JPH08277440A - Production of high strength steel excellent in stress corrosion cracking resistance in electron beam weld zone and steel structural body thereof - Google Patents

Abstract

PURPOSE: To provide a method for producing a high strength steel excellent in stress corrosion cracking resistance in the electron beam weld zone and having >=780MPa yield strength and to provide a method for producing a steel structural body thereof. CONSTITUTION: By incorporating 0.03 to 0.12% C, 0.02 to 0.50% Si, 0.1 to 0.8% Mn, <=0.008% P, <=0.005% S, 3.0 to 7.5% Ni, 0.2 to 1.5% Cr, 0.1 to 1.5% Mo, 0.01 to 0.15% V, 0.01 to 0.06% Al and <=0.0050% N into a steel, a high strength steel excellent in stress corrosion cracking resistance in the electron beam weld zone can be obtd., and moreover, in a steel structural body using the same steel, electron beam welding is executed, and thereafter, heat treatment is executed, by which the high strength structural body excellent in stress corrosion cracking resistance can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel having a yield strength of 780 MPa or more excellent in stress-corrosion cracking resistance of electron beam welds and a method for producing a steel structure thereof.

[0002]

2. Description of the Related Art In recent years, as energy demand has increased, interest in marine development such as ocean resource development and submarine crust geological survey has increased to secure a stable supply of steel. The concept of the structure is becoming active. Steel materials used for these structures include
Structurally, high weldability, high strength, and high toughness are required, and further, it is desired to have stress corrosion cracking resistance even under use environment conditions such as seawater. The demand is of course
It is also applied to the welded portion that constitutes a part of the structure.

The conventional welding in the structure is mainly MIG welding, TIG welding, covered arc welding (SMAW), or latent arc welding (SAW). In these weldings, as the plate thickness increases, the number of laminated layers increases at an accelerating rate, resulting in an enormous amount of construction time. Also, welding defects such as blowholes and slag winding are likely to occur, resulting in poor reliability. In order to respond to these improvements in welding construction efficiency and reliability, application of electron beam welding has come to be considered.

Electron beam welding is a conventional welding method (MIG,
Compared with (TIG, SMAW, SAW), particularly in the range where the weld thickness exceeds 50 mm, it is a region advantageous in terms of cost even at the current state of the art, and the effect increases as the plate thickness increases. In addition, unlike conventional welding, electron beam welding involves melting and joining the steel sheet itself, so when manufacturing the structure, component design or construction considering the strength, toughness, and stress corrosion cracking resistance of the welded part. Need to do. However, it is no exaggeration to say that no consideration has been given to this point in the past.

So far, for example, Japanese Patent Publication No. 60-246.
As disclosed in Japanese Patent Publication No. 41 and Japanese Patent Publication No. 1-25371, Pcm (welding crack susceptibility index) is limited on the assumption that welding is performed by a conventional welding method to improve the weldability of the heat affected zone. However, no study has been made in consideration of coarse grain martensite structure in the electron beam welded portion having a high cooling rate, which significantly reduces toughness and yield strength, and stress corrosion cracking in seawater.

In order to solve such a problem, the inventors of the present invention have already proposed in JP-A-2-282446 a high toughness and high strength steel having an excellent electron beam weldability, and the strength of the electron beam weld portion is high. Although the toughness could be improved, it is further desired that the yield strength of the electron beam welded portion be increased and the resistance to stress corrosion cracking be improved.

[0007]

DISCLOSURE OF THE INVENTION The present invention is to solve the above-mentioned problems, and the yield strength, low temperature toughness and stress corrosion cracking resistance of the welded portion are good even if the welding is carried out by electron beam welding. The present invention provides a high-strength steel excellent in stress corrosion cracking resistance of an electron beam welded portion and a method for manufacturing the steel structure thereof.

[0008]

The present invention, in% by weight, comprises C:
0.03 to 0.12%, Si: 0.02 to 0.50%,
Mn: 0.1 to 0.8%, P: 0.008% or less, S:
0.005% or less, Ni: 3.0 to 7.5%, Cr:
0.2-1.5%, Mo: 0.1-1.5%, V: 0.
01-0.15%, Al: 0.01-0.06%, N:
Steel containing 0.0050% or less, with the balance being Fe and inevitable impurities, and Cu: 0.
1-1.5%, Nb: 0.005-0.05%, Ti:
A high-strength steel excellent in stress corrosion cracking resistance of an electron beam weld, characterized by containing one or more of a strength improving element group consisting of 0.005 to 0.03%.
Further, it is a method for producing a steel structure using the above-mentioned steel, which is heat-treated at a temperature of 300 ° C. to Ac ₁ point or less after electron beam welding.

[0009]

The present invention will be described in detail below. Unlike the conventional welding method, the electron beam welding is not to supply another material to the welded portion and improve the characteristics of the welded portion, but to melt and weld the object to be welded. Therefore, when manufacturing a welded object such as high strength steel with a yield strength of 780 MPa or more, austenite grains are refined by quenching and tempering and the tempered martensite structure (or tempered lower betenite structure) provides high strength and high toughness. However, in the electron beam welded part, the austenite grains become remarkably coarse due to the melting and solidifying structure, and the rapid cooling rate contributes to the martensite structure (or lower part It becomes a bainite structure) and the yield strength is significantly reduced. Further, in the case of a martensite-based structure, the toughness is significantly reduced as compared with the lower bainite-based structure. Furthermore, since the as-welded state, the resistance to stress corrosion cracking in seawater tends to decrease.

The inventors have conducted various studies on steel materials having resistance to stress corrosion cracking in seawater or salt water at electron beam welds and having better yield strength and low temperature toughness, and as a result, coarse grain martensite has been obtained. If C and P are high in the main structure, the grain boundaries and the inside of the grains are significantly embrittled, and the low temperature toughness and stress corrosion cracking resistance of the electron beam welded portion are reduced.

Further, the yield strength of the electron beam welded portion is remarkably lowered in the as-welded state, but the post-heat treatment at an appropriate temperature after welding remarkably improves the yield strength, and further, the hardness is lowered by the post-heat treatment and the carbide fineness is increased. It has been found that stress corrosion cracking resistance is also improved by precipitation (the carbide is considered to be a trap site for hydrogen and is considered to have an effect of suppressing hydrogen from accumulating at grain boundaries and embrittlement). The steel material may be any of thick plate, steel pipe, shaped steel, and bar steel.

FIG. 1 shows steel H (0.09% C-0.25% Si-) shown in Table 1.
0.8% Mn-5.0% Ni-0.5% Cr-0.5% Mo-0.06% V-0.03% Al-0.0030%
N-type) as a basic component, and the C and P contents were changed, the results were obtained by conducting a stress corrosion cracking test (KIscc test) in artificial seawater of 3.5% NaCl at electron beam welds. It is a figure which shows the influence of C and P which influences a KIscc value. By setting the C content to 0.13% or less and the P content to 0.008% or less, good stress corrosion cracking resistance of KIscc ≧ 125 MPa√m can be obtained. Further, as described above, even if the C content and the P content are appropriate, if the basic components are not appropriate, excellent strength and toughness at a yield strength of 780 MPa or more cannot be obtained.

The reasons for limiting the components will be described below. C: C is an element necessary to secure strength, 0.03%
If it is less than the above, it becomes difficult to secure the target yield strength. But,
If it exceeds 0.13%, the stress corrosion cracking resistance of the electron beam welded portion is significantly lowered. Therefore, the content of C is 0.03
˜0.13%.

Si: Si is effective for improving the strength, but if it is high, the low temperature toughness and weldability are deteriorated, so the upper limit is set to 0.
50%. However, 0.02% is necessary for steelmaking. Therefore, the Si content is set to 0.02 to 0.50%.

Mn: Mn is an element that increases the strength, and its lower limit is 0.1%. However, if it exceeds 0.8%, the low temperature toughness and the stress corrosion cracking resistance of the electron beam welded portion deteriorate. Therefore, the Mn content is set to 0.1 to 0.8%.

P: P is an important element for improving the low temperature toughness and stress corrosion cracking resistance of the electron beam welded portion, as described above, and it is necessary to reduce it to 0.008% or less. S: S is an element harmful to toughness, and 0.005%
It needs to be reduced to:

Ni: Ni is an element effective for improving the low temperature toughness and the strength, but 3.0% or more is necessary to secure the yield strength of the electron beam welded portion. However, if it exceeds 7.5%, the effect is saturated, and the cost is rather increased. Therefore, the Ni content is set to 3.
It was set to 0 to 7.5%.

Cr: Cr is effective for ensuring strength. However, if less than 0.2%, the effect is reduced. Also, 1.5
%, The low temperature toughness decreases. Therefore, the content of Cr is set to 0.2 to 1.5%.

Mo: Mo, like Cr, is also effective for securing strength. However, if it is less than 0.1%, the effect is reduced. Further, if it exceeds 1.5%, the low temperature toughness decreases. Therefore, the content of Mo is set to 0.1 to 1.5%.

V: V is an element effective for securing strength. Particularly, it is effective in securing the yield strength of the electron beam welded portion by forming carbide in the post heat treatment after electron beam welding and precipitation hardening, and the lower limit is 0.01%. But 0.1
If it exceeds 5%, the toughness of the electron beam welded portion is lowered.

Al: Al is an element that combines with N to refine austenite grains and improve toughness. Therefore, 0.01% is required to make the base material finer. But,
In the electron beam welded portion, since the cooling rate after melting is fast, even if a large amount of Al is added, N and Al do not bond with each other, and solute Al tends to increase and the toughness tends to decrease.
The upper limit is 0%. Therefore, the Al content is 0.01 to
It was set to 0.06%.

N: N is an element detrimental to low temperature toughness and stress corrosion cracking resistance of electron beam welds. Further, if the N content is high, welding defects such as poor penetration easily occur at the beam tip of the electron beam weld. From these things, N content was 0.0050% or less.

Although the above are the basic components of the steel in the present invention, in the present invention, in order to further improve the strength and toughness, one or more of the following strength / toughness improving element groups can be added. Cu: Cu is effective in increasing the strength without lowering the toughness, and its lower limit is 0.1%. However, 1.
If it exceeds 5%, hot workability and hot cracking of electron beam welds may occur. Therefore, the Cu content is set to 0.
It was set to 1 to 1.5%.

Nb: Nb is effective for securing strength and toughness by fine graining, and its lower limit is 0.005%.
However, if it exceeds 0.05%, the toughness of the electron beam welded portion is lowered. Therefore, the Nb content is 0.005
It was set to 0.05%.

Ti: Ti is an element that combines with N to make austenite grains finer, and has a base material toughness and HAZ.
(Welding heat affected zone) Add to improve toughness. 0.0
If it is less than 05%, its effect is small, and if it exceeds 0.03%, the toughness of the base material and the HAZ toughness are rather lowered. Therefore, the content of Ti is set to 0.005 to 0.03%.

In melting this steel, an electric furnace,
Any of the converters may be used. Further, in manufacturing the steel material, any of forging, rolling and forming may be used. The heat treatment of the steel material is preferably a quenching and tempering treatment. The quenching treatment in this case may be repeated twice or more by reheating, or may be performed by direct quenching treatment after rolling, or may be performed by reheating quenching treatment after direct quenching treatment.

Next, a method for manufacturing a structure, which is another skeleton of the present invention, will be described. It is desirable to heat-treat the electron beam welded part of the above-mentioned steel structure after the electron beam welding in order to obtain the desired yield strength and further favorable low temperature toughness and stress corrosion cracking resistance.

That is, the electron beam welded steel structure is then subjected to post heat treatment at a temperature of 300 ° C. to Ac ₁ point or lower. FIG. 2 is a diagram showing the influence of the post heat treatment temperature on the yield strength of the electron beam welded portion of the composition steel B of the present invention. If the post heat treatment temperature is less than 300 ° C., the as-welded martensite structure is not tempered so that precipitation of fine carbide does not occur, and the desired yield strength of 780 MPa or more cannot be obtained.
At an Ac of ₁ or more, the yield strength is significantly reduced due to the formation of retained austenite.

Further, in the temperature range of 300 ° C. to Ac ₁ point or lower, the stress corrosion cracking resistance is improved. The cooling after the post heat treatment may be any of water cooling, air cooling, and slow cooling at 300 ° C. to 500 ° C. or less, but water cooling is preferable at 500 ° C. to Ac ₁ point or less from the viewpoint of toughness, but is not particularly limited. .

[0030]

EXAMPLES Among the chemical components shown in Table 1, steels A to R are steels of the present invention, and steels S to X are comparative steels. Steel was melted in a converter, made into a slab by continuous casting and a steel ingot decomposition method, and then rolled into a steel plate (plate thickness 50 to 100 mm) by plate rolling. The heat treatment conditions for the steel sheet are quenching treatments 840 to 91.
Following 0 ° C water cooling, tempering treatment (tempering temperature 550-6
50 ° C.). Then, after electron beam welding to these steel sheets, post heat treatment was performed at each heat treatment temperature of the method of the present invention and the comparative method shown in Table 2. However, the electron beam welding conditions are an acceleration voltage of 150 kV and a beam current of 100 to 180.
mA, welding speed is 20 to 30 cm / min.

[0031]

[Table 1]

[0032]

[Table 2]

Table 3 shows the tensile test (according to JISZ2201), the Charpy impact test (according to JISZ2202) and the ASTM E 39 of artificial seawater of 3.5% NaCl of the base metal part and electron beam welded part of these steel sheets.
The Iscc test result using the test piece shown in 9 is shown. still,
The sampling position and the notch position of each test piece are the center of the plate thickness in the case of the base metal, and the center of the center of the thickness of the weld metal in the case of the electron beam welded part.

[0034]

[Table 3]

[0035]

[Table 4]

The thick underlined portions in the table indicate locations outside the scope of the invention and those with insufficient characteristics. Examples of the present invention (A-1 to R-1 in which the steel of the present invention and the method of the present invention are combined
In 8), good yield strength of electron beam welded part,
It has toughness and stress corrosion cracking resistance.

On the other hand, even in the method of the present invention, the comparative steels (S to S) which deviate from the chemical composition range limited by the present invention.
In the comparative example combined with X), the limit [KIscc] of the electron beam weld is low in Example S-19 due to the high [C] content. Example T-20 has a high [P] content and low toughness and limit KIscc of the electron beam weld. In Example U-21, since [V] is not added, precipitation hardening cannot be expected in the post heat treatment after electron beam welding, and a yield strength of 780 MPa class cannot be obtained.

In Example V-22, the [N] content is high and the toughness and limit KIscc of the electron beam weld are low. Example W-23 is [N
i) 88 in the base metal and electron beam welds due to the low amount
Yield strength of 0 MPa class cannot be obtained. Example X-24 is [M
o] Low amount and 880MP in base metal and electron beam weld
A class yield strength cannot be obtained.

Next, even in the case of the steel of the present invention, Examples B-25, H-28, N in Comparative Examples in combination with the comparative method (manufacturing conditions No. 25 to 33) deviating from the scope of the method of the present invention. -3
In No. 1, the post-heat treatment after electron beam welding is not performed, and therefore the yield strength and the limit KIscc of the electron beam welded portion are low.
Examples B-26, H-29 and N-32 have low yield strength and critical KIscc due to the low post heat treatment temperature after electron beam welding. In Examples B-27, H-30, and N-33, the post heat treatment temperature after electron beam welding exceeded the Ac ₁ point, and the yield strength was
Remarkably low toughness and critical KIscc.

[0040]

The combination of the component range of the present invention and the post heat treatment after electron beam welding stabilizes the high strength steel having good yield strength, toughness and stress corrosion cracking resistance of the electron beam welded portion. It is provided as a result and has a great industrial effect.

[Brief description of drawings]

FIG. 1 is a chart showing the influence of P amount and C amount on a limit KIscc value of an electron beam welded portion.

FIG. 2 is a chart showing the effect of post heat treatment temperature on the yield strength of electron beam welds.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C22C 38/50 C22C 38/50 38/60 38/60

Claims (4)

[Claims] 1. C .: 0.03 to 0.13% Si: 0.02 to 0.50% Mn: 0.1 to 0.8% P: 0.008% or less S: 0.0. 005% or less Ni: 3.0 to 7.5% Cr: 0.2 to 1.5% Mo: 0.1 to 1.5% V: 0.01 to 0.15% Al: 0.01 to 0 0.06% N: 0.0050% or less, the balance being Fe and unavoidable impurities, and a high-strength steel excellent in stress corrosion cracking resistance of an electron beam weld. 2. C .: 0.03 to 0.13% Si: 0.02 to 0.50% Mn: 0.1 to 0.8% P: 0.008% or less S: 0.0. 005% or less Ni: 3.0 to 7.5% Cr: 0.2 to 1.5% Mo: 0.1 to 1.5% V: 0.01 to 0.15% Al: 0.01 to 0 0.06% N: 0.0050% or less is contained, and further, Cu: 0.1-1.5% Nb: 0.005-0.05% Ti: 0.005-0.03% Strength and toughness improvement A high-strength steel excellent in stress corrosion cracking resistance of an electron beam weld, which contains one or more elements from the element group and the balance is iron and inevitable impurities. 3. By weight%, C: 0.03 to 0.13% Si: 0.02 to 0.50% Mn: 0.1 to 0.8% P: 0.008% or less S: 0.0. 005% or less Ni: 3.0 to 7.5% Cr: 0.2 to 1.5% Mo: 0.1 to 1.5% V: 0.01 to 0.15% Al: 0.01 to 0 0.06% N: 0.0050% or less is contained, and after the electron beam welding is performed on a steel structure having the balance of Fe and inevitable impurities, post heat treatment is performed at a temperature of 300 ° C. to Ac ₁ point or less. Of high strength steel structure with excellent resistance to stress corrosion cracking of electron beam welds. 4. C .: 0.03 to 0.13% Si: 0.02 to 0.50% Mn: 0.1 to 0.8% P: 0.008% or less S: 0.0. 005% or less Ni: 3.0 to 7.5% Cr: 0.2 to 1.5% Mo: 0.1 to 1.5% V: 0.01 to 0.15% Al: 0.01 to 0 0.06% N: 0.0050% or less is contained, and further, Cu: 0.1-1.5% Nb: 0.005-0.05% Ti: 0.005-0.03% Strength and toughness improvement A steel structure containing one or more elements selected from the group of elements, the balance being iron and unavoidable impurities, and electron beam welding, followed by post heat treatment at a temperature of 300 ° C. to Ac ₁ point or less. Of high strength steel structure with excellent resistance to stress corrosion cracking of electron beam welds.