WO2010082361A1

WO2010082361A1 - Steel for weld construction having excellent high-temperature strength and low-temperature toughness and process for producing the steel

Info

Publication number: WO2010082361A1
Application number: PCT/JP2009/050906
Authority: WO
Inventors: 渡部義之; 植森龍治
Original assignee: 新日本製鐵株式会社
Priority date: 2009-01-15
Filing date: 2009-01-15
Publication date: 2010-07-22
Also published as: EP2380997A1; KR101189263B1; EP2380997A4; JP4834149B2; CN101849026B; EP2380997B1; KR20100105821A; CN101849026A; BRPI0901011B1; JPWO2010082361A1; US20110262298A1; BRPI0901011A2

Abstract

Disclosed is a steel for weld construction that has excellent high-temperature strength and low-temperature toughness and can be produced at low cost. The steel for weld construction is produced by heating a steel product comprising C: 0.003 to 0.05%, Si: not more than 0.60%, Mn: 0.6 to 2.0%, P: not more than 0.020%, S: not more than 0.010%, Cr: 0.20 to 1.5%, Nb: 0.005 to 0.05%, Al: not more than 0.060%, N: 0.001 to 0.006%, and Mo as an impurity limited to not more than 0.03% with the balance consisting of iron and unavoidable impurities, the steel product having a weld cracking susceptibility composition (P_CM) value, defined by P_CM = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 + 5B, of not more than 0.22%, to 1000 to 1300°C, terminating the hot rolling at a temperature of 800°C or above, and then cooling the steel product.

Description

Welded structural steel excellent in high temperature strength and low temperature toughness and its manufacturing method

The present invention is mainly intended for building construction refractory steel for the purpose of maintaining proof stress at high temperatures such as fires, but is not limited to architectural use, and is widely used for marine structures, ships, bridges, various storage tanks, etc. Applicable to welded structural steel. In addition, the strength level of the steel plate as a target is a class generally called as 40 kg or 50 kg steel, which has a yield strength of 235 to 475 MPa and a tensile strength of 400 to 640 MPa.

A number of techniques have been disclosed for so-called fire-resistant steel in architectural applications aimed at securing high-temperature proof stress, including Japanese Patent Application Laid-Open No. 2-77523. However, most of them contain Mo. Certainly, Mo is an element that is extremely effective in securing the high-temperature proof stress of steel, but is also an expensive element.
By the way, since the strength of general structural steel standardized by Japanese Industrial Standards (JIS) or the like decreases from about 350 ° C., the allowable temperature is about 350 ° C. In other words, when using the steel materials described above for buildings such as buildings, offices, residences, and multistory parking lots, it is obliged to apply sufficient fireproof coating to ensure safety in the event of a fire. The various building-related laws and regulations stipulate that the temperature of steel materials should not exceed 350 ° C in the event of a fire. This is because the steel material has a yield strength of about 2/3 of room temperature at about 350 ° C., which is lower than the required strength. For this reason, when using a general steel material for a building, it is necessary to apply a fireproof coating so that the temperature of the steel material does not reach 350 ° C. during a fire.
In order to omit or reduce this fireproof coating, the high-temperature strength at the time of a high-temperature tensile test at 600 ° C. or the like (hereinafter, unless otherwise specified, high temperature refers to 600 ° C., and high-temperature strength refers to high-temperature strength). ) Refractory steel with increased resistance is being used.
In general, Mo is added to refractory steel for the purpose of maintaining high-temperature strength. However, Mo has a large change in market conditions, and depending on the amount added, it is often more expensive than the fireproof coating cost. For this reason, development and practical use of an inexpensive refractory steel not containing Mo have been awaited.
An object of this invention is to obtain the steel for welded structures which is excellent also in the low temperature toughness which is one of the basic performances of steel materials with the outstanding high temperature strength, without adding expensive Mo. Therefore, by limiting the steel components to a specific range and further limiting the production method, refractory steel that has excellent high-temperature strength, suppressed weld cracking susceptibility, and secured low-temperature toughness is industrially stable and low-cost. It provides a method that can be supplied by
According to the present invention, a large amount of welded structural steel having sufficient proof strength can be supplied even in an environment exposed to a high temperature such as a fire, and this contributes to improving the safety of a wide range of welded steel structures for various applications. Is possible.
The point of the present invention is to secure a high-temperature strength at 600 ° C. In order to stably secure a high temperature strength, a relatively small amount of C instead of expensive Mo and combined addition of Cr and Nb, transformation structure strengthening and Cr and Nb precipitates It is to use precipitation strengthening by (carbonitride).
That is, it has been found that by adding an appropriate amount of Cr under the Mo-free condition, the hardenability of the steel is improved, the transformation temperature is lowered, and the hard structure containing cementite becomes bainitic.
As a result, the strength at room temperature and high temperature is increased, and the matrix is a fine bainitic structure transformed at a relatively low temperature. Therefore, at high temperatures, Cr and Nb combined with Cr and Nb alone or combined with carbonitride are combined. The inventors have found that it is very finely precipitated in the matrix, and that high-temperature strength can be secured and maintained at a high level, resulting in the present invention.
As described above, the refractory steel not containing Mo is very innovative in itself, and at the same time, the basic performance (strength and toughness) as a welded structural steel is low by not containing Mo with high hardenability. Of course, it also leads to improved weldability and gas cutting performance.
The present invention, Cr, not Nb only defines C, Si, individual element content and weld crack susceptibility composition P _CM, including Mn, by further limiting the manufacturing conditions, the use of costly Mo Not only has excellent high-temperature strength and low-temperature toughness compatible, but also ensures various use performances as welded structural steel. The summary is as follows.
(1) The component is mass%,
C: 0.003 to 0.05%
Si: 0.60% or less Mn: 0.6 to 2.0%
P: 0.020% or less S: 0.010% or less Cr: 0.20 to 1.5%
Nb: 0.005 to 0.05%
Al: 0.060% or less N: 0.001 to 0.006%,
Furthermore, Mo is limited to 0.03% or less as an impurity, and the balance consists of iron and inevitable impurities,
P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
Steel in defined weld crack susceptibility composition P _CM value is is less than 0.22%, was heated to a temperature of 1000 ~ 1300 ° C., to exit the hot rolling at 800 ° C. or higher, that then cooled A method for producing a welded structural steel having excellent high temperature strength and low temperature toughness.
(2) After finishing the hot rolling, accelerated cooling is started from a temperature of 750 ° C. or higher, and accelerated cooling is stopped at a temperature of 550 ° C. or lower. A method for producing welded structural steel with excellent low temperature toughness.
(3) Furthermore, in mass%,
V: 0.01 to 0.10%
Ti: 0.005 to 0.025%
(1) or (2), the method for producing a welded structural steel having excellent high-temperature strength and low-temperature toughness.
(4) Further, by mass% Ni: 0.05 to 0.50%
Cu: 0.05 to 0.50%
B: 0.0002 to 0.003%
Mg: 0.0002 to 0.005%
The method for producing a steel for welded structure having excellent high-temperature strength and low-temperature toughness according to any one of (1) to (3), characterized by containing any one or more of the above.
(5) Further, in mass% Ca: 0.0005 to 0.004%
REM: 0.0005 to 0.008%
The method for producing a steel for welded structure having excellent high-temperature strength and low-temperature toughness according to any one of (1) to (4), characterized by containing any one of the following.
(6) The component is mass%,
C: 0.003 to 0.05%
Si: 0.60% or less Mn: 0.6 to 2.0%
P: 0.020% or less S: 0.010% or less Cr: 0.20 to 1.5%
Nb: 0.005 to 0.05%
Al: 0.060% or less N: 0.001 to 0.006%,
Furthermore, Mo is limited to 0.03% or less as an impurity, and the balance consists of iron and inevitable impurities,
P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
Resulting in a steel weld crack susceptibility composition P _CM value is less than 0.22%, which is defined, by heating to a temperature of 1000 ~ 1300 ° C., to exit the hot rolling at 800 ° C. or higher, and then cooled Steel for welded structures with excellent high-temperature strength and low-temperature toughness.

The range of addition of each alloy element defined in the present invention will be described.
C: 0.003 to 0.05%
C is limited to a very low level as a high strength steel. This is closely related to the production method along with other components. Among steel components, C has the greatest influence on the properties of steel materials. The lower limit of 0.003% is a minimum value for preventing the heat-affected zone such as securing strength and welding from being softened more than necessary.
If the amount of C is too large, the hardenability is increased more than necessary, which adversely affects the balance between strength and toughness of the steel material and weldability. In addition, as will be described later, depending on the target plate thickness and strength, there are cases where accelerated cooling is stopped at a relatively low temperature, in order to suppress extreme hardening near the front and back surfaces of the steel material and material variation in the plate thickness direction. The upper limit was made 0.05%.
The lower limit is preferably 0.005%, more preferably 0.01%, in order to avoid strength reduction from the balance with operational fluctuations and other components. In order to avoid excessive quench hardening and material fluctuation, the upper limit is preferably 0.04%, and more preferably 0.03%.
Si: 0.60% or less Si is an element contained in steel for deoxidation, but if added in large amounts, weldability and HAZ toughness deteriorate, so the upper limit was made 0.60%. Since deoxidation of steel is possible with Ti and Al, the content may be determined by balance with these elements. However, it is preferably as low as possible from the viewpoints of HAZ toughness, hardenability, etc., and may be free of additives. For this reason, the upper limit may be limited to 0.40%, 0.20%, and 0.10%. In addition, when manufacturing steel in a steelmaking factory, it is common that 0.01% or more of Si is contained even when it is deoxidized by Ti and Al without adding Si.
Mn: 0.6 to 2.0%
Mn is an indispensable element for securing the strength and toughness at room temperature, and its lower limit is 0.6%. Preferably it is 0.8% or more or 1.0% or more. However, if the amount of Mn is too large, not only the hardenability is increased and the weldability and HAZ toughness are deteriorated, but also the center segregation of the continuously cast slab is promoted, so the upper limit was made 2.0%. Preferably it is 1.8% or less, More preferably, it is 1.6% or less or 1.4% or less.
P: 0.020% or less Since the amount of P tends to decrease the grain boundary fracture in HAZ when the amount is small, the smaller the amount, the better. If the content is large, the low temperature toughness of the base metal and the weld zone is deteriorated, so the upper limit is made 0.020%. More preferably, the content is 0.015% or less, 0.010% or less, or 0.008% or less. Of course, no additives may be added.
S: 0.010% or less S is more preferable from the viewpoint of low temperature toughness of the base material. If the content is large, the low temperature toughness of the base metal and the welded portion is deteriorated, so the upper limit is made 0.010%. 0.008% or less, 0.006%, and 0.004% are more preferable. Of course, no additives may be added.
Cr: 0.20 to 1.5%
Cr is one of the most important elements in the present invention. It is essential to add Cr together with Nb to ensure high temperature strength. This is because the transformation temperature is lowered due to the effect of improving the hardenability of Cr, and the hard structure containing cementite becomes bainitic, so the strength at room temperature and high temperature is increased, and further, Cr precipitates (carbonitrides at high temperatures). This is because the precipitation strengthening due to the above is utilized.
In order to enjoy these effects, the Cr content should be at least 0.20%. Preferably it is 0.35% or more, more preferably 0.50% or more, 0.8% or 1.0% or more. However, if the addition amount is too large, the base material, the toughness of the welded portion and the weldability are deteriorated and the economical efficiency is also lost, so the upper limit was made 1.5%. Preferably it is 1.3% or less.
Nb: 0.005 to 0.05%
Nb is the most important element in the present invention together with Cr. This is because, like Cr, precipitation strengthening by Nb precipitates (carbonitrides) is used to ensure high temperature strength.
For this reason, at least 0.005% or more is necessary. Preferably it is 0.010% or more. However, if the addition amount is too large, the toughness of the weld zone is deteriorated, so the upper limit was made 0.05%. Preferably it is 0.045% or less, Furthermore, it is good in it being 0.030% or less. In addition, Nb addition raises the non-recrystallization temperature of austenite and contributes also to exhibiting the effect of the controlled rolling at the time of hot rolling to the maximum.
By the combined addition of Cr and Nb described above, high temperature strength can be secured even without Mo. Therefore, intentional addition of Mo is not performed in the present invention. Moreover, even if Mo is not intentionally mixed as an impurity, it is limited to 0.03% or less.
Al: 0.060% or less Al is an element generally contained in steel for deoxidation. Since deoxidation is also performed with Si or Ti, the amount may be determined in balance with these elements. However, when the amount of Al increases, not only the cleanliness of steel deteriorates but also the toughness of the weld metal deteriorates, so the upper limit is made 0.060%. Preferably it is 0.040% or less. The smaller the amount, the better. In addition, when manufacturing steel in a steelmaking factory, even when it does not deoxidize by Al, it is common that 0.001% or more of Al is contained.
N: 0.001 to 0.006%
N is contained in the steel as an unavoidable impurity, but combines with Nb to form carbonitride to increase the strength, and TiN to increase the properties of the steel as described above. . For this reason, the N amount is required to be at least 0.001%. Preferably it is 0.0015% or more. However, an increase in the amount of N is detrimental to the weld heat affected zone toughness and weldability, and the upper limit of the steel of the present invention is 0.006%. More preferably, it is 0.0045% or less.
Next, the reason for the addition of V and Ti that can be contained as required will be described.
V: 0.01 to 0.10%
V has substantially the same effect as Nb, and the role of V in the present invention complements Nb. However, since V is less effective than Nb and affects hardenability, the upper and lower limits are limited. The lower limit was set to 0.01% as the minimum amount that can surely enjoy the effect of V addition. Preferably, it is 0.025% or more. The upper limit is set to 0.10% in consideration the influence on the weld crack susceptibility composition P _CM which will be described later. Preferably, it is 0.08% or less, further 0.05% or less.
Ti: 0.005 to 0.025%
Ti is preferably added to improve the toughness of the base material and the weld heat affected zone. This is because when Ti has a small amount of Al (for example, 0.003% or less), it combines with O to form precipitates mainly composed of Ti2O3, which becomes the nucleus of intragranular transformation ferrite formation and has a weld heat affected zone toughness. Improve.
Further, Ti is combined with N and finely precipitated in the steel as TiN, which suppresses the coarsening of γ grains during heating and is effective for making the rolled structure finer. Further, the fine TiN present in the steel material refines the weld heat affected zone structure during welding and improves toughness. In order to obtain these effects, Ti needs to be at least 0.005%. However, if it is too much, TiC is formed and low temperature toughness and weldability are deteriorated, so the upper limit was made 0.025%. Preferably, it is 0.020% or less.
Next, the reason for adding Ni, Cu, B, and Mg will be described.
The main purpose of adding these elements to the basic components is to improve properties such as strength and toughness without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount added is of a nature that should naturally be limited.
Ni: 0.05 to 0.50%
If Ni is not added excessively, it improves the strength and toughness of the base material without adversely affecting the weldability and weld heat affected zone toughness. In order to exert these effects, addition of at least 0.05% is essential.
On the other hand, excessive addition is not only expensive, but is not preferable for weldability. Further, it has been pointed out that the addition of a large amount of Ni may induce stress corrosion cracking (SCC) in liquid ammonia. According to the experiments by the inventors, addition up to 1.0% does not significantly deteriorate the weldability and SCC in liquid ammonia, and the effect of improving strength and toughness is greater. Was 0.50%. Furthermore, when priority is given to economy, it may be limited to 0.35%.
Cu: 0.05 to 0.50%
Cu exhibits substantially the same effects and phenomena as Ni, with the upper limit of 0.50% being restricted in terms of weldability deterioration and excessive addition because Cu-cracks are generated during hot rolling, making it difficult to manufacture. The lower limit should be the minimum amount for obtaining a substantial effect, and is 0.05%. When giving priority to economy, the upper limit may be limited to 0.30%.
B: 0.0002 to 0.003%
B segregates at austenite grain boundaries and suppresses the formation of ferrite, thereby improving hardenability and contributing to strength improvement. In order to enjoy this effect, at least 0.0002% is necessary.
However, too much addition not only saturates the effect of improving hardenability but also may form B precipitates that are harmful to toughness, so the upper limit is made 0.003%. Preferably it is 0.002% or less. In cases where stress corrosion cracking is a concern, such as for tank steel, reduction of the hardness of the base metal and the weld heat affected zone is often the point (for example, prevention of sulfide stress corrosion cracking (SCC)). Therefore, HRC ≦ 22 (HV ≦ 248) is essential in Rockwell hardness). In such a case, addition of B which increases hardenability is not preferable. In addition, although B has the above-mentioned strength improvement effect, since there is a problem of material deterioration such as heat-affected zone toughness due to addition of B, in order to avoid these problems, B is 0.0003% or less. It is more desirable to limit or not add.
Mg: 0.0002 to 0.005%
Mg suppresses the growth of austenite grains in the weld heat-affected zone and has the effect of making the grains finer, so that the weld zone can be strengthened. In order to enjoy such an effect, Mg needs to be 0.0002% or more. On the other hand, since the effect cost for the added amount decreases as the added amount increases, the upper limit is set to 0.005% because this is not a cost effective measure. Preferably it is 0.0035% or less.
Next, the reason for adding Ca or REM will be described.
Ca: 0.0005 to 0.004%
REM: 0.0005 to 0.008%
Ca and REM control the morphology of MnS, improve the low temperature toughness of the base material, and reduce the susceptibility to hydrogen induced cracking (HIC, SSC, SOHIC) in a wet hydrogen sulfide environment. In order to exert these effects, 0.0005% is necessary at least.
However, too much addition adversely deteriorates the cleanliness of the steel, and increases the base metal toughness and susceptibility to hydrogen-induced cracking (HIC, SSC, SOHIC) in a wet hydrogen sulfide environment. REM was limited to 0.004% and 0.008%, respectively. Preferably, they are 0.003% and 0.006% or less, respectively. In addition, since Ca and REM have a substantially equivalent effect, any one should just be added in the said range, and you may add both.
Even if the individual components of the steel are limited, excellent properties cannot be obtained unless the entire component system is appropriate. In the present invention, to limit the content of each element (mass%), the value of the weld crack susceptibility composition P _CM, which is defined by the following expression below 0.22%.
P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60
+ Cr / 20 + Mo / 15 + V / 10 + 5B
PCM is an index representing weldability. The lower the _{CM, the} better the weldability. In JIS G 3106 “Rolled steel for welded structure”, it varies depending on the strength level and thickness, but the most severe one is specified to be 0.24% or less.
According to various weld cracking test in a wide range of steels of the present inventors, those with limited P _CM below 0.22% as a condition can prevent more severe constraints, reliably welded cold cracking even at ambient conditions is there. Although the lower limit is not particularly limited, the lower limit is naturally limited by the limited range of each component.
Subsequently, manufacturing conditions will be described.
The reason why the heating temperature prior to hot rolling is limited to 1000 to 1300 ° C. is to keep the austenite grains during heating small and to refine the rolled structure. 1300 ° C. is an upper limit temperature at which the austenite during heating is not extremely coarsened. When the heating temperature is exceeded, the austenite grains are coarsely mixed and the structure after transformation is also coarsened, so that the toughness of the steel is remarkably deteriorated.
On the other hand, if the heating temperature is too low, depending on the plate thickness, it is difficult not only to secure the rolling end temperature, which will be described later, but also to increase the non-recrystallization temperature of austenite and to cause the solution of Nb to develop precipitation strengthening. From the viewpoint, the lower limit was set to 1000 ° C. The most preferable heating temperature range is 1050 to 1250 ° C.
The steel material heated on the above conditions is cooled after 800 degreeC or more complete | finishes hot rolling. The cooling means is not particularly limited. Although it may be cooled by leaving it in the atmosphere, the properties of the steel material can be further improved by performing accelerated cooling from a temperature of 750 ° C. or higher to a temperature of 550 ° C. or lower.
When the rolling end temperature is lower than 800 ° C., in the steel of the present invention having a relatively small amount of C, ferrite is likely to undergo transformation transformation and the ferrite may be processed (rolled), which is not preferable in terms of ensuring low temperature toughness. For this reason, the rolling end temperature is limited to 800 ° C. or higher. Preferably it is 820 degreeC or more.
After hot rolling at 800 ° C. or higher, so-called 40 kg class steel (for example, JIS standard SM400, SN400 steel) with relatively low strength satisfies the specified strength even when cooled in the air. it can.
However, even with 50kg steel (for example, JIS standard SM490, SN490 steel) and 40kg steel, if the plate thickness is too thick, it will be difficult to ensure stable strength when left in the atmosphere. After the hot rolling is finished, it is desirable to accelerate cooling from a temperature of 750 ° C. or higher. The accelerated cooling after rolling is intended to enhance the properties of the steel material, and does not impair the excellent features of the present invention.
In the first place, accelerated cooling is intended to refine the structure by increasing the cooling speed in the transformation region and simultaneously improve strength and toughness. Therefore, it does not have any meaning unless it is started before the start of transformation or at least before the end of transformation. For this reason, the accelerated cooling start temperature is limited to 750 ° C. or higher. This accelerated cooling requires cooling to a temperature of 550 ° C. or lower in order to enjoy the effect. This is because when the temperature exceeds 550 ° C., the transformation during accelerated cooling does not proceed sufficiently, and the structure is not sufficiently refined. The preferred accelerated cooling start temperature is 760 ° C. or higher, and the preferred accelerated cooling stop temperature range is 520 or lower and 300 ° C. or higher.
The cooling rate during accelerated cooling depends on the steel composition, the intended strength, and the low temperature toughness level, but from the accelerated cooling start temperature to 550 ° C. at a position 1/4 of the plate thickness from the surface in the plate thickness direction. The average cooling rate is desirably 3 ° C./second or more.
Moreover, even if the tempering process below Ac1 temperature is added with respect to the rolled material after cooling, the outstanding characteristic of this invention is not impaired. It is rather preferable in order to cancel the non-uniformity of cooling and increase the in-plate uniformity of the material.

Steel sheets of various steel components (thickness 19 to 100 mm) were manufactured in the converter-continuous casting-thick plate process, and the materials were investigated.
Table 1 shows the steel components of the present invention steel together with the comparative steel, and Table 2 shows the production conditions and various properties of the steel sheet.
All the steel plates manufactured according to the present invention (present invention steel) have good characteristics. On the other hand, it can be seen that a steel sheet (comparative steel) manufactured without using the present invention is inferior in any of the characteristics.
Since the comparative steel 11 has a high C content, both the base material and the reproduced HAZ are inferior in low-temperature toughness compared to the steel of the present invention.
Since the comparative steel 12 is not added with Nb, and the comparative steel 13 has a low Cr content, the high temperature strength is low.
Since the comparative steel 14 has a low C content, the high temperature strength is low.
Since the comparative steel 15 has a high Cr content, both the base material and the reproduced HAZ are inferior in toughness.
Since comparative steel 16 has high Nb, it is inferior in HAZ toughness.
The components of the comparative steels 17-1 to 17-3 are the same as those of the present invention steel 5. However, the comparative steel 17-1 has a low rolling end temperature, and as a result, the accelerated cooling start temperature cannot be ensured and has become low, so both the room temperature and the high temperature strength are low. Since the comparative cooling steel 17-2 has a low accelerated cooling start temperature, both the room temperature and the high temperature strength are low. Since comparative steel 17-3 has a high accelerated cooling stop temperature, both normal temperature and high temperature strength are low.
Comparative Steel 18, the individual elements and production methods are within the present invention range, normal temperature, although such temperature strength and toughness satisfies the required properties as 490MPa class, because of the high P _CM, weldability (oblique Cracks occurred in the y-type weld crack test).

The present invention has made it possible to provide a large amount and a low cost of welded structural steel having excellent high temperature strength and low temperature toughness. As a result, it has become possible to reduce or omit the fireproof coating for building structures. In addition to the basic properties such as strength and toughness in applications other than construction, it also has high-temperature strength. Can be further improved.

Claims

Ingredient is% by mass
C: 0.003 to 0.05%
Si: 0.60% or less Mn: 0.6 to 2.0%
P: 0.020% or less S: 0.010% or less Cr: 0.20 to 1.5%
Nb: 0.005 to 0.05%
Al: 0.060% or less N: 0.001 to 0.006%,
Furthermore, Mo is limited to 0.03% or less as an impurity, and the balance consists of iron and inevitable impurities,
The P CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + steel weld crack susceptibility composition P CM value defined by 5B is less than 0.22%, was heated to a temperature of 1000 ~ 1300 ° C., A method for producing a steel for welded structure excellent in high-temperature strength and low-temperature toughness, characterized in that hot rolling is finished at a temperature of 800 ° C. or higher, followed by cooling.
The high temperature strength and low temperature toughness according to claim 1, wherein after the hot rolling is finished, accelerated cooling is started from a temperature of 750 ° C. or higher, and accelerated cooling is stopped at a temperature of 550 ° C. or lower. Excellent method for producing welded structural steel.
Furthermore, in mass%,
V: 0.01 to 0.10%
Ti: 0.005 to 0.025%
Any one or two of these are contained, The manufacturing method of the steel for welded structures which is excellent in the high temperature strength and low temperature toughness of Claim 1 or 2 characterized by the above-mentioned.
Furthermore, in mass% Ni: 0.05 to 0.50%
Cu: 0.05 to 0.50%
B: 0.0002 to 0.003%
Mg: 0.0002 to 0.005%
4. The method for producing a steel for welded structure having excellent high-temperature strength and low-temperature toughness according to any one of claims 1 to 3, wherein any one or more of these are contained.
Furthermore, in mass% Ca: 0.0005 to 0.004%
REM: 0.0005 to 0.008%
The method for producing a steel for welded structure having excellent high-temperature strength and low-temperature toughness according to any one of claims 1 to 4, wherein any one of the above is contained.
Ingredient is% by mass
C: 0.003 to 0.05%
Si: 0.60% or less Mn: 0.6 to 2.0%
P: 0.020% or less S: 0.010% or less Cr: 0.20 to 1.5%
Nb: 0.005 to 0.05%
Al: 0.060% or less N: 0.001 to 0.006%,
Furthermore, Mo is limited to 0.03% or less as an impurity, and the balance consists of iron and inevitable impurities,
The P CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + steel weld crack susceptibility composition P CM value defined by 5B is less than 0.22%, was heated to a temperature of 1000 ~ 1300 ° C., A welded structural steel excellent in high temperature strength and low temperature toughness, characterized by being obtained by finishing hot rolling at a temperature of 800 ° C. or higher and then cooling.