JP5098317B2 - Manufacturing method of welded structural steel with excellent high temperature strength and low temperature toughness - Google Patents

Description

  The present invention is mainly intended for building structure refractory steel for the purpose of maintaining proof stress at high temperatures such as fires, but is not limited to architectural applications, and is widely used for marine structures, ships, bridges, various storage tanks, etc. The present invention relates to a method for manufacturing high-strength steel for welded structures that can be used for various purposes. In addition, the strength level which the welded structural steel according to the present invention is mainly targeted is a class generally called 40-50 kilo steel having a yield strength of 235 to 475 MPa and a tensile strength of 400 to 640 MPa.

Many techniques have been disclosed for so-called refractory steel in architectural applications aimed at ensuring high temperature proof stress. For example, in mass%, C: 0.04 to 0.15%, Si: 0.6% or less, Mn: 0.5 to 1.6%, Nb: 0.005 to 0.04%, Mo: 0 .4 to 0.7%, Al: 0.1% or less, N: 0.001 to 0.006%, the steel slab composed of Fe and inevitable impurities as the balance is heated in the temperature range of 1100 to 1300 ° C., and then heated. Invention of the manufacturing method of the low yield ratio steel material for construction excellent in fire resistance which complete | finishes hot rolling in the temperature range of 800-1000 degreeC (for example, refer patent document 1), C: 0.05-0. 15%, Si: 0.6% or less, Mn: 0.8 to 1.6%, P: 0.03% or less, S: 0.005% or less, Mo: 0.35 to 0.80%, Ti : Hot-rolled steel slab containing 0.005 to 0.025%, Al: 0.06% or less, N: 0.006% or less, and then Ac ₃ transformation point to 100 Invention of a method for producing a steel excellent in high temperature strength and low temperature toughness with a thickness of 50 mm or more by reheating and quenching in a temperature range of 0 ° C. and tempering in a temperature range of 450 ° C. to Ac ₁ transformation point (See, for example, Patent Document 2). 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 JIS or the like decreases from about 350 ° C., the allowable temperature is about 500 ° C. In other words, when using the above steel materials 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 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.

  Therefore, in the manufacture of refractory steel, it is premised that it is commensurate with general steel + refractory coating and its construction cost. However, Mo, which is generally added for the purpose of maintaining high-temperature proof stress, has a large change in market conditions, and depending on the amount of addition, there may be a situation that does not match the fireproof coating cost. 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 JP-A-5-339632

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 base by limiting the specific range along with the weld crack susceptibility composition P _CM steel components, industrially stable, yet producing method of welding structural steel excellent in possible high temperature strength and low temperature toughness provided at low cost The issue is to provide.

  The point of the present invention is that it is extremely effective in securing and maintaining high-temperature proof stress. In order to stably secure high-temperature proof strength without adding Mo, which is usually used, transformation structure teaching by relatively high combined addition of Cr and Nb And a precipitate (carbonitride) of Cr or Nb. The high-temperature proof strength steel that does not contain Mo is extremely innovative in itself, and at the same time, by not containing Mo with high hardenability, of course, the basic performance (strength, toughness) as a welded structural steel, It also leads to improved weldability and gas cutting performance.

The present invention, Cr, not Nb only, 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 Of course, the high temperature strength and the low temperature toughness are both achieved, and the gist thereof is as follows.

(1) The component is mass%,
C: 0.005-0.05%,
Si: 0.60% or less,
Mn: 0.8 to 2.0%,
P: 0.020% or less,
S: 0.010% or less,
Cr: more than 0.50 to 3.0%,
Nb: 0.05 to 0.50% and Nb ≧ 2 C,
Al: 0.060% or less,
N: 0.001 to 0.006%
Containing the balance being iron and unavoidable _{impurities, P CM = C + Si /} 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B and _{P CM} value defining comprises 0.24% or less slab or steel The piece is heated to a temperature of 1000 to 1300 ° C, the cumulative reduction in the austenite non-recrystallization temperature region is set to 30% or more, and after hot rolling is finished at a temperature of 750 ° C or more, it is allowed to cool. A method for producing welded structural steel with excellent high temperature strength and low temperature toughness.

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

(3) 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%
The method for producing a steel for welded structure having excellent high temperature strength and low temperature toughness according to the above (1) or (2), comprising 1 type or 2 types or more in the range described above.

(4) 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 the above (1) to (3), comprising 1 type or 2 types within the range of.

  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. Became possible. As a result, it has become possible to reduce or omit the fireproof coating for building structures. In addition to building construction, it has basic performances such as strength and toughness, and as a steel for welded structures that may be exposed to high temperatures, the safety of structures can be further enhanced. .

The present invention will be described in detail below.
First, the reason why the present invention limited the steel components as described in the claims will be described.

  C is limited to a very low level as a high-strength steel, and is one of the features of the present invention. This is closely related to the manufacturing method together with other components described later. Among steel components, C has the greatest influence on the properties of the steel material, and the lower limit of 0.005% is the minimum amount for ensuring that the heat-affected zone such as securing the strength and welding is softened more than necessary. . However, if the amount of C is too large, the hardenability is increased more than necessary, and the upper limit is set to 0.05% because it adversely affects the strength, toughness balance, weldability, etc. that the steel material should originally have.

  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 0.60%. Deoxidation of steel can be sufficiently performed only with Ti and Al, and is preferably as low as possible from the viewpoints of HAZ toughness, hardenability, and the like, and it is not always necessary to add them.

  Mn is an element indispensable for securing the strength and toughness at room temperature, and its lower limit is 0.8%. 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%.

  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 HAZ, so it is preferable that it is as small as possible. 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%.

  Cr is one of the most important elements in the present invention. In the steel of the present invention which does not substantially contain Mo, a relatively large amount of Cr needs to be added to ensure high temperature strength. This is because the transformation temperature is lowered due to the effect of improving the hardenability of Cr to increase the normal temperature and high temperature strength, and the solid solution effect of Cr and Cr carbide are used at high temperatures. In order to enjoy these effects, at least 0.50% is necessary. However, if the addition amount is too large, the base material, the toughness of the welded portion and the weldability are deteriorated, and the economic efficiency is also lost, so the upper limit was made 3.0%.

  Nb, like Cr, is one of the important elements in the present invention. This is because, in the steel of the present invention that does not substantially contain Mo, solid solution Nb and Nb precipitates (carbonitrides) are used to ensure high temperature proof stress. In order to increase the normal temperature strength by the precipitation effect of Nb precipitates, a relatively small amount may be used. However, in order to ensure high temperature proof stress, it is 0.05% or more and at least twice the amount of C. The above Nb (Nb ≧ 2C) is necessary, and in order to secure solid solution Nb stably, it is preferable to contain 5 times or more the amount of C. The upper limit is not necessarily determined, but it is 0.50% in the present invention as a range that does not cause a significant deterioration in toughness of the welded portion by experiments of the present inventors. 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 point.

  Al is an element generally 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 Al increases, not only the cleanliness of steel deteriorates but also the toughness of the western metal deteriorates, so the upper limit 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 enjoy the effect of V addition. only was 0.20% taking into account the impact of the P _CM, which will be described later and it is the complementary role of Nb is.

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 Al (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 the formation of intragranular transformed ferrite and influence of welding heat. 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%.

  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.

  If Ni is not added excessively, the strength and toughness of the base material are improved without adversely affecting the weldability and weld heat affected zone toughness. In order to exert these effects, it is necessary to add at least 0.05% or more. 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%.

  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%.

  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, a reduction in 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 reducing the size of the particles, 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.

  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, either one 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 steel to limit the value of _{P CM} below 0.24%. 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 0.25% or less, the limit of the present invention is obtained by intended to more clearly the features of the present invention. The lower limit is not particularly limited, but is naturally limited by the limited range of each component.

  In order to obtain a high-strength steel for welded structures that achieves 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 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 becomes difficult to secure the rolling end temperature, which will be described later, and the non-recrystallization temperature of austenite is raised to maximize the effect of controlled rolling during hot rolling. The lower limit was limited to 1000 ° C. from the viewpoint of solutionization of Nb for exhibiting the above-described effects and for causing precipitation effects.

  The slab or steel slab heated under the above conditions is allowed to cool after the hot rolling at 750 ° C. or higher is completed at a cumulative reduction 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 750 ° 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 750 degreeC or more.

  Although it was allowed to cool after completion of the hot rolling, if it is a component of the present invention, it is allowed to cool, generally 235 to 475 MPa in yield strength and 400 to 640 MPa in tensile strength, so-called generally 40-50 kg class steel. This is because the following strength can be obtained. Therefore, although accelerated cooling after rolling is not necessarily denied, the cooling is clearly superior from the viewpoint of ease of manufacturing such as cooling uniformity, and the present invention is limited to the cooling.

The present invention will be specifically described below based on examples.
Steel sheets (thickness 19 to 80 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 sheets (invention steels) produced according to the method of the present invention have good characteristics. On the other hand, the comparative steel not according to the present invention is inferior in any of the characteristics.

  That is, the comparative steel 11 is inferior in the base metal toughness because of the high C content. Since the absolute value of Nb amount is low, the comparative steel 12 is inferior in high temperature strength. Although each component of the comparative steel 13 is within the limited range of the present invention, the high temperature strength is inferior because the Nb amount is lower than the C amount. Since the comparative steel 14 has a low C content, both the room 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, the comparative steel 15-1 is inferior in toughness because the cumulative reduction amount in the γ non-recrystallization temperature region is small, and the comparative steel 15-2 is inferior in toughness because the rolling end temperature is low.

Since P _CM is lower in the present invention steels as well comparative steels, weldability (oblique y-groove weld cracking test: JIS Z 3158) are both good, clear difference was observed.

Claims (4)

Ingredient is% by mass
C: 0.005-0.05%,
Si: 0.60% or less,
Mn: 0.8 to 2.0%,
P: 0.020% or less,
S: 0.010% or less,
Cr: more than 0.50 to 3.0%,
Nb: 0.05 to 0.50% and Nb ≧ 2 C,
Al: 0.060% or less,
N: 0.001 to 0.006%
Containing the balance being iron and unavoidable _{impurities, P CM = C + Si /} 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B and _{P CM} value defining comprises 0.24% or less slab or steel The piece is heated to a temperature of 1000 to 1300 ° C, the cumulative reduction in the austenite non-recrystallization temperature region is set to 30% or more, and after hot rolling is finished at a temperature of 750 ° C or more, it is allowed to cool. A method for producing welded structural steel with excellent high temperature strength and low temperature toughness.
In addition to the above ingredients,
V: 0.01-0.20%,
Ti: 0.005-0.025%
The method for producing a steel for welded structure having excellent high temperature strength and low temperature toughness according to claim 1, comprising 1 type or 2 types within the range of 1 above. 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%
The method for producing a steel for welded structure having excellent high-temperature strength and low-temperature toughness according to claim 1, wherein the steel contains 1 type or 2 types or more. Furthermore, in mass%,
Ca: 0.0005 to 0.004%,
REM: 0.0005 to 0.008%
The method for producing a welded structural steel excellent in high-temperature strength and low-temperature toughness according to any one of claims 1 to 3, wherein the material contains 1 type or 2 types.