JPH07305140A - Composite steel sheet excellent in low temperature toughness and corrosion resistance - Google Patents

Abstract

PURPOSE:To stably produce a composite steel sheet excellent in strength, low temp. toughness and corrosion resistance by producing a composite steel sheet in which dead soft low alloy steel is used as an internal layer, stainless steel is used as both external layers and a transition layer having a componental transition layer is provided on the space between boss by a casting method. CONSTITUTION:A composite steel sheet in which the internal layer is constituted of low carbon alloy steel contg., by weight, 0.05 to 0.020% C, <0.50% Si, 1.0 to 2.2% Mn, <0.03% P, <0.01% S, 0.005 to 0.10% Nb, 0.05 to 1.0% Ni, 0.05 to 0.50% Mo, 0.005 to 0.50% Ti. <0.05% Al and 0.002 to 0.006% N or furthermore contg. one or >=two kinds among 0.05 to 1.0% Cu, 0.05 to 0.5% Cr and 0.001 to 0.005% Ca, and both sides are constituted of stainless steel contg. <0.020% C or an Ni alloy is produced by a casting method. As for the boundary part of both layers, a componental transition layer in which mutual components are coexistent at the time of casting both molten metals is formed at a thickness of 2 to 10% that of the composite steel sheet, and the composite steel sheet excellent in the adhesion of both layers and having high strength and high toughness by the low alloy steel as the internal layer and excellent corrosion resistance by the stainless steel as the external layer can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a composite steel sheet excellent in low temperature toughness and corrosion resistance in which a high corrosion resistant alloy such as stainless steel or nickel alloy is used for the surface layer and a specific low alloy steel is used for the inner layer. is there. The steel plate of the present invention is most suitable for a line pipe in cold regions.

[0002]

2. Description of the Related Art From the viewpoint of safety and economy, a large diameter line pipe for transporting crude oil and natural gas containing a large amount of corrosive substances (H ₂ S, CO ₂ , Cl) is combined with stainless steel and nickel alloy. The adoption of high alloy clad steel pipes is increasing. Conventionally, such a clad steel plate is rolled by a rolling method (for example, JP-A-60-216984, JP-A-6-216984).
No. 2-16892, JP-A No. 63-130283), the explosive deposition method and the like. However, the clad steel sheet based on these rolling methods and explosion-bonding methods has a problem in securing the density of the entire contact surface between the low alloy steel and the high alloy, and securing stable adhesion is a major problem. ing.

[0003]

DISCLOSURE OF THE INVENTION The present invention is a high alloy clad steel sheet which can simultaneously achieve excellent corrosion resistance of a high alloy, strength / toughness of a base material and adhesion of a low alloy steel and a high alloy. It is intended to provide.

[0004]

SUMMARY OF THE INVENTION The gist of the present invention is that in a steel sheet having a multi-layered structure in the order of two kinds of component systems A, B and A, A located on the surface layer is C: 0.020
% Or less of stainless steel or nickel alloy,
The component of B located in the inner layer is% by weight, and C: 0.005 to
0.020%, Si: 0.5% or less, Mn: 1.0 to
2.2%, P: 0.03% or less, S: 0.01% or less,
Nb: 0.005 to 0.10%, Ni: 0.05 to 1.
0%, Mo: 0.05-0.5%, Ti: 0.005-
0.03% Al: 0.05% or less, N: 0.002-
0.006%, and optionally Cu:
0.05-1.0%, Cr: 0.05-0.5%, C
a: 0.001 to 0.005% of 1 type or 2 types or more, the balance consisting of iron and unavoidable impurities, and a thickness of 2 to 10% of the steel plate thickness at the boundary between the surface layer and the inner layer. It is a composite steel sheet excellent in low temperature toughness and corrosion resistance, which is characterized by having a transition layer of sag.

[0005]

The steel sheet of the present invention is a so-called cast clad in which the surface layer (both sides) is made of ultra-low C stainless steel or nickel alloy and the inner layer is made of low-alloy steel (extremely low C-Mn-Nb-Mo-trace Ti). It is a steel plate that ensures corrosion resistance, high strength and high toughness and adhesion between low alloy steel and high alloy.

In the present invention, the stainless steel (surface layer component A) refers to austenite-based, ferrite-based, martensite-based, and two-phase system, and nickel alloy (surface layer component A) refers to nickel alloys such as Incoloy 825 and Inconel 625. It is a high alloy material with excellent corrosion resistance.
Further, the low alloy steel (inner layer component B) has a characteristic (value in the direction perpendicular to the rolling direction) of strength X52 or more (API standard),
Low temperature toughness 2 vE- ₃₀ ≧ 10 kgf-m, vTrs ≦ -6
It is a high-strength, high-toughness material that can reach 0 ° C.

The steel of the present invention has a multi-layer structure in which two kinds of components A, B and A are arranged in this order, but a transition layer having a thickness of 2 to 10% of the steel plate thickness at the boundary between A and B. It is necessary to have This is because peeling or cracking does not occur at the boundary between A and B. When A and B are in direct contact with each other in the casting method, elements such as C diffuse, martensite is generated in the A layer, and hydrogen is trapped at the interface between A and B, so that hydrogen cracking occurs. When martensite is generated in the A layer, the C content of stainless steel or nickel alloy is limited to 0.020% or less in order to prevent hydrogen cracking.

The reason why the thickness of the transition layer is 2% or more of the plate thickness of the steel sheet is that 2% is the limit value for preventing the occurrence of peeling and cracking, and when the thickness of the transition layer exceeds 10%. It becomes difficult to secure the necessary base material properties (strength, low temperature toughness, weldability). Therefore, the thickness of the transition layer is 2 to the steel plate thickness.
Limited to 10%. This transition layer is obtained by the casting method, in which A first starts solidification from the surface and B subsequently starts solidification at the site where the solidification ends. Therefore, it is necessary that the A and B layers sandwiching the transition layer are solidified continuously without being mixed with each other and are macroscopically separated from the viewpoint of component analysis. The transition layer formed during casting is retained even after heating, rolling and heat treatment.

Next, the reasons for limiting the components of the low alloy steel of the present invention will be explained. In order to secure the strength and low temperature toughness of low alloy steel and the corrosion resistance of high alloy, C, Mn, Nb, Mo,
The amount of Ti is 0.005-0.020% and 1.0, respectively.
.About.2.2%, 0.005 to 0.10%, 0.05 to 0.
It is limited to 5% and 0.005-0.03%.

The upper limit of the C content is a limit value for preventing martensite cracking in the transition layer. If the C content of the low alloy steel is too high, martensite is generated in the transition layer during the production of a multi-layer cast product, and internal cracking occurs. Therefore, it is necessary to limit the C content of the low alloy steel to 0.020% or less. The lower limit of the amount of Mn is the minimum amount for exerting the desired strength and toughness of the base metal and welded portion and the effect of refining the crystal grains. The upper limit is a limit value for obtaining excellent low temperature toughness and field weldability of the base material.

In the present invention, Nb:
0.005-0.10%, Ti: 0.005-0.03
% Is included. Nb contributes to grain refinement and precipitation hardening in controlled rolling, and has the effect of strengthening the steel.
By adding Nb in an amount of 0.005% or more, grain refinement and precipitation hardening progress, and excellent strength and toughness can be obtained as compared with the conventional clad steel sheet. However, if Nb is 0.
If over 10% is added, the field weldability and the toughness of the welded portion deteriorate, so the upper limit was made 0.10%.

Mo is an element that improves the strength of the base material, but it has no effect unless added in an amount of 0.05% or more. But,
When Mo exceeds 0.5%, the toughness and weldability of the welded portion are deteriorated, which is not preferable, so the upper limit is limited to 0.5%. Ti forms fine TiN and is effective in improving the toughness of the base material and the weld heat affected zone (HAZ) by suppressing the coarsening of γ grains in the welded portion during reheating of the slab. This effect is particularly important when rolling is finished at high temperature. TiN
In order to bring out the full effect of T, at least 0.005% T
i addition is required. However, if the Ti content is too large, the T
Since iN coarsening and precipitation hardening due to TiC occur and low temperature toughness deteriorates, it is necessary to limit the upper limit to 0.03%.

Next, the reasons for limiting other elements will be described. Si is an element that strengthens the steel, but if added in a large amount, it deteriorates the weldability and HAZ toughness, so the upper limit was made 0.5%. Ti alone is sufficient for deoxidizing steel,
Si does not necessarily have to be added. In the steel of the present invention, P and S which are impurities are 0.03% or less, 0.0
The reason for setting the content to 05% or less is to further improve the low temperature toughness of the base material and the welded portion. Reduction of P prevents grain boundary destruction, and reduction of S prevents deterioration of toughness due to MnS.
Preferred P and S contents are 0.01% or less and 0.00, respectively.
It is 3% or less.

Ni improves the strength and toughness of the base metal without adversely affecting the weldability and HAZ toughness.
If it is less than 5%, the effect is small, and if it exceeds 1.00%, the weldability is not preferable.
%. Al is an element usually contained in steel as a deoxidizing agent, but deoxidation can also be performed with Ti or Si, and it is not always necessary to add it. If the amount of Al exceeds 0.05%, Al-based nonmetallic inclusions increase and impair the cleanliness of steel, so the upper limit was made 0.05%.

N forms TiN and improves the base material toughness and HAZ toughness through the effect of suppressing the coarsening of γ grains. The minimum amount of N for this purpose is 0.002%. However, too much N causes deterioration of HAZ toughness due to solid solution N, so it is necessary to suppress the content to 0.006% or less. Next Cu,
The reason for adding Cr and Ca will be described.

The main purpose of adding these elements in addition to the basic components is to improve the properties such as strength and low temperature toughness of the base metal without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount of addition is limited by itself. Cu has almost the same effect as Ni, and also has an effect on corrosion resistance, hydrogen-induced cracking resistance, etc., but if it is less than 0.05%, the effect is weak, and if it exceeds 1.00%, Cu- during hot rolling is used. Cracks occur, making manufacturing difficult. Therefore, the range of Cu is 0.05 to 1.00.
%.

Cr is an element that enhances the strength of the base material,
It is necessary to add 0.05% or more. However, if the Cr content exceeds 0.5%, the weldability and HAZ toughness deteriorate, so the upper limit was made 0.5%. Ca controls the morphology of sulfide (MnS), increases Charpy absorbed energy to improve low temperature toughness, and is also effective in improving hydrogen-induced cracking resistance. However, if Ca is less than 0.001%, there is no practical effect, and if it exceeds 0.005%, Ca is
O and CaS are produced in large amounts to form large inclusions, which not only impair not only the toughness of steel but also the cleanliness, and also have an adverse effect on weldability. Therefore, the range of Ca addition amount is 0.001 to 0.00.
It was set to 5%.

In order to improve the hydrogen-induced cracking resistance, the S and O contents are reduced to 0.001% or less and 0.002% or less, respectively, and ESSP = (Ca) [1-124 (O)].
It is particularly effective to set /1.25(S)≧0.7.

[0019]

EXAMPLES Clad steel plates having various components shown in Tables 1 and 2 (continued from Table 1) were manufactured, and strength of low alloy steel base material, low temperature toughness (Charpy impact test), and corrosion resistance of high alloys (pitting corrosion) With or without, test condition: 10% FeCl ₃ .6H ₂ O solution in SUS316L at 15 ° C. for 48 hours, Incoloy 8
No. 25 was immersed at 30 ° C. for 48 hours), and the adhesion between the low alloy steel and the high alloy (flaw detection by ultrasonic waves) was investigated. Table 3 shows the results of various tests.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

In Tables 1 to 3, steels 1 to 10 are steels of the present invention and 11 to 20 are comparative steels. The steels 1 to 10 of the present invention have good properties for both low alloy steels and high alloys. On the other hand, the comparative steels 11 to 20 are inferior in the properties of the low alloy steel or the high alloy. Since the comparative steel 11 had a high C content in the inner layer, martensite was generated in the transition layer and cracking occurred. Since the comparative steel 12 has a high C content in the surface layer, martensite was generated in the transition layer and cracking occurred. Comparative Steel 13 had a reduced strength because Mo was not added to the inner layer. In Comparative Steel 14, since Ti was not added to the inner layer, the low temperature toughness deteriorated. Comparative Steel 15 had a low Mn content in the inner layer, so the low temperature toughness deteriorated. Steel 16 did not contain Nb, so its low temperature toughness deteriorated. Steel 17 had a low N content in the inner layer, so the low temperature toughness deteriorated. Steel 18 contained too much Al in the inner layer, so the low temperature toughness deteriorated. Since the ratio of the transition layer in Comparative Steel 19 was too small, cracks occurred in the transition layer. Comparative steel 20
Since the ratio of the transition layer was too large, the strength and toughness deteriorated.

[0024]

EFFECTS OF THE INVENTION The clad steel sheet of the present invention improves various properties such as strength and low temperature toughness of low alloy steel and corrosion resistance of high alloy, and remarkably improves the safety of pipeline.

Claims (2)

[Claims] 1. In a steel sheet having a multilayer structure in the order of two kinds of component systems A, B and A, A located on the surface layer is C:
In a stainless steel or nickel alloy containing 0.020% or less, the content of B in the inner layer is% by weight, C: 0.005 to 0.020%, Si: 0.5% or less, Mn: 1. 0 to 2.2%, P: 0.03% or less, S: 0.01% or less, Nb: 0.005 to 0.10%, Ni: 0.05 to 1.0%, Mo: 0.05 .About.0.5%, Ti: 0.005 to 0.03%, Al: 0.05% or less, N: 0.002 to 0.006%, and the balance consisting of iron and unavoidable impurities, A composite steel sheet having excellent low temperature toughness and corrosion resistance, which has a transition layer having a thickness of 2 to 10% of the steel sheet thickness at the boundary between the surface layer and the inner layer. 2. In a steel sheet having a multilayer structure in the order of two kinds of component systems A, B and A, A located on the surface layer is C:
In a stainless steel or nickel alloy containing 0.020% or less, the content of B in the inner layer is% by weight, C: 0.005 to 0.020%, Si: 0.5% or less, Mn: 1. 0 to 2.2%, P: 0.03% or less, S: 0.01% or less, Nb: 0.005 to 0.10%, Ni: 0.05 to 1.0%, Mo: 0.05 -0.5%, Ti: 0.005-0.03%, Al: 0.05% or less, N: 0.002-0.006%, and further Cu: 0.05-1.0%. , Cr: 0.05 to 0.5%, Ca: 0.001 to 0.005%, containing 1 or 2 or more kinds, the balance consisting of iron and unavoidable impurities, and the boundary between the surface layer and the inner layer A composite steel sheet having excellent low temperature toughness and corrosion resistance, characterized by having a transition layer having a thickness of 2 to 10% of the steel sheet thickness.