separation by introducing processed ferrite is used, and the average ferrite grain size is 5 μm.
It is necessary to miniaturize below. Even in ultra high strength steel,
It was found that the introduction of processed ferrite causes separation on the fracture surface such as in the Charpy impact test, resulting in a dramatic decrease in the fracture transition temperature (separation is a delamination phenomenon that occurs on the fracture surface such as in the Charpy impact test. , It is believed that the triaxial stress level at the brittle crack tip is reduced and the brittle crack propagation arresting property is improved).

Further, it was found that by setting the average ferrite grain size to 5 μm or less, the martensite / bainite structure other than ferrite can be simultaneously refined, and the transition temperature can be remarkably improved and the yield strength can be increased. .

From the above, ferrite and martensite of Nb-Cu steel, which was conventionally considered to have poor low temperature toughness,
We have succeeded in greatly improving the balance of strength and low temperature toughness of bainite hard-soft mixed structure.

However, even if the microstructure of steel is strictly controlled as described above, a steel material having desired characteristics cannot be obtained. For this purpose, it is necessary to limit the chemical composition as well as the microstructure. The reasons for limiting the constituent elements will be described below.

The C content is limited to 0.05 to 0.10%.
Carbon is extremely effective in improving the strength of steel, and at least 0.05% is necessary to obtain the target strength in the ferrite and martensite-bainite hard-soft mixed microstructure. Further, this amount is also the minimum amount for achieving the precipitation effect due to the addition of Nb and V, the effect of refining the crystal grains, and ensuring the weld strength. However, if the amount of C is too large, the low temperature toughness of the base metal and HAZ and the field weldability are significantly deteriorated, so the upper limit was made 0.10%.

Si is an element added for deoxidation and improvement of strength, but if added in a large amount, HAZ toughness and field weldability are significantly deteriorated, so the upper limit was made 0.6%. Deoxidation of steel is sufficiently possible with Al or Ti, and Si is not necessarily added.

Mn is an essential element for ensuring the excellent balance of strength and low temperature toughness by making the microstructure of the steel of the present invention a fine ferrite and martensite / bainite hard-soft mixed structure, and its lower limit is 1.7. %. However, if the Mn content is too large, not only the hardenability of the steel is increased to deteriorate the HAZ toughness and field weldability, but also the center segregation of the continuously cast steel piece is promoted and the low temperature toughness of the base material is also deteriorated. It was set to 2.2%. Desirable Mn amount is 1.9-
It is 2.1%.

The purpose of adding Ni is to improve the low carbon steel of the present invention without deteriorating the low temperature toughness and field weldability. Compared to Mn, Cr, and Mo additions, addition of Ni is less likely to form a hardened structure detrimental to low temperature toughness in the rolled structure (especially the central segregation zone of continuously cast steel slab), and is 0.1% or more. It was found that the addition of a trace amount of Ni is also effective in improving the HAZ toughness. From the viewpoint of HAZ toughness, the particularly effective amount of Ni added is 0.3% or more. However, if the addition amount is too large, not only economic efficiency but also HAZ toughness and field weldability are deteriorated, so the upper limit was made 1.0%. Further, addition of Ni is also effective for preventing Cu cracking during continuous casting and hot rolling. In this case, Ni is Cu
It is necessary to add 1/3 or more of the amount.

Cu significantly increases the strength by hardening and precipitation strengthening of the martensite / bainite phase in the mixed structure of ferrite and martensite / bainite hard / soft. If it is less than 0.8%, the effect is not sufficient. On the other hand, if it is added excessively, the toughness of the base material and HAZ decreases due to precipitation hardening, and Cu cracking occurs during hot working, so the upper limit is 1.2%. did.

The reason for adding Mo is to improve the hardenability of steel and to obtain the desired hard-soft mixed structure. Further, Mo coexists with Nb to suppress recrystallization of austenite during controlled rolling, and is also effective for refining the austenite structure. In order to obtain such effects, Mo must be at least 0.35%. However, if excessive Mo is added, H
Since it deteriorates AZ toughness and field weldability, its upper limit was made 0.5%.

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 coexists with Mo and not only suppresses recrystallization of austenite during controlled rolling to refine the structure, but also contributes to precipitation hardening and increase in hardenability and strengthens steel. If it is less than 0.01%, the effect is not sufficient. On the other hand, if the amount of Nb added is too large, it adversely affects the HAZ toughness and field weldability, so the upper limit was made 0.10%.

On the other hand, addition of Ti forms fine TiN,
When the slab is reheated and the austenite grains of the HAZ are suppressed from becoming coarse, the microstructure is refined, and the base metal and HAZ
Improve the low temperature toughness of. Further, when the amount of Al is small (for example, 0.005% or less), Ti forms an oxide and HA
Acts as an intragranular ferrite formation nucleus in Z, and HAZ
It also has the effect of refining the structure. In order to bring out such an effect of TiN, it is necessary to add at least 0.005% Ti. 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. Therefore, the upper limit was limited to 0.03%.

Al is an element usually contained in steel as a deoxidizing material, and has an effect of refining the structure. However, if the Al content exceeds 0.06%, Al-based nonmetallic inclusions increase and impair the cleanliness of the steel, so the upper limit was made 0.06%. Deoxidation is also possible with Ti or Si, and Al does not necessarily have to be added.

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

Further, in the present invention, the amounts of P and S which are impurity elements are set to 0.015% and 0.003% or less, respectively. The main reason for this is to further improve the low temperature toughness of the base material and HAZ. Reduction of the amount of P reduces the center segregation of the continuously cast slab, prevents grain boundary fracture, and improves the low temperature toughness. Further, the reduction of the amount of S has an effect of reducing MnS which is drawn by hot rolling and improving ductility and toughness.

Next, the purpose of adding V, Cr and Ca will be described. The main purpose of adding these elements to the basic composition is to further improve the strength and toughness and expand the size of steel that can be produced without impairing the excellent characteristics of the steel of the present invention. Therefore, it is not always necessary to contain it, and the addition amount is of a nature that should be limited by itself.

V has almost the same effect as Nb, but its effect is weaker than that of Nb. However, the effect of V addition in the ultra-high strength steel is great, and the combined addition of Nb and V makes the excellent characteristics of the steel of the present invention more remarkable. Also, V
Has been found to undergo strain-induced precipitation by the processing of ferrite (hot rolling), and to strengthen ferrite significantly. From the viewpoint of HAZ toughness and on-site weldability, the upper limit is 0.10%, and a range of 0.03 to 0.08% is particularly desirable.

[0028] Cr increases the strength of the base material and the welded portion, but if it is too much, it significantly deteriorates the HAZ toughness and field weldability. Therefore, the upper limit of the amount of Cr is 0.8%. V, C
The lower limit of the amount of r is not particularly limited, but V is 0.01% and Cr is 0.1% as the minimum amount at which the effect on the material becomes remarkable.

Ca controls the morphology of sulfide (MnS),
Improves low temperature toughness (increased absorbed energy in Charpy test, etc.). However, when the amount of Ca is 0.001% or less, there is no practical effect. Also, when added in excess of 0.006%, CaO-CaS is produced in large amounts and large clusters,
Not only does it impair the cleanliness of the steel and becomes a large inclusion, it also adversely affects the field weldability. Therefore, the upper limit of the amount of Ca added is limited to 0.006%. In the case of ultra-high strength line pipe, the S and O contents were 0.001% and 0.00, respectively.
2% or less, and ESSP = (Ca) [1-12
4 (O)] / 1.25S 0.5 ≦ ESSP ≦ 10.0
Is especially effective. Here, ESSP is
Abbreviation for effective sulfide morphology control parameter.

In the present invention, in addition to the above limitation of the individual additive elements, P = 2.7C + 0.4Si + Mn + 0.
8Cr + 0.45 (Ni + Cu) + Mo + V-1 2.
Limit to 0 ≦ P ≦ 3.5. This is the desired strength
This is to achieve a low temperature toughness balance. The lower limit of the P value is to obtain strength of 950 MPa or more and excellent low temperature toughness. In addition, the upper limit of the P value is 3.5, which is excellent for HA.
This is to maintain Z toughness and field weldability.

Next, claim 2 will be described. According to a second aspect, the steel of the _first aspect is tempered at a temperature not higher than the Ac ₁ point. Ductility and toughness are appropriately restored by the tempering treatment. The tempering treatment does not change the microstructure fraction itself, does not impair the excellent features of the present invention, and has the effect of narrowing the softening width of the weld heat affected zone.

[0032]

EXAMPLES Next, examples of the present invention will be described. Slabs of various steel components were produced by laboratory melting (50 kg, 100 mm thick steel ingot) or converter-continuous casting method (240 mm thick).
These slabs were rolled into steel plates having a thickness of 15 to 25 mm under various conditions and tempered in some cases to investigate various properties and microstructures.

Mechanical properties of the steel sheet (yield strength: YS, tensile strength: TS, absorbed energy at -40 ° C. in Charpy test: vE _-40 and 50% fracture surface transition temperature: vTrs) were in the direction perpendicular to rolling. I researched in. HAZ toughness (Charpy test -4
The absorbed energy at 0 ° C: vE _-40 was evaluated by the HAZ reproduced by the simulated thermal cycler (maximum heating temperature: 140).
Cooling time at 0 ° C, _800-500 ° C [Δt _800-500 ]:
25 seconds). The field weldability was evaluated in the Y-slit weld cracking test (JIS G3158) by the minimum preheating temperature necessary for preventing low temperature cracking of HAZ (welding method: gas metal arc welding, welding rod: tensile strength 100 MPa, heat input:
0.3 kJ / mm 2, hydrogen content of deposited metal: 3 cc / 100 g metal).

Examples are shown in Tables 1 and 2. The steel sheet produced according to the method of the present invention has an excellent strength / low temperature toughness balance,
Shows HAZ toughness and field weldability. On the other hand, it is clear that the comparative steels are inferior in any one of the properties due to the improper chemical composition or microstructure.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

According to the present invention, a super-strength line pipe with a low yield ratio and excellent low temperature toughness and field weldability (tensile strength 950
Steels for MPa or more and API standard X100 or more) can be stably manufactured in large quantities. As a result, the safety of the pipeline was significantly improved, and the transport efficiency and construction efficiency of the pipeline were dramatically improved.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshio Terada 2-6-3 Otemachi, Chiyoda-ku, Tokyo Shin Nippon Steel Corporation

Claims (4)

[Claims] 1. By weight%, C: 0.05 to 0.10%, Si: 0.6% or less, Mn: 1.7 to 2.5%, P: 0.015% or less, S: 0 0.003% or less, Ni: 0.1 to 1.0%, Cu: 0.8 to 1.2%, Mo: 0.35 to 0.50%, Nb: 0.01 to 0.10%, Ti : 0.005 to 0.030%, Al: 0.06% or less, N: 0.001 to 0.006%, with the balance being iron and inevitable impurities, and the P value defined by the following formula Is in the range of 2.0 or more and 3.5 or less, and its microstructure is composed of martensite, bainite and ferrite, the ferrite fraction is 20 to 90%, and the processed ferrite in the ferrite is 50 to 100%. Contains,
A high-strength line pipe steel excellent in low-temperature toughness having a low yield ratio, characterized by having an average ferrite grain size of 5 μm or less. P = 2.7C + 0.4Si + Mn + 0.8Cr + 0.4
5 (Ni + Cu) + Mo + V-1 2. In addition to the components of claim 1, weight%
A high-strength line pipe having a low yield ratio and excellent low temperature toughness according to claim 1, characterized in that it contains one or two of V: 0.10% or less and Cr: 0.6% or less. For steel. 3. In addition to the component according to claim 1 or 2,
The steel for high strength line pipes having a low yield ratio and excellent low temperature toughness according to claim 1 or 2, further containing Ca: 0.001 to 0.006% by weight. 4. The high-strength line pipe having a low yield ratio and excellent low-temperature toughness, characterized in that the steel according to claim 1, 2, or 3 is made of steel tempered at a temperature of Ac ₁ or less. For steel.