JPH08209290A - High tensile strength steel for welding excellent in low temperature toughness - Google Patents

Abstract

PURPOSE: To produce a ultrahigh tensile strength steel for welding excellent in the balance of strength and low temp. toughness, easily capable of applying field welding and excellent in low temp. toughness by specifying the microstructure of a steel having a specified compsn. contg. Ni, Mo, Nb, Ti or the like. CONSTITUTION: The compsn, of a steel is constituted of the one contg., by weight, 0.05 to 0.10% C, <=0.6% Si, 1.8 to 2.5% Mn, <=0.015% P, <=0.003% S, 0.1 to 1.0% Ni, 0.35 to 0.60% 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, Cu, Cr and Ca, and the balance iron with inevitable impurities, and in which the value of P shown by the formula lies in the range of 1.9 to 2.8. Moreover, the microstructure of the steel is formed of the one contg. >=60mol.% tempered martensite with <=10μm grain size transformed from unrecrystallized austenite, and in which the total of the tempered martensite fractional rate and tempered bainite fractional rate is regulated to >=90%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. 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 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 for long-distance pipelines of crude oil and natural gas are (1) improved in transportation efficiency by increasing pressure and (2) locally reduced in outer diameter and weight of the line pipe. There is a tendency for tensile strength to become higher and higher in order to improve construction efficiency. To date, the American Petroleum Institute (AP
I) Standard line pipes up to X80 (yield strength 551 MPa or more, tensile strength 620 MPa or more) have been put into practical use, but there is a growing need for line pipes with even 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), Volume
V, pp179-185), but it is considered that the production of X100 (yield strength 689 MPa or more, tensile strength 760 MPa or more) line pipe is the limit at most.

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 a typical ultra-high tension line pipe (X100 or more).

[0005]

DISCLOSURE OF THE INVENTION The present invention provides an ultrahigh-strength welding steel having an excellent balance of strength and low-temperature toughness and having a tensile strength of 950 MPa or more (API standard X100 or more) that facilitates on-site welding. is there.

[0006]

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

That is, the gist of the present invention is, in% by weight, C
: 0.05-0.10%, Si: 0.6% or less, Mn: 1.8-2.5%, P: 0.0
15% or less, S: 0.003% or less, N
i: 0.1 to 1.0%, Mo: 0.35 to 0.60%,
Nb: 0.01 to 0.10%, Ti: 0.005
~ 0.030%, Al: 0.06% or less, N: 0.
001 to 0.006%, and if necessary, V: 0.01 to 0.10%, Cu: 0.1 to
0.7%, Cr: 0.1 to 0.8%, Ca:
It contains 0.001 to 0.006% of one or more kinds, and the balance is iron and inevitable impurities, and P =
2.7C + 0.4Si + Mn + 0.8Cr + 0.45
(Ni + Cu) + Mo + V-1 has a steel composition satisfying 1.9 ≦ P ≦ 2.8, and its microstructure is tempered by 60% or more transformed from unrecrystallized austenite having an average austenite grain size of 10 μm or less. A weldable high-strength steel excellent in low-temperature toughness, characterized by containing martensite and having a sum of a tempered martensite fraction and a tempered bainite fraction of 90% or more.

The features of the present invention are (1) Ni-Nb-Mo-
A low carbon / high Mn system (1.8% or more) in which a small amount of Ti is added in combination, (2) A fine tempered martensite transformed from unrecrystallized austenite whose microstructure has an average austenite grain size of 10 μm or less, and It consists of bainite.

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 at most 750 MPa. It was There is no ultrahigh-strength steel having a fine tempered martensite / bainite mixed structure in this basic component system. This is Nb
This is because, in the tempered martensite / bainite mixed structure of -Mo steel, not only tensile strength of 950 MPa or more is impossible but also low temperature toughness and field weldability are considered to be insufficient.

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 the steel material a certain amount or more of tempered martensite, and the fraction thereof must be 60% or more. 60% tempered martensite fraction
When it is below, not only sufficient strength cannot be obtained, but also it becomes difficult to secure good low temperature toughness (in terms of strength and low temperature toughness, the most desirable tempered martensite fraction is 70 to 90%). . However, even if the tempered martensite fraction is 60% or more, the desired strength and low temperature toughness cannot be achieved if the remaining structure is inappropriate.
Therefore, the sum of the tempered martensite fraction and the tempered bainite fraction is set to 90% or more.

However, even if the types of microstructures are limited as described above, good low temperature toughness cannot always be obtained. In order to obtain excellent low temperature toughness, it is necessary to optimize the austenite structure before γ-α transformation (former austenite structure) and effectively refine the final structure of the steel material. Therefore, the old austenite structure is unrecrystallized austenite,
And the average particle diameter (dγ) is limited to 10 μm or less.
As a result, Nb, which was conventionally considered to have poor low temperature toughness,
It has been found that an extremely excellent balance between strength and low temperature toughness can be obtained even in a mixed structure of tempered martensite and bainite of -Mo steel.

The refinement of the unrecrystallized austenite grain size is N
It is particularly effective for improving the low temperature toughness of the b-Mo steel of the present invention. In order to obtain the desired low temperature toughness (for example, −80 ° C. or lower at the transition temperature of the V-notch Charpy impact test), the average grain size must be 10 μm or less. Here, the apparent average austenite grain size is defined as shown in FIG. 1, and in the measurement of the austenite grain size, a deformation zone and a twin boundary having the same action as the austenite grain boundary are also included. Specifically, the total length of a straight line drawn in the lengthwise direction of the steel sheet was divided by the number of intersections with austenite grain boundaries existing on the straight line to obtain dγ. It was found that the austenite average grain size thus obtained has a very good correlation with the low temperature toughness (transition temperature of Charpy impact test).

Further, by strictly controlling the chemical composition (high Mn-Nb-high Mo addition) and microstructure (unrecrystallized austenite) of the steel material as described above,
It was also clarified that separation occurred on the fracture surface in the Charpy impact test and the transition temperature of the fracture surface was further improved. Separation is believed to reduce the triaxial stress level at the brittle crack tip by a delamination phenomenon parallel to the plate surface that occurs on the fracture surface such as in the Charpy impact test, and improve the brittle crack propagation stopping property.

However, as described above, even if the microstructure of the steel material is strictly controlled, a steel material having the 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%.
C is an extremely effective element for improving the strength of steel, and at least 0.05% is necessary to obtain the target strength in the tempered martensite / bainite mixed structure. Further, this amount is also the minimum amount for precipitation hardening by the addition of Nb and V, manifestation of 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 will be significantly deteriorated.
It was set to 0%.

Si is an element added for deoxidation and strength improvement, 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 Ti or Al, and Si is not necessarily added.

Mn is an element essential for ensuring a fine martensite / bainite mixed structure in the microstructure of the steel of the present invention and ensuring an excellent balance of strength and low temperature toughness, and its lower limit is 1.8%. However, if the Mn content is too large, not only the hardenability of the steel increases and the HAZ toughness and field weldability deteriorate, but also the center segregation of the continuously cast steel slab is promoted and the low temperature toughness of the base metal also deteriorates. The upper limit was 2.5%.
(A desirable Mn amount is 1.9 to 2.1%).

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 to Mn, Cr, and Mo additions, Ni addition does not often form a hardened structure harmful to low temperature toughness in the rolling structure (especially the central segregation zone of the slab), and addition of a small amount of Ni improves HAZ toughness. It has been found that it is also effective (a particularly effective Ni addition amount is 0.3% or more in terms of HAZ toughness). However, if the addition amount is too large, not only the economical efficiency but also the HAZ toughness and the field weldability are deteriorated, so the upper limit was made 1.0%. Further, addition of 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 and to obtain the desired martensite / bainite mixed structure. Further, Mo coexists with Nb to strongly suppress recrystallization of austenite during controlled rolling, and is also effective for refining the austenite structure. In order 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 0.6%.

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 make crystal grains fine, but also contributes to precipitation hardening and increase in hardenability and has an action of strengthening steel. However, if the amount of Nb added is too large, it adversely affects HAZ toughness and field weldability.
The upper limit was 0.10%.

On the other hand, addition of Ti forms fine TiN,
When the slab is reheated and the austenite grains of the welded HAZ are suppressed from coarsening, the microstructure is refined, and the base metal and H
Improves low temperature toughness of AZ. Further, when the amount of Al is small (for example, 0.005% or less), Ti forms an oxide and acts as an intragranular ferrite formation nucleus in HAZ, and also has the effect of refining the HAZ structure. In order to develop such Ti addition effect, at least 0.005% T
i addition is required. However, if the amount of Ti is too large, Ti
Since coarsening of N and precipitation hardening due to TiC occur and the low temperature toughness deteriorates, the upper limit was limited to 0.03%.

Al is an element usually contained in steel as a deoxidizing agent and has an effect of refining the structure. However, the amount of Al is 0.
If it exceeds 06%, Al-based nonmetallic inclusions increase and impair the cleanliness of the steel, so the upper limit was made 0.06%. Deoxidation is T
It is also possible to use i or Si, and it is not always necessary to add Al.

N forms TiN and suppresses coarsening of austenite grains in the slab during 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 the amount of N is too large, it may cause a flaw in the slab surface or deterioration of the HAZ toughness due to solid solution N, so the upper limit must be suppressed to 0.006%.

Furthermore, in the present invention, P and S which are impure elements are used.
The amounts are 0.015% or less 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 drawn by hot rolling to improve ductility and toughness.

Next, the purpose of adding V, Cu, Cr and Ca will be described. The main purpose of adding these elements to the basic composition is to further improve the properties such as strength and low temperature toughness and to expand the size of the manufacturable steel material 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 weaker than that of Nb. However, the effect of V addition in 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. (In the steel of the present invention, addition of 0.03 to 0.08% V is particularly desirable). The upper limit is 0. 0 from the viewpoint of HAZ toughness and field weldability.
Up to 10% is acceptable.

Cu has almost the same effect as Ni, and also has an effect of improving the corrosion resistance and hydrogen-induced cracking resistance. Also, the strength is significantly increased by Cu precipitation hardening. However, if added excessively, the base metal and HA will be precipitated and hardened.
Since the toughness of Z decreases and Cu cracks occur during hot rolling, the upper limit was made 0.7%.

[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 0.6%. V, C
The lower limits of 0.01%, 0.1%, and 0.1% of the amounts of u and Cr are the minimum amounts in which the effect on the material due to the addition of the respective elements becomes remarkable.

Ca controls the morphology of sulfide (MnS),
Improved low temperature toughness (absorbed energy in Charpy impact test)
Increase). Particularly, in the steel of the present invention mainly used for ultra-high strength line pipe, high Charpy absorbed energy is required to prevent the propagation of unstable ductile fracture.
Reduction of the amount and Ca treatment are important. However, if the amount of Ca added is 0.001% or less, there is no practical effect, and
If added in excess of 5%, a large amount of CaO-CaS is formed to form large clusters and large inclusions, which not only impairs the cleanliness of the steel but also adversely affects on-site weldability. Therefore, the upper limit of the amount of Ca added is limited to 0.005%. Ultra high strength steel contains 0.001% of S and O,
Reduced to 0.002% or less and ESSP = (Ca) [
1-124 (O)] / 1.25 (S) 0.5 ≦ ESS
It is particularly effective to set P ≦ 10.0. ES
SP is an 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
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 strength of 950 MPa or more and excellent low temperature toughness. Moreover, the upper limit of the P value is set to 2.8 in order to maintain excellent HAZ toughness and field weldability.

[0031]

Next, an embodiment of the present invention will be described. Slabs of various steel compositions were produced by laboratory melting (50 kg, 120 mm thick steel ingot) or converter-continuous casting (240 mm thick). These slabs were rolled into steel plates having a thickness of 15 to 28 mm under various conditions and tempered (550 ° C. to 620 ° C. × 20 minutes air cooling) to investigate various mechanical properties and microstructures.

Mechanical properties of the steel sheet (yield strength: YS, tensile strength: TS, absorbed energy at -40 ° C. in Charpy impact test: vE _-40 and transition temperature: vTrs) were investigated in the direction perpendicular to rolling. . HAZ toughness (Charpy impact test -20 ° C
The absorbed energy at: vE _-20 was evaluated by the HAZ reproduced by the simulated thermal cycler (maximum heating temperature: 1400).
℃, _800-500 ℃ cooling time [△ t _800-500 ]: 2
5 seconds). The field weldability was evaluated in the Y-slit welding crack test (JIS G3158) by the minimum preheating temperature required to prevent low temperature cracking of HAZ (welding method: gas metal arc welding, welding rod: tensile strength 100 MPa, heat input:
0.5 kJ / mm, hydrogen content of deposited metal: 3 cc / 100 g).

Examples are shown in Tables 1 and 2. The steel sheet produced according to the present invention has an excellent strength / low temperature toughness balance,
HAZ toughness and field weldability. On the other hand, the comparative steels have an inadequate chemical composition or microstructure, and either characteristic is significantly inferior.

Steel 9 contains too much C, so that the base metal and H
The Charpy absorbed energy of AZ is low and the preheating temperature during welding is high. Since Steel 10 does not contain Ni, the low temperature toughness of the base material and HAZ is poor. Steel 11 is Mn
Since the added amount and P value are too high, the low temperature toughness of the base metal and HAZ is poor, and the preheating temperature during welding is also extremely high.

Since the steel 12 contains too much Mo, it requires preheating during welding. Steel 13 does not have Nb added, and therefore has insufficient strength, has a large austenite grain size, and has poor toughness as the base material. Steel 14 has an excessive amount of S, so that the absorbed energy of the base metal and HAZ is low.

Steel 15 cannot achieve the target strength because the amount of Mo added is too small. Steel 16 has an austenite grain size that is too large, resulting in poor low temperature toughness of the base material. Steel 17
Since the tempered martensite fraction is too small, the strength is insufficient and the Charpy transition temperature of the base material is inferior. Steel 18 has insufficient strength because the fraction of tempered martensite and tempered bainite is too small. Steel 19 has a large austenite grain size and a too small tempered martensite fraction, so that the strength and low temperature toughness do not reach the targets.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

Industrial Applicability According to the present invention, it has become possible to stably manufacture a large amount of steel for ultra high strength line pipes (tensile strength 950 MPa or more, API standard X100 or more) excellent in low temperature toughness and field weldability. As a result, the safety of the pipeline is significantly improved, and the transportation efficiency of the pipeline,
It has become possible to dramatically improve construction efficiency.

[Brief description of drawings]

FIG. 1 is a diagram showing the definition of an average austenite grain size (dγ).

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

Claims (3)

[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 0.003% 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 : 0.06% or less, N: 0.001 to 0.006%, the balance consisting of iron and unavoidable impurities, and P defined by the following formula
The value is in the range of 1.9 to 2.8, and further contains tempered martensite transformed from unrecrystallized austenite having an average austenite grain size of 10 μm or less as a microstructure of steel in a volume fraction of 60% or more, and A weldable high-strength steel with excellent low-temperature toughness, characterized in that the sum of the tempered martensite fraction and the tempered bainite fraction is 90% or more. 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%
And V: 0.01 to 0.10%, Cu: 0.1 to 0.7%, Cr: 0.1 to 0.8%, and one or more of them are contained. A weldable high-strength steel excellent in low temperature toughness as set forth in Item 1. 3. The low temperature toughness according to claim 1 or 2, further comprising Ca: 0.001 to 0.006% by weight in addition to the components according to claim 1 or 2. Excellent weldability high strength steel.