JP2007191746A - Fire-resistant steel material superior in weldability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fire-resistant steel material having both of excellent fire resistance and weldability in spite of high strength of a 50 to 60 kilogram grade (tensile strength of 490 to 590 MPa at room temperature). <P>SOLUTION: The fire-resistant steel material comprises, by mass%, more than 0.02 but 0.15% or less C, 0.1-1.0% Si, 1.0-2.0% Mn, 0.020% or less P, 0.010% or less S, 0.005-0.050% Al, 0.015-0.10% Ti, 0.2-2.0% Cr, 0.002-0.010% N, 0.0005-0.0050% B, 0% or more but less than 0.3% Mo, 0% or more but less than 0.005% Nb, 0% or more but less than 0.005% V, while controlling the following TS value into 0 to 0.10% and the following CS value into 0.12 to 0.22: TS=[Ti]-3.4×[N], and CS=6.4×[C]-1.4[Ti]+0.004×[Cr]+3.2×[N], (wherein [X] represents mass% of an element (X)), and the balance Fe with unavoidable impurities. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a refractory steel material suitable for, for example, a building structure material that may be exposed to a high temperature such as a fire.

In general, structural steel materials are designed so that sufficient strength can be secured at room temperature (normal temperature), but the strength is significantly reduced when the steel is in a high temperature state of 500 ° C. or higher. For this reason, in steel materials for building structures that are exposed to high temperatures due to fire or the like, the steel materials are provided with a fireproof coating so that the steel materials do not become brittle at high temperatures and the structure does not collapse or deform significantly.
Since such a fireproof coating increases the construction cost and leads to a prolonged construction period, in recent years, a fireproof steel material has been developed that maintains strength even at high temperatures without applying this type of fireproof coating. For example, Japanese Patent Publication No. 4-50362 (Patent Document 1) describes a steel material that is precipitation strengthened by fine carbides such as Mo, Nb, and V. However, the steel material strengthened with such a precipitation strengthening element has a problem of reducing weldability, particularly HAZ toughness, required as a steel material for construction.

  In contrast, as described in Japanese Patent Application Laid-Open No. 2002-249854 (Patent Document 2), the applicant uses a refractory steel material in which Cu and other elements such as Mo are reduced using precipitation of Cu at a high temperature. Proposed. This steel material has made it possible to cope with welding with a heat input of about 5 kJ / mm, but sufficient weldability is obtained in the present day when higher heat input welding is desired from the viewpoint of reducing welding work efficiency. It cannot be said that it is done.

  In addition, as a refractory steel material that can cope with high heat input welding, Japanese Patent Laid-Open No. 2001-262269 (Patent Document 3) discloses a Mo-containing refractory with improved HAZ toughness by utilizing a composite inclusion of oxide and TiN. Although steel materials have been proposed, there is a problem that the HAZ toughness is improved, in particular, the HAZ toughness level of high-strength steel materials of 60 kg class is insufficient, and it is difficult to manufacture oxide inclusions uniformly. .

On the other hand, as a 60 kg-class high HAZ toughness steel material for the construction field, Japanese Patent Application Laid-Open No. 2005-36295 (Patent Document 4) defines the carbon equivalent (CE _N ) and average aspect ratio representing the curability of welded HAZ. However, high heat input architectural steel materials in which island-like martensite phase and ε-Cu phase are dispersed in the matrix have been proposed, but the structure control in the manufacturing stage is complicated and high temperature strength (high temperature proof stress) Therefore, it cannot be said that it has sufficient fire resistance.
Japanese Patent Publication No. 4-50362 JP 2002-249854 A JP 2001-262269 A JP-A-2005-36295

  As described above, a high-strength steel material that has both sufficient high-temperature strength (fire resistance) and weldability (weld HAZ toughness) has not yet been developed, and the present invention has a 50-60 kg class (tensile strength at room temperature). An object of the present invention is to provide a refractory steel material having both excellent fire resistance and weldability while having a high strength of 490 to 590 MPa.

In the refractory steel material of the present invention, the chemical component is mass%, C: more than 0.02 to 0.15%, Si: 0.1 to 1.0%, Mn: 1.0 to 2.0%, P : 0.020% or less, S: 0.010% or less, Al: 0.005 to 0.050%, Ti: 0.015 to 0.10%, Cr: 0.2 to 2.0%, N: 0.002 to 0.010%, B: 0.0005 to 0.0050%, Mo: 0 to less than 0.30%, Nb: 0 to less than 0.005%, V: 0 to less than 0.005% In addition, the following TS value is 0 to 0.10%, CS value is 0.12 to 0.22%, and the balance is composed of the remaining Fe and inevitable impurities. However, [X] indicates mass% of the element X.
TS = [Ti] -3.4 × [N]
CS = 6.4 × [C] −1.4 [Ti] + 0.004 × [Cr] + 3.2 × [N]

  According to the refractory steel material of the present invention, since the contents of the three elements Mo, Nb, and V that coarsen the bainite structure of the welded HAZ are limited, HAZ toughness can be ensured. In addition, the amount of S is suppressed, Ti is sufficiently added to N in the range where the TS value is 0 or more and 0.10 or less, and the CS value is in the range of 0.12 to 0.22%. Since an appropriate amount of Cr is added, TiC is first finely precipitated at a high temperature, Cr carbide is finely precipitated as a core, and addition of an appropriate amount of B suppresses the coarsening of Cr carbide, which is said to be easily coarsened. In combination with the fine precipitation of TiC, excellent high temperature strength (high temperature proof stress) can be ensured by making the Cr carbide fine.

  In the above-mentioned refractory steel material, from an element of Group A (Zr: 0.005 to 0.050%, Ca, Mg, REM (rare earth element): 0.0005 to 0.0050% each) as a HAZ toughness improving element or at a high temperature From elements of group B (Cu, Ni: 0.1 to 2.0% each, but AS value (= [Mn] + [Ni] + 2 * [Cu]): 4.5% or less) as strength improving elements One or more elements can be further added.

  According to the refractory steel material of the present invention, these carbides are heated to a high temperature by adding a predetermined amount of Ti, Cr, and B while suppressing the addition of Mo, Nb, and V, which coarsens the weld HAZ structure and degrades its toughness. It can be finely deposited below and has excellent weldability and high temperature strength (high temperature proof stress). For this reason, it is suitable as a refractory steel material, its manufacturing method is easy, and productivity is also good.

First, the chemical components of the refractory steel material of the present invention will be described. Hereinafter, the unit is mass%. C: more than 0.02 to 0.15%
C is added as a strengthening element. If it is 0.02% or less, it becomes difficult to secure a strength of 490 MPa or more. On the other hand, if it exceeds 0.15%, the hard MA structure (mixed structure of martensite and austenite) in HAZ increases. HAZ toughness decreases. Therefore, the C content is more than 0.02%, preferably 0.03% or more, and the upper limit is 0.15%, preferably 0.12%.

Si: 0.1 to 1.0%
Si is added for securing strength and deoxidation. If it is less than 0.1%, these effects are insufficient, while if it exceeds 1.0%, the hard MA structure increases and the HAZ toughness deteriorates. For this reason, the lower limit of Si is 0.1%, preferably 0.12%, while the upper limit is 1.0%, preferably 0.80%.

Mn: 1.0-2.0%
Mn is added to ensure strength. If it is less than 1.0%, the predetermined strength cannot be obtained. On the other hand, if it exceeds 2.0%, the strength of the HAZ becomes too high, and the ductility returns and deteriorates. For this reason, the lower limit of Mn is 1.0%, preferably 1.2%, while the upper limit is 2.0%, preferably 1.8%.

P: 0.020% or less P is an impurity element that promotes grain boundary fracture, and is 0.020% or less, preferably 0.012% or less, to ensure HAZ toughness.

S: 0.010% or less S is an impurity element that promotes hot cracking of HAZ, and is combined with Ti to reduce the amount of Ti. From the viewpoints of securing HAZ toughness and preventing a decrease in Ti amount, it is 0.010% or less, preferably 0.008%.

Al: 0.005 to 0.050%
Al is added as a deoxidizing element. If it is less than 0.005%, deoxidation becomes insufficient, so that the ductility is lowered. On the other hand, if it exceeds 0.050%, the HAZ toughness deteriorates as in the case of Si. For this reason, the lower limit of the Al amount is 0.005%, preferably 0.010%, and the upper limit is 0.050%, preferably 0.040%.

Ti: 0.015-0.10%
Ti improves the HAZ toughness by forming a nitride. Moreover, fine TiC is produced | generated and it contributes to fine precipitation of Cr carbide | carbonized_material using this as a nucleus, and high temperature strength is improved by these effect | actions. If the amount of Ti is less than 0.015%, these effects are too small. On the other hand, if it exceeds 0.10%, the HAZ toughness returns and decreases. For this reason, the lower limit of the Ti amount is 0.015%, preferably 0.020%, and the upper limit is 0.10%, preferably 0.080%.

Cr: 0.2 to 2.0%
Cr contributes to securing high temperature strength by forming Cr carbide. If it is less than 0.2%, such an action is insufficient. On the other hand, if it exceeds 2.0%, the coarsening of Cr carbide is unavoidable and the high-temperature strength is lowered. For this reason, the lower limit of the Cr content is 0.2%, preferably 0.5%, while the upper limit is 2.0%, preferably 1.5%.

N: 0.002 to 0.010%
N combines with part of Ti to form TiN and contributes to the improvement of HAZ toughness. If it is less than 0.002%, such an effect is too small. On the other hand, if it exceeds 0.010%, the precipitation amount of TiC is lowered, so that the high-temperature strength is lowered. Toughness also decreases. For this reason, the lower limit of the N amount is 0.0020%, preferably 0.0025%, while the upper limit is 0.010%, preferably 0.008%.

B: 0.0005 to 0.0050%
Since B suppresses the growth of Cr carbide, it contributes to the improvement of high temperature strength. If it is less than 0.0005%, such an effect is insufficient. On the other hand, if it exceeds 0.0050%, the HAZ toughness returns and decreases.

Mo: 0 to less than 0.30% Since Mo coarsens the bainite structure of HAZ and lowers toughness, it is preferable that it be less. There may be no additive. In the component system of the present invention, it is allowed to be less than 0.30%, but is preferably limited to less than 0.20%.

Nb and V: 0 to less than 0.005%, respectively. Like Mo, the HAZ bainite structure is coarsened and the toughness is lowered, so that the smaller one is preferable. There may be no additive. In the component system of the present invention, it is allowed to be less than 0.005%, but is preferably limited to 0.004% or less.

TS value (= [Ti] -3.4 * [N]): 0 to 0.10%
The TS value is an index indicating whether Ti carbide can be precipitated. In the present invention, since the amount of S is limited, Ti hardly forms sulfides and most of them precipitate as carbonitrides. The TS value corresponds to a value obtained by subtracting the Ti amount fixed as nitride from the total Ti amount. If the TS value is not 0% or more, TiC cannot be secured, but if it exceeds 0.10%, TiC becomes excessive, and the HAZ toughness returns and decreases. For this reason, the TS value is set to 0% or more and 0.10% or less. Preferably, the lower limit is 0.01% and the upper limit is 0.08%.

CS value (= 6.4 * [C] -1.4 [Ti] + 0.004 * [Cr] + 3.2 * [N]): 0.12-0.22%
The CS value quantifies the amount of Cr carbide calculated by thermodynamic calculation and the influence of main elements, and is an index of the balance between TiC and Cr carbide. If it is less than 0.12%, almost no Cr carbide is generated, and if it exceeds 0.22%, the Cr carbide becomes coarse and the high-temperature strength decreases. For this reason, the lower limit of the CS value is 0.12%, preferably 0.13%, and the upper limit is 0.22%, preferably 0.20%.

The refractory steel material of the present invention is composed of the balance of Fe and unavoidable impurities in addition to the above basic components, and further from elements of Group A (Zr, Ca, Mg, REM (rare earth element)) as a HAZ toughness improving element, or at a high temperature. By adding a predetermined amount of one or more elements from the group B (Cu, Ni) elements as the strength improving elements, the following components (1), (2) and (3) can be obtained.
(1) Basic component + one or more elements from group A
(2) Basic component + one or more elements from group B
(3) Component (1) above + one or more elements from Group B

The addition amount of the above characteristic improving element and a more specific action will be described.
Zr: 0.005 to 0.050%
Zr improves HAZ toughness by forming nitrides, so 0.005% or more is preferable. On the other hand, when it is excessively added to exceed 0.050%, the nitride becomes coarse, and the HAZ toughness returns and decreases. For this reason, it is desirable to stop at 0.050% or less.
Ca, Mg, REM: 0.0005 to 0.0050% each
These elements have the effect of improving toughness by spheroidizing inclusions. Therefore, addition of 0.0005% or more is desirable. On the other hand, if added in excess of 0.0050% each, an oxide is formed, and the HAZ toughness returns and decreases. For this reason, it is good to stop at 0.0050% or less.

Cu, Ni: 0.1 to 2.0% each
These elements do not affect the HAZ toughness and improve the strength. If the content is less than 0.1%, the effect of improving the strength is too small. If the content exceeds 2.0%, the effect is saturated, resulting in high material costs. For this reason, it is desirable to add 0.1% or more and 2.0% or less, respectively.

AS value (= [Mn] + [Ni] + 2 * [Cu]): 4.5% or less When Cu and Ni are added, it is effective for improving the strength as described above. Although these can be obtained, these are austenite stabilizing elements like Mn, and excessive addition tends to austenite at a high temperature, and the high temperature strength decreases. For this reason, when adding Ni and Cu, it is desirable to stop AS value to 4.5% or less.

The refractory steel material of the present invention is manufactured by hot-rolling the above components with steel by a conventional method. That is, it is manufactured by melting the steel of the above components and heating the steel piece to a temperature of about 1100 to 1200 ° C., then finishing the hot rolling at a finish rolling temperature of about 850 ° C. and cooling by air cooling. The
The structure of the steel material thus manufactured is a structure composed of ferrite and a low-temperature transformation product (bainite or / and martensite), but the refractory steel material of the present invention may be any structure.
Next, the hot-rolled steel sheet and the method for producing the same according to the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the examples.

Steel types shown in Table 1 and Table 2 are melted, the steel pieces are heated to 1150 ° C., hot rolled, the finish rolling finish temperature is 850 ° C., the rolling is finished, air-cooled, and the sheet thickness is 50 mm. Steel plates were manufactured.
Using the steel plate of each sample, a normal temperature tensile test and a high temperature tensile test at 600 ° C. were performed, and the yield strength (YS) and tensile strength (TS) at normal temperature and YS at high temperature were examined. The tensile test was performed in accordance with JISZ2241, by processing a tensile test piece from a ¼ portion of the steel plate thickness according to JISZ2201. These measurement results are shown in Table 3. A normal temperature tensile strength of 490 MPa or higher and a high temperature YS / normal temperature YS of 75% or higher are evaluated as acceptable levels.

  In addition, a thermal cycle test was performed on the sample steel plate in order to investigate weldability. The thermal cycle test is a thermal cycle in which the heat input of welding is equivalent to 65 kJ / cm, and is given a thermal cycle that is heated to 1400 ° C. and then cooled from 800 ° C. to 500 ° C. in 500 seconds. After the thermal cycle test, An impact test piece was collected from the steel plate, subjected to a Charpy impact test (test temperature 0 ° C.), and an impact absorption characteristic absorption energy (vE 0) was measured. A vE0 of 150 J or higher is evaluated as an acceptable level of HAZ toughness. The test results are also shown in Table 3.

  From Table 3, Sample Nos. 1 to 20 of the invention example have a normal temperature tensile strength of 490 MPa or more and a high temperature YS / normal temperature YS of 75% or more, and a considerable strength is ensured even in a high temperature state at 600 ° C. You can see that Moreover, as a result of the thermal cycle test, an impact absorption energy of 150 J or more is secured, and it has excellent HAZ toughness. In particular, Sample Nos. 15 to 20 to which Cu and Ni are added at an AS value of 4.5% or less have a normal temperature strength of 600 MPa or more, but a high high temperature strength (high temperature proof stress) is secured, and HAZ toughness is also achieved. Also excellent.

Claims (3)

Chemical component is mass%, C: more than 0.02 to 0.15%, Si: 0.1 to 1.0%,
Mn: 1.0 to 2.0%, P: 0.020% or less,
S: 0.010% or less, Al: 0.005 to 0.050%,
Ti: 0.015-0.10%, Cr: 0.2-2.0%,
N: 0.002-0.010%, B: 0.0005-0.0050%,
Mo: 0 to less than 0.30%, Nb: 0 to less than 0.005%,
V: 0 to less than 0.005%, the following TS value is 0 to 0.10%, CS value is 0.12 to 0.22%, and consists of the balance Fe and inevitable impurities, excellent weldability Refractory steel.
TS = [Ti] -3.4 × [N]
CS = 6.4 × [C] −1.4 [Ti] + 0.004 × [Cr] + 3.2 × [N]
However, [X] indicates mass% of the element X.   The chemical component is further Zr: 0.005 to 0.050%, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, REM: 0.0005 to 0.0050% The refractory steel material according to claim 1, comprising more than seeds. The chemical component further includes one or more of Cu: 0.1 to 2.0% and Ni: 0.1 to 2.0%, and the following AS value is 4.5% or less. 2. The refractory steel material described in 2.
AS = [Mn] + [Ni] + 2 × [Cu]