Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite

Definitions

• 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.
• 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.
• the above-mentioned steel material has a proof strength of about / of normal temperature at about 350 ° C., which is lower than required strength.
• 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.
• the cost of refractory coating is higher than the cost of steel, and construction costs are unavoidably increased significantly.
• Japanese Patent Application Laid-Open No. Hei 2-77552 / Japanese Patent Application Laid-Open No. Hei 10-68044 has been invented.
• the temperature is 600 ° C. or higher, it is generally referred to as refractory steel.
• 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.
• the yield strength at 600 ° C. is equal to or higher than the normal temperature yield strength of 2 Z 3.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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).
• 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.
• 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 ⁇ 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.
• PCM C + S i / 30 + Mn / 20 + C u / 20 + N i / 60 + C r / 2 O + M o / 15 + V / 10 +5
• PCM 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).
• 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.
• 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.
• the steel has a non-dimensional stress reduction ratio from the yield stress at room temperature to the yield stress at high temperature.
• 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.
• 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).
• 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%.
• Ni 0.05 to 1.0%
• Cu 0.05 to: L. 0%
• Cr 0.05 to: I. 0%
• V 0.01 to 0.1%
• Ca 0.005 to 0.04%
• REM 0
• 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.
• 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.
• 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.
• 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.
• the bainite single structure may be used to enhance the room temperature strength and emphasize the characteristics as a high-tensile steel.
• 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.
• 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.
• the microstructure 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.
• 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.
• 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.
• 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.
• % 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.
• 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.
• 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.
• 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
• 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%.
• 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.
• 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%.
• 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.
• M 0 is added in a relatively large amount.
• T i is also effective for increasing the high-temperature strength similarly to N b.
• 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.
• Ti combines with N and precipitates finely in the slab as TiN, which suppresses coarsening of ⁇ 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.
• Ding1 needs at least 0.005%.
• 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.
• 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.
• 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%.
• 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%.
• 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%.
• 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.
• 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.
• the former austenite grain size of the final transformed structure is limited to an average circle equivalent diameter of 15 ⁇ 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 ⁇ ⁇ -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 ⁇ or less, it is lower than the present invention. It can secure toughness comparable to that of Mo steel.
• 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
• 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.
• the temperature should be 110 ° C or more and 125 ° C or less from the viewpoint of securing the toughness of the base metal.
• 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.
• 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.
• the toughness is insufficient, so the upper limit is set to 110 ° C.
• the steel sheet 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.
• 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.
• 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.
• 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 ⁇ m. Furthermore, the actual yield strength ratios are excellent values of more than 64% and 23% at 700 ° C and 800 ° C, respectively.
• 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%.
• the high temperature yield strength 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.
• 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%.
• 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.
• 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.
• 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.
• 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.
• the ductile-brittle transition temperature of the base metal is near 0 ° C, and the reproduced HAZ toughness is low.
• the reproduced HAZ toughness is low because the N content exceeds 0.006%.
• 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.
• 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.
• JIS Z 3158 Oblique y-shaped weld cracking test. Table 4
• 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. .

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