KR20040089746A - High tensile steel excellent in high temperature strength and method for production thereof - Google Patents

Abstract

The present invention provides a low alloying carbon additive having excellent high temperature strength in a relatively short time of about 1 hour in a temperature range of 600 ° C. to 800 ° C. used for general structures such as construction, civil engineering, offshore structures, shipbuilding, and storage tanks. The present invention relates to a method for producing high-strength steel (steel sheet, steel pipe, section steel, wire rod) for building structures, wherein, 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: 0.1-1.5%, Nb: 0.3-0.3%, Ti: 0.025% or less, B: 0.0005-0.003%, Al: 0.06% or less, N: 0.006% or less The stress reduction rate (high-temperature yield stress / normal-temperature yield stress) which consists of Fe and an unavoidable impurity, and has made the yield stress at normal temperature non-dimensionalized from the yield stress at normal temperature: p, steel temperature T (degreeC) is 600 degreeC or more 800 P ≥ -00029 × T in the range below It relates to a high tensile strength steel having excellent high temperature strength and a method for producing the same, characterized by satisfying +2.80.

Description

High tensile strength steel with excellent high temperature strength and its manufacturing method {HIGH TENSILE STEEL EXCELLENT IN HIGH TEMPERATURE STRENGTH AND METHOD FOR PRODUCTION THEREOF}

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

In other words, when the steel materials are used in buildings such as buildings, offices, houses, multi-level parking lots, etc., it is obliged to provide sufficient fireproof coating to ensure safety in fires. It is prescribed that steel temperature does not exceed 350 degreeC.

This is because in the steel, the yield strength is about 2/3 of the normal temperature at about 350 ° C., and is less than the required strength. When steel is used for drying, fireproof coating is used to prevent the temperature of the steel from reaching 350 ° C in case of fire. Therefore, the fireproof coating cost becomes high with respect to the steel cost, and the construction cost cannot be largely avoided.

In order to solve the said subject, Unexamined-Japanese-Patent No. 2-77523, Unexamined-Japanese-Patent No. 10-68044, etc. are invented, for example.

When it is 600 degreeC or more, it is generally called refractory steel, For example, in invention of Unexamined-Japanese-Patent No. 2-77523, it has high temperature strength of 2/3 (about 70%) or more of normal-temperature yield strength at 600 degreeC. A refractory steel has been proposed. Even in the example of another 600 degreeC refractory steel, it is common to make the yield strength in 600 degreeC into 2/3 or more of normal temperature yield strength.

However, 700 degreeC refractory steel and 800 degreeC refractory steel do not show a general side at the time of setting high temperature intensity (ratio with normal-temperature yield strength). For example, Japanese Unexamined Patent Application Publication No. 2-77523 discloses a steel to which a considerable amount of Mo and Nb are added, and a 600 ° C proof strength ensures 70% or more of normal temperature proof strength, but a 700 ° C and 800 ° C yield strength. Not.

In addition, if the proof strength at 600 ° C. is about 70% of the normal temperature proof, considering the increase of the temperature at the time of fire, the amount of fireproof coating can be reduced, but the building which can be omitted is limited to open spaces such as a three-dimensional parking lot or atrium, Use in fire-resistant coatings is significantly limited.

Japanese Patent Laid-Open No. 10-68044 discloses that the microstructure is bainite with steel added with a considerable amount of Mo and Nb, so that the yield strength at 700 ° C ensures 56% or more of the room temperature yield strength. The yield strength of ° C is not shown.

That is, the steel which secured the high temperature strength of about 600 degreeC like this example is already used in the market, The invention of the steel material which ensures the constant strength at 700 degreeC is made, but the high temperature strength in 700 degreeC and 800 degreeC is made It was difficult to manufacture a stable practical steel.

On the other hand, the 850 degreeC refractory steel disclosed by Unexamined-Japanese-Patent No. 2002-105585 was recently disclosed by the present inventors. Although this steel ensures an effective precipitate even at high temperature by adding a relatively large amount of alloying elements such as Al and Ti to obtain fire resistance at 850 ° C., it is not suitable as a steel for welding structures.

As described above, when steel is used in buildings, since high-temperature strength is low in ordinary steel, it cannot be used as an uncoated or translucent cloth and an expensive fireproof coating must be applied.

In addition, even in a refractory steel, the fire resistance temperature is limited to 600-700 degreeC, and the development of the steel material which can use the fire-resistant coating at 700 degreeC and 800 degreeC, and can abbreviate | omit the fireproof coating process by this was desired.

The present invention provides a low alloying carbon additive having excellent high temperature strength in a relatively short time of about 1 hour in a temperature range of 600 ° C. to 800 ° C. used for general structures such as construction, civil engineering, offshore structures, shipbuilding, and storage tanks. The present invention relates to a method for manufacturing high strength steel (steel sheet, steel pipe, section steel, wire rod) for building structures.

The present invention provides a high tensile strength steel for use in applications such as construction civil engineering, which is excellent in high temperature strength and weldability in a temperature range of 600 ° C to 800 ° C, and a manufacturing method that enables the industrially stable supply of the steel. There is. The gist of the present invention is as follows.

(1) In mass%, C: 0.005% or more and less than 0.08%, Si: 05% 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 a high strength steel having excellent high temperature strength, characterized by consisting of the remainder part Fe and unavoidable impurities. .

(2) Stress reduction rate (high-temperature yield stress / normal-temperature yield stress) which said steel made dimensionless yield stress at high temperature from yield stress at normal temperature: p is steel material temperature T (degreeC) of 600 degreeC or more and 800 degrees C or less. High tensile strength excellent in the high-temperature strength as described in (1) characterized by satisfy | filling p≥-0.0029 * T + 2.48 in the range.

(3) The temperature at which the steel is bainite monostructure at room temperature or a mixed structure of ferrite and bainite at room temperature at the time of high-temperature heating equivalent to fire, and at a high temperature of fire corresponding to the reverse transformation into austenite (Ac ₁ ) Is more than 800 ° C, and the stress ratio (high-temperature yield stress / normal-temperature yield stress) in which the yield stress at high temperature is dimensionless from the yield stress at room temperature: p is the steel temperature T (° C) of 600 ° C or more and 800 ° C or less. The high tensile strength excellent in the high-temperature strength as described in (1) characterized by satisfy | filling p≥-0.0029 * T + 2.48 in the range of.

(4) In the high temperature region of 600 ° C or more and 800 ° C or less, the stress lowering ratio (high-temperature yield stress / normal-temperature yield stress) in which the yield stress at high temperature is non-dimensionalized from the yield stress at normal temperature: p is the steel temperature. In the range of T (° C) of 600 ° C or more and 800 ° C or less, the material satisfies p ≥ -0.0029 × T + 2.48, and bainite monostructure or ferrite at room temperature at high temperature heating corresponding to fire. And a thermodynamically stable carbonitride precipitation phase in the bainite monostructure, or in the mixed structure of ferrite and bainite, having a structure in which the mixed structure of bainite is reverse transformed into austenite (Ac ₁ ) above 800 ° C. high temperature as described in the same time to hold more than 5 × 10 ^-4 by molar fraction, solid solution Mo, Nb, (1), characterized in that the total amount of Ti less than 1 × 10 ^-3 in a molar concentration in the ferrite structure It degrees excellent high-strength steel.

(5) The stress reduction ratio (high temperature yield stress / room temperature yield stress) in which the steel is non-dimensionalized from the yield stress at room temperature to the yield stress at room temperature in the high temperature region of 600 ° C. or more and 800 ° C. or less: p is the steel temperature. In the range of T (° C) of 600 ° C or more and 800 ° C or less, the material satisfies p ≥ -0.0029 × T + 2.48, and bainite monostructure or ferrite at room temperature at high temperature heating corresponding to fire. And a structure in which the mixed structure of bainite is reverse transformed into austenite (Ac ₁ ) is higher than 800 ° C., and the average circle equivalent diameter of the old austenite particles is 120 μm or less, and the bainite monostructure or In the mixed structure of ferrite and bainite, the thermodynamically stable carbonitride precipitation phase is held at a mole fraction of 5 × 10 ⁻⁴ or more, and the total amount of Mo, Nb, and Ti that is solidly melted in the ferrite structure It is 1 * 10 <^-3> or more by molar concentration, The high tensile strength excellent in the high temperature strength as described in (1).

(6) Weld Crack Susceptibility Composition where the steel is defined as PCM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B: PCM is 0.20 It is% or less, The high tensile strength excellent in the high temperature strength in any one of (1)-(5).

(7) The steel further contains one or two or more kinds of Ni: 0.05 to 1.0%, Cu: 0.05 to 1.0%, Cr: 0.05 to 1.0%, and V: 0.01 to 0.1% by mass%. A high tensile strength steel having excellent high temperature strength according to any one of (1) to (7).

(8) The steel further contains one or two or more kinds of Ni: 0.05 to 1.0%, Cu: 0.05 to 1.0%, Cr: 0.05 to 1.0%, and V: 0.01 to 0.1% by mass%. It is excellent in the high temperature strength in any one of (1)-(7) characterized by containing 1 type (s) or 2 or more types of Ca: 0.0005 to 0.004%, REM: 0.0005 to 0.004%, Mg: 0.0001 to 0.006%. High tensile strength steel.

(9) In the high temperature region of 600 ° C or more and 800 ° C or less, the stress lowering ratio (high-temperature yield stress / normal-temperature yield stress) in which the yield stress at high temperature is dimensionless from the yield stress at normal temperature: p is the steel temperature In the range of T (° C) of 600 ° C or more and 800 ° C or less, the material satisfies p ≥ -0.0029 × T + 2.48, and bainite monostructure or ferrite at room temperature at high temperature heating corresponding to fire. And a structure having a temperature (Ac ₁ ) at which the mixed structure of bainite is reverse transformed into austenite is higher than 800 ° C., and the average circle equivalent diameter of the old austenite particles is 120 μm or less, and the bainite monostructure or mole fraction of ferrite and bay the thermodynamically stable carbo-nitride precipitates in a mixed structure of the night, while not more than 5 × 10 ^-4, the total amount of Mo, Nb, Ti, which solid solution in the ferrite structure Claims, characterized in that not less than 1 × 10 ^-3 at a concentration (7) or (8) an excellent high temperature strength, high-strength steel as described in.

(10) After reheating the casting member or steel member having the steel component composition according to any one of (1) to (9) to a temperature range of 1100 to 1250 ° C, the cumulative reduction amount at 1100 ° C or less is 30% or more. Hot-rolled at a temperature of 850 ° C or higher, and after completion of hot-rolling, cooling from a temperature range of 800 ° C or higher to a temperature range of 650 ° C or lower at a cooling rate of 0.3 Ks ^-1 or higher, and the microstructure of the steel is made of bainite monostructure or ferrite and bainite. A method for producing high tensile strength steel having excellent high temperature strength, characterized by comprising a mixed structure.

(11) Mass: 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 03%, Ti: 0.025% or less, B: 0.0005 to 0.003%, Al: 0.06% or less, N: 0.006% or less, and the remainder is composed of Fe and unavoidable impurities, and at the time of high temperature heating corresponding to fire High temperature strength, characterized in that the mixed structure of ferrite and bainite having a bainite fraction at room temperature of 20 to 95% has a resistivity ratio in a structure whose temperature (Ac ₁ ) at which the reverse transformation into austenite is higher than 800 ° C. Excellent high tensile strength steel.

(12) The steel further contains one or two or more kinds of Ni: 0.05 to 1.0%, Cu: 0.05 to 1.0%, Cr: 0.05 to 1.0%, and V: 0.01 to 0.1% by mass%. A high tensile strength steel having excellent high temperature strength according to (11).

(13) The steel further contains one or two or more kinds of Ni: 0.05 to 1.0%, Cu: 0.05 to 1.0%, Cr: 0.05 to 1.0%, and V: 0.01 to 0.1% by mass%. A high tensile strength steel having excellent high temperature strength according to (11) or (12), which contains one kind or two or more kinds of Ca: 0.0005 to 0.004%, REM: 0.0005 to 0.004%, and Mg: 0.0001 to 0.006%.

(14) After reheating the casting member or steel member having the steel component composition according to any one of (11) to (13) to a temperature range of 1100 to 1250 ° C, the cumulative reduction amount at 1100 ° C or less is 30% or more. Hot-rolled at a temperature of 850 ° C or higher, and after completion of hot-rolling, cooling from a temperature range of 800 ° C or higher to a temperature range of 650 ° C or lower at a cooling rate of 0.3 Ks ^-1 or higher, and the microstructure of the steel is bainite monostructure or ferrite and bainite. The mixed structure of ferrite and bainite having a bainite fraction at room temperature of 20 to 95% at a high temperature of fire equivalent temperature is converted to austenite (Ac ₁ ) at 800 ° C. A method for producing high tensile strength steel having excellent high temperature strength, characterized by having a resistive ratio with the excess tissue.

The inventors have already found a steel having excellent high temperature strength of 600 ° C. and 700 ° C., and the steel having high temperature strength of 600 ° C. is already used in most fields including construction, but in the market, it is also a steel that can withstand high temperatures. There is a very strong demand. At the same time, there is also a greater need for higher strength for steels having good high temperature strength.

In the fireproof design, it is sufficient to maintain high strength within the duration of the fire, and it is not necessary to consider the long-term strength as in the conventional heat-resistant steel, and it is only necessary to be able to maintain a relatively short time high temperature yield strength. For example, if the holding time at 800 ° C. can secure a short time high temperature yield strength of about 30 minutes, it can be sufficiently used as an 800 ° C. refractory steel.

In conventional refractory steels, the performance has been determined so that the high temperature yield strength is 2/3 of the normal temperature. However, considering that the actual design range of the steel structure is about 0.2 to 0.4 times the lower limit of the normal temperature yield strength, The stress reduction rate (high temperature yield stress / room temperature yield stress) which dimensioned the yield stress at high temperature: p <=-0.0029 x T + 2.48 in the range of p and steel temperature T (degreeC) of 600 degreeC or more and 800 degrees C or less. It is necessary to be satisfied.

For the increase in high temperature strength, the complex addition of Mo and Nb promotes precipitation of stable carbonitride at high temperature, and at the same time, bainization of the microstructure is effective. In order to emphasize the characteristics as a high tensile strength steel by increasing the normal temperature strength, it may be a bainite single structure.

However, as the fraction of hard bainite increases, the strength of the room temperature increases. Therefore, when the upper limit of the yield ratio (YR) is required, the microstructure may be a bainite monostructure or an appropriate bainite depending on the required room temperature strength and various characteristics. It is preferable to use a mixed structure of ferrite and bainite having a fraction.

Lowering C is effective to make an appropriate microstructure and to achieve the required room temperature strength range. Lower C also has the effect of increasing the thermodynamic stability at high temperature of the mixed structure of bainite or ferrite and bainite, and raising the reverse transformation temperature (Ac ₁ ) to austenite. In this case, however, the microstructure and the material were easily affected by the rolling conditions and the subsequent cooling conditions, and it was found that stable production was difficult.

Therefore, the present inventors have found that an appropriate amount of B addition is effective for production stabilization as a result of confronting the microstructure control and the increase in the high temperature strength.

As general welded steel, weldability needs to be provided as conventionally, and therefore steel having excellent high temperature strength of 700 ° C to 800 ° C has been a very difficult problem.

In order to solve this problem, the present inventors earnestly examine, and the high-temperature strength of 700 to 800 degreeC is the potential of precipitation strengthening by complex addition of alloying elements, such as Mo, Nb, V, Ti, and the bainitization of a micro structure. It was found that the increase in density or the potential recovery delay due to solid melting Mo, Nb, and V was effective, and Ti also had some effect.

In order to simultaneously secure both the strength of 700 ° C to 800 ° C, the strength of normal temperature, and the ratio of strength of normal temperature and high temperature (p), the microstructure is called a mixed structure of ferrite and bainite or bainite monostructure, and an addition alloy As the optimum amount of elements, it was found that it is important to obtain the thermal stability of the parent-like structure at a high temperature, an appropriate matching precipitation strengthening effect, and a potential recovery delay effect. In addition, in order to secure a resistance ratio, it is necessary to make micro structure into the mixed structure of an appropriate ferrite and bainite.

Although the yield strength of steel materials generally drops rapidly from around 450 ° C, this is because the thermal activation energy decreases with temperature rise, and the effective resistance at low temperatures with respect to slip movement of dislocation becomes invalid.

Usually, Cr carbides, Mo carbides, etc. used for reinforcement in the temperature range of about 700 ° C act as effective resistances to high temperatures of about 600 ° C with respect to the slip motion of dislocations. As I do it, I cannot almost maintain the reinforcement effect.

MEANS TO SOLVE THE PROBLEM The present inventors examined various single or complex precipitates higher than stability in high temperature. As a result, it was found that the composite precipitates with Mo, Nb, Ti, and V had high stability at high temperatures and had a high reinforcing effect even at 700 to 800 ° C. That is, by adding an appropriate amount of Mo, Nb, Ti, and V and increasing the heating temperature at the time of rolling, these are sufficiently solid-melted, and by the introduction of an appropriate rolled structure having a high dislocation density, a precipitation site capable of depositing precipitates is secured. Thereby, the composite precipitate with Mo, Nb, Ti, V precipitates finely at the time of reheating, for example, during the temperature rising by a fire.

Such a composite precipitate also grows coarsened during 700 to 800 ° C. holding, while the reinforcing effect is small, but when present in a very fine and high density dispersion, the above-mentioned 700 to 800 ° C. yield strength within a holding time of about 30 minutes. You can get enough of the target value.

In addition, Mo, Nb, V, and Ti, which are solid-melted in the BCC phase, are effective against the potential recovery delay and have an effect of increasing the temperature at which the sudden drop in yield strength starts. The inventors have studied in detail the effect of such high temperature reinforcement factors on the yield stress at 700 ° C to 800 ° C. As a result, the inventors have come to obtain the following findings. That is, at 700 ° C. to 800 ° C., the steel material temperature is T (° C.), where the high temperature room temperature yield stress ratio p (= high temperature yield stress / room temperature yield stress) satisfies p ≥ -0.0029 × T + 2.48, that is, the yield stress ratio In order to become 45% and 16% or more, respectively, at 700 degreeC and 800 degreeC, the composite carbonitride of Mo, Nb, V, Ti at the said temperature is 5 * 10 <^-4> or more as a mole fraction, and in BCC phase The total amount of Mo, Nb, V and Ti to be melted solid must be 1 × 10 ⁻³ or more in molar concentration.

The composition of the composite carbonitride precipitate phase which is important for high-temperature strength expression can be easily identified by, for example, analysis by electron microscopy or EDX. The equilibrium production amount of the thermodynamically stable precipitated phase and the amount of the solid molten alloy element in the BCC phase can be calculated more easily than the amount of the added alloying element by using a commercially available thermodynamic calculation database software or the like.

However, even if the precipitate itself is stable, if the foundation is transformed by the temperature rise, the coherence between the precipitate and the ground may be lost, and thus, misalignment may occur. Therefore, the reinforcing action of the precipitate is sharply lowered. That is, in order to utilize the strengthening effect by the composite precipitate which is stable even at high temperature, it is essential for the material not to transform the basic structure even at 800 ° C, which is a design temperature.

Accordingly, specifically, it is necessary by adjustment of the alloy elements, such as to decrease the amount of austenite formers Mn, that lectures Ac ₁ transformation temperature to more than 800 ℃.

In addition, since it is the idea of enhancing the high temperature strengthening by utilizing precipitates and solid molten elements, the amount of addition of alloying elements that have been added to conventional high-temperature steels such as Cr, Mn, and Mo can be suppressed to a low level. Alloy design is not possible.

In addition, the strength of the bainite single-structure steel increases, so the resistance ratio ratio obtained in the steel for construction cannot be satisfied. For this reason, in the steel of the present invention, when a resistance ratio is required, the microstructure is a mixed structure of ferrite and bainite, and the fraction of bainite is in the range of 20% to 95%. If the fraction of ferrite in the microstructure becomes excessive, it is difficult to secure the strength of the room temperature and the high temperature due to the increase of the additive alloy element.

The reason for limitation of each component in this invention is demonstrated below. In addition,% means mass%.

C is an element having the most remarkable effect on the properties of the steel, and at least 0.005% is required because it is essential for forming a composite precipitate (carbide) with Mo, Nb, Ti, and V. In the amount of C below this, strength is insufficient. However, if over 0.08%, the addition of Ac ₁ transformation temperature is lowered, so it is difficult to obtain a strength at 800 ℃, addition is limited to 0.005% or more and less than 0.08% since the toughness is also degraded. In addition, in order to secure the reinforcing effect by maintaining the thermodynamic and stable mixed matrix structure of ferrite and bainite at the high temperature of fire equivalent, and maintaining the consistency with the composite carbonitride precipitates of Mo, Nb, V, and Ti. It is preferable to set it as less than 0.04%.

Si is an element included in the deoxidized steel and is effective for improving the strength of the base metal at room temperature because it has a substitutional solid melt strengthening action, but it is not particularly effective in improving the high temperature strength above 600 ° C. In addition, since a large amount added, weldability and HAZ toughness deteriorate, the upper limit was limited to 0.5%. Deoxidation of steel is possible only by Ti and Al, and it is so preferable that it is low from a viewpoint of HAZ toughness, hardenability, etc., and does not necessarily need to add.

Although Mn is an indispensable element in securing strength and toughness, Mn, a substitutional solid melt strengthening element, is effective for increasing strength at room temperature, but there is no improvement effect that is too large, especially at high temperature strengths above 600 ° C. . Therefore, it was limited to 1.6% or less from the viewpoint of weldability improvement, ie, PCM reduction, in steel containing a relatively large amount of Mo as in the present invention. By suppressing the upper limit of Mn low, it becomes advantageous also from the point of center segregation of a continuous casting slab. In addition to the Ac ₁ transformation temperature to above 800 ℃ it is necessary to suppress the addition, it is preferable that the upper limit to 0.9%. Moreover, it is although it does not specifically limit about a minimum, It is preferable to add 0.1% or more on the strength and toughness adjustment of a base material.

In order to obtain an appropriate bainite structure fraction, it is necessary to make cooling rate from the temperature of 800 degreeC or more to the temperature of 650 degreeC or less after completion | finish of rolling into 0.3 Ks <^-1> or more. That is, relatively thin steel sheets having a plate thickness of less than about 25 mm should be manufactured by an air cooling or accelerated cooling (water cooling) process, and relatively thick steel plates of more than about 25 mm should be manufactured by applying an accelerated cooling (water cooling) process.

P is an impurity in the steel of the present invention, and a decrease in the amount of P tends to extinguish grain boundary fracture in HAZ. When there is much content, since the low-temperature toughness of a base material and a weld part deteriorates, an upper limit was made into 0.02%.

S is an impurity in the steel of the present invention similarly to P, and is preferably less from the viewpoint of low temperature toughness of the base material. When there is much content, since the low-temperature toughness of a base material and a weld part deteriorates, an upper limit was made into 0.01%.

Mo is a basic element constituting the composite precipitate which increases the high temperature strength, and is an essential element in the steel of the present invention. In order to obtain a high density of composite precipitates with Mo, Nb, Ti, or composite precipitates with Mo, Nb, Ti, and V, it is necessary to add 0.1% or more. Since control becomes difficult and deteriorates the toughness of the weld heat-affecting zone and economic efficiency is lost, the Mo addition amount is more than 0.1%, 1.5% or less, preferably 0.2% or more and 1.1% or less.

Nb is an element which plays an important role in securing the high temperature strength of 700 degreeC and 800 degreeC in this invention which adds comparatively large amount of Mo. First, as a general effect, it is an element useful in raising the recrystallization temperature of austenite and exhibiting the effect of the controlled rolling at the time of hot rolling to the maximum. In addition, prior to rolling, it also contributes to refining of the heated austenite during reheating, annealing or quenching.

Moreover, it has a strength improvement effect as precipitation hardening, and contributes also to high temperature strength improvement by the composite addition with Mo. If it is less than 0.03%, hardening of precipitation hardening in 700 to 800 degreeC is few, and 0.1% or more of addition is preferable. On the other hand, since the toughness of a base material may fall when it exceeds 0.2%, an upper limit shall be 0.3%. Therefore, 0.03 to 0.3% is made into a limited range.

Ti, like Nb, is also effective for increasing the high temperature strength. In particular, when the demand for the base metal and the weld part toughness is severe, it is preferable to add it. This is because when Ti is small (for example, 0.003% or less), Ti binds with O to form a precipitate containing Ti ₂ O ₃ as a main component, and becomes a nucleus for the formation of metamorphic ferrite in the particles, thereby improving the weld toughness. In addition, Ti is finely precipitated in the slab as TiN in combination with N, suppresses coarsening of γ particles during heating, and is effective for refining the rolled structure, and fine TiN present in the steel sheet is affected by welding heat during welding. To fine-tune the sub-organization. In order to acquire such an effect, Ti is required at least 0.005% or more. However, when too much, TiC is formed to deteriorate low-temperature toughness or weldability. Preferably, the upper limit is 0.02% or less.

B is very important in controlling the strength through the generation fraction of bainite. That is, B is effective in improving quenchability through segregation at the austenite grain boundary and suppressing the formation of ferrite, and stably producing bainite even when the cooling rate such as air cooling is relatively small. In order to smell this effect, at least 0.0005% or more is required. However, an excessively large amount of additives not only saturates the effect of improving the quenching property, but also forms a B precipitate that becomes detrimental to the embrittlement and toughness of the old austenite grain boundary.

Al is generally an element contained in the deoxidized steel, but deoxidation is sufficient even if only Si or Ti is used. In the steel of the present invention, the lower limit thereof is not limited (including 0%). However, when the amount of Al increases, not only the cleanliness of the steel deteriorates, but also the toughness of the weld metal deteriorates, so the upper limit is made 0.06%.

N is contained in steel as an unavoidable impurity, and the lower limit is not particularly determined, but an increase in the amount of N is very detrimental to HAZ toughness and weldability, and the upper limit is 0.006% in the steel of the present invention.

Next, the reason for addition and the range of addition amount of Ni, Cu, Cr, V, Ca, REM, and Mg which can be contained as needed is demonstrated. The main purpose of adding such an element to the base component is to improve the properties such as strength and toughness without compromising the characteristics of the steel of the present invention. Therefore, the addition amount is a property which should be naturally limited.

Ni improves the strength and toughness of the base material without adversely affecting weldability and HAZ toughness. In order to exhibit these effects, addition of at least 0.05% is essential. On the other hand, when it adds too much, it not only impairs economical efficiency, but also is unfavorable for weldability, so the upper limit was made into 10%.

Cu exhibits almost the same effects and developments as Ni, and 1.0% of the upper limit is regulated because excessive addition of weldability deterioration occurs due to the occurrence of Cu-cracks during hot rolling. The lower limit is 0.05% which should be the minimum amount for the practical effect to be obtained.

Cr together improves the strength and toughness of the base material. However, when the addition amount is too large, the toughness and weldability of the base material and the welded portion deteriorate, so the limited range is made 0.05 to 1.0%.

The above-mentioned Cu, Ni, and Cr are effective not only in terms of strength and toughness of the base material, but also in weather resistance, and for such purpose, it is preferable to add Cu, Ni, and Cr in a range that does not impair weldability.

V has a compound precipitation effect almost the same as that of Nb, but its effect is smaller than that of Nb. In addition, V also influences hardenability and contributes to the improvement of high temperature strength. The effect similar to Nb is less effective at less than 0.01%. On the other hand, when too much, a base material toughness may fall. Therefore, the minimum of V in this invention steel is 0.01%, and an upper limit is 0.1%.

Ca and REM combine with S, which is an impurity, to improve toughness and to inhibit organic cracking due to diffusion hydrogen in the weld zone. However, Ca and REM form coarse inclusions and adversely affect them. 0.004% is an appropriate range.

Mg has the effect of suppressing the growth of austenite particles and miniaturizing in the weld heat affected zone, and thus, the welded portion can be strengthened. Mg is required 0.0001% or more in order to cope with this effect. On the other hand, when the added amount is increased, the value of the effect on the added amount is reduced and economic efficiency is lost. Therefore, the upper limit was made 0.006%.

In addition, securing a high temperature strength by adding an appropriate amount of W, such as Mo, Nb, V is also an effective means for improving the properties of the steel of the present invention. W is required at least 0.01% in order to obtain the effect, but if it exceeds 1%, the effect is saturated, so the upper limit is 1% from the viewpoint of economics.

In order to ensure cracking susceptibility at normal temperature and to enable welding in preheat-free, the value of PCM is further limited to 0.20% or less. PCM is an index indicating weldability, and the lower the quality, the better the weldability. In the steel of the present invention, excellent weldability can be ensured as long as PCM is in the range of 0.20% or less. The weld crack susceptibility composition PCM is defined by the following equation.

PCM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B

In addition, in the plate thickness cross-sectional direction 1/4 thickness position of the final rolling direction of a steel plate, the former austenite particle diameter of a final transformation structure is limited to 150 micrometers or less by an average circle equivalent diameter. This is because the former austenite grain size has a great influence on the toughness together with the structure, and in order to increase the toughness, particularly in the Mo-added steel like the present invention, it is important and essential to control the small austenite grain size. The reason for limiting the former austenite particle diameter is based on the experimental results of various modifications of the manufacturing conditions of the inventors. If the average circle equivalent diameter is 120 µm or less, the steel having a lower Mo than the present invention and toughness can be secured. have. In addition, there are not a few cases where the old austenite particles are not easily distinguished. In such a case, a notch imparting impact test piece, such as JIS Z 2202 4 test piece (2 mmV notch) or the like, which is taken in a direction perpendicular to the final rolling direction of the steel sheet, centered on the sheet thickness 1/4 thickness position is used. In addition, the wavefront unit at the time of brittle fracture at low temperature is defined as the effective crystal grain size which can be corrected as the old austenite grain size, and the average equivalent circle diameter is measured. In this case, it is required to be 150 µm or less.

In the method for producing high tensile strength steel having excellent high temperature strength, the heating temperature at the time of rolling the steel member or the cast member is preferably a high temperature in order to sufficiently melt Mo, Nb, Ti, and V, but to secure the toughness of the base metal. It is 1100 degreeC or more and 1250 degrees C or less from a viewpoint of the point of view.

Next, in the temperature range of 1100 degrees C or less, hot rolling which ensures the cumulative reduction ratio of 30% or more with respect to the finish plate thickness is performed, and rolling completes at 850 degreeC or more. When excessively reducing the low-temperature region, the ferrite transformation is promoted, the ferrite fraction becomes too large, making it difficult to secure the strength, and Nb, Ti, and V precipitate as carbides during rolling, and the required solid melt Mo, Nb, Ti, V Since 850 degreeC is a lower limit for rolling completion temperature, when rolling is complete | finished by temperature exceeding 1100 degreeC, toughness will run out, and an upper limit shall be 1100 degreeC.

After the end of rolling, the average cooling rate of the steel plate surface is cooled from the temperature range of 800 degreeC or more to the temperature range of 650 degreeC or less to 0.3 Ks <^-1> or more. The objective is to obtain a rolling structure containing a large number of deformation zones or dislocations which become precipitation sites and to freeze it by water cooling to obtain a high density of complex precipitates of Mo and Nb, Ti, and V, which are fine and matched to the basis at elevated temperatures. There is.

In addition, even after the steel of the present invention is manufactured, the characteristics of the steel of the present invention are not impaired even if it is reheated to a temperature below the Ac ₁ transformation point for dehydrogenation or the like.

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

In addition, the steel of the present invention can sufficiently enjoy the effects of the present invention in addition to steel sheets such as steel sheets, steel pipes, thin steel sheets, and shaped steels.

<Example>

In the converter-continuous casting-thick plate process, steel sheets of various steel components (thickness 15 to 50 mm) were produced, and the strength and toughness of the yield strength at 700 ° C. and 800 ° C., and no preheating (room temperature) The presence or absence of the root crack at the crack test was examined.

The steel components of the steel of the present invention together with the comparative steels in Table 1 and Table 2 show the manufacturing conditions and structure of the steel sheet in Table 3, and the results of the investigation of various properties in Table 4.

In the examples of steel Nos. 1 to 9 of the present invention, all of the microstructures are mixed structures of ferrite bainite, and the average circle equivalent diameter of the old austenite particle diameter is 120 µm or less. Moreover, also about an earnings yield strength ratio, it is a value excellent in 64% and 23% or more at 700 degreeC and 800 degreeC, respectively.

In the examples of steel Nos. 10 to 18 of the present invention, the microstructure is a bainite monostructure or a mixed structure of ferrite bainite, and the average circle equivalent diameter of the old austenite particle diameter is 120 µm or less, Regarding the ratio, 61% and 25% or more are excellent values at 700 ° C and 800 ° C, respectively.

In Comparative Steel No. 19, C is excessive, and since the reverse transformation start temperature Ac ₁ to austenite becomes 800 ° C. or lower, a high value can be obtained for room temperature strength, but the yield strength ratio p at room temperature / high temperature is p <-0.0029 × T + 2.48.

In Comparative Steel No. 20, C was insufficient, yield strength was 490 MPa, and the composite carbon nitride phase at a high temperature of 600 ° C. or higher was less than 5 × 10 ⁻⁴ , and the yield strength ratio at room temperature / high temperature ( p) is also low as p <-0.0029 × T + 2.48.

In Comparative Steel No. 21, since the Mn amount exceeds 1.6%, Ac ₁ is less than 800 ° C and the normal temperature / high temperature yield strength ratio p is p <−0.0029 × T + 2.48 at a temperature of 700 ° C or higher.

In Comparative Steel No. 22, the Mn content was less than 0.1%, and the solid melt strengthening effect at room temperature was insufficient, and the yield strength and tensile strength at room temperature were lower than the lower limit of the standard value of 490 MPa class.

In Comparative Steel No. 23, since P exceeded 0.02%, both the soft brittle transition temperature of the base material and the absorbed energy value of the reproduced HAZ at 0 ° C. deteriorated.

In Comparative Steel No. 24, since S exceeds 0.01%, together with Comparative Steel No. 23, both the soft brittle transition temperature of the base material and the absorbed energy values of the reproduced HAZ at 0 ° C. are deteriorated.

In Comparative Steel No. 25, due to the lack of Mo addition amount, both the molten mol of Mo in the carbonitride precipitated phase and the BCC phase was insufficient, so the room temperature strength was a good result, but it was low as 15% for the high temperature room temperature yield strength ratio at 800 ° C. .

In Comparative Steel No. 26, due to the excessive amount of Mo, the non-uniformity of the base material was increased, and even though the weld crack susceptibility composition PCM was 0.18%, root cracking occurred in the y crack test without preheating. In addition, the absorbed energy value of reproduced HAZ is low.

In Comparative Steel No. 27, since the amount of Nb was insufficient and sufficient precipitation hardening effect could not be obtained at 700 ° C and 800 ° C, the yield strength ratio p at room temperature / high temperature was p <−0.0029 × T + 2.48.

In Comparative Steel No. 28, since the amount of Nb is excessive, a high value can be obtained for the high temperature strength, but the absorption energy value of the reproduced HAZ is low.

In Comparative Steel No. 29, since the gamma kernel is coarse, the absorption energy value of the reproduced HAZ is low.

In Comparative Steel No. 30, since the Ti amount is excessive, both the soft brittle transition temperature and the reproduced HAZ absorbed energy value of the base material are deteriorated.

In Comparative Steel No. 31, the amount of B added was insufficient, sufficient hardenability was not obtained, and the bainite fraction of the microstructure was too small, so the yield strength at room temperature was lower than the lower limit of the standard value of 490 MPa class.

In Comparative Steel No. 32, since the amount of B added was excessive, the soft brittle transition temperature of the base metal was in the vicinity of 0 ° C., and the absorption energy value of the reproduced HAZ was low.

In Comparative Steel No. 33, since the Al content exceeds 0.06%, the soft brittle transition temperature of the base metal is in the vicinity of 0 ° C., and the reproduction HAZ toughness is also low.

In Comparative Steel No. 34, since the amount of N exceeds 0.006%, the reproduced HAZ toughness is low.

In Comparative Steel No. 35, the P _CM value exceeded 0.20%, and root cracking occurred in the y crack test without preheating. In addition, the reproduction HAZ absorbed energy value is also low.

In Comparative Steel No. 36, due to the reheating temperature of less than 1100 ° C, sufficient alloy reinforcement could not be obtained without solid melting in the austenite at the time of reheating, and both yield strength and tensile strength were good at room temperature. However, the yield strength ratio p at room temperature / high temperature is p <−0.0029 × T + 2.48.

In Comparative Steel No. 37, since the reheating temperature exceeded 1250 ° C, the austenite particles coarsened at the time of reheating, and the absorption energy value of the reproduced HAZ was low.

In Comparative Steel No. 38, since the cumulative reduction in the amount of 1100 ° C. or less is less than 30%, the old austenite particles are coarse and the reproduction HAZ toughness is low.

In Comparative Steel No. 39, since rolling was carried out at a temperature of less than 850 ° C, precipitation of Nb, Ti, and V was promoted, and sufficient precipitation strengthening could not be obtained, and the room temperature strength satisfies the standard value of 490 MPa class. The yield strength ratio p at high temperature is p <-0.0029 × T + 2.48.

In Comparative Steel No. 40, since the reheating temperature was as high as 1250 ° C, the austenitic particles after the end of rolling were coarse more than 120 µm and the base metal toughness was low.

In Comparative Steel No. 41, an increase in the room temperature strength was achieved by performing water cooling after rolling, but the ferrite fraction is excessive because the sheet thickness is large and the cooling rate near the γ / α transformation temperature in the 1/4 thickness portion is insufficient. (> 80%: bainite fraction <20%), the solid-melt strengthening effect in normal temperature was insufficient, and the tensile strength of normal temperature was less than the lower limit of the standard value of 490 Mpa class steel for construction.

In Comparative Steel No. 42, since the plate thickness was more than 25 mm, accelerated cooling was applied to secure a cooling rate of 0.3 Ks ⁻¹ or higher, but the water cooling start temperature was less than 700 ° C., after the end of rolling to the start of cooling (690 ° C.). Since the cooling rate became 0.3 Ks ⁻¹ or less and the transformation of ferrite proceeded before the start of water cooling, the bainite fraction became less than 20%, and the normal temperature tensile strength was less than 490 MPa.

TABLE 1

TABLE 2

TABLE 3

Table 3 continued

1) Phase mole fraction thermodynamic calculation at 700 ° C

2) Molar fraction thermodynamic calculation at 700 ° C

3) Average circle equivalent diameter of the old austenite particles at a quarter thickness position in the thickness direction of the plate thickness in the final rolling direction of the steel sheet

4) JIS Z 3158: warp y type weld crack test

TABLE 4

[Continued Table 4]

1) Room temperature yield strength ≥ 325 MPa.

2) Room temperature tensile strength ≥ 490 MPa.

3) Ratio (p) ≥ 45% with respect to yield strength performance at normal temperature of yield strength at 700 degreeC.

4) Ratio (p)> 16% with respect to yield strength performance at normal temperature of yield strength in 800 degreeC.

5) PT: 1400 ° C., Δt 8/5 = 99S, _v E _o ≥ 27J.

The steel material produced by the chemical component and production method of the present invention is a microstructure of mixed structure of ferrite bainite or bainite monostructure, and has a high temperature strength of 490 MPa or higher, and a high temperature / room temperature at 600 to 800 ° C. Stress ratio (high temperature yield stress / room temperature yield stress): p has the necessary characteristics as a building fire resistant steel having the characteristic of satisfying p≥-0.0029 x T + 2.48 as steel temperature as T (degreeC). Not a very new steel.

Claims (14)

C: 0.005% or more and less than 0.08% by mass%, 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% A high tensile strength steel having excellent high temperature strength, comprising: Ti: 0.025% or less, B: 0.0005 to 0.003%, Al: 0.06% or less, and N: 0.006% or less and the remainder being Fe and unavoidable impurities. The stress lowering ratio (high temperature yield stress / normal temperature yield stress) in which the steel is dimensionless from the yield stress at normal temperature to the yield stress at high temperature: p is a steel temperature T (° C.) of 600 ° C. or higher and 800. High tensile strength high tensile strength steel, satisfy | filling p≥-0.0029 * T + 2.80 in the range below ° C. 2. The temperature according to claim 1, wherein the steel is bainite monostructure at room temperature or a mixed structure of ferrite and bainite at room temperature upon high temperature heating corresponding to fire, and reverse transformation into austenite at high temperature heating corresponding to fire ( Ac ₁ ) is higher than 800 ° C., and the stress lowering rate (high temperature yield stress / normal temperature yield stress) in which the yield stress at high temperature is dimensionless, p is the steel temperature T (° C.) of 600 ° C. or higher. A high tensile strength, high tensile strength steel that satisfies p ≧ −0.0029 × T + 2.80 in the range of 800 ° C. or lower. The stress lowering ratio (high temperature yield stress / normal temperature yield stress) according to claim 1, wherein the steel has a dimensionless yielding stress at high temperature from yield stress at room temperature in a high temperature region of 600 ° C or higher and 800 ° C or lower: In the range of 600 degrees C or more and 800 degrees C or less, steel material temperature T (degreeC) has intensity | strength which satisfy | fills p≥-0.0029 x T + 2.80, and bainite stage at normal temperature at the time of high temperature heating of a fire equivalent Tissue or a mixture of ferrite and bainite having a temperature (Ac ₁ ) at which the reverse transformation to austenite is higher than 800 ° C., and also thermodynamically stable carbonylation in the bainite monostructure or in the mixed structure of ferrite and bainite high temperature to the precipitation phase to the molar fraction at the same time to hold more than 5 × 10 ^-4, characterized in that the total amount of solid solution Mo, Nb, Ti in the ferrite structure, which is not less than 1 × 10 ^-3 in a molar concentration It degrees excellent high-strength steel. The stress lowering ratio (high temperature yield stress / normal temperature yield stress) according to claim 1, wherein the steel has a dimensionless yielding stress at high temperature from yield stress at room temperature in a high temperature region of 600 ° C or higher and 800 ° C or lower: In the range of 600 degrees C or more and 800 degrees C or less, steel material temperature T (degreeC) has intensity | strength which satisfy | fills p≥-0.0029 x T + 2.80, and bainite stage at normal temperature at the time of high temperature heating of a fire equivalent The tissue or the mixed tissue of ferrite and bainite has a structure at which the temperature (Ac ₁ ) reverse transformation into austenite is higher than 800 ° C, and the average circle equivalent diameter of the old austenite particles is 120 µm or less, and the bainite Mo, Nb, and T that hold a thermodynamically stable carbonitride precipitated phase at a molar fraction of 5 × 10 ^-4 or more in a monostructure or a mixed structure of ferrite and bainite, and solid melt in a ferrite structure A high tensile strength steel having excellent high temperature strength, wherein the total amount of i is 1 × 10 ⁻³ or more in molar concentration. The steel according to any one of claims 1 to 5, wherein the steel is PCM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B Weld crack susceptibility composition defined as: PCM is 0.20% or less, high tensile strength steel having excellent high temperature strength. The steel according to any one of claims 1 to 7, wherein the steel is also in mass% of Ni: 0.05 to 1.0%, Cu: 0.05 to 1.0%, Cr: 0.05 to 1.0%, and V: 0.01 to 0.1%. A high tensile strength steel having excellent high temperature strength, characterized by containing one kind or two or more kinds. The steel according to any one of claims 1 to 7, wherein the steel is also in mass% of Ni: 0.05 to 1.0%, Cu: 0.05 to 1.0%, Cr: 0.05 to 1.0%, and V: 0.01 to 0.1%. It contains one kind or two or more kinds, and one or two kinds or more of Ca: 0.0005% to 0.004%, REM: 0.0005% to 0.004%, and Mg: 0.0001% to 0.006%. River. The stress reduction rate (high temperature yield stress / room temperature yield stress) of Claim 7 or 8 which dimensioned the yield stress at high temperature from the yield stress at normal temperature in the high temperature range of 600 degreeC or more and 800 degrees C or less. ): p has a strength that satisfies p ≧ −0.0033 × T + 2.80 in the range of 600 ° C. or higher and 800 ° C. or lower, and at room temperature at the time of high temperature heating corresponding to fire. The bainite monostructure of or a mixed structure of ferrite and bainite has a structure at which the temperature (Ac ₁ ) of reverse transformation into austenite is higher than 800 ° C, and the average circle equivalent diameter of the old austenite particles is 120 µm or less, in addition, the bainite ferrite and a single organization or a mole fraction bay thermodynamically stable carbo-nitride precipitates in a mixed structure of the night, while not more than 5 × 10 ^-4, solid-dissolved in the ferrite structure Is Mo, Nb, high-temperature high-strength steel excellent in strength, characterized in that the total amount of Ti less than 1 × 10 ^-3 in a molar concentration. 10. The cumulative rolling reduction at 1100 DEG C or lower is 850 as 30% or more after reheating the casting member or steel member having the steel component composition according to any one of claims 1 to 9 to a temperature range of 1100 to 1250 DEG C. Hot-rolled at a temperature of not less than ℃, and after completion of hot-rolling, cooled from a temperature range of 800 ℃ or more to a temperature range of 650 ℃ or less at a cooling rate of 0.3 Ks ^-1 , and the microstructure of the steel is mixed with bainite monostructure or ferrite and bainite A method for producing high tensile strength steel having excellent high temperature strength, characterized by forming a structure. C: 0.005% or more and less than 0.08% by mass%, 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 the remainder is composed of Fe and inevitable impurities, and at the time of heating at a high temperature corresponding to fire, at room temperature The structure in which the mixed structure of ferrite and bainite having a bainite fraction in the range of 20 to 95% is inversely transformed into austenite (Ac ₁ ) is more than 800 ° C, and has a high yield strength, characterized by having a resistance ratio. High tensile strength steel. The said steel further contains 1 type (s) or 2 or more types of Ni: 0.05-1.0%, Cu: 0.05-1.0%, Cr: 0.05-1.0%, V: 0.01-0.1% by mass%. High tensile strength steel having excellent high temperature strength, characterized in that. The said steel is 1 type or 2 of Claim 11 or 12 which is also mass: Ni: 0.05-1.0%, Cu: 0.05-1.0%, Cr: 0.05-1.0%, V: 0.01-0.1%. A high tensile strength steel having excellent high-temperature strength, containing at least one kind and containing at least one kind of Ca: 0.0005 to 0.004%, REM: 0.0005 to 0.004%, and Mg: 0.0001 to 0.006%. After the reheating the casting member or the steel member having the steel component composition according to any one of claims 11 to 13 to a temperature range of 1100 to 1250 ° C, the cumulative reduction amount at 1100 ° C or lower is 30% or more as 850 ° C. Hot-rolled at the above-mentioned temperature, and after completion of hot-rolling, cooling from the temperature range of 800 degreeC or more to the temperature range below 650 degreeC at a cooling rate of 0.3 Ks <^-1> , and the micro structure of steel is bainite monostructure or mixed structure of ferrite and bainite. In addition, at the time of high temperature heating equivalent to a fire, a structure (Ac ₁ ) at which the mixed structure of ferrite and bainite having a bainite fraction at room temperature of 20 to 95% is reversely transformed into austenite is higher than 800 ° C. The high-temperature strength excellent strength steel manufacturing method characterized by having a resistance ratio.