JP5494090B2 - Refractory steel material excellent in reheat embrittlement resistance and low temperature toughness and method for producing the same - Google Patents

Description

  The present invention is used in steel structures, particularly construction structures, and has a high yield strength at 600 ° C. when exposed to fire, and at the same time, SR (Stress Relief) crack resistance of the heat affected zone. The present invention relates to a refractory steel material excellent in (reheat embrittlement resistance) and excellent in low-temperature toughness and a method for producing the same.

  In recent years, steel materials used for building structures have been required to have so-called “fireproof steel” characteristics (fireproof performance) that are excellent in high-temperature strength. This is a property decided by the Ministry of Land, Infrastructure, Transport and Tourism based on the “New Fire Resistance Design Law”, which uses steel without fireproof coating in consideration of environmental issues, etc., and is based on Ministry of Land, Infrastructure, Transport and Tourism Notification No. 333 (2004) It is based on performance.

  Fire resistance performance means that when a steel material is exposed to a fire or the like in a state without a coating, the steel material continues to exhibit the strength required for a certain period of time. By improving the fire resistance of the steel material, it is possible to prevent the collapse of a building structure or the like. In particular, when a fireproof coating is not provided on the steel material, various fire scales, environmental temperatures, and the like are assumed. Therefore, the steel material that supports the strength of the structure is required to have as high a high temperature strength as possible.

  Conventionally, alloys such as Mo, Cr, Nb, and Cu that contribute to high-temperature strength and refractory steel added with B have been proposed (for example, see Patent Documents 1 to 5). However, when a large amount of alloy element is added, when a steel material is welded, there is a problem that when a heat affected zone (referred to as Heat Affected Zone, HAZ) is exposed to a high temperature, it becomes extremely brittle. HAZ embrittlement at high temperatures is a phenomenon that impairs ductility during high temperature deformation and is called reheat embrittlement.

  Moreover, in order to apply steel material to a building structure, the low temperature toughness of a base material and HAZ is also required. In particular, in recent years, the use of thick steel materials and the application of high heat input welding are increasing in order to increase the size and height of buildings and improve welding efficiency. Therefore, even when the steel material becomes thick and the welding heat input becomes high, it is necessary to increase the low temperature toughness of the base material and the HAZ in order to obtain sufficient earthquake resistance.

Japanese Patent Laid-Open No. 08-143156 JP 09-137218 A JP-A-11-279683 JP 2007-051321 A JP 2007-191746 A

  In order to obtain high temperature strength, conventional refractory steel materials have added Mo, Cu, Cr, Nb, etc., and the alloy cost is high. Moreover, when these elements are added excessively, the deformation resistance of hot rolling is increased, the manufacturability is impaired, and the low temperature toughness of the base material and HAZ may be lowered due to precipitates. Furthermore, Mo, Cr, and Nb are elements that form carbides, and Cu is an element that precipitates in the steel by heating. Therefore, when the amount of addition increases, the time of exposure to high temperatures increases. , Ductility may decrease.

  The present invention has been made in view of such circumstances, and limits the content of alloying elements that increase the high-temperature strength, and has a sufficient fire resistance and low temperature toughness, that is, reheat brittleness. The present invention provides a refractory steel material excellent in heat resistance and low-temperature toughness and a method for producing the same. In the present invention, the high-temperature strength is evaluated by the yield strength at 600 ° C., the reheat embrittlement resistance by the HAZ drawn value held at 600 ° C. for 1 hour, and the low-temperature toughness by the Charpy absorbed energy at 0 ° C.

  The present inventors maintain manufacturability and low temperature toughness without adding a large amount of elements conventionally used for improving high temperature strength such as Mo, Cu, Cr, and Nb, and effectively increase high temperature strength. Considered to increase. As a result, the combined addition of Mo and B is effective, and in order to suppress reheat embrittlement when held at 600 ° C. for 1 hour while ensuring high temperature strength, the contents of Mo and B It was found that the ratio Mo / B must be limited. This invention is made | formed based on such knowledge, The summary is as follows.

(1) In mass%,
C: 0.002% or more, 0.050% or less,
Si: 0.01% or more, 0.50% or less,
Mn: 0.50% or more, 2.00% or less,
Mo: 0.05% or more and less than 0.20%,
B: 0.0003% or more, 0.0020% or less,
N: 0.0010% or more, 0.0100% or less,
Ti: 0.005% or more, 0.030% or less,
Al: 0.002% or more and 0.100% or less, and further each content of P, S, O,
P: less than 0.0200%,
S: less than 0.0100%,
O 2: Reheat embrittlement resistance, characterized by being limited to less than 0.0100%, the balance being Fe and inevitable impurities, and the content [% by mass] of Mo and B satisfying the following (formula 1) Refractory steel with excellent heat resistance and low temperature toughness.
Mo / B ≦ 286 (Formula 1)

(2) Furthermore, in mass%,
Cu: 0.30% or less,
Ni: 0.30% or less,
Nb: less than 0.020%,
The content of Mo, Cu, Ni, Nb, and B [mass%] satisfying the following (formula 2): Refractory steel with excellent reheat embrittlement and low temperature toughness.
(Mo + 1.6Cu + 1.3Ni + 8.5Nb) / B ≦ 286 (Formula 2)
(3) Si content is
Si: 0.040% or more, 0.50% or less
The fire-resistant steel material excellent in reheat embrittlement resistance and low-temperature toughness as described in (1) or (2) above.

( 4 ) Furthermore, in mass%,
V: 0.20% or less,
Zr: 0.10% or less,
Cr: 0.20% or less,
W: Excellent in reheat embrittlement resistance and low-temperature toughness according to any one of (1) to (3) above, characterized by containing one or more of 0.30% or less Refractory steel.

( 5 ) Furthermore, in mass%,
Mg: 0.0005 to 0.0050%,
Ca: 0.0005 to 0.0050%,
Y: 0.001 to 0.050%,
La: 0.001 to 0.050%,
Ce: 0.001 to 0.050%
The fireproof steel material having excellent reheat embrittlement resistance and low temperature toughness according to any one of the above (1) to ( 4 ), characterized by containing one or more of the above.

(6) Furthermore, Ceq calculated | required by the following (Formula 3) is 0.20-0.40, and Pcm calculated | required by the following (Formula 4) is 0.05-0.20. The fire resistant steel material excellent in reheat embrittlement resistance and low temperature toughness according to any one of (1) to ( 5 ).
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (Formula 3)
Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15
+ V / 10 + 5B (Formula 4)
Here, C, Si, Mn, Ni, Cu, Cr, Mo, V, and B are the content [% by mass] of each element.

(7) The steel slab having the steel component described in any one of (1) to (6 ) above is heated to a temperature of 1100 ° C. or higher and 1300 ° C. or lower, and hot rolling is performed at a finishing temperature of 800 ° C. or higher. A method for producing a refractory steel material excellent in reheat embrittlement resistance and low temperature toughness, characterized in that the method is performed and then allowed to cool.

( 8 ) Further, it is excellent in reheat embrittlement resistance and low temperature toughness as described in ( 7 ) above, characterized by performing tempering heat treatment in a temperature range of 400 ° C. or more and less than 650 ° C. for 5 minutes or more and 360 minutes or less. A method for producing refractory steel.

  According to the present invention, there is provided a fire resistant steel material excellent in fire resistance, particularly having a yield strength at 600 ° C. that is 2/3 or more of the yield strength at room temperature, excellent in reheat embrittlement resistance and low temperature toughness, and a method for producing the same. In addition, the industrial contribution is extremely significant, such as being able to provide a refractory steel material having excellent productivity and suppressing deformation resistance during hot rolling.

It is a figure which shows the relationship between ratio of content of Mo and B, and reheat embrittlement resistance. It is a figure which shows the relationship between the relational expression of content of Mo, Cu, Ni, Nb, and B and reheat embrittlement resistance.

  The present inventors first examined a method for increasing the high-temperature strength and ensuring reheat embrittlement resistance without adding alloy elements such as Cu, Ni, Nb, V, Cr, and W. The high temperature strength of a steel material is improved by solid solution strengthening, precipitation strengthening, and an increase in dislocation density, similar to the strengthening mechanism at room temperature. C is an element that is extremely effective for improving the strength. However, in a refractory steel material, if a large amount of carbide precipitates, the low temperature toughness of the base material and HAZ is impaired, and reheat embrittlement becomes significant. Therefore, a large amount of C is added. I can't. According to the study by the present inventors, it was found that the upper limit of the C amount needs to be 0.05% by mass or less in order to prevent reheat embrittlement.

  Mo is known as an element that contributes to solid solution strengthening, but precipitation strengthening by Mo carbide is used to improve high-temperature strength. On the other hand, when Mo is added excessively, the reheat embrittlement resistance is lowered, and therefore, it was found that the upper limit of the appropriate amount of Mo is less than 0.20% by mass. Furthermore, the present inventors paid attention to B in order to obtain high temperature strength while suppressing precipitation of Mo carbides. That is, if B and Mo are added at the same time, a structure having a high dislocation density such as bainite and acicular ferrite can be generated without performing accelerated cooling after hot rolling, and high temperature strength can be secured. Thought.

  It was found that when B and Mo were added simultaneously, the hardenability was remarkably improved, and if B was added, high temperature strength could be ensured even if the amount of Mo added was suppressed. Therefore, the addition of B is effective to ensure the high temperature strength of the refractory steel material, while the Mo amount must be suppressed to ensure the reheat embrittlement characteristics of the refractory steel material. Therefore, the present inventors consider that the ratio Mo / B of Mo and B content [% by mass] needs to be in an appropriate range, and examine the relationship between Mo / B and reheat embrittlement resistance. Went.

  FIG. 1 is a diagram showing the relationship between Mo / B and reheat embrittlement resistance. The vertical axis in FIG. 1 is a drawing value when a test piece is taken from a steel material in which the contents of Mo and B are changed, a reproducible HAZ thermal cycle is given, and an SR drawing test is performed. The reproducible HAZ thermal cycle specifically assumes a welding heat input of 2 kJ / mm, sets the heating rate to 100 ° C./s, heats it to 1400 ° C., holds it for 2 s, then from 800 ° C. to 500 ° C. It is the heat processing which cools the temperature range of 16 seconds in 16s. In the SR drawing test, a test piece subjected to a reproducible HAZ thermal cycle is immediately heated from room temperature to 600 ° C., which is the expected fire temperature, at 1 ° C./s, held for 1 hour, and then subjected to tensile stress to break. And the aperture value (SR aperture value) is measured. In the course of research and development, the present inventors have confirmed that this test is suitable for evaluating the hot ductility of the weld heat affected zone (HAZ).

As shown in FIG. 1, when the Mo / B is 286 or less, the SR aperture value is 30% or more, and the reheat embrittlement resistance is good. That is, when the Mo content is relatively decreased with respect to the B content, reheat embrittlement resistance can be ensured. The reason for this is not clear, but it is estimated that the formation of grain boundary precipitates containing Mo and B simultaneously, for example, (Fe, Mo) ₂₃ (C, B) ₆ , is suppressed, and reheat cracking can be suppressed. doing.

  Next, the contents of Cu, Ni, Nb, V, Cr, and W that are selectively added to improve the hardenability were examined. In particular, when Cu and Ni are added excessively, the grain boundaries of HAZ are locally transformed when held at a high temperature to impair reheat embrittlement resistance. Further, Nb generates carbides at the grain boundaries, particularly when the amount of C added is large, and the reheat embrittlement resistance is significantly reduced. Therefore, the present inventors considered that the contents of Mo, B, Cu, Ni, and Nb need to be in an appropriate range, and examined the influence on the reheat embrittlement resistance.

FIG. 2 is a diagram showing the relationship between (Mo + 1.6Cu + 1.3Ni + 8.5Nb) / B and reheat embrittlement resistance. The vertical axis in FIG. 2 is the SR aperture value as in FIG. As shown in FIG. 2, the content [% by mass] of B, Mo, Cu, Ni, and Nb is
(Mo + 1.6Cu + 1.3Ni + 8.5Nb) / B ≦ 286
Is satisfied, the SR aperture value is 30% or more, and the reheat embrittlement resistance is good. That is, in order to improve hardenability, when selectively adding an alloy, in addition to Mo / B, the contents of B, Mo, Cu, Ni, and Nb need to be in the above relationship.

  Hereinafter, the refractory steel material excellent in reheat embrittlement resistance and low temperature toughness of the present invention will be described. First, chemical components will be described. In addition, in the following description, all the addition amounts of each element are represented by mass%.

C: 0.002% or more and 0.050% or less C is an element effective for improving the strength of the steel material, and 0.002% or more is added in order to ensure the strength at room temperature and high temperature. A preferable lower limit of the amount of C is 0.005% or more, and more preferably 0.010% or more is added. C precipitates a stable carbide at 600 ° C. and contributes to securing high-temperature strength. However, if added over 0.050%, the low-temperature toughness and reheat embrittlement resistance of the base material and HAZ decrease, The upper limit is 0.050% or less. The upper limit with preferable C amount is 0.045% or less, More preferably, it is 0.040% or less.

Si: 0.01% or more and 0.50% or less Si is a deoxidizing element and also an element contributing to improvement of hardenability, and 0.01% or more is added. The minimum with the preferable amount of Si is 0.05% or more, and addition of 0.10% or more is still more preferable. On the other hand, when Si is added excessively, retained austenite is generated in the HAZ, and the low temperature toughness is lowered. Therefore, the upper limit is made 0.50% or less. The upper limit with the preferable amount of Si is 0.45% or less, and 0.40% or less is still more preferable.

Mn: 0.50% or more, 2.00% or less Mn is an element that contributes to improvement of hardenability, and 0.50% or more is added to improve strength and toughness. In order to increase the high temperature strength, it is preferable to add 0.80% or more of Mn. A more preferable lower limit of the amount of Mn is 1.00% or more. On the other hand, when Mn is added in excess of 2.00%, the Ac ₁ transformation point is lowered, and when reheated to 600 ° C., the grain boundaries of HAZ are transformed into austenite, thereby impairing reheat embrittlement resistance. The upper limit of the amount of Mn is preferably 1.80% or less, and more preferably 1.70% or less.

Mo: 0.05% or more and less than 0.20% Mo is an element that remarkably improves hardenability when added simultaneously with B. Mo is an element that forms carbides, and 0.05% or more is added in order to ensure high temperature strength. A preferable lower limit of the amount of Mo is 0.08% or more. On the other hand, if added excessively, the carbides become coarse when heated to a high temperature of about 600 ° C., and the reheat embrittlement resistance is lowered, so the upper limit is made less than 0.20%. The upper limit with preferable Mo amount is 0.15% or less.

B: 0.0003% or more and 0.0030% or less B is an element that increases the hardenability by adding a small amount. In particular, when it is added simultaneously with Mo, the hardenability is remarkably improved. In order to obtain the effect, it is necessary to add 0.0003% or more of B. In order to promote the formation of a structure having a high dislocation density such as acicular ferrite and bainite and increase the high temperature strength, it is preferable to add 0.0005% or more of B. On the other hand, if the amount of B exceeds 0.0030%, B nitride precipitates in the HAZ and impairs the reheat embrittlement resistance, so the upper limit is made 0.0030% or less. The upper limit with the preferable amount of B is 0.0020% or less, and 0.0015% or less is still more preferable.

N: 0.0010% or more, 0.0100% or less N is an element that forms nitrides, and in order to refine the structure of the steel material and to increase the low temperature toughness, particularly the low temperature toughness of HAZ, the lower limit of the content 0.0010% or more. On the other hand, if N is added excessively, the nitride becomes coarse and the low temperature toughness, particularly the low temperature toughness of HAZ, is impaired, so the upper limit is made 0.0100% or less. Further, when the amount of N is large, a nitride of B is formed in the HAZ and the reheat embrittlement resistance may be impaired. Therefore, the upper limit of the amount of N is preferably set to 0.0080% or less. A more preferable upper limit of the N amount is 0.0070% or less.

Ti: 0.005% or more and 0.030% or less Ti is an element that forms carbides and nitrides, and in order to refine the structure of a steel material and improve low-temperature toughness, particularly HAZ low-temperature toughness. Add 005% or more. The lower limit of the Ti amount is preferably 0.010% or more. On the other hand, if Ti is added excessively, coarse carbides and nitrides are formed, and low temperature toughness and reheat embrittlement resistance are impaired. Therefore, the upper limit is made 0.030% or less. The upper limit of Ti content is preferably 0.025% or less, and more preferably 0.020% or less.

Al: 0.002% or more and 0.100% or less Al is an element necessary for deoxidation of steel materials, and 0.002% or more is added. The lower limit of the amount of Al is preferably 0.005% or more. On the other hand, if the Al content exceeds 0.100%, coarse oxide clusters may be formed and the low temperature toughness of the steel material may be impaired, and the upper limit is made 0.100% or less. The upper limit of the amount of Al is preferably 0.050% or less.

P: less than 0.0200% S: less than 0.0100% O: less than 0.0100% P, S, and O are impurities. When excessively contained, the low temperature toughness of the base material and HAZ is affected, Since reheat embrittlement impairs the decrease, it is limited to less than 0.0200%, less than 0.0100%, and less than 0.0100%, respectively. The upper limit with preferable P, S, and O is 0.015% or less, 0.008% or less, and 0.003% or less. The lower the content of P, S, and O, the better, so the lower limit is not specified, but industrially unavoidably includes 0.0005% or more, 0.0001% or more, and 0.0005% or more, respectively. .

Mo / B ≦ 286
The present invention suppresses the addition amount of Mo in order to ensure reheat embrittlement resistance, and adds B to increase the high temperature strength, so that the B amount is relatively increased with respect to the Mo amount. is necessary. The present inventors experimentally determined the upper limit of (Mo / B) to be 286 or less. The upper limit of (Mo / B) is preferably 250 or less, more preferably 200 or less, still more preferably 150 or less, and most preferably 100 or less. The lower limit of (Mo / B) is determined by the lower limit of the Mo amount and the upper limit of the B amount.

  In order to increase the strength at room temperature and high temperature, one or more of Cu, Ni, Nb, V, Cr, and W that contribute to improvement of hardenability and precipitation strengthening may be added. However, in this invention, it is preferable to suppress the addition amount of these alloys from a viewpoint of cost, manufacturability, low temperature toughness, and reheat embrittlement resistance.

Cu: 0.30% or less Cu is an element that enhances hardenability and contributes to precipitation strengthening, and 0.01% or more may be added to obtain an effect. However, if Cu is added excessively, precipitates are generated and the low temperature toughness and reheat embrittlement resistance are impaired. Therefore, when Cu is added, the upper limit is made 0.30% or less. Moreover, when Cu is added excessively, the grain boundary of HAZ transforms to austenite, the grain boundary strength is lowered, and reheat embrittlement resistance may be impaired.

Ni: 0.30% or less Ni is an element that improves hardenability and contributes to the improvement of low-temperature toughness, and 0.01% or more may be added to obtain the effect. On the other hand, if Ni is added excessively, the grain boundary of HAZ is transformed to austenite, the grain boundary strength is lowered, and the reheat embrittlement resistance is impaired. Therefore, when Ni is added, the upper limit is 0.30%. The following.

Nb: less than 0.020% Nb generates carbides and nitrides, and contributes to improvement of low-temperature toughness by refining the structure of steel materials and improvement of strength by precipitation strengthening. Nb is also an element that enhances the hardenability of the steel material, and 0.001% or more may be added to obtain an effect. On the other hand, when the amount of Nb added is 0.020% or more, coarse NbC precipitates at the grain boundaries of HAZ and causes significant reheat embrittlement. Therefore, when Nb is added, the upper limit is less than 0.020%. And The Nb amount is preferably 0.010% or less, and more preferably 0.0050% or less. In addition, when adding Nb, it is preferable to suppress content of C to 0.010% or less.

When one or more of Cu, Ni, and Nb are contained, the content [% by mass] of Mo, Cu, Ni, Nb, and B needs to satisfy the following (formula 2).
(Mo + 1.6Cu + 1.3Ni + 8.5Nb) / B ≦ 286 (Formula 2)
Since Cu, Ni, and Nb are elements that cause reheat embrittlement, it is necessary to satisfy the above (Formula 2) in addition to the limitation of Mo / B. The upper limit of {(Mo + 1.6Cu + 1.3Ni + 8.5Nb) / B} was experimentally determined to be 286 or less, similar to the upper limit of (Mo / B). The upper limit of {(Mo + 1.6Cu + 1.3Ni + 8.5Nb) / B} is preferably 250 or less, more preferably 200 or less, still more preferably 150 or less, and most preferably 100 or less.

V: 0.20% or less V is an element that generates carbides and nitrides and contributes to low-temperature toughness and strength improvement of steel materials. V is also an element that enhances the hardenability of the steel material, and 0.01% or more may be added to obtain an effect. On the other hand, when the addition amount of V exceeds 0.20%, the reheat embrittlement resistance is impaired. Therefore, when V is added, the upper limit is made 0.20% or less.

Zr: 0.10% or less Zr is an element that generates carbides and nitrides and contributes to improvement of low-temperature toughness by refinement of the structure of steel materials and improvement of strength by precipitation strengthening. 0.001% or more may be added. On the other hand, if the amount of Zr added exceeds 0.10%, the precipitates become coarse, and particularly the low temperature toughness of HAZ is impaired. Therefore, when Zr is added, the upper limit is made 0.10% or less.

Cr: 0.20% or less Cr is an element that enhances the hardenability of the steel material, and 0.01% or more may be added to obtain an effect. On the other hand, Cr is also an element that generates carbides, and if added excessively, resistance to reheat embrittlement is impaired. Therefore, when adding Cr, the upper limit is made 0.20% or less.

W: 0.30% or less W is an element that enhances the hardenability of the steel material. When added simultaneously with B, W has an effect of significantly improving the hardenability of B. Therefore, even if 0.01% or more is added. Good. On the other hand, when W is added excessively, a coarse intermetallic compound is precipitated and the reheat embrittlement resistance is impaired. Therefore, when W is added, the upper limit is made 0.30% or less.

  Furthermore, one or more of Mg, Ca, Y, La, and Ce, which are elements that control the form of inclusions such as oxides and sulfides and contribute to improvement of hot workability and low temperature toughness May be added.

Mg: 0.0005 to 0.005% or less Ca: 0.0005 to 0.005% or less Y: 0.001 to 0.050% or less La: 0.001 to 0.050% or less Ce: 0.001 0.050% or less Mg, Ca, Y, La, and Ce are powerful deoxidizing elements and are effective in preventing the coarsening of the HAZ particle size by generating fine oxides. Mg, Ca, Y, La, and Ce are also elements that generate sulfides, and suppress the generation of MnS stretched in the rolling direction, thereby contributing to improvement in hot workability and low temperature toughness. In order to obtain these effects, Mg: 0.0005% or more, Ca: 0.0005% or more, Y: 0.001% or more, La: 0.001% or more, Ce: 0.001% or more Or you may add 2 or more types. On the other hand, if Mg, Ca, Y, La, and Ce are added excessively, coarse oxides may be generated and low-temperature toughness may be impaired. Therefore, the upper limit is set to Mg: 0.005% or less, Ca: It is preferable to make 0.005% or less, Y: 0.050% or less, La: 0.050% or less, and Ce: 0.050% or less. In order to ensure low temperature toughness, the upper limits are respectively Mg: 0.004% or less, Ca: 0.004% or less, Y: 0.040% or less, La: 0.040% or less, Ce: 0.0. It is preferable to make it 040% or less.

  Furthermore, in the present invention, the hardenability is enhanced, the generation of a structure having a high dislocation density such as acicular ferrite and bainite enclosing a lath-like structure is promoted, and room temperature strength and high temperature strength are ensured. Is preferably 0.20 to 0.40. A more preferable range of the carbon equivalent Ceq is 0.22 to 0.38, and 0.23 to 0.36 is more preferable. The carbon equivalent Ceq is calculated by the following (formula 3) from the content [mass%] of C, Mn, Ni, Cu, Cr, Mo, and V.

Further, from the viewpoint of weldability, it is preferable to set the weldability index Pcm to 0.20 or less. A more preferable upper limit of the weldability index Pcm is 0.15. On the other hand, if the weldability index Pcm is too low, the hardenability may be insufficient, and the lower limit value of the weldability index Pcm is preferably 0.05 or more. A more preferable lower limit of the weldability index Pcm is 0.10. The weldability index Pcm is calculated by the following (formula 4) from the contents [mass%] of C, Si, Mn, Cu, Cr, Ni, Mo, V, and B.
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (Formula 3)
Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15
+ V / 10 + 5B (Formula 4)
Here, C, Si, Mn, Ni, Cu, Cr, Mo, V, and B are the content [% by mass] of each element. Among these, when Ni, Cu, Cr, and V, which are selectively contained elements, are not intentionally added, the calculation is made as 0 in the above (formula 3) and (formula 4).

  The metal structure of the steel of the refractory steel material of the present invention produces polygonal ferrite, acicular ferrite and bainite containing a lath-like structure, depending on the component composition and production conditions, particularly the cooling rate after hot rolling. These acicular ferrites and bainite have a high dislocation density and are effective in improving not only room temperature strength but also high temperature strength. Since bainite has carbides formed in the grains, it can be distinguished from bainite and acicular ferrite by the presence or absence of carbides in the grains.

  In addition, the refractory steel material excellent in reheat embrittlement resistance and low temperature toughness of the present invention is a steel material excellent in high temperature proof stress at 600 ° C. Such a high temperature proof stress changes for every temperature with the composition of steel materials. For example, a steel material excellent in high temperature yield strength at a temperature of 700 ° C. or higher does not necessarily exhibit high high temperature yield strength at a temperature below 700 ° C. This is because, when the material is exposed to a fire environment, the high-temperature proof stress is greatly influenced by the temperature range in which precipitation of carbide or the like previously contained as an alloy component (called secondary hardening) occurs. Because. The present invention newly proposes an alloy composition for obtaining a high temperature yield strength of 600 ° C., and is based on a design philosophy that is completely different from a steel material excellent in high temperature yield strength in other temperature ranges.

  Moreover, the target value of the strength of the refractory steel material excellent in reheat embrittlement resistance and low temperature toughness according to the present invention is that the tensile strength (TS) at room temperature is 400 to 550 MPa, and the proof stress at room temperature (YS) is 235 MPa. That's it. Further, the high temperature strength (high temperature proof stress) is set such that the lower limit value of the proof stress at 600 ° C. is 2/3 of the lower limit value 235 MPa of the proof stress at room temperature, that is, 157 MPa or more. Moreover, about low temperature toughness, the Charpy absorbed energy in 0 degreeC needs to be 27J or more. These mechanical property and high temperature property ranges and lower limit criteria are not necessarily defined in actual industrial standards, but are values estimated by design calculations, and include safety factors.

  Regarding the reheat embrittlement resistance, the SR lower limit is 30% or more as the target lower limit. The SR aperture value is the HAZ aperture value at 600 ° C., and is evaluated by performing a tensile test at 600 ° C. using a test piece containing HAZ or a test piece subjected to a reproduction thermal cycle. The lower limit of the target SR aperture value is the aperture value when the fracture surface of the unstable grain boundary fracture observed at the time of observation of the cross-section after fracture is 50% of the total fracture surface. As 30% or more. That is, when the SR drawing value was 30% or more, it was determined by experimentally confirming that the grain boundary fracture surface ratio of the fracture surface was 50% or less and that high-temperature ductility could be obtained.

  Next, the manufacturing method of the refractory steel material excellent in reheat embrittlement resistance and low temperature toughness of the present invention will be described. The refractory steel material is roughly classified into a thick steel plate and an H-shaped steel according to the shape, and is manufactured by heating a steel piece and performing hot rolling. A tempering heat treatment may be performed after the hot rolling.

  Steel is melted and cast into steel pieces. From the viewpoint of productivity, continuous casting is preferable. The obtained steel slab is formed into a steel plate or a shaped steel by hot rolling and cooled. In addition, steel materials which this invention makes object include shape steels, such as a rolled steel plate, H-shape steel, I-shape steel, angle steel, groove shape steel, an unequal side unequal thickness angle steel. Of these, H-shaped steel is particularly suitable for building materials that require fire resistance and reheat embrittlement resistance.

Billet heating temperature: 1100 ° C to 1300 ° C In order to perform hot rolling in the austenite region, the billet is heated to 1100 to 1300 ° C. The reason why the heating temperature of the steel slab is 1100 ° C. or higher is to sufficiently dissolve the precipitate in the steel. On the other hand, if the heating temperature of the steel slab is too high, the structure becomes coarse and the low-temperature toughness decreases, so the heating temperature is set to 1300 ° C. or lower.

Hot rolling finishing temperature: 800 ° C. or higher After heating, hot rolling is performed a plurality of times to a predetermined plate thickness, and the finishing temperature is set to 800 ° C. or higher. This is because the deformation resistance increases when the hot rolling finish temperature is less than 800 ° C. Moreover, when the finishing temperature of hot rolling is less than 800 ° C., the cooling start temperature is lowered, the structure cannot be controlled, and the low temperature toughness may be impaired.

  The refractory steel material of the present invention can ensure strength and low temperature toughness by allowing to cool after hot rolling. That is, the refractory steel material of the present invention is excellent in hardenability because Mo and B are added at the same time, and since the content of precipitation strengthening elements is suppressed, the dislocation density is high even when air-cooled, and the acicular ferrite. And bainite are formed, and the formation of precipitates is suppressed. In addition, although accelerated cooling, such as water cooling, may be performed after hot rolling, in this case, it is preferable that a cooling rate shall be 3-15 degreeC / s.

  When manufacturing H-section steel, hot rolling is performed by a breakdown process by hole-type rolling, an intermediate rolling process by an edger rolling mill and a universal rolling mill, and a finish rolling process by a universal rolling mill. In addition, you may include the skew roll rolling process which controls the web height of H-section steel.

Tempering heat treatment temperature: 400 ° C. or more and less than 650 ° C. Tempering heat treatment holding time: 5 minutes or more and 360 minutes or less Hot rolling may be performed and allowed to cool, followed by tempering heat treatment. By performing the tempering heat treatment at 400 ° C. or higher, the low temperature toughness can be remarkably enhanced. In order to increase the low temperature toughness, it is preferable that the holding time of the tempering heat treatment be 5 minutes or longer. On the other hand, if the temperature of the tempering heat treatment is too high, the precipitates are coarsened and the high temperature strength and reheat embrittlement resistance may be impaired. Therefore, it is preferable to perform the tempering heat treatment at less than 650 ° C. Further, when the holding time of the tempering heat treatment exceeds 360 minutes, the productivity is lowered, and depending on the heating temperature, the precipitates may be coarsened to impair the high temperature strength and reheat embrittlement resistance.

  Steel having the composition shown in Tables 1 and 3 was melted, and a steel piece was cast by continuous casting. Tables 2 and 4 show the calculated values of (Mo + 1.6Cu + 1.3Ni + 8.5Nb) / B, Mo / B, Ceq, and Pcm for each steel. The obtained steel slab was hot-rolled under the conditions shown in Tables 5 and 6 and allowed to cool to produce a steel plate. Some steel plates were tempered and others were hot rolled. A sample for examining room temperature tensile properties, Charpy properties, and high temperature tensile properties was collected from the central portion of the plate thickness of the steel plate, with the plate width direction perpendicular to the rolling direction as the longitudinal direction.

  The tensile properties at room temperature were evaluated according to JIS Z 2241. The average number of yield strength and tensile strength was calculated with 2 tests. Regarding the strength at room temperature, a proof stress of 235 MPa or more and a tensile strength of 400 to 550 MPa were evaluated as good. The low temperature toughness of the base material was evaluated by conducting a Charpy impact test at a test temperature of 0 ° C. in accordance with JIS Z 2242. As for the low temperature toughness of the base material, the average value of Charpy absorbed energy was calculated by setting the number of tests to 3, and 27 J or more was evaluated as good.

  As for the high-temperature strength (high-temperature proof stress), a tensile strain rate of 0.5% /% is used based on the JIS G 0567 high-temperature tensile test using a high-temperature tensile test piece having a parallel part diameter of 6 mm and a parallel part length of 30 mm. The test piece was deformed in minutes, and a stress-strain diagram was collected to measure the high temperature proof stress. The yield strength at this time was all 0.2% yield strength. As for the yield strength at 600 ° C., 157 MPa or more, which is 2/3 of the threshold value 235 MPa of the yield strength at room temperature, was evaluated as good.

  Furthermore, a test piece having a diameter of 10 mm was taken from the steel material, provided with a reproducible thermal cycle, and after the tensile test, a drawing value (SR drawing value) was measured. In the reproduction heat cycle, the temperature rise rate is set to 100 ° C./s, heated to 1400 ° C. and held for 2 s, and cooled from 800 ° C. to 500 ° C. in 16 s. The SR drawing value was measured by heating a test piece provided with a reproducible heat cycle from room temperature to 600 ° C. at 1 ° C./s and holding it for 1 hour, then applying a tensile stress and breaking the test piece. The SR aperture value was evaluated as good when 30% or more.

  Moreover, about the low temperature toughness of HAZ, the steel plate which processed 45 degree | times X groove | channel was submerged-arc-welded, and it evaluated using the obtained welded joint. At this time, it was confirmed by calculating from the output, current, and voltage values during welding that the welding heat input was always 2 k to 3 kJ / mm. In accordance with JIS Z 3128, a test piece was collected from the welded joint and evaluated by performing a Charpy test at 0 ° C. The V notch of the test piece was provided at a position of 1 mm on the base metal side from the melting line. As for the low temperature toughness of HAZ, the average number of Charpy absorbed energy was calculated with 3 tests, and 27J or more was evaluated as good.

The results are shown in Tables 7 and 8. “YS” and “TS” of “room temperature tensile properties” are proof stress and tensile strength at room temperature, respectively, and vE ₀ is Charpy absorbed energy at 0 ° C. “YS” and “SR aperture value” of “High-temperature characteristics (600 ° C.)” are the yield strength and SR aperture value at 600 ° C., respectively. Steel No. 1 , 4-6, 8, 10-29 are within the scope of the present invention in terms of component composition and production conditions, and are excellent in room temperature tensile properties, low temperature toughness, high temperature strength, and reheat embrittlement resistance.

  On the other hand, the steel material No. Since 30 has a large amount of C, the strength is high, and the low temperature toughness and reheat embrittlement resistance of HAZ are reduced. Steel No. No. 31 has a small amount of C, and the strength at room temperature and high temperature is low. Steel No. Since 32 has a large amount of Si, the low temperature toughness of HAZ is lowered. Steel No. No. 33 has a small amount of Si, lacks hardenability, and has reduced room temperature strength and low temperature toughness. Steel No. Since 34 has a large amount of Mn, the reheat embrittlement resistance is low. Steel No. No. 35 has a small amount of Mn and has reduced room temperature strength and high temperature strength.

  Steel No. No. 36 has a large amount of Mo. Since 38 has a large amount of B, its resistance to reheat embrittlement is lowered. Steel No. No. 37 has a small amount of Mo, and steel material No. Since No. 39 has a small amount of B, the high-temperature strength is lowered. Steel No. No. 40 has a large amount of Ti. Since 42 has a large amount of N, the low temperature toughness and reheat embrittlement resistance of HAZ are lowered. Steel No. No. 41 has a small amount of Ti. Since 43 has a small amount of N, the low temperature toughness of HAZ is lowered.

  Steel No. No. 44 has a large amount of Cr. No. 45 has a large amount of V. No. 46 has a large amount of Ni. 47 has a large amount of Cu. Since 48 has a large amount of W, the resistance to reheat embrittlement is lowered. Steel No. No. 49 has a large amount of Zr, and the low temperature toughness of HAZ is lowered. Steel No. No. 50 has a large amount of O. No. 51 has a large amount of P. Since No. 52 has a large amount of S, the low-temperature toughness of the base material and HAZ is low, and the reheat embrittlement resistance is also lowered. Steel No. Nos. 53 to 57 have a high (Mo + 1.6Cu + 1.3Ni + 8.5Nb) / B. Since 58 and 59 have high Mo / B, the reheat embrittlement resistance is low.

  Steel having the component composition shown in Table 9 was melted, and a steel piece was cast by continuous casting. Table 10 shows the calculated values of (Mo + 1.6Cu + 1.3Ni + 8.5Nb) / B, Mo / B, Ceq, and Pcm for each steel. The obtained steel slab was hot-rolled by a universal rolling equipment line under the conditions shown in Table 11 and allowed to cool to produce an H-section steel. Some H-section steels were tempered and other H-section steels were hot rolled.

Samples were collected for investigating normal temperature tensile characteristics, Charpy characteristics, and high temperature tensile characteristics from a quarter of the flange thickness length at the center of the flange thickness of the H-shaped steel. In addition, the welded joint which evaluated the low temperature toughness of HAZ evaluated by extracting the sample from the flange of H-section steel. The results are shown in Table 12. Steel No. 101-104 and 107-117 have the component composition and manufacturing conditions within the scope of the present invention, and are excellent in room temperature tensile properties, low temperature toughness, high temperature strength, and reheat embrittlement resistance.

Claims (8)

% By mass
C: 0.002% or more, 0.050% or less,
Si: 0.01% or more, 0.50% or less,
Mn: 0.50% or more, 2.00% or less,
Mo: 0.05% or more and less than 0.20%,
B: 0.0003% or more, 0.0020% or less,
N: 0.0010% or more, 0.0100% or less,
Ti: 0.005% or more, 0.030% or less,
Al: 0.002% or more and 0.100% or less, and further each content of P, S, O,
P: less than 0.0200%,
S: less than 0.0100%,
O 2: Reheat embrittlement resistance, characterized by being limited to less than 0.0100%, the balance being Fe and inevitable impurities, and the content [% by mass] of Mo and B satisfying the following (formula 1) Refractory steel with excellent heat resistance and low temperature toughness.
Mo / B ≦ 286 (Formula 1) Furthermore, in mass%,
Cu: 0.30% or less,
Ni: 0.30% or less,
Nb: less than 0.020%,
2 or more types, and the content [% by mass] of Mo, Cu, Ni, Nb, and B satisfies the following (formula 2). Refractory steel with excellent thermal embrittlement and low temperature toughness.
(Mo + 1.6Cu + 1.3Ni + 8.5Nb) / B ≦ 286 (Formula 2) Si content is
Si: 0.040% or more, 0.50% or less
The fire-resistant steel material excellent in reheat embrittlement resistance and low-temperature toughness according to claim 1 or 2. Furthermore, in mass%,
V: 0.20% or less,
Zr: 0.10% or less,
Cr: 0.20% or less,
W: 0.30% or less of 1 type or 2 types or more, The fireproof steel material excellent in reheat embrittlement resistance and low temperature toughness according to any one of claims 1 to 3 . Furthermore, in mass%,
Mg: 0.0005 to 0.0050%,
Ca: 0.0005 to 0.0050%,
Y: 0.001 to 0.050%,
La: 0.001 to 0.050%,
Ce: 0.001 to 0.050%
The fireproof steel material excellent in reheat embrittlement resistance and low temperature toughness according to any one of claims 1 to 4 , characterized by containing at least one of the following. Furthermore, Ceq calculated | required by the following (Formula 3) is 0.20-0.40, and Pcm calculated | required by the following (Formula 4) is 0.05-0.20. 5. A fire-resistant steel material excellent in reheat embrittlement resistance and low-temperature toughness according to any one of 5 above.
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (Formula 3)
Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15
+ V / 10 + 5B (Formula 4)
Here, C, Si, Mn, Ni, Cu, Cr, Mo, V, and B are the content [% by mass] of each element. A steel slab having the steel component according to any one of claims 1 to 6 is heated to a temperature of 1100 ° C or higher and 1300 ° C or lower, hot-rolled at a finishing temperature of 800 ° C or higher, and then allowed to cool. A method for producing a refractory steel material excellent in reheat embrittlement resistance and low temperature toughness. Furthermore, the tempering heat treatment for 5 minutes or more and 360 minutes or less is performed in the temperature range of 400 degreeC or more and less than 650 degreeC, The refractory steel material excellent in reheat embrittlement resistance and low temperature toughness of Claim 7 characterized by the above-mentioned. Production method.

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