JP3522564B2 - Steel plate with excellent toughness in weld heat affected zone - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a welding heat affected zone (He).
at Affected Zone: HAZ) A high strength steel sheet of 500 to 600 MPa class having excellent toughness. The steel sheet of the present invention is used for construction, bridges, shipbuilding, line pipes, construction machinery,
Used for various welded steel structures such as offshore structures and tanks. The steel sheet of the present invention has good HAZ toughness under a wide range of welding conditions from small heat input welding to super large heat input welding.

[0002]

2. Description of the Related Art In HAZ, the heating temperature during welding becomes higher as it gets closer to the melting line.
Heating austenite (γ) in the region heated above ℃
Is significantly coarsened, and the HAZ structure after cooling is coarsened and the toughness deteriorates. This tendency becomes more remarkable as the welding heat input increases.

As a means for solving such a problem,
JP-A-60-245768, JP-A-60-152
Japanese Patent Laid-Open No. 626, JP-A-63-210235, JP-A-2-250917, and Japanese Patent Application No. 1-73320 disclose that Ti oxide or TiN and M are contained inside coarse γ grains.
The HAZ toughness has been improved by positively generating intragranular transformation ferrite having nS composite precipitates as nuclei. However, the steel produced by these techniques also has a large heat input welding amount H such that the heat input for welding exceeds 20 kJ / mm.
It was difficult to obtain sufficient toughness in AZ.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a high tensile strength steel plate of 500 to 600 MPa class having good HAZ toughness even in large heat input welding where the heat input for welding exceeds 20 kJ / mm. And

[0005]

Means for Solving the Problems The inventors of the present invention diligently studied the relationship between the precipitation behavior of TiN and the oxide composition, and found a means for precipitating TiN finely and in large numbers in base metal. This makes it possible to enhance the effect of TiN for suppressing the growth of γ grains as compared with the conventional technique, and it has become possible to keep the γ grains in the HAZ near the melting line small. At the same time, the inventors discovered a means to disperse a large number of MnS finely through an oxide and to use this as a nucleus to promote the generation of intragranular transformation ferrite more than ever before. By achieving both the γ grain refinement and the intragranular transformation ferrite formation at a high level, a steel having excellent HAZ toughness was invented.

The gist of the present invention is as follows. (1) C: 0.03 to 0.2% and Si: 0.
4% or less, Mn: 1.0 to 2.0%, P: 0.015%
Hereinafter, S: 0.0005 to 0.006%, Al: 0.0
05% or less, Ti: 0.007 to 0.025%, Mg:
0.0001 to 0.003%, O: 0.001 to 0.0
04%, N: 0.002 to 0.006%, and the effective TiN amount calculated by the following formulas (1) to (4) using weight% is 0.007% or more, and , The balance has a chemical composition consisting of iron and unavoidable impurities, and size
The but that there average composition by weight% (a) Ti ≦ 15% (b) Mg ≧ 5% (c) Mn ≧ 5% der Ru oxide 20 / mm ² or more in 0.5~10μm A steel plate with excellent weld heat-affected zone toughness. O-0.66M
When g-0.89Al ≧ 0, [Ti] = Ti-2 (O-0.66Mg-0.89Al) (1) When O-0.66Mg-0.89Al <0, [Ti] ] = Ti (2) When [Ti] ≧ 3.4N, effective TiN amount = 4.4N (3) When [Ti] <3.4N, effective TiN amount = 1.3 [ Ti] (4)

(2) C by weight%: 0.03 to 0.2
%, Si: 0.4% or less, Mn: 1.0 to 2.0%,
P: 0.015% or less, S: 0.0005 to 0.006
%, Al: 0.005% or less, Ti: 0.007 to 0.
025%, Mg: 0.0001 to 0.003%, O:
0.001-0.004%, N: 0.002-0.00
6%, and further Ca: 0.0003 to 0.00
3%, REM: 0.0003 to 0.003%, Zr:
0.0003 to 0.003% Cu: 1.5% or less, N
i: 1.5% or less, Mo: 1.0% or less, Cr: 1.0
% Or less, Nb: 0.05% or less, V: 0.05% or less, B: 0.002% or less, or one or more of them is contained, and the following (1) to ( The effective TiN amount calculated by the equation 4) is 0.007% or more,
Then, a chemical component and the balance being iron and unavoidable impurities, the average composition in% by weight magnitude 0.5~10μm (a) Ti ≦ 15% (b) Mg ≧ 5% (c) REM + Ca + Zr ≧ 5% (d) Mg + REM + Ca + Zr ≧ 15% (e) Mn steel sheet excellent in weld heat-affected zone toughness ≧ 5% der Ru oxide is characterized by the presence 20 / mm ² or more. O-0.66M
g-0.40Ca-0.17REM-0.18Zr-
When 0.89Al ≧ 0, [Ti] = Ti-2 (O-0.66Mg-0.40Ca
-0.17REM-0.18Zr-0.89Al) ...
-(1) O-0.66Mg-0.40Ca-0.17REM-
When 0.18Zr-0.89Al <0, [Ti] = Ti ... (2) When [Ti] ≧ 3.4N, effective TiN amount = 4.4N (3) [Ti] < In case of 3.4N, effective TiN amount = 1.3 [Ti] (4)

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below.

For example, iron and steel, 65 (1979), 12
As reported in 32, in heating for a short time such as welding, the difference in the dispersed state of TiN in the steel sheet is carried over to the time of welding heating. Therefore, if TiN can be more finely dispersed in the steel sheet than in the conventional case, the pinning force during welding can be improved. The TiN dispersion state in the steel sheet is determined by Ti during solidification cooling in the casting process.
The initial precipitation behavior of N or the Ostwald growth behavior of TiN during reheating of the slab in the plate manufacturing process can be achieved, but the former is dominant. Precipitation of TiN during solidification cooling occurs with δ ferrite or γ. At this time, Ti
The preferential precipitation site of N is regarded as a lattice defect in a broad sense. The lattice defects here are roughly classified into inclusions such as oxides of about 0.5 to 10 μm, dislocations in the base iron, grain boundaries, and microsegregation. Comparing the dispersion states of both, the latter is overwhelmingly large in number, and it can be said that it is effective from the viewpoint of fine dispersion of TiN to utilize the lattice defects in the base metal. Therefore, the inventors studied to preferentially precipitate TiN not on the coarsely dispersed oxide but on the lattice defects in the densely dispersed base iron. As a result, it was found that TiN was less likely to be deposited on the oxide in the following specific oxide composition.

(A) Ti ≦ 15% by weight (b) Mg ≧ 5% by weight (c) REM + Ca + Zr ≧ 5% by weight (d) Mg + REM + Ca + Zr ≧ 15% by weight By controlling the composition of the oxide within the above range, TiN Cannot be precipitated on these oxides, and its reaction promotes precipitation to lattice defects in the base iron. As shown in (a), when the Ti content in the oxide is 15% or less, the force of the oxide to attract the solid solution N is weakened, and the amount of Ti that can be supplied to the surface from the inside of the oxide is small. Therefore, TiN is hard to precipitate on the oxide. When the Ti content exceeds 15%, the force of the oxide to attract the solid solution N is strengthened, and the solid solution N collected on the oxide surface is combined with a large amount of Ti discharged from the inside of the oxide to form a layer on the oxide. It easily produces TiN. As a result, less TiN can be precipitated in the base iron. Moreover, as shown in (b), (c), and (d),
When Mg is mixed with REM, Ca and Zr and exists in a certain amount or more in the oxide, a negative interaction works between the oxide and the solid solution N, so that the oxide does not attract the solid solution N. You will have power. If the Mg content is less than 5%,
When the sum of the contents of REM, Ca, and Zr is less than 5%, or the sum of the contents of Mg, REM, Ca, and Zr is less than 15%, the oxide has a force of eliminating solid solution N. It weakens and a certain amount of solute N can collect on the oxide surface and is consumed as TiN on the oxide.
Even if the conditions of (b), (c), and (d) are satisfied, if the condition of (a) is not satisfied, the oxide is a solid solution N.
The force of repelling the oxygen and the force of the oxide attracting it cancel each other out, and a certain amount of TiN is generated on the oxide. Therefore, by devising the steel composition and deoxidizing method, the above (a),
It is necessary to produce an oxide that satisfies (b), (c) and (d). The oxide at this time does not have to be a simple substance, and may have a form in which a plurality of oxides are combined.

Even if the oxide composition is appropriate as described above, it is necessary to secure a sufficient amount of TiN for pinning. The Ti added to the steel is first consumed by deoxidation in the molten steel. At this time, if Mg, Ca, REM, Zr, Al or the like having a stronger deoxidizing power than Ti is present, these contribute to deoxidizing in preference to Ti. Then, the remaining Ti consumed by the deoxidation is once solid-solved in the base iron immediately after solidification, and in the subsequent cooling process, it is combined with the solid-solution N that does not stick to the oxide to form TiN in the base iron. At this time, TiN that can be stoichiometrically generated can be calculated in consideration of the quantitative balance between solid solution Ti and solid solution N.
The amount of TiN produced calculated in this way is calculated as "effective Ti
N amount ”. The effective TiN amount is calculated by the following equations (1) to (4) using the weight% of the chemical components.

O-0.66Mg-0.40Ca-0.1
When 7REM-0.18Zr-0.89Al ≧ 0, [Ti] = Ti-2 (O-0.66Mg-0.40Ca-0.17REM-0.18Zr-0.89Al) (1) O-0.66Mg-0.40Ca-0.17REM-
When 0.18Zr-0.89Al <0, [Ti] = Ti ... (2) When [Ti] ≧ 3.4N, effective TiN amount = 4.4N ... (3) [Ti] < In case of 3.4N, effective TiN amount = 1.3 [Ti] (4)

Here, [Ti] is the remaining amount of Ti consumed by deoxidation, that is, the amount of Ti that once becomes solid solution Ti immediately after solidification and contributes to TiN formation in the subsequent cooling process. FIG. 1 shows the effect of the amount of effective TiN on the heated γ particle size when held at 1400 ° C. for 10 s. Effective TiN
When the amount is 0.007% or more, a small γ particle size of 200 μm or less is obtained. At this time, it is effective to further improve the HAZ toughness by setting the effective TiN amount to 0.010% or more and reducing the γ grain size to 150 μm or less. When the amount of effective TiN is less than 0.007%, the γ grain growth suppressing effect is insufficient and the effect is unstable, so that the obtained γ particle diameter is coarse and has a large variation.

Further, HAZ finely divided by the above means
If it is possible to promote the generation of intragranular transformation ferrite in the γ grains, the HAZ structure is further refined and the toughness is enhanced. MnS is known to be effective as a nucleus for forming intragranular ferrite, and a technique for dispersing MnS as finely as possible through precipitation into oxide or TiN is disclosed in, for example, JP-A-60-
245768, JP-A-60-152626, JP-A-63-210235, JP-A-2-25
It is invented in Japanese Patent Application No. 0917, Japanese Patent Application No. 1-73320, and the like. However, these techniques aim at the formation of intragranular transformation ferrite within coarse γ grains.
Here, we searched for an oxide that easily precipitates MnS while satisfying the above four conditions (a) to (d), and examined the formation of intragranular transformation ferrite in fine γ grains. . As a result, in addition to the above four conditions (a) to (d), it was found that MnS is frequently precipitated on the oxide when the Mn content in the oxide is 5% by weight or more. At this time, since these oxides contain a large amount of Mg, the oxide itself is finely dispersed, and MnS can be finely dispersed in a large number during solidification cooling by using this as a precipitation site. Such MnS acts as nuclei for the intragranular transformation ferrite in the small γ grains in the HAZ, and further HA
It brings about the refinement of the Z structure. At this time, the size of oxides effective as MnS precipitation sites is 0.5 to 10 μm, and the number of oxides is required to be 20 pieces / mm ² or more. If the number of oxides is less than this, the number of nuclei of intragranular transformation is insufficient and the HAZ structure is not sufficiently refined. Further, the oxide at this time does not have to be a simple substance, and may have a form in which a plurality of oxides are combined.

As described above, TiN and Mn precipitated during solidification cooling by controlling the oxide composition and dispersion state.
It becomes possible to disperse a large amount of S more finely than in the past, and it has become possible to dramatically improve the HAZ toughness by achieving both γ-fine graining and generation of intragranular transformation ferrite.

Next, the reasons for limiting each chemical component will be described.

The lower limit of C is the minimum amount for ensuring the strength and toughness of the base material and the welded portion. However, if the amount of C is too large, the toughness of the base material and HAZ is reduced and the weldability is deteriorated, so the upper limit is made 0.2%.

Si is contained in steel for deoxidation, but if it is too much, the weldability and HAZ toughness deteriorate, so the upper limit is made 0.4%. The deoxidation of the present invention is sufficiently possible with only Ti, and in order to obtain good HAZ toughness, Si should be reduced to 0.
It is desirable to be 3% or less.

Mn is indispensable for securing the strength and toughness of the base material and the welded portion. In addition, when 5% or more of Mn is contained in the oxide, MnS easily precipitates on the oxide. From the above viewpoint, Mn needs to be at least 1.0%. M
When n is less than 1.0%, strength and toughness cannot be secured, and the Mn content in the oxide becomes extremely low,
It becomes difficult for MnS to be deposited on the oxide. However, if Mn is too much, the HAZ toughness is deteriorated, the center segregation of the slab is promoted, and the weldability is deteriorated, so the upper limit is made 2.0%.

In the present invention, P is an impurity element, and is 0.
015% or less. Reduction of P improves mechanical properties of the base material and HAZ through reduction of slab center segregation, and further suppresses intergranular fracture of HAZ.

S is an essential element for forming MnS on the oxide and promoting intragranular transformation. Therefore, at least 0.0005% is necessary, but 0.002% or more is preferable. However, if the amount of S is too large, center segregation is promoted and a large amount of stretched MnS is generated, and the mechanical properties of the base material and HAZ are deteriorated, so the upper limit is made 0.006%.

Al is an undesirable element in the present invention, and it is necessary to reduce it as much as possible. Since Al acts as a deoxidizing agent, when a large amount is contained in the oxide, M in the oxide is contained.
The contents of g, RENM, Ca, Zr, Mn, etc. are lowered, and it becomes difficult to achieve the oxide composition of the present invention. Also, the remaining Al consumed for deoxidation is Al in the base iron.
Attempts to create N hinders the formation of TiN. From the above viewpoint, the upper limit of Al is 0.005%.

Ti is essential for producing TiN as pinning particles. Even if an element that is more strongly deoxidized than Ti is added, since Ti is substantially consumed as an oxide to some extent, that amount cannot contribute to the production of TiN. Therefore, at least 0.007% Ti is required to secure a substantially effective TiN production amount. It is known that a part of TiN is melted at the time of welding heating, and that Ti once solid-dissolved at this time precipitates TiC at the time of cooling to embrittle the HAZ. When Ti exceeds 0.025%, TiC is excessively precipitated to cause remarkable HAZ embrittlement. Also, increase the Ti content in the oxide. Therefore, the upper limit of Ti is 0.025%. TiN also contributes to improving the strength and toughness of the steel sheet by refining the base metal structure by suppressing the γ grain growth during slab heating during plate rolling.

Mg plays an important role in the present invention. Since Mg is a stronger deoxidizing element than Ti, it is deoxidized preferentially over Ti to reduce the content of Ti in the oxide. In addition, the presence of REM, Ca, and Zr in the oxide in a complex manner strengthens the force for the oxide to repel solid solution N. Furthermore, Mg has the effect of finely dispersing the oxide itself. For this,
The content of Mg in the oxide needs to be 5% or more, and therefore 0.0001% or more, preferably 0.0003% or more of Mg must be contained in the steel. However, when Mg is contained in the steel in an amount of more than 0.003%, an oxide mainly composed of Mg is generated and REM, Ca, Zr, and Mn in the oxide are formed.
The content of is extremely low. As a result, the oxide is a solid solution N
Is weakened, and MnS is less likely to precipitate on the oxide. Therefore, the upper limit of Mg in steel is 0.003%.

O is necessary because it produces a predetermined number of oxides of a predetermined size, and this functions as a MnS precipitation site. When O is less than 0.001%, the number of oxides is small, MnS cannot be finely dispersed, and the amount of intragranular transformed ferrite is not sufficient. On the other hand, O is 0.
If it exceeds 004%, many relatively large oxides of 5 μm or more are generated. Oxides of such a size may act as a starting point for occurrence of brittle fracture, so it is not preferable from the viewpoint of toughness that the number of oxides is too large. Therefore,
O needs to be controlled to 0.001 to 0.004%.

N is necessary for forming TiN as pinning particles. When Al is contained in steel, N is substantially consumed even if it is AlN. Therefore, in order to secure a substantially effective TiN production amount,
At least 0.002% N is required. It is known that part of TiN is melted at the time of welding heating, and at this time, N once solid-solved cannot be precipitated as a nitride at the time of cooling and remains in a solid-solution state to embrittle the HAZ. N is 0.
If it exceeds 006%, such solid solution N becomes excessive, resulting in remarkable HAZ embrittlement. Therefore, the upper limit of N is 0.0
It is 06%.

Ca, REM, and Zr are present in the oxide by being mixed with Mg in any one or in plurals,
It lowers the Ti content in the oxide and strengthens the ability to eliminate solid solution N. For this purpose, the sum of the contents of these elements in the oxide should be 5% or more. For that purpose, at least one of these elements must be contained in the steel in an amount of 0.0003% or more. However, when at least one of these elements is contained in steel in an amount of more than 0.003%, Ca,
An oxide mainly composed of either REM or Zr is generated, the content of Mg or Mn in the oxide is extremely low, the content of Ti is increased, and the oxide has a force of eliminating solid solution N. As it weakens, it becomes difficult for MnS to precipitate on the oxide. Therefore, the upper limit of Ca, REM, and Zr in steel is 0.003%. The REM in the present invention refers to lanthanoid-based elements such as La and Ce. When adding these elements, the effect does not change even if a misch metal containing these elements is used.

Subsequently, Cu, Ni, Mo, Cr, Nb,
The reason for adding V and B will be described.

Cu and Ni improve the strength and toughness of the base material without adversely affecting the weldability and HAZ toughness.
However, if it exceeds 1.5%, the weldability and HAZ toughness deteriorate.

Mo and Cr improve the strength and toughness of the base material. However, if it exceeds 1.0%, the base material toughness, weldability and HAZ toughness deteriorate.

Nb is an element effective for refining the structure of the base material and improves the strength and toughness of the base material. But 0.05%
If it exceeds, the HAZ toughness deteriorates.

V improves the strength of the base material. But 0.
If it exceeds 05%, the weldability and HAZ toughness deteriorate.

B enhances the hardenability and improves the strength and toughness of the base material and HAZ. However, if added over 0.002%, HAZ toughness and weldability deteriorate.

The steel of the present invention is manufactured by adjusting the chemical composition to a predetermined chemical composition in the steelmaking process of the steel industry, performing continuous casting, reheating the slab, and then imparting the shape and base material by slab rolling. It If necessary, various heat treatments may be applied to the steel sheet to form the base material. It is also possible to carry out hot charge rolling without reheating the slab.

The number and average composition of the oxides of 0.5 to 10 μm specified in the present invention are measured, for example, by the following method. A small piece sample is cut out from an arbitrary place on the base material steel plate, and the inclusions other than oxides are solutionized by holding this at 1400 to 1450 ° C. for 10 minutes or more, and then water cooling. This is mirror-polished, and then 1000 using an optical microscope.
Observation is performed at a magnification of at least 1 mm ² over an area of at least 1 mm ² , the number of target oxides of 0.5 to 10 μm is measured, and converted into the number per unit area. Further, the composition of at least 10 or more of the oxides of the target size is analyzed by using the EDS attached to EPMA, and the sum of the Ti and Mg contents in the average composition of the oxides is calculated in% by weight. At this time, the analysis value of the oxide composition shows that
When e is detected, Fe is excluded from the analysis value to obtain the average composition of the oxide.

[0036]

EXAMPLES Table 1 shows the chemical composition and the composition and number of oxides of continuously cast steel, and Table 2 shows the manufacturing conditions and mechanical properties of steel sheets. The steel of the present invention has TS of 520 to 660 MPa, and has excellent HAZ toughness that has never been obtained in the fusion line of the electrogas welded portion or the electroslag welded portion having a welding heat input of 20 to 100 kJ / mm. This is because the steel according to the present invention controls the dispersion state of TiN and MnS through the composition and number of oxides by optimizing the chemical composition, and suppresses the γ grain growth suppression effect in HAZ and the intragranular transformation ferrite formation. This is due to having achieved both at an unprecedented level. On the other hand, the chemical composition of the comparative steel is not appropriate, so the base metal and H
The toughness of AZ is inferior. Steel 11 has an excessively low C content, steel 12 has an excessively high C content, and steel 13 has an excessively high Si content, so that the base material or HAZ has poor toughness. Since the Mn content of Steel 14 is too low, the Mn content in the oxide is low, MnS does not precipitate on the oxide, and the intragranular transformation is not promoted. As a result, the HAZ toughness is not good.

Steel 15 has an excessively high Mn content.
Since the P content is too high, the toughness of the base material or HAZ is inferior. Since the amount of S in steel 17 is too low, the amount of MnS that functions as an intragranular transformation nucleus is small, and the HAZ toughness is not good. Steel 18 is inferior in toughness of the base material or HAZ because the S content is too high. Since the amount of Al in Steel 19 is too high, the composition of the oxide is unsuitable and the number is small, and the precipitation of AlN in the base iron prevents the precipitation of TiN, resulting in low effects in both intragranular transformation and pinning. Mari, H
HAZ toughness is poor due to coarsening of the AZ structure. Steel 20 is Ti
Since the amount is too low, the amount of TiN that is substantially formed is small while securing the lower limit of the effective TiN amount, the pinning force of TiN does not increase, and the HAZ structure becomes coarse and the HAZ toughness deteriorates. Since the amount of Ti in steel 21 is too high, the composition of the oxide is inappropriate and the pinning force of TiN does not increase.
The organization becomes coarse. Furthermore, the base material and HAZ are embrittled due to excessive precipitation of TiC. Steel 22 is Mg
Since the amount is too low, the amount of Mg in steel 23 is too high, the amount of Ca in steel 24 is too high, and the amount of REM in steel 25 is too high. Is less, MnS is hard to precipitate on the oxide, and TiN is not finely precipitated, so that the effect is small in either intragranular transformation and pinning, or both.
HAZ toughness is poor due to coarsening of the Z structure.

Since the amount of O in steel 26 is too low, a sufficient number of oxides to finely disperse MnS are not formed, and the HAZ
Not good toughness. Since the amount of O in steel 27 is too high, many relatively large oxides that are the starting points of brittle fracture are generated, and the toughness of the base material and HAZ is poor. Since the amount of N in steel 28 is too low, the amount of TiN that is substantially formed is small while securing the lower limit of the amount of effective TiN, the pinning force of TiN does not increase, and the HAZ structure coarsens, resulting in poor HAZ toughness. Steel 29 has an excessively high N content.
Is generated and the HAZ is brittle. Steel 30 is effective TiN
Since the amount is too low, a sufficient amount of TiN is not formed, and H
HAZ toughness is poor due to coarsening of the AZ structure.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

According to the present invention, it becomes possible to provide a steel sheet of 500 to 600 MPa class having good HAZ toughness regardless of welding heat input, and the safety of various welded structures is remarkably improved. Further, by using the steel of the present invention, the application area of high-efficiency welding is expanded, and it becomes possible to significantly reduce the welding construction cost.

[Brief description of drawings]

FIG. 1 is a diagram showing the effect of the amount of effective TiN on the γ particle size heated at 1400 ° C.

Front page continued (72) Inventor Takashi Sawai 1 Fuji-machi, Hirohata-ku, Himeji City Nippon Steel Corporation Hirohata Works (72) Inventor Rikio Senjoiwa 1 Kimitsu, Kimitsu New Japan Kimitsu Steel Co., Ltd. In-house (58) Fields surveyed (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. C: 0.03 to 0.2% by weight, Si: 0.4% or less, Mn: 1.0 to 2.0%, P: 0.015% or less, S: 0.0. 0005 to 0.006%, Al: 0.005% or less, Ti: 0.007 to 0.025%, Mg: 0.0001 to 0.003%, O: 0.001 to 0.004%, N: 0.002 to 0.006% by weight, the effective TiN amount calculated by the following formulas (1) to (4) using weight% is 0.007% or more, and the balance is iron and It has a chemical composition consisting of unavoidable impurities, size of Ru average composition by weight% (a) Ti ≦ 15% (b) Mg ≧ 5% (c) Mn ≧ 5% der in 0.5~10μm A steel sheet having excellent weld heat-affected zone toughness, characterized by the presence of 20 or more oxides / mm ² . O-0.66M
When g-0.89Al ≧ 0, [Ti] = Ti-2 (O-0.66Mg-0.89Al) (1) When O-0.66Mg-0.89Al <0, [Ti] ] = Ti (2) When [Ti] ≧ 3.4N, effective TiN amount = 4.4N (3) When [Ti] <3.4N, effective TiN amount = 1.3 [ Ti] (4) 2. C: 0.03 to 0.2% by weight, Si: 0.4% or less, Mn: 1.0 to 2.0%, P: 0.015% or less, S: 0.0. 0005 to 0.006%, Al: 0.005% or less, Ti: 0.007 to 0.025%, Mg: 0.0001 to 0.003%, O: 0.001 to 0.004%, N: 0.002 to 0.006%, further, Ca: 0.0003 to 0.003%, REM: 0.0003 to 0.003%, Zr: 0.0003 to 0.003% Cu: 1 0.5% or less, Ni: 1.5% or less, Mo: 1.0% or less, Cr: 1.0% or less, Nb: 0.05% or less, V: 0.05% or less, B: 0.002 % Effective TiN content containing any one or more of the following, and calculated by the following formulas (1) to (4) using weight%. And 0.007% or more, and has a chemical composition and the balance being iron and unavoidable impurities, the average composition in% by weight magnitude 0.5~10μm (a) Ti ≦ 15% (b) mg ≧ 5% (c) REM + Ca + Zr ≧ 5% (d) mg + REM + Ca + Zr ≧ 15% (e) Mn ≧ 5% der Ru oxide 20 / mm ² or more weld heat-affected zone toughness, characterized in that there Excellent steel plate. O-0.66M
g-0.40Ca-0.17REM-0.18Zr-
When 0.89Al ≧ 0, [Ti] = Ti-2 (O-0.66Mg-0.40Ca
-0.17REM-0.18Zr-0.89Al) ...
-(1) O-0.66Mg-0.40Ca-0.17REM-
When 0.18Zr-0.89Al <0, [Ti] = Ti ... (2) When [Ti] ≧ 3.4N, effective TiN amount = 4.4N (3) [Ti] < In case of 3.4N, effective TiN amount = 1.3 [Ti] (4)