KR100482214B1 - Method for manufacturing high strength steel plate having superior toughness in weld heat-affected zone by nitriding treatment - Google Patents

Abstract

The present invention fundamentally blocks the generation of cast surface cracks in high nitrogen steel through the immersion treatment of low-nitrogen steel slabs, while the difference in toughness of the base metal and the heat affected zone is minimal in the heat input range from medium heat to super heat. It is an object of the present invention to provide a method for producing a high strength welded structural steel by refining the ferrite grain size of the structure.

The present invention for achieving the above object, in the weight% C: 0.03-0.17%, Si: 0.01-0.5%, Mn: 0.4-2.0%, Ti: 0.005-0.2%, Al: 0.0005-0.1%, N: A steel slab that satisfies 0.005% or less, B: 0.0003-0.01%, W: 0.001-0.2%, P: 0.03% or less, S: 0.03% or less and O: 0.005% or less,

Heating the slab at a temperature of Ar ₃ + 25 ° C. to 1150 ° C. for 60 to 180 minutes to make N, 0.008 to 0.03% of the steel, and soaking N, Ti, B, and Al so as to satisfy the following relation: 1.2 ≤Ti / N≤2.5, 10≤N / B≤40, 2.5≤Al / N≤7, 6.5≤ (Ti + 2Al + 4B) / N≤14

The heated slab is cooled to a temperature range of (Ar ₃ + 25 ° C.) to (Ar ₃ -25 ° C.) at a rate of 25 ° C./sec or less, hot rolled at a reduction ratio of 60% or more, and 5-30 ° C./sec. Technical aspect of the present invention relates to a method for manufacturing a welded structural steel material by immersion treatment comprising cooling to room temperature at a cooling rate.

Description

Method for manufacturing high strength steel plate having superior toughness in weld heat-affected zone by nitriding treatment}

The present invention relates to structural steel used in welded structures, such as construction, bridges, shipbuilding, offshore structures, steel pipes, line pipes. More specifically, the present invention relates to a method for producing a welded structural steel material which can improve the toughness of the weld heat affected zone by refining the ferrite grain size of the base material while securing the fine TiN precipitate by depositing the low nitrogen steel slab.

Recently, steel materials used in accordance with the trend of high-rise buildings, structures are being replaced by thick materials. In order to weld such thick materials, high-efficiency welding is inevitable. As a technique for welding thickened steel materials, a high-pass heat submerged welding method and an electro-welding method capable of 1-pass welding are widely used. In addition, in the field of shipbuilding and bridges, even in the case of welding a steel plate having a plate thickness of 25 mm or more, the above-described high heat input welding method capable of one-pass welding is applied.

In general, in welding, the larger the amount of heat input, the larger the amount of welding, so that the number of welding passes decreases. Therefore, it is advantageous to enable high heat input welding in consideration of welding productivity. In other words, increasing the amount of heat input in the welding will be able to widen the range of use. The range of high heat input currently used corresponds to approximately 100-200 kJ / cm, and in order to weld steel with a thicker plate thickness of 50 mm or more, it is possible to have a super heat input range of 200-500 kJ / cm.

When the heat input is applied to the steel, the heat affected zone formed during welding, particularly the heat affected zone near the fusion boundary, is heated to a temperature close to the melting point by the amount of heat input. Accordingly, since the grains of the weld heat-affected zone grow and coarse, and microstructures that are vulnerable to toughness, such as upper bainite and martensite, are formed during the cooling process, the weld heat-affected zone becomes the site where the toughness of the weld is most degraded.

Therefore, in order to secure the stability of the welded structure, it is necessary to suppress the growth of the austenite grains in the weld heat affected zone and to keep it fine. As a means to solve this problem, there is known a technique for delaying grain growth of the weld heat affected zone during welding by appropriately distributing an oxide or Ti-based carbonitride, which is stable at high temperature, to steel. Such techniques include Japanese Patent Laid-Open No. 11-140582, Hei 10-298708 Hei 10-298706 Hei 9-194990 Hei 9-324238 Hei 8-60292, (S) 60-245768, (Square) 5-186848, (S) 58-31065, (S) 61-79745, Journal of the Japan Welding Society, Vol. 52, No. 2, 49 and Japanese Patent Laid-Open And 64-15320.

Japanese Patent Laid-Open No. 11-140582 is a representative technique using TiN precipitates, and has a toughness of about 200J at 0 ° C when 100 J / cm of heat input (maximum heating temperature of 1400 ° C) is applied. Is about 300J). In this prior art, Ti / N is substantially managed at 4-12 so that TiN precipitates of 0.05 µm or less are 5.8 × 10 ³ pieces / mm 2 to 8.1 × 10 ⁴ pieces / mm 2, and TiN precipitates of 0.03 to 0.2 μm are 3.9 ×. The toughness of the welded portion is secured by making the ferrite fine by depositing 10 ³ pieces / mm 2 to 6.2 × 10 ⁴ pieces / mm 2.

However, according to this prior art, when the 100 kJ / cm high heat input welding is applied, the toughness of the base material and the heat affected zone is generally low (0 ° C. impact toughness of the base material: up to 320 J, heat affected zone: up to 220 J). Since the toughness difference of the heat affected zone is about 100J, there is a limit in securing the reliability of the steel structure due to superheated welding of the thickened steel. In addition, as a method for securing the desired TiN precipitate, the slab is heated at a temperature above 1050 ° C. and rapidly cooled and then reheated for hot rolling. Becomes In addition, in this prior art, since the high-nitrogen molten steel containing 0.005-0.02% of N is continuously cast into an ingot, the surface crack of a cast steel is high. That is, since N is an austenite stabilizing element and austenite is maintained for a long time in the solidification process of the ingot, there is a problem that impurities such as P and S may cause segregation cracks by promoting segregation in the uncoagulated portion.

Japanese Patent Laid-Open No. 9-194990 discloses a composite oxide composed of Al, Mn, and Si by controlling the ratio of Al and O to 0.3 ≦ Al / O ≦ 1.5 in low nitrogen steel (N ≦ 0.005%). The ferrite nucleation function in the austenite mouth in the heat affected zone and the transition temperature of the heat affected zone (maximum heating temperature of 1450 ℃, cooling cycle of 800 to 500 ℃ for 60 seconds) is -50 to -60. Welded structural steels are presented. This steel has a rather low transition temperature. In this prior art, high nitrogen steels use less than 0.005% of low nitrogen steels because they deteriorate toughness.

To date, many techniques for improving the toughness of the weld heat affected zone during high heat input welding have been known. However, there have been no reports of a significant improvement in the toughness of the weld heat affected zone during long heat input welding maintained at 1350 ° C. or longer. In particular, while the toughness of the weld heat affected zone shows the same level as that of the base material, there are few technologies that have achieved high strength of the base material.

The present invention fundamentally blocks the generation of cast surface cracks in high nitrogen steel through the immersion treatment of low-nitrogen steel slabs, while the difference in toughness of the base metal and the heat affected zone is minimal in the heat input range from medium heat to super heat. It is an object of the present invention to provide a method for producing a high strength welded structural steel by refining the grain size of ferrite.

The present invention for achieving the above object, in the weight% C: 0.03-0.17%, Si: 0.01-0.5%, Mn: 0.4-2.0%, Ti: 0.005-0.2%, Al: 0.0005-0.1%, N: A steel slab that satisfies 0.005% or less, B: 0.0003-0.01%, W: 0.001-0.2%, P: 0.03% or less, S: 0.03% or less and O: 0.005% or less,

Heating the slab at a temperature of Ar ₃ + 25 ° C. to 1150 ° C. for 60 to 180 minutes so that N of the steel becomes 0.008 to 0.03%, and N, Ti, B, and Al are immersed to satisfy the following relation,

1.2≤Ti / N≤2.5, 10≤N / B≤40, 2.5≤Al / N≤7, 6.5≤ (Ti + 2Al + 4B) / N≤14

The heated slab is cooled to a temperature range of (Ar ₃ + 25 ° C.) to (Ar ₃ -25 ° C.) at a rate of 25 ° C./sec or less, hot rolled at a reduction ratio of 60% or more, and 5-30 ° C./sec. It comprises a step of cooling to room temperature at a cooling rate.

Hereinafter, the present invention will be described in detail.

In the present invention, the term "prior Austenite" refers to austenite formed in the weld heat affected zone when the heat input welding is applied to the steel, and the austenite formed in the manufacturing process of the steel (hot rolling process). Use for convenience to distinguish from.

The present inventors studied a method for improving the toughness of the weld heat affected zone while preventing surface cracks generated in high nitrogen steel, while uniformly distributing TiN precipitates having excellent high temperature stability through immersion in low-nitrogen steel slabs. Refining the ferrite grains by recrystallization-controlled rolling reduces the grain size of the old austenite grains below the threshold (approximately 80 µm) during welding, so that the toughness of the welded heat affected zone is not a problem. It was confirmed that the strength and toughness were improved.

Starting from this point of view, the present inventors have been able to derive the following methods for improving the toughness and strength of the base material together with the toughness of the heat affected zone.

[1] While minimizing the distribution of fine TiN precipitates through nitriding in low nitrogen steel, the solubility product showing stability of precipitates at high temperatures is reduced.

[2] The recrystallization-controlled rolling refines the ferrite grains of the base material to manage the old austenite of the weld heat affected zone at about 80 µm or less. By refining the ferrite grains of the steel material, the strength and toughness of the base material are simultaneously improved. Also,

[3] By reducing the Ti / N ratio, BN and AlN precipitates can be effectively precipitated to increase ferrite production in the welding heat direction, and in particular, to induce ferrite shapes into needles or polygons effective for toughness improvement. . [1] [2] [3] will be described in detail below.

[1] TiN precipitate management through nitriding

When heat input welding is applied to structural steel (base material), the weld heat affected zone near the melting line is heated to a high temperature of about 1400 ° C or higher, and TiN precipitates precipitated in the base material are partially dissolved by welding heat or Oswald lifeening phenomenon ( Only some precipitates are coarsened by ostwald ripening, the smaller precipitates are decomposed and diffused into larger precipitates, resulting in larger precipitates. The inhibitory effect of disappears.

The inventors noticed that this phenomenon is caused by the diffusion of solid solution Ti atoms, which are dispersed in the base metal, by the heat of welding. As a result of examining the characteristics of the TiN precipitates according to the Ti / N ratio, the high nitrogen environment (Ti / In the case of low ratio of N), it has been found that the concentration of solid solution Ti, the diffusion rate of solid Ti atoms, and the high temperature stability of TiN precipitates are improved. More interestingly, even if steel slabs are made of low-nitrogen steel of 0.005% or less, which is less prone to cast surface cracking, and then made into high-nitrogen steel by immersion treatment in slab heating furnace during the rolling process, the ratio of Ti / N is 1.2 to 2.5. In this case, the amount of solid solution Ti was extremely reduced and the high temperature stability of TiN precipitates was increased, resulting in the distribution of fine TiN precipitates of 0.01-0.1㎛ size over 1.0x10 ⁷ / mm2 at intervals of 0.5㎛ or less. . This is because when the nitrogen content is increased through immersion treatment at the same Ti content, all of the dissolved Ti atoms are easily combined with the nitrogen atoms, and the amount of solid solution Ti decreases in a high nitrogen environment. It was analyzed that the solubility level at which the precipitate was stabilized was lowered.

Furthermore, the present inventors have focused on the importance of finely depositing TiN after molten steel without dissolving Ti as an oxide in molten steel and dissolving it finely in order to uniformly distribute a large amount of fine TiN precipitates. As a result, when the amount of dissolved oxygen affecting the formation behavior of the oxide was controlled to 30 ppm or less, it was confirmed that Ti was dissolved in the steel without forming it as an oxide.

According to the present invention, by managing the deoxidation conditions so that Ti is dissolved in steel as much as possible and the ratio of Ti / N is controlled through the sedimentation treatment, the fine TiN precipitates having a size of 0.01-0.1 μm are 1.0 × ^{10 7} / The result distributed in mm <2> or more can be obtained.

[2] recrystallization-controlled rolling (fine grains in ferrite grains)

According to the present invention, it is important to refine the size of the ferrite in the base material structure of ferrite + pearlite together with the management of the precipitates in order to make the size of the old austenite at 80 μm while improving the strength and toughness of the base material. . In the present invention, the refining of the ferrite is achieved by using recrystallization controlled rolling. Recrystallization controlled rolling uses the precipitate (TiN) during high temperature rolling to suppress grain growth, grain refining by static recrystallization during rolling, and ferrite due to carbide precipitation (WC, Fe ₃ C, VC, etc.) during austenite-ferrite transformation. Grain the grain.

[3] microstructure of weld heat affected zone

The facts of the present invention revealed that the toughness of the weld heat affected zone had a significant effect not only on the size of the austenite grains when the base material was heated above 1400 ° C, but also on the amount and shape of the ferrite precipitated at the austenite grain boundaries. will be. In particular, it is important to induce the transformation of polygonal ferrite and acicular ferrite in the austenite mouth. In the present invention, AlN, Fe ₂₃ (B, C) ₆ , BN precipitates are used for this purpose.

Hereinafter, the present invention will be described in detail by dividing the steel composition and its manufacturing method.

[Composition of Welding Structural Steels]

It is preferable to make content of carbon (C) into 0.03 to 0.17%.

If the content of C is less than 0.03%, securing strength as a structural steel is insufficient. In addition, when C exceeds 0.17%, microstructures susceptible to toughness such as upper bainite, martensite and degenerate pearlite during transformation are transformed to lower the low temperature impact toughness of structural steel, and Increasing hardness or strength results in toughness degradation and generation of weld cracks.

The content of silicon (Si) is preferably limited to 0.01-0.5%.

If the Si content is less than 0.01%, the deoxidation effect of the molten steel is insufficient during steelmaking and the corrosion resistance of the steel is deteriorated. If the content of Si is more than 0.5%, the effect is saturated, and the degree of quenchability during cooling after rolling is increased. It promotes the transformation of martensite and lowers the low temperature impact toughness.

The content of manganese (Mn) is preferably limited to 0.4-2.0%.

Mn is an effective element which deoxidizes in steel and improves weldability, hot workability, and strength. Mn forms a substituted solid solution in the matrix to strengthen the matrix to secure the strength and toughness. For this purpose, Mn is preferably added at least 0.4%. However, when the Mn content is more than 2.0%, it has a detrimental effect on the toughness of the weld heat affected zone due to tissue heterogeneity due to manganese segregation rather than a solid solution strengthening effect. In addition, when the steel solidifies, macro segregation and micro segregation occur depending on the segregation mechanism, which promotes the formation of a central segregation zone in the center of rolling, thereby causing a low temperature transformation structure in the center of the base metal. In particular, manganese is an element that precipitates in the form of MnS around the Ti-based oxide and affects the formation of acicular and polygonal ferrites effective for improving the toughness of the weld heat affected zone.

The content of aluminum (Al) is preferably limited to 0.0005-0.1%.

Al is not only an element necessary for deoxidation, but also an indispensable element for forming fine AlN precipitates in steel. In addition, Al is an element which reacts with oxygen to form an Al oxide, and thus is necessary for Ti to form fine TiN precipitates without reacting with oxygen. In order to form a fine AlN precipitate, the Al should be added 0.0005% or more, but if the content exceeds 0.1%, Weedman Stetten is vulnerable to toughness during the cooling of the welding heat affected zone of AlN precipitated and the remaining solid Al It promotes the production of ferrite (Widmanstatten ferrite) and phase martensite to lower the toughness of the high heat input welding heat affected zone.

The content of titanium (Ti) is preferably limited to 0.005-0.2%.

Ti is indispensable in the present invention because it combines with N to form fine TiN precipitates that are stable at high temperatures. In order to obtain such a fine TiN precipitation effect, Ti should be added more than 0.005%, but when the content exceeds 0.2%, coarse TiN precipitates and Ti oxides are formed in molten steel, which does not inhibit austenite grain growth of the weld heat affected zone. It is not preferable because it is.

The content of boron (boron, B) is preferably limited to 0.0003-0.01%.

B forms a BN precipitate, which hinders the growth of the old austenite grains and forms Fe carbide in the grain boundary and in the mouth to promote ferrite transformation of acicular and polygons having excellent toughness. If the B content is less than 0.0003%, such an effect cannot be expected, and if it exceeds 0.01%, it is not preferable because the hardenability increases due to hardening and low temperature cracking of the weld heat affected zone.

The content of nitrogen (N) is preferably limited to 0.008-0.03%.

N is an indispensable element for forming TiN, AlN, BN, VN, NbN, etc., and it suppresses austenite grain growth of welding heat affected zone at the time of high heat input welding and precipitates such as TiN, AlN, BN, VN, NbN Increase the amount. In particular, since the TiN and AlN precipitates have a remarkable effect on the size, precipitate interval, precipitate distribution, complex precipitation frequency with oxide, and high temperature stability of the precipitate itself, the content is preferably set to 0.008% or more. However, when the N content exceeds 0.03%, the effect is saturated, toughness is reduced due to the increase in the amount of solid solution nitrogen distributed in the weld heat affected zone, and it is mixed in the weld metal due to dilution during welding to reduce the toughness of the weld metal. It is not preferable because it causes. In the present invention, N is managed in the steel slab to less than 0.005% of low probability of cast surface cracking, and then made into high nitrogen steel of 0.008-0.03% through the immersion treatment in the slab reheating process.

The content of tungsten (W) is preferably limited to 0.001-0.2%.

W is an element that uniformly precipitates in the base material as tungsten carbide (WC) after hot rolling to effectively suppress ferrite grain growth after ferrite transformation, and also suppress growth of austenite grains in the initial heating of the weld heat affected zone. If the content is less than 0.001%, the tungsten carbide for inhibiting ferrite grain growth is less distributed during the hot rolling and it is not preferable because the effect is saturated when more than 0.2% is added.

The content of phosphorus (P) and sulfur (S) is preferably limited to 0.030% or less.

P is preferably as low as possible because it is an impurity element that promotes central segregation during rolling and hot cracking during welding. In order to improve the toughness of the base metal, the toughness of the weld heat affected zone, and to reduce the center segregation, it is recommended to manage it to 0.03% or less.

Since S forms a low melting point compound such as FeS when present in a large amount, it is preferable to manage S as low as possible. In order to reduce the base material toughness, weld heat affected zone toughness and central segregation, it is recommended that the S content be 0.03% or less. However, in the case of sulfur, it precipitates in the form of MnS around the Ti-based oxide and thus affects the formation of needle-shaped and polygonal ferrites, which are effective for improving the toughness of the weld heat affected zone. It is desirable to limit the amount from 0.003% to 0.03% or less.

Oxygen (O) is preferably at most 0.005%.

If the oxygen content exceeds 0.005%, the Ti element is not preferable because the Ti element is formed of Ti oxide in molten steel and thus does not form a TiN precipitate. Also, inclusions such as coarse Fe oxide and Al oxide are formed, which adversely affects the toughness of the base metal. It is not desirable because it is crazy.

In the present invention, the nitrogen ratio is set so that the ratio of Ti / N of steel is 1.2 to 2.5, the ratio of N / B is 10 to 40, the ratio of Al / N is 2.5 to 7, and the ratio of (Ti + 2Al + 4B) / N is 6.5 to 14. It is preferable to soak.

The ratio of Ti / N is preferably 1.2 to 2.5.

In the present invention, the Ti / N ratio is lowered to 2.5 or less, which has two advantages. First, it is possible to distribute uniformly while increasing the number of TiN precipitates. In other words, if the nitrogen content is increased at the same Ti content, all the Ti atoms in the solid solution combine with the nitrogen atoms introduced through the immersion treatment to increase the fine TiN precipitates. Secondly, the solubility product showing stability at high temperature is small, thereby preventing re-use of the Ti. In other words, in a high nitrogen environment, Ti is more strongly bound to N than solid solution, and thus TiN precipitates are stable at high temperatures. However, when the Ti / N ratio is less than 1.2, the amount of solid solution nitrogen in the base material increases, which is detrimental to the toughness of the weld heat-oriented part. In addition, when the Ti / N ratio is higher than 2.5, excess Ti remaining without precipitation as TiN exists in a solid solution state, which adversely affects the toughness of the weld heat affected zone.

It is preferable to make ratio of N / B into 10-40.

In the present invention, if the N / B ratio is less than 10, the precipitation amount of BN which promotes the ferrite transformation of polygons at the austenite grain boundary during the cooling process after welding is insufficient, and when the N / B ratio exceeds 40, the effect is saturated and dissolved. This is because the amount of nitrogen is increased to lower the toughness of the weld heat affected zone.

It is preferable to make Al / N ratio into 2.5-7.

In the present invention, when the Al / N ratio is less than 2.5, AlN precipitates for inducing needle-like ferrite transformation are insufficient, and the amount of solid solution nitrogen in the weld heat affected zone may increase, resulting in a weld crack, and an Al / N ratio of more than 7 In the case the effect is saturated.

It is preferable that ratio of (Ti + 2Al + 4B) / N is 6.5-14.

In the present invention, when the ratio of (Ti + 2Al + 4B) / N is less than 6.5, the growth inhibition of the austenite grain growth of the weld heat affected zone, the generation of fine polygonal ferrite at the grain boundary, the amount of solid solution nitrogen, the needle shape in the grain and the polygonal ferrite The size and number of distributions of TiN, AlN, BN, and VN precipitates for formation and control of the tissue fraction are insufficient, and the effect is saturated when (Ti + 2Al + 4B) / N exceeds 14. On the other hand, when V is added, it is preferable to set the ratio of (Ti + 2Al + 4B + V) / N to 7-17.

In order to further improve the toughness of the steel material (base material) and the heat-affected portion formed as described above, V is further added.

The content of vanadium (V) is preferably limited to 0.01-0.2%.

V is an element that combines with N to form VN to promote ferrite formation in the weld heat affected zone, and VN precipitates alone or precipitates in TIN precipitates to promote ferrite transformation. In addition, V combines with C to form VC, which acts to inhibit ferrite grain growth after ferrite transformation. When the V content is less than 0.01%, it is difficult to obtain the ferrite transformation promoting effect in the weld heat affected zone because the VN deposition amount is small. On the other hand, exceeding 0.2% is not preferable because it causes toughness of the base metal and the weld heat affected zone (HAZ) and improves the weld hardenability, which may cause the low temperature crack of the weld.

In addition, it is preferable that the ratio of V / N after a immersion process shall be 0.3-9.

In the present invention, when the V / N ratio is less than 0.3, it is difficult to secure an appropriate number and size of VN precipitates deposited and distributed at the TiN + MnS precipitate boundary for improving the toughness of the weld heat affected zone. On the other hand, when the V / N ratio exceeds 9, the weld heat influences because the size of the VN precipitates deposited at the TiN + MnS precipitate boundary is coarsened, and the number of VN precipitations deposited at the TiN + MnS composite precipitate boundary is reduced. Reduce the ferrite phase fraction effective for negative toughness.

In the present invention, in order to further improve the mechanical properties in the steel composition as described above, one or more selected from the group of Ni, Cu, Nb, Mo, Cr is further added.

The content of nickel (Ni) is preferably limited to 0.1-3.0%.

Ni is an effective element for improving the strength and toughness of the base material by solid solution strengthening. To achieve this effect, Ni should be added more than 0.1%, but if the content exceeds 3.0%, the hardenability increases to decrease the toughness of the weld heat affected zone, and there is a possibility of high temperature crack in the weld heat affected zone and the weld metal. Because it is not desirable.

The content of copper (Cu) is preferably limited to 0.1-1.5%.

Cu is an element that is effective at securing the strength and toughness of the base material due to solid solution at the base. In order to achieve this effect, Cu should be added more than 0.1%, but if the content exceeds 1.5%, it is desirable because it increases the hardenability in the weld heat affected zone and lowers the toughness and promotes high temperature crack in the weld heat affected zone and the weld metal. I can't. In particular, the Cu precipitates in the form of CuS around the Ti-based oxide together with sulfur, thereby affecting the formation of needle-shaped and polygonal ferrites, which are effective for improving the toughness of the weld heat affected zone, and thus the content is 0.3-1.5%. desirable.

In addition, when adding Cu and Ni compositely, it is preferable to make these sum total less than 3.5%. The reason for this is that when it exceeds 3.5%, the hardenability becomes large, which adversely affects the weld heat affected zone toughness and weldability.

The content of niobium (Nb) is preferably limited to 0.01-0.10%.

Nb is an effective element from the viewpoint of securing the base material strength, and this effect cannot be obtained when the Nb content is less than 0.01%. On the other hand, if it exceeds 0.1%, it causes undesired precipitation of coarse NbC, which is detrimental to the toughness of the base metal.

Chromium (Cr) is preferably made 0.05 to 1.0%.

Cr increases the hardenability and also improves the strength, when the content is less than 0.05%, strength cannot be obtained. Exceeding 1.0% results in degradation of the base metal and HAZ toughness.

Molybdenum (Mo) is preferably 0.05-1.0%.

Mo is an element that has an effect of increasing hardenability and improving strength. The content of Mo is preferably set to 0.05% or more for securing strength, but in order to suppress HAZ hardening and welding low temperature cracking, the upper limit thereof is 1.0%. It is preferable to set it as.

In addition, in the present invention, one or two kinds of Ca and REM are further added to suppress grain growth of austenite during heating.

Ca and REM form an oxide having excellent high temperature stability, thereby suppressing austenite grain growth when heated in the base metal and improving the toughness of the weld heat affected zone. In addition, Ca has the effect of controlling the coarse MnS shape during steelmaking. To this end, it is preferable to add more than 0.0005% of calcium (Ca) and more than 0.005% of REM, but if Ca exceeds 0.005% of REM of more than 0.05%, large inclusions and clusters are generated to harm the cleanliness of the steel. As REM, 1 type, or 2 or more types, such as Ce, La, Y, and Hf, may be used, and any of the above effects can be obtained.

[Method of manufacturing welded structural steel]

Refining (Deoxidation, Degassing) Process

In general, the steel refining process consists of an out-of-furnace refining process after the first refining of the converter and the second refining of the molten steel of the converter by ladle. ). Usually deoxidation takes place between primary and secondary refining.

A feature of the present invention is that in such a deoxidation process, the dissolved oxygen is adjusted to an appropriate level or lower, and then Ti is added, so that Ti is mostly dissolved in molten steel without forming Ti as an oxide. For this purpose, it is preferable to inject and deoxidize an element having a greater deoxidizing power than Ti before adding Ti. The deoxidizing power of the deoxidizer is as follows.

Cr <Mn <Si <Ti <Al <REM <Zr <Ca ≒ Mg

The amount of dissolved oxygen is greatly influenced by the formation behavior of the oxide, and the deoxidizer having a high affinity with oxygen has a very high rate of bonding with oxygen in the molten steel. Therefore, if deoxidation is performed using an element having a greater deoxidizing power before adding Ti, it is possible to prevent Ti from forming an oxide as much as possible. Of course, before the element Al having a greater deoxidizing power than Ti is introduced, Mn, Si, and the like, which are the five major elements of steel, are added and deoxidized, and then Al is added and deoxidized to reduce the amount of deoxidizer added.

On the other hand, floating separation of inclusions in molten steel is generally known to proceed in the following order. (Dissolution of deoxygenation element in molten steel) → (Involvement of nucleation) → (Growth of inclusions) → (Continuous growth and injury due to collisions between inclusions) → (Removal of slag from slag on molten steel surface) Since the speed of each step varies depending on the type, deoxidation using a strong deoxidation element can lower the dissolved oxygen amount more easily.

In the present invention, the amount of dissolved oxygen is added as low as possible by injecting a strong deoxidation element before the introduction of Ti, and in order to maximize the amount of Ti dissolved in molten steel, it is preferable to set it as at least 30 ppm or less. The reason for this is that when the dissolved oxygen amount exceeds 30 ppm, the oxygen in the molten steel and Ti are combined to form Ti oxide easily when Ti is added, and the amount of solid solution Ti decreases.

On the other hand, according to the 'Stoke Law' widely used in steelmaking, the higher the density of inclusions, the more difficult the inclusions are. Injuries are difficult Accordingly, since inclusions increase in the steel, the addition of a deoxidation element forming a high density inclusion may be utilized as an additional advantage according to the distribution of oxides. It does not affect the effect of.

After adjusting the amount of dissolved oxygen according to the present invention, it is preferable to add Ti within about 10 minutes so that the content is 0.005-0.2%. If less than 0.005% of Ti is contained in the molten steel after deoxidation, it is difficult to form a large amount of fine TiN in the casting process, and if it contains more than 0.2%, the effect is saturated and the TiN is coarsened to expect the austenite grain suppression effect. Difficult to do If Ti is added in excess of 10 minutes, the Ti oxide is formed as the time after Ti is added, and thus the amount of solid solution Ti decreases, which is not preferable. When the vacuum degassing treatment ('RH') is performed in the refining process, the addition of Ti can be performed either before or after the vacuum degassing treatment.

Casting process

In the present invention, the molten steel refined as described above is continuously cast into slabs. Since the molten steel is low nitrogen, the casting speed may be either high speed or low speed during continuous casting. In order to obtain a good internal quality, it is preferable to set it as 0.9-1.3 m / min.

Slab reheating process

In the present invention, by adjusting the ratio of Ti and N through the nitriding treatment in the slab furnace, by increasing the amount of very fine TiN precipitates and reducing the amount of solid solution Ti in the weld heat affected zone during welding, Oswald life Maximum inhibition of (Ostwald ripening).

In addition to the fact that the nitriding effect in slab furnaces can make slabs from low-nitrogen molten steel, it is possible to fundamentally prevent the problem of slab surface cracks commonly encountered in high-nitrogen steels. have. The first is to increase the amount of fine TiN precipitates, the second is to stabilize the fine precipitated TiN at high temperatures. In other words, by increasing the nitrogen content in the base metal at the same Ti content through the nitriding treatment, all Ti atoms can be combined with nitrogen atoms in the heat treatment in the slab furnace to increase the amount of fine TiN precipitates.

On the other hand, in the present invention, before the hot rolling is subjected to a immersion treatment in the reheating step of the slab. The slab is immersed with heating at a temperature of Ar ₃ + 25 ° C. to 1150 ° C. for 60 to 180 minutes to bring the slab nitrogen concentration to 0.008-0.03%. Nitrogen should be contained more than 0.008% in order to secure an appropriate level of TiN deposition in the slab, but if it exceeds 0.03%, the amount of nitrogen that is deposited on the surface of the slab increases more than the amount of nitrogen that is diffused into the slab. Hardening occurs on the surface, which affects the subsequent rolling process.

The slab heating temperature for the immersion treatment is preferably set to Ar ₃ +25 ℃ to 1150 ℃. The reason is that if the heating temperature is less than Ar ₃ + 25 ℃, the driving force for the impregnated nitrogen can be diffused and the number of fine TiN precipitates is small. On the other hand, if the heating temperature is higher than 1150 ℃, the austenite grains of the slabs This is because it grows during heating and affects recrystallization during rolling. More preferably, in order to shorten the immersion treatment time, the heating temperature is preferably 1100 ° C or more.

It is preferable to heat the slab at this temperature for 60 to 180 minutes. The reason is that when the slab heating time is less than 60 minutes, the sedimentation effect is not exerted, and when the heating time is longer than 180 minutes, not only increases the operating cost, but also causes austenite grain growth in the slab, resulting in the subsequent rolling process. Because it affects.

When subjected to the nitriding treatment according to the present invention, the ratio of Ti / N in the slab is 1.2 to 2.5, the ratio of N / B is 10 to 40, the ratio of Al / N is 2.5 to 7, and the ratio of V / N is 0.3. It is preferable to impregnate N so that the ratio of -9 and (Ti + 2Al + 4B + V) N may be 7-17.

Hot rolling process (recrystallization controlled rolling)

In the present invention, the ferrite grains are refined by recrystallization control rolling. Recrystallization-controlled rolling sets a rolling condition that can fine-tune ferrite grains by static recrystallization of a material in which TiN is uniformly distributed through a immersion treatment.

First, the slab heated as described above is cooled to a temperature range of (Ar ₃ +25) to (Ar ₃ -25 ° C.) at a rate of 25 ° C./sec or less, and then (Ar ₃ +25) to (Ar ₃ −). Rolling is started at a temperature of 25 ° C). When the cooling rate is faster than 25 ° C / sec, it is difficult to secure the austenite recrystallization region, it is difficult to secure fine ferrite grains in the abnormal reverse rolling.

By the cooling rate (Ar ₃ + 25 ℃) - by cooling to the temperature range of (Ar ₃ -25 ℃) it is desirable that the step-down swallowing. When the rolling temperature is below Ar ₃ -25 ° C, a large amount of coarse ferrite precipitates in the steel sheet before rolling and remains in a coarse stretched form after rolling, which has a detrimental effect on the ferrite granulation. At a rolling temperature of Ar ₃ + 25 ° C. or higher, strain organic transformation does not occur, so that it is difficult to obtain fine ferrite, and even if strain organic transformation occurs, the grain size of ferrite is not preferable.

At this time, the rolling reduction is preferably at least 60%. If the reduction ratio is less than 60%, the austenite grains are coarse during recrystallization, which is not preferable because the ferrite grains formed at the austenite grain boundary become large.

After hot rolling as described above, and cooled to room temperature at a cooling rate of 5 ~ 30 ℃ / sec. If the cooling rate is less than 5 ℃ / sec, it is difficult to suppress the growth of ferrite grains, and if the cooling rate is faster than 30 ℃ / sec is not preferable because residual austenite or martensite is formed and affects the base material toughness. Can not do it.

· Microstructure of steel

Steel produced according to the present invention has a microstructure of ferrite + pearlite, the size of the ferrite grains is preferably to have 8㎛ or less. This is because the finer the ferrite, the finer the austenite grains of the weld heat affected zone during the high heat input welding can be atomized to 80 µm or less.

In addition, the higher the percentage of ferrite in the composite structure of ferrite + perlite increases the toughness and elongation of the base material. In consideration of this, the phase fraction of the ferrite is preferably limited to 80% or more.

Distribution of precipitates

The former austenite grains of the weld heat affected zone are greatly influenced by the size and number of nitrides distributed in the base metal and their distribution when the austenite grain size of the base material is constant. In addition, 30-40% of the nitrides distributed in the base material when the heat input welding is higher than the heat input temperature (above the heating temperature of 1400 ℃) are re-used as the base material, thereby reducing the austenite grain growth inhibiting effect of the welding heat affected zone. There is a need for a uniform distribution of further nitrides taking into account the nitrides that become. In order to suppress the growth of austenite in the weld heat affected zone, it is important to uniformly distribute the fine TiN precipitate to suppress the Ostwald ripening phenomenon in which some precipitates are coarsened. For this purpose, the TiN precipitates should be controlled to 0.5 μm or less to make the TiN distribution uniform.

In addition, it is preferable to limit the particle diameter and the critical number of TiN to 0.01-0.1 μm and 1.0 × 10 ⁷ or more per 1 mm ² . The reason for this is that less than 0.01 μm is easily re-used to the base metal during the high heat input welding, so that the effect of inhibiting the growth of the austenite grains is insufficient. If the thickness exceeds 0.1 μm, pinning of the austenite grains occurs. This is because the growth inhibition effect is reduced and the same behavior as coarse nonmetallic inclusions has a detrimental effect on the mechanical properties. In addition, when the number of precipitates is less than 1.0 × 10 ⁷ per 1 mm ² , it is difficult to control the size of the austenite grains of the weld heat-affected zone at the time of welding higher than the heat input to be less than or equal to the threshold of 80 μm.

In the hot rolling process of the present invention, hot charge rolling and direct rolling, which are well-known according to a user's use, may be applied, and various techniques such as known control rolling and control cooling may be applied. In addition, in order to improve the mechanical properties of the hot rolled sheet produced according to the present invention, heat treatment may be applied. Thus, even if the known techniques are applied to the present invention, it is natural to interpret that this is a simple change of the present invention substantially within the technical idea of the present invention.

Hereinafter, the present invention will be described in detail by way of examples.

EXAMPLE

Steel grades having the composition as shown in Table 1 were prepared as slabs according to the conditions of Tables 2 and 4 by melting in a converter, and the composition ratios between the alloying elements of the steel types according to the conditions of the present invention are shown in Table 3 to show the effects of the present invention. .

The test pieces for evaluating the mechanical properties of the base metal from the hot rolled plates as described above were taken from the center of the plate thickness of the rolled material, the tensile test piece in the rolling direction, and the Charpy impact specimen in the direction perpendicular to the rolling direction. Was collected.

Tensile test piece was used KS standard (KS B 0801) No. 4 test piece and the tensile test was tested at the cross head speed (5mm / mim). The impact test piece was manufactured according to KS (KS B 0809) No. 3 test piece, and the notch direction was processed on the side of the rolling direction (L-T) in the case of the base material and in the welding line direction on the welding material. In addition, in order to investigate the austenite grain size according to the maximum heating temperature of the weld heat affected zone, it is heated to the maximum heating temperature (1200 ~ 1400 ℃) to 140 ℃ / sec condition by using a simulated welding simulator (1). Hold for a second and then quenched with He gas. The quenched specimens were ground and corroded to determine the austenite grain size at the highest heating temperature condition by KS (KS D 0205).

TiN precipitate size, number, and spacing, which have a significant effect on the microstructure analysis and the toughness of the weld affected zone after cooling, were measured by the point counting method using an image analyzer and an electron microscope. At this time, the test surface was evaluated based on 100 mm ² .

Impact toughness evaluation of the welding heat affected zone is 800-500 ℃ cooling after heating the welding conditions corresponding to about 80 kJ / cm, 150 kJ / cm, 250 kJ / cm, that is, the maximum heating temperature to 1400 ℃ After the welding heat cycles of 60 seconds, 120 seconds, and 180 seconds were applied, the surface of the test piece was polished, processed into an impact test piece, and evaluated through a Charpy impact test at -40 ° C.

Chemical composition (% by weight) C Si Mn P S Al Ti B (ppm) N (ppm) W Cu Ni Cr Mo Nb V Ca REM O (ppm) Invention 1 0.12 0.13 1.54 0.006 0.005 0.04 0.014 7 40 0.005 - - - - - 0.01 - - 11 Invention 2 0.09 0.12 1.50 0.006 0.005 0.07 0.05 10 42 0.002 - 0.2 - - - 0.01 - - 13 Invention 3 0.14 0.10 1.48 0.006 0.005 0.06 0.015 3 39 0.003 0.1 - - - - 0.02 - - 11 Invention 4 0.10 0.12 1.48 0.006 0.005 0.02 0.02 5 41 0.001 - - - - - 0.05 - - 10 Invention 5 0.11 0.15 1.52 0.006 0.004 0.09 0.05 15 40 0.002 0.1 - 0.1 - - 0.05 - - 14 Invention 6 0.12 0.14 1.50 0.007 0.005 0.025 0.02 10 44 0.004 - - - 0.1 - 0.09 - - 10 Invention 7 0.13 0.14 1.48 0.007 0.005 0.04 0.015 8 45 0.15 0.1 - - - - 0.02 - - 14 Invention Material 8 0.11 0.15 1.52 0.007 0.005 0.06 0.018 10 43 0.001 - - - - 0.015 0.01 - - 13 Invention 9 0.13 0.21 1.50 0.007 0.005 0.025 0.02 4 38 0.002 - - 0.1 - - 0.02 0.001 - 15 Invention 10 0.12 0.16 1.45 0.008 0.006 0.045 0.025 6 42 0.05 - 0.3 - - 0.01 0.02 - 0.01 12 Inventive Steel 11 0.12 0.21 1.48 0.006 0.003 0.047 0.019 10 48 0.01 - 0.1 - - 0.01 - - - 18 Conventional Steel 1 0.05 0.13 1.31 0.002 0.006 0.0014 0.009 1.6 22 - - - - - - - - - 22 Conventional Steel 2 0.05 0.11 1.34 0.002 0.003 0.0036 0.012 0.5 48 - - - - - - - - - 32 Conventional Steel 3 0.13 0.24 1.44 0.012 0.003 0.0044 0.010 1.2 127 - 0.3 - - - 0.05 - - - 138 Conventional Steel 4 0.06 0.18 1.35 0.008 0.002 0.0027 0.013 8 32 - - - 0.14 0.15 - 0.028 - - 25 Conventional Steel 5 0.06 0.18 0.88 0.006 0.002 0.0021 0.013 5 20 - 0.75 0.58 0.24 0.14 0.015 0.037 - - 27 Conventional Steel 6 0.13 0.27 0.98 0.005 0.001 0.001 0.009 11 28 - 0.35 1.15 0.53 0.49 0.001 0.045 - - 25 Conventional Steel 7 0.13 0.24 1.44 0.004 0.002 0.02 0.008 8 79 - 0.3 - - - 0.036 - - - - Conventional Steel 8 0.07 0.14 1.52 0.004 0.002 0.002 0.007 4 57 - 0.32 0.35 - - 0.013 - - - - Conventional Steel 9 0.06 0.25 1.31 0.008 0.002 0.019 0.007 10 91 - - - 0.21 0.19 0.025 0.035 - - - Conventional Steel 10 0.09 0.26 0.86 0.009 0.003 0.046 0.008 15 142 - - 1.09 0.51 0.36 0.021 0.021 - - - Conventional Steel 11 0.14 0.44 1.35 0.012 0.012 0.030 0.049 7 89 - - - - - - 0.069 - - - Conventional steels (1, 2, 3) are invention steels (5, 32, 55) of Japanese Patent Application Laid-Open No. 9-194990, and conventional steels (4, 5, 6) are inventions of Japanese Patent Application Laid-Open No. 10-298708. Steels (14, 24, 28) and conventional steels (7, 8, 9, 10) are invention steels (48, 58, 60, 61) of JP-A-8-60292. Inventive Steel F of Patent Publication No. 11-140582

Steel grade used division Primary deoxidation sequence Dissolved oxygen after Al addition (ppm) Ti addition after finishing deoxidation (%) Casting speed (m / min) Specific quantity (ℓ / kg) Inventive Steel 1 Invention 1 Mn → Si 19 0.014 1.1 0.35 Invention 2 Mn → Si 18 0.014 1.1 0.35 Invention 3 Mn → Si 18 0.014 1.1 0.35 Comparative Material 1 Mn → Si 25 0.014 0.5 0.15 Comparative Material 2 Mn → Si 21 0.014 1.8 0.75 Inventive Steel 2 Invention 4 Mn → Si 16 0.05 1.1 0.35 Invention Steel 3 Invention 5 Mn → Si 15 0.015 1.2 0.35 Inventive Steel 4 Invention 6 Mn → Si 15 0.02 1.2 0.35 Inventive Steel 5 Invention 7 Mn → Si 12 0.05 1.2 0.35 Inventive Steel 6 Invention Material 8 Mn → Si 17 0.02 1.2 0.35 Inventive Steel 7 Invention 9 Mn → Si 18 0.015 1.0 0.35 Inventive Steel 8 Invention 10 Mn → Si 14 0.018 1.0 0.35 Inventive Steel 9 Invention 11 Mn → Si 19 0.02 1.1 0.35 Inventive Steel 10 Invention Material12 Mn → Si 23 0.025 1.1 0.35 Inventive Steel 11 Invention Material 13 Mn → Si 21 0.019 1.1 0.35 The manufacturing conditions of the conventional steel (1-11) are not specifically described.

Steel grade used division Heating temperature (℃) Sedimentary atmosphere (ℓ / min) Heating time (min) Slab cooling rate (℃ / sec) Rolling Start Temperature (℃) Rolling end temperature (℃) Cumulative pressure drop (%) Cooling rate (℃ / sec) Base nitrogen (ppm) Inventive Steel 1 Invention 1 1000 550 150 10 920 780 85 15 105 Invention 2 1100 600 140 10 920 780 85 15 115 Invention 3 1100 650 140 10 920 780 85 15 120 Comparative Material 1 900 250 100 10 890 780 85 0.5 72 Comparative Material 2 1350 950 160 10 920 780 85 50 316 Inventive Steel 2 Invention 4 1100 650 130 10 930 780 85 16 275 Invention Steel 3 Invention 5 1100 600 140 9 930 770 85 15 112 Inventive Steel 4 Invention 6 1110 550 110 11 930 780 85 16 80 Inventive Steel 5 Invention 7 1120 690 150 10 920 770 80 16 300 Inventive Steel 6 Invention Material 8 1110 600 120 11 920 770 80 14 100 Inventive Steel 7 Invention 9 1150 600 130 11 920 790 85 15 115 Inventive Steel 8 Invention 10 1150 620 120 12 930 780 80 17 120 Inventive Steel 9 Invention 11 1140 450 120 10 920 770 80 17 90 Inventive Steel 10 Invention Material12 1130 600 110 9 930 765 80 15 100 Inventive Steel 11 Invention Material 13 1120 650 130 11 930 770 80 16 130 Conventional Steel 11 1200 - - Ar ₃ or higher 960 Cooling - Conventional steel (1-11) is a hot rolled material produced without immersion treatment, conventional steel (1-10) is not specifically presented the hot rolling conditions

Alloying element ratio after nitriding treatment to show the effect of the present invention Ti / N N / B Al / N V / N (Ti + 2Al + 4B + V) / N Invention 1 1.3 15.0 3.8 1.0 10.2 Invention 2 1.2 16.4 3.5 0.9 9.3 Invention 3 1.2 17.1 3.3 0.8 8.9 Comparative Material 1 1.9 10.3 5.6 1.4 14.8 Comparative Material 2 0.4 45.1 1.3 0.3 3.4 Invention 4 1.8 28.0 2.5 0.4 7.3 Invention 5 1.4 36.7 5.5 1.8 14.2 Invention 6 2.5 16.0 2.5 6.3 14.0 Invention 7 1.7 20.0 3.0 1.7 9.5 Invention Material 8 2.0 10.0 2.5 9.0 16.4 Invention 9 1.3 14.4 3.5 1.7 10.3 Invention 10 1.5 12.0 5.0 0.8 12.7 Invention 11 2.2 22.5 2.8 2.2 10.2 Invention Material12 2.5 16.7 4.5 2.0 13.7 Invention Material 13 1.5 11.8 3.6 - 9.0 Conventional Steel 1 4.1 13.8 0.6 - 5.7 Conventional Steel 2 2.5 96.0 0.8 - 4.0 Conventional Steel 3 0.8 105.8 0.4 - 1.5 Conventional Steel 4 4.1 4.0 0.8 8.8 15.5 Conventional Steel 5 6.5 4.0 1.1 18.5 28.1 Conventional Steel 6 3.2 2.6 0.4 16.1 21.6 Conventional Steel 7 1.0 9.9 2.5 - 6.5 Conventional Steel 8 1.2 14.3 0.4 - 2.2 Conventional Steel 9 0.8 9.1 2.1 3.9 9.2 Conventional Steel 10 0.6 9.5 3.2 1.5 8.9 Conventional Steel 11 5.5 12.7 3.4 7.8 20.3

division  Precipitate properties Base material texture characteristics Base material mechanical properties Count (pcs / mm ² ) Average size (㎛) Thickness (㎛) AGS FGS Ferrite Percentage (%) Thickness (mm) Yield strength (MPa) Tensile Strength (MPa) Elongation (%) -40 ℃ impact toughness (J) Invention 1 2.3 X ^{10 8} 0.016 0.26 17 6 82 20 554 673 35 364 Invention 2 3.1 X ^{10 8} 0.017 0.26 15 5 84 20 575 681 36 355 Invention 3 2.5 X ^{10 8} 0.012 0.24 13 4 83 20 544 680 36 358 Comparative Material 1 4.3X10 ⁶ 0.154 1.4 38 27 50 20 387 584 28 210 Comparative Material 2 5.4 X 10 ⁶ 0.155 1.5 34 23 55 20 794 980 9 45 Invention 4 3.2 X ^{10 8} 0.025 0.35 15 6 83 25 564 684 35 358 Invention 5 2.6 X ^{10 8} 0.013 0.32 14 6 82 25 554 685 35 349 Invention 6 3.3 X ^{10 8} 0.026 0.42 15 6 84 25 561 683 35 230 Invention 7 4.6 X ^{10 8} 0.024 0.45 16 5 83 30 542 684 35 346 Invention Material 8 4.3X10 ⁸ 0.014 0.35 15 6 82 30 565 682 36 352 Invention 9 5.6 X ^{10 8} 0.028 0.36 15 6 81 30 542 686 36 348 Invention 10 5.2 X ^{10 8} 0.021 0.35 15 5 82 30 534 686 35 358 Invention 11 3.7 X ^{10 8} 0.029 0.29 14 5 84 35 551 696 36 362 Invention Material12 3.2 X ^{10 8} 0.025 0.25 16 5 83 35 563 682 35 347 Invention Material 13 3.2 X ^{10 8} 0.024 0.34 15 5 87 35 555 668 36 362 Invention 14 3.2 X ^{10 8} 0.025 0.35 15 5 84 35 574 659 35 350 Invention 15 3.2 X ^{10 8} 0.023 0.36 14 6 81 30 555 662 38 364 Conventional Steel 1 35 406 436 - Conventional Steel 2 35 405 441 - Conventional Steel 3 25 629 681 - Conventional Steel 4 Precipitates of MgO-TiN 3.03 × 10 ⁶ pcs / mm ² 40 472 609 32 Conventional Steel 5 Precipitates of MgO-TiN 4.07 × 10 ⁶ pcs / mm ² 40 494 622 32 Conventional Steel 6 Precipitates of MgO-TiN 2.80 × 10 ⁶ pcs / mm ² 50 812 912 28 Conventional Steel 7 25 629 681 - Conventional Steel 8 50 504 601 - Conventional Steel 9 60 526 648 - Conventional Steel 10 60 760 829 - Conventional Steel 11 0.2μm or less 11.1 × 10 ³ 50 401 514 18.3

As shown in Table 5, in the case of the present invention, the TiN precipitate showed a range of 1.0 × 10 ⁷ / mm ² or more, and the base metal strength of about 650 MPa or more. In addition, it can be seen that the ferrite grain size in the base metal state of the present invention is very fine, about 8 μm or less, and the percentage of ferrite is 80% or more.

division Austenitic grain size of welding heat affected zone (㎛) Microstructure with welding heat effect of 100kJ / cm heat input Weldment Mechanical Properties Reproduction Weld Heat Affected Zone -40 ℃ Impact Toughness (J) (Maximum Heating Temperature: 1400 ℃) 1200 (℃) 1300 (℃) 1400 (℃) Ferrite Percentage (%) Ferrite Average Grain Size (㎛) Δt _800-500 = 180 seconds Δt _800-500 = 120 seconds Δt _800-500 = 180 seconds Yield strength (kg / ㎡) Tensile Strength (kg / ㎡) Impact Toughness (J) Transition temperature (℃) Impact Toughness (J) Transition temperature (℃) Invention 1 23 33 56 73 16 370 -74 328 -67 294 -62 Invention 2 22 34 55 76 15 383 -76 343 -69 301 -63 Invention 3 23 32 56 74 17 365 -72 333 -67 298 -63 Comparative Material 1 54 84 182 36 32 126 -43 48 -34 26 -27 Comparative Material 2 65 91 198 37 35 104 -40 39 -32 18 -26 Invention 4 25 37 65 75 18 353 -71 323 -68 287 -64 Invention 5 26 40 57 74 16 362 -71 333 -67 296 -61 Invention 6 25 31 53 76 17 386 -73 352 -69 305 -62 Invention 7 24 34 55 74 18 367 -71 336 -67 293 -63 Invention Material 8 27 36 51 73 14 364 -71 334 -67 294 -61 Invention 9 24 36 52 74 17 367 -72 335 -67 285 -62 Invention 10 22 35 55 73 18 385 -72 340 -66 294 -61 Invention 11 26 34 63 74 16 358 -71 324 -68 285 -63 Invention Material12 27 38 63 74 18 355 -71 323 -67 284 -62 Invention Material 13 24 32 55 75 16 367 -72 334 -68 285 -63 Invention 14 25 31 55 72 17 365 -72 331 -68 280 -63 Invention 15 24 32 54 76 14 368 -72 345 -68 286 -63 Conventional Steel 1 187 -51 Conventional Steel 2 156 -48 Conventional Steel 3 148 -50 Conventional Steel 4 230 93 143 -48 132 (0 ℃) Conventional Steel 5 180 87 132 -45 129 (0 ℃) Conventional Steel 6 250 47 153 -43 60 (0 degrees Celsius) Conventional Steel 7 141 -54 -61 Conventional Steel 8 156 -59 -48 Conventional Steel 9 145 -54 -42 Conventional Steel 10 138 -57 -45 Conventional Steel 11 141 -43 219 (0 ℃)

 As shown in Table 6, the austenitic grain size in the conditions of the highest heating temperature of 1400 ℃, such as the welding heat affected zone has a range of 53-64㎛ for the steel of the present invention, while the conventional range of about 175㎛ or more Have In the case of the present invention, the ferrite phase fraction is about 72% or more, and the ferrite grain size is about 15 μm or less. In the case of the invention, the impact toughness of the weld heat affected zone, which is subjected to a high heat input welding heat cycle with a cooling time of 800-500 ° C. for 180 seconds, exhibits excellent toughness of about 276 J or more, and a transition temperature of about -62 ° C. or less. Showed excellent impact toughness. On the other hand, in the case of conventional materials, the impact toughness showed a value of about 100J or less, and the transition temperature also showed a level of about -49 ° C or more. Therefore, it can be seen that the present invention can significantly improve the impact toughness and transition temperature of the weld heat affected zone during the heat input welding as well as improving the base material strength compared to the conventional steel.

As described above, the present invention further provides the impact toughness of the high heat input welding heat affected zone as it distributes fine and uniform TiN precipitates, which are stable and fine even at high temperatures, by immersion in low nitrogen steel slabs, and through recrystallization control rolling. By refining the ferrite of the base material is a useful effect that can provide a steel for welding structure excellent in strength and toughness of the steel (base material).

Claims (5)

By weight% C: 0.03-0.17%, Si: 0.01-0.5%, Mn: 0.4-2.0%, Ti: 0.005-0.2%, Al: 0.0005-0.1%, N: 0.005% or less, B: 0.0003-0.01% Making steel slab satisfying W: 0.001-0.2%, P: 0.03% or less, S: 0.03% or less, O: 0.005% or less, Heating the slab at a temperature of Ar ₃ + 25 ° C. to 1150 ° C. for 60 to 180 minutes so that N of the steel becomes 0.008 to 0.03%, and N, Ti, B, and Al are immersed to satisfy the following relation, 1.2≤Ti / N≤2.5, 10≤N / B≤40, 2.5≤Al / N≤7, 6.5≤ (Ti + 2Al + 4B) / N≤14 The heated slab is cooled to a temperature range of (Ar ₃ + 25 ° C.) to (Ar ₃ -25 ° C.) at a rate of 25 ° C./sec or less, hot rolled at a reduction ratio of 60% or more, and 5-30 ° C./sec. Method for producing a welded structural steel material by the immersion treatment comprising the step of cooling to room temperature at a cooling rate. The steel slab according to claim 1, wherein V is contained in an amount of 0.01 to 0.2%, and the ratio of V and N (V / N) is 0.3 to 9 and 7≤ (Ti + 2Al + 4B) / N≤ after immersion treatment. 17. A method of manufacturing a welded structural steel by immersion treatment, characterized by satisfying 17. The steel slab according to claim 1 or 2, wherein the steel slab comprises Ni: 0.1 to 3.0%, Cu: 0.1 to 1.5%, Nb: 0.01 to 0.1%, Mo: 0.05 to 1.0%, and Cr: 0.05 to 1.0%. 1 or 2 or more selected from Ca, 0.0005-0.005%, REM: 0.005 to 0.05% selected from the group containing one or two, wherein the REM is one or two of Ce, La, Y and Hf Method for manufacturing a welded structural steel by the immersion treatment, characterized in that the paper phase. According to claim 1, The steel slab is deoxidized element having a greater deoxidizing power than Ti in molten steel prior to the Ti input, deoxidized dissolved oxygen of molten steel to 30ppm or less and added within 10 minutes so that Ti is 0.005 ~ 0.2% A method of manufacturing a welded structural steel by immersion treatment, characterized in that it is made by continuous casting of gas-treated molten steel. The steel material of claim 1 or 4, wherein the steel is composed of a composite structure of ferrite and perlite having a microstructure of 8 µm or less, and 1.0 × 10 ⁷ Ti / Si precipitates of 0.01-0.1 µm at intervals of 0.5 µm or less. Method for producing a welded structural steel by the immersion treatment distributed over 2 mm 2.