WO2003087414A1 - High tensile steel excellent in high temperature strength and method for production thereof - Google Patents

Abstract

A high tensile steel excellent in high temperature strength, characterized in that it has the chemical composition, in mass %: C: 0.005 % or more and less than 0.08 %, Si: 0.5 % or less, Mn: 0.1 to 1.6 %, P: 0.02 % or less, S: 0.01 % or less, Mo: 0.1 to 1.5 %, Nb: 0.03 to 0.3 %, Ti: 0.025 % or less, B: 0.0005 to 0.003 %, Al: 0.06 % or less, N: 0.006 % or less, and balance: Fe and inevitable impurities, and preferably, the rate of reduction of the stress thereof from an ordinary temperature to a high temperature (yield stress at a high temperature/yield stress at an ordinary temperature): p satisfies p ≥ -0.0029 × T + 2.80, wherein T represents the temperature (°C) of a steel product, in the range of T of 600 to 800°C; and a method for producing the high tensile steel. The high tensile steel (a steel plate, a steel pipe, a shaped steel product or a wire) has a low content of alloy carbon, and is excellent in the high temperature strength in a temperature range of 600 to 800°C for a relatively short time of about one hour, and thus can be used for general structures in wide fields of building, civil engineering, marine structures, shipbuilding, storage tanks and the like.

Description

 Description High tensile strength steel with excellent high-temperature strength and its manufacturing method

 The present invention is applicable to general structures such as architecture, civil engineering, marine structures, shipbuilding, storage tanks, etc., in a temperature range of 600 ° C. or more and 800 ° C. or less, for about one hour. The present invention relates to a method for manufacturing high-strength steel (steel plate, steel pipe, section steel, wire rod) for building structures with excellent low-temperature carbon content and excellent high-temperature strength in a short time. Background art

 For example, in the fields of construction, civil engineering, and the like, steel materials standardized by JIS and the like are widely used as various building steel materials. Since the strength of general steel for building structures decreases from about 350 ° C, the allowable temperature is 550 ° C.

 In other words, when the above-mentioned steel materials are used in buildings, offices, dwellings, multi-story parking lots, and other buildings, it is mandatory to apply sufficient fireproof coating to ensure fire safety. Various laws and regulations stipulate that the temperature of steel products should not exceed 350 ° C in the event of a fire.

This is because the above-mentioned steel material has a proof strength of about / of normal temperature at about 350 ° C., which is lower than required strength. When steel is used for buildings, it is used with a fireproof coating so that the temperature of the steel does not reach 350 in the event of a fire. As a result, the cost of refractory coating is higher than the cost of steel, and construction costs are unavoidably increased significantly. In order to solve the above-mentioned problems, for example, Japanese Patent Application Laid-Open No. Hei 2-77552 / Japanese Patent Application Laid-Open No. Hei 10-68044 has been invented.

 When the temperature is 600 ° C. or higher, it is generally referred to as refractory steel. For example, in the invention described in Japanese Patent Application Laid-Open No. 2-77553, the yield strength at room temperature at 600 ° C. is 2 3 ( A refractory steel having a high temperature strength of about 70% or more has been proposed. In other examples of the invention relating to 600 ° C. refractory steel, it is general that the yield strength at 600 ° C. is equal to or higher than the normal temperature yield strength of 2 Z 3.

 However, there is no general rule for the setting of high-temperature strength (ratio with normal-temperature yield strength) for 700 ° C refractory steel and 800 ° C refractory steel at present. For example, in Japanese Patent Application Laid-Open No. 2-77532, a steel to which a considerable amount of Mo and Nb is added ensures that the proof stress at 600 ° C is 70% or more of the normal temperature proof stress. However, resistance to temperatures of 700 ° C. and 800 ° C. is not shown.

 Also, if the proof stress at 600 ° C is about 70% of the normal temperature proof strength, considering the rise in temperature in the event of a fire, it is possible to reduce the amount of fireproof coating, but buildings that can be omitted can be omitted. Because it is limited to open spaces such as parking lots of atrium, its use in non-fireproof coatings is significantly limited.

 In Japanese Patent Application Laid-Open No. H10-68044, the microstructure is made bainite with steel to which a considerable amount of Mo and Nb are added, so that the yield strength at 700 ° C can be maintained at room temperature. Although it is disclosed that the proof strength is maintained at 56% or more, the proof strength of 800 ° C is not shown.

 In other words, steel with a high-temperature strength of about 600 ° C as in these examples has already been used in the market, and the invention of a steel material that has a certain strength at 700 ° C has been made. However, stable production of practical steel that can secure high temperature strength at 700 ° C and 800 ° C was difficult.

On the other hand, the present inventors recently disclosed a 850 ° C. refractory steel disclosed in Japanese Patent Application Laid-Open No. 2002-105585. This steel is A1, Ti etc. Although the addition of a relatively large amount of alloying elements ensures effective precipitates even at high temperatures and obtains fire resistance at 850 ° C, it is not suitable for welding structural steel.

 As described above, when steel is used for buildings, ordinary steel has low strength at high temperatures, so it cannot be used with no coating or light coating, and expensive fireproof coating must be applied.

 Even for fire-resistant steel, the fire resistance temperature is limited to a maximum of 600 to 700 ° C, and use of non-fireproof coatings at 700 ° C and 800 ° C and There has been a demand for the development of a steel material that can omit the refractory coating process. Disclosure of the invention

 The present invention relates to a high-tensile steel used for applications such as architectural civil engineering excellent in high-temperature strength and weldability in a temperature range of 600 ° C. to 800 ° C., and an industrially stable steel. An object of the present invention is to provide a manufacturing method that enables the supply. The gist of the present invention is as follows.

(1) Mass ⁰ /. And C: 0.005% or more and less than 0.8%, S i: 0.5% or less, Mn: 0.1 to 1.6%, P: 0.02% or less, S: 0.01% or less, Mo: 0.1 to 1.5%, Nb: 0.03 to 0.3%, Ti: 0.025% or less, B: 0.00 0 Excellent high-temperature strength, characterized by containing 5 to 0.03%, A1: 0.06% or less, N: 0.06% or less, and the balance of Fe and unavoidable impurities High strength steel.

(2) The stress reduction rate of the steel in which the yield stress at high temperature is made dimensionless from the yield stress at normal temperature (high temperature yield stress / normal temperature yield stress): P is steel temperature T (° C) is 60 In the range of 0 ° C or more and 800 ° C or less, p ≥—0.029 XT + 2.48 is satisfied. High tensile strength steel with excellent temperature strength.

 (3) The steel has a bainite single structure at room temperature or a mixed structure of ferrite and bainite at normal temperature when heated to a high temperature equivalent to a fire, and reversely transforms to austenite when heated to a high temperature equivalent to a fire ( A c!) Is more than 800 ° C, and the stress ratio obtained by making the yield stress at high temperature non-dimensional from the yield stress at normal temperature (high temperature yield stress / normal temperature yield stress): P is the steel material temperature T ( (° C) is in the range of 600 ° C or more and 800 ° C or less, and satisfies p ≥ 0.02 XT + 2.48. High strength steel with excellent strength.

 (4) In the high-temperature region of 600 ° C. or more and 800 ° C. or less, the steel has a dimension in which the yield stress at normal temperature is reduced from the yield stress at high temperature to the dimensionless stress reduction rate.

(High temperature yield stress: normal temperature yield stress): P≥0.02 9 XT + 2. if P is steel temperature T (° C) in the range of 600 ° C or more and 800 ° C or less. The temperature at which the bainite single structure or the mixed structure of ferrite and bainite at room temperature reversely transforms to austenite when heated to a high temperature equivalent to a fire. _{C l)} has a tissue is 8 0 0 ° C greater, further, the base intragastric preparative single tissue or thermodynamically stable carbonitride precipitation phase in a mixed tissue Blow I bets and downy Inai preparative It holds the mole fraction 5 XI 0 ⁴ or more, M o which forms a solid solution with Fe Rye preparative tissues, N b, the total amount of T i is 1 X 1 0 molarity - and features a Dearuko ³ or more (1) A high-tensile steel excellent in high-temperature strength according to (1).

 (5) In the high-temperature region of not less than 600 ° C. and not more than 800 ° C., the steel has a dimension in which the yield stress at normal temperature is reduced from the yield stress at high temperature to a dimensionless stress reduction rate.

(High-temperature yield stress / room-temperature yield stress): ΐ>, p≥—0.002 XT + when steel temperature T (° C) is in the range of 600 ° C or more and 800 ° C or less. 4 8) It has strength that satisfies 8), and when heated to a high temperature equivalent to a fire, bainite single tissue at room temperature or a mixture of ferrite and bainite. The composite structure has a structure in which the temperature (A c) at which reverse transformation to austenite is more than 800 ° C., and the average equivalent circle diameter of the former austenite grains is 120 μπι or less, and base Inai DOO single tissue or in a mixed structure in ferrite and downy Inai preparative thermodynamically stable carbonitride precipitation phase mole fraction holds 5 X 1 0- ⁴ or more, the solid in the ferrite structure (1) The high-strength steel excellent in high-temperature strength according to (1), wherein the total amount of dissolved Mo, Nb, and Ti is 1 X 10 or more in molar concentration.

 (6) The steel is PCM = C + S i / 30 + Mn / 20 + C u / 20 + N i / 60 + C r / 2 O + M o / 15 + V / 10 +5 B: Weld crack susceptibility composition defined by: PCM is 0.20% or less, characterized by high temperature strength excellent in high-temperature strength according to any one of (1) to (5). Tension steel.

 (7) The steel further contains, by mass%, Ni: 0.05 to 1.0%, Cu; 0.05 to 1.0%, Cr: 0.05 to 1.0%. %, V: 0.01% to 0.1%, wherein one or more kinds are contained, and the high tensile strength excellent in high-temperature strength according to any one of (1) to (7), steel.

(8) The steel further contains, by mass%, Ni: 0.05 to 1.0%, Cu: 0.05 to: 1.0%, Cr: 0.05 to: L 0%, V: 0.01 to 0.1%, 1 or 2 or more types, and Ca: 0.005 to 0.04%, REM: 0. It is characterized by containing one or more of 0.005% to 0.004%, Mg: 0.00001 to 0.006% (1) to (7) A high-tensile steel excellent in high-temperature strength according to any one of the above items.

 (9) In the high-temperature region of 600 ° C. or more and 800 ° C. or less, the steel has a non-dimensional stress reduction ratio from the yield stress at room temperature to the yield stress at high temperature.

(High-temperature yield stress / normal-temperature yield stress): P ≥ 0.02 9 XT + 2. when P is steel temperature T (° C) in the range of 600 ° C or more and 800 ° C or less. Four 8. The temperature at which a single structure of payinite or a mixed structure of ferrite and bainite at normal temperature reversely transforms to austenite when it has a strength that satisfies the above and is heated to a high temperature equivalent to a fire (Aci). Is greater than 800 ° C, the average equivalent circle diameter of the former austenite grains is 120 / zm or less, and the bainite single structure, or the ferrite and bainite together thermodynamically stable carbonitride precipitation phase in a mixed tissue is kept in a molar fraction 5 X 1 0- ⁴ or more, M o to solid solution during Fuwerai preparative tissues, N b, the total amount of T i is The high-strength steel excellent in high-temperature strength according to (7) or (8), wherein the high-strength steel has a molar concentration of 1 × 10 ³ or more.

(10) After re-heating the slab or the slab having the steel component composition according to any one of (1) to (9) to a temperature range of 1100 to 1250 ° C, Hot rolling is performed at a temperature of 85 ° C or more with the cumulative rolling reduction at 110 ° C or less being 30% or more, and after the completion of hot rolling, from the temperature range of 800 ° C or more, 65 up to below 0 ° C temperature range is cooled in 0. 3 K s ¹ or more cooling rate, and characterized in that the steel microstructure organizations the base Inai preparative single tissue, or ferrite preparative base Inai preparative mixed structure of To produce high-strength steel with excellent high-temperature strength.

(11) In mass%, C: 0.05% or more and less than 0.08%, Si: 0.5% or less, Mn: 0.1 to 1.6%, P: 0.0 2% or less, S: 0.0 1% or less, Mo: 0.1 to 1.5%, Nb: 0.03 to 0

3%, Ti: 0.025% or less, B: 0.005 to 0.003%, A1: 0.06% or less, N: 0.06% or less The mixture of ferrite and bainite, which contains Fe and the balance of Fe and unavoidable impurities, and has a bainite fraction of 20 to 95% at room temperature when heated to a high temperature equivalent to a fire, High tensile strength steel with excellent high temperature strength characterized by having a low yield ratio and a structure with a temperature (Ac!) Of reverse transformation to austenite exceeding 800 ° C. (12) The steel further contains, by mass%, Ni: 0.05 to: 0.5%, Cu: 0.05 to: 1.0%, Cr: 0.05 to : L. 0%, V: 0.01 ~ 0.1% 1 or more types

A high-tensile steel having excellent high-temperature strength according to (11).

 (13) The steel further contains, by mass%, Ni: 0.05 to 1.0%, Cu: 0.05 to: L. 0%, Cr: 0.05 to: I. 0%, V: 0.01 to 0.1%, 1 or 2 or more kinds, and Ca: 0.005 to 0.04%, REM: 0 (1 1) or characterized in that it contains one or more of 0.005 to 0.004%, Mg: 0.00001 to 0.006%. (12) A high-tensile steel excellent in high-temperature strength according to any one of the above (12).

(14) The slab or the slab having the steel composition described in any of (11) to (13) is returned to a temperature range of 110 to 125 ° C. After heating, hot rolling is performed at a temperature of 850 ° C or more, with the cumulative rolling reduction at 110 ° C or less being 30% or more, and from the temperature range of 800 ° C or more after the completion of hot rolling. 6 to 5 0 ° temperature range of C hereinafter cooled at 0. 3 K s ¹ or more cooling rate, the steel Miku port tissue base Inai preparative single tissue, or ferrite and downy Inai bets mixed structure of The temperature (Aci) at which the mixed structure of ferrite and payinite having a bainite fraction at room temperature of 20 to 95% at room temperature during heating to a high temperature equivalent to a fire reversely transforms to austenite (Aci) is 80%. A method for producing a high-tensile steel excellent in high-temperature strength, characterized by having a structure with a temperature of over 0 ° C and a low yield ratio. BEST MODE FOR CARRYING OUT THE INVENTION

The present inventors have already found steel excellent in high-temperature strength at 600 ° C and 700 ° C, and steel excellent in high-temperature strength at 600 ° C has already been used in many fields including construction. However, the market is facing extremes in steels that can withstand higher temperatures. There is a strong demand. At the same time, there is a greater need for higher strength steels with superior high temperature strength.

 In a fire-resistant design, it is only necessary to maintain high strength within the duration of the fire. Unlike conventional heat-resistant steel, there is no need to consider long-term strength, as long as high-temperature yield strength can be maintained for a relatively short time. For example, if a short-time high-temperature yield strength of about 30 minutes at 800 ° C can be secured, it can be sufficiently used as 800 ° C refractory steel.

 In conventional refractory steels, the performance was determined so that the high-temperature yield strength was 2 to 3 at room temperature.However, the actual design range of a steel frame structure was 0.2 to 0.4 times the lower limit of room temperature yield strength. Taking into account that, there is a stress reduction rate in which the yield stress at high temperature is made dimensionless from the yield stress at normal temperature (high temperature yield stress Z normal temperature yield stress): P is steel temperature T (° C) is 600 It is necessary to satisfy p ≥ 0.02 XT + 2.48 within the range of ° C to 800 ° C.

 To increase the high-temperature strength, it is effective to promote the precipitation of carbonitride stable at high temperatures by the combined addition of Mo and Nb, and to make the microstructure of the orifice micropainite. The bainite single structure may be used to enhance the room temperature strength and emphasize the characteristics as a high-tensile steel.

 However, since the strength at room temperature increases as the fraction of the hard bainite increases, when the upper limit of the yield ratio (YR) is required, the mix opening is determined according to the required room temperature strength and various characteristics. It is desirable that the organization be a single veneer organization or a mixed organization of flite and bainite with an appropriate veneer fraction.

It is effective to lower the C to create a suitable microstructure and achieve the required room temperature strength range. Lowering C increases the thermodynamic stability of bainite or a mixed structure of ferrite and bainite at high temperatures, and also has the effect of increasing the reverse transformation temperature (A _C1 ) to austenite. However, in this case, it was found that the microstructure and the material were easily affected by the rolling conditions and the subsequent cooling conditions, and that stable production was difficult.

 Thus, the present inventors have worked on microstructure control and increased high-temperature strength, and as a result, have found that the addition of an appropriate amount of B is effective for stabilizing production, and reached the present invention.

 As a general welded structural steel, it is necessary to provide the same weldability as before, so a steel with excellent high-temperature strength at 700 ° C to 800 ° C was an extremely difficult task. .

 In order to solve this problem, the present inventors have studied diligently, and the high-temperature strength of 700 ° C. to 800 ° C. depends on the composite addition of alloy elements such as M 0, Nb, V, and Ti. Increased dislocation density due to precipitation strengthening and micropainite microstructure, and the effect of solid solution Mo, Nb, and V on dislocation recovery delay were found to be effective, and Ti was also found to have some effect. .

 In order to simultaneously secure the strength of 700 ° C to 800 ° C and the strength at room temperature, and the strength ratio p between room temperature and high temperature, the microstructure must be a mixed structure of ferrite and veneite or venaite. It has been found that it is important to obtain the thermal stability of the matrix structure at high temperatures, the appropriate coherent precipitation strengthening effect, and the dislocation recovery delay effect with the amount of the added alloy element in the optimum range, in addition to the single-structured alloy. In addition, in order to ensure a low yield ratio, it is necessary to make the microstructure of the mouth mouth an appropriate mixed structure of ferrite and bainite.

The yield strength of steel materials generally drops sharply from around 450 ° C. This is because the thermal activation energy decreases as the temperature rises, and the resistance that was effective at low temperatures against the slip motion of dislocations. Cr is usually used for strengthening in the temperature range of less than 700 ° C. Although it acts as an effective resistance to dislocation sliding motion up to a high temperature of about 600 ° C, it re-solid-dissolves at a high temperature of about 800 ° C. The effect cannot be maintained. The present inventors have studied various single or composite precipitates having higher stability at high temperatures. As a result, it has been found that the composite precipitate of Mo, Nb, Ti, and V has high stability at high temperatures and has a high strengthening effect even at 700 to 800 ° C. That is, by adding an appropriate amount of Mo, Nb, T i, and V to increase the heating temperature during rolling, these are sufficiently dissolved to introduce a suitable rolling structure having a high dislocation density. By securing a precipitation site where precipitates can be deposited, composite precipitates of Mo, Nb, Ti, and V become finer when the temperature rises again, for example, during a temperature rise due to a fire. Precipitates.

 These composite precipitates also grow coarse while holding at 700 to 800 C, and the strengthening effect eventually decreases, but when they are very fine and densely dispersed, 3 Within the holding time of about 0 minutes, the above-mentioned target value of yield strength of 700 to 800 ° C. can be sufficiently obtained.

In addition, Mo, Nb, V, and Ti dissolved in the BCC phase are effective against dislocation recovery delay and have the effect of raising the temperature at which the yield strength sharply decreases. . The inventors have conducted detailed studies on the effects of these high-temperature strengthening factors on the yield stress at 700 ° C to 800 ° C, and as a result, have obtained the following knowledge. That is, in the temperature range of 700 ° C to 800 ° C, assuming that the steel material temperature is T (° C), the high-temperature normal-temperature yield stress ratio p (= high-temperature yield stress / normal-temperature yield stress) is P ≥—0.00 In order to satisfy 29 XT + 2.48, that is, to obtain a yield stress ratio of 45% and 16% or more at 700 ° C and 800 ° C, respectively, o, N b, V, both the complex carbonitride of T i is the a molar fraction 5 X 1 0- ⁴ or more, forms a solid solution with BCC phase M o, N b, V , The total amount of the composition of 1 X 1 0 _ ³ or more is required should be important in high-temperature strength development complex carbonitride precipitation phase in the molar concentration of T i are readily identified by analysis, for example by electron microscopy or EDX It is possible. In addition, the equilibrium generation amount of the thermodynamically stable precipitated phase and the amount of solid solution alloy element in the BCC phase are determined by using commercially available thermodynamic calculation database software, etc. It can be easily calculated.

 However, even if the precipitate itself is stable, if the base material is transformed by a rise in temperature, the consistency between the precipitate and the base material is lost and becomes inconsistent, so that the strengthening action of the precipitate is rapidly reduced. In other words, in order to utilize the strengthening effect of the composite precipitate that is stable even at high temperatures, it is essential for the material not to transform the base structure even at the design temperature of 80 ° C.

Therefore, specifically, by adjusting the alloy elements, such as reducing the amount of M n is austenite Tofoma, it is necessary that the A _{C l-varying} state temperature of the steel and 8 0 0 ° C or higher.

 In addition, since the idea is to enhance high-temperature strengthening by utilizing precipitates and solid-solution elements, the amount of alloying elements, such as Cr, Mn, and Mo, which were conventionally added in high-temperature steels, should be kept low. Therefore, it is possible to design an alloy that does not reduce the weldability.

In addition, since the strength of the steel with bainite single structure is high, the low yield ratio condition required for building steel cannot always be satisfied. For this reason, when a low yield ratio is required in the steel of the present invention, the microstructure should be a mixed structure of bright and bainite, and the fraction of bainite should be within the range of 20% to 95%. I do. If the fraction of ferrite in the microstructure becomes excessive, it becomes difficult to secure the strength at room temperature and high temperature due to the increase of the added alloying elements. Hereinafter, the reasons for limiting each component in the present invention will be described. In addition,% means mass%.

 C is an element that has the most remarkable effect on the properties of steel materials, and is at least 0.0 because it is essential to form complex precipitates (carbides) with Mo, Nb, Ti, and V. 0 5% is required. If the C content is less than this, the strength will be insufficient. However, if it exceeds 0.08%, A c! Since the transformation temperature decreases, it is difficult to obtain strength at 800 ° C, and the toughness also decreases. Therefore, the content is limited to not less than 0.05% and not more than 0.08%. In addition, during heating to a high temperature equivalent to a fire, the mixed matrix structure of ferrite and bainite is thermodynamically stable and maintains compatibility with the complex carbonitride precipitates of Mo, Nb, V, and Ti. In order to secure the reinforcing effect, the content is preferably set to less than 0.04%.

 Si is an element contained in the deoxidized upper steel and has a substitution-type solid solution strengthening effect, which is effective in improving the base metal strength at room temperature. No improvement. Also, the addition of a large amount deteriorates the weldability and HAZ toughness, so the upper limit was limited to 0.5%. Deoxidation of steel is possible only with T i and A 1, and the lower it is, the better from the viewpoint of HAZ toughness and hardenability, and it is not always necessary to add it.

Mn is an indispensable element for securing strength and toughness, but Mn, a substitutional solid solution strengthening element, is effective for increasing the strength at room temperature, but is particularly effective at 600 ° C. There is no significant improvement in high-temperature strength exceeding C. Therefore, in a steel containing a relatively large amount of Mo as in the present invention, the content is limited to 1.6% or less from the viewpoint of improving the weldability, that is, reducing the PCM. By keeping the upper limit of Mn low, it is advantageous from the viewpoint of center segregation of the continuous structure slab. Further, in order to keep the _ACl transformation temperature at 800 ° C. or higher, it is necessary to suppress the addition, and the upper limit is desirably set to 0.9%. The lower limit is not particularly limited. In order to adjust the strength and toughness, it is desirable to add 0.1% or more.

To obtain a proper base Inai preparative tissue fraction is required to be the cooling rate from the end of rolling after 8 0 0 ° C or higher temperatures up to 6 5 0 ° C temperatures below 0. 3 K s one ¹ or more is there. In other words, relatively thin steel sheets with a thickness of less than about 25 mm are manufactured using the air-cooling or accelerated cooling (water cooling) process, and relatively thick steel sheets with a thickness of more than about 25 mm are manufactured using the accelerated cooling (water cooling) process. Need to be

 P is an impurity in the steel of the present invention, and the lower the P content, the lower the intergranular rupture in HAZ. If the content is large, the low-temperature toughness of the base metal and the welded portion is deteriorated, so the upper limit was made 0.02%.

 S, like P, is an impurity in the steel of the present invention and is preferably as small as possible from the viewpoint of the low-temperature toughness of the base metal. If the content is large, the low-temperature toughness of the base metal and the welded portion is deteriorated, so the upper limit was set to 0.01%.

 Mo is a basic element constituting the composite precipitate which enhances the high-temperature strength, and is an essential element in the steel of the present invention. To obtain a high-density composite precipitate of Mo and Nb, Ti, or a composite precipitate of Mo and Nb, Ti, and V, and to increase the high-temperature strength, use 0.1%. On the other hand, if it exceeds 1.5%, it becomes difficult to control the uniformity of the base metal material, and the toughness of the heat affected zone is deteriorated, further losing economic efficiency. Therefore, the amount of Mo added is more than 0.1% and 1.5% or less, preferably 0.2% or more and 1.1% or less.

Nb is an element that plays an important role in ensuring a high-temperature strength of 700 ° C. and 800 ° C. in the present invention in which M 0 is added in a relatively large amount. First, as a general effect, it is a useful element for raising the recrystallization temperature of austenite and maximizing the effect of controlled rolling during hot rolling. Also, re-heating or quenching prior to rolling Also contributes to the refinement of heated austenite.

 In addition, it has the effect of improving the strength as precipitation hardening, and contributes to the improvement of the high-temperature strength by addition of Mo in combination. If the content is less than 0.03%, the hardening of precipitation hardening at 700 to 800 ° C is small, and the addition of 0.1% or more is preferable. On the other hand, if the content exceeds 0.2%, the toughness of the base material may decrease, so the upper limit is set to 0.3%. Therefore, the limited range is from 0.03 to 0.3%.

T i is also effective for increasing the high-temperature strength similarly to N b. In particular, if the requirements for base metal and weld toughness are severe, it is preferable to add them. Is because if T i when A 1 amount is small (e.g. 0.0 0 3% or less), forming a precipitate which was coupled with O as a main component T i ₂ 0 _3, intragranular transformation ferrite generation of It serves as a nucleus and improves weld toughness. In addition, Ti combines with N and precipitates finely in the slab as TiN, which suppresses coarsening of γ grains during heating and is effective for reducing the grain size of the rolled structure. The fine TiN is to refine the structure of the weld heat affected zone during welding. To achieve these effects, Ding1 needs at least 0.005%. However, if it is too large, TiC is formed and the low-temperature toughness / weldability deteriorates. Therefore, the content is preferably 0.02% or less, and the upper limit is 0.025%.

B is crucial in controlling strength through the formation fraction of bainite. In other words, B segregates at the austenite grain boundaries and suppresses the formation of ferrite, thereby improving hardenability and stabilizing the payite even when the cooling rate is relatively low such as air cooling. It is effective to generate in. To enjoy this effect, at least 0.005% is required. However, the addition of too much not only saturates the hardenability improving effect, but also may cause the embrittlement of old austenite grain boundaries and the formation of B precipitates that are harmful to toughness. 3% and did.

 A 1 is an element generally contained in deoxidized upper steel, but deoxidation is sufficient with only Si or T i, and the lower limit is not limited in the steel of the present invention (including 0% ). However, when the amount of A 1 increases, not only does the cleanliness of the steel deteriorate, but also the toughness of the weld metal deteriorates, so the upper limit was set to 0.06%.

 N is contained in steel as an unavoidable impurity, and the lower limit is not specified.However, an increase in the amount of N is extremely harmful to HAZ toughness and weldability, and the upper limit is set in the steel of the present invention. 0.06%.

 Next, the reason for addition of Ni, Cu, Cr, V, Ca, REM, and Mg, which can be contained as needed, and the range of the added amount will be described. The main purpose of adding these elements to the basic components is to improve properties such as strength and toughness without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount added is of a nature that should be naturally restricted.

 Ni improves the strength and toughness of the base metal without adversely affecting weldability and HAZ toughness. In order to exert these effects, it is essential to add at least 0.05% or more. On the other hand, an excessive addition not only impairs economic efficiency but also is not favorable for weldability, so the upper limit was set to 1.0%.

 Cu exhibits almost the same effects and phenomena as Ni, with the upper limit of 1.0% in addition to the deterioration of weldability, and excessive addition causes Cu-cracks during hot rolling and makes production difficult. Be regulated. The lower limit should be the minimum for a substantial effect to be achieved and is 0.05%.

Cr improves both the strength and toughness of the base metal. However, if the addition amount is too large, the toughness and weldability of the base metal and the welded portion are degraded, so the limiting range is set to 0.05 to 1.0%. The above Cu, Ni, and Cr are effective not only in terms of the strength and toughness of the base metal but also in weather resistance, and for such purposes, should be added within a range that does not impair weldability. Is preferred.

 V has almost the same composite precipitation action as Nb, but its effect is smaller than that of Nb. V also affects hardenability and contributes to improvement of high-temperature strength. The effect similar to N b is less effective at less than 0.01%. On the other hand, if it is excessive, the base material toughness may be reduced. Therefore, the lower limit of V in the steel of the present invention is 0.01%, and the upper limit is 0.1%.

 C a and R EM combine with S, which is an impurity, to improve toughness and suppress cracks induced by diffusion of hydrogen in the welded part.However, if too large, coarse inclusions are formed and adverse effects are caused. The appropriate ranges are 0.0 005 to 0.004% and 0.005 to 0.004%, respectively.

 Mg has the effect of suppressing the growth of austenite grains in the heat-affected zone of the welding and reducing the size of the austenitic grains, and can strengthen the toughness of the welded portion. In order to enjoy such effects, Mg needs to be at least 0.001%. On the other hand, as the amount of addition increases, the effect on the amount of addition decreases, losing economic efficiency. Therefore, the upper limit was set to 0.006%.

As in the case of Mo, Nb, and V, adding an appropriate amount of W to ensure high-temperature strength is also an effective means for improving the properties of the steel of the present invention. W must be at least 0.01% in order to obtain its effect, but if it exceeds 1%, the effect will be saturated, so the upper limit is set to 1% from the viewpoint of economy. In order to secure cracking susceptibility at room temperature and enable welding without preheating, the value of should be further limited to a range of 0.20% or less. PCM is an index that indicates weldability. The lower the value, the better the weldability. In the steel of the present invention, excellent weldability can be ensured in the range of PCf ^ ^s 0.20% or less. The weld crack susceptibility composition P CM is given by the following equation. More defined.

 P CM = C + S i / 3 O + Mn / 2 O + C u / 20 + N i / 60 + C r / 20 + M o / l 5 + V / l 0 +5 B

 In addition, at the 1/4 thickness position in the thickness direction in the final rolling direction of the steel sheet, the former austenite grain size of the final transformed structure is limited to an average circle equivalent diameter of 15Ομππι or less. This is because the prior austenite grain size has a great effect on the toughness together with the microstructure. In particular, in order to increase the toughness of the 添 ο-added steel as in the present invention, the prior austenite grain size is reduced. Control is important and essential. The reason for limiting the former austenite particle size is based on the results of experiments conducted by variously changing the manufacturing conditions of the inventors. If the average circle equivalent diameter is 120 μππ or less, it is lower than the present invention. It can secure toughness comparable to that of Mo steel. In some cases, it is not always easy to distinguish old austenite grains. In such a case, a notched impact test piece taken in a direction perpendicular to the final rolling direction of the copper plate, centered on the thickness position of the plate 14, for example, a JISZ224-4 test piece (2 mm Using a V notch, etc., define the fracture surface unit at the time of brittle fracture at sufficiently low temperature as the effective crystal grain size that can be read as the old austenite grain size, and measure the average equivalent circle diameter However, also in this case, it is necessary to be equal to or less than 150 μπι.

 In the method for producing a high-strength steel excellent in high-temperature strength according to the present invention, the heating temperature during rolling of a billet or a slab is set to a high temperature in order to sufficiently form a solid solution of Mo, Nb, Ti, and V. Desirably, the temperature should be 110 ° C or more and 125 ° C or less from the viewpoint of securing the toughness of the base metal.

Next, hot rolling is performed in a temperature range of 110 ° C or less to secure a cumulative draft of 30% or more with respect to the finished plate thickness, and the rolling is completed at 850 ° C or more. Excessive reduction in the low temperature range promotes ferrite transformation. The ferrite fraction becomes excessive, making it difficult to secure the strength.In addition, Nb, Ti, and V precipitate as carbides during rolling, and the necessary solid solution M0, Nb, Ti, and V are obtained. Therefore, the lower limit of the rolling end temperature is 850 ° C. On the other hand, if the rolling is completed at a temperature exceeding 110 ° C, the toughness is insufficient, so the upper limit is set to 110 ° C.

After the end of rolling, the steel sheet is cooled from the temperature range of 800 ° C or higher to the temperature range of 600 ° C or lower at an average cooling rate of 0.3 Ks- ¹ or higher. The purpose is to obtain a rolled structure containing many deformation zones and dislocations, which become precipitation sites, and freeze it by water cooling to obtain Mo, Nb, Nb, The object is to obtain a composite precipitate with Ti and V at a high density.

In addition, even if the steel of the present invention is reheated to a temperature lower than the _ACl transformation point for the purpose of dehydrogenation or the like after production, the characteristics of the steel of the present invention are not impaired at all.

 After water cooling, the steel sheet may be subjected to a tempering heat treatment in a temperature range of 500 ° C or less for 30 minutes or less.

 Further, the steel of the present invention can sufficiently enjoy the effects of the present invention not only as a thick steel plate but also as a steel material such as a steel pipe, a thin steel plate, and a section steel. ' Example

 Converter continuous production-steel plate of various steel components (thickness: 15 to 50 mm) is manufactured in a thick plate process, and its strength, toughness, and yield strength at 700 ° C and 800 ° C are manufactured. The presence of root cracks during the y crack test without preheating (room temperature) was investigated.

 Tables 1 and 2 show the steel composition of the steel of the present invention together with the comparative steel, Table 3 shows the manufacturing conditions and structure of the steel sheets, and Table 4 shows the results of investigations on various properties.

In the example of the steel No. 1 to 9 of the present invention, the microstructure was all It has a mixed structure of bainite, and the average equivalent circle diameter of the prior austenite particle size is less than 120 μm. Furthermore, the actual yield strength ratios are excellent values of more than 64% and 23% at 700 ° C and 800 ° C, respectively.

In the example of the steel No. 10 to 18 of the present invention, the microstructure is a single bainite structure or a mixed structure of ferrite and bainite, and the average circle of the prior austenite grain size is obtained. equivalent is below 1 2 0 _M m diameter, for the ratio of the track record yield strength, 7 0 0 ° C, 8 0 0 ° C , respectively 61% and an excellent value of more than 25%.

 In the comparative steel No. 19, C was excessive, and the onset of the reverse transformation to austenite, Aci, was 800 ° C or less. The high temperature yield strength ratio (P) is P <— 0.029 XT + 2.48.

In the comparative steel No. 20, C is insufficient, the yield strength is insufficient at 490 MPa class, and the production amount of the composite carbonitrided phase at a high temperature of 600 ° C or more is 5 × 10 — Less than ⁴ , and the yield strength ratio (P) at room temperature and high temperature is as low as 0.00.029 XT + 2.48.

 In the comparative steel No. 21, since the 1 ^ 11 amount exceeds 1.6%, the A ci is less than 800 ° C, and at a temperature of 700 ° C or more, normal temperature / high temperature yield strength The ratio (P) is p—0.029 XT + 2.48 Comparative steel No. 22 has a solid solution strengthening effect at room temperature because the Mn content is less than 0.1% The yield strength and tensile strength at room temperature were below the lower limit of the standard of 490 MPa.

In the comparative steel No. 23, since P exceeds 0.02%, both the ductile-brittle transition temperature of the base metal and the absorbed energy value of the HAZ reproduced at a high temperature are deteriorated. In the comparative steel No. 24, S exceeds 0.01%, so the ductile brittle transition temperature of the base material and the HAZ absorption at 0 ° C, as in the comparative steel No. 23 Both energy values have deteriorated.

 In the comparative steel No. 25, the room temperature strength was good because the amount of Mo added was insufficient and the amount of solid solution Mo in the carbonitriding precipitation phase and the BCC phase was insufficient. The yield ratio at high temperature and room temperature is as low as 15%.

In Comparative steel N o. 2 6, since M o amount is excessive, increases the non-uniformity of matrix material, regardless weld crack susceptibility composition P _CM is involved in a 1 8% 0., without preheating Root cracking occurred in the y-cracking test. Also, the absorption energy value of the reproduction HAZ is low.

 In the comparative steel No. 27, the amount of Nb was insufficient, and a sufficient precipitation hardening effect could not be obtained at 700 ° C and 800 ° C. P) is p-0.02 9 XT + 2.48.

 The comparative steel No. 28 has a high Nb content, so a high value is obtained for the high-temperature strength, but the absorption energy value of the reproduced HAZ is low. Is large, the absorption energy value of the reproduced HAZ is low.

 In the comparative steel No. 30, the amount of Ti was excessive, so that both the ductile-brittle transition temperature of the base metal and the reproduced HAZ absorption energy value were deteriorated. In comparative steel No. 31, the amount of B added is insufficient, sufficient hardenability cannot be obtained, and the bainite fraction of the microstructure is too small, so that the yield strength at room temperature is 490 MP. It has fallen below the lower limit of the a-class specification.

In the comparative steel No. 32, the transition temperature of ductile brittleness of the base metal is near 0 ° C due to the excessive amount of B added, and the absorbed energy value of the reproduced HAZ is low. In the comparative steel No. 33, since the amount of 81 exceeds 0.06%, the ductile-brittle transition temperature of the base metal is near 0 ° C, and the reproduced HAZ toughness is low. In 4, the reproduced HAZ toughness is low because the N content exceeds 0.006%.

 In comparative steel No. 35,? (; „Value exceeds 0.20%, root crack occurred in y crack test without preheating. Also, reproduced HAZ absorption energy value is low.

 In Comparative Steel No. 36, since the reheating temperature was lower than 110 ° C, the added alloying element did not form a solid solution in the austenite during reheating, and sufficient precipitation strengthening was not obtained. Is a good result for both yield strength and tensile strength, but the yield strength ratio (P) at room temperature and high temperature is P <_ 0.02 9 XT + 2.48.

 In the comparative steel No. 37, the reheating temperature exceeded 125 ° C, so the austenite grains became coarse during reheating, and the absorption energy value of the reconstructed HAZ was low.

 In the comparative steel No. 38, since the cumulative rolling reduction at 110 ° C or lower is less than 30%, the prior austenite grains are coarse and the reproduced HAZ toughness is low.

Since the comparative steel No. 39 was rolled at a temperature lower than 850 ° C., the precipitation of Nb, Ti, and V was promoted, and sufficient precipitation strengthening was not obtained. The comparative steel No. 40, which satisfies the standard value of 90MPa class, but has a yield strength ratio (p) of 0.0000 XT + 2.48 at room temperature and high temperature. However, since the reheating temperature is as high as 125 ° C, the austenite grains after rolling are over 20 and coarse, and the base metal toughness is high. Poor.

 In comparison steel N 0.41, the room temperature strength was increased by performing water cooling after rolling, but the sheet thickness was large and the cooling rate near the γΖα transformation temperature in the 1Z4 thick part was insufficient, so The ferrite fraction is excessive (> 80%: bainite fraction is 20%), and the effect of solid solution strengthening at room temperature is insufficient, and the tensile strength at room temperature for construction is 49 OMPa grade steel. Below the specified lower limit.

Since the comparative steel ^No. 42 has a plate thickness of more than 25 mm, accelerated cooling was applied to ensure a cooling rate of 0.3 Ks ¹ or more, but the water cooling start temperature was 700 °. less than C, the cooling rate of the finish rolling after-cooling start (6 9 0 ° C) becomes 0. 3 Κ 3 · ¹ or less, the varying state of the ferrite before water cooling initiation has progressed, bainite fraction It was less than 20%, and the room temperature tensile strength was lower than 49 OMPa.

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Table 3

Section Steel Heating Rolling 1000 ° C Acceleration Acceleration Plate thickness BC No preheating Duplex

Min; pan 'end cooling below C phase

 Y division in S

ACT a

 ; Dish ίχ.

 Reduction amount Dish nn.fx.

(%) + Soluble particle size cracking Yes I Bok original ^free) products

 Hata

 Fraction amount

 1)

 (° C) (° c) (° C) (.c) (mm) (%) (.c) X X (μm)

10 ³ 10 ³

 1 1150 880 70 25 45 891 1.35 7.06 55 No crack

2 1200 900 60 15 62 877 0.57 4.62 72 No cracK

3 1100 880 50 850 450 40 41 829 0.82 2.92 45 No crack

4 1150 910 70 20 40 833 1.03 6.24 56 No crack

5 1100 870 50 25 59 815 0.62 2.00 88 No crack

6 1100 90 "θ" 40 880 495 50 46 863 1.00 4.47 43 ¹ No crsck

7 1100 970 30 820 500 30 63 803 1.40 2.33 51 No crack Book 8 ϊ 950 950 50 820 500 32 44 839 1.06 2.84 66 No crack request 9 1150 880 60 18 50 854 1.12 6.20 55 No crack Departure 10 1150 870 70 25 85 815 1.33 7.08 55 No crack Light 11 1100 1000 30 30 73 805 2.73 5.90 51 No crack Steel 12 1150 960 65 20 55 821 1.03 6.24 56 No crack

13 1100 920 50 850 580 50 85 805 1.84 1.92 82 No crack

14 1100 900 50 850 480 40 75 812 4.08 4.93 59 No crack

15 1100 880 60 820 650 65 100 832 0.73 4.85 76 No crack

16 1100 900 60 860 600 32 81 828 2.27 7.22 78 No crack

17 1150 860 60 810 590 28 88 808 1.20 3.96 73 No crack

18 1150 960 50 900 620 45 89 817 2.46 6.65 62 No crsck

Table 3 continued

1 ) 700°C における相モル分率熱力学計算値

1) Calculated thermodynamics of phase mole fraction at 700 ° C

 2) Calculated mole fraction thermodynamics at 700 ° C

 3) Average circle equivalent diameter of old austenite grains at 1/4 thickness position in the thickness direction in the final rolling direction of the steel sheet.

 4) JIS Z 3158: Oblique y-shaped weld cracking test. Table 4

Ward steel

 Room temperature strength 700 ° C 800¾ min HAZ S3

Kei ^, Y ¾ 4¾ Ύ

Yield strength ^υ tensile strength ²⁾ yield ratio yield strength yield strength of _V E. ⁵ ) Ratio ^3> Ratio ⁴⁾

 (Pa) (MPa) (%) (MPa) (%) (MPa) (%)

 1 ■ 366 499 73-51 236 64 85 23 220

2 409 530 77 -40 267 65 94 23 210

3 353 489 72-32 232 66 86 24 199

4 348 486 72 1 35 225 65 81 23 187

5 408 530 77 -37 263 64 93 23 225

6 362 496 73 -40 237 65 85 24 218

7 421 539 78 -35 274 65 97 23 155 8 357 492 73 -41 233 65 84 24 230 Application 9 375 506 74-33 246 65 88 24 224 Dep. 10 516 699 74-51 337 65 135 26 250 Akira 11 521 689 76 -45 374 72 137 26 205 Steel 12 468 686 68 -45 325 69 121 26 227

13 535 723 74 -42 327 61 121 23 238

14 483 729 66 1 40 335 69 124 26 241

15 551 680 81 -42 377 68 136 25 254

16 492 703 70 -43 346 70 128 26 271

17 524 721 73 -46 386 74 143 27 242

18 506 699 72-52 343 68 128 25 227

4 Continuation

1) Room temperature yield strength ≥ 3 2 5 M Pa

 2) Tensile strength at room temperature ≥ 490 MPa

 3) Ratio of yield strength at 700 ° C to actual yield strength at room temperature (P) ≥ 45%

 4) Ratio of yield strength at 800 ° C to actual yield strength at room temperature (P) ≥ 16%

5) PT 1400 ° C Δt 8/5 = 99S _v E ≥ 27 J Industrial availability TJP03 / 04040 The steel composition manufactured by the chemical composition and the manufacturing method of the present invention has a microstructure of a mixed structure of ferrite and bainite or a single structure of bainite.

A high-strength steel with a normal-temperature strength of more than 490 MPa, and a high-temperature Z normal-temperature stress ratio at 600 to 800 ° C (high-temperature yield stress / normal-temperature yield stress): p is the steel material temperature TC) Therefore, it has characteristics satisfying p≥—0.029 XT + 2.48, and also has the characteristics required for building refractory steel, making it a completely new steel material. .

Claims

The scope of the claims 1. In mass%, C: 0.005% or more and less than 0.8%, Si: 0.5% or less, Mn: 0.1 to; 1.6%, P: 0.02 % Or less, S: 0.01% or less, Mo: 0.1 to: 1.5%, Nb: 0.03 to 0.3%, Ti: 0.025% or less, B : 0.000 5 to 0.03%, A1: 0.06% or less, N: 0.06% or less, characterized by the balance Fe and unavoidable impurities High strength steel with excellent high temperature strength.  2. The stress reduction ratio of the steel in which the yield stress at high temperature is made dimensionless from the yield stress at normal temperature (high temperature yield stress Z normal temperature yield stress): P is steel temperature T (° C) is 600 2. The high-tensile steel excellent in high-temperature strength according to claim 1, wherein p≥0.02 XT + 2.80 is satisfied in the range of not less than ° C and not more than 800 ° C. .  3. The steel is a single structure of bainite or a mixed structure of ferrite and bainite at room temperature when heated to a high temperature equivalent to a fire. The temperature (A) reverses to austenite when heated to a high temperature equivalent to a fire. c,) is more than 800 ° C, and the stress reduction rate is the non-dimensional yield stress at high temperature from the yield stress at normal temperature (high temperature yield stress Z normal temperature yield stress): P is the steel material temperature T Claim 1 characterized by satisfying p ≥ 1.0 0.029 XT + 2.80, when (° C) is in the range of 600 ° C or more and 800 ° C or less. High tensile steel with excellent high temperature strength.  4. In the high temperature range of 600 ° C or more and 800 ° C or less, the steel is reduced from the yield stress at normal temperature to the non-dimensional yield stress at high temperature. (High temperature yield stress Kano room temperature yield stress): p≥_0.02 0 XT + 2 when p is steel temperature T (° C) in the range of 600 ° C or more and 800 ° C or less. 80, and at room temperature when heated to a high temperature equivalent to a fire A single structure or a mixed structure of ferrite and bainite in which the temperature (A _C1 ) at which austenite reversely transforms is higher than 800 ° C; Maintains a thermodynamically stable carbonitrided precipitate phase at a mole fraction of 5 X 10 or more in an initite single structure or a mixed structure of ferrite and bainite and dissolves in the ferrite structure. M o, N b, high-tensile steel the total amount of T i is excellent in high temperature strength according to claim 1, wherein a is 1 X 1 0- ³ or more on a molar 5. In the high-temperature range of 600 ° C or more and 800 ° C or less, the steel is reduced from the yield stress at room temperature to the non-dimensional yield stress at high temperatures. (High-temperature yield stress Z room-temperature yield stress): P≥0.02 XT +2 when P is steel temperature T (° C) in the range of 600 ° C or more and 800 ° C or less. At a temperature (A) at which a single bainite structure or a mixed structure of ferrite and bainite at room temperature reversely transforms to austenite at a high temperature equivalent to a fire, having a strength that satisfies _{C l} ) is more than 800 ° C, and the average circle equivalent diameter of the former austenite grains is 120 in or less and the bainite single tissue, or ferrite and bainite. together thermodynamically stable carbonitride precipitation phase mixed structure in preparative held in a molar fraction 5 X 1 0- ⁴ or more, Mo for solid solution in ferrite tissues, N b, the total amount of T i 2. The high-tensile steel having excellent high-temperature strength according to claim 1, wherein is a molar concentration of 1 × 10 or more.  6. The steel is PCM = C + Si / 3O + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 2O + Mo / 15 + V / 10 + The high tensile strength steel excellent in high-temperature strength according to any one of claims 1 to 5, characterized in that a weld crack susceptibility composition defined by 5B: PCM is 0.20% or less. 7. The steel further has a mass of ⁰ /. And N i: 0.05 to 1.0%, C u: 0.05 to 1.0%, Cr: 0.05 to 1.0%, V: 0.01 to 0.1% One or more of the following: A high-tensile steel excellent in high-temperature strength according to any one of claims 1 to 7.  8. The steel further contains, by mass%, Ni: 0.05 to 1.0%, Cu: 0.05 to: L. 0%, Cr: 0.05 to 1.0%. %, V: 0.01 to 0.1%, contains one or more of them, and C a: 0.0005 to 0.004%, REM: 0.00 The composition according to any one of claims 1 to 7, wherein the composition contains one or more of 0.5 to 0.004%, Mg: 0.00001 to 0.06%. High-strength steel with excellent high-temperature strength as described in the item.  9. In the high-temperature range of 600 ° C or more and 800 ° C or less, the steel has a dimension in which the yield stress at normal temperature is reduced from the yield stress at high temperature to the dimensionless dimension. (High temperature yield stress Z room temperature yield stress): P≥0.03 XT + 2. when P is steel temperature T (° C) in the range of 600 ° C or more and 800 ° C or less. At a temperature (A) at which the bainite single structure or the mixed structure of ferrite and bainite at room temperature reversely transforms to austenite when it has a strength that satisfies 80 and is heated to a high temperature equivalent to a fire. ci) is greater than 800 ° C, the average austenite grains have an average equivalent circle diameter of 120 / zm or less, and the bainite single tissue, or ferrite and bainite. together thermodynamically stable carbonitride precipitation phase mixed structure in preparative held in a molar fraction 5 X 1 0- ⁴ or more, the sum of M o, N b, T i of solid solution in ferrite tissue high tensile steel amount and excellent high temperature strength according to claim 7 or 8, characterized in der Rukoto 1 X 1 0- ³ or more on a molar. 110. After re-heating the slab or the slab having the steel component composition according to any one of claims 1 to 9 to a temperature range of 110 to 125 ° C., Temperature of 850 ° C or more, with the cumulative reduction at 0 ° C or less as 30% or more In hot rolled, from 8 0 0 ° C or higher temperature region after hot rolling finished up 6 5 0 ° C the following temperature range is cooled at a cooling rate of 0. 3 K s ^1, the steel microstructure A method for producing a high-strength steel excellent in high-temperature strength, characterized in that it has a single-unit structure or a mixed structure of ferrite and bainite.  1 1. In mass%, C: 0.05% or more 0.08. /. Less than, S i: 0.5% or less, 1: 0.1 to 1.6%, P: 0.02% or less, S: 0.01% or less, Mo: 0.1 to 1.5 %, Nb: 0.03 to 0.3%, Ti: 0.025% or less, B: 0.0000 to 5%, A1: 0.06% or less , N: 0.06% or less, ferrite consisting of the balance Fe and inevitable impurities, and having a bainite fraction of 20 to 95% at normal temperature when heated to a high temperature equivalent to a fire A structure in which the mixed structure of ぉ and veneite has a temperature (Ac!) At which the reverse transformation to austenite exceeds 800 ° C, and has a low yield ratio, and is characterized by having a high yield strength. Tension steel.  1 2. The steel further contains, by mass%, Ni: 0.05 to 1.0%, Cu: 0.05 to 1.0%, Cr: 0.05 to 1.0. The high-tensile steel excellent in high-temperature strength according to claim 11, characterized in that it contains one or more of 0% and V: 0.01 to 0.1%.  1 3. The steel further contains, by mass%, Ni: 0.05 to: 0.5%, Cu: 0.05 to 1.0%, Cr: 0.05 to: L 0%, V: 0.01-0.1% 1 or 2 or more kinds, and C a: 0.0005 to 0.04%, REM: 0. Claim 11 or Claim 1 characterized in that it contains one or more of 0.0005% to 0.004%, Mg: 0.00001 to 0.006%. 2. The high-tensile steel according to any one of the above items 2, which is excellent in high-temperature strength. 1 4. A piece or a piece of steel having the steel composition according to any one of claims 11 to 13 is 110 to 125. (After reheating to the temperature range of Hot rolling at a temperature of 850 ° C or higher with the cumulative rolling reduction at 110 ° C or lower being 30% or more, and from the temperature range of 800 ° C or higher after the end of hot rolling ° C under until: temperature region is cooled at a cooling rate of 0. 3 K s ^1, the steel microstructure of the base Inai preparative single tissue or a woven ferrite and downy Inai bets mixed sets of, and fire considerable When heated at a high temperature, the mixed structure of ferrite and bainite having a bainite fraction of 20 to 95% at room temperature has a temperature (A _C1 ) at which the austenite reversely transforms to over 800 ° C. A method for producing a high tensile strength steel having a high yield strength, wherein the high tensile strength steel has a low yield ratio.