JPH08158006A - High strength steel excellent in toughness in weld heat-affected zone - Google Patents

Abstract

PURPOSE: To obtain excellent toughness at a large heat input welded joint by specifying, in a steel plate of specific composition, the C concentration in a segregation zone and also cleanliness and the inclusions and precipitates in a cross section, respectively. CONSTITUTION: This steel has a composition consisting of, by weight, 0.02-0.15% C, 0.005-0.50% Si, 0.8-2% Mn, 0.005-0.05% Al, 0.005-0.06% Nb, 0.005-0.02% Ti, 0.003-0.015% N, 0.003% O, and the balance Fe with inevitable impurities and satisfying -0.375C+0.035=Nb<=-0.375C+0.065, C+Mn/6+1.7Nb<=0.30, and C+Mn-/6<=0.36. Further, the average C concentration in the segregation zone in the central part of the steel plate [(plate thickness/50)mm×10×10mm] is regulated to a value <=1.2 times the average concentration and also the cleanliness is regulated to <=0.03%. Moreover, in the cross section, the number of oxide inclusions of >=10μm average diameter and the number of 0.05 to 5μm precipitates of oxide and nitride are regulated to <=1 piece/mm<2> and >=100 pieces/mm<2> , respectively. By this method, the high strength steel, used for a welded structure and excellent in HAZ toughness, can be provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used for welded structures such as ships, offshore structures, low temperature tanks and bridges.
The present invention relates to a high-strength steel having excellent toughness in a heat-affected zone (hereinafter abbreviated as HAZ).

[0002]

2. Description of the Related Art Conventionally, it has been a problem that the HAZ toughness at a welded joint portion of a steel structure is significantly deteriorated as compared with the base metal portion, and improvement of this HAZ toughness has become an urgent issue. . To date, various means for improving HAZ toughness have been proposed, for example, steel using nitrides such as TiN and BN, JP 62-109948 A, JP 1-159356 A, and JP H09-159356 A. There is proposed a steel using a Ti oxide as disclosed in JP-A-3-162522 and JP-A-4-99248.

The use of Ti oxide is that TiN particles are HAZ.
This is a method considered to solve the problem of melting when heated to a high temperature of about 1400 ° C or higher in the vicinity of the melting line of, and toughness deterioration in a portion heated to a high temperature of about 1400 ° C or higher is caused. It is effective in reducing the size. Also, HAZ
Since the upper bainite structure and island-like martensite that are generated in the HAZ are harmful to the HAZ toughness, it is effective to reduce the amount of C, the amount of alloying elements and the carbon equivalent (Ceq) in order to reduce the formation of these structures. However, on the other hand, H
There is a problem that the strength of AZ decreases.

[0004]

When Ti oxide is used, HAZ in a minute region heated to 1400 ° C. or higher is used.
Although it is possible to prevent deterioration of toughness, good toughness is not always obtained in the entire HAZ, and 1350
The toughness in most of the HAZ region heated to below 0 ° C may be hardly improved, or even deteriorated, as compared with the case of utilizing the nitride.

Further, since it is necessary to add a large amount of alloying elements to a high-strength steel sheet, the HAZ toughness is greatly deteriorated, and how to improve the contradictory characteristics of high strength and HAZ toughness improvement of the material. Is a problem. Nb is generally used to increase the strength of the steel sheet, but Nb deteriorates the HAZ toughness of the steel, so that Nb is usually not added as much as possible in the steel sheet where the HAZ toughness is strictly required. The present invention solves the above-mentioned problems in the prior art and improves the HAZ toughness while positively adding Nb that is effective for increasing the strength of steel, and thereby improves the HAZ toughness of ships, marine structures, low temperature tanks and bridges. H used for welded structures such as
It is intended to provide a high strength steel having excellent AZ toughness.

[0006]

[Means for Solving the Problems] In order to achieve the above-mentioned object, the inventors of the present invention have conducted extensive studies, and as a result, have found that the optimum balance of C content and Nb content in steel as a base material and the optimal balance of each alloying element. The inventors have found that the addition amount and the size and number of inclusions and precipitated particles in the steel sheet are closely related to the high strength of the steel and the improvement of HAZ toughness, and have completed the present invention. The present invention made based on the above findings has the following eight aspects.

First invention C: 0.02 to 0.15% by weight, Si: 0.00
5 to 0.50%, Mn: 0.8 to 2.0%, P ≦ 0.0
10%, S ≦ 0.003%, Al: 0.005-0.0
5%, Nb: 0.005 to 0.060%, Ti: 0.0
05-0.02%, N: 0.003-0.015%, O
≦ 0.0030%, balance Fe and unavoidable impurities, and −0.375C + 0.035 ≦ Nb ≦ −0.375C +
0.065 C + Mn / 6 + 1.7 Nb ≧ 0.30 Satisfaction of C + Mn / 6 ≦ 0.36, center segregation portion of steel sheet (sheet thickness / 50) mm thickness × 1
The C concentration of the average chemical analysis value of 0 mm × 10 mm is 1.2 times or less of the average C concentration of the steel material, the cleanliness of inclusions measured by JIS standard is 0.03% or less, and it is observed in the steel plate cross section. The number of oxide inclusions with an average diameter of 10 μm or more is 1
A high-strength steel excellent in toughness of HAZ, characterized in that the number of oxide / nitride precipitates of not more than 1 piece / mm ² and 0.05 to 5 μm is not less than 100 piece / mm ² .

Second invention C: 0.02 to 0.15% by weight, Si: 0.00
5 to 0.50%, Mn: 0.8 to 2.0%, P ≦ 0.0
10%, S ≦ 0.003%, Al: 0.005-0.0
5%, Nb: 0.005 to 0.060%, Ti: 0.0
05-0.02%, B: 0.0003-0.0030
%, N: 0.003 to 0.015%, O ≦ 0.0030
%, With the balance being Fe and unavoidable impurities, and -0.375C + 0.035≤Nb≤-0.375C +.
0.065 C + Mn / 6 + 1.7 Nb + 20B ≧ 0.30 C + Mn / 6 ≦ 0.36 is satisfied, and the center segregated portion (sheet thickness / 50) mm thickness of the steel sheet × 1
The C concentration of the average chemical analysis value of 0 mm × 10 mm is 1.2 times or less of the average C concentration of the steel material, the cleanliness of inclusions measured by JIS standard is 0.03% or less, and it is observed in the steel plate cross section. The number of oxide inclusions with an average diameter of 10 μm or more is 1
A high-strength steel excellent in toughness of HAZ, characterized in that the number of oxide / nitride precipitates of not more than 1 piece / mm ² and 0.05 to 5 μm is not less than 100 piece / mm ² .

Third invention C: 0.02 to 0.15% by weight, Si: 0.00
5 to 0.50%, Mn: 0.8 to 2.0%, P ≦ 0.0
10%, S ≦ 0.003%, Al: 0.005-0.0
5%, Nb: 0.005 to 0.060%, Ti: 0.0
05-0.02%, N: 0.003-0.015%, O
≦ 0.0030%, further, Cu: 0.05 ~
2.0%, Ni: 0.05 to 3.5%, Cr: 0.01
~ 0.5%, Mo: 0.01 to 0.5%, V: 0.00
5 to 0.10% selected from the group consisting of one or more selected from the balance Fe and inevitable impurities, and -0.375C + 0.035≤Nb≤-0.375C +.
0.065 C + Mn / 6 + 1.7 Nb + (Cu + Ni) / 15 +
(Cr + Mo) /10+V/2.5≧0.30 C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo +
V) /5≦0.36 is satisfied, and the center segregated portion of the steel sheet (sheet thickness / 50) mm thickness × 1
The C concentration of the average chemical analysis value of 0 mm × 10 mm is 1.2 times or less of the average C concentration of the steel material, the cleanliness of inclusions measured by JIS standard is 0.03% or less, and it is observed in the steel plate cross section. The number of oxide inclusions with an average diameter of 10 μm or more is 1
A high-strength steel excellent in toughness of HAZ, characterized in that the number of oxide / nitride precipitates of not more than 1 piece / mm ² and 0.05 to 5 μm is not less than 100 piece / mm ² .

Fourth invention C: 0.02 to 0.15%, Si: 0.00 by weight%
5 to 0.50%, Mn: 0.8 to 2.0%, P ≦ 0.0
10%, S ≦ 0.003%, Al: 0.005-0.0
5%, Nb: 0.005 to 0.060%, Ti: 0.0
05-0.02%, B: 0.0003-0.0030
%, N: 0.003 to 0.015%, O ≦ 0.0030
%, Further, Cu: 0.05 to 2.0%, N
i: 0.05 to 3.5%, Cr: 0.01 to 0.5%,
Mo: 0.01-0.5%, V: 0.005-0.10
%, Containing 1 or 2 or more selected from the
e and unavoidable impurities, and is −0.375C + 0.035 ≦ Nb ≦ −0.375C +
0.065 C + Mn / 6 + 1.7 Nb + (Cu + Ni) / 15 +
(Cr + Mo) /10+V/2.5+20B≧0.30 C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo +
V) /5≦0.36 is satisfied, and the center segregated portion of the steel sheet (sheet thickness / 50) mm thickness × 1
The C concentration of the average chemical analysis value of 0 mm × 10 mm is 1.2 times or less of the average C concentration of the steel material, the cleanliness of inclusions measured by JIS standard is 0.03% or less, and it is observed in the steel plate cross section. The number of oxide inclusions with an average diameter of 10 μm or more is 1
A high-strength steel excellent in toughness of HAZ, characterized in that the number of oxide / nitride precipitates of not more than 1 piece / mm ² and 0.05 to 5 μm is not less than 100 piece / mm ² .

Fifth Invention C: 0.02 to 0.15% by weight%, Si: 0.00
5 to 0.50%, Mn: 0.8 to 2.0%, P ≦ 0.0
10%, S ≦ 0.003%, Al: 0.005-0.0
5%, Nb: 0.005 to 0.060%, Ti: 0.0
05-0.02%, N: 0.003-0.015%, O
≦ 0.0030%, further Ca: 0.000
5 to 0.0030%, REM: 0.005 to 0.050
%, Containing 1 or 2 or more selected from the
e and unavoidable impurities, and is −0.375C + 0.035 ≦ Nb ≦ −0.375C +
0.065 C + Mn / 6 + 1.7 Nb ≧ 0.30 Satisfaction of C + Mn / 6 ≦ 0.36, center segregation portion of steel sheet (sheet thickness / 50) mm thickness × 1
The C concentration of the average chemical analysis value of 0 mm × 10 mm is 1.2 times or less of the average C concentration of the steel material, the cleanliness of inclusions measured by JIS standard is 0.03% or less, and it is observed in the steel plate cross section. The number of oxide inclusions with an average diameter of 10 μm or more is 1
A high-strength steel excellent in toughness of HAZ, characterized in that the number of oxide / nitride precipitates of not more than 1 piece / mm ² and 0.05 to 5 μm is not less than 100 piece / mm ² .

Sixth Invention C: 0.02 to 0.15% by weight, Si: 0.00
5 to 0.50%, Mn: 0.8 to 2.0%, P ≦ 0.0
10%, S ≦ 0.003%, Al: 0.005-0.0
5%, Nb: 0.005 to 0.060%, Ti: 0.0
05-0.02%, B: 0.0003-0.0030
%, N: 0.003 to 0.015%, O ≦ 0.0030
%, And further Ca: 0.0005 to 0.003
0%, REM: 0.005 to 0.050%, containing one or more selected from the group consisting of the balance Fe and inevitable impurities, and -0.375C + 0.035≤Nb≤-0. 375C +
0.065 C + Mn / 6 + 1.7 Nb + 20B ≧ 0.30 C + Mn / 6 ≦ 0.36 is satisfied, and the center segregated portion (sheet thickness / 50) mm thickness of the steel sheet × 1
The C concentration of the average chemical analysis value of 0 mm × 10 mm is 1.2 times or less of the average C concentration of the steel material, the cleanliness of inclusions measured by JIS standard is 0.03% or less, and it is observed in the steel plate cross section. The number of oxide inclusions with an average diameter of 10 μm or more is 1
A high-strength steel excellent in toughness of HAZ, characterized in that the number of oxide / nitride precipitates of not more than 1 piece / mm ² and 0.05 to 5 μm is not less than 100 piece / mm ² .

Seventh Invention C: 0.02 to 0.15% by weight%, Si: 0.00
5 to 0.50%, Mn: 0.8 to 2.0%, P ≦ 0.0
10%, S ≦ 0.003%, Al: 0.005-0.0
5%, Nb: 0.005 to 0.060%, Ti: 0.0
05-0.02%, N: 0.003-0.015%, O
≦ 0.0030%, further, Cu: 0.05 ~
2.0%, Ni: 0.05 to 3.5%, Cr: 0.01
~ 0.5%, Mo: 0.01 to 0.5%, V: 0.00
5 to 0.10%, one or more selected from, and further contains Ca: 0.0005 to 0.0030%,
REM: contains one or more selected from 0.005 to 0.050%, consists of balance Fe and unavoidable impurities, and -0.375C + 0.035≤Nb≤-0.375C +
0.065 C + Mn / 6 + 1.7 Nb + (Cu + Ni) / 15 +
(Cr + Mo) /10+V/2.5≧0.30 C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo +
V) /5≦0.36 is satisfied, and the center segregated portion of the steel sheet (sheet thickness / 50) mm thickness × 1
The C concentration of the average chemical analysis value of 0 mm × 10 mm is 1.2 times or less of the average C concentration of the steel material, the cleanliness of inclusions measured by JIS standard is 0.03% or less, and it is observed in the steel plate cross section. The number of oxide inclusions with an average diameter of 10 μm or more is 1
A high-strength steel excellent in toughness of HAZ, characterized in that the number of oxide / nitride precipitates of not more than 1 piece / mm ² and 0.05 to 5 μm is not less than 100 piece / mm ² .

Eighth invention C: 0.02 to 0.15% by weight, Si: 0.00
5 to 0.50%, Mn: 0.8 to 2.0%, P ≦ 0.0
10%, S ≦ 0.003%, Al: 0.005-0.0
5%, Nb: 0.005 to 0.060%, Ti: 0.0
05-0.02%, B: 0.0003-0.0030
%, N: 0.003 to 0.015%, O ≦ 0.0030
%, Further, Cu: 0.05 to 2.0%, N
i: 0.05 to 3.5%, Cr: 0.01 to 0.5%,
Mo: 0.01-0.5%, V: 0.005-0.10
%, One or two or more selected from%, and further Ca: 0.0005 to 0.0030%, REM:
1 or 2 selected from 0.005 to 0.050%
Containing at least one species, consisting of balance Fe and unavoidable impurities, and -0.375C + 0.035≤Nb≤-0.375C +
0.065 C + Mn / 6 + 1.7 Nb + (Cu + Ni) / 15 +
(Cr + Mo) /10+V/2.5+20B≧0.30 C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo +
V) /5≦0.36 is satisfied, and the center segregated portion of the steel sheet (sheet thickness / 50) mm thickness × 1
The C concentration of the average chemical analysis value of 0 mm × 10 mm is 1.2 times or less of the average C concentration of the steel material, the cleanliness of inclusions measured by JIS standard is 0.03% or less, and it is observed in the steel plate cross section. The number of oxide inclusions with an average diameter of 10 μm or more is 1
A high-strength steel excellent in toughness of HAZ, characterized in that the number of oxide / nitride precipitates of not more than 1 piece / mm ² and 0.05 to 5 μm is not less than 100 piece / mm ² .

[0015]

The structure and operation of the present invention will be described. The reason for limiting the chemical composition range of the steel according to the present invention will be described. The chemical content shown here is a component analysis value in molten steel or a component analysis value of a portion excluding the central segregation portion of the steel sheet. C: C increases the amount of island martensite in the HAZ, is an element harmful to the HAZ toughness, and hinders weldability.
%. Further, if the content is less than 0.02%, the predetermined strength of steel cannot be secured. Therefore, the C content is 0.02 to 0.15%. Preferably from 0.03
0.09% is good.

Si: Si is an element useful for deoxidizing and strengthening steel, and if the content is less than 0.005%, its effect is not exerted, and if it exceeds 0.50%, it is steel. Weldability and toughness of the steel deteriorate. Therefore, the Si content is set to 0.005 to 0.50%. Mn: Mn is an element effective in increasing the strength of steel, but if its content is less than 0.8%, sufficient strength cannot be obtained, and if it exceeds 2.0%, it is contained. , Deteriorates the weldability of steel. Therefore, the Mn content is 0.8-2.
0%

P: P is an element which easily segregates and segregates into the open grains to deteriorate the toughness. Therefore, the P content is set to 0.010% or less. S: S forms MnS inclusions and deteriorates toughness and ductility in the plate thickness direction, so the S content is made 0.003% or less. Al: Al is a deoxidizing element, and if the content is less than 0.005%, its effect is small, and if it is added in excess of more than 0.05%, coarse alumina-based inclusions are produced. Becomes the starting point of fracture and deteriorates toughness. Therefore,
The Al content is 0.005 to 0.05%.

Nb: Nb, together with C, is an important element in the present invention. Nb is extremely effective in increasing the strength of steel, and by controlling the temperature during rolling, it is possible to promote the refinement of the austenite grain size and the introduction of strain into the austenite grains, and to reduce the ferrite grain size after cooling. It is also effective in improving the toughness of the base material by making it finer. In order to obtain the above effect, it is necessary to contain 0.005% or more. On the other hand, if the content exceeds 0.060%, the precipitation amount of Nb carbonitrides becomes too large and the HAZ toughness deteriorates, so the Nb content is made 0.005 to 0.060%.

Ti: Ti is an important element effective in improving the HAZ toughness, and TiN precipitated in the steel structure is
It prevents coarsening of austenite grains when heated by welding and promotes ferrite transformation. Content is 0.0
If it is less than 05%, this effect is not obtained, and if it is added in excess of 0.020%, the toughness deteriorates. Therefore, Ti
The content is 0.005 to 0.020%.

N: N produces TiN together with Ti,
It is an element having an effect of improving the HAZ toughness by suppressing coarsening of the austenite grain size of HAZ and promoting ferrite formation. If the content is less than 0.003%, such an effect does not occur. Further, as the N content increases, the austenite grain size becomes finer and it is effective for improving the toughness. However, if the N content exceeds 0.015%, the toughness-deteriorating free N content increases, and conversely the toughness deteriorates. Therefore,
The N content is 0.003 to 0.015%.

O: O is effective for forming a Ti oxide which prevents coarsening of the austenite grain size of HAZ, but if it is contained in excess of 0.0030%, it is a coarse oxide-based inclusion. Since a large amount is generated, the oxide becomes a starting point of fracture generation and deteriorates toughness. Therefore, the O content is set to 0.0030% or less.

In addition to the constituent elements described above, B can be contained in the high-strength steel having excellent HAZ toughness according to the present invention. B: B not only increases the strength of the base metal but also precipitates BN in the cooling process and promotes ferrite transformation from within the austenite grains even when TiN is melted in the part of HAZ heated to 1400 ° C or higher. Can be made. To obtain this effect, the B content is 0.0003.
% Or more is required. However, if added in a large amount of 0.0030% or more, the toughness deteriorates. Therefore, the B content is 0.
0003 to 0.0030%.

Further, in the steel of the present invention, due to the increase in strength, C
One or more of u, Ni, Cr, Mo and V may be contained. Cu: Cu is effective for increasing the strength. Addition of less than 0.6% can utilize the solid solution strengthening, and addition of 0.6% or more can utilize the aging precipitation strengthening effect, and can also significantly increase the strength. . If the content is less than 0.05%, this effect does not occur.
If it is contained in a large amount exceeding 0%, the toughness is reduced. Therefore, the Cu content is 0.05 to 2.0%.

Ni: Ni improves the toughness of the base material and is effective for increasing the strength. If the content is less than 0.05%, this effect does not occur, and since it is expensive, it must be 3.5% or less. Therefore, the Ni content is 0.05 to 3.5.
%. Cr: Cr is an element effective for increasing strength, and if the content is less than 0.01%, such an effect is small, and
If it is contained in a large amount exceeding 0.50%, the weldability is impaired. Therefore, the Cr content is 0.01 to 0.50.
%.

Mo: Mo is an element effective for increasing the strength, and if the content is less than 0.01%, such an effect is small, and if it exceeds 0.50% in a large amount, the weldability is impaired. Come to do. Therefore, the Mo content is 0.01
~ 0.50%. V: V is an element effective in increasing strength, and its content is 0.0
If it is less than 1%, such an effect is small and 0.50.
If it is contained in a large amount in excess of%, the weldability and toughness will be impaired. Therefore, the V content is set to 0.01 to 0.50%.

The steel of the present invention may contain one or two of Ca and REM for controlling the morphology of inclusions. Ca: Ca reduces inclusions that are the starting point of fracture occurrence,
It is an element effective in improving the HAZ toughness by refining and controlling the morphology. Particularly harmful Al ₂ O
₃ It has the effect of flocculating coarse (Alumina) -based inclusions in molten steel, reducing the amount of alumina-based inclusions and finely dispersing them. Also, the morphology of MnS inclusions is controlled. If the content is less than 0.0005%, there is no such effect, and if the content exceeds 0.003%, an extremely coarse inclusion of Ca and alumina is generated. Therefore, the Ca content is 0.0005 to 0.0030%. REM: REM is effective in controlling the morphology of MnS inclusions. If the content is less than 0.005%, this effect is small,
Further, if contained in excess of 0.050%, harmful coarse inclusions are generated. Therefore, the REM content is 0.005
~ 0.050%.

It is generally said that Nb is an element that significantly impairs the toughness of the welded portion of steel, and it is necessary to limit the addition of Nb to the smallest possible amount. However, this is the case where the C content is relatively large, and when the C content is less than about 0.06%, the effect of preventing coarsening of the ferrite crystal grain size of the HAZ becomes stronger, rather. The HAZ toughness becomes better as the Nb content is increased. That is, the Nb content needs to be determined in balance with the C content.

In the steel of the present invention, the balance of Nb and C contents was clarified by the following mathematical formula. −0.375C + 0.035 ≦ Nb ≦ −0.375c + 0.065 For example, when the C content is 0.04%, the Nb content is 0.02 to 0.05% and the C content is 0. When it is 08%,
When the Nb content is 0.005 to 0.035% and the C content is 0.12%, the Nb content needs to be 0.02% or less. This is an extremely important point for producing a high strength steel having excellent HAZ toughness.

Next, in the high strength steel of the present invention having excellent HAZ toughness, the content of the constituent elements must be limited within the range of the following formula. First, in order to obtain a sufficiently high strength, C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo) /10+V/2.5+20B≧0.30 ... Must be satisfied. Further, in order to obtain excellent HAZ toughness, C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) ≦ 0.36 ... Must be satisfied. The present invention steel, these,
, The content of ingredients must be adjusted to satisfy all the formulas.

At the center segregated portion of the steel sheet, the constituent elements are concentrated, so that the portion hardens and becomes brittle, and becomes a starting point of fracture occurrence. The components of the central segregation portion greatly differ depending on the area to be analyzed, and a high-concentration analysis result is obtained by analysis in a minute area by EPMA or CMA. In the steel of the present invention, after the center segregation portion of the steel sheet is macro-etched, the average chemical analysis value in the most segregated portion (sheet thickness / 50) mm thickness × 10 mm × 10 mm is used as the component of the center segregation portion. Although segregation elements include C, P, Mn, etc., the concentration of C element that represents the amount of hardening was adopted, and the degree of segregation was based on the ratio with the average C concentration of steel.

Here, the average C concentration of the steel material is a component analysis value in molten steel or a component analysis value of a portion excluding the center segregated portion of the steel material. There is no such thing as center segregation, but in actual steel materials, it is impossible to eliminate center segregation because the center solidifies last in the solidification process of continuous casting. Therefore, in the steel of the present invention, the degree of center segregation, which is harmful to HAZ toughness, was clearly defined. That is, when the C concentration of the central segregation portion exceeds 1.2 times the average C concentration of the steel material, a clear black line appears on the macrostructure, and that portion often becomes the starting point of fracture occurrence. The C concentration of the average chemical analysis value of the center segregated portion (plate thickness / 50) mm thickness × 10 mm × 10 mm is 1.2 times or less of the average C concentration of the steel material.

If non-metallic inclusions are present in the steel,
This may be the point of destruction. Particularly, in the case of relatively large inclusions or a series of inclusions, when welding heat is applied, the interface is separated due to the heat and stress, and breakage easily occurs. From this point, it is desirable that the steel having excellent HAZ toughness has few nonmetallic inclusions. Generally, as a method for evaluating the amount of inclusions, there is a cleanliness degree defined by JIS G 0555 "Microscopic test method for non-metallic inclusions in steel". That is, using a microscope having 20 grid lines in each of the vertical and horizontal directions, observe 60 fields of view at a magnification of 400 times, measure the number of center points of the grid points occupied by inclusions, and calculate the inclusions by the following formula. The area percentage occupied by is calculated and used as the cleanliness (d%). d = n ÷ (p + f) × 100 where p: total number of lattice points on the glass plate in the visual field f: number of visual fields n: number of lattice point centers occupied by all inclusions in f visual fields

For example, in the case where the cleanliness is very good such that only one inclusion hangs over one grid point in 60 fields of view,
The cleanliness d is 0.004%. This cleanliness is 0.03
%, The toughness deteriorates, so the cleanliness of inclusions measured according to JIS standard is 0.03% or less.

The relatively fine oxide-based inclusions (oxide-based precipitates) having an average diameter of 5 μm or less are ferrite from the austenite grains in the HAZ heated to 1400 ° C. or higher as described above. Although effective for promoting transformation, oxide-based inclusions having an average diameter of 10 μm or more are harmful as they may be the starting point of fracture. This average diameter 10
Existence frequency of oxide inclusions of μm or more is 1 piece / 1 mm ²
If it exceeds 1.0, the HAZ toughness deteriorates, so the number of oxide inclusions having an average diameter of 10 μm or more observed in the steel plate cross section is set to 1 or less / 1 mm ² .

On the other hand, fine precipitates of oxides and nitrides having an average diameter of 5 μm or less promote the suppression of coarsening of austenite crystal grains in the HAZ and the promotion of ferrite formation from within the austenite grains to improve the HAZ toughness. To obtain this effect, the number of oxide and nitride precipitates having an average diameter of 0.05 to 5 μm must be 100 or more / 1 mm ² .

The oxide inclusions in the prior art may be produced by Al deoxidation or Ti deoxidation in the steelmaking process, or may be mixed in from slag, casting flux or the like. These inclusions range from several hundred μm to over 1 mm, and 10
Inclusions of μm or more are 2 or more / 1m in the ordinary steelmaking method.
m ² exists. In particular, Ti deoxidation is likely to produce many such coarse inclusions. In order to reduce the coarse inclusions, it is necessary to perform a sufficient degassing process to cause the produced oxide to flocculate and float.

Fine precipitates having an average diameter of 5 μm or less are targeted because nitrides and oxides containing Ti as a main component are effective in improving the HAZ toughness. In order to generate a large amount of fine precipitates, it is necessary to contain TI and N in an appropriate balance and to increase the solidification cooling rate, and it is necessary to manufacture by continuous casting instead of ingot casting.

The method for producing the steel of the present invention is not particularly limited, but it is desirable to carry out accelerated cooling after hot rolling or water cooling such as direct quenching (DQ) in order to achieve high strength. Tempering after cooling may or may not be performed. Further, after rolling, it may be air-cooled or may be heat-treated by quenching and tempering.

[0039]

EXAMPLES Examples of the present invention will be described, but the present invention is not limited thereto. Example The number of inclusions and the number of precipitates in the examples were made differently by changing the deoxidizing method, the degassing conditions, the casting conditions, the slab sampling position, and the like. Steels having the alloy components and component content ratios shown in Tables 1 and 2 below were melted by a conventional method, then continuously cast, and the manufactured slabs were heated, rolled, and cooled to produce test steel sheets. .

[0040]

[Table 1]

[0041]

[Table 2]

From these steel sheets, the component analysis of the central segregation portion, the observation of inclusions in the steel sheet cross section, the tensile test and the Charpy impact test were conducted, and after one pass of large heat input welding, the test piece was taken from the center portion of the plate thickness. Take a sample, make a notch in the bond where the HAZ and weld metal are divided 1: 1 at the fusion line, -4
A Charpy impact test was performed at 0 ° C. Table 3 shows the parameters indicating the conditions of the component ratios, and Table 4 shows the test results.

[0043]

[Table 3]

[0044]

[Table 4]

From the above table, it can be seen that the steel of the present invention has high strength, high toughness and excellent Charpy impact characteristics in the welded joint. That is, the steel of the present invention has a base metal tensile strength of 510 MPa or more and a high heat input welded joint bond portion of -4.
It has excellent HAZ toughness with a Charpy impact absorbed energy of 90 J or more at 0 ° C.

On the other hand, the comparative steel BA has a high C content, and the comparative steel BB has a Nb content exceeding the upper limit corresponding to the C concentration, so that the toughness of the welded joint is low. The comparative steel BC satisfies the conditions for the content of each component element, but the strength parameter calculated from the ratio of the content of the component elements is lower than the lower limit value, and the comparative steel BD has the Nb content at the C concentration. Since the strength parameter is lower than the corresponding lower limit value and the strength parameter is also low, the strength is low and the toughness of the welded joint is also low. Comparative steel B
Since E has a low Ti content, the toughness of the welded joint is low.
Since the comparative steel BF has a low N content, TiN is not sufficiently produced and the toughness of the welded joint is low. Since the comparative steel BG has a high O content, a large amount of inclusions are formed and the toughness of the welded joint is low. Further, the comparative steels A2, B2, C2, L2, and N2 satisfy all the conditions for the amounts of the respective constituent elements, but the degree of center segregation is so severe that the toughness of the welded joint is low. Comparative steel A
3, B3, C3, L3, and N3 satisfy all the conditions for the amount of each elemental element, but the toughness of the welded joint is low because there are many inclusions having an average diameter of 10 μm or more.

[0047]

EFFECTS OF THE INVENTION Since the present invention is constituted as described above, it is possible to maintain high tensile strength of 510 MPa or more as a steel sheet and to have excellent toughness in a large heat input welded joint portion. This is an extremely useful invention in industry.

Claims (8)

[Claims] 1. C: 0.02 to 0.15% by weight,
Si: 0.005 to 0.50%, Mn: 0.8 to 2.0
%, P ≦ 0.010%, S ≦ 0.003%, Al: 0.
005-0.05%, Nb: 0.005-0.060
%, Ti: 0.005 to 0.02%, N: 0.003 to
0.015%, O ≦ 0.0030%, balance Fe
And unavoidable impurities, and -0.375C + 0.035≤Nb≤-0.375C +
0.065 C + Mn / 6 + 1.7 Nb ≧ 0.30 Satisfaction of C + Mn / 6 ≦ 0.36, center segregation portion of steel sheet (sheet thickness / 50) mm thickness × 1
The C concentration of the average chemical analysis value of 0 mm × 10 mm is 1.2 times or less of the average C concentration of the steel material, the cleanliness of inclusions measured by JIS standard is 0.03% or less, and it is observed in the steel plate cross section. The number of oxide inclusions with an average diameter of 10 μm or more is 1
A high-strength steel with excellent toughness in the heat-affected zone of welding, characterized in that the number of precipitates of oxides and nitrides of not more than 1 piece / mm ² and 0.05 to 5 μm is not less than 100 pieces / mm ² . 2. C .: 0.02 to 0.15% by weight,
Si: 0.005 to 0.50%, Mn: 0.8 to 2.0
%, P ≦ 0.010%, S ≦ 0.003%, Al: 0.
005-0.05%, Nb: 0.005-0.060
%, Ti: 0.005 to 0.02%, B: 0.0003
~ 0.0030%, N: 0.003 to 0.015%, O
≦ 0.0030%, balance Fe and unavoidable impurities, and −0.375C + 0.035 ≦ Nb ≦ −0.375C +
0.065 C + Mn / 6 + 1.7 Nb + 20B ≧ 0.30 C + Mn / 6 ≦ 0.36 is satisfied, and the center segregated portion (sheet thickness / 50) mm thickness of the steel sheet × 1
The C concentration of the average chemical analysis value of 0 mm × 10 mm is 1.2 times or less of the average C concentration of the steel material, the cleanliness of inclusions measured by JIS standard is 0.03% or less, and it is observed in the steel plate cross section. The number of oxide inclusions with an average diameter of 10 μm or more is 1
A high-strength steel with excellent toughness in the heat-affected zone of welding, characterized in that the number of precipitates of oxides and nitrides of not more than 1 piece / mm ² and 0.05 to 5 μm is not less than 100 pieces / mm ² . 3. C: 0.02 to 0.15% by weight,
Si: 0.005 to 0.50%, Mn: 0.8 to 2.0
%, P ≦ 0.010%, S ≦ 0.003%, Al: 0.
005-0.05%, Nb: 0.005-0.060
%, Ti: 0.005 to 0.02%, N: 0.003 to
0.015%, O ≦ 0.0030%, and
Cu: 0.05-2.0%, Ni: 0.05-3.5
%, Cr: 0.01 to 0.5%, Mo: 0.01 to 0.
5%, V: 0.005 to 0.10%, containing 1 or 2 or more kinds, consisting of the balance Fe and unavoidable impurities, and -0.375C + 0.035≤Nb≤-0. 375C +
0.065 C + Mn / 6 + 1.7 Nb + (Cu + Ni) / 15 +
(Cr + Mo) /10+V/2.5≧0.30 C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo +
V) /5≦0.36 is satisfied, and the center segregated portion of the steel sheet (sheet thickness / 50) mm thickness × 1
The C concentration of the average chemical analysis value of 0 mm × 10 mm is 1.2 times or less of the average C concentration of the steel material, the cleanliness of inclusions measured by JIS standard is 0.03% or less, and it is observed in the steel plate cross section. The number of oxide inclusions with an average diameter of 10 μm or more is 1
A high-strength steel with excellent toughness in the heat-affected zone of welding, characterized in that the number of precipitates of oxides and nitrides of not more than 1 piece / mm ² and 0.05 to 5 μm is not less than 100 pieces / mm ² . 4. C: 0.02 to 0.15% by weight,
Si: 0.005 to 0.50%, Mn: 0.8 to 2.0
%, P ≦ 0.010%, S ≦ 0.003%, Al: 0.
005-0.05%, Nb: 0.005-0.060
%, Ti: 0.005 to 0.02%, B: 0.0003
~ 0.0030%, N: 0.003 to 0.015%, O
≦ 0.0030%, further, Cu: 0.05 ~
2.0%, Ni: 0.05 to 3.5%, Cr: 0.01
~ 0.5%, Mo: 0.01 to 0.5%, V: 0.00
5 to 0.10% selected from the group consisting of one or more selected from the balance Fe and inevitable impurities, and -0.375C + 0.035≤Nb≤-0.375C +.
0.065 C + Mn / 6 + 1.7 Nb + (Cu + Ni) / 15 +
(Cr + Mo) /10+V/2.5+20B≧0.30 C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo +
V) /5≦0.36 is satisfied, and the center segregated portion of the steel sheet (sheet thickness / 50) mm thickness × 1
The C concentration of the average chemical analysis value of 0 mm × 10 mm is 1.2 times or less of the average C concentration of the steel material, the cleanliness of inclusions measured by JIS standard is 0.03% or less, and it is observed in the steel plate cross section. The number of oxide inclusions with an average diameter of 10 μm or more is 1
A high-strength steel with excellent toughness in the heat-affected zone of welding, characterized in that the number of precipitates of oxides and nitrides of not more than 1 piece / mm ² and 0.05 to 5 μm is not less than 100 pieces / mm ² . 5. C .: 0.02 to 0.15% by weight,
Si: 0.005 to 0.50%, Mn: 0.8 to 2.0
%, P ≦ 0.010%, S ≦ 0.003%, Al: 0.
005-0.05%, Nb: 0.005-0.060
%, Ti: 0.005 to 0.02%, N: 0.003 to
0.015%, O ≦ 0.0030%, and
Ca: 0.0005 to 0.0030%, REM: 0.0
It contains one or more selected from the range of 0.05 to 0.050%, consists of the balance Fe and unavoidable impurities, and -0.375C + 0.035≤Nb≤-0.375C +.
0.065 C + Mn / 6 + 1.7 Nb ≧ 0.30 Satisfaction of C + Mn / 6 ≦ 0.36, center segregation portion of steel sheet (sheet thickness / 50) mm thickness × 1
The C concentration of the average chemical analysis value of 0 mm × 10 mm is 1.2 times or less of the average C concentration of the steel material, the cleanliness of inclusions measured by JIS standard is 0.03% or less, and it is observed in the steel plate cross section. The number of oxide inclusions with an average diameter of 10 μm or more is 1
A high-strength steel with excellent toughness in the heat-affected zone of welding, characterized in that the number of precipitates of oxides and nitrides of not more than 1 piece / mm ² and 0.05 to 5 μm is not less than 100 pieces / mm ² . 6. C: 0.02 to 0.15% by weight,
Si: 0.005 to 0.50%, Mn: 0.8 to 2.0
%, P ≦ 0.010%, S ≦ 0.003%, Al: 0.
005-0.05%, Nb: 0.005-0.060
%, Ti: 0.005 to 0.02%, B: 0.0003
~ 0.0030%, N: 0.003 to 0.015%, O
≦ 0.0030%, further Ca: 0.000
5 to 0.0030%, REM: 0.005 to 0.050
%, Containing 1 or 2 or more selected from the
e and unavoidable impurities, and is −0.375C + 0.035 ≦ Nb ≦ −0.375C +
0.065 C + Mn / 6 + 1.7 Nb + 20B ≧ 0.30 C + Mn / 6 ≦ 0.36 is satisfied, and the center segregated portion (sheet thickness / 50) mm thickness of the steel sheet × 1
The C concentration of the average chemical analysis value of 0 mm × 10 mm is 1.2 times or less of the average C concentration of the steel material, the cleanliness of inclusions measured by JIS standard is 0.03% or less, and it is observed in the steel plate cross section. The number of oxide inclusions with an average diameter of 10 μm or more is 1
A high-strength steel with excellent toughness in the heat-affected zone of welding, characterized in that the number of precipitates of oxides and nitrides of not more than 1 piece / mm ² and 0.05 to 5 μm is not less than 100 pieces / mm ² . 7. C .: 0.02 to 0.15% by weight,
Si: 0.005 to 0.50%, Mn: 0.8 to 2.0
%, P ≦ 0.010%, S ≦ 0.003%, Al: 0.
005-0.05%, Nb: 0.005-0.060
%, Ti: 0.005 to 0.02%, N: 0.003 to
0.015%, O ≦ 0.0030%, and
Cu: 0.05-2.0%, Ni: 0.05-3.5
%, Cr: 0.01 to 0.5%, Mo: 0.01 to 0.
5%, V: 0.005 to 0.10%, containing 1 or 2 or more selected, and further Ca: 0.0005
~ 0.0030%, REM: 0.005-0.050%
Containing one or more selected from the balance Fe
And unavoidable impurities, and -0.375C + 0.035≤Nb≤-0.375C +
0.065 C + Mn / 6 + 1.7 Nb + (Cu + Ni) / 15 +
(Cr + Mo) /10+V/2.5≧0.30 C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo +
V) /5≦0.36 is satisfied, and the center segregated portion of the steel sheet (sheet thickness / 50) mm thickness × 1
The C concentration of the average chemical analysis value of 0 mm × 10 mm is 1.2 times or less of the average C concentration of the steel material, the cleanliness of inclusions measured by JIS standard is 0.03% or less, and it is observed in the steel plate cross section. The number of oxide inclusions with an average diameter of 10 μm or more is 1
A high-strength steel with excellent toughness in the heat-affected zone of welding, characterized in that the number of precipitates of oxides and nitrides of not more than 1 piece / mm ² and 0.05 to 5 μm is not less than 100 pieces / mm ² . 8. C: 0.02 to 0.15% by weight,
Si: 0.005 to 0.50%, Mn: 0.8 to 2.0
%, P ≦ 0.010%, S ≦ 0.003%, Al: 0.
005-0.05%, Nb: 0.005-0.060
%, Ti: 0.005 to 0.02%, B: 0.0003
~ 0.0030%, N: 0.003 to 0.015%, O
≦ 0.0030%, further, Cu: 0.05 ~
2.0%, Ni: 0.05 to 3.5%, Cr: 0.01
~ 0.5%, Mo: 0.01 to 0.5%, V: 0.00
5 to 0.10%, one or more selected from, and further contains Ca: 0.0005 to 0.0030%,
REM: contains one or more selected from 0.005 to 0.050%, consists of balance Fe and unavoidable impurities, and -0.375C + 0.035≤Nb≤-0.375C +
0.065 C + Mn / 6 + 1.7 Nb + (Cu + Ni) / 15 +
(Cr + Mo) /10+V/2.5+20B≧0.30 C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo +
V) /5≦0.36 is satisfied, and the center segregated portion of the steel sheet (sheet thickness / 50) mm thickness × 1
The C concentration of the average chemical analysis value of 0 mm × 10 mm is 1.2 times or less of the average C concentration of the steel material, the cleanliness of inclusions measured by JIS standard is 0.03% or less, and it is observed in the steel plate cross section. The number of oxide inclusions with an average diameter of 10 μm or more is 1
A high-strength steel with excellent toughness in the heat-affected zone of welding, characterized in that the number of precipitates of oxides and nitrides of not more than 1 piece / mm ² and 0.05 to 5 μm is not less than 100 pieces / mm ² .