JP2007211278A - Fire-resistant thick steel plate and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fire-resistant thick steel plate which has excellent short-time high-temperature strength in a temperature range not higher than 700°C even without the addition of Mo thereto and also has improved base-material toughness and weld heat-affected zone toughness and also to provide a manufacturing method therefor. <P>SOLUTION: The fire-resistant thick steel plate has a composition which consists of, by mass, 0.02 to 0.2% C, 0.03 to 1% Si, 0.3 to 2% Mn, 0.001 to 0.05% Al, 0.005 to 0.2% Nb, 0.03 to 0.3% V, 0.005 to 0.03% N, P and S both limited to ≤0.02% and ≤0.01%, respectively, and the balance Fe with inevitable impurities and also contains, if necessary, one or more elements selected from the group consisting of Ti, Ta, Zr, Cr, W, Ni, Cu, B, Mg, Ca and REM. Further, the steel plate has a microstructure containing ≥20 vol.%, in total, of bainite and martensite. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a refractory thick steel plate used as a structural member of a welded structure in the fields of architecture, civil engineering, offshore structures, shipbuilding, storage tanks, and the like, and a method for manufacturing the same.

  In general, building steel structures are designed to ensure safety in the event of a fire in response to the results of the Ministry of Construction's comprehensive technology development project “Development of fire prevention design methods for buildings” (Fire Protection General Professional) reported in 1987. In view of the high temperature strength of steel and the load actually applied to the construction steel structure itself, the design considering the fire resistance of the steel is required.

  For these reasons, as steel materials for building steel structures, in the fireproof design of building steel structures, the cost reduction required for fireproof coating such as rock wool on the steel material surface, the omission of the construction process, and the appearance From the above point of view, fireproofing can be omitted, and the use of refractory steel, which has higher proof stress at a higher temperature than ordinary steel and has excellent fireproof performance, is expanding.

  Depending on the design philosophy of each building steel structure, for fireproof applications, the yield strength or yield strength is usually (1/2) to (2/3) or more of the standard yield point (room temperature yield point) at high temperatures. Use of the steel material which has is useful. Therefore, conventionally, a method for producing a low yield ratio steel material for construction with an increased short-time high-temperature strength in a temperature range of 600 ° C. has been developed (see, for example, Patent Document 1) and applied. In the manufacturing method of the low yield ratio steel for construction described in Patent Document 1, C: 0.04-0.15%, Si: 0.6% or less, Mn: 0.5-1. 6%, Nb 0.005 to 0.04%, Mo: 0.4 to 0.7%, Al: 0.1% or less and N: 0.001 to 0.006%, the balance being Fe and inevitable After heating a steel slab composed of mechanical impurities in a temperature range of 1100 to 1300 ° C. and performing hot rolling at an end temperature of 800 to 1000 ° C., a coating thickness of 20 to 50% of the uncoated or conventional fireproof coating Now we have a steel that can achieve the fireproof purpose.

  Recently, a refractory steel having refractory characteristics in a higher temperature range up to 800 ° C. has been proposed (see, for example, Patent Document 2). The 490 MPa class high-strength steel described in Patent Document 2 has a steel component of C: 0.005% or more and less than 0.04% in order to obtain excellent high-temperature strength and weldability in a temperature range of 600 to 800 ° C., Si : 0.5% or less, Mn: 0.1 to 0.5%, P: 0.02% or less, S: 0.01% or less, Mo: 0.3 to 1.5%, Nb: 0.03 -0.15%, B: 0.0005-0.003%, Al: 0.06% or less and N: 0.006% or less, with the balance being Fe and inevitable impurities, steel temperature When T is in the range of 600 to 800 ° C., the high temperature normal temperature yield stress ratio p (= high temperature yield stress / normal temperature yield stress) obtained by making the yield stress at high temperature dimensionless by the normal temperature yield stress satisfies the following formula (1). I have to.

Japanese Patent Laid-Open No. 2-77523 JP 2004-43961 A

  However, all of the above-described conventional refractory steels have a Mo content of about 1% (0.4 to 0.7% for the steel described in Patent Document 1 and Patent Document 2) in order to obtain the desired fire resistance characteristics. However, Mo is an expensive element, and when a large amount of Mo is added, the toughness of the steel material, particularly the weld heat affected zone (HAZ: Heat) Affected Zone) has the problem of adversely affecting the toughness. For this reason, from the viewpoint of reducing manufacturing costs and securing HAZ toughness, it is desired to develop a thick steel plate that has excellent fire resistance characteristics and excellent high-temperature strength without adding Mo.

  The present invention has been devised in view of the above-mentioned problems, and an excellent short-time high-temperature strength can be obtained in a temperature range of 700 ° C. or less without adding Mo, and the base material toughness and welding can be achieved. It aims at providing the fire-resistant thick steel plate with favorable heat-affected zone toughness, and its manufacturing method.

  The refractory thick steel plate according to the present invention is in mass%, C: 0.02 to 0.2%, Si: 0.03 to 1%, Mn: 0.3 to 2%, Al: 0.001 to 0.00. 05%, Nb: 0.005 to 0.2%, V: 0.03 to 0.3% and N: 0.005 to 0.03%, P: 0.02% or less, and S: It is limited to 0.01% or less, and the balance is Fe and inevitable impurities, and the volume fraction of bainite and martensite in the microstructure is 20% or more in total.

  In the present invention, the steel composition is optimized, and the total volume fraction of bainite and martensite in the microstructure is 20% or more, so that the high temperature strength can be improved by precipitation strengthening with V carbonitride. In addition, the room temperature strength can be maintained at a desired level as a welded structural steel material. Thereby, even if it does not add Mo, while being able to improve high temperature strength for a short time, both base material toughness and welding heat affected zone toughness can be made favorable.

  This refractory thick steel plate is 1% selected from the group consisting of Ti: 0.002 to 0.02%, Ta: 0.002 to 0.2% and Zr: 0.002 to 0.2% by mass%. It may contain seeds or two or more elements.

  Further, in terms of mass%, Cr: 0.01 to 3%, W: 0.01 to 3%, Ni: 0.01 to 3%, Cu: 0.01 to 1.5%, and B: 0.0003 to One or two or more elements selected from the group consisting of 0.005% can also be contained.

  Furthermore, 1% selected from the group consisting of Mg: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, and REM (rare earth element): 0.0005 to 0.05% by mass%. It may contain seeds or two or more elements.

  The manufacturing method of the fire-resistant thick steel plate which concerns on this invention is the mass%, C: 0.02-0.2%, Si: 0.03-1%, Mn: 0.3-2%, Al: 0.001. -0.05%, Nb: 0.005-0.2%, V: 0.03-0.3% and N: 0.005-0.03%, P: 0.02% or less And S: After restricting to 0.01% or less, the steel slab or slab of the composition which the remainder consists of Fe and an inevitable impurity is heated to 1150-1300 degreeC, The cumulative reduction rate in 1100 degrees C or less is 10-90. %, Hot rolling is performed at a rolling end temperature of Ar3 transformation point or higher.

  In the present invention, the composition of the steel slab or slab is optimized, and after heating to 1150 to 1300 ° C., the cumulative rolling reduction at 1100 ° C. or lower is set to 10 to 90%, and the rolling end temperature is set to the Ar3 transformation point or higher. Since it is hot-rolled, the volume fraction of bainite and martensite in the microstructure of the thick steel plate can be made 20% or more in total. Accordingly, the high temperature strength can be improved by precipitation strengthening with V carbonitride, and the room temperature strength can be maintained at a desired level as a steel material for welded structure. It is possible to produce a thick steel plate having excellent high-temperature strength for a short time in a temperature range of 0 ° C. or less and excellent base material toughness and weld heat affected zone toughness.

  In this method for producing a refractory thick steel plate, tempering can be performed at 400 to 650 ° C. after the hot rolling.

  Alternatively, after the hot rolling, accelerated cooling may be performed from the rolling end temperature to a temperature range of 550 ° C. or lower at a cooling rate of 3 to 100 ° C./second. In that case, it can also temper at 400-650 degreeC after the said accelerated cooling.

  Further, the steel slab and the slab are further in mass%, from Ti: 0.002 to 0.02%, Ta: 0.002 to 0.2%, and Zr: 0.002 to 0.2%. One or two or more elements selected from the group may be contained.

  Furthermore, the said steel slab and the said slab are the mass%, Cr: 0.01-3%, W: 0.01-3%, Ni: 0.01-3%, Cu: 0.01-1. One or two or more elements selected from the group consisting of 5% and B: 0.0003 to 0.005% may be added.

  Still further, the steel slab and the slab are composed of Mg: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, and REM: 0.0005 to 0.05% by mass. One or more elements selected from the group can be contained.

  According to the present invention, since the steel composition is optimized and the volume fraction of bainite and martensite in the microstructure is 20% or more in total, welding with room temperature strength and the like without containing expensive Mo is performed. Sufficient characteristics as a structural steel plate can be obtained, and a Mo-free refractory steel plate having excellent base metal toughness and HAZ toughness with excellent short-time high-temperature strength at 700 ° C. or less can be obtained.

  Hereinafter, the best mode for carrying out the present invention will be described in detail. When using refractory steel as a thick steel plate for welded structures used in construction steel structures, etc., it is necessary to ensure a sufficiently high high-temperature strength after ensuring the desired room temperature strength and good toughness. In conventional refractory steels, by adding appropriate amounts of precipitation strengthening elements such as Mo and Nb, high temperature strength is suppressed while suppressing excessive rise in room temperature strength and deterioration of base metal toughness and HAZ toughness due to these precipitation strengthening elements. The method of improving is taken. Among these precipitation strengthening elements, especially Mo is in a solid solution state at room temperature, so it does not cause an excessive increase in room temperature strength, and the amount of precipitation increases in the high temperature range of 600 to 700 ° C., which requires fire resistance. High strength. For this reason, in conventional fireproof steel, Mo is an essential element from the viewpoint of achieving both room temperature strength and high temperature strength.

  However, the present inventor examined the high-temperature strengthening mechanism of Mo added as an essential element in the conventional refractory steel, and the precipitation strengthening by Mo is sufficient in the temperature range of 600 ° C. or lower, but 600 ° C. In order to increase the high temperature strength of the steel material in the temperature range exceeding about 700 ° C., the conventional addition amount is insufficient, and it is necessary to add a large amount of Mo and to enhance the strengthening mechanism mainly by solid solution strengthening of Mo Found that there is. Further, the inventor of the present invention has high hardenability compared to other precipitation strengthening elements. In particular, in order to significantly harden HAZ, Mo is used to improve the high temperature strength of the steel material in a temperature range up to about 700 ° C. It was also confirmed that there is a problem that the HAZ toughness and weldability deteriorate when added.

  Therefore, the present inventor diligently investigated Mo-free refractory steel using a high-temperature strengthening mechanism that can achieve both room temperature strength and high-temperature strength without adding Mo and can achieve good HAZ toughness. Went. As a result, by including a predetermined amount or more of a bainite phase and a martensite phase having a high dislocation density in the microstructure, precipitation strengthening mainly by V carbonitride without using a high temperature strength development mechanism by Mo, It has been found that high temperature strength can be improved and toughness can be improved in a temperature range up to 700 ° C. while maintaining room temperature strength at a desired level as a steel material for welded structures. V and Nb form not only pure carbides and nitrides but also carbonitrides in which C and N are dissolved, and further, any one of these precipitates is used as a precipitation nucleus. A composite precipitate is also formed in which precipitates are deposited, but the effect on the material of the steel sheet is almost the same for any precipitate. Therefore, in the following explanation, all of these are referred to as carbonitrides or precipitates. Collectively.

  The refractory thick steel plate (hereinafter, also simply referred to as a thick steel plate) of the present invention is based on these findings, and is in mass%, C: 0.02 to 0.2%, Si: 0.03. -1%, Mn: 0.3-2%, Al: 0.001-0.05%, Nb: 0.005-0.2%, V: 0.03-0.3% and N: 0.00. 005 to 0.03%, P: 0.02% or less and S: 0.01% or less, the balance is composed of Fe and inevitable impurities, and bainite and marten in the microstructure The volume fraction of the site is over 20% in total.

  First, the reason for limiting the component composition in the thick steel plate of the present invention will be described. Unless otherwise specified, “%” shown below means “% by mass”.

  The thick steel plate of the present invention has C: 0.02 to 0.2%, Si: 0.03 to 1%, Mn: 0.3 to 2%, Al: 0.001 to 0.05 as basic chemical components. %, Nb: 0.005 to 0.2%, V: 0.03 to 0.3% and N: 0.005 to 0.03%. Further, P, which is an impurity, is limited to 0.02% or less and S is limited to 0.01% or less, and the balance is Fe and inevitable impurities.

C: 0.02-0.2%
C is an element that greatly affects the strength and toughness of the thick steel plate. However, when the C content is less than 0.02%, neither the room temperature strength nor the high temperature strength of the thick steel plate can be sufficiently increased. On the other hand, when the C content in the thick steel plate exceeds 0.2%, the toughness of the base material and the HAZ is remarkably deteriorated and the weldability is also deteriorated. For this reason, in this invention, C content in a thick steel plate is limited to 0.02-0.2%.

Si: 0.03 to 1%
Si is a deoxidizing element and is an important element for maintaining the soundness of steel. Si also has the effect of increasing the strength of the thick steel plate by solid solution strengthening. However, if the Si content in the thick steel sheet is less than 0.03%, deoxidation becomes insufficient, and the possibility of degrading the toughness by degrading the cleanliness of the thick steel sheet is increased, and further, the increase in strength is not increased. It becomes clear. On the other hand, if the Si content exceeds 1%, the HAZ toughness is particularly deteriorated, which is not preferable. Therefore, in the thick steel plate of the present invention, the Si content is limited to 0.03 to 1%.

Mn: 0.3-2%
If used properly, Mn is an element that can simultaneously improve the strength and toughness of a thick steel plate through refinement of the structure. However, if the Mn content is less than 0.3%, the structure is not sufficiently refined and the improvement of strength and toughness does not appear clearly. On the other hand, if the Mn content exceeds 2%, the hardenability becomes excessive, the strength of the thick steel plate becomes excessive, the base material toughness is deteriorated, and the HAZ toughness and weldability are also deteriorated. For this reason, in the thick steel plate of this invention, Mn content shall be 0.3 to 2% of range.

Al: 0.001 to 0.05%
Al is a deoxidizing element like Si, and it is necessary to contain 0.001% or more in order to maintain the cleanliness of the thick steel plate. On the other hand, since Al forms a nitride and fixes N, the amount of addition should be limited in the steel plate of the present invention that uses precipitation strengthening of V carbonitride described later. Specifically, when the Al content exceeds 0.05%, the precipitation strengthening effect by V carbonitride is significantly reduced due to the formation of Al nitride. Therefore, in the thick steel plate of the present invention, the Al content is set to 0.001% to 0.05%. In addition, when the Si content is sufficient and there is no possibility that the soundness of the steel is impaired due to insufficient deoxidation, the Al content is limited to 0.001 to 0.01% in order to effectively use the precipitation strengthening of V. More preferably.

Nb: 0.005 to 0.2%
Nb has the effect of stably developing high strength by transformation strengthening and precipitation strengthening in a wide temperature range from room temperature to high temperature by adding a trace amount. It is a basic element added. However, if the Nb content in the thick steel plate is less than 0.005%, the high temperature strength in the temperature range up to 700 ° C. cannot be sufficiently improved. Nb also has the effect of increasing the room temperature strength, and the addition of Nb improves the room temperature strength to the same extent as the high temperature strength. For this reason, when the Nb content in the thick steel plate exceeds 0.2%, the room temperature strength becomes excessively high, and the toughness of the base material and the HAZ is significantly deteriorated. Therefore, in the thick steel plate of the present invention, the Nb content is in the range of 0.005 to 0.2%.

V: 0.03-0.3%
V is an essential element that is added to sufficiently develop high-temperature strength in a temperature range up to 700 ° C. in the thick steel plate of the present invention, that is, Mo-free refractory steel, like Nb described above. is there. Further, in the range of the N content in the steel plate of the present invention, that is, when the N content is 0.005 to 0.03%, V carbonitride in a temperature range of 600 to 700 ° C. in a short time. In order to develop high temperature strength mainly by precipitation strengthening of V carbonitride, it is necessary to contain V by 0.03% or more. When the V content is less than 0.03%, the deposition rate of V carbonitride at 600 to 700 ° C. is not sufficient, and the strength is not manifested by short-time heating, which is a premise for ensuring fire resistance in a fire. . Moreover, even if heated and held for a long time, the amount of precipitation of V carbonitride is small, so a significant improvement in high temperature strength cannot be expected.

  On the other hand, if the V content is 0.03% or more, the higher the strength is, the higher the high temperature strength is. However, if V is excessively contained exceeding 0.3%, the following problems occur. Occurs. That is, the first problem is that V carbonitride precipitates in the product steel even at room temperature, so the room temperature strength of the steel sheet becomes excessive and the base material toughness deteriorates, and the steel plate for welded structures such as buildings. It is that it becomes impossible to secure the characteristics as. The second problem is that V carbonitrides precipitated in the product steel at room temperature are likely to be coarsened, resulting in significant deterioration in toughness. The third problem is that both HAZ toughness and weldability are significantly degraded because the HAZ is hardened significantly. For the above reasons, in the thick steel plate of the present invention, the V content is limited to 0.03 to 0.3%.

N: 0.005 to 0.03%
N combines with V described above in the steel to produce V carbonitride effective for precipitation strengthening. Thus, N is an important element for developing high-temperature strength in the temperature range up to 700 ° C. using precipitation strengthening, but V carbonitride is unnecessarily precipitated in the product steel at room temperature. First, in order to quickly deposit V carbonitride and develop high temperature strength by short-time heating at 600 to 700 ° C., it is necessary to optimize the N content with respect to the V content. Specifically, when the N content in the thick steel plate is less than 0.005%, the solubility product of V carbonitride becomes small, and the precipitation amount and precipitation rate at 600 to 700 ° C. become insufficient. On the other hand, when the N content in the thick steel plate exceeds 0.03%, V carbonitride precipitates at the room temperature stage, so that the room temperature strength becomes excessive or the V carbonitride precipitated at room temperature After that, adverse effects such as coarsening and severe deterioration of toughness, or significant hardening of HAZ toughness and weldability due to remarkable hardening of HAZ become unacceptable. For this reason, N content in the thick steel plate of this invention shall be 0.005-0.03%. In order to stably improve the high temperature strength at 600 to 700 ° C., the N content range is preferably 0.007 to 0.02%.

P: 0.02% or less P is an impurity element and inhibits toughness, so its content needs to be reduced as much as possible. In particular, if the P content in the thick steel plate exceeds 0.02%, the adverse effect on toughness exceeds the allowable range, so the P content is regulated to 0.02% or less in the thick steel plate of the present invention.

S: 0.01% or less S is an impurity element as in the above-described P, and if it is excessively present in the steel sheet, the toughness and ductility are deteriorated. Therefore, the content is preferably reduced as much as possible. In particular, if the S content in the thick steel plate exceeds 0.01%, the adverse effects on toughness and ductility exceed the allowable range. Therefore, in the thick steel plate of the present invention, the S content is restricted to 0.01% or less. .

  The above is the basic chemical composition in the thick steel plate of the present invention.In the thick steel plate of the present invention, in order to increase the high-temperature strength of the steel plate by precipitation strengthening, in addition to these basic components, if necessary, Ti: 0.002-0.02%, Ta: 0.002-0.2% and Zr: One or more elements selected from the group consisting of 0.002-0.2% Can be made.

Ti: 0.002 to 0.02%
Ti has an effect of increasing strength by precipitation strengthening, and further has an effect of generating TiN and refining the HAZ structure by a pinning effect, and is an element effective in improving HAZ toughness. When Ti is contained in a thick steel plate, if the content is less than 0.002%, the above effect does not appear clearly. Therefore, the Ti content is preferably 0.002% or more. On the other hand, Ti, like Al, has a stronger affinity with N than V and easily forms Ti nitride. Therefore, when Ti is excessively contained in excess of 0.02%, in the temperature range up to 700 ° C. This is not preferable because the above-described precipitation of V carbonitride is suppressed. Therefore, in the thick steel plate of the present invention, when Ti is contained in the thick steel plate, it is preferable to be in the range of 0.002 to 0.02%. In addition, in order to suppress the production | generation and precipitation of V carbonitride at high temperature, both Al and Ti couple | bond with N and produce | generate a nitride, When adding Ti, the total amount of Al and Ti is 0.01. It is more preferable to regulate to not more than%.

Ta: 0.002-0.2%, Zr: 0.002-0.2%
Ta and Zr exhibit an intermediate solid solution precipitation behavior between Nb and V, and when the appropriate amount is added, the same effect as Nb and V is exhibited. When adding Ta and / or Zr, it is necessary to contain 0.002% or more in order to increase the high temperature strength. On the other hand, when the Ta content or the Zr content exceeds 0.2%, an excessive increase in the room temperature strength and deterioration of the base metal toughness and the HAZ toughness are caused. Therefore, when adding Ta and / or Zr, the content is made 0.002 to 0.2%, respectively.

  In addition, in the thick steel plate of the present invention, for the purpose of adjusting the room temperature strength and toughness, and imparting other properties, as necessary, in addition to the above components, Cr: 0.01 to 3%, One or two selected from the group consisting of W: 0.01 to 3%, Ni: 0.01 to 3%, Cu: 0.01 to 1.5% and B: 0.0003 to 0.005% More than seed elements can be contained.

Cr: 0.01 to 3%
Cr has the effect of improving hardenability and improving room temperature strength. Further, when a relatively large amount of Cr is contained, the heating transformation point is increased, so that the structural stability at high temperature is improved and the high temperature strength of 700 ° C. or higher is improved. However, when the Cr content is less than 0.01%, these effects are not clearly exhibited. On the other hand, if the Cr content exceeds 3%, the toughness of the base material and the HAZ deteriorates, which is not preferable. Therefore, in the thick steel plate of the present invention, when Cr is contained, the content is set to 0.01 to 3%.

W: 0.01 to 3%
W has an effect almost similar to Cr. Therefore, when W is contained in the thick steel plate, the content is limited to 0.01 to 3% for the same reason as Cr described above.

Ni: 0.01 to 3%
By adding an appropriate amount of Ni, the strength and toughness of the thick steel plate can be improved at the same time, and Ni is an element effective for securing HAZ toughness. However, when the Ni content is less than 0.01%, these effects cannot be obtained. Moreover, when Ni is contained excessively exceeding 3%, a base material and HAZ may harden | cure excessively and may deteriorate toughness on the contrary. Therefore, in the thick steel plate of the present invention, when Ni is contained, the content is limited to 0.01 to 3%.

Cu: 0.01 to 1.5%
Cu is an element effective for improving the strength of the thick steel plate mainly by improving hardenability. However, when the Cu content is less than 0.01%, a clear effect cannot be obtained. On the other hand, when Cu is contained excessively exceeding 1.5%, the toughness of the base material and the HAZ deteriorates, and hot workability deteriorates, resulting in a problem in the productivity of the thick steel plate. Therefore, when making the thick steel plate of this invention contain Cu, the content shall be 0.01 to 1.5% of range.

B: 0.0003 to 0.005%
B is an element that is very effective for increasing the strength of thick steel plates, since the hardenability can be greatly enhanced by adding a small amount. However, when it is contained for the purpose of increasing the strength of the thick steel plate, the B content needs to be 0.0003% or more. On the other hand, if B exceeds 0.005% and HA is excessively contained, HAZ hardens abnormally, and HAZ toughness and weldability are remarkably deteriorated, and HAZ reheat embrittlement and reheat cracking susceptibility increase. It is not preferable. Therefore, in the thick steel plate of the present invention, when B is contained, the content is limited to 0.0003 to 0.005%.

  Furthermore, in the thick steel plate of the present invention, Mg: 0.0005 to 0.01%, Ca: 0.0005, in addition to the above components, as necessary, for the purpose of improving the ductility of the steel plate and improving the HAZ toughness. ˜0.01% and REM (rare earth element): One or more elements selected from the group consisting of 0.0005 to 0.05% can be contained.

  Mg, Ca, and REM all have the effect of forming inclusions containing S to render S in the steel harmless, and inclusions can be controlled by adding an appropriate amount, especially for fine dispersion of inclusions. Contribute. However, in order to ensure the effect of addition, it is necessary to contain 0.0005% or more of each. On the other hand, if the Mg content or Ca content exceeds 0.01%, or the REM content exceeds 0.05%, coarse inclusions are generated, and ductility and toughness deteriorate. Therefore, when adding Mg, Ca and / or REM, the Mg content is 0.0005 to 0.01%, the Ca content is 0.0005 to 0.01%, and the REM content is 0.0005. Limited to ~ 0.05%. In addition, when adding several REM, let the total content be the said range.

  In addition, in order to make the volume fraction of bainite and martensite in the microstructure in the steel plate of the present invention stably within the range described later, after satisfying the above-described component composition, the following formula ( The carbon equivalent (Ceq.) Defined in 2) is more preferably 0.3 to 0.5%. In the following formula (2), [C] is the C content (%), [Si] is the Si content (%), [Mn] is the Mn content (%), and [Ni] is Ni content. The amount (%), [Cr] indicates the Cr content (%), [Mo] indicates the Mo content (%), and [V] indicates the V content (%).

  When the carbon equivalent (Ceq.) Defined by the above formula (2) is less than 0.3%, sufficient hardenability cannot be obtained, and a microstructure containing 20% by volume or more of bainite and martensite in total. It may not be obtained stably. On the other hand, when the carbon equivalent (Ceq.) Defined by the above formula (2) exceeds 0.5%, the hardening of HAZ cannot be ignored, and the HAZ toughness and weldability may be deteriorated. Therefore, the carbon equivalent (Ceq.) Defined by the above formula (2) is preferably 0.3 to 0.5%.

  Next, the microstructure in the thick steel plate of the present invention will be described. In order to increase the high-temperature strength mainly by precipitation strengthening of V carbonitride without adding Mo to the steel sheet, each steel component is limited to the above-described range, and the volume fraction of bainite and martensite in the microstructure. The rate needs to be 20% or more in total.

  In the thick steel plate of the present invention, bainite and martensite in the microstructure have a higher dislocation density than ferrite and pearlite, and therefore V and N are present even in a short time in the temperature range of 600 to 700 ° C. with dislocations as the core. Bonds to form a carbonitride and is likely to precipitate. Moreover, since there are many production nuclei by a dislocation density, V carbonitride tends to precipitate finely. The precipitation strengthening of the V carbonitride can improve the high-temperature strength at 600 to 700 ° C. Furthermore, in the manufacture of thick steel plates, bainite and martensitic transformations start at a lower temperature than ferrite and pearlite, so V tends to exist in a solid solution state in steel plate products at room temperature after steel plate manufacture, There exists an effect | action which suppresses raising a intensity | strength too much and a base material toughness falling. Further, it is advantageous for fine precipitation at high temperatures of precipitation strengthening elements such as Nb other than V, and the high temperature strength expression effect of the precipitation strengthening elements in general is effectively exhibited. Furthermore, since dislocations in bainite and martensite have many movable dislocations, they also have the effect of maintaining the characteristics of a low yield ratio required for construction steel.

  In the thick steel plate of the present invention, in order to sufficiently exhibit the above-described effects, the total volume fraction of bainite and martensite in the microstructure needs to be 20% or more. When the volume fraction of bainite and martensite in the microstructure is less than 20% in total, ferrite and pearlite with low dislocation density increase, and at a high temperature of 600 to 700 ° C. for a short time, such as in a fire. In addition, the amount of V dissolved in room temperature in the steel plate product after manufacturing the steel plate cannot be sufficiently precipitated.

  In addition, when the V content and the N content are outside the above-described suitable ranges, V carbonitrides are likely to be coarsely precipitated in the steel sheet during production, and the solid solution V in the steel sheet product at room temperature is not sufficiently secured. Therefore, even if the total volume fraction of bainite and martensite in the microstructure of the steel sheet is within the above range, the amount of V carbonitride that precipitates in a short time at a high temperature of 600 to 700 ° C. is reduced. Needless to say, the effect of improving the high-temperature strength is reduced compared to the room temperature strength, and the toughness is deteriorated.

  Therefore, in the thick steel plate of the present invention, in order to increase the high temperature strength mainly by precipitation strengthening of V carbonitride without containing Mo, the component composition is limited to the above-described range, and further, the bainite in the microstructure. And the volume fraction of martensite is 20% or more in total. In addition, since the effect on the solid solution / precipitation behavior of V is not significantly different between bainite and martensite, if the volume fraction of bainite and martensite is 20% or more in total, the structure other than ferrite and pearlite In addition, the above-described effects can be effectively exhibited by bainite alone, martensite alone, or a mixed structure of both.

  In addition, the above effect is sufficiently exhibited when the total volume fraction of bainite and martensite is in the range of 20 to 100%. However, when the tensile strength level needs to be 570 MPa class or less, it is stable for architectural use. When it is desired to lower the yield ratio, the upper limit of the total volume fraction of bainite and martensite is preferably limited to 90%.

  The above is the component composition in the refractory thick steel sheet of the present invention, the structural requirements of the structure, and the reasons for limitation. As described above, in the thick steel sheet of the present invention, the steel composition is optimized, and since bainite and martensite having a high dislocation density is contained in a microstructure, a predetermined amount or more is contained, so that Mo is not added. Further, the high temperature strength can be improved by precipitation strengthening with V carbonitride, and the room temperature strength can be maintained at a desired level as a steel material for welded structure.

  Next, the manufacturing method of the refractory thick steel plate of the present invention configured as described above will be described.

  In the method for producing a thick steel plate of the present invention, C: 0.02 to 0.2%, Si: 0.03 to 1%, Mn: 0.3 to 2%, Al: 0.001 to 0.05% , Nb: 0.005 to 0.2%, V: 0.03 to 0.3% and N: 0.005 to 0.03%, P: 0.02% or less, and S: 0.0. It is limited to not more than 01%, and further, if necessary, Ti: 0.002 to 0.02%, Ta: 0.002 to 0.2%, Zr: 0.002 to 0.2%, Cr: 0.00. 01-3%, W: 0.01-3%, Ni: 0.01-3%, Cu: 0.01-1.5%, B: 0.0003-0.005%, Mg: 0.0005 -0.01%, Ca: 0.0005-0.01% and REM: One or two or more elements selected from the group consisting of 0.0005-0.05%, the balance being After heating the steel slab or slab of the composition consisting of e and inevitable impurities to 1150 to 1300 ° C., hot rolling at a cumulative reduction rate of 1 to 100 ° C. or less at 10 to 90% and a rolling end temperature of Ar 3 transformation point or higher Is a basic requirement.

  Hereinafter, the reason for limiting the numerical value of each condition in the method for producing a thick steel plate of the present invention will be described.

Heating temperature: 1150-1300 ° C
When heating the steel slab or slab before hot rolling, if the heating temperature is less than 1150 ° C., some of the precipitate-forming elements such as Nb and V are not present in the thick steel plate at room temperature after the end of rolling. Since it remains as a coarse precipitate as a solid solution, both the room temperature strength and the high temperature strength are not sufficiently high, and the base material toughness may further deteriorate. On the other hand, when the heating temperature exceeds 1300 ° C., the solution of the element is sufficient, but the grain size of the heated austenite becomes extremely coarse and is not uniformly refined even in the subsequent hot rolling. There is a high possibility that the base material toughness of the thick steel plate is not uniform and deteriorates. Further, the surface properties of the thick steel plate are also deteriorated. For the above reasons, the heating temperature of the steel slab or slab is 1150 to 1300 ° C.

Cumulative rolling reduction at 1100 ° C. or lower: 10 to 90%, rolling finish temperature: Ar3 transformation point or higher Hot rolling to obtain a thick steel plate with a desired thickness after heating a steel slab or cast slab is uniform by recrystallization of austenite In order to refine the transformation structure through refinement and introduction of dislocations into the austenite (non-recrystallization zone rolling), and to improve the strength and toughness of the thick steel plate, the cumulative rolling reduction particularly at 1100 ° C. or less is 10 to 10%. It is necessary to set it to 90% and to make the rolling end temperature equal to or higher than the Ar3 transformation point.

  At temperatures exceeding 1100 ° C., austenite grows rapidly after recrystallization, so it does not contribute to the refinement of the austenite grain size. Therefore, in the method for producing a thick steel plate of the present invention, the cumulative rolling reduction in the temperature range of 1100 ° C. or less related to the refinement of the austenite grain size is specified. When the cumulative rolling reduction in the temperature range of 100 ° C. or lower is less than 10%, austenite in the steel cannot be effectively refined by recrystallization. In addition, austenite is refined as the cumulative rolling reduction at 1100 ° C. or less increases. However, if the cumulative rolling reduction in this temperature range exceeds 90%, the effect of refining austenite is saturated and the rolling time is long. Since time is spent, productivity is reduced and the rolling mill is loaded. Furthermore, from the relationship between the thickness of the steel slab or cast slab and the thickness of the thick steel plate, setting a cumulative reduction ratio exceeding 90% causes a practical problem. Therefore, the cumulative rolling reduction at 1100 ° C. or lower is set to 10 to 90%.

  On the other hand, in hot rolling, when the rolling end temperature is low temperature rolling less than the Ar3 transformation point, the ratio of unrecrystallized austenite in the steel sheet and the introduction of dislocations in this austenite become excessive, so Nb and V There is a possibility that a part of the precipitate is excessively precipitated during or after rolling and the high-temperature strength is not sufficiently increased. In addition, when the rolling end temperature is lower than the Ar3 transformation point, the transformation structure in the steel sheet is excessively refined and the yield ratio (yield stress / tensile strength) becomes excessively high, which may not be preferable for architectural use. For this reason, in the manufacturing method of the thick steel plate of this invention, in hot rolling, rolling completion temperature is limited to an Ar3 transformation point or more. In hot rolling, in order to stably suppress the precipitation of part of Nb and V in the steel sheet due to the introduction of work dislocations and the anisotropy of the material and sufficiently increase the high temperature strength, the rolling is completed. The temperature is preferably 850 ° C. or higher.

  In the method for producing a thick steel plate of the present invention, even if some of the rolling is performed at a temperature exceeding 1100 ° C. during the hot rolling, the properties of the thick steel plate are not adversely affected as long as the above-described hot rolling conditions are satisfied. . Therefore, for the convenience of the relationship between the thickness of the steel slab or cast slab and the thickness of the thick steel plate, other than hot rolling where the cumulative rolling reduction at 1100 ° C. or less is 10 to 90% and the rolling end temperature is not less than the Ar3 transformation point In addition, there is no problem in performing rolling at a temperature exceeding 1100 ° C. In addition, before hot rolling, steel pieces or cast slabs made of ingots, slabs, etc. may be subjected to heat treatment for the purpose of partial rolling and segregation diffusion for the purpose of shape adjustment, etc. The effect of the present invention described above is not impaired by the treatment.

  In addition, the cooling conditions after the hot rolling step described above need not be particularly limited, but precipitation strengthening elements such as Nb and V contained in the steel plate product at room temperature after the production of the thick steel plate are dissolved as much as possible. In order to obtain a state and a total volume fraction of bainite and martensite in the microstructure is 20% or more, it is preferable to cool at a cooling rate equal to or higher than air cooling. Specifically, it is more preferable to cool under the condition that the average cooling rate from the end of rolling to 400 ° C. is 0.5 ° C./second or more.

  Furthermore, in the method for producing a thick steel plate of the present invention, after hot rolling, the cooling is accelerated at a cooling rate of 3 to 100 ° C./second from the cooling start temperature not lower than the Ar3 transformation point to the cooling end temperature not higher than 550 ° C. It is preferable to cool. Thus, after hot rolling, by accelerated cooling from a temperature range (cooling start temperature) above the Ar3 transformation point, the structural transformation in the steel sheet becomes the entire low temperature side, and precipitation of Nb, V, etc. is suppressed. Therefore, even when hot rolling is performed at a lower temperature, precipitation strengthening elements such as Nb and V contained in the steel plate product can be held in a solid solution state at room temperature thereafter, so that the high temperature strength of the steel plate Can be improved stably.

  At this time, if the cooling rate at the time of accelerated cooling after hot rolling is less than 3 ° C./second, the effect of transformation point reduction or microstructure refinement due to accelerated cooling is not sufficient, and appropriate bainite and / or in the steel sheet It becomes difficult to generate martensite at the above-described volume fraction, and it becomes difficult to sufficiently suppress precipitation of Nb carbonitride and V carbonitride in the steel sheet. On the other hand, as the cooling rate increases, the structure control by accelerated cooling is achieved. However, even if the cooling rate is increased as it exceeds 100 ° C./second, the above-described effect is saturated, and further cooling is performed at such an excessive cooling rate. This leads to an increase in manufacturing cost and deterioration of the steel plate shape. Therefore, in the manufacturing method of the thick steel plate of the present invention, when accelerated cooling is performed after hot rolling, the cooling rate is preferably 3 to 100 ° C./second.

  Further, in the accelerated cooling after hot rolling, when the cooling start temperature is lower than the Ar3 transformation point, precipitation elements such as Nb and V are precipitated before the start of accelerated cooling, and cannot contribute to the increase in the high temperature strength. There is a fear. On the other hand, if the accelerated cooling end temperature (stop temperature) exceeds 550 ° C., precipitation elements such as Nb and V may be deposited after the accelerated cooling is stopped. Therefore, when accelerated cooling is performed after hot rolling, it is preferable that the cooling start temperature is not lower than the Ar3 transformation point and the accelerated cooling end temperature (stop temperature) is not higher than 550 ° C.

  Furthermore, after completion of hot rolling or after accelerated cooling, the heating temperature is 400 to 650 ° C. with respect to the thick steel plate for the purpose of adjusting the room temperature strength of the thick steel plate, improving toughness, releasing residual stress, and the like. May be tempered. When tempering a thick steel plate, if the tempering temperature is less than 400 ° C., this tempering treatment has little effect on material adjustment and residual stress release. On the other hand, when the tempering temperature exceeds 650 ° C., precipitation occurs in the tempering stage, and the room temperature strength of the thick steel plate may excessively increase, which may deteriorate the toughness. Moreover, carbonitride may be in an over-aged precipitation state, and high temperature strength may not increase effectively, which is not preferable.

  The tempering holding time and cooling conditions need not be specified as long as they are in practical and industrial conditions because the influence on the material is extremely small compared to the heating temperature. From the viewpoint of suppressing coarse precipitation by increasing the amount, the holding time in tempering is preferably 48 hours or less when the heating temperature is 500 ° C. or less, and 4 hours or less when the heating temperature exceeds 500 ° C., and cooling conditions About, it is preferable to set it as the cooling rate more than air cooling or air cooling.

  As described above, in the method for producing a thick steel plate according to the present invention, a steel slab or cast slab whose composition is optimized is heated to 1150 to 1300 ° C., and then the cumulative rolling reduction at 1100 ° C. or less is 10 to 90%. In addition, since hot rolling is performed at a rolling end temperature of Ar3 transformation point or higher, the volume fraction of bainite and martensite in the microstructure of the pressed steel sheet can be made 20% or more in total. Thereby, while high temperature strength can be improved by precipitation strengthening by V carbonitride, room temperature strength can be maintained at a desired level as a steel material for welded structure. As a result, it is possible to obtain a thick steel plate having excellent high-temperature strength for a short time in the temperature range of 700 ° C. or less and excellent base material toughness and weld heat affected zone toughness without adding Mo.

  Hereinafter, the effects of the present invention will be described in detail by way of examples of the present invention. In this example, steel pieces having a chemical composition shown in Table 1 below were used to produce steel plates having a plate thickness of 25 mm or 50 mm. In addition, No. 1-No. No. 8 steel slab has a chemical composition within the scope of the present invention. 9-No. The steel slab of 17 has a chemical composition outside the scope of the present invention. Further, the balance in the chemical composition shown in Table 1 below is Fe and inevitable impurities.

  The steel slab was produced by subjecting an ingot or slab melted by vacuum melting or a converter as it was or once to rolling in pieces. And the steel piece whose thickness is 100-200 mm was made into the thick steel plate whose plate | board thickness is 25 mm or 50 mm by hot rolling. At this time, after hot rolling, accelerated cooling by air cooling or water cooling was performed, and then tempering treatment was performed as necessary. As conditions at that time, conditions were selected in which the room temperature strength was intended to fall within the range of tensile strength of 490 to 570 MPa class steel, but in some of the comparative examples, the chemical composition deviated from the scope of the present invention. Therefore, some of the room temperature strengths deviated from the range of tensile strength of 490 to 570 MPa grade steel. Table 2 below shows the manufacturing conditions for each thick steel plate and the total amount of the volume fractions of bainite and martensite in the microstructure of the steel plate. In addition, the total amount of the volume fraction of bainite and martensite in the microstructure shown in Table 2 below was determined by observing the Nital corrosion structure in the cross section in the thickness direction perpendicular to the rolling direction with an optical microscope with a magnification of 100 to 500 times. The area ratio was obtained. Then, at least 5 visual fields were observed at a position of 2 mm from the surface, a position of 1/4 of the plate thickness, and a position of the center portion of the plate thickness, and an average area ratio thereof was adopted. In addition, the Ar3 transformation point was measured under a simulated temperature history and processing conditions using a small test by a processing for master and an actual rolling process.

  In addition, No. 2 shown in Table 2 above. A1-No. The steel plate of A12 is a thick steel plate according to an example in which both the chemical composition and the structure form are within the scope of the present invention. B1-No. The steel plate of B10 is a thick steel plate of a comparative example in which either the chemical composition or the structure form is out of the scope of the present invention.

  Next, a round bar tensile test piece and a 2 mm V notch Charpy impact test piece are collected so that the longitudinal direction of the test piece is in a direction orthogonal to the rolling direction of the thick steel plate from the center of the plate thickness of these thick steel plates, The mechanical properties of the base material were investigated. At that time, the tensile properties were tested at room temperature, 600 ° C. and 700 ° C., and the 2 mm V notch Charpy impact properties were conducted at 0 ° C. The tensile test at high temperature was performed according to JIS G0567.

  As for HAZ toughness, the thickness of the 25 mm thick steel plate is reduced as it is, the thickness of the 50 mm thick steel plate is reduced to 25 mm, and the joint is formed by single-sided one-pass submerged arc welding with a heat input of about 15 kJ / mm using copper alloy. Produced. Then, a 2 mm V notch Charpy impact test specimen was taken so that the boundary between the weld metal and the HAZ at the center of the steel plate thickness of each joint (fusion part: FL) was the notch position, and the notch entered a plane perpendicular to the steel plate surface. The test was performed at 0 ° C. in the same manner as the base material. The one-pass submerged arc welding performed in this example is a high heat input welding, and the cooling time from 800 ° C. to 500 ° C. is about 250 seconds or more. The above results, that is, the mechanical properties and HAZ toughness of the base material are summarized in Table 3 below. In addition, the base material toughness and the HAZ toughness shown in Table 3 below are average values of 0 ° C. absorbed energy of three test pieces.

  Example No. shown in Table 3 above. A1-No. The steel sheet of A12 had a high temperature strength at 600 ° C. and 700 ° C. sufficiently high for a tensile strength of 490 to 570 MPa class steel, and the base material and HAZ toughness were all sufficiently satisfying 200J or more. From this result, Example No. A1-No. It became clear that the steel plate of A12 has sufficient characteristics as a fire-resistant thick steel plate without containing Mo.

  On the other hand, the comparative example No. shown in Table 3 above. B1-No. The steel plate of B10 is the above-mentioned Example No. A1-No. The characteristics were inferior compared to the A12 steel plate. Specifically, Comparative Example No. Since the steel plate of B1 does not contain V, the high-temperature strength is low as a tensile strength 490 MPa class steel, and particularly the yield strength at 700 ° C. is low. Comparative Example No. Since the steel sheet of B2 has an excessive V content, the room temperature strength is excessive even as a steel having a tensile strength of 570 MPa, and furthermore, both the base metal toughness and the HAZ toughness are significantly deteriorated. It was not preferable.

  Comparative Example No. The B3 steel sheet has an excessively low N content, so the precipitation of V carbonitride at high temperatures is not sufficient. Therefore, the high temperature strength is lower than that of the steel sheets of the same room temperature strength level, and is refractory. High-temperature strength was insufficient as a tensile strength 570 MPa class steel. Comparative Example No. On the contrary, in the steel sheet of B4, since the N content was excessive, V was coarsely precipitated at an early stage, and the high temperature strength was low, and particularly, the decrease in the high temperature strength at 700 ° C. was remarkable. Comparative Example No. The steel plate of B4 had extremely low base metal toughness and HAZ toughness due to an increase in solute N and coarse V carbonitride. Comparative Example No. Since the steel sheet of B5 does not contain Nb, the high-temperature strength was not sufficient even though the contents of V and N were within the scope of the present invention. Comparative Example No. On the contrary, the steel sheet of B6 has an excessive Nb content, so that the room temperature strength is excessive as a tensile strength 570 MPa grade steel, and furthermore, both the base metal toughness and the HAZ toughness have been significantly deteriorated. It was not preferable as a thick steel plate.

  Comparative Example No. The steel sheet of B7 has an excessive C content. Since the steel plate of B8 has an excessive Mn content, both the base metal toughness and the HAZ toughness are remarkably deteriorated, which is not preferable as a thick steel plate for welded structure. Comparative Example No. The steel plate of B9 is an example of Mo-containing steel that is outside the scope of the present invention. This Comparative Example No. The steel plate of B9 has no particular problem with respect to the high temperature strength, but the high heat input HAZ toughness was clearly inferior to the steel plates of the above-described examples. Comparative Example No. Since the total amount of the bainite and martensite volume fractions in the microstructure of the steel sheet B10 was too small, the high-temperature strength was insufficient as a tensile strength 490 MPa grade steel.

  Also according to the above examples, according to the present invention, in the thick steel plate for welded structure having a tensile strength of 490 to 570 MPa without containing Mo, about 2/3 or more of the standard yield point at 600 ° C., and standard yield at 700 ° C. It is possible to provide a thick steel plate having a yield strength of 1/2 or more of the point and having a HAZ toughness of the base metal and the high heat input welded joint of 70 J or more in absorbed energy at 0 ° C. in a 2 mmV notch Charpy impact test. Therefore, the industrial effect is extremely large.

Claims (11)

% By mass
C: 0.02 to 0.2%,
Si: 0.03 to 1%,
Mn: 0.3-2%,
Al: 0.001 to 0.05%,
Nb: 0.005 to 0.2%,
V: 0.03-0.3% and N: 0.005-0.03%,
P: 0.02% or less and S: 0.01% or less,
The balance consists of Fe and inevitable impurities,
A refractory thick steel sheet having a total volume fraction of bainite and martensite of 20% or more in a microstructure.   Further, one or two selected from the group consisting of Ti: 0.002 to 0.02%, Ta: 0.002 to 0.2% and Zr: 0.002 to 0.2% by mass% The refractory thick steel plate according to claim 1, comprising the above elements.   Further, in terms of mass%, Cr: 0.01 to 3%, W: 0.01 to 3%, Ni: 0.01 to 3%, Cu: 0.01 to 1.5%, and B: 0.0003 to The refractory thick steel plate according to claim 1 or 2, comprising one or more elements selected from the group consisting of 0.005%.   Furthermore, by mass%, one or two selected from the group consisting of Mg: 0.0005-0.01%, Ca: 0.0005-0.01% and REM: 0.0005-0.05% The refractory thick steel plate according to any one of claims 1 to 3, comprising the above elements.   In mass%, C: 0.02-0.2%, Si: 0.03-1%, Mn: 0.3-2%, Al: 0.001-0.05%, Nb: 0.005- 0.2%, V: 0.03-0.3% and N: 0.005-0.03%, P: 0.02% or less and S: 0.01% or less, After heating the steel slab or slab of the composition which consists of Fe and an unavoidable impurity to 1150-1300 degreeC, the cumulative reduction at 1100 degrees C or less is 10-90%, and rolling end temperature is more than Ar3 transformation point A method for producing a refractory thick steel sheet, characterized by performing hot rolling.   6. The method for producing a refractory thick steel plate according to claim 5, wherein tempering is performed at 400 to 650 [deg.] C. after the hot rolling.   6. The method for producing a refractory thick steel plate according to claim 5, wherein after the hot rolling, accelerated cooling is performed from the rolling end temperature to a temperature range of 550 ° C. or lower at a cooling rate of 3 to 100 ° C./second.   The method for producing a refractory thick steel plate according to claim 7, wherein tempering is performed at 400 to 650 ° C after the accelerated cooling.   The steel slab and the cast slab further comprise, in mass%, Ti: 0.002-0.02%, Ta: 0.002-0.2%, and Zr: 0.002-0.2%. The manufacturing method of the refractory thick steel plate of any one of Claims 5-8 containing 1 type, or 2 or more types of elements selected from these.   Further, the steel slab and the cast slab are in mass%, Cr: 0.01 to 3%, W: 0.01 to 3%, Ni: 0.01 to 3%, Cu: 0.01 to 1. It contains 1 type, or 2 or more types of elements selected from the group which consists of 5% and B: 0.0003-0.005%, The fireproof of any one of Claims 5-9 characterized by the above-mentioned. Manufacturing method of thick steel plate.   The steel slab and the cast slab further comprise, in mass%, Mg: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, and REM: 0.0005 to 0.05%. The manufacturing method of the fire-resistant thick steel plate of any one of Claims 5-10 containing 1 type, or 2 or more types of elements selected from these.