JP2003003228A - Steel having excellent low temperature toughness in welded joint and stress corrosion cracking property and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel which satisfies both characteristics of low temperature toughness in a welded joint and stress corrosion cracking properties. SOLUTION: The steel having excellent low temperature toughness in a welded joint and stress corrosion cracking properties consists of steel containing, by mass, 0.05 to 0.18% C, 0.05 to 0.3% Si, 0.4 to 2% Mn, <=0.02% P, <=0.02% S, 0.1 to 1% Cu, 0.1 to 1% Ni, 0.005 to 0.04% Al, 0.005 to 0.03% Ti and 0.0005 to 0.003% Ca. In the steel, oxide grains having a diameter equivalent to a circle of 0.005 to 2.0 μm are contained in 100 to 3,000 pieces/mm<2> by piece density per unit area, and the composition of the oxide grains at least contains Ca, Al and O, and, as to the elements other than O, by mass, >=5% Ca and >=5% Al are respectively contained, and the total of Ca an Al is >=50%. Also, the steel has a bainitic microstructure of >=50% by area.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to LPG, LNG,
The present invention relates to a steel material having excellent stress corrosion cracking properties and weld joint low temperature toughness suitable for use in ships for transporting low temperature liquefied gas such as liquid ammonia and tanks for storage tanks, and a manufacturing method thereof.

[0002]

2. Description of the Related Art In recent years, demands for material characteristics of welding steel materials used in ships for transporting liquefied gas and tanks for liquefied gas storage tanks have become increasingly severe. Although it depends on the type of the liquefied gas, the liquefaction temperature of the gas is generally low at normal pressure, for example, −48 ° C. in the case of LPG, so excellent low temperature toughness is required not only in the base metal but also in the welded joint.

It is known that liquid ammonia causes stress corrosion cracking of steel materials, and IGC CODE
17.13 (International Code
for the Construction and
Equipment of Ships Carryi
ng Requested Gases in Bul
In k), the operating conditions such as oxygen partial pressure and temperature during storage are regulated, and the Ni content of steel is limited to 5% or less and the yield strength is limited to 440 N / mm ² or less. is doing.

However, since LPG and liquid ammonia are required to be mixedly loaded in a liquefied gas transportation ship or a tank for a storage tank, it is naturally necessary to satisfy the specifications required for both.

However, in order to achieve a low yield ratio as a component of the steel material, high C may be used, but high C deteriorates weldability and low temperature toughness. In other words, the low temperature toughness and the yield ratio for preventing stress corrosion cracking are incompatible with each other, and it is extremely difficult to make them compatible with each other.

Further, in order to increase the size of ships and the capacity of tanks, the steel materials used for these are required to have a tensile strength of 500 MPa or more, and further 600 MPa or more.

In order to achieve a tensile strength of 500 MPa or more, it is necessary to increase C particularly in the steel component system, but if these components are adjusted, the problem of low temperature toughness of the welded joint occurs. .

[0008]

In view of the above situation, it is an object of the present invention to provide a steel material satisfying both properties of the welded joint low temperature toughness and stress corrosion cracking property, and a method for producing the same. is there.

[0009]

The present inventors have found that the tensile strength is 5
Yield strength of 440MPa while securing more than 00MPa
The following was studied to prevent stress corrosion cracking. As a result, by adjusting the steel composition and dispersing the oxide particles in the steel, stress corrosion cracking in the weld heat affected zone can be prevented, and the low temperature toughness of the welded joint can be improved, and the steel structure can be improved. It was found that, when the bainite has a quenched structure of 50% or more, the tensile strength can be increased to 500 MPa or more and the yield strength can be lowered to prevent stress corrosion cracking of the base material.

The present invention has been completed based on the above findings, and the gist of the invention is as follows.

(1) C: 0.05 to 0.1 in mass%
8%, Si: 0.05 to 0.3%, Mn: 0.4 to 2.
0%, P: 0.02% or less, S: 0.02% or less, C
u: 0.1 to 1.0%, Ni: 0.1 to 1.0%, A
1: 0.005 to 0.04%, Ti: 0.005 to 0.
03%, Ca: 0.0005-0.003% is contained,
The balance is steel composed of Fe and unavoidable impurities, and oxide particles having an equivalent circle diameter of 0.005 to 2.0 μm are contained in the steel at a number density per unit area of 100 to 3000 particles / m 2.
m ² and the composition of the oxide particles is at least Ca,
The mass ratio of elements including Al and O and excluding O is Ca: 5.
%, Al: 5% or more, respectively, Ca and Al
And 50% or more in total, and the steel microstructure is composed of a bainite structure with an area ratio of 50% or more, a steel material excellent in low-temperature toughness and stress corrosion cracking characteristics of a welded joint.

(2) The composition of the oxide particles contains at least Ca, Al, O, and S, and the elements excluding O are in a mass ratio of Ca: 5% or more, Al: 5% or more, S: 1%. Excellent in low-temperature toughness and stress corrosion cracking properties of the welded joint according to (1) above, characterized in that the total content of Ca, Al, and S is 51% or more, and the balance consists of other unavoidable impurities. Steel material.

(3) C: 0.05 to 0.1 in mass%
8%, Si: 0.05 to 0.3%, Mn: 0.4 to 2.
0%, P: 0.02% or less, S: 0.02% or less, C
u: 0.1 to 1.0%, Ni: 0.1 to 1.0%, A
1: 0.005 to 0.04%, Ti: 0.005 to 0.
03%, Ca: 0.0005 to 0.003%, Mg:
A steel containing 0.002% or less, the balance being Fe and inevitable impurities, and having a circle equivalent diameter of 0.0
It contains 100 to 3000 particles / mm ^{2 in} number density per unit area of oxide particles of 05 to 2.0 μm, and the composition of the oxide particles contains at least Ca, Al, Mg and O,
The mass ratio of elements excluding O is Ca: 5% or more, Al: 5
% Or more, Mg: 1% or more, respectively, Ca and Al
A steel material excellent in low-temperature toughness and stress corrosion cracking characteristics of a welded joint, characterized in that the total amount of Mg and Mg is 51% or more and the microstructure of the steel is a bainite structure having an area ratio of 50% or more.

(4) The composition of the oxide particles contains at least Ca, Al, Mg, O, and S, and the elements excluding O are in a mass ratio of Ca: 5% or more, Al: 5% or more, Mg: 1
%, S: 1% or more, respectively, the total of Ca, Al, Mg, and S is 52% or more, and the balance is composed of other unavoidable impurities. Steel with excellent toughness and stress corrosion cracking properties.

(5) In mass%, Nb: 0.05% or less, V: 0.1% or less, Cr: 0.6% or less, Mo: 0.6
% Or less, and a steel material excellent in low temperature toughness and stress corrosion cracking characteristics of the welded joint according to any one of the above (1) to (4), characterized by containing one or more kinds.

(6) B: 0.0005 to 5% by mass
0.003% is contained, and the above (1) to
A steel material having excellent low temperature toughness and stress corrosion cracking characteristics according to any one of (5).

(7) The oxide particles have an equivalent circle diameter of 0.
1 to 2.0 μm, characterized in that (1) to
A steel material having excellent low-temperature toughness and stress corrosion cracking properties according to any one of (6).

(8) After rolling the steel having the component described in any one of (1), (3), (5) or (6), water cooling is started from the austenite temperature range to 250 ° C.
A method for producing a steel material excellent in low temperature toughness and stress corrosion cracking characteristics of a welded joint, characterized in that water cooling is stopped at 400 ° C.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. The present inventors examined the achievement of reheat austenite grain refinement in the HAZ region heated to 1400 ° C. or higher by using an oxide as a metallographic factor that improves the low temperature toughness of the welded joint.

In order to refine the reheated austenite grains, it is necessary to suppress the growth of austenite grains at high temperature. The most effective method is to pin the austenite grain boundaries with dispersed particles and stop the movement of the grain boundaries. As one of the dispersed particles having such a function, it has been conventionally considered that Ti nitride and oxide are effective. However, since Ti nitride has a large proportion of solid solution at a high temperature of 1400 ° C. or higher, it has a small pinning effect and is not suitable for pinning particles. Therefore, we decided to utilize oxides that are stable at high temperatures as pinning particles.

Further, the pinning effect of the crystal grain boundaries by the dispersed particles becomes larger as the volume ratio of the dispersed particles becomes larger and the diameter of each particle becomes larger. However, since the volume ratio of dispersed particles has an upper limit depending on the concentration of the elements that make up the particles contained in the steel, if the volume ratio is assumed to be constant, a smaller particle size is more effective for pinning. . From such a viewpoint, the present inventors have made various studies so as to increase the volume fraction of the oxide and to obtain an appropriate particle size.

One of the means for increasing the volume fraction of oxide is to increase the amount of oxygen. Since increasing the amount of oxygen causes a large amount of coarse oxide, which is harmful to the material, Not an effective means. Therefore, the present inventors examined the utilization of an element having a small solubility product with oxygen in order to maximize the utilization of oxygen. Al is generally used as a strong deoxidizing element, which has a small solubility product with oxygen. However, Al alone is not sufficient to sufficiently utilize oxygen, and a deoxidizing element stronger than Al is necessary, and it is important to utilize Ca that is generally used in the deoxidizing step of steel. Since Ca has a small solubility product with oxygen, it is possible to generate a larger amount of oxide than Al for the same amount of oxygen. As a result of conducting an experiment using Ca as a deoxidizing element, it is necessary that the composition of the oxide particles formed in the steel is such that Ca is 5% or more and Al is 5% or more, and the total is It has been found that the inclusion of 50% or more makes it possible to increase the volume fraction of oxide, that is, the amount of oxide. Based on this result,
The composition of the oxide particles contained in the steel is at least Ca,
The elements containing Al and O and excluding O are 5% Ca by mass.
As described above, Al is set to 5% or more and the total is set to 50% or more.

It is also effective to use Mg together with Ca to produce a large number of oxides. Mg is Ca
Although not so effective, it is a deoxidizing element stronger than Al and has a small solubility product with oxygen. Therefore, Mg to C
By combining with a and using it for deoxidation, the number of oxides can be further increased. The inventors conducted an experiment using Ca as a deoxidizing element, and as a result, as a composition of oxide particles generated in steel, Ca was 5% or more and Al was 5% or more,
It has been found that when the content of Mg is 1% or more and the total content thereof is 51% or more, the volume fraction of oxide, that is, the amount of oxide can be further increased. Based on this result,
The composition of the oxide particles contained in the steel is at least Ca,
Elements containing Al, Mg, and O, except O, are Ca in mass ratio.
Was 5% or more, Al was 5% or more, Mg was 1% or more, and the total was 51% or more.

Furthermore, the present inventors can further increase the volume fraction of oxides and sulfides by depositing sulfides such as CaS and MgS around the oxides. I found that. Based on this result
The composition of particles contained in steel is at least Ca, Al,
The mass ratio of the elements including O and S, excluding O, is 5% or more for Ca, 5% or more for Al, and 1% or more for S, and the total is 51%.
% Or more, or at least Ca, Al, Mg, O,
The elements including S and excluding O contain Ca in a mass ratio of 5% or more,
Al was 5% or more, Mg was 1% or more, S was 1% or more, and the total thereof was 52% or more.

Next, the size of oxide particles effective for pinning will be described.

The pinning effect of the crystal grain boundaries by the dispersed particles is larger as the volume ratio of the dispersed particles is larger and the particle diameter of one particle is larger, but when the volume ratio of the particles is constant, the size of one oxide particle is large. It was thought that the smaller the value, the larger the number of particles and the greater the pinning effect. As a result of detailed examination of the austenite grain size when heated to a high temperature using test pieces having various grain sizes changed, it was found that the grain size of 0.005 to 2.0 μm was found for pinning. He found that the effect was great. Further, it has been found that the pinning force for stopping the movement of the austenite grain boundaries is stronger as the size of the dispersed particles is larger, and the particle size of 0.1 to 2.0 μm is particularly preferable among the particle diameters of 0.005 to 2.0 μm. We came to discover that it was effective. When it is less than 0.1 μm, the pinning effect gradually decreases, and when it is less than 0.005 μm, the pinning effect is hardly exhibited. Also,
Oxide particles larger than 2.0 μm have a pinning effect, but may be the starting point of brittle fracture, and are unsuitable for the properties of steel materials. From this result, the required particle size was set to 0.
It was 005 to 2.0 μm. Among them, 0.1 to 2.0 μm is particularly preferable.

Next, the number of pinning particles required for HAZ toughness was examined.

The larger the number of oxide particles, the finer the structural unit, and the larger the number of particles, the better the low temperature toughness of the welded joint. However, the steel material of a liquefied gas transportation ship, which is considered to have particularly severe required characteristics, has a high strength. Weld joint low temperature toughness required when large heat input welding is performed at, for example, test temperature -40
In order to satisfy the absorbed energy of 50 J or more at ℃, as shown in FIG.
It was found that the number of μm oxide particles needs to be 100 particles / mm ² or more. However, the larger the number of particles, the smaller the effect of improving the toughness, and considering that increasing the number of particles more than necessary increases the possibility that coarse particles harmful to the toughness will be generated. Is 3000 /
mm ² is suitable.

Since the particles such as oxides are dispersed, the reheated austenite in the heat-affected zone of the weld becomes finer and the hardenability deteriorates as described above. Prone. Since the ferrite structure has low hardness, stress corrosion cracking resistance is improved. That is, the oxide particle dispersion of the present invention exerts an effect not only on the low temperature toughness of the weld heat affected zone but also on the stress corrosion cracking resistance.

The size and number of the oxide particles are measured, for example, in the following manner. The size and number of the oxides are measured by making an extraction replica from a steel plate which is a base material, and observing it with an electron microscope at a magnification of 10,000 times for 20 fields or more and an observation area of 1000 μm ² or more. To measure the size, for example, the equivalent circle diameter is obtained based on a photograph of particles. At this time, the extracted replica collected from any portion from the surface layer portion to the central portion of the steel sheet may be used. Further, if the particles can be properly observed, the observation magnification may be lowered.

In the process for producing a steel material, cooling is started from the austenite temperature range in water cooling after rolling and the stop temperature is set to 250 to 400 ° C., so that bainite having a microstructure of 50% or more in area ratio can be obtained. The steel has a hardened structure in which ferrite and / or martensite, etc. are present, and has a high strength, while the yield strength is low and the steel material has excellent stress corrosion cracking resistance.

If the cooling stop temperature is less than 250 ° C., the yield strength of 450 MPa or less cannot be achieved. On the other hand, if the cooling stop temperature exceeds 400 ° C., the tensile strength decreases and the tensile strength of 500 MPa or more can be achieved. Can not. Therefore, although the cooling stop temperature is set to 250 to 400 ° C. in the present invention, it is preferable to set the cooling stop temperature to 300 to 350 ° C. in order to optimize the relationship between the yield strength and the tensile strength.

The cooling rate is 4 to 15 ° C./s.
ec, especially 4 to 7.5 ° C./sec is preferable.

Further, the range of basic components of the present invention will be described.

C is an effective component for improving the strength of steel. If C is less than 0.05%, the strength cannot be ensured, and excessive addition significantly reduces the weldability of steel materials and the low temperature toughness of welded joints. Therefore, the upper limit was made 0.18%.

Si is 0.0 for securing the strength of the base material and deoxidizing.
5% or more is a necessary component, but the upper limit was set to 0.3% in order to prevent deterioration of toughness due to hardening of HAZ.

Mn must be added in an amount of 0.4% or more as an effective component for securing the strength and toughness of the base metal, but the upper limit is 2.0% within the allowable range of toughness and cracking of the welded portion. And

The smaller the content of P is, the more preferable it is. However, since it is very costly to industrially reduce this as an impurity, 0.02% is made the upper limit.

As with P, the smaller the content of S is, the more desirable it is. However, it takes a great cost to reduce this industrially, so 0.02% was made the upper limit.

Cu is effective for improving the strength of the steel material and is required to be 0.1% or more. However, if it exceeds 1.0%, the HAZ toughness decreases, so 1.0% is made the upper limit. .

Ni is effective for improving the strength and toughness of the steel material and is required to be 0.1% or more. However, since an increase in the amount of Ni increases the manufacturing cost, 1.0% was made the upper limit.

Al is an important deoxidizing element, and the lower limit value is 0.005%. Also, when Al is present in a large amount,
Since the surface quality of the slab deteriorates, the upper limit was made 0.04%.

Ti is combined with N to form Ti nitride, which is finely precipitated in the slab and is effective for grain refinement of the rolling structure. The Ti nitride present in the steel sheet is HAZ during welding.
To refine the tissue. 0.0 to obtain these effects
Add more than 05%. However, as the amount of solid solution Ti increases, the HAZ toughness decreases, so 0.03% was made the upper limit.

Ca is required to be added in an amount of 0.0005% or more in order to form a Ca-based oxide. However, since excessive addition produces coarse inclusions, 0.0
The upper limit was 03%.

Mg is an element that increases the number of oxides when used in combination with Ca for deoxidation. However,
Since excessive addition produces coarse inclusions, Mg is less than 0.1%.
Although it was 002% or less, preferably 0.0001 to
It is 0.002%.

Nb, V, Cr, and Mo have the effect of improving the strength and toughness of the steel, but in the HAZ part, excessive addition significantly lowers the toughness, so 0.05% each.
%, 0.1%, 0.6%, and 0.6% were made the upper limits.

B is an element which improves the hardenability of steel and also improves the strength. However, if it is less than 0.0005%, a sufficient effect cannot be obtained, and if it exceeds 0.003%, the hardenability is increased. Not only is the improvement effect saturated, but the toughness is reduced by forming B precipitates that are harmful to the toughness, so B is 0.0005.
Was made 0.003%.

[0048]

EXAMPLE A steel plate was made as a trial with the chemical composition shown in Table 1.
1 to 8 are steels of the present invention, and 9 to 20 are comparative steels. The trial steel is melted in a converter and deoxidized during vacuum degassing at RH. After casting into a 280 mm thick slab by continuous casting,
It was manufactured as a steel plate having a plate thickness of 30 mm through hot rolling and water cooling. The obtained steel plate was subjected to CO2 welding for a total of 15 passes using a general-purpose welding material. The heat input is about 30 kJ / cm for each pass.

Tables 2 and 3 show the composition of oxides, the number of particles, and
Hot rolling water cooling conditions, base material properties, bainite fraction, base material stress corrosion cracking (SCC) properties, HAZ toughness, HAZ hardness,
And HAZSCC characteristics are shown. The Charpy value for toughness evaluation is the test temperature -60 at the fusion line site.
The average value is obtained by conducting three tests at ° C.

As is clear from Tables 2 and 3, the inventive steels 1 to 8 have absorbed energy (vE-60) as compared with the comparative steels.
It can be seen that the alloy has excellent HAZ toughness of 50 J or more, a YP of 440 MPa or less and a bainite fraction of 50% or more, and good stress corrosion cracking resistance (SCC characteristic evaluation). Also, the HAZ has good resistance to stress corrosion cracking.

On the other hand, Comparative Examples 9 to 20 do not simultaneously satisfy HAZ toughness and stress corrosion cracking resistance as shown in Tables 2 and 3. Nos. 9 to 16 had a composition of oxides or the number of oxides was out of the range of the present invention.
The toughness and HAZ SCC properties are poor. 17, 1
No. 8 was Y because the water cooling stop temperature was out of the range of the present invention.
P exceeds 440 MPa, and the SCC characteristics of the base material are inferior. In Nos. 19 and 20, since the water cooling stop temperature was out of the range of the present invention, YP exceeded 440 MPa, the SCC characteristics of the base material were poor, and the strength was insufficient.

[0052]

[Table 1]

[0053]

[Table 2]

[0054]

[Table 3]

[0055]

INDUSTRIAL APPLICABILITY The present invention can provide a 500 MPa class steel material which is excellent in both low temperature toughness of welded joints and stress corrosion cracking properties, and is manufactured by welding liquefied gas transportation vessels and liquefied gas storage tanks. It can be applied to structures that require that.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the number of oxide particles in steel and HAZ toughness.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yuji Funazu             No. 1 Nishinoshu, Oita-shi, Nippon Steel Corporation             Company Oita Works F-term (reference) 4K032 AA01 AA02 AA04 AA05 AA08                       AA11 AA14 AA16 AA19 AA22                       AA23 AA27 AA29 AA31 AA35                       AA36 BA01 CA02 CC03 CD02                       CD03 CD06

Claims (8)

[Claims] 1. C: 0.05 to 0.18% by mass%,
Si: 0.05 to 0.3%, Mn: 0.4 to 2.0%,
P: 0.02% or less, S: 0.02% or less, Cu: 0.
1 to 1.0%, Ni: 0.1 to 1.0%, Al: 0.0
05-0.04%, Ti: 0.005-0.03%, C
a: 0.0005 to 0.003% is contained, and the balance is Fe
And an unavoidable impurity, wherein the steel contains oxide particles having an equivalent circle diameter of 0.005 to 2.0 μm in a number density per unit area of 100 to 3000 particles / mm ² , and the oxide thereof. The composition of particles is at least Ca, Al, O
And the elements excluding O are in a mass ratio of Ca: 5% or more,
Al: 5% or more, respectively, the total of Ca and Al is 50% or more, and the microstructure of steel is 50% in area ratio.
A steel material excellent in low-temperature toughness and stress corrosion cracking properties of a welded joint characterized by comprising the above bainite structure. 2. The composition of the oxide particles is at least C
The mass ratio of elements including a, Al, O and S, excluding O,
Ca: 5% or more, Al: 5% or more, S: 1% or more, respectively, and the total of Ca, Al and S is 51% or more,
The steel material excellent in low temperature toughness and stress corrosion cracking characteristics of the welded joint according to claim 1, characterized in that the balance comprises other unavoidable impurities. 3. In mass%, C: 0.05 to 0.18%,
Si: 0.05 to 0.3%, Mn: 0.4 to 2.0%,
P: 0.02% or less, S: 0.02% or less, Cu: 0.
1 to 1.0%, Ni: 0.1 to 1.0%, Al: 0.0
05-0.04%, Ti: 0.005-0.03%, C
a: 0.0005 to 0.003%, Mg: 0.002%
A steel containing the following, the balance being Fe and unavoidable impurities, and having a circle equivalent diameter of 0.005 to 2.0 μm in the steel.
The number density of the oxide particles of m is 100 to 100 per unit area.
Containing 3000 pieces / mm ² , the composition of the oxide particles includes at least Ca, Al, Mg, and O, and the elements excluding O are mass ratio, Ca: 5% or more, Al: 5% or more, Mg:
1% or more, respectively, the total of Ca, Al and Mg is 51% or more, and the microstructure of steel is 50% in area ratio.
A steel material excellent in low-temperature toughness and stress corrosion cracking properties of a welded joint characterized by comprising the above bainite structure. 4. The composition of the oxide particles is at least C.
a, Al, Mg, O, S, elements other than O are contained in a mass ratio of Ca: 5% or more, Al: 5% or more, Mg: 1% or more, S: 1% or more, respectively. Ca, Al and Mg
The steel material excellent in low temperature toughness and stress corrosion cracking properties of the welded joint according to claim 3, wherein the total of S and S is 52% or more, and the balance is composed of other unavoidable impurities. 5. Nb: 0.05% or less by mass% V:
One or more of 0.1% or less, Cr: 0.6% or less, and Mo: 0.6% or less are contained, and any one of claims 1 to 4 is included. A steel material with excellent low temperature toughness and stress corrosion cracking properties. 6. In mass%, B: 0.0005 to 0.00
3% is contained, The steel material excellent in the low temperature toughness of a welded joint and stress corrosion cracking characteristic in any one of Claims 1-5 characterized by the above-mentioned. 7. The oxide particles have a circle equivalent diameter of 0.1 to 0.1.
It is 2.0 micrometers, Claims 1-6 characterized by the above-mentioned.
A steel material excellent in low temperature toughness and stress corrosion cracking properties according to any one of 1. 8. A steel having the composition according to claim 1, 3, 5 or 6 is rolled, and then water cooling is started from an austenite temperature range and stopped at 250 ° C. to 400 ° C. A method for manufacturing a steel material having excellent low temperature toughness and stress corrosion cracking characteristics.