WO2007015311A1

WO2007015311A1 - 490 MPa GRADE HIGH-TENSILE STEEL FOR WELDED STRUCTURE EXCELLENT IN HIGH-TEMPERATURE STRENGTH AND METHOD FOR PRODUCING SAME

Info

Publication number: WO2007015311A1
Application number: PCT/JP2005/014461
Authority: WO
Inventors: Yasushi Mizutani; Yoshiyuki Watanabe; Ryuuji Uemori; Tatsuya Kumagai
Original assignee: Nippon Steel Corporation
Priority date: 2005-08-01
Filing date: 2005-08-01
Publication date: 2007-02-08
Also published as: CN101098978A; TW200706659A

Abstract

Provided is a 490 MPa grade high-tensile steel for welded structures which is excellent in high-temperature strength in a range of 600 to 800˚C. This 490 MPa grade high-tensile steel for welded structures is characterized by having a chemical composition consisting of not less than 0.005% and less than 0.040% of C, 0.5% or less of Si, not less than 0.1% and less than 0.5% of Mn, 0.02% or less of P, 0.01% or less of S, 0.3 to 1.5% of Mo, 0.03 to 0.15% of Nb, 0.06% or less of Al, 0.006 % or less of N and the balance of iron and inevitable impurities, while optionally containing, if necessary, one or more of Cu, Ni, Cr, V, Ti, Ca, REM and Mg. This high-tensile steel is further characterized by having a weld cracking sensitivity composition (PCM), which is defined as PCM = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 + 5B, of not more than 0.15%, and substantially containing no B. This high-tensile steel is still further characterized in that it has a micro structure mainly composed of a mixed structure of ferrite and bainite, and bainite accounts for 20 to 90 % of the mixed structure. A method for producing such a high-tensile steel is also provided.

Description

490MPa class high strength steel for welded structure with high temperature strength and its manufacturing method

Law.

Technical field

The present invention relates to architecture, civil engineering, offshore structures, shipbuilding, various storage tanks.

High-strength steel for welded structures with excellent strength and high strength in a hot plate in a relatively short time of about 1 hour in a temperature range of 600 ° C to 800 ° C It is about the script. This invention is mainly intended for thick plates (thick steel plates), but also includes steel pipes and shaped steel.

Background art

The strength of general welded structural steel has been reduced from about 350 ° C, and its allowable temperature is about 500 ° C. For this reason, when these steel materials are used in buildings, offices, residences, multi-storey parking lots, and other buildings, it is mandatory to provide fireproof coatings to ensure safety in fires. The building-related laws and regulations stipulate that the temperature of a steel material does not exceed 350 "€ in the event of a fire. This is because the steel material has a proof stress (yield strength) strength of about s' 350 ° C and 2/3 of room temperature. This is because the fireproof coating has a great influence on the construction cost.

In order to solve such problems, “refractory steel” having resistance against high temperatures has been developed (see, for example, JP-A-2-77523 and JP-A-10-68044). The proof stress at 600 ° C and 700 ° C can be maintained at 2 Z 3 or more, the standard minimum resistance (yield strength) at room temperature. is there. However, in all cases, the proof stress at a specific temperature is shown, and the proof stress at a higher temperature is not mentioned at all. In particular, since temperatures exceeding 700 ° C fall within the temperature region where transformation is partially initiated depending on the steel composition, it can be reflected in the design to the extent that there is a concern about a sudden drop in yield strength (yield strength). It has been extremely difficult to produce a stable and practical steel. Previously, the present inventors have made an invention as a steel capable of securing high-temperature strength at 700 to 800 ° C. and a method for producing the same (for example, 2004-439 6 1)). As a steel component, B addition is essential, making it easy to control the structure and achieving a low yield ratio, especially as steel for building structures. However, as is generally known, B is in the midst of merits and demerits, such as increasing hardenability. For example, during low heat input welding, the weld heat affected zone hardens significantly, resulting in poor toughness. Conversely, if the heat input becomes too high, it precipitates at the austenite grain boundaries and the hardenability of B cannot be used effectively. The problem was that the weld heat input range was limited due to the coarse structure and poor toughness.

By the way, as steel for building structures, there is a demand for a low yield ratio from the viewpoint of earthquake resistance, and it is specified as 80% or less in the “Rolled steel for building structures” standard in IIS. The previous invention by the present inventors was made with these in mind. However, the revised Building Standards Law, which came into effect in June 2001, has been revised from the previous usage rules to performance specifications, and includes the early commercialization of new technologies and materials. Regarding building steel, Article 37 of the Building Standards Law, Article 1:; IIS materials that are permitted to be used for building structures, Section 2: After evaluating the performance of steel according to various performance requirements The one approved by the Minister of Land, Infrastructure, Transport and Tourism can be used. Therefore, the present inventors are not limited by the definition of the yield ratio in JIS steel for construction. The present inventors have intensively studied a steel material having excellent weld toughness in a wide heat input range, and have reached the present invention.

Disclosure of the invention

As described above, when steel is used for buildings, normal steel has low high-temperature strength (resistance to yield stress), so it cannot be used for uncoated or fire-resistant coating reduction. I had to give it. In addition, even with newly developed steel materials, the limit of the fireproof temperature is limited to 600 to 700 ° C, which makes it possible to use a fireproof coating at 700 to 800 ° C and to eliminate the fireproof coating process. The development of steel was desired.

An object of the present invention is to provide a high-strength steel for welded structure having excellent high-temperature strength in a temperature range of 600 to 800 ° C, and a production method capable of industrially supplying the steel. .

In order to overcome the above-mentioned problems, the present invention achieves the object by limiting the steel components, microstructures, etc. to appropriate ranges, and the gist thereof is as follows.

(1) Steel component is mass%,

C: 0.005% or more and less than 0.040%,

S i: 0.5% or less,

Mn: 0.1 to less than 0.5%

P: 0.02% or less,

S: 0.01% or less,

Mo: 0.3-1.5%,

Nb: 0.03 to 0.15%,

A1: 0.06% or less,

N: 0.006% or less, And,

P _CM = C + Si / 30 + Mn / 20 + CU / 20 + N1 / 60 + Cr / 20 + Mo / 15+ V / 10 + 5 B

Weld crack susceptibility composition P _CM define the 15% or less 0.1 and contains substantially no B, with a remainder being iron and unavoidable impurities, microstructure was a mixed structure mainly of ferrite preparative base Inai Bok 490MPa class high-strength steel for welded structures with excellent high-temperature strength, characterized by a 20-90% portion of the vane.

(2) The above steel is in% by mass.

Cu: 0.05-1.0%

Ni: 0.05-1.0%

Gr: 0.05-1.0%

V: 0.0 to 0.1%,

Ti: 0.005 to 0.025%,

Ca: 0.0005 to 0.004%,

REM: 0.0005-0.004%,

Mg: 0.0001 to 0.006%

490 MPa class high-tensile steel for welded structures having excellent high-temperature strength as described in (1) above, characterized by containing one or more of any of the above.

(3) In the above (1) or (2), the average equivalent circle diameter of the old austenite grains having a cross section parallel to the rolling direction at a thickness of 1/4 thickness is 120 m or less. 490MPa class high strength steel for welded structures with excellent high temperature strength as described.

(4) After reheating the slab or slab of the steel component described in (1) or (2) above to the temperature range of 1100 to 1250, the cumulative reduction amount at 1100 ° C or less is 30% or more. Rolled at a temperature of 850 ° C or higher and then allowed to cool, or accelerated from 800 ° C or higher to the following temperature at 650 490MPa class high strength steel for welded structures with excellent high temperature strength. BEST MODE FOR CARRYING OUT THE INVENTION

Details of the present invention will be described below.

High-temperature strength is enhanced by the addition of Mo and Nb, which promotes the precipitation of stable carbonitrides at high temperatures and increases the dislocation density due to the formation of chromite structure, and further dislocations by solid solution Mo and Nb. It is effective to delay recovery. In particular, in order to develop strength at an extremely high temperature of 700 to 800 ° C., which is the object of the present invention, a large amount of Mo is indispensable in the extension of conventional knowledge, but excellent welding as a steel for welded structures. Contrary to the viewpoint of securing the weldability and weld toughness, it is extremely difficult to achieve high temperature strength.

According to the study by the present inventors, the alloy element is optimized and the structure is controlled, particularly by the thermal stability of the matrix structure at a high temperature, the appropriate coherent precipitation strengthening effect, and the dislocation recovery delay effect. We have found that it is possible to achieve both high weldability, weld toughness and high-temperature strength.

First, the reason why the present invention limits the steel components as claimed will be described.

C has the most prominent effect on the properties of steel, and must be controlled within a narrow range, with a limited range of 0.005% or more and less than 0.040%. When the amount of C is less than 0.005%, the strength is insufficient, and when it exceeds 0.040%, the amount of Mo. added is large. In the present invention, the weldability and weld toughness are deteriorated, and the cooling rate after the rolling is finished. If it is too large, the yield of the bainty increases and the risk of excess strength increases. Furthermore, during high-temperature heating equivalent to a fire, the mixed matrix structure of Paynite and Ferai 卜 is kept thermodynamically stable and consistent with Mo, Nb, V, and Ti composite carbonitride precipitates. Maintenance Therefore, C is required to be less than 0.040% in order to secure the strengthening effect.

Si is an element contained in deoxidized upper steel and has a substitutional solid solution strengthening action, so it is effective in improving the strength of the base metal at room temperature, but is particularly effective in improving the high temperature strength above 600 ° C. There is no. In addition, since the weldability and weld toughness deteriorate when a large amount is added, the upper limit is limited to 0.5%. Deoxidation of steel is possible with Ti and A1 alone, and is preferably as low as possible from the viewpoint of weld toughness and hardenability, and is not necessarily added.

Mn is an indispensable element for securing strength and toughness, but Mn, a substitutional solid solution strengthening element, is effective in increasing the strength at room temperature, but it is particularly suitable for high-temperature strength at 600. There is not much improvement effect. Therefore, it is necessary to relatively viewpoint from less than 0.5% of the weldability improving i.e. P _CM reduced in steel containing a large amount of Mo such as in the present invention. By keeping the upper limit of M n low, it is advantageous from the viewpoint of center segregation of the continuous forged slab. As for the lower limit, addition of 0.1% or more is necessary to adjust the strength and toughness of the base metal.

P and S are impurities in the steel of the present invention, and the lower the better. P segregates at the grain boundaries and promotes grain boundary fracture, and S forms sulfides typified by MnS and degrades the toughness of the base metal and welds, so the upper limit is 0.02% and 0.01% respectively. %.

Mo is an indispensable element along with Nb from the viewpoint of high temperature strength development and maintenance in the steel of the present invention. It is more advantageous to add more from the standpoint of high-temperature strength, but it should be constrained in consideration of the base metal strength, weldability, and weld toughness. In the present invention to reduce the C, as long as it is within the range of P _CM section later (0.16% or less), Mo is allowed up to 1.5%. The lower limit is the combined addition with Nb, or the addition of V and Ti, which are effective for improving the high temperature strength described later, ensuring stable high temperature strength. In order to achieve this, it is necessary to add 0.3% or more.

Nb is an element that must be added in combination with Mo. First, as a general effect of Nb, it is an element useful for raising the recrystallization temperature of austenite and maximizing the effect of controlled rolling during hot rolling. It also contributes to the refinement of the heated austenite cake during reheating prior to rolling. In addition, it has the effect of improving high temperature strength by strengthening precipitation and suppressing dislocation recovery, and by adding it together with Mo, it contributes to further improvement of high strength. If less than 0.03%, the effect of suppressing precipitation hardening and dislocation recovery at 700 ° C and 800 ° C is small <, and if exceeding 0.15%, the degree of hardening decreases with respect to the amount added, which is also economically favorable. Not only does it deteriorate, but the toughness of the welds also deteriorates. For these reasons, Nb is limited to the range of 0.03 to 0.15%.

A 1 is an element generally contained on deoxidation, but deoxidation is performed by Si or

Ti alone is sufficient, and in the present invention, the lower limit is not limited.

(Including 0%). However, if the amount of A 1 is increased, not only the cleanliness of the steel is deteriorated, but also the toughness of the welded portion deteriorates, so the upper limit was set to 0.06%.

N is contained in steel as an unavoidable impurity. However, when Nb and T i described later are added, Nb combines with Nb to form carbonitride to increase the strength. To enhance the properties of steel. For this reason, at least 0.001% is necessary as the N amount. However, an increase in the amount of N is detrimental to weld zone toughness and weldability, and in the present invention the upper limit is 0.006%. This upper limit is not necessarily limited in terms of characteristics, and is limited within the range confirmed by the present inventors. Next, Cu, Ni, which can be contained if necessary The reasons for the addition of Cr, V, Ti, Ca, REM, and Mg and the range of the amount added are explained. The main purpose of adding these elements to the basic components is This is because the characteristics such as strength and toughness are improved without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount of addition is of a nature that should be restricted naturally.

Improves the strength and toughness of the base metal without significantly affecting the weldability and weld toughness. In order to exert these effects, addition of at least 0.05% is essential. On the other hand, excessive addition leads to a deterioration of weldability and also increases the risk of Cu cracking during hot rolling, so the upper limit was limited to 1.0%. It is known that the Cu crack itself can be avoided by adding appropriate Ni depending on the amount of Cu. Since the weldability is also related to the amount of other alloy elements including the C amount, the upper limit is not necessarily limited. It does not have a limit taste.

Ni exhibits almost the same effect as Cu, and has a particularly large effect on improving the toughness of the base metal. In order to enjoy these effects reliably, at least

On the other hand, addition of 0.05% or more is essential. On the other hand, excessive addition deteriorates weldability even with Ni, and is also a relatively expensive element. In the present invention, 490 MPa class steel is used.

-In consideration of the fact that it is a get-together, the upper limit is 1.0%.

Cr can be added as necessary to improve the strength of the base material. It can be clearly distinguished from the small amount of playing cards / elements from scrap, etc.

% Or more must be added. Too much addition, like other elements, degrades weldability and weld toughness, so the upper limit is limited to 1.0%.

The above Cu, Ni and Cr are effective not only in terms of mechanical properties of the base metal but also in weather resistance. For such purposes, weldability and weld toughness are greatly impaired. It is preferable to add it as much as possible.

V has almost the same effect and action as Nb, including improved high-temperature strength. The effect is small compared to that of Nb. In addition, V is as can be seen from the fact that also contains the expression of P _CM, hardenability,及boss the impact on the weldability. Therefore, the lower limit is set to 0.01% to ensure the effect of V addition, and the upper limit is set to 0.1% to eliminate adverse effects.

Ti, like Nb and V, is effective in improving high temperature strength. In addition to this, it is preferable to add them particularly when the requirements for the base metal and the weld zone toughness are severe. Because, Ti, when the amount of A1 is small (e.g., 0.003%), combined with O form precipitates composed mainly of Ti ₂ 0 _3, becomes intragranular ferrite formation of nuclei, weld zone toughness Improve. In addition, Ti combines with N and precipitates finely in the slab as TiN, which suppresses the coarsening of austenite grains during heating and is effective in refining the rolling structure. The fine TiN present in the steel sheet・ Refine the grain structure of the heat affected zone during welding. To obtain these effects, Ti must be at least 0.005%. However, if the amount is too large, TiC is formed and low temperature toughness deteriorates weldability, so the upper limit is 0.025%.

Ca and REM combine with the impurity S to improve toughness and suppress cracking due to diffusible hydrogen in the weld, but if too much, coarse inclusions are formed, which adversely affects toughness. Therefore, both are limited to the range of 0.0005 to 0.004%. Since both elements have almost the same effect, at least one of them should be added in order to enjoy the above-described effect.

Mg has the effect of suppressing the growth of austenite grains in the weld heat-affected zone and making it finer, making it possible to strengthen the weld zone. In order to enjoy such effects, Mg needs to be 0.0001% ≥. On the other hand, as the additive amount increases, the effect on the additive amount decreases and the economic efficiency is lost, so the upper limit was set to 0.006%.

In the present invention, B is not made intentionally, and steelmaking The point is that it does not contain substantially beyond the level included as a contamination in the process. B is extremely advantageous in terms of microstructure control and strength improvement when used in high-strength steel because B significantly increases hardenability, but it also has the risk of degrading weldability and weld toughness. Have both. In the present invention, in order to further improve the use performance as a welded structural steel in addition to the high temperature characteristics, the graphical addition of B is avoided and the material is substantially B-free.

Even if the individual components of the steel are limited as described above, the excellent characteristics that are the characteristics of the present invention cannot be obtained unless the entire component system is appropriate. In particular, according to a previous patent by the present inventors (Japanese Patent Application No. 2004-43961), it is intended to greatly improve weldability and weld toughness, so the PCM value is limited to 0.15% or less. To do. Here, P c M is an index representing weld crack sensitivity and is defined by the following equation.

P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15+ V / 10+ δ B

In general, P _CM has excellent low as weldability, not more than 0.22%, preheating (for welding cold cracking prevention) during welding are said unnecessary. ■ In high-strength steels, especially high-strength steels with excellent high-temperature strength, such as the present invention, and that does not substantially contain B, an element that significantly increases hardenability, Pe _M is 0 15% or less is a very low value

Furthermore, in the present invention, ≤ □ □ organization is also limited. With only steel components, excellent weldability and weld toughness for welded structures can be ensured, but high-temperature properties and above all the basic properties (particularly strength) of 490 MPa grade steel are not satisfied. Can not. For this reason, to meet the object of the present invention, the microstructure is mainly composed of a mixture of ferrite and Payne cake, and the fraction of the Payne cake is limited to 20 to 90%. This is because 490MPa class room temperature is a little higher than the pay rate fraction. It is difficult to ensure the strength and high temperature strength, and the experiment results of the present inventors have shown that the risk of exceeding the strength range of 490MPa class steel specified by JIS increases if the ratio of the paynite is too high. The present invention is limited to clarify the characteristics of the present invention, and is not necessarily limited.

These microstructures are representative of 14 thickness positions in the thickness direction. In addition, the name of “Bainite” as an organization name is unclear at certain points in the area when measuring fractions due to the variety of force variations that are widely used by those skilled in the art. May occur. In that case, there is also a method of judging with “Ferrai 卜”, which is another organization in the organizational structure. The X-ray distribution ratio of this place a is 10% to 80%. It is a polygonal or pseudopolygonal ferrule that does not contain any light (not including needle-shaped ferrules).

The austenite grain size before transformation after rolling needs to be appropriately limited in order to control (toughen) the toughness of a relatively high Mo-added steel such as the present invention. The finer the austenite, the finer the final transformation structure and the better the toughness. In order to obtain toughness comparable to that of ordinary steel with low Mo, the austenite grain size at the position of 1 to 4 'thickness in the sheet thickness cross-section direction in the final rolling direction of the steel sheet is limited to an average equivalent circle diameter of 120 m or less. Depending on the plate thickness and steel composition, there is a case where sufficient toughness can be obtained even if it exceeds 120 ^ m. It is limited as a particle size that can ensure stable toughness reliably, and does not necessarily have a limit meaning. . In many cases, it is not always easy to determine the austenity particle size. In such a case, use an impact test piece with a notch sampled in the direction perpendicular to the final rolling direction of the steel sheet, centering on the position where the thickness is 1/4 thickness, for example, JISZ 2202 2mmV notch test piece, Brittle fracture at sufficiently low temperature In this case, the average equivalent circle diameter should be 120 m or less as well. is there.

The excellent properties aimed at by the present invention, including the structure (structure, tissue fraction, former austenite particle size, etc.) and the high temperature characteristics as limited to the above, are as follows. It can be easily obtained.

The reheating of billets or slabs with a given steel composition is limited to a temperature range of 1 100-1250 ° C. The lower limit of 1100 is to make Mo, Nb, and V and Ti added as needed, in a solid solution state with the primary purpose of ensuring high-temperature properties. For this purpose, the higher the reheating temperature, the better, but the heating austenite grains are coarsened and are not preferred from the standpoint of base material toughness, so the upper limit is limited to 1250.

The limitation of the rolling conditions is directly to control the austenite grain size before the transformation after rolling to a relatively fine grain as described above, mainly for ensuring toughness. For this reason, rolling requires that the cumulative reduction at 1 100 ° C or less be 30% or more. The rolling end temperature is limited to 850: or more as the lower limit temperature for precipitation of Mo, Nb, or V, Ti added as needed as carbides under low temperature pressure.

Cooling after rolling should also be limited from the viewpoint of structure control. Although it depends on the steel composition, a relatively thin material can obtain a specified structure even at a cooling rate that is about the same as that of cooling. However, if it is thicker, the cooling rate becomes slower when cooling is needed, and accelerated cooling is required. There is. In this case, accelerated cooling is most commonly water cooling in the production of thick steel sheets, but it is not always necessary to use water cooling. In addition, accelerated cooling is aimed at increasing the cooling speed in the transformation region for controlling the structure, so it is necessary to perform from 800 to above 650 ° C. In the present invention, the high temperature strength is obtained from 600 ° C to 800 ° C in the evening, and the quantitative target is the ratio P (= high temperature yield stress) of the yield stress at high temperature to the room temperature yield stress. (Yield stress at normal temperature) Force Steel temperature T (° C) Force Within the range of 600 ° C to 800 ° C, p≥—0.0033 XT + 2.80. Example

Steel sheets of various steel components (thickness 12 to 80 mm) are manufactured in a continuous forging process of a converter-thick plate process. The mechanical properties, weldability, and weld toughness are evaluated according to JIS. The presence or absence of root cracks in the shape weld cracking test and the HAZ toughness of reproduction equivalent to small heat input and super large heat input welding by the welding reproducible heat cycle were investigated. Table 1 shows the steel components of the present invention together with the comparative example, Table 2 shows the manufacturing conditions, and Table 3 shows the results of the investigation of the structure and various properties.

In the examples of the present invention, all satisfy the limited range of the present invention, and various properties including high temperature strength and reproducible HAZ toughness are extremely good. In contrast, the comparative example is inferior in characteristics to the inventive example because at least one of the steel components, production conditions, and structure deviates from the limited range of the present invention. I understand. That is, in Comparative Example 19, the amount of C is low, and thus the fraction of the vein is low, and both the room temperature strength and the high temperature strength (ratio) are low. In Comparative Example 20, the amount of C is high, so the bainitic fraction is high, and the room temperature strength is high. In addition, the base metal toughness, reproducible HAZ toughness is also inferior. Since Comparative Example 21 has a low Mo content and a low acceleration cooling start temperature, the high-temperature strength (ratio) is low due to the low payin fraction. Comparative Example IV has a low Nb content, a low heating temperature and a rolling end temperature, and a high accelerated cooling stop temperature, resulting in low normal temperature strength and high temperature strength (ratio). In Comparative Example 23, since B is added, when accelerated cooling is applied, the bainite fraction is high and the base metal toughness is poor. In addition, the reproducible HAZ toughness is inferior. Comparative Example 24 has a high amount of Mn, P _CM In addition, the cumulative reduction amount below 1 100 is also low, and therefore the base material strength becomes excessive as 490MPa grade steel with a high percentage of baseite, resulting in poor base metal toughness and reproducible HAZ toughness.

The root crack in the oblique y-type weld crack test was about 0.185% in Comparative Example 24 although Pc _M was higher than the limited range of the present invention, and it did not occur in any case.

2

Table 3

Tensile specimen: JIS Z 2201 No. 1A (total thickness), board thickness over 50 J IS Z 2201 No. 4

(1/4 thickness), perpendicular to the rolling direction

Charpy impact test piece: JIS I 2202 2mniV notch, rolling direction

High-temperature tensile specimen: Round bar (8ωηι or 10mm <, 1/4 thickness position, perpendicular to the rolling direction Thermal history 1: 1400 at X 1 sec, '800 → 500 cooling time 8 sec

Thermal history 2: 1400 X30 seconds, 800 → 500 cooling time 330 seconds Industrial applicability

The steel materials produced by the steel components and production methods based on the present invention have been demonstrated in the examples that the microstructure also satisfies the limited range of the present invention and is excellent not only in high temperature strength but also in weldability and weld toughness. It was. In other words, it is possible to industrially stably mass-produce welded structural steels with high-temperature properties far exceeding the conventional refractory steels that guarantee high-temperature properties up to about 600. For architectural use, for example, it is expected that applied buildings and completely fire-resistant coatings will be greatly expanded.

Claims

The scope of the claims

1. Steel component is mass%,

C: 0.005% or more and less than 0.040%,

Si: 0.5% or less,

Mn: 0.1 to less than 0.5%

P: 0.02% or less.

S: 0.01% or less,

Mo: 0.3-1.5%,

Nb: 0.03 ~ 0.15%,

A1: 0.06% or less,

: 0.006% or less,

And,

\ [Rho weld crack susceptibility composition defined _Μ is 15% or less 0.1 and contains substantially no B, with a remainder being iron and unavoidable impurities, Miku b mixed structure mainly organizations ferrite preparative base Inai Bok 490MPa class high-strength steel for welded structures with excellent high-temperature strength, characterized in that the fraction of the vein is 20 to 90%.

2 The steel is

Cu: 0.05-1.0%

Ni: 0.05-1.0%,

Cr: 0.05-1.0%

V: 0.01% to 0.1%,

Ti: 0.005-0.025%,

Ca: 0.0005% to 0.004%, REM: 0.0005% to 0.004%,

Mg: 0.0001 to 0.006%

The 490 MPa class high-tensile steel for welded structures excellent in high-temperature strength according to claim 1, comprising one or more of any of the above.

3. Plate thickness 14 Excellent in high-temperature strength according to claim 1 or 2, characterized in that the average equivalent circular diameter of the old austenite grains having a cross section parallel to the rolling direction at a thickness of 14 is 120 m or less. For welded structures 490MPa: Grade high strength steel.

4. After reheating the steel slab or slab comprising the steel component according to claim 1 or 2 to a temperature range of 1100 to 1250, the cumulative reduction at 1100 ° C or less is 30% or more, and 850 ° C Manufacture of 490 MPa class high strength steel for welded structures with excellent high-temperature strength, characterized by rolling at the above temperature and then allowing it to cool, or accelerated cooling from 800 ° C or higher to 650 or lower Method