JP2003253380A - High-strength steel superior in toughness of heat- affected zone by ultra-high heat input welding - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength steel which has an excellent toughness in a weld heat affected zone, even when the high-strength steel with a tensile strength of 570 MPa or higher, is subjected to welding of such an ultra-high heat input as exceeding 600 kJ/cm. <P>SOLUTION: The high-strength steel comprises, by mass% 0.01-0.15% C, 0.05-0.50% Si, 0.1-2.0% Mn, 0.05-1.0% Al, 0.05% or less Ti, 0.0070% or less N, 0.020% or less P, and 0.0050% or less S; satisfying the following expressions (1), (2), and (3): 0.1-0.125×(%Al)≤(%C)≤0.09+0.13×(%Al)...(1) for the case of (%Al)≤0.8, -0.20+0.25×(%Al)≤(%C)≤0.09+0.13×(%Al)...(2) for the case of (%Al)>0.8, and (%N)-(%Ti)/3.4<0.0015...(3); and having the carbon equivalent Ceq of 0.36 or more, which is defined by the following expression (4): Ceq=(%C)+(%Si)/24+(%Mn)/6...(4). <P>COPYRIGHT: (C)2003,JPO

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 excellent in toughness of a heat-affected zone of super-high heat input welding suitable for use in various fields such as shipbuilding, construction and civil engineering, and particularly, carbon equivalent (C _eq )
Of 0.36 or more and a tensile strength (TS) of 570 MPa or more, the welding heat input exceeds 600 kJ / cm without deterioration of the toughness of the weld heat affected zone. This makes it possible to carry out heat welding.

[0002]

2. Description of the Related Art Steel materials used in various fields such as shipbuilding, construction and civil engineering are generally finished by welding to form a structure having a desired shape. In such a structure,
From the viewpoint of safety, not only the toughness of the base material of the steel material used but also the toughness of the weld heat affected zone is required.

At that time, the most important problem is the toughness at the bond portion of the heat affected zone. This is because this bond portion is exposed to a high temperature just below the melting point during the high heat input welding, and the austenite crystal grains are most likely to be coarsened, and further, it is a position where it is likely to be transformed into a brittle upper bainite structure by cooling. Further, in this bond portion, a brittle structure such as a Woodmansstatten structure or island-like martensite is likely to be generated, which also causes a decrease in toughness.

Conventionally, as a measure for improving the toughness of the bond part,
A technique has been put into practical use in which TiN is finely dispersed to suppress coarsening of austenite and is used as a nucleus of ferrite transformation. In addition, Japanese Patent Publication No. 3-53367,
Heat input: 230 kJ / cm Weld joints with the aim of improving toughness are disclosed in Japanese Patent Laid-Open No. 6-184663 in which a rare earth element (REM) is added.
A method is disclosed in which fine particles are dispersed in steel to prevent grain growth of austenite and improve the toughness of a welded portion by adding Ti in combination.

Further, a technique for dispersing Ti oxide,
A technology combining the ferrite nucleation ability of BN has also been developed. In addition, there is also known a technique for improving toughness by adding Ca or REM and controlling the sulfide morphology.

[0006] However, in each of the above-mentioned prior arts, it is difficult to manufacture a steel material that can obtain stable toughness, and in high heat input welding exceeding 100 kJ / cm, sufficient toughness can be obtained in the weld heat affected zone. There was a problem of not having.
That is, in the technology that mainly uses TiN,
The effect disappeared at the welded part heated to the temperature range where M melts, and the toughness was remarkably reduced due to the embrittlement of the matrix structure due to solid solution Ti and N. Further, the technique using the oxide of Ti has a problem that the fine dispersion of the oxide cannot be sufficiently homogenized. Furthermore, even with the technique of adding Ca and REM, it was difficult to secure high toughness in the heat-affected zone of welding with high heat input welding exceeding 100 kJ / cm 2.

On the other hand, in recent years, in manufacturing welded structures such as steel frames of ships and buildings, further improvement in efficiency has been demanded. The reduction of welding time is directly linked to the improvement of efficiency, and therefore the application of large heat input welding is increasing. However, there is no steel material which can stably secure high toughness of the heat affected zone when such large heat input welding is performed, and its development has been desired.

[0008] In this respect, the inventors previously developed a low alloy structural steel excellent in toughness of a weld heat affected zone having a new composition as a solution to the above-mentioned demand, and applied for a patent 2001.
-396979. This technique achieves improvement in toughness by widening the temperature range of two phases of austenite phase and ferrite phase at high temperature by effectively adjusting the composition, and effectively suppressing coarsening of austenite grains. With this technology, welding heat input
Even when high heat input welding exceeding 100 kJ / cm is performed, excellent toughness can be stably obtained not only in the base metal but also in the heat affected zone of the welding.

[0009]

By the way, recently,
Due to the higher strength of steel materials and the further increase of welding heat input, the efficiency of the design and manufacturing of welded structures is being promoted. Among them, ultra-high heat input welding has been started to be applied to high-strength materials with a tensile strength of 570 MPa or more so that the welding heat input exceeds 600 kJ / cm. For example, in the field of construction, when manufacturing a box pillar, a skin plate-
Ultra-high heat input welding with a welding heat input of more than 1000 kJ / cm, such as electroslag welding, is applied to the diaphragm joint.

Generally, the higher the strength of steel, the more difficult it becomes to secure the toughness of the heat-affected zone of welding. However, by applying the technique disclosed in the above-mentioned Japanese Patent Application No. 2001-396979, Good toughness can be obtained even when high-strength heat-welding is applied to high-strength steel. However, even in the above technique, when a high-strength material having a tensile strength (TS) of 570 MPa or more is subjected to ultra-high heat input welding with a welding heat input exceeding 600 kJ / cm, sufficient toughness is not always obtained. Was not always obtained.

[0011]

The present invention advantageously solves the above-mentioned problems, and the welding heat input exceeds 600 kJ / cm for a high strength material having a tensile strength of 570 MPa or more. A high-strength steel with excellent toughness in the heat-affected zone of super-high heat input, which can obtain excellent toughness not only in the base metal but also in the heat-affected zone of the weld, even when extremely high heat-input welding is performed. The purpose is to propose.

That is, the gist of the present invention is as follows. 1. % By mass, C: 0.01 to 0.15%, Si: 0.05 to 0.50%,
Mn: 0.1-2.0%, Al: 0.05-1.0%, Ti: 0.05% or less, N: 0.0070% or less, P: 0.020% or less and S: 0.
0050% or less, the balance is Fe and unavoidable impurities composition, and when the amount of C and Al is the following formulas (1), (2) (% Al) ≤ 0.8 0.1-0.125 × (% Al ) ≦ (% C) ≦ 0.09 + 0.13 × (% Al) --- (1) (% Al)> 0.8 −0.20 + 0.25 × (% Al) ≦ (% C) ≦ 0.09 + 0.13 × (% Al) --- (2) where (% M) satisfies the relationship of M element content (mass%), and N and Ti contents are expressed by the following formula (3) (% N) -(% Ti) /3.4 <0.0015 --- (3) is satisfied, and the following formula (4) C _eq = (% C) + (% Si) / 24 + (% Mn) / 6 --- A high-strength steel excellent in toughness in the heat-affected zone of ultra-high heat input welding, which has a carbon equivalent C _eq specified by (4) of 0.36 or more and a tensile strength (TS) of 570 MPa or more.

2. In the above 1, in terms of mass%, the steel material further contains Nb: 0.1% or less, V: 0.1% or less, Cu: 1.5% or less, Ni: 3.5% or less, Cr: 0.8% or less, Mo: 0.5% or less, B: 0.0030% or less, Ca: 0.0030% or less, REM: 0.02
% Or less and Mg: 0.010% or less, and a composition containing one or more selected from the following formula (4) 'C _eq = (% C) + (% Si) / 24 + (% Mn) / 6 + (% Ni) / 40 + (% Cr) / 5 + (% Mo) / 4 + (% V) / 4 --- (4) ', the carbon equivalent C _eq is 0.36 or more. High-strength steel with excellent toughness in the heat-affected zone.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. As disclosed in the above-mentioned Japanese Patent Application No. 2001-396979, in the equilibrium state of 910 ° C or higher, the temperature range width of the two phases of the austenite phase and the ferrite phase is 25 ° C or higher, and on the low temperature side of the two-phase range. If there is a temperature range of austenite single phase, coarsening of austenite grains is effectively suppressed, and consequently, generation of coarse grains is also suppressed in the weld heat affected zone after welding, and refinement of the structure is achieved. Therefore, good toughness can be obtained. This invention also basically follows the technique disclosed in the above-mentioned Japanese Patent Application No. 2001-396979.

1 (a) and 1 (b), Fe-C of the conventional steel and the steel of the present invention calculated by commercially available calculation software, respectively.
The state diagrams are shown for comparison. As shown in (a) of the figure, in the conventional steel, the (α + γ) two-phase region in the high temperature region is very narrow, and the temperature range width of the (α + γ) two-phase region is 25 ° C or more at 910 ° C or higher. There is no C amount range. On the other hand, as shown in Fig. 2 (b), in the steel whose composition was adjusted according to the present invention, the (α + γ) two-phase region was significantly expanded,
It can be seen that at 910 ° C. or higher, there is a wide C amount range in which the temperature range width of the (α + γ) two-phase region is 25 ° C. or higher. Here, the temperature range width of the (α + γ) two-phase region is 25 ° C.
The above is defined because grain growth at high temperature cannot be effectively suppressed unless the temperature is 25 ° C or higher.

In the present invention, it was further found that in order to obtain high toughness of the weld heat affected zone, it is necessary to have an austenite single phase region on the low temperature side of the above (α + γ) two phase region. That is, the weld heat affected zone is exposed to a high temperature and then cooled. However, when the (α + γ) 2 phase region once changes to the austenite single phase region, the austenite grain boundaries and intragranular grains are formed in the subsequent cooling process. From the transformation nuclei of γ to α transformation, high toughness is achieved by the refinement of the structure. However, in the case of only the (α + γ) 2 phase region in the high temperature region, there is no transformation from nucleation, and coarsening of ferrite grains existing from the high temperature heating stage occurs during cooling to room temperature, so high toughness cannot be obtained.

Incidentally, FIG. 1 (b) is a Fe-C phase diagram in the steel containing Al: 1.0 mass%, and C≤0.22 mass%
It can be seen that, in the range of, the temperature range width, which is the (α + γ) two-phase region, is 25 ° C or more at 1300 ° C or more. However, C
When <0.03 mass%, there is no austenite single-phase temperature range, so a weld heat-affected zone with high toughness cannot be obtained.

The target steel type of the present invention is so-called low alloy welded structural steel. For example, in SM 570 shown in JIS G 3160, when the plate thickness is 100 mm or less, the chemical composition is C: 0.18. mass% or less, Si: 0.55 mass% or less, Mn: 1.
60 mass% or less, P: 0.035 mass% or less, S: 0.035 mass
%, Steel with TS of 570 MPa or more and 720 MPa or less is specified. Also, as a 590 MPa class high-performance steel (SA 440) for building structures, the chemical composition is C: 0.18 mass% or less, S
i: 0.55 mass% or less, Mn: 1.60 mass% or less, P: 0.035 m
ass% or less, S: 0.008 mass% or less, TS is 590 MPa
Above, steel of 740 MPa or less is specified. In addition, 11ma
The ferritic stainless steel containing more than ss% Cr is
The temperature range which is the (α + γ) two-phase region at 910 ° C. or higher is 25 ° C. or higher, and the austenite single phase also exists on the low temperature side, but since it is a high alloy steel, it is outside the technical scope of the present invention.

In the present invention, specifically, it can be obtained by adjusting the composition of the steel in the range of low alloy steel, but since FIGS. 1 (a) and 1 (b) are calculation state diagrams, It is necessary to define the composition of ingredients by careful experiment. Hereinafter, the particularly preferable composition range of the high-strength steel according to the present invention will be described. In addition, "%" about ingredient
Unless otherwise specified, the indication means mass%. C: 0.01 to 0.15% C is an element useful for obtaining the strength required for structural steel, but if it is less than 0.01%, its addition effect is poor,
On the other hand, if it exceeds 0.15%, the toughness of the base material and the welded portion deteriorates, so C is limited to the range of 0.01 to 0.15%.

Si: 0.05 to 0.50% Si requires at least 0.05% in steel making, while 0.50%.
%, The toughness of the base material deteriorates.
It is limited to the range of 0.50%.

Mn: 0.1-2.0% Mn needs to be 0.1% or more to secure the strength of the base metal, but if it exceeds 2.0%, the toughness of the welded portion is deteriorated, so Mn is 0.1-2.0%. It is limited to the range of.

Al: 0.05 to 1.0% Al is an important alloy component in the present invention, and is 0.05%.
Although the above-mentioned addition is required, contrary to the effect of suppressing the growth of austenite grains at high temperature, the addition of Al tends to cause embrittlement of the matrix. Therefore, in the case of high-strength steel having a carbon equivalent C _eq ≧ 0.36, which is the object of the present invention, the upper limit must be 1.0%. Therefore, Al is 0.05-1.
It is limited to the range of 0%.

Ti: 0.05% or less Ti is an element useful for increasing the strength of the steel sheet, and also effectively contributes as a fixed element of solid solution N that deteriorates the toughness of the heat affected zone. However, if the content exceeds 0.05%, the toughness deteriorates, so the content was made 0.05% or less. The N content described later is 0.0015.
In the case where the content is not more than%, it is not necessary to fix the solid solution N, so that it is not always necessary to add Ti.

N: 0.0070% or less N is an element inevitably mixed in the steel as an impurity, but if the content exceeds 0.0070%, the toughness of the steel material is deteriorated, so N is suppressed to 0.0070% or less. I decided to do it.

P: 0.020% or less, S: 0.0050% or less If P content exceeds 0.020%, the toughness of the welded portion is deteriorated, so P is limited to 0.020% or less. Similarly, if S also exceeds 0.0050%, it deteriorates the toughness of the base material and the welded portion, so S was suppressed to 0.0050% or less.

Although the proper composition range of the basic components has been described above, it is not sufficient in the present invention that each of the components simply satisfies the above composition range, and the austenite phase and the ferrite phase at an equilibrium state of 910 ° C. or higher. It is necessary that the temperature range width of the two phases is 25 ° C. or more, and that the temperature range of the austenite single phase exists on the low temperature side of the two phases. To do so, if the following equations (1), (2) (% Al) ≤ 0.8 0.1-0.125 x (% Al) ≤ (% C) ≤ 0.09 + 0.13 x (% Al) --- (1) When (% Al)> 0.8 −0.20 + 0.25 × (% Al) ≦ (% C) ≦ 0.09 + 0.13 × (% Al) --- (2) where (% M) is the M element It is important to satisfy the relationship of content (mass%).

As shown in Table 1, steels having fixed amounts of Si and Mn and varying amounts of C and Al were manufactured and subjected to a simulated heat cycle test simulating a welding heat affected zone. After that, a Charpy impact test was conducted to investigate the toughness of each steel. FIG. 2 shows a reproduced thermal cycle pattern applied to the test piece. This pattern simulates the heat-affected zone of ultra-high heat input welding of about 1000 kJ / cm.

FIG. 3 shows the result of the Charpy impact test. Each symbol in the figure shows the value of the absorbed energy (vE ₀ ) at 0 ° C of the steel having the composition of Al and C content at that position, and ○ means vE _-40 ≧ 150 J, △ means Δ 150J>
vE _-40 ≧ 70 J, and x is vE _-40 <70 J.

[0029]

[Table 1]

As shown in FIG. 3, the amounts of C and Al in the steel satisfy the relations of the above formulas (1) and (2), and in the region of Al ≦ 1.0%, especially in the weld heat affected zone. Excellent high toughness is obtained. However, even within the above range, it was occasionally found that sufficient toughness could not be obtained when the carbon equivalent C _eq was 0.36 or more.

Then, the inventors next solve this point.
As a result of further studies, C_eqTS ≧ 570 with ≧ 0.36
For high-strength steel of MPa, welding heat input exceeds 600 kJ / cm.
To improve toughness during super heat input welding
Not only the relationship between C and Al content, but also the relationship between N and Ti content.
Therefore, after satisfying the above expressions (1) and (2), N and
And Ti amount, the following equation (3)     (% N)-(% Ti) /3.4 <0.0015 --- (3) Here, (% M) is the relation of the content of M element (mass%)
Charpy absorption at 0 ℃ by satisfying
Energy (vE ₀) Is 150 J or more, excellent toughness is stable
It was clarified that it can be obtained.

Although the basic components have been described above, other elements described below can be appropriately contained in the present invention. Nb: 0.1% or less, V: 0.1% or less Nb and V are both useful elements for increasing the strength of the steel sheet, but if the content exceeds 0.1%, the toughness deteriorates, so both are 0.1%. It shall be contained below.

Cu: 1.5% or less, Ni: 3.5% or less, Cr: 0.
8% or less, Mo: 0.5% or less Cu, Ni, Cr and Mo are all useful elements for improving the strength of the steel sheet, but if the Cu content exceeds 1.5%, hot brittleness occurs and It deteriorates the surface properties, Ni is expensive, Cr content deteriorates the toughness of the heat-affected zone when the content exceeds 0.8%, and Mo content adversely affects the toughness when the content exceeds 0.5%. It should be contained within the above range.

B: 0.0030% or less, Ca: 0.0030% or less, R
EM: 0.02% or less, Mg: 0.010% or less B is a useful element that contributes to strengthening with a trace amount, but if the content exceeds 0.0030%, the toughness of the weld heat affected zone deteriorates, so B is 0.0030%. % Or less.
Ca is an element useful for improving toughness by fixing S,
If the content exceeds 0.0030%, the effect will be saturated, so
Ca should be contained at 0.0030% or less. REM effectively contributes to the improvement of toughness, but its effect saturates when the content exceeds 0.02%, so the upper limit was made 0.02%. Mg
Is an element useful for refining crystal grains, but its content is
The effect reaches saturation when the content exceeds 0.010%, so the upper limit was made 0.010%.

In this invention, the tensile strength is 570 M.
Although high-strength steel of Pa or higher is targeted, it is essential to set the carbon equivalent (C _eq ) to 0.36 or higher in order to obtain such high strength. Therefore, the following formula (4) or (4) 'C _eq = ( % C) + (% Si) / 24 + (% Mn) / 6 --- (4) C _eq = (% C) + (% Si) / 24 + (% Mn) / 6 + (% Ni) / 4 + ( It is also a requirement of the present invention to adjust the components so that C _eq defined by% Cr) / 5 + (% Mo) / 4 + (% V) / 4 --- (4) 'is 0.36 or more.

The method for manufacturing the welded structural steel of the present invention is not particularly limited, and may be manufactured by a conventionally known method. For example, the molten steel prepared in the above preferred composition, after slab continuous casting, 1000 ~
It may be produced by heating at 1250 ° C. and then hot rolling, or may be produced by heat-treating after hot rolling to obtain a desired plate thickness and shape. As the heat treatment, known quenching, tempering, or quenching or tempering twice is advantageously suitable.

[0037]

[Examples] Steel slabs adjusted to various component compositions shown in Table 2 were heated to 1100 ° C and then hot rolled into thick steel plates of 85 mm. A JIS No. 4 tensile test piece was sampled from the 1/4 position of the thickness of each steel sheet obtained, and a tensile test was performed to determine the yield strength (YP) and tensile strength (TS) of the base material. Similarly, a JIS No. 4 impact test piece was sampled from the plate thickness 1/4 position of each steel sheet, and a Charpy test was performed to determine the absorbed energy (vE ₀ ) of the base material at 0 ° C. Further, a groove as shown in FIG. 4 was prepared on a joint test plate taken from each steel plate, and a welding joint was produced by electroslag welding (welding heat input: 1000 kJ / cm). After that, as shown in FIG. 5, a JIS No. 4 impact test piece having a bond at the notch position was taken from the welded joint, and a Charpy impact test was performed at a test temperature of 0 ° C. The absorbed energy (vE ₀ ) at ℃ was obtained. The results obtained are shown in Table 3.

[0038]

[Table 2]

[0039]

[Table 3]

As shown in Table 3, the invention examples are
Welding heat input: Even when ultra-high heat input welding of 1000 kJ / cm is performed, it can be seen that excellent weld heat-affected zone toughness with vE ₀ of 150 J or more at the bond portion is obtained.

[0041]

Thus, according to the present invention, C _eq ≧ 0.
Excellent toughness in the heat-affected zone, not to mention the base metal, even when ultra-high heat input welding exceeding 600 kJ / cm is applied to high strength steel with TS ≧ 570 MPa of 36 Can be obtained. Therefore, the present invention largely contributes to the increase in size of the structure and the improvement of construction efficiency.

[Brief description of drawings]

FIG. 1 is an equilibrium diagram (a) of a conventional steel and an equilibrium diagram (b) of the present invention calculated by commercially available calculation software to explain the concept of the present invention.

FIG. 2 is a diagram showing a reproduced thermal cycle pattern applied to a test piece.

FIG. 3 is a graph showing the influence of C content and Al content on the toughness of the heat affected zone of welding.

FIG. 4 is a view showing a groove shape in electroslag welding.

FIG. 5 is a diagram showing a procedure for collecting JIS No. 4 impact test pieces from a welded joint.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Shinichi Amano             1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama             Shi) Kawasaki Steel Co., Ltd. Mizushima Steel Works

Claims (2)

[Claims] 1. In mass%, C: 0.01 to 0.15%, Si: 0.05 to 0.50%, Mn: 0.1 to 2.0%, Al: 0.05 to 1.0%, Ti: 0.05% or less, N: 0.0070% or less, P : 0.020% or less and S: 0.0050% or less, the balance is composed of Fe and inevitable impurities, and the C and Al contents are in the following formulas (1), (2) (% Al) ≤ 0.8 0.1−0.125 × (% Al) ≦ (% C) ≦ 0.09 + 0.13 × (% Al) --- (1) (% Al)> 0.8 −0.20 + 0.25 × (% Al) ≦ (% C) ≦ 0.09 + 0.13 × (% Al) --- (2) Here, (% M) satisfies the relationship of the content (mass%) of the M element, and the N and Ti contents are as follows. (3) (% N)-(% Ti) /3.4 <0.0015 --- (3) is satisfied, and the following equation (4) C _eq = (% C) + (% Si) / 24 + (% Mn) / 6 --- (4) carbon equivalent C _eq defined by is not less 0.36 or more, tensile strength (TS) is equal to or is 570 MPa or more ultra rafters High-strength steel excellent in toughness of the weld heat affected zone. 2. The steel material according to claim 1, further comprising Nb: 0.1% or less, V: 0.1% or less, Cu: 1.5% or less, Ni: 3.5% or less, Cr: 0.8% or less, Mo: 0.5% or less, B: 0.0030% or less, Ca: 0.0030% or less, REM: 0.02% or less, and Mg: 0.010% or less. A composition containing one or more selected from the following formula (4) 'C _eq = (% C) + (% Si) / 24 + (% Mn) / 6 + (% Ni) / 4 + (% Cr) / 5 + (% Mo) / 4 + (% V) / 4 --- ( High strength steel with excellent toughness in the heat-affected zone of ultra-high heat input welding, characterized in that the carbon equivalent C _eq specified in 4) 'is 0.36 or more.