JP5849846B2 - Refractory steel and manufacturing method thereof - Google Patents

Description

  The present invention relates to a refractory steel material suitable for steel structures such as buildings and a method for producing the same.

  Steel structures such as buildings are required to exhibit the required strength for a certain period of time in order to prevent collapse and allow occupants to escape when exposed to fire. In general, since strength of steel materials decreases when exposed to high temperatures, conventionally, a method has been employed in which steel materials are covered with a fireproof coating to suppress the temperature rise of the steel materials during a fire. However, with the establishment of the new fireproof design law in March 1987, steel materials with high high-temperature strength at 600 ° C. have been used in order to simplify or reduce the fireproof coating.

  Conventional steel materials having high-temperature strength, so-called refractory steel materials, were positively added with Mo that increases high-temperature strength by precipitation strengthening. However, since the price of Mo is likely to fluctuate and the cost increases when the price rises, a technique based on an alloy design that does not necessarily depend on the addition of Mo has been proposed (see, for example, Patent Document 1). Further, as a method for increasing the high-temperature strength without depending on the addition of alloy components, a method using accelerated cooling at the time of manufacturing a steel material has been proposed (for example, see Patent Document 2).

  Furthermore, when a steel structure is exposed to a fire, there is an example in which a weld heat affected zone (hereinafter referred to as “HAZ”) of a welded joint does not follow the deformation and breaks. Degradation of deformability when HAZ is exposed to high temperatures (hereinafter, sometimes referred to as HAZ reheat embrittlement) may be particularly noticeable in steels to which Mo or B is added. For this reason, steel has been proposed in which the high-temperature strength is increased by solid solution strengthening of Nb and the addition of Mo and B is suppressed (see, for example, Patent Documents 3 and 4). In addition, a refractory steel material to which Cr is added for suppressing reheating embrittlement of HAZ has been proposed (for example, Patent Document 5).

JP 2007-197794 A Japanese Patent Laid-Open No. 2002-11502 JP 2008-121121 A JP 2008-179881 A JP 2011-190506 A

  In recent years, buildings have become larger and taller, and with the increase in the size of welded structures, the thickness of steel materials has increased, and in order to increase the efficiency of welding, the adoption of high heat input welding has increased. ing. In high heat input welding, the HAZ temperature during welding rises and the cooling rate decreases, so the coarsening of the grain size of prior austenite (hereinafter sometimes referred to as former γ) and the former γ grain boundary of HAZ Precipitation of carbide and the like on the surface is promoted. In particular, when Mo, B, or the like is added to increase the high-temperature strength, carbonitrides are generated in the HAZ, and the HAZ reheat embrittlement and the toughness decrease become significant.

  The present invention has been made in view of such a situation, without adding a large amount of alloying elements, to improve the high-temperature strength of the steel material, even when high heat input welding is performed, An object of the present invention is to provide a fire-resistant steel material excellent in reheat embrittlement resistance and toughness of HAZ, and a production method thereof.

  The present inventors have conventionally added Mo and B that have been actively used to ensure high temperature strength, prevent reheat embrittlement of high heat input HAZ, ensure toughness, and provide high temperature strength. The chemical composition and production conditions to ensure were repeated. As a result, by limiting the content of C, Si, Mn, Ti, Al, and N and optimizing accelerated cooling conditions, stable refractory steel materials with a tensile strength at room temperature of 400-510 MPa and a high-temperature strength of 157 MPa or more. It was found that it can be manufactured. In addition, an extremely high level of reheat embrittlement resistance is realized by keeping the amount of C low without adding Mo or B.

The gist of the present invention is as follows.
(1) In mass%,
C: 0.001 to 0.030%,
Si: 0.01 to 0.50%,
Mn: 0.50 to 1.50%,
Ti: 0.005 to 0.030%,
Al: 0.005 to 0.100%,
N: 0.0010 to 0.0100%
In addition, the content of each of P, S, O is
P: 0.030% or less,
S: 0.020% or less,
O: limited to 0.010% or less, the balance consisting of Fe and inevitable impurities,
The metal structure includes pseudo-polygonal ferrite having an area ratio of 50% or more and polygonal ferrite having an area ratio of 40% or less, and the total of the area ratio of the pseudo-polygonal ferrite and the area ratio of the polygonal ferrite is 90%. A refractory steel material characterized by the above.
(2) Furthermore, in mass%,
Nb: 0.030% or less,
V: 0.30% or less,
Zr: 0.010% or less,
Cr: 0.50% or less,
W: 0.50% or less,
Cu: 0.50% or less,
Ni: 0.51% or less of 1 type or 2 types or more are contained, The fireproof steel materials as described in said (1) characterized by the above-mentioned.
(3) Furthermore, in mass%,
Mg: 0.005% or less,
Ca: 0.005% or less,
Y: 0.050% or less,
La: 0.050% or less,
Ce: The refractory steel material according to (1) or (2) above, containing one or more of 0.050% or less.
(4) A method for producing a refractory steel material according to any one of (1) to (3) above , wherein the steel slab has the steel component according to any one of (1) to (3) above. Is heated to 1000 to 1300 ° C., then hot-rolled at a rolling ratio in the temperature range of 800 to 1000 ° C. of 20% or more, and the hot rolling is finished at 800 ° C. or more, and then 620 ° C. A method for producing a refractory steel material, characterized by accelerated cooling to the following temperature range.

  The refractory steel material of the present invention has a tensile strength at room temperature of 400 to 510 MPa, a yield strength at 600 ° C. of 157 MPa, a HAZ 600 ° C. squeeze drawing value of 50% or more, and resistance to reheat embrittlement. Excellent. In particular, toughness is ensured even in HAZ by welding with a heat input of 10 kJ / mm. Therefore, according to the present invention, it is possible to provide a fire-resistant steel material suitable for architectural use and a method for manufacturing the same, which can ensure the requirements in building design and have a sufficient safety margin in a fire, and contribute industrially. Extremely prominent.

FIG. 1 is a schematic diagram showing an optical microscope image of an example of pseudopolygonal ferrite. FIG. 2 is a schematic diagram showing an optical microscope image of an example of polygonal ferrite. FIG. 3 is an optical micrograph corresponding to FIG. FIG. 4 is an optical micrograph corresponding to FIG.

  Conventionally, accelerated cooling is often employed to increase the strength at room temperature, and has been applied, for example, to the manufacture of thick steel plates and H-section steels having a tensile strength at room temperature of 500 MPa or more. However, even if accelerated cooling is not applied, it is possible to produce a thick steel plate having a tensile strength of 400 to 500 MPa at room temperature, resulting in poor shape and residual stress of the steel plate, and reducing the load of the production process. From the viewpoint, accelerated cooling tended to be avoided.

  However, in order to increase the high-temperature strength of the steel material without adding a large amount of alloy elements, it is considered preferable to perform accelerated cooling after hot rolling to obtain a structure with a high dislocation density. Therefore, the inventors investigated the high temperature strength of various steel materials having a room temperature strength in the range of 400 to 500 MPa. As a result, it has been found through experiments that a significant difference in high-temperature strength may be observed when steel manufactured by applying accelerated cooling is compared with steel manufactured by air cooling. That is, it was found that a steel material manufactured by performing accelerated cooling under appropriate conditions can obtain high high-temperature strength even when the room temperature strength is comparable to that of steel to which accelerated cooling is not applied.

  The present inventors have examined the cause of the difference in high-temperature strength depending on whether or not accelerated cooling is applied. As a result, it has been found that steel materials that can obtain high-temperature strength by applying accelerated cooling have a metal structure mainly composed of pseudopolygonal ferrite. However, even when accelerated cooling is applied, if the stop temperature of accelerated cooling is high or the selection of alloy elements is not appropriate, it becomes a metal structure mainly composed of polygonal ferrite, and high high-temperature strength may not be obtained. all right.

  FIGS. 1 and 3 show schematic diagrams and photographs of examples of pseudo-polygonal ferrite observed with an optical microscope, and FIGS. 2 and 4 show schematic diagrams and photographs of examples of polygonal ferrite, respectively. As shown in FIGS. 1 to 4, the shape of the crystal grains of the pseudo-polygonal ferrite is more angular than that of the polygonal ferrite. Moreover, as a result of analyzing the refractory steel material of the present invention by the X-ray diffraction method, it was found that the half-value width of the diffraction peak of ferrite was large and the dislocation density was high. The balance of pseudopolygonal ferrite and polygonal ferrite is one or more of acicular ferrite, bainite, martensite and retained austenite.

  Furthermore, the present inventors have examined in detail, through experiments and analyses, the effects of various alloy elements of refractory steel on the reheat embrittlement of HAZ. Specifically, first, C: 0.001 to 0.070%, Si: 0.01 to 0.80%, Mn: 0.50 to 2.00%, N: 0.0010 to 0.0100% , Ti: 0.003 to 0.035%, Al: 0.005 to 0.10%, and steel materials having various component compositions with the balance being Fe were manufactured.

  Next, a test piece was collected from the obtained steel material and applied with a heat cycle assuming welding with a heat input of 10 kJ / mm. A heat cycle assuming welding with a heat input of 10 kJ / mm is a cooling rate from 800 ° C. to 500 ° C. when cooled at 20 ° C./s from room temperature to 1400 ° C., held at 1400 ° C. for 2 s, and then cooled. Is a heat history of 3 ° C./s. Thereafter, the temperature was raised from room temperature to 600 ° C. over 60 minutes, held at 600 ° C. for 30 minutes, and then held at 600 ° C., a tensile test was performed with a stress increase rate of 1 MPa / s, The aperture value was measured. The aperture value was used as an index of HAZ reheat embrittlement, and 50% or more was considered good.

  As a result, it was found that if the C content is limited to 0.03% or less, the formation of carbide is suppressed and the reheat embrittlement resistance of HAZ is remarkably improved. Further, since Si promotes the formation of a hard embrittlement phase in HAZ, and Mn segregates at the grain boundary, if the Si content is 0.50% or less and the Mn content is 1.50% or less, HAZ It was found that the deterioration of reheat embrittlement resistance can be suppressed. Furthermore, Ti is effective for fixing C and N, and the addition of 0.005 to 0.030% suppresses the precipitation of carbides and nitrides at the grain boundaries and improves reheat embrittlement resistance. I understood.

  Hereinafter, the present invention will be described in detail.

  First, the chemical components of the refractory steel of the present invention will be described. In the following description, the content of each element is expressed in mass%.

C: 0.001 to 0.030%
C is an element effective for improving the hardenability of the steel material, and 0.001% or more is added to obtain a metal structure mainly composed of pseudopolygonal ferrite. In order to increase the hardenability and increase the high temperature strength, it is preferable to add 0.005% or more of C, more preferably 0.010% or more, and more preferably 0.015% or more. It is preferable. When improvement in high temperature strength is important, the C content is more preferably 0.020% or more. On the other hand, when C is added over 0.030%, in the HAZ during high heat input welding, a lot of martensite-austenite mixed structure (hereinafter sometimes referred to as MA phase) and carbides are generated. It may reduce the toughness of the HAZ and cause reheat embrittlement. Therefore, the upper limit of the C amount is 0.030% or less. A more preferable upper limit of the amount of C is 0.025% or less.

Si: 0.01 to 0.50%
Si is a deoxidizing element and also an element contributing to improvement of hardenability, and 0.01% or more is added. In order to increase the strength, 0.05% or more of Si is preferably added, and more preferably 0.10% or more. On the other hand, Si is an element that promotes the formation of the MA phase of HAZ during high heat input welding, and in order to ensure reheat embrittlement resistance and toughness, the upper limit may be made 0.50% or less. Necessary, and more preferably 0.40% or less. In order to increase the toughness of the HAZ, it is preferable to set the upper limit of the Si amount to 0.30% or less.

Mn: 0.50 to 1.50%
Mn is effective in improving hardenability and is added in an amount of 0.50% or more in order to obtain a metal structure mainly composed of pseudopolygonal ferrite. In order to increase room temperature strength and high temperature strength, it is preferable to add 0.70% or more of Mn, and more preferably 1.00% or more. On the other hand, Mn segregates at grain boundaries and promotes reheating embrittlement of HAZ, so the upper limit is made 1.50% or less. A more preferable upper limit of Mn is 1.40%.

Ti: 0.005-0.030%
Ti precipitates as carbides and nitrides and contributes to an increase in high temperature strength. Ti also precipitates carbides and nitrides in the grain boundaries and grains of the HAZ, suppresses the formation of coarse precipitates at the grain boundaries, and contributes to the suppression of reheat embrittlement of the HAZ. In order to obtain these effects, it is necessary to add 0.005% or more of Ti. A preferable lower limit of the Ti amount is 0.008% or more, and a more preferable lower limit is 0.010%. On the other hand, if Ti is added in an amount exceeding 0.030%, the toughness of HAZ decreases, so the upper limit was limited to 0.030%. The upper limit with preferable Ti amount is 0.025% or less.

Al: 0.005 to 0.100%
Al is an element necessary for deoxidation of steel materials. Since the effect of controlling the oxygen concentration in the molten steel is obtained by addition of 0.005% or more, the lower limit value of Al is set to 0.005%. The minimum with preferable Al amount is 0.010% or more, More preferably, it is 0.020% or more, More preferably, it is 0.030% or more. On the other hand, when the Al content exceeds 0.100%, coarse oxide clusters are formed, and the toughness of the steel material may be impaired. Therefore, the upper limit is defined as 0.100%. The upper limit with preferable Al amount is 0.070% or less, and a more preferable upper limit is 0.050% or less.

N: 0.0010 to 0.0100%
N forms 0.0010% or more in order to form various alloy elements and nitrides and contribute to the improvement of high temperature strength. A preferable lower limit of the N amount is 0.0020% or more. However, if the N content is excessive, the nitride precipitated at the grain boundaries of the HAZ becomes coarse and reheat embrittlement of the HAZ becomes prominent, so the upper limit is limited to 0.0100%. The upper limit with preferable N amount is 0.0070% or less, More preferably, it is 0.0060% or less, More preferably, it is 0.0050% or less.

P: 0.030% or less P is an impurity, lowers the toughness of the steel material, and promotes reheat embrittlement of the HAZ. Therefore, the upper limit of the content is limited to 0.030% or less. The upper limit with the preferable amount of P is 0.020% or less. The lower limit of the amount of P is not specified, but is preferably 0.001% or more in order to suppress an increase in cost.

S: 0.0200% or less S is also an impurity, and when coarse MnS is produced, the toughness of the steel material is reduced and the reheating embrittlement of the HAZ is promoted, so the upper limit of the content is 0.020%. Restrict to: The upper limit with the preferable amount of S is 0.010% or less. The lower limit of the amount of S is not specified, but is preferably 0.0001% or more in order to suppress an increase in cost.

O: 0.0100% or less O is also an impurity, and when a coarse oxide is formed, the toughness of the steel material is lowered and the reheating embrittlement of the HAZ is promoted, so the upper limit of the content is 0.010. % Or less. The upper limit with preferable O amount is 0.0050% or less, More preferably, it is 0.0030% or less. The lower limit of the amount of O is not specified, but is preferably 0.0001% or more in order to suppress an increase in cost.

  In the present invention, in addition to the above essential elements, one or more elements as described below can be selectively added.

Nb: 0.030% or less Nb increases the hardenability of the steel material, promotes the formation of pseudopolygonal ferrite, increases the dislocation density, further precipitates as carbide or nitride, and has a tensile strength at room temperature. Contributes to the improvement of high temperature strength. In order to increase the room temperature strength and the high temperature strength, Nb is preferably added in an amount of 0.001% or more, more preferably 0.005% or more. However, if Nb is added in excess of 0.030%, HAZ toughness and reheating embrittlement of HAZ due to NbC coarse precipitation at the grain boundaries become prominent, so the amount added should be 0.030% or less. Is preferable, 0.020% or less is more preferable, and 0.015% or less is still more preferable.

V: 0.30% or less V is an element that forms carbides by reheating in the event of a fire. In order to improve high temperature strength, it is preferable to add 0.01% or more. More preferably, 0.05% or more of V is added in order to increase the uniform elongation in the high temperature tensile test. On the other hand, when V is added exceeding 0.30%, carbides precipitated at the grain boundaries of HAZ become coarse, and reheat embrittlement of HAZ becomes prominent, so the addition amount may be made 0.30% or less. preferable. A more preferable upper limit of the V amount is 0.25% or less, and further preferably 0.20% or less.

Zr: 0.010% or less Zr is an element that forms carbide and nitride, and 0.002% or more is preferably added to improve room temperature strength and high temperature strength. On the other hand, if Zr exceeding 0.010% is added, carbides precipitated at the grain boundaries may become coarse and promote reheat embrittlement of the HAZ, so the upper limit of the Zr amount should be 0.010% or less. Is preferred. A more preferable upper limit of the amount of Zr is 0.005% or less.

Cr: 0.50% or less Cr is an element that improves hardenability, and 0.01% or more is preferably added to improve room temperature strength and high temperature strength. Moreover, Cr forms fine Cr carbide, suppresses the formation of carbide at the grain boundaries of HAZ, and has the effect of suppressing reheat embrittlement of HAZ, so 0.10% or more may be added. preferable. On the other hand, if Cr is added excessively, the HAZ may be hardened or the MA phase may increase and the toughness may decrease, so the upper limit is preferably made 0.50% or less. More preferably, the Cr amount is 0.35% or less, and further preferably 0.20% or less.

W: 0.50% or less W is an element that improves hardenability, and in order to improve room temperature strength and high temperature strength, it is preferable to add 0.01% or more. Further, W has an effect of forming carbides, suppressing the formation of carbides at the grain boundaries of HAZ, and suppressing reheat embrittlement of HAZ, and therefore it is preferable to add 0.10% or more. On the other hand, if W is added excessively, the HAZ may be hardened or the MA phase may increase and the toughness may decrease, so the upper limit is preferably made 0.50% or less. More preferably, the W amount is set to 0.35% or less, and further preferably set to 0.20% or less.

Cu: 0.50% or less Cu is effective in improving room temperature strength and high temperature strength by improving hardenability, and it is preferable to add 0.01% or more. More preferably, 0.10% or more is added. On the other hand, Cu is also an element that promotes reheat embrittlement of HAZ, and the upper limit is preferably made 0.50%. A more preferable upper limit of the amount of Cu is 0.30% or less.

Ni: 0.50% or less Ni is effective for improving room temperature strength and high temperature strength by improving hardenability, and 0.01% or more is added. Ni is also an element that improves toughness, and more preferably 0.10% or more is added. On the other hand, Ni is also an element that promotes reheat embrittlement of HAZ, and the upper limit is preferably made 0.50% or less. A more preferable upper limit of the Ni amount is 0.40% or less, and further preferably 0.20% or less.

Mg: 0.005% or less
Ca: 0.005% or less Mg and Ca have an effect of controlling the form of sulfide in the steel material and suppressing reduction in base metal toughness due to sulfide. Moreover, in order to acquire such an effect, it is preferable to add 0.0005% or more of Mg and Ca, respectively. On the other hand, if Mg and Ca are added in excess of 0.005%, the effect is saturated, so the upper limit is preferably made 0.005% or less.

Y: 0.050% or less
La: 0.050% or less
Ce: 0.050% or less Y, La, and Ce have an effect of controlling the form of sulfide in the steel material and suppressing a decrease in base material toughness due to sulfide. In order to obtain such an effect, it is preferable to add 0.001% or more of Y, La, and Ce, respectively. On the other hand, even if Y, La and Ce are added in excess of 0.050%, the effect is saturated, so the upper limit is preferably made 0.050% or less, more preferably 0.030%. The following.

  In the present invention, due to the limitation of the chemical component composition as described above, the room temperature tensile strength is in the range of 400 to 510 MPa, and even when exposed to fire, it has a high yield strength at a temperature of 600 ° C. At the same time, reheat embrittlement in the weld heat affected zone of the welded joint is suppressed, and a refractory steel material excellent in the low temperature toughness of the base material and the welded joint can be obtained.

  Next, the metal structure of the steel material of the present invention will be described.

  In general, it is considered that the high-temperature strength of a steel material is manifested by dislocation strengthening due to dislocations existing in the steel material, and precipitates and crystal grain boundaries that hinder dislocation movement. If the temperature of the steel material exceeds 550 ° C. and the dislocations are coalesced due to the upward movement of the dislocations, the high temperature strength may suddenly decrease. For this reason, in order to ensure high high-temperature strength, the steel material has a sufficient amount of dislocations at the time before being exposed to fire, that is, at room temperature, and further, the dislocation movement is obstructed. It is effective to include a large number of structures, specifically, precipitates and grain boundaries.

  In addition, as will be described in detail in the manufacturing method described later, in the present invention, there are severe restrictions on the addition of the alloy in order to suppress the reheat cracking of the HAZ to the limit. For this reason, high temperature strength is ensured by accelerated cooling. Mainly strengthening by dislocation. For this reason, when the steel material structure (metal structure) is observed with an optical microscope, 50% or more of the area ratio becomes pseudopolygonal ferrite. Pseudopolygonal ferrite has higher high temperature strength than polygonal ferrite even when the room temperature strength is similar. The area ratio of the pseudopolygonal ferrite is more preferably 60% or more, further preferably 70% or more, and most preferably 80% or more.

  Furthermore, the metal structure of the refractory steel material of the present invention has a polygonal ferrite having an area ratio of 40% or less, preferably 30% or less, more preferably 20% or less. The total with the area ratio of null ferrite is 90% or more. By setting it as such a metal structure, it becomes possible to ensure high temperature strength. If the area ratio of polygonal ferrite exceeds 40% or the total area ratio of pseudo-polygonal ferrite and polygonal ferrite is less than 90%, it is difficult to ensure high-temperature strength. In addition, the balance excluding pseudo-polygonal ferrite and polygonal ferrite in the metal structure is one or more of acicular ferrite, bainite, martensite and retained austenite, but their area ratio is small. Has little effect on properties.

  Next, the mechanical characteristics of the steel material of the present invention will be described.

  The refractory steel material of the present invention has a tensile strength at room temperature of 400 to 510 MPa and a yield stress at a temperature of 600 ° C. of 157 MPa or more. Furthermore, the refractory steel material of the present invention is also excellent in reheat embrittlement resistance. As a result, it is possible to realize a fire-resistant steel material having a sufficient safety margin in a fire and ensuring various requirements in building design in a building application.

  In the refractory steel material of the present invention, the reheat embrittlement resistance is given a thermal history assuming welding with a heat input of 10 kJ / mm, and then a tensile test at a temperature of 600 ° C. is performed, and the fracture drawing value is measured. The value is evaluated based on the value. In the present invention, a refractory steel value at which the fracture drawing value at a temperature of 600 ° C. is 50% or more and the HAZ of the welded joint has sufficient deformability when reheated to an assumed temperature of 600 ° C. in a fire can be obtained. .

 Next, the manufacturing method of the fireproof steel material of this invention is demonstrated.

  The refractory steel material of the present invention is manufactured by melting and casting steel by a conventional method, hot rolling the obtained steel slab, and performing accelerated cooling. Since the high temperature strength is ensured by the dislocation density introduced by accelerated cooling, the refractory steel material of the present invention can reduce costs by alloying and further prevent reheating embrittlement of HAZ.

Heating temperature: 1000-1300 ° C
The heating temperature of the steel slab is set to 1000 ° C. or higher in order to promote solid solution of the alloy element. In order to promote re-dissolution of carbides and nitrides, the heating temperature is preferably set to 1100 ° C. or higher. On the other hand, when the steel slab is heated to over 1300 ° C., the amount of scale generated increases and the crystal grain size becomes coarse, so the upper limit of the heating temperature is made 1300 ° C. or less. Preferably, the heating temperature is 1250 ° C. or lower.

End temperature of hot rolling: 800 ° C. or higher Hot rolling ends at 800 ° C. or higher in order to suppress the formation of precipitates. In order to generate pseudopolygonal ferrite by accelerated cooling, it is preferable to set the end temperature of hot rolling to 850 ° C. or higher. The upper limit of the end temperature of hot rolling is 1000 ° C. or less.

Rolling ratio in the temperature range of 800 to 1000 ° C .: 20% or more In hot rolling, the rolling ratio in the temperature range of 800 to 1000 ° C. is set to 20% or more in order to increase the dislocation density that becomes a nucleation site of pseudopolygonal ferrite. And The lower limit of the temperature range is a temperature at which hot rolling is finished. When hot rolling is finished at 1000 ° C., the rolling ratio at 1000 ° C. needs to be 20% or more. The reduction ratio is preferably 30% or more, more preferably 40% or more. In the present invention, the rolling reduction at 800 ° C. to 1000 ° C. may be 20% or more, and it does not hinder starting rolling in a temperature range exceeding 1000 ° C. The rolling reduction of the present invention may be a value obtained by dividing the difference between the plate thickness at 1000 ° C. and the plate thickness at 800 ° C. by the plate thickness at 1000 ° C.

Accelerated cooling stop temperature: 620 ° C. or less Immediately after hot rolling, accelerated cooling is started. If the stop temperature for accelerated cooling is too high, dislocations are recovered and the formation of pseudopolygonal ferrite is hindered. Therefore, in this invention, the stop temperature of accelerated cooling shall be 620 degrees C or less. The stop temperature of accelerated cooling is preferably 550 ° C. or lower, more preferably 500 ° C. or lower, and accelerated cooling may be performed to room temperature. The cooling rate of the accelerated cooling is not particularly limited because it should be faster than air cooling and changes depending on the plate thickness. The accelerated cooling can be performed by water cooling, mist cooling, or the like.

Average cooling rate of accelerated cooling: 3 to 50 ° C./s
Since the refractory steel material of the present invention has a low C content, the change in the cooling rate on the strength is not significant, but in order to promote the formation of pseudopolygonal ferrite, the lower limit of the average cooling rate of accelerated cooling is limited. It is preferable to be 3 ° C./s or more. The lower limit of the average cooling rate of accelerated cooling is more preferably 5 ° C./s or more, and further preferably 10 ° C./s or more. In addition, when the average cooling rate of accelerated cooling is increased, it becomes difficult to control the stop temperature, so the upper limit is preferably 50 ° C./s or less. The upper limit of the average cooling rate of accelerated cooling is more preferably 40 ° C./s or less, and further preferably 30 ° C./s or less. The average cooling rate of accelerated cooling is an average of cooling rates from the start to the end of accelerated cooling.

 Steel was melted by a conventional method, deoxidation and desulfurization of the molten steel were performed, chemical components were adjusted, and a steel slab having the chemical composition shown in Table 1 was manufactured by continuous casting. Next, according to the manufacturing conditions shown in Table 2, the steel slab was heated to a predetermined temperature and held for 1 hour, and then hot-rolled and subjected to accelerated cooling to manufacture a steel plate having a predetermined thickness. In Tables 1 and 2, items that are outside the scope of the present invention are displayed with an underline.

  Table 2 shows the thickness of the manufactured steel sheet (production thickness), the heating temperature of the steel slab (steel slab heating temperature), the rolling reduction (rolling ratio) at 800 to 1000 ° C., the rolling finishing temperature (end temperature), The stop temperature of accelerated cooling (accelerated cooling stop temperature) and the average cooling rate of accelerated cooling are shown. The reduction ratio shown in Table 2 was determined from the change in plate thickness in the temperature range of 800 to 1000 ° C. That is, the value obtained by dividing the difference between the plate thickness at 1000 ° C. and the plate thickness at 800 ° C. by the plate thickness at 1000 ° C. is expressed as a percentage.

  Samples were collected from each of the obtained steel materials, corroded with a nital solution, observed with an optical microscope, the type of metal structure was determined, and the area ratios of pseudopolygonal ferrite and polygonal ferrite were measured. The size of the measured area is 500 μm × 500 μm. The balance of pseudopolygonal ferrite and polygonal ferrite was one or more of acicular ferrite, bainite, martensite, and retained austenite.

  Next, a tensile test is performed in accordance with JIS Z 2241. If an upper yield point appears on the stress-strain curve, the upper yield point is taken as the yield strength at room temperature. Yield strength. In addition, a high-temperature tensile test was performed at a temperature of 600 ° C. in accordance with JIS G 0567, and the measured 0.2% proof stress was taken as 600 ° C. yield strength. In the Charpy test of the base material, a V-notch test piece was sampled from the thickness 1 / 2t of each steel material in accordance with JIS Z 2242, and a Charpy impact test was performed at 0 ° C. At this time, the threshold of absorbed energy was set to 27 J in consideration of the earthquake resistance of the building structure.

  Moreover, the thermal cycle which assumed the welding of heat input 10kJ / mm was given, and the toughness and reheat embrittlement characteristic of HAZ were evaluated. The thermal cycle is a condition in which a range from 800 ° C. to 500 ° C. is cooled at 3 ° C./s after heating from room temperature to 1400 ° C. at 20 ° C./s, holding at 1400 ° C. for 2 s, and then cooling. .

  The toughness of HAZ was evaluated by collecting a V-notch test piece in accordance with JIS Z 2222 and applying a Charpy impact test at 0 ° C. after applying a thermal cycle. The threshold of absorbed energy of HAZ was also set to 27J in consideration of the earthquake resistance of the building structure. Furthermore, the 600 ° C. tensile drawing value of HAZ is determined by increasing the stress from 60 ° C. to 60 ° C. after performing a heat cycle, holding at 600 ° C. for 30 minutes, and then maintaining the temperature at 600 ° C. Was set to 1 MPa / s, and a tensile test was performed. The drawing threshold value, which is an index of HAZ reheat embrittlement, was set to 50% or more.

Table 3 shows the area ratios of pseudo-polygonal ferrite and polygonal ferrite, and their total, tensile strength (TS) and yield strength (YS) at room temperature, yield strength (600 ° C YS) at 600 ° C, base material And the Charpy absorbed energy (vE ₀ ) at 0 ° C. of HAZ, and the fracture drawing value (drawing) of the 600 ° C. tensile test of HAZ, respectively. Production No. 1 to 15 are examples of the present invention. 16 to 25 are comparative examples.

  Production No. 16 to 18 are comparative examples in which the production conditions are outside the scope of the present invention. Production No. 16 is an example in which the rolling ratio at 800 to 1000 ° C. is too low, and the toughness of the base material is lowered. Production No. No. 17 is an example in which the stop temperature of accelerated cooling is too high, and since the generation of pseudopolygonal ferrite is insufficient, the room temperature strength and the high temperature strength are reduced. Production No. No. 18 is an example of air cooling after hot rolling, pseudo-polygonal ferrite is not generated, and the room temperature strength and high temperature strength are reduced.

  Production No. 19 to 25 are comparative examples in which the components are outside the scope of the present invention. Production No. 19 and 25 are examples in which the amount of C is large, a low-temperature transformation structure having a higher strength than that of pseudopolygonal ferrite is generated, the strength at room temperature is increased, and reheat embrittlement occurs. Production No. No. 20 is an example in which the amount of Si is large, an embrittlement phase is generated in the HAZ, and the toughness of the HAZ is lowered. Production No. No. 21 is an example in which the amount of Mn is large, the low-temperature transformation structure increases, the pseudopolygonal ferrite decreases, the strength at room temperature increases, and reheat embrittlement occurs. Production No. No. 22 is an example in which the amount of Ti is large and the HAZ toughness is lowered. Production No. No. 23 is an example in which the amount of N is large and reheat embrittlement occurs. Production No. No. 24 is an example in which the amount of Mn is small and the structure is mainly composed of polygonal ferrite, and the room temperature strength and high temperature strength are lowered.

Claims (4)

% By mass
C: 0.001 to 0.030%,
Si: 0.01 to 0.50%,
Mn: 0.50 to 1.50%,
Ti: 0.005 to 0.030%,
Al: 0.005 to 0.100%,
N: 0.0010 to 0.0100%
In addition, the content of each of P, S, O is
P: 0.030% or less,
S: 0.020% or less,
O: limited to 0.0100% or less, the balance being Fe and inevitable impurities,
The metal structure includes pseudo-polygonal ferrite having an area ratio of 50% or more and polygonal ferrite having an area ratio of 40% or less, and the total of the area ratio of the pseudo-polygonal ferrite and the area ratio of the polygonal ferrite is 90%. A refractory steel material characterized by the above. Furthermore, in mass%,
Nb: 0.030% or less,
V: 0.30% or less,
Zr: 0.010% or less,
Cr: 0.50% or less,
W: 0.50% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
1 or 2 types or more of these are contained, The refractory steel materials of Claim 1 characterized by the above-mentioned. Furthermore, in mass%,
Mg: 0.005% or less,
Ca: 0.005% or less,
Y: 0.050% or less,
La: 0.050% or less,
Ce: 1 type or 2 types or more of 0.050% or less are contained, The refractory steel materials of Claim 1 or 2 characterized by the above-mentioned. It is a manufacturing method of the refractory steel materials of any one of Claims 1-3, Comprising: The steel slab which has the steel component of any one of Claims 1-3 is made into the temperature of 1000-1300 degreeC. After heating, hot rolling is performed with a reduction ratio in the temperature range of 800 to 1000 ° C. being 20% or more, hot rolling is finished at 800 ° C. or more, and then accelerated cooling to a temperature range of 620 ° C. or less. A method for producing a refractory steel material.