JP5838693B2 - Manufacturing method of steel with excellent high temperature strength - Google Patents

Description

  The present invention relates to a welded structural steel material widely used in welded structures such as buildings, bridges, ships, and the like, and more particularly, to a so-called fireproof steel material that is excellent in high-temperature strength during fire (referred to as fireproof performance). The “steel material” here includes thick plates, H-section steel, and the like.

  In general, a steel material for welded structure has a necessary and sufficient strength at room temperature, but when exposed to a temperature exceeding 350 ° C., the strength usually decreases. For this reason, when such a welded structural steel material is used, for example, as a structural member of a building structure, in order to ensure safety in the event of a fire, a fireproof coating is applied to the structural member to increase the temperature of the steel material. Ingenuity such as suppression is carried out.

However, recently, from the viewpoint of cost reduction and aesthetics of building structures, reduction of fireproof coating and further omission of fireproof coating (uncovering) have been demanded. In response to these demands, a welded structural steel material that does not require a fireproof coating, that is, a refractory steel material has been developed, and refractory steel materials having excellent high temperature strength at 600 ° C, which is more than 2/3 of the normal temperature strength standard value, are widely used. It has been.
Examples of such a refractory steel material are described in Patent Document 1, Patent Document 2, Patent Document 3, Patent Document 4, and the like. All of the refractory steel materials described in Patent Documents 1 to 4 contain Mo essentially, and the desired excellent strength (high-temperature strength) is secured at 600 ° C. by precipitation of Mo carbides. In addition to the precipitation strengthening of Mo carbides, the above-mentioned refractory steel materials are used in combination with refinement of ferrite grains, solid solution strengthening with alloy elements other than Mo, precipitation strengthening, and strengthening by dispersion of the hard phase.

However, since Mo is a scarce resource and expensive, a refractory steel material containing a large amount of Mo often loses the economic superiority of the refractory steel material due to soaring Mo raw material. Therefore, studies are being made on whether to use low Mo or no addition of Mo.
For example, in Patent Document 5 , C: 0.001 to 0.030%, Si: 0.05 to 0.50%, Mn: 0.4 to 2.0%, Nb: 0.03 to 0.50%, Ti: 0.005% or more and less than 0.040%, N: 0.0001 to 0.0050 %, Al: 0.005 to 0.030%, Mg: 0.005% or less, REM: 0.010% or less, one or two of them, P and S are limited low, C, Nb, Ti, N Refractory steel materials adjusted to satisfy specific relationships are described. In the technique described in Patent Document 5, a high amount of Nb is contained, and the amount of C is kept low so as not to form carbides. The high temperature strength equivalent to that of Mo-containing steel is obtained by utilizing the drag effect of solute Nb. It can be secured. In the technique described in Patent Document 5, the content of Mo: less than 0.10% is allowed.

In Patent Document 6 , C: 0.005 to 0.030%, Si: 0.05 to 0.40%, Mn: 0.40 to 1.85%, Nb: 0.01% or more and less than 0.35%, N: 0.0001 to 0.0045%, and Zr: 0.005 to Contains one or two of 0.060% or REM: 0.001 to 0.01%, Al: 0.03% or less, further restricts P and S, and Zr, REM and N satisfy a specific relationship, and C , Refractory H-section steel adjusted so that Nb satisfies a specific relationship is described. According to the technique described in Patent Document 6 , excellent high-temperature strength can be secured and reheat embrittlement can be prevented without containing expensive Mo.

Japanese Patent No. 2828054 Japanese Patent No.3596473 Japanese Patent Laid-Open No. 6-10040 JP-A-9-137218 JP 2008-121120 A JP 2008-179881 A

  However, in the techniques described in Patent Documents 5 and 6, it is necessary to adjust C to be low, and there is a problem that the refining time (decarburization time) becomes long. It is necessary to use Mn, and there is a problem that the material cost increases. In addition, in the techniques described in Patent Documents 5 and 6, there are problems such that reheat embrittlement easily occurs due to extremely low C and high Nb content, and further increase in high-temperature strength is required. is there. Moreover, in the technique described in Patent Document 7, it is necessary to adjust C to be low, and there is a problem that the refining time (decarburization time) becomes long. For example, as a secondary material, it is necessary to use a raw material that does not contain C. There is a problem that the material cost increases.

  The present invention provides a refractory steel material excellent in high-temperature strength and a method for manufacturing the same, which can solve the problems of the prior art and can stably secure a desired high-temperature strength even with a composition in which the Mo content is reduced as much as possible. Objective.

  In order to achieve the above-mentioned object, the present inventors diligently studied a method for stably securing a desired high-temperature strength even when the Mo content is reduced as much as possible. Conventionally, in order to improve the fire resistance with a steel composition with reduced or reduced Mo content, Nb was included and the drag effect of solute Nb was used. Instead, attention was paid to Nb carbonitride Nb (C, N) and V carbonitride V (C, N).

  By further research by the present inventors, by depositing an appropriate amount of fine precipitates (Nb carbonitride and V carbonitride) of less than 20 nm, the amount of Mo deposited during a fire (during high-temperature heating) The amount of Mo precipitated as carbide) increases dramatically, and even if the Mo content is small, the high-temperature strength increases stably, that is, the ratio of the Mo content to the amount of Mo precipitated as Mo carbide during a fire, It was found that the precipitation Mo ratio (Mo yield) improved dramatically, and the high-temperature strength improved even when the Mo content was small.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows .

( 1 ) When a steel material is hot-rolled to obtain a steel material having a predetermined size, the steel material is mass%, C: 0.01 to 0.1%, Si: 0.01 to 1.0%, Mn: 0.1 to 2.0%, P: 0.030% or less, S: 0.030% or less, A1: 0.003-0.1%, Mo: 0.010-0.30%, Nb: 0.010-0.20%, V: 0.005-0.50% %) = C + Si / 24 + Mn / 6 + Mo / 4 + V / 14 + Ni / 40 + Cr / 5 (1)
(Here, C, Si, Mn, Mo, V, Ni, Cr: content of each element (mass%) . In addition, when Ni and Cr in the formula (1) are contained as selective elements, (If the content is not contained as a selective element, the content shall be calculated as zero.)
The carbon equivalent Ceq defined in the above is adjusted to satisfy 0.46% or less, and the steel material has a composition composed of the balance Fe and inevitable impurities, and the hot rolling is performed at a temperature in the range of 1000 to 1350 ° C. And after the hot rolling step, after the hot rolling step, the range of (Ar3 transformation point-30 ° C) to (Ar3 transformation point-130 ° C). A cooling step of air-cooling to a temperature, and heat of at least one pass after the cooling step at a temperature in the range of (Ar3 transformation point −30 ° C.) to (Ar3 transformation point −130 ° C.): 1.0 to 10% A method for producing a refractory steel material excellent in high-temperature strength, which is a step of sequentially performing a precipitation inducing treatment step of performing hot rolling.

( 2 ) In ( 1 ), it replaces with the said air cooling in the said cooling process, It is set as the accelerated cooling which cools with an average cooling rate of 20 degrees C / s or less, The manufacturing method of the refractory steel materials characterized by the above-mentioned.
( 3 ) In ( 1 ) or ( 2 ), in addition to the above composition, Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Cr: 0.05 to 1.0%, B: 0.0001 to 0.003% A method for producing a refractory steel material, comprising one or more selected from among the above.

( 4 ) The method for producing a refractory steel material according to any one of ( 1 ) to ( 3 ), further comprising Ti: 0.003 to 0.02% by mass% in addition to the above composition.
( 5 ) In any one of ( 1 ) to ( 4 ), in addition to the above composition, in addition to mass, one or two selected from Ca: 0.0005 to 0.005% and REM: 0.0005 to 0.005% A method for producing a refractory steel material, comprising:

( 6 ) In any one of ( 1 ) to ( 5 ), in addition to the above composition, in addition to the above composition, in addition, by mass%, selected from Zr: 0.0001 to 0.003%, Mg: 0.0001 to 0.003% A method for producing a refractory steel material characterized by containing one or two of them.

  According to the present invention, even with a composition that contains Nb, V, and Mo in an essential manner and the Mo content is reduced as much as possible, a desired high-temperature strength can be secured stably, and a steel material having excellent high-temperature strength can be easily obtained. It can be manufactured inexpensively and stably, and has a remarkable industrial effect.

First, the reasons for limiting the composition of the steel of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.
C: 0.01 to 0.1%
C is an element that increases the strength of steel. In the present invention, Nb (C, N) and V (C, N) are effectively precipitated in a rolled state, and at a predetermined amount during high-temperature heating (fire). It is an indispensable element for precipitating the above Mo carbides and increasing the high temperature strength. In order to acquire such an effect, 0.01% or more of content is required. On the other hand, even if it contains exceeding 0.1%, the effect is saturated and weldability falls. Therefore, C is limited to a range of 0.01 to 0.1%. In addition, Preferably, it is 0.02 to 0.08%.

Si: 0.01-1.0%
Si is an effective element that increases the strength by solid solution. In order to acquire such an effect, 0.01% or more of content is required. On the other hand, if the content exceeds 1.0%, the toughness of the heat affected zone (HAZ) is significantly reduced. For this reason, Si was limited to the range of 0.01 to 1.0%. In addition, Preferably, it is 0.05%-0.6%.

Mn: 0.1-2.0%
Mn, like Si, is a relatively inexpensive element that increases strength, and is an important element for increasing the strength of steel for welded structures. In order to obtain such an effect, the content of 0.1% or more is required. Below 0.1%, the effect is small. On the other hand, if the content exceeds 2.0%, the upper bainite transformation is promoted, so that the base metal toughness and the toughness of the heat affected zone are reduced. For this reason, Mn was limited to the range of 0.1 to 2.0%. In addition, Preferably, it is 0.5 to 1.8%.

P: 0.030% or less P has a strong tendency to segregate at grain boundaries, final solidification positions, and the like, and particularly has adverse effects such as lowering the toughness of the tough ductility and weld heat affected zone at the center segregation part. For this reason, it is desirable to reduce P as much as possible. However, since the adverse effect is small if it is 0.030% or less, P is limited to 0.030% or less. In addition, Preferably, it is 0.025% or less.

S: 0.030% or less S is present in steel as sulfide inclusions such as MnS, and has an adverse effect on reducing ductility and toughness. Moreover, like P, it promotes embrittlement of the steel material. Therefore, like P, it is desirable to reduce as much as possible. However, the adverse effect is acceptable if it is 0.030% or less. For this reason, S was limited to 0.030% or less. In addition, Preferably, it is 0.025% or less.

Al: 0.003-0.1%
Al is an element that acts as a deoxidizer. In order to acquire such an effect, 0.003% or more of content is required. On the other hand, even if the content exceeds 0.1%, the effect is saturated and an effect commensurate with the content cannot be expected. For this reason, Al was limited to the range of 0.003 to 0.1%. In addition, Preferably it is 0.010 to 0.050%.

Mo: 0.010-0.30%
Mo is an element that forms Mo carbides in the event of a fire (when heated at 600 ° C.), has the effect of stably ensuring high-temperature strength, and is an effective element for fully exhibiting the fire resistance performance of steel. In order to secure such an effect, the content of 0.010% or more is required. On the other hand, if the content exceeds 0.30%, the material cost increases. For this reason, Mo was limited to the range of 0.010 to 0.30%. In addition, Preferably, it is 0.05%-0.30%.

Nb: 0.010-0.20%
Nb is an extremely important element in the present invention. In the present invention, a large amount of fine Nb (C, N) is precipitated in the rolled and cooled steel material to promote the precipitation of Mo carbides at the time of fire. Thereby, even if there is little Mo content, the high temperature intensity | strength obtained can be made high and Mo content can be reduced. In order to obtain such an effect, a content of 0.010% or more is required. On the other hand, if the content exceeds 0.20%, the effect is saturated, and not only the effect commensurate with the content can be expected, but also coarse Nb (C, N) precipitates, the steel material becomes brittle, and the welded part also has There are adverse effects such as embrittlement. For this reason, Nb was limited to 0.010 to 0.20% of range. In addition, Preferably it is 0.015 to 0.15%.

V: 0.005-0.50%
V precipitates finely as V (C, N), increases the normal temperature strength and the high temperature strength, and contributes to an increase in the strength of the steel material. Also, like Nb, it finely precipitates as V (C, N) and promotes the precipitation of Mo carbides at the time of fire. Thereby, even if there is little Mo content, the high temperature intensity | strength obtained can be made high and Mo content can be reduced. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, if the content exceeds 0.50%, the surface quality deteriorates and the welded thermal shadow becomes brittle. For this reason, V was limited to the range of 0.005 to 0.50%. In addition, Preferably it is 0.010 to 0.30%.

  The above components are basic components. In addition to the basic components, Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Cr: 0.05 to 1.0%, B: 0.0001 to 0.003% One or more selected from among them and / or Ti: 0.003-0.02% and / or Ca: 0.0005-0.005%, REM: 0.0005-0.005% selected from one or two Species and / or one or two selected from Zr: 0.0001 to 0.003%, Mg: 0.0001 to 0.003% can be selected and contained.

One or more selected from Cu: 0.01-1.0%, Ni: 0.01-1.0%, Cr: 0.05-1.0%, B: 0.0001-0.003%
Cu, Ni, Cr, and B are all elements that increase the strength of the steel material, and can be selected to contain one or more as required.
Cu: 0.01-1.0%
When Cu is contained up to about 0.5%, it contributes to increasing the strength of the steel material by solid solution strengthening, and beyond that by precipitation strengthening. In order to acquire such an effect, 0.01% or more of content is required. On the other hand, the content exceeding 1.0% deteriorates the surface quality of the steel material. For this reason, when it contains, it is preferable to limit Cu to 0.01 to 1.0% of range.

Ni: 0.01-1.0%
Ni contributes to increasing the strength of steel by solid solution strengthening, and also contributes to improving low temperature toughness. In order to acquire such an effect, it is preferable to contain 0.01% or more, but even if it contains exceeding 1.0%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Become. For this reason, when it contains, it is preferable to limit Ni to 0.01 to 1.0%.

Cr: 0.05-1.0%
Cr contributes to an increase in the strength of the steel material by solid solution strengthening and has an action of suppressing the decomposition of cementite, thereby contributing to an improvement in softening resistance during tempering. In order to acquire such an effect, it is desirable to contain 0.05% or more, but when it contains exceeding 1.0%, weldability will fall, scale peelability will be inhibited and surface quality will fall. For this reason, when it contains, it is preferable to limit Cr to 0.05 to 1.0% of range.

B: 0.0001-0.003%
B is contained in a small amount and is an effective element that enhances hardenability, and contributes to increasing the strength of steel materials such as thick-walled H-section steel. In order to acquire such an effect, it is desirable to contain 0.0001% or more, but inclusion exceeding 0.003% forms carbonitride and reduces toughness. For this reason, when it contains, it is preferable to limit B to 0.0001 to 0.003% of range.

Ti: 0.003-0.02%,
Ti is an element that forms TiN, refines the microstructure of the high heat input welding heat-affected zone, and contributes effectively to improving toughness, and can be contained as necessary. In order to obtain such an effect, it is preferable to contain 0.003% or more, but if it exceeds 0.02%, coarse TiN is likely to precipitate, and precipitates containing Nb and V (Nb carbonitride and Nb V fine carbon dispersion is inhibited. For this reason, when it contains, it is preferable to limit Ti to 0.003 to 0.02% of range.

One or two selected from Ca: 0.0005-0.005%, REM: 0.0005-0.005%
Both Ca and REM are elements that contribute effectively to the inclusion shape control, and are particularly effective in improving the toughness and ductility of segregated parts and toughness of weld heat affected parts through the form control of inclusions. It is an element which contributes, and it can contain 1 type or 2 types by selecting as needed. In order to acquire such an effect, it is desirable to contain 0.0005% or more of each, but the content exceeding 0.005% respectively reduces the cleanliness of steel. For this reason, when it contains, it is preferable to limit to Ca: 0.0005-0.005% and REM: 0.0005-0.005%, respectively.

One or two selected from Zr: 0.0001-0.003%, Mg: 0.0001-0.003%
Zr and Mg are elements that form oxides and contribute to improving the toughness of the high heat input weld zone by refining the microstructure of the high heat input weld zone. Or it can contain 2 types. In order to acquire such an effect, it is desirable to contain 0.0001% or more of each, but if it contains more than 0.003%, the cleanliness of the steel material is lowered. For this reason, when it contains, it is preferable to limit to the range of Zr: 0.0001-0.003% and Mg: 0.0001-0.003%.

In the steel of the present invention, the components in the above range are further divided into the following formula (1): carbon equivalent Ceq (%) = C + Si / 24 + Mn / 6 + Mo / 4 + V / 14 + Ni / 40 + Cr / 5 (1)
(Here, C, Si, Mn, Mo, V, Ni, Cr: Content of each element (mass%))
The carbon equivalent Ceq defined by is adjusted to satisfy 0.46 or less. When the carbon equivalent Ceq defined by the above formula (1) exceeds 0.46, weldability and weld heat affected zone toughness are lowered. For this reason, the carbon equivalent Ceq was limited to 0.46 or less. Preferably it is 0.44 or less. In addition, about Ni and Cr in (1) Formula, when it contains as a selective element, the content shall be calculated as zero when not containing as a selective element.

The balance other than the components described above consists of Fe and inevitable impurities. Inevitable impurities include O: 0.0030% or less and N: 0.0070% or less.
Next, the reason for limiting the structure of the steel material of the present invention will be described.
The steel material of the present invention has the above-described composition, and further, Nb carbonitride and V carbonitride are precipitates having a particle size of less than 20 nm by mass% in the steel. 0.15% has a precipitated structure.

When the particle size of Nb carbonitride and V carbonitride is 20 nm or more, the contribution to the desired high temperature strength improvement is small. In the present invention, the particle sizes of Nb carbonitride and V carbonitride are limited to less than 20 nm.
The amount of Nb carbonitride and V carbonitride precipitated as precipitates with a particle size of less than 20 nm in steel is the total amount in terms of Nb and V, that is, the amount of Nb and V deposited as precipitates with a particle size of less than 20 nm. However, if it is less than 0.03% in total, the precipitation of Mo carbide during high-temperature heating is not sufficiently promoted. On the other hand, if it exceeds 0.15%, the yield ratio increases and the earthquake resistance of the structure decreases. Moreover, in order to precipitate such fine Nb precipitates and fine V carbonitrides in large quantities, it is necessary to highly manage the hot rolling conditions and the subsequent cooling conditions, thereby reducing productivity. For this reason, the amount of precipitation of Nb carbonitride and V carbonitride having a particle size of less than 20 nm in the as-rolled state, that is, the total amount of Nb and V precipitated as precipitates of particle size of less than 20 nm is 0.03 to Limited to a range of 0.15%. In addition, Preferably it is 0.03-0.12%.

The amount of precipitation of Nb carbonitride and V carbonitride having a particle size of less than 20 nm is determined as follows.
A test piece for electrolytic extraction is collected from the target steel material, and the electrolytic residue is extracted using the electrolytic extraction residue method to determine the amount of precipitated Nb and the amount of precipitated V. In the electrolytic extraction residue method, first, the test piece is subjected to constant current electrolysis in a 10% AA-based electrolytic solution (10 vol% acetylacetone-1 mass% tetramethylammonium chloride / methanol). Then, the obtained electrolytic solution is filtered, and the filtered electrolytic solution is analyzed using an ICP emission spectroscopic analyzer, and the Nb amount and the V amount in the electrolytic solution are respectively measured.

  In addition, filtration of electrolyte solution first captures residues (particles) having a size of 100 nm or more with a 100 nm porous filter, then filters using a 20 nm porous filter, and has a size of 100 nm to 20 nm. Residues (particles) were captured, and then the residue (particles) that could not be captured by the 20 nm filter was analyzed as Nb and V amounts for the resulting filtered electrolyte solution as fine precipitates of less than 20 nm. To do. Then, the total amount of Nb and V obtained is divided by the electrolysis weight to obtain the amount of Nb and V (mass%) precipitated as fine precipitates having a particle size of less than 20 nm.

Below, the preferable manufacturing method of this invention refractory steel material is demonstrated.
The steel material having the above composition is subjected to a hot rolling step, a cooling step, and a precipitation promoting treatment step sequentially as hot rolling to obtain a steel material having a predetermined size and shape.
About the manufacturing method of the steel material, it is not particularly limited, and the molten steel having the above composition is melted by using a conventional melting method such as a converter, and a conventional casting method such as a continuous casting method, a slab, It is preferable to use a steel material such as a beam blank.

The steel material is first reheated and then subjected to a hot rolling process.
The heating temperature for hot rolling is a temperature in the range of 1000 to 1350 ° C. When the heating temperature is less than 1000 ° C., the solid solution of Nb is insufficient, fine Nb precipitates cannot be dispersed in a large amount, and Mo carbide precipitation acceleration cannot be achieved. On the other hand, at a high temperature exceeding 1350 ° C., scale loss increases, yield decreases, and fuel intensity for heating decreases. For this reason, the reheating temperature of the steel material was limited to a temperature in the range of 1000 to 1350 ° C. In addition, Preferably it is 1100-1250 degreeC. From the viewpoint of complete solid solution of Nb, it is preferably 1200 ° C. or higher.

  In the hot rolling process, the heated steel material is hot-rolled with a reverse rolling mill such as a thick plate rolling mill or an H-shaped steel rolling mill to obtain a desired steel plate such as a thick steel plate or H-shaped steel. . Hot rolling is rolling in the austenite region with a rolling end temperature of 850 ° C. or higher. When the rolling end temperature is less than 850 ° C., the structure becomes finer and the yield ratio at room temperature tends to increase, and the earthquake resistance of the structure decreases. For this reason, the rolling end temperature of hot rolling was limited to 850 ° C. or higher. In addition, Preferably it is 900 degreeC or more.

After completion of the hot rolling process, the obtained steel material is then subjected to a cooling process. The cooling step is a step of air cooling to a temperature in the range of (Ar ₃ transformation point−30 ° C.) to (Ar ₃ transformation point−130 ° C.). Note that accelerated cooling may be used instead of air cooling. A reduction in rolling efficiency can be suppressed by performing accelerated cooling instead of air cooling. For this reason, the accelerated cooling is preferably cooling at a cooling rate of 20 ° C./s or less. On the contrary, if the cooling is accelerated beyond 20 ° C./s, the amount of fine Nb and V precipitates decreases.

  After the cooling step, a precipitation induction treatment step is performed. The precipitation inducing treatment step is a step of performing at least one pass of hot rolling at a reduction ratio of 1.0 to 10% in a temperature range of (Ar3 transformation point −30 ° C.) to (Ar3 transformation point −130 ° C.). . As a result, Nb (C, N), which is an Nb precipitate, and V (C, N), which is a V precipitate, are strain-induced and precipitate finely and in large quantities in the α phase. This increases the strength at room temperature and promotes precipitation of Mo carbides when heated to a high temperature during a fire, increasing the high-temperature proof stress.

At a high temperature at which hot rolling at least one pass exceeds (Ar3 transformation point −30 ° C.), the amount of Nb and V precipitates produced is small. On the other hand, at a low temperature lower than (Ar3 transformation point -130 ° C), the rolling load increases and hot rolling is often difficult, and the yield strength increases and the earthquake resistance decreases due to the increase in processed ferrite. To do. For this reason, the temperature of the precipitation promoting treatment was limited to a temperature range of (Ar3 transformation point−30 ° C.) to (Ar3 transformation point−130 ° C.). The temperature range is preferably from (Ar3 transformation point −30 ° C.) to (Ar3 transformation point −100 ° C.), more preferably from (Ar3 transformation point −30 ° C.) to (Ar3 transformation point −70 ° C.). It is. The Ar3 transformation point is
Ar3 transformation point (℃) = 910−273C + 25Si−74Mn−54Ni−8Cu−15Cr−10Mo
(Here, C, Si, Mn, Ni, Cu, Cr, Mo: content of each element (mass%))
It shall be calculated using In addition, when the element described in the above formula is not included, the calculation is performed with the element as zero.

In order to efficiently generate strain-induced precipitation of Nb and V precipitates up to the center of the wall thickness, in the precipitation induction treatment step, the surface temperature and the center temperature of the steel material are reduced after at least one pass of hot rolling. It is desirable that the temperature difference be 30 ° C. or higher, that is, the surface temperature be 30 ° C. lower than the center temperature.
Further, when the rolling reduction of at least one pass of hot rolling is less than 1.0%, the amount of strain-induced precipitation of Nb and V precipitates is small. On the other hand, if it exceeds 10%, the rolling load increases and hot rolling becomes difficult. For this reason, the rolling reduction of at least one pass of hot rolling is limited to a range of 1.0 to 10%. It is preferably 2 to 8%.

After the precipitation inducing treatment step, it is cooled to room temperature by air cooling.
Hereinafter, based on an Example, this invention is demonstrated further.

Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab (slab or beam blank) by a continuous casting method. These slabs are heated to the heating temperature shown in Table 2, and then subjected to a hot rolling step, a cooling step, and a precipitation inducing treatment step under the conditions shown in Table 2, and a steel material (thickness) shown in Table 2 (thickness) Steel plate, H-shaped steel). From the obtained steel, in accordance with the provisions of JIS Z 2201, a JIS 1A test piece is collected so that the direction parallel to the rolling direction is the length direction of the test piece, and at room temperature in accordance with the provisions of JIS Z 2241. A tensile test was performed at (25 ° C.) to determine tensile properties (yield strength YP, tensile strength TS, yield ratio). In addition, in accordance with the provisions of JIS G 0567, a high-temperature tensile test piece (diameter: 10 mmφ, GL: 50 mm) is taken from the obtained steel so that the direction parallel to the rolling direction is the length direction of the test piece. In accordance with the provisions of JIS G 0567, a high temperature tensile test (test temperature: 600 ° C.) was carried out to obtain a high temperature proof stress (600 ° C.) (0.2% proof stress σ ₆₀₀ ).

  Moreover, the test piece was extract | collected from the rolled state of the obtained steel materials, and the electrolytic extraction residue analysis of Nb and V was performed. Electrolytic extraction conditions were 10% AA electrolyte (10vol% acetylacetone-1mass% tetramethylammonium chloride / methanol) under constant current electrolysis, and the resulting electrolyte was finally used with a 20nm porous filter. Filter and analyze the filtered electrolyte using an ICP emission spectrophotometer, measure the amount of Nb and V in the electrolyte, divide the amount of Nb and V obtained by the electrolytic weight, and less than 20 nm The amount of Nb and V in the form of fine precipitates (the amount of precipitated Nb and the amount of precipitated V (mass%)) was used.

In addition, the obtained steel material (as-rolled) was further subjected to heat treatment at 600 ° C. for 30 minutes, and an electrolytic extraction residue analysis of Mo was performed. Electrolytic extraction residue analysis was performed in the same manner as Nb electrolytic extraction residue analysis, and a fine amount of precipitated Mo (% by mass) of less than 20 nm was obtained. Using these values, the ratio of the obtained precipitated Mo amount to the Mo content and the precipitated Mo ratio (Mo yield) were calculated.
Table 4 shows the obtained results.

In all of the examples of the present invention, the ratio of precipitated Mo is high, and high high temperature proof stress (proof strength at 600 ° C.) can be secured. In all of the examples of the present invention, excellent fire resistance can be secured with a low Mo content. On the other hand, in the comparative example which is out of the scope of the present invention with few fine precipitates of less than 20 nm, the precipitated Mo ratio is low, and the desired excellent fire resistance performance cannot be secured .

Claims (6)

In hot rolling the steel material into a steel material of a predetermined size,
The steel material in mass%,
C: 0.01 to 0.1%, Si: 0.01 to 1.0%,
Mn: 0.1 to 2.0%, P: 0.030% or less,
S: 0.030% or less, A1: 0.003-0.1%,
Mo: 0.010 to 0.30%, Nb: 0.010 to 0.20%,
V: 0.005-0.50%
Is adjusted so that the carbon equivalent Ceq defined by the following formula (1) satisfies 0.46% or less, and a steel material having a composition consisting of the balance Fe and inevitable impurities,
After the hot rolling is heated to a temperature in the range of 1000 to 1350 ° C., a hot rolling step in which the rolling end temperature is 850 ° C. or higher, and after the hot rolling step,
A cooling step of air-cooling to a temperature in the range of (Ar3 transformation point-30 ° C) to (Ar3 transformation point-130 ° C), and (Ar3 transformation point-30 ° C) to (Ar3 transformation point-130 ° C) after the cooling step. A precipitation inducing treatment step in which at least one pass of hot rolling is performed at a temperature in the range of 1.0 to 10%;
Sequentially subjected to the process der the is,
The steel material is a steel material having the above composition and a structure in which the total amount of Nb and V precipitated as precipitates having a particle size of less than 20 nm in mass% in the steel is 0.03 to 0.15%. A method for producing refractory steel with excellent high-temperature strength.
Carbon equivalent Ceq (%) = C + Si / 24 + Mn / 6 + Mo / 4 + V / 14 + Ni / 40 + Cr / 5 (1)
Here, C, Si, Mn, Mo, V, Ni, Cr: Content of each element (% by mass) .
In addition, about Ni and Cr in (1) Formula, when it contains as a selective element, the content shall be calculated as zero when not containing as a selective element. Wherein instead of the air cooling in the cooling step, the production method of the refractory steel according to claim 1, characterized in that the accelerated cooled cooling at an average at 20 ° C. / s or less cooling rate. In addition to the above composition, in addition to mass, one or more selected from Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Cr: 0.05 to 1.0%, B: 0.0001 to 0.003% The method for producing a refractory steel material according to claim 1 or 2 , characterized by comprising: The method for producing a refractory steel material according to any one of claims 1 to 3 , further comprising Ti: 0.003 to 0.02% by mass% in addition to the composition. In addition to the above composition, by mass%, Ca: 0.0005 to 0.005%, REM: claims 1, characterized by containing one or two selected from among 0.0005 to 0.005% to either 4 A method for producing a refractory steel material according to claim 1. In addition to the above composition, by mass%, Zr: 0.0001 to 0.003%, Mg: claims 1, characterized by containing one or two selected from among 0.0001 to 0.003% to either 5 A method for producing a refractory steel material according to claim 1.