JP3244981B2 - Weldable high-strength steel with excellent low-temperature toughness - Google Patents

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 in low-temperature toughness and weldability. It can be widely used as welding steel 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 to (1) improve transport efficiency under high pressure and (2) reduce the outside diameter and weight of line pipes. In order to improve construction efficiency, the strength tends to be further increased. The American Petroleum Institute (AP)
I) Although line pipes up to X80 (tensile strength of 620 MPa or more) have been put to practical use in the standard, there is a growing need for line pipes having higher strength.

[0003] At present, research on ultra-high-strength line pipe manufacturing methods is based on conventional X80 line pipe manufacturing techniques (for example, NK).
K Technique No.138 (1992), pp24-31, and The 7th Offshore
Mechanics and Arctic Engineering (1998), Volume V, p
p179-185), but the production of an X100 (tensile strength of 760 MPa or more) line pipe is considered to be the limit at most. The ultra-high strength of pipeline has many problems such as strength-low temperature toughness balance, welding heat affected zone (HAZ) toughness, on-site weldability, and softening of joints. There is a demand for early development of a strength line pipe (above X100).

[0004]

SUMMARY OF THE INVENTION In order to satisfy the above-mentioned demands, the present invention has an excellent balance between strength and low-temperature toughness, and a tensile strength of 950 MPa or more which facilitates on-site welding (API standard X).
(More than 100) ultra-high strength welding steel.

[0005]

Means for Solving the Problems The present inventors have determined 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 on-site weldability. The enthusiastic research on the microstructure led to the invention of a new ultra-high strength welding steel.

That is, the gist of the present invention is that, by weight%, C: 0.05 to 0.10%, Si:
0.6% or less, Mn: 1.7 to 2.2%, P
: 0.015% or less, S: 0.003% or less,
Ni: 0.1-1.0%, Mo: 0.15-0.
50%, Nb: 0.01 to 0.10%, Ti:
0.005 to 0.030%, B: 0.0003 to
0.0020%, Al: 0.06% or less,
N: 0.001 to 0.006%, or further, if necessary, V: 0.01 to 0.10%, Cu:
0.1 to 1.0%, Cr: 0.1 to 0.6%, contains one or more kinds, and optionally further contains Ca:
0.001 to 0.006%, P = 2.7C +
0.4Si + Mn + 0.8Cr + 0.45 (Ni + C
u) + 2Mo satisfies 2.5 ≦ P ≦ 4.0, the balance has a steel component consisting of iron and unavoidable impurities, and the microstructure has an apparent average austenite grain size (dγ).
Is transformed from unrecrystallized austenite of 10 μm or less,
This is a weldable high-strength steel excellent in low-temperature toughness, characterized in that its microstructure contains 90% or more of martensite and that this steel is tempered at a temperature of ₁ point or less of Ac.

[0007]

Hereinafter, the contents of the present invention will be described in detail.
The features of the present invention are: (1) a low-carbon, high-Mn system to which Ni-Mo-Nb-trace B-trace Ti is added in combination;
(2) Its microstructure has an average austenite grain size of 10
mainly a fine martensite structure transformed from unrecrystallized austenite of not more than μm (tempered martensite structure after tempering).

Conventionally, very low carbon and high Mn-Nb- (M
o)-(Ni) -trace B-trace Ti steel is known as a linepipe steel having a fine bainite-based structure, but the upper limit of its tensile strength is at most 750 MPa. There is no ultrahigh-strength steel having a structure mainly composed of fine martensite in this basic component system. This is because not only the bainite-based structure cannot at all achieve a tensile strength of 950 MPa or more, but also it is thought that the low-temperature toughness deteriorates as the martensite structure increases.

First, the microstructure of the present invention will be described. In order to achieve an ultra-high tensile strength of 950 MPa or more, the microstructure of the steel must be a certain degree or more of martensite, and its fraction must be 90% or more. When the martensite fraction is 90% or less, not only sufficient strength is not obtained, but also it is difficult to secure good low-temperature toughness.

[0010] However, even if the type of microstructure is 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 (former austenite structure) before the γ-α transformation and to effectively refine the final structure of the steel material. For this reason, the former austenite structure is unrecrystallized austenite,
In addition, the average particle size (dγ) was limited to 10 μm or less.
As a result, it has been clarified that an extremely excellent strength and low-temperature toughness balance can be obtained even in a structure mainly composed of martensite, which was conventionally considered to be poor in low-temperature toughness. Refinement of the unrecrystallized austenite grain size is particularly effective for improving the low-temperature toughness of martensite-based steel. Target low temperature toughness (eg transition temperature vTrs for V notch Charpy test)
In this case, the average particle size must be 10 μm or less.

Here, the apparent average austenite grain size is defined as shown in FIG.
Deformation zones and twin boundaries having the same effect as austenite grain boundaries were also included. Specifically, the total length of the straight line drawn in the thickness direction of the steel sheet was reduced by the number of intersections with the austenite grain boundaries existing on the straight line to obtain dγ. It was found that the average austenite grain size thus obtained had a very good correlation with the low-temperature toughness (transition temperature in the Charpy impact test). Furthermore, by controlling the morphology of the microstructure strictly as described above, it is believed that the triaxial stress at the brittle crack tip on the fracture surface such as the Charpy impact test is reduced, and the brittle crack propagation arrest property is improved. It was also found that the delamination phenomenon called separation occurred and the fracture surface transition temperature became even lower.

However, as described above, it is not possible to obtain a steel material having desired characteristics by strictly controlling the microstructure of the steel. For this purpose, it is necessary to limit the chemical composition simultaneously with the microstructure. The reasons for limiting the component 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 required to obtain the target strength in the martensite structure. However, if the C content is too large, the low-temperature toughness and the on-site weldability of the base material and HAZ are remarkably deteriorated, so the upper limit is set to 0.10%. However, it is desirable to limit the upper limit to 0.08%.

[0014] Si is an element added for deoxidation and improvement of strength, but if added in a large amount, HAZ toughness and on-site weldability are significantly deteriorated, so the upper limit was made 0.6%. Deoxidation of steel is sufficiently possible with Al or Ti, and Si need not always be added.

Mn is an element indispensable for ensuring a good balance between strength and low temperature toughness by making the microstructure of the steel of the present invention a structure mainly composed of martensite, and the lower limit thereof is 1.7.
%. However, if the Mn content is too high, the hardenability of the steel increases, and not only deteriorates HAZ toughness and on-site weldability,
The upper limit was set to 2.2% because it promotes center segregation of the continuously cast steel slab and degrades the low-temperature toughness of the base material.

The purpose of adding Ni is to improve the low-carbon steel of the present invention without deteriorating low-temperature toughness and on-site weldability. Compared with the addition of Mn, Cr, or Mo, Ni addition not only causes less formation of a hardened structure that is harmful to low-temperature toughness in the rolled structure (particularly, the central segregation zone of a continuously cast steel slab), but also has a content of 0.1% or more. It has been found that the addition of a small amount of Ni is also effective for improving the HAZ toughness (in terms of HAZ toughness, the particularly effective Ni addition amount is 0.3% or more). However, if the addition amount is too large, not only economic efficiency but also HAZ toughness and on-site weldability are deteriorated. Therefore, the upper limit is set to 1.0%. The addition of Ni is also effective in preventing Cu cracking during continuous casting and hot rolling. In this case, Ni is Cu
It is necessary to add at least 1/3 of the amount.

The reason for adding Mo is to improve the hardenability of the steel and obtain the desired structure mainly composed of martensite. In the B-added steel, the effect of improving the hardenability of Mo is enhanced, and the multiple of Mo in the P value described later is 2 in the B steel compared to 1 in the non-B steel, and therefore, the addition of Mo is particularly effective in the steel of the present invention. . Further, Mo coexists with Nb to suppress recrystallization of austenite during controlled rolling, and is also effective in refining the austenite structure. In order to obtain such an effect, Mo must be at least 0.15%. But,
Excessive Mo addition degrades HAZ toughness and on-site weldability,
Further, since the effect of improving the hardenability of B may be lost, the upper limit is set to 0.5%.

B is an indispensable element in the steel of the present invention in order to dramatically improve the hardenability of the steel in a very small amount and to obtain the desired structure mainly composed of martensite. There is an effect corresponding to 1 in the P value described later, that is, 1% Mn. Further, B enhances the effect of improving the hardenability of Mo, and synergistically increases the hardenability together with Nb. In order to obtain such an effect, B should be at least 0.1.
0003% is required. On the other hand, if it is added excessively, it not only deteriorates the low-temperature toughness, but also sometimes makes the hardenability improving effect of B disappear.
0020%.

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 to suppress the recrystallization of austenite during controlled rolling, not only to refine the structure, but also to contribute to precipitation hardening and hardenability and toughen the steel. In particular, when Nb and B coexist, the effect of improving hardenability increases synergistically. However, if the Nb addition amount is too large,
Since the HAZ toughness and on-site weldability are adversely affected, the upper limit is set to 0.10%. On the other hand, the addition of Ti
N is formed to suppress the coarsening of the austenite grains of the HAZ during reheating of the slab and to refine the microstructure, thereby improving the low-temperature toughness of the base material and the HAZ. Also, it has a role of fixing solid solution N harmful to the effect of improving the hardenability of B as TiN. For this purpose, it is desirable to add Ti in an amount of 3.4N (each wt%) or more. When the amount of Al is small (for example, 0.005% or less), Ti forms an oxide, acts as an intragranular ferrite generation nucleus in the HAZ, and has an effect of making the HAZ structure finer. In order to exert such an effect of TiN, at least 0.00
5% Ti addition is required. However, if the amount of Ti is too large, coarsening of TiN and precipitation hardening due to TiC occur, deteriorating low-temperature toughness. Therefore, the upper limit is set to 0.03%.

Al is an element usually contained in steel as a deoxidizing material, and also has an effect on 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 can be performed with Ti or Si, and Al need not always be added.

N forms TiN and suppresses the coarsening of austenite grains in the HAZ during reheating of the slab and in the HAZ.
Improves the low-temperature toughness of AZ. The minimum required for this is 0.001%. However, if the amount of N is too large, it causes deterioration of the HAZ toughness due to slab surface flaws and solid solution N, and lowers the effect of improving the hardenability of B, so the upper limit is 0.0.
It is necessary to suppress it to 06%.

Further, in the present invention, P, which is an impurity element,
The S content is 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 the HAZ. The reduction of the P content reduces the center segregation of the continuously cast slab, and also prevents the intergranular fracture and improves the low-temperature toughness. Further, the reduction of the S content has the effect of reducing MnS to be stretched by hot rolling and improving ductility and toughness.

Next, the purpose of adding V, Cu, Cr and Ca will be described. The main purpose of further adding these elements to the basic components is to further improve the strength and toughness and expand the size of the steel material that can be manufactured without impairing the excellent characteristics of the steel of the present invention. Therefore,
The amount added is of a nature that should be naturally restricted.

V has almost the same effect as Nb, but the effect is weaker than Nb. However, the effect of V addition on ultra-high strength steel is great, and the combined addition of Nb and V makes the excellent features of the steel of the present invention more remarkable. The upper limit is H
From the point of AZ toughness and on-site weldability, 0.10% can be tolerated, but the addition of 0.03 to 0.08% is particularly desirable.

[0025] Cu increases the strength of the base material and the welded portion, but if it is too large, the HAZ toughness and on-site weldability are remarkably deteriorated. Therefore, the upper limit of the amount of Cu is 1.0%. Cr increases the strength of the base material and the weld, but if too much, HAZ
It significantly deteriorates toughness and on-site weldability. Therefore, the upper limit of the Cr content is 0.6%. Lower limit of V, Cu, Cr amount
01%, 0.1%, and 0.1% are the minimum amounts at which the effect on the material by adding each element becomes remarkable.

Ca controls the form of sulfide (MnS),
Improves low-temperature toughness (increased energy absorbed in Charpy test, etc.). However, if the Ca content is 0.001% or less, there is no practical effect, and if the Ca content exceeds 0.005%, CaO—CaS is generated in large quantities and becomes large clusters and large inclusions, which only impairs the cleanliness of steel. In addition, it has an adverse effect on the on-site weldability. For this reason, the upper limit of the amount of Ca added was limited to 0.006%. In the case of an ultra-high-strength line pipe, the amounts of S and O are 0.001% and 0.002%, respectively.
% And ESSP = (Ca) [1-124
(O)] / 1.25S is particularly effective to satisfy 0.5 ≦ ESSP ≦ 10.0.

In the present invention, in addition to the limitation of the individual additive elements described above, P = 2.7C + 0.4Si + Mn + 0.
8Cr + 0.45 (Ni + Cu) + 2Mo is 2.5 ≦ P
Limited to ≤4.0. This is to achieve the desired strength-low temperature toughness balance. Lower limit of P value is 950
This is for obtaining strength equal to or higher than MPa and excellent low-temperature toughness.
The upper limit of the P value is set at 4.0 in order to maintain excellent HAZ toughness and on-site weldability.

Next, claim 4 will be described. In a fourth aspect, the steel according to any one of the first to third aspects is subjected to a tempering treatment at a temperature of not more than _one point Ac. The ductility and toughness are appropriately recovered by tempering. The tempering treatment does not change the microstructure fraction itself, does not impair the excellent features of the present invention, and also has the effect of narrowing the softening width of the heat affected zone.

[0029]

Next, embodiments of the present invention will be described. Slabs (240 mm thick) of various steel components were produced by laboratory melting (50 kg, 100 mm thick steel ingot) or converter-continuous casting method.
These slabs were rolled into steel sheets having a thickness of 15 to 25 mm under various conditions, and in some cases, a tempering treatment was performed to investigate various properties and microstructure. Mechanical properties of steel sheet (Yield strength: Y
S, tensile strength: TS, absorption energy at -40 ℃ Charpy test: vE _{-4 0} 50% fracture appearance transition temperature: vTrs) were examined in the rolling direction perpendicular. The HAZ toughness (absorbed energy at −40 ° C. in the Charpy test: vE ₋₄₀ ) was evaluated by HAZ reproduced by a reproducible heat cycler (maximum heating temperature: 1400 ° C., cooling time at 800 to 500 ° C. [Δt]
_800-500 ]: 25 seconds). The on-site weldability was evaluated at the minimum preheating temperature required for preventing low-temperature cracking of the HAZ in a Y-slit welding crack test (JIS G3158) (welding method: gas metal arc welding, welding rod: tensile strength 100M).
Pa, heat input: 0.3KJ / mm, hydrogen content of deposited metal: 3cc /
100 g metal).

Examples are shown in Tables 1 and 2. The steel sheet manufactured according to the method of the present invention has excellent strength-low temperature toughness balance,
Shows HAZ toughness and field weldability. In contrast, it is clear that the comparative steel is significantly inferior in any of its properties due to inadequate chemical composition or microstructure.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

Industrial Applicability According to the present invention, steel for super-strength line pipe (tensile strength of 950 MPa or more, API standard X100 or more) excellent in low-temperature toughness and on-site weldability can be stably manufactured in large quantities. As a result, the safety of the pipeline has been significantly improved, and the transportation efficiency of the pipeline,
Dramatic improvement in construction efficiency has become possible.

[Brief description of the drawings]

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

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Yoshio Terada 2-6-3 Otemachi, Chiyoda-ku, Tokyo Nippon Steel Corporation (58) Field surveyed (Int.Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 6/00 C21D 8/00-8/10

Claims (4)

(57) [Claims] C: 0.05 to 0.10%, Si: 0.6% or less, Mn: 1.7 to 2.2%, P: 0.015% or less, S: 0% by weight 0.003% or less, Ni: 0.1 to 1.0%, Mo: 0.15 to 0.50%, Nb: 0.01 to 0.10%, Ti: 0.005 to 0.030%, B : 0.0003 to 0.0020%, Al: 0.06% or less, N: 0.001 to 0.006%, the balance being Fe
And the inevitable impurities, the P value defined by the following formula is in the range of 2.5 or more and 4.0 or less, and the apparent average austenite grain size (d
A weldable high-strength steel excellent in low-temperature toughness, characterized by containing at least 90% by volume fraction of martensite transformed from unrecrystallized austenite having γ) of 10 μm or less. P = 2.7C + 0.4Si + Mn + 0.8Cr + 0.4
5 (Ni + Cu) + 2Mo 2. In addition to the components of claim 1, V: 0.01 to 0.10%, Cu: 0.1 to 1.0%, Cr: 0.1 to 0. The weldable high-strength steel excellent in low-temperature toughness according to claim 1, comprising one or more of 6%. 3. The low temperature according to claim 1 or 2, further comprising, by weight%, 0.001 to 0.006% of Ca in addition to the component according to claim 1 or 2. Weldable high strength steel with excellent toughness. 4. The weldable high-strength steel with excellent low-temperature toughness according to claim 1, wherein the steel is tempered at a temperature of not more than _one point Ac.