JP2007277680A - Method for producing steel for welded structure excellent in high temperature strength and low temperature toughness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a steel for welded structure excellent in high temperature strength and low temperature toughness by which the steel for welded structure excellent in the high temperature strength and also in the low temperature toughness being one of the basic performance of steel can be supplied inexpensively without adding Mo having large fluctuation in a market condition. <P>SOLUTION: A cast slab or a steel slab comprising, by mass%, 0.003-0.05% C, ≤0.40% Si, 0.5-2.0% Mn, ≤0.020% P, ≤0.010% S, 0.05-0.50% Nb, the Nb being ≥3C, ≤0.060% Al, 0.001-0.006% N and the balance Fe with inevitable impurities, wherein a P<SB>CM</SB>value is 0.22 or below , is heated to 1,000-1,300°C, and after finishing hot-rolling at ≥850°C as ≥30% accumulated rolling-reduction ratio in austenite non-recrystallization temperature zone, an air-cooling is performed. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention is mainly intended for a manufacturing method of fireproof steel for building structures intended to maintain the yield strength at high temperatures such as fires, but is not limited to building applications, but also marine structures, ships, bridges, various storage tanks It can be applied to the manufacturing method of steel for welded structure that has excellent high temperature strength and low temperature toughness for a wide range of uses.

  Many techniques have been disclosed for so-called refractory steel in architectural applications aimed at ensuring high-temperature proof stress (see, for example, Patent Document 1). 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, which is standardized by JIS and the like, decreases from about 350 ° C., its allowable temperature is about 500 ° C. In other words, when the steel materials described above are used for buildings such as buildings, offices, residences, and multistory parking lots, it is obliged to provide sufficient fireproof coating to ensure safety in the event of a fire. Related laws and regulations stipulate that the temperature of steel materials does 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 general steel materials for buildings, it is necessary to apply fireproof coating so that the temperature of the steel materials does not reach 350 ° C in the event of a fire.

  Therefore, in the production of refractory steel, it is assumed that it is appropriate for the general steel + refractory coating and its construction cost. However, for the purpose of maintaining high-temperature proof stress, Mo added generally changes greatly in market conditions, and depending on the amount of addition, there may be situations where it does not match the cost of refractory coating. For this reason, the development and commercialization of inexpensive high-temperature strength-guaranteed steel not containing Mo has been awaited.

Japanese Patent Laid-Open No. 2-77523

In order to obtain a welded structural steel having excellent low temperature toughness, which is one of the basic performances of steel materials, without adding Mo, which has a large market fluctuation, the present invention has a relatively low C and a relatively high Nb. the steel compositions were limited to a specific range with weld crack susceptibility composition P _CM to the base over scan, further by limiting the manufacturing method, industrially stable, yet the available supply high-temperature strength and low temperature toughness at a low cost The present invention provides an excellent method for producing welded steel.

The point of the present invention is that Nb precipitates (carbonitrides) by relatively high Nb addition in order to stably secure high-temperature proof stress without adding Mo that is usually used because it is extremely effective for securing and maintaining high-temperature proof stress. ). High-temperature proof strength steel that does not contain Mo is extremely innovative in itself. At the same time, it does not contain Mo, which has high hardenability, so that the basic performance (strength and toughness) of welded steel is not to mention. This also leads to an improvement in gas and gas cutting properties. The present invention, Nb, not only Mo, C, Si, limiting the individual amounts of alloying elements and P _CM, including Mn, by further limiting the production conditions, various use performance as welding structural steel Needless to say, the high temperature strength and the low temperature toughness are both achieved. The gist of the invention is as follows.

(1) The component is mass%,
C: 0.003 to 0.05%,
Si: 0.40% or less,
Mn: 0.5 to 2.0%
P: 0.020% or less,
S: 0.010% or less,
Nb: 0.05 to 0.50% and Nb ≧ 3C,
Al: 0.060% or less,
N: 0.001 to 0.006%,
Furthermore, it is 0.03% or less of the extent that Mo is contained as contamination, substantially does not contain Mo, and the balance is composed of iron and inevitable impurities. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + a V / 10 + 5B and _{P CM} value defined consisting of 0.22% slab or steel strip is heated to a temperature of 1000 to 1300 ° C., the cumulative reduction ratio of austenite non-recrystallization temperature region 30% or more As a method for producing a steel for welded structure excellent in high-temperature strength and low-temperature toughness, characterized in that after hot rolling is completed at a temperature of 850 ° C. or higher, it is allowed to cool.

(2) In addition to the above steel components,
V: 0.01-0.20%,
Ti: 0.005 to 0.025%,
The method for producing high-strength steel for welded structures having excellent high-temperature strength and low-temperature toughness as described in (1) above, which contains 1 type or 2 types within the range described above.

(3) Further, Ni by mass: 0.05 to 0.50%,
Cu: 0.05 to 0.50%,
Cr: 0.05 to 0.50%,
B: 0.0002 to 0.003%,
Mg: 0.0002 to 0.005%,
The method for producing high-strength steel for welded structures having excellent high-temperature strength and low-temperature toughness as described in (1) or (2) above, comprising 1 type or 2 types or more in a range of

(4) Furthermore, Ca: 0.0005 to 0.004% by mass%,
REM: 0.0005 to 0.008%,
The method for producing a high-strength steel for welded structures having excellent high-temperature strength and low-temperature toughness according to any one of the above (1) to (3), characterized by containing at least one of the following.

  According to the present invention, a steel for welded structure excellent in high temperature strength and low temperature toughness can be provided in a large amount and at low cost. 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, so it can be used as a steel for welded structures that may be exposed to high temperatures. Can be further improved.

  The present invention will be described in detail below.

  First, the reason why the steel components are limited as described in the claims will be described. In addition, content of a component means the mass%.

  C is limited to a very low level as a general welded structural steel and is one of the features of the present invention. This is closely related to other components described later. Among steel components, C has the greatest influence on the properties of steel materials, and the lower limit of 0.003% is the minimum amount to ensure that the heat-affected zone such as securing strength and welding is not softened more than necessary. is there. However, if the amount of C is too large, the hardenability is unnecessarily increased, which adversely affects the strength, balance of toughness, weldability, etc. that the steel material should have, and further, Nb for securing high-temperature strength described later. In the use of the precipitate and the solid solution Nb, the upper limit was set to 0.05% in view of the quantitative balance of C and Nb.

  Si is an element contained in the deoxidized upper steel, but if added in a large amount, weldability and HAZ toughness deteriorate, so the upper limit was limited to 0.40%. Deoxidation of steel is sufficiently possible only with Ti and A1, and is preferably as low as possible from the viewpoints of HAZ toughness, hardenability, etc., and it is not always necessary to add them.

  Mn is an indispensable element for securing the strength and toughness at room temperature, and its lower limit is 0.5%. However, if the amount of Mn is too large, not only the hardenability is increased and the weldability and toughness of the welded portion are deteriorated, but also the center segregation of the continuously cast slab is promoted, so the upper limit was made 2.0%.

  P is an impurity in the steel of the present invention, and a reduction in the amount of P tends to reduce the grain boundary fracture in the welded portion, so the smaller the better. If the content is large, the low temperature toughness of the base metal and the welded portion is deteriorated, so the upper limit was made 0.020%.

  S, like P, is an impurity in the steel of the present invention, and is preferably as small as possible from the viewpoint of the 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 was made 0.010%.

  Nb is one of the most important elements in the present invention. This is because the Mo-free steel according to the present invention uses solute Nb at the same time as Nb precipitates (carbonitrides) in order to secure high-temperature proof stress. In order to increase the normal temperature strength by precipitation hardening with Nb precipitates, a relatively small amount is sufficient, but in order to ensure the yield strength at high temperature, it is 0.05% or more and at least three times the amount of C. It is necessary to contain the above Nb. In particular, when the amount of Nb exceeds 5 times the amount of C, the ratio of high temperature strength to normal temperature strength is improved, and the effect of Nb addition to high temperature strength is further improved. The upper limit is not necessarily determined. However, according to the present inventors' experiment, in the steel of the present invention with a relatively small amount of C, Nb is used in the present invention as a range that does not cause significant toughness deterioration of the weld. The upper limit of 0.50%. 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.

  A1 is generally an element contained in deoxidized upper steel, but Si or Ti is sufficient for deoxidation, and the lower limit is not limited in the steel of the present invention. However, when the amount of A1 increases, not only the cleanliness of steel deteriorates but also the toughness of the weld metal deteriorates, so the upper limit was made 0.060%.

  N is contained in the steel as an unavoidable impurity, but combines with Nb to form carbonitride to increase the strength, and TiN is formed to enhance the properties of the steel as described above. For this reason, the N amount is required to be at least 0.001%. 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%.

  Next, the reason for the addition of V and Ti that can be contained as required will be described.

V has substantially the same effect as Nb, and the role of V in the present invention complements Nb. However, V is less effective than Nb and affects hardenability, so the upper and lower limits are limited. However, the lower limit is 0.01% as the minimum amount that can surely enjoy the effect of V addition, the upper limit is only was it and taking into account the 0.20% impact on the P _CM, which will be described later is the complementary role of Nb.

Ti is essential for improving the toughness of the base metal and the weld heat affected zone. This is because when Ti has a small amount of A1 (for example, 0.003% or less), it combines with O to form a precipitate mainly composed of Ti ₂ O ₃ , which becomes the nucleus of intragranular transformation ferrite formation, and is a weld heat affected zone. Improve toughness. Ti is combined with N and finely precipitated in the slab as TiN, which suppresses the coarsening of γ grains during heating and is effective for refining the rolled structure. The fine TiN present in the steel sheet is welded. This is to sometimes refine the weld heat affected zone structure. In order to obtain these effects, Ti needs to be at least 0.005%. However, if it is too much, TiC is formed and the low temperature toughness and weldability are deteriorated, so the upper limit is 0.025%. The fact that Mo is not substantially added in the present invention means that Mo is not intentionally added, but when it is inevitably contained from the manufacturing process, it is acceptable within a range of 0.03% or less. .

Next, the reason for adding Ni, Cu, Cr, 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.

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 unfavorable 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 present inventors, addition up to 1.0% does not significantly deteriorate the weldability and SCC in liquid ammonia, and the effect of improving the strength and toughness is greater. The upper limit was 0.5%.

  Cu exhibits substantially the same effects and phenomena as Ni, and the upper limit of 0.50% is restricted because weldability deteriorates, and excessive addition causes Cu-cracks during hot-rolling rolling, which makes it difficult to manufacture. The lower limit should be the minimum amount for obtaining a substantial effect and is 0.05%.

  Cr improves both the strength and toughness of the base material. In order to enjoy these effects, at least 0.05% is necessary when added. 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 0.50%.

  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 was made 0.003%. 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), and in such a case, B addition for increasing the hardenability is not preferable.

  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, as the amount added increases, the effect on the amount added decreases, so this is not a cost-effective measure, so the upper limit was made 0.005%.

  Next, the reason for adding Ca or REM according to claim 4 will be described.

  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. Since Ca and REM have substantially the same effect, at least one of them may be added in the above range, or both may be added.

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 value of _{P CM} below 0.20%. PCM is an index representing weldability. The lower the _{CM, the} better the weldability. - in general, P _CM is possible to secure excellent weldability if 0.5% or less, the limit in the present invention has contemplated to more clearly the features of the present invention, It is an epoch-making low for welded structural steel. The lower limit is not particularly limited, but is naturally limited by the limited range of each component.

  In order to obtain a welded structural steel having both excellent high-temperature strength and low-temperature toughness in the limited steel components, it is necessary to limit the production conditions as in the present invention. The reason will be described below.

  The reason why the heating temperature prior to rolling is limited to 1000 to 1300 ° C. is to keep the austenite grains during heating small and to refine the rolling 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 will be difficult to secure the end temperature of rolling, which will be described later, and the final recrystallization temperature of austenite will be raised to maximize the effect of controlled rolling during hot rolling. The lower limit was limited to 1000 ° C. from the viewpoint of solution of Nb for exhibiting to the limit or for causing precipitation hardening.

  The slab or steel slab heated under the above conditions is allowed to cool after the hot rolling at 850 ° C. or higher is completed at a cumulative reduction amount in the austenite non-recrystallization temperature range of 30% or higher. By rolling in the austenite non-recrystallization temperature range, the austenite grains are remarkably refined, so that a cumulative reduction amount of at least 30% or more is required. If the rolling end temperature is lower than 850 ° C., ferrite may be transformed and precipitated, and the ferrite may be processed (rolled), which is not preferable in terms of securing low temperature toughness. For this reason, rolling end temperature is limited to 850 degreeC or more.

  After hot rolling at 850 ° C. or higher, it is limited to cooling. This is because, in the steel of the present invention with a relatively small amount of C, accelerated cooling that accelerates the cooling speed in the transformation region is less effective as transformation enhancement, while productivity decreases, steel plate shape defects due to accelerated cooling, Since there are negative aspects such as residual stress, it is limited to cooling. Therefore, although accelerated cooling after rolling does not distinguish from the present invention, it is limited to cooling after rolling in order to clarify the characteristics of the present invention.

  Hereinafter, the present invention will be described in detail based on examples.

  Steel sheets (thickness 12 to 100 mm) of various steel components were manufactured in the converter-continuous casting-thick plate process, and the materials were investigated.

  Table 1 shows the steel components of the steel of the present invention together with the comparative steel, and Table 2 shows the manufacturing conditions and various properties of the steel sheet.

  All the steel plates manufactured according to the method of the present invention (present invention steel) have good characteristics. On the other hand, the comparative steel not according to the present invention is inferior in any of the characteristics.

  Since the comparative steel 11 has a high C content, it is inferior in low-temperature toughness compared to the steel of the present invention. Since the absolute value of Nb amount is low, the comparative steel 12 is inferior in high temperature strength. The comparative steel 13 is inferior in high-temperature strength because the Nb content is lower than the C content, although the individual components are within the limited range of the present invention. Since the comparative steel 14 has a low C content, both the normal temperature and the high temperature strength are low. The comparative steel 15 is the same as the steel 5 of the present invention in terms of composition. However, comparative steel 15-1 is not subjected to accelerated cooling after rolling, and is low in both normal temperature and high temperature strength. The comparative steel 15-2 has a low rolling end temperature, and as a result, the accelerated cooling start temperature cannot be ensured and has been lowered. Therefore, both the normal temperature and the high temperature strength are low. Since comparative steel 15-3 has a low accelerated cooling start temperature, both the normal temperature and the high temperature strength are low. Since comparative steel 15-4 has a high accelerated cooling stop temperature, both normal temperature and high temperature strength are low.

Since the present invention steels P _CM is lower in course comparative steels, both weldability (oblique y-groove weld cracking test) is good, clear difference was observed.

Claims (4)

Ingredient is% by mass
C: 0.003 to 0.05%,
Si: 0.40% or less,
Mn: 0.5 to 2.0%
P: 0.020% or less,
S: 0.010% or less,
Nb: 0.05 to 0.50% and Nb ≧ 3C,
Al: 0.060% or less,
N: 0.001 to 0.006%,
Furthermore, it is 0.03% or less of the extent that Mo is contained as contamination, substantially does not contain Mo, and the balance is composed of iron and inevitable impurities. P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + a V / 10 + 5B and _{P CM} value defined consisting of 0.22% slab or steel strip is heated to a temperature of 1000 to 1300 ° C., the cumulative reduction ratio of austenite non-recrystallization temperature region 30% or more As a method for producing a steel for welded structure excellent in high-temperature strength and low-temperature toughness, characterized in that after hot rolling is completed at a temperature of 850 ° C. or higher, it is allowed to cool. In addition to the above steel components,
V: 0.01-0.20%,
Ti: 0.005-0.025%
The method for producing a high-strength steel for welded structures having excellent high-temperature strength and low-temperature toughness according to claim 1, comprising 1 type or 2 types in a range of Further, Ni by mass: 0.05 to 0.50%,
Cu: 0.05 to 0.50%,
Cr: 0.05 to 0.50%,
B: 0.0002 to 0.003%,
Mg: 0.0002 to 0.005%,
The method for producing a high-strength steel for welded structures having excellent high-temperature strength and low-temperature toughness according to claim 1 or 2, wherein one or more of them are contained in the range. Furthermore, Ca: 0.0005 to 0.004% in mass%,
REM: 0.0005 to 0.008%
The method for producing a high-strength steel for welded structures having excellent high-temperature strength and low-temperature toughness according to any one of claims 1 to 3, wherein at least one of the above is contained.