JP2002317243A - High tensile strength steel for welding structure for low temperature use having excellent heat affected zone toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel for welding structures used under low temperature which has excellent toughness in the weld zone subjected to superhigh heat input welding. SOLUTION: The steel has a composition containing, by mass, 0.04 to 0.12% C, 0.01 to 0.5% Si, 0.5 to 2.0% Mn, 0.001 to 0.01% S, 0.005 to 0.02% Ti, 0.015 to 0.050% sol.Al, 0.001 to 0.004% Ca, 0.001 to 0.005% N and 0.001 to 0.007% O, and if required, containing one or more kinds selected from Cu, Ni, Cr, Mo, V, Nb and B, and in which the ratio (X) of the content of oxygen existing as CaO to the total oxygen content is controlled to 0.08 to 0.25, and the ratio (Y) of the content of sulfur present as CaS to the total sulfur content is controlled to 0.1 to 0.3; wherein, X=(the content of oxygen existing as CaO)/(the total content of oxygen contained in the steel), and Y=(the content of sulfur present as CaS)/(the total content of sulfur contained in the steel), wherein, each content denotes their content (mass%) in the steel.

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 used for large-scale welded steel structures such as storage tanks, ships, bridges and buildings, and particularly to a large heat input weld heat affected zone having a heat input of about 60 kJ / cm. -40 ° C. or lower toughness.

[0002]

2. Description of the Related Art In recent years, steel structures have been increasing in size, and in view of reducing construction costs, the number of man-hours for welding work has been reduced.
Large heat input welding that can improve efficiency has been applied.

[0003] When high heat input electrogas welding or submerged arc welding is applied to high-tensile steel, deterioration of the toughness of the weld heat affected zone becomes a problem. 1. coarsening of austenite crystal grains; Formation of upper bainite,
3. The formation of island martensis is mentioned.

[0004] Japanese Patent Publication No. 55-26164 discloses a technique for precipitating fine TiN, suppressing the coarsening of γ crystal grains, and securing the toughness of the heat affected zone of the weld. However, when TiN exceeds 1400 ° C., most of the solid solution forms a solid solution with the base material, so that the crystal grains in the BOND portion are coarsened and deterioration of toughness is inevitable.

JP-A-61-79745 discloses that Ti oxide particles serve as nucleation sites for intragranular ferrite, and JP-A-5-287374 discloses that Ca oxide and Ca oxysulfide are used for intragranular acicular ferrite. It is described that the microstructure of a weld is refined to improve toughness.

Japanese Patent Application Laid-Open No. 10-183295 discloses that a Ti—Al—Ca oxide is used as a nucleus, and TiN or MnS is used.
Is described to suppress the coarsening of γ crystal grains, promote the formation of intragranular ferrite, and improve the toughness of the heat affected zone in welding.

[0007] The present technology is characterized in that in a deoxidizing step, the amount of dissolved oxygen in molten steel is adjusted with Si and then deoxidized in the order of Ti, Al, and Ca.

However, in the method disclosed in Japanese Patent Application Laid-Open No. 5-287374, in order to secure a stable Ca oxide, O is set to 0.0040% or less and Al as a strong deoxidizing element is set to a very small amount of 0.007% or less. Must be controlled.

Furthermore, in order to form Ca oxide, the amount of Al must be adjusted so that O remains appropriately, and when applied to an actual machine, Al cannot be contained.

In order to uniformly disperse finer oxides in steel substantially containing no Al, it is necessary to strictly control the deoxidation method and the addition procedure of various elements. , Heavy burden.

When the amount of Al in the steel is small, the toughness of the weld metal may be deteriorated in a general welding material.

In the method disclosed in Japanese Patent Application Laid-Open No. 10-183295, in order to disperse a large number of Ti-Al-Ca oxides, TiN and MnS uniformly and finely, the dissolved oxygen concentration is reduced to 20 to 20 by preliminary deoxidation with Si. It is necessary to add Ti to deoxidation treatment by adding Ti to 80 ppm of molten steel, and then to add an appropriate amount of Al or Ca within a short time, which is often difficult in actual operation.

Further, the low-temperature toughness of the target weld is-
It is only about 20 ° C and cannot be applied to low temperature structures such as LPG tanks.

[0014]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and has an excellent low temperature toughness (at -50.degree. C.) in a BOND portion of a large heat input welding weld having a heat input of 60 kJ / cm or more. It is an object of the present invention to provide a high-tensile steel having a Charpy absorbed energy of 50 J or more.

[0015]

Means for Solving the Problems The present inventors have developed a heat input 60.
The structure of the weld heat-affected zone, especially the BOND zone, with kJ / cm or more was examined in detail,
By optimizing the amount of aO and CaS, the crystal grains are refined,
It was also found that the formation of intragranular ferrite was promoted and good low-temperature toughness was obtained.

The present invention has been made by further study based on the above findings. That is, the present invention provides: In mass%, C: 0.04 to 0.12%, Si: 0.
01-0.5%, Mn: 0.5-2.0%, S: 0.0
01-0.01%, Ti: 0.005-0.02%, s
ol. Al: 0.015 to 0.050%, Ca: 0.0
01-0.004%, N: 0.001-0.005%,
O: 0.001 to 0.007%, and CaO
(X) of the amount of oxygen present as the total amount of oxygen present as:
0.08 to 0.25, the ratio of the amount of sulfur present as CaS to the total amount of sulfur (Y): toughness of the weld heat affected zone substantially consisting of iron and unavoidable impurities satisfying 0.1 to 0.3 High-strength steel for low-temperature welded structures with excellent performance.

However, X = (amount of oxygen present as CaO) / (total amount of oxygen contained in steel) Y = (amount of sulfur present as CaS) / (total amount of sulfur contained in steel) The amount is the content in steel (mass%).

2. Further, in terms of mass% as a steel component, Cu ≦
0.5%, Ni ≦ 1.0%, Cr ≦ 0.5%, Mo ≦
0.5%, V ≦ 0.1%, Nb ≦ 0.03%, B: 0.
2. A high-strength steel for a welded structure for low-temperature use having excellent toughness in a weld heat-affected zone according to 1, wherein the high-strength steel contains one or more of 0003 to 0.003%.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the reasons for limiting the components in the present invention will be described in detail.

The present invention is characterized in that nucleation of ferrite is promoted in the heat affected zone of welding and the structure thereof is refined.

In order to secure a proper amount of ferrite nucleation sites, CaO and CaS
Are defined as the respective ratios to the total oxygen amount and the total sulfur amount.

X ｛= (amount of oxygen present as CaO)
/ (Total amount of oxygen contained in steel)｝: 0.08 to 0.25 Y ｛= (amount of sulfur present as CaS) / (total amount of sulfur contained in steel)｝: 0.1 to 0 However, each amount is the content in steel (mass%).

X is (the amount of oxygen present as CaO)
If / (total oxygen content in steel) is less than 0.08,
The proportion of alumina increases and ferrite formation is not promoted. On the other hand, if it exceeds 0.25, inclusions become coarse and the toughness is reduced.

Y is (amount of sulfur present as CaS)
// (total sulfur content in the steel), where X is 0.08 to 0.
Within the range of 25, if it is less than 0.1, solid solution S increases and toughness decreases, and if it exceeds 0.3, CaS coarsens and ferrite formation is suppressed.
And

The amount of oxygen present as CaO [OasC
aO] is obtained by subjecting a residue extracted and separated from steel by the bromine methanol method or the iodine methanol method to alkali dissolution treatment, and then performing CaP emission spectroscopic analysis on the Ca concentration ([Ca]).
(Br-M, ppm) is measured, and the oxygen amount (OasCaO, ppm) that becomes CaO from the Ca concentration is determined by the following equation. The total oxygen content in the steel is measured by an inert gas melting-infrared absorption method.

[OasCaO] = 2.5 × [Ca] The amount of sulfur present as Br-MCaS [SasCaS] is given by the following equation.

[SasCaS] = 0.8 × ([Ca] MS−
[Ca] Br-M) However, the Ca concentration ([Ca] MS, ppm) is determined by subjecting the residue extracted and separated from steel by 4% MS electrolytic extraction to alkali dissolution treatment and ICP emission spectroscopy. The total sulfur content in steel is measured by combustion in an oxygen stream-infrared absorption method.

Ca constituting the Ca—Al-based composite oxide
When O and CaS are specified as described above, the reason why the BOND toughness of the weld heat affected zone is improved is that the form of the precipitate is suitable for the nucleation site of ferrite and the nucleation site of ferrite is increased. Probable, but details are unknown.

Further, the present oxide is very stable thermally,
Since the influence of welding is small, the precipitation form specified in the base material is not impaired in the welded portion, and the effect is exhibited.

The method for producing steel satisfying the above X and Y is not particularly limited, but after adding Ca, the temperature of the molten steel treatment is increased to 1620 ° C.
It is desirable to adjust the molten steel temperature, for example, by raising the temperature to 1670 ° C. After continuous casting, there is no problem in applying the direct rolling in which the slab is rolled at a temperature exceeding Ar3.

The steel of the present invention promotes ferrite nucleation by the above-described Ca-Al-based composite oxide, and has the necessary mechanical properties as a high-strength steel material for low-temperature welding structures applied to large structures. In order to satisfy them, the following composition is used.

C C is an element effective for ensuring strength. In order to obtain the effect, 0.04% or more is added. On the other hand, 0.12
%, The addition of a large amount in excess of 0.04 to 0.12% (0.04% to 0.04%) may cause the formation of island martensite in the heat affected zone.
4% or more and 0.12% or less).

Si Si is added because it secures strength and is necessary as a deoxidizing agent in the steelmaking process. If it is less than 0.01%, the effect is insufficient, and 0.01% or more is added. On the other hand, if added in excess of 0.5%, high carbon island martensite is generated and the toughness of the heat affected zone is deteriorated.

Mn Mn is added to secure strength. If it is less than 0.5%, the effect is insufficient, and 0.5% or more is added. on the other hand,
When added in excess of 2.0%, the quenching properties are increased, and the weldability and the toughness of the heat affected zone deteriorate, so
%.

S S is necessary for generating CaS which becomes a nucleation site of ferrite in the heat affected zone. In order to obtain the effect, the content is made 0.001% or more. On the other hand, 0.01%
If it exceeds, the toughness of the base metal and the weld will deteriorate, and
To reduce the crack sensitivity of the weld,
0.01%.

Ti Ti suppresses the coarsening of γ grains in the heat affected zone of welding and, when added in combination with Ca, forms TiN which promotes the nucleation of ferrite, using Ca-Al complex oxide by Ca as nucleus. To be added. To obtain the effect,
Add 005% or more. On the other hand, if it exceeds 0.02%,
Coarse Ti that degrades toughness of base metal and HAZ
Since C precipitates, the content is made 0.005 to 0.02%.

Sol. Al Al is added in order to generate a deoxidizing agent and inclusions serving as precipitation sites for intragranular ferrite. In order to obtain the effect, 0.015% or more is added. On the other hand, if 0.050% or more is added, the inclusions become coarse and the toughness decreases, so the content is 0.015 to 0.050%.

Ca Ca has the effect of forming a Ca—Al-based composite oxide, using the nucleus as a nucleus to precipitate CaO (which subsequently becomes a precipitation nucleus of TiN) and CaS, thereby precipitating intragranular ferrite. In the present invention, it is an essential element and essential addition. 0.001%
If less than 0.004%, the effect cannot be sufficiently obtained.
When added in excess of 0.001%, large inclusions and clusters are formed, and the cleanliness of the steel deteriorates.
4%.

NN forms TiN, suppresses coarsening of γ crystal grains in the heat affected zone of welding, causes nucleation of ferrite, and precipitates intragranular ferrite. On the other hand, if it exceeds 0.005%, the amount of dissolved N increases,
In order to degrade the toughness of the base material and weld,
0.005%.

O O is Ca—Al which serves as a nucleation site for TiN and CaS.
Generates composite oxides and improves the toughness of the heat affected zone. In order to obtain the effect, the content is made 0.001% or more. On the other hand, if the content exceeds 0.007%, O in steel becomes excessive and the base material toughness deteriorates.

The above is the basic component composition of the present invention. In order to improve the strength characteristics, Cu, Ni, Cr, M
One, two or more of o, V, Nb, and B can be contained.

Cu Cu is added in an amount of 0.5% or less, because excessive addition of Cu impairs weldability and deteriorates the toughness of the base material and the weld heat affected zone due to precipitation hardening.

Ni Ni is added in an amount of 1.0% or less because excessive addition of Ni impairs weldability and raises production costs.

Cr, Mo, V, Nb If Cr, Mo, V, Nb is excessively added, the weldability is impaired and the toughness of the heat affected zone is deteriorated.
≦ 0.5%, Mo ≦ 0.5%, V ≦ 0.1%, Nb ≦
0.03%.

BB suppresses the growth of the ferrite side plate which is harmful to the toughness of the weld heat affected zone, and fixes solid solution N as BN to prevent the toughness of the weld heat affected zone from deteriorating. However, excessive addition increases solid solution B, impairs weldability, improves the hardenability of the weld heat affected zone, and degrades toughness.
0003 to 0.003%.

Incidentally, in the present invention, it is possible to contain a trace element as long as the function and effect are not impaired.

The steel of the present invention can be subjected to controlled rolling, controlled cooling, quenching and tempering in accordance with the desired mechanical properties of the base material, and its manufacturing method is not specified. The characteristics of are not impaired.

[0048]

EXAMPLES Steel having the chemical composition shown in Table 1 was melted and then melted.
After heating to 0 to 1250 ° C., controlled rolling, and air cooling, a steel sheet having a thickness of 20 mm was obtained.

With respect to the obtained steel sheet, the strength and toughness of the base material were investigated, and joints were manufactured by electrogas arc welding at a heat input of 60 kJ / cm. Samples were taken from a 厚 of the plate thickness, and the impact absorption energy was determined at a test temperature of −50 ° C.
Furthermore, a weld cycle reproduction heat cycle test (maximum heating temperature 140
0 ° C, 800-500 ° C cooling time 80 sec, plate thickness 2
0 mm, heat input 60 kJ / cm) at the same test temperature of -50.
C. was performed.

Table 2 shows the test results. No. of the steel of the present invention.
Sample Nos. 1 to 11 have excellent absorption energy at a tensile strength of 500 N / mm @ 2 and -50 DEG C. of 200 J or more.

On the other hand, the comparative steel No. 12 to 15, 17, 1
Sample Nos. 8 and 22 have a component composition out of the range of the present invention, and have the same base metal properties as the steel of the present invention, but are inferior to the welded portion reproduction thermal cycle test of less than 40 J.

Comparative steel No. No. 16, Ti and N are comparative steel No. In No. 23, S was out of the range of the present invention, and the base metal toughness or the results of the weld cycle reproduction thermal cycle test were inferior.

Comparative steel No. Comparative Steel No. 21 has a low C content outside the range of the present invention, a low base metal strength, and a comparative steel No. 21. 19, 20, 2
In Nos. 4 to 26, both the X and Y values were out of the range of the present invention, and the welding cycle reproduction heat cycle test results were inferior.

[0054]

[Table 1]

[0055]

[Table 2]

[0056]

According to the present invention, it is possible to obtain a welding structural steel excellent in large heat input weld toughness at a low temperature with a low-cost component composition and without requiring a complicated manufacturing process. It is.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor ▲ Kaku ▼ Hiroyuki 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Inside Nihon Kokan Co., Ltd.

Claims (2)

[Claims] C: 0.04 to 0.12% by mass%
Si: 0.01 to 0.5%, Mn: 0.5 to 2.0%,
S: 0.001 to 0.01%, Ti: 0.005 to 0.
02%, sol. Al: 0.015 to 0.050%, C
a: 0.001 to 0.004%, N: 0.001 to 0.
005%, O: 0.001 to 0.007%, and the ratio of the amount of oxygen present as CaO to the total amount of oxygen (X): 0.08 to 0.25, the amount of sulfur present as CaS Ratio to total sulfur (Y): 0.1 to 0.3
A high-strength steel for welding structures for low-temperature use, which is excellent in the toughness of the weld heat-affected zone substantially consisting of iron and unavoidable impurities. Here, X = (amount of oxygen present as CaO) / (total oxygen content in steel) Y = (amount of sulfur present as CaS) / (total sulfur content in steel) Content in steel (mass%). 2. The steel composition further comprises, in mass%, Cu ≦ 0.
5%, Ni ≦ 1.0%, Cr ≦ 0.5%, Mo ≦ 0.5
%, V ≦ 0.1%, Nb ≦ 0.03%, B: 0.000
2. The high-strength steel for welded structures for low-temperature use having excellent toughness of the weld heat-affected zone according to claim 1, comprising one or more of three to 0.003%.