JP2002020832A - High tensile strength steel excellent in high temperature strength and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide high tensile strength steel excellent in high temperature strength and also good in weldability and toughness and to provide its production method. SOLUTION: This steel contains, as components, C, Si, Mn, Mo, Al, Nb, V and Ti, and the balance iron and, if required, contains Cu, Ni, Cr, Mg, Ca and rare earth metals. In its production method, after reheating at 1,000 to 1,250 deg.C, the cumulative draft at <=1,000 deg.C is controlled to >=30%, rolling is finished at >=750 deg.C, after that, air cooling or accelerated cooling from >=700 to <=600 deg.C is performed, or, subsequently to the hot rolling, normalizing or quenching is performed at Ac3 to 950 deg.C, and, if required, tempering is thereafter performed at <Ac1. For the purpose of reducing its yield ratio, the above steel sheet is reheated to a two phase coexistent region of α and γ of >Ac1 to <Ac3, is then cooled to <=600 deg.C by air cooling or at a cooling rate of that of air cooling or more and is, if required, further tempered at <Ac1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a high-strength steel capable of withstanding a demand as a building steel or the like capable of guaranteeing a high yield strength at the time of a fire at the same time as a low yield ratio and a high toughness from the viewpoint of earthquake resistance. It is related to the production method, and is most suitable in the steel industry for a plate mill. It can be applied not only to the construction field but also to a wide range of applications as general welding structural steel such as civil engineering, marine structures, shipbuilding, and various storage tanks.

[0002]

2. Description of the Related Art With the shift from elastic design (allowable stress design) to ultimate strength design based on a new seismic design standard implemented in June 1981, low yield ratios are required for building steel materials. I have. In order to achieve a low yield ratio, generally, a dual phase (dual phase) of the steel structure is formed, that is, a soft phase (usually ferrite) that controls yield and a hard phase (pearlite, bainite) for securing tensile strength. , Martensite, etc.) are widely used. Specifically, the steel after hot rolling or quenching, including controlled rolling, is reheated to the two-phase temperature of ferrite and austenite to form austenite in which ferrite and C are concentrated, and then air-cooled. Japanese Patent Laid-Open No. 2-266378 discloses a method of cooling at the above-described cooling speed (and further tempering thereafter).
No., for example. At this time,
The higher the C content, the easier the two-phase organization becomes,
The hard phase hardens more and the low yield ratio becomes easy. But,
There is a problem that increasing the C content is disadvantageous for weldability and low-temperature toughness. On the other hand, in order to improve low temperature toughness,
Although reduction in C and controlled rolling are effective, they all increase the yield ratio, so that improvement in low-temperature toughness and reduction in yield ratio are incompatible, and it is extremely difficult to achieve both. Conventionally, for architectural applications,
Although the required level of toughness was low and there was no particular problem with high-C steel, which is advantageous for lowering the yield ratio, the demand for seismic performance in recent years following the Great Hanshin Earthquake was not sufficient. There was a problem that it could not be handled.

As for so-called refractory steels for architectural use for the purpose of guaranteeing high-temperature strength, a method for producing Mo-containing steel is disclosed in Japanese Unexamined Patent Publication No. 2-77523 and many other publications. However, Mo remarkably enhances the hardenability of steel, and has an extremely strong interaction with C. Therefore, material changes are sensitive to changes in manufacturing conditions, and the strength-toughness balance at room temperature and its variation, and room temperature strength. In consideration of the balance between high-temperature strength and high-temperature strength, it is effective in terms of high-temperature strength, but was not added much as a general welded structural steel. Further, the addition of a large amount of Mo significantly deteriorates the toughness of the base material and the welded portion in addition to the remarkable deterioration of the weldability, so that the Mo was not added much even for the purpose of improving the high-temperature strength.

[0004]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems of the prior art, the present invention aims to obtain a high-strength steel having excellent high-temperature strength and excellent toughness and weldability. One of Nb, V and Ti which are carbide forming elements
More species in terms of the combined addition, even weld cracking susceptibility composition P _CM limitation, further by limiting the manufacturing process, the steel having the above-described composite properties, and steel can be industrially stable supply of It provides a method.

[0005]

SUMMARY OF THE INVENTION The point of the present invention is that a relatively large amount of Mo and carbon (nitride) forming elements Nb, V,
The primary purpose is to stably secure high-temperature strength by adding one or more kinds of Ti in combination, and in order to guarantee the deterioration of weldability and toughness due to the addition of a large amount of Mo, C, S
i, limiting the individual amounts of alloying elements and P _CM, including Mn, further by limiting the manufacturing conditions is to be compatible excellent high-temperature strength and weldability, the composite properties such as toughness.

[0006] For this purpose, the manufacturing method including the steel component is limited according to the present invention, and the gist is as follows.

(1) The steel component is represented by mass% and C: 0.03
0.15%, Si: 0.6% or less, Mn: 1.6% or less, P: 0.02% or less, S: 0.01% or less, Mo:
0.7-1.5%, Al: 0.06% or less, N: 0.0
06% or less, and [C-0.13Nb-0.24V-0.25 (Ti-
Nb: 0.005 to 0.1%, V: 0.01 to 0 so that the amount defined as 3.4N)] / (0.063Mo) satisfies the range of 0.5 to 1.0. .
2%, Ti: at least one or more in the range of 0.005 to 0.1%, and P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni
/ 60 + Cr / 20 + Mo / 15 + V / 10 + 5B and definition to weld cracking susceptibility composition P _CM is below 0.25%, a high tensile steel balance and excellent high-temperature strength, characterized in that it consists of iron and inevitable impurities.

(2) In addition to the above steel components, C
u: 0.05 to 1.0%, Ni: 0.05 to 1.0%,
And １ ／ or more of the addition amount of Cu, Cr: 0.05-1.
0%, B: 0.0002 to 0.003%, Mg: 0.0
High-tensile steel excellent in high-temperature strength according to the above item (1), wherein one or more kinds are contained in the range of 002 to 0.005%.

(3) Ca: 0.0005 to 5% by mass
The high-tensile steel excellent in high-temperature strength according to the above item (1) or (2), further comprising any one of 0.004% and REM: 0.0005 to 0.004%.

(4) A slab or a slab made of the steel component according to any one of the above items (1) to (3) is
After reheating to a temperature range of １２1250 ° C., rolling is completed at a temperature of 750 ° C. or higher with a cumulative rolling reduction at a temperature of 1000 ° C. or lower being 30% or more. A method for producing a high-strength steel excellent in high-temperature strength, wherein the high-temperature steel is accelerated to a desired temperature of 600 ° C. or less at a cooling rate.

(5) A steel slab or a slab comprising the steel component according to any one of the above items (1) to (3) is hot-rolled and then normalized at a temperature of from Ac _{3 to} 950 ° C. A method for producing a high-strength steel excellent in high-temperature strength.

(6) After hot rolling a slab or a slab made of the steel component according to any one of the above (1) to (3), after reheating to a temperature of Ac _{3 to} 950 ° C. And a method for producing high-strength steel excellent in high-temperature strength, characterized by quenching.

(7) Any one of the above items (4) to (6), wherein the steel sheet is tempered at a temperature lower than Ac ₁ for the purpose of adjusting strength, improving toughness, or removing residual stress of the steel sheet. 2. The method for producing a high-tensile steel having excellent high-temperature strength according to claim 1.

(8) For the purpose of lowering the yield ratio,
After reheating to the two-phase coexistence region of ferrite and austenite of more than ₁ Ac _{3 and} less, it is allowed to cool to a temperature of 600 ° C. or less at a cooling rate of not less than ₁ and then tempered at a temperature of less than Ac ₁ if necessary. (4)-characterized in that
(6) The method for producing a high-tensile steel excellent in high-temperature strength according to any one of the above (6).

According to the present invention, not only large plastic deformability as a result of lowering the yield ratio (earthquake resistance in architectural uses, etc.) but also sufficient strength in an environment exposed to high temperatures such as fires. Since high-tensile steel excellent in toughness and weldability can be supplied in large quantities and at low cost, it has become possible to contribute to improving the safety of a wide range of welded steel structures for various applications.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.

The reason why the present invention limits the steel composition and the production method as described in the claims will be described.

C is most remarkably effective for the properties of steel materials. The lower limit of 0.03% is M, which is a carbon (nitride) forming element.
In the present invention in which o and at least one of Nb, V, and Ti are added in combination, this is the minimum amount for forming a precipitate. However, if the C content is too large, not only the weldability but also the hardenability will be increased more than necessary, and this will adversely affect the balance of strength and toughness that the steel material should originally have, so the upper limit was made 0.15%.

Si is an element contained in the deoxidized upper steel, but if added in a large amount, the weldability and the HAZ toughness deteriorate, so the upper limit was limited to 0.6%. Deoxidation of steel is Ti, Al
Alone is sufficiently possible, and the lower the better, from the viewpoints of HAZ toughness, hardenability, etc., and it is not always necessary to add.

Mn is an indispensable element for securing the strength and toughness of the base material. However, Mn, which is a substitution-type solid solution strengthening element, has a remarkably large effect of improving the high-temperature strength, particularly above 600 ° C. without were limited in terms of P _CM reduced to less than 1.6% in weldability improvement that invention in steel containing a relatively large amount of Mo such as in the present invention. By keeping the upper limit of Mn low, it is advantageous also from the viewpoint of the center segregation of the continuously cast slab. The lower limit is not particularly limited, but is preferably added in order to adjust the strength and toughness of the base material at normal temperature.

P is an impurity in the steel of the present invention,
Since a reduction in the amount of P tends to reduce grain boundary fracture in HAZ, a smaller amount is more preferable. If the content is large, the low-temperature toughness of the base material and the welded portion is deteriorated, so the upper limit is 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 material. If the content is large, the low-temperature toughness of the base material and the welded portion is deteriorated, so the upper limit was made 0.01%.

Mo is an indispensable element for securing the high-temperature strength of steel, and is one of the most important elements in the present invention. If only high-temperature strength up to about 600 ° C. is considered, the lower limit can be relaxed. However, ferrite + austenite for maintaining high-temperature strength exceeding 600 ° C. (for example, about 700 ° C.) and lowering the yield ratio described below. In order to ensure high strength and high toughness at room temperature even after the two-phase region heat treatment and subsequent tempering as necessary, the lower limit is set to 0.7%. If the addition is too large, it becomes difficult to control the base material (control of variation and deterioration of toughness) and also deteriorates weldability.

Al is an element generally contained in the deoxidized upper steel, but deoxidation is sufficient with only Si or Ti. In the steel of the present invention, the lower limit 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 was made 0.06%.

N is contained in steel as an unavoidable impurity. However, in the steel of the present invention to which at least one of Ti, Nb and V is added as described later, TiN is formed to enhance the properties of the steel. Or, it combines with Nb and V to form a carbonitride and increases the strength. For this reason, at least 0.001% is required as the amount of N. However, an increase in the N content is extremely harmful to HAZ toughness and weldability, and the upper limit of the steel of the present invention is 0.006%.

In the present invention, in addition to the above-described elements,
[C-0.13Nb-0.24V-0.25 (Ti-
3.4N)] / (0.063Mo), so that at least one of Nb, V, and Ti within the range described below so that the amount defined as 0.5 / 1.0 satisfies the range of 0.5 to 1.0. Addition is mandatory.

The implication of the above equation is that the added Mo
Is calculated stoichiometrically based on the atomic weight, the ratio of which precipitates as carbide (Mo ₂ C), and the molecules are NbC, V
It shows the remaining amount of C after being consumed as C and TiC (considering the consumption of Ti as TiN). When the calculated value of the above equation is 0.
5 to 1.0 means that Mo is calculated (stoichiometrically) 50
It means that １００100% is precipitated as Mo ₂ C.

That is, the present invention intends that M
o does not exist in a solid solution state more than necessary,
The purpose is to ensure an amount of C sufficient for at least 50% or more to precipitate as Mo ₂ C. This is because at a high temperature exceeding 600 ° C., the contribution of solid solution strengthening is small, and the contribution of precipitation strengthening by precipitates is greater.

The stoichiometric excess of C relative to the amount of Mo added
Has an upper limit of 1.0, which is stoichiometrically equivalent to the amount of Mo added in the above formula, since it affects the hardenability and the toughness degradation accompanying the increase in the amount of cementite produced.

Hereinafter, the limited range of the added amounts of Nb, V, and Ti will be described. These are all carbide forming elements, and the addition of one or more of them is indispensable together with Mo.

Nb is a useful element for increasing the austenite recrystallization temperature as a general effect and maximizing the effect of the controlled rolling during hot rolling. Need to be added. It also contributes to reheating prior to rolling, fine graining of heated austenite during normalizing and quenching. Furthermore, it has an effect of improving strength as precipitation hardening, and contributes to improvement of high-temperature strength by addition of Mo. However, excessive addition causes toughness degradation of the welded portion, so the upper limit was made 0.1%. Note that Mo, which is an essential element in the present invention, also has the effect of increasing the recrystallization temperature of austenite, and the addition of Nb is not necessarily essential.

V has almost the same effect as Nb, but its effect is smaller than that of Nb. V also affects the hardenability and contributes to the improvement in high-temperature strength. The effect similar to that of Nb is small when it is less than 0.01%, and the upper limit is acceptable up to 0.2%.

It is preferable to add Ti when the requirements for the base metal and the weld toughness are severe. The reason for this is that when the amount of Al is small (for example, 0.003%
The following is combined with O to form a precipitate mainly composed of Ti ₂ O _3, which serves as a nucleus for the formation of intragranular transformed ferrite and improves weld toughness. Further, Ti combines with N to form fine precipitates in the slab as TiN, which suppresses coarsening of γ grains during heating and is effective for reducing the rolling structure. Fine TiN present in the steel sheet is welded. This is because the structure of the heat affected zone is sometimes refined. In order to obtain these effects, Ti should be at least 0.1.
005% is required. However, if it is too large, a large amount of TiC is formed, and the low-temperature toughness and weldability are deteriorated. Therefore, the upper limit is 0.1%.

Next, the reasons for adding Ni, Cu, Cr, B, and Mg, which can be contained as needed, 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.

If Ni is not excessively added, weldability,
Improves the strength and toughness of the base material without adversely affecting HAZ toughness. In order to exert these effects, it is essential to add at least 0.05% or more. On the other hand, excessive addition is not only expensive but also unfavorable for weldability, so the upper limit was made 1.0%. In addition, when adding Cu, in order to prevent Cu-crack at the time of hot rolling, it is necessary to satisfy the above-mentioned addition range and at the same time, make the addition amount of Cu equal to or more than の.

Cu exhibits almost the same effects and phenomena as Ni. The upper limit of 1.0% is not only deteriorated in weldability, but excessive addition causes Cu-cracks to occur during hot rolling, which makes production difficult. Is done. The lower limit should be 0.05%, which should be the minimum for a substantial effect to be obtained. This is the Cr
The same applies to.

Cr is added in an amount of 0.05% or more in order to improve both the strength and the toughness of the base material. However, if the added amount is too large, the toughness and weldability of the base material and the welded portion are deteriorated, so the upper limit was made 1.0%.

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

B segregates at austenite grain boundaries and suppresses the formation of ferrite, thereby improving hardenability and contributing to strength improvement. To enjoy this effect, at least 0.0002% is required. However, too much addition not only saturates the effect of improving hardenability but also may form B precipitates that are harmful to toughness, so the upper limit was made 0.003%. In addition, in cases where stress corrosion cracking is a concern as steel for tanks, reduction of the hardness of the base metal and the weld heat affected zone is often the point (for example, to prevent sulfide stress corrosion cracking (SCC)). HR for
C ≦ 22 (HV ≦ 248) is essential. In such a case, the addition of B which increases the hardenability is not preferable.

Mg has the effect of suppressing the growth of austenite grains in the heat affected zone and reducing the size of the austenitic grains, thereby toughening the weld. In order to enjoy such effects, Mg needs to be 0.0002% or more. On the other hand, as the addition amount increases, the effect cost on the addition amount decreases,
The upper limit is set to 0.005% because it is not advantageous in terms of cost.

Further, Ca and REM control the morphology of MnS, improve the low-temperature toughness of the base material, and also cause hydrogen-induced cracking (HIC, SSC, SOC) in a wet hydrogen sulfide environment.
HIC) reduces susceptibility. To achieve these effects, a minimum of 0.0005% is required. However, too much addition will conversely increase the cleanliness of the steel, increase base metal toughness and hydrogen-induced cracking in wet hydrogen sulfide environments (HIC, SSC, SSC).
OHIC) To increase the sensitivity, the upper limit of the amount added is 0.00
Limited to 4%. Since Ca and REM have almost the same effect, one of them may be added in the above range.

Even if the individual components of the steel are limited, excellent properties cannot be obtained unless the entire component system is appropriate. Therefore, P
Limit the value of _CM to 0.25% or less. P _CM is a indicator of the weldability, the lower the weldability is good. In the present invention steels, it is possible to ensure excellent weldability if P _CM is 0.25% or less. Incidentally, the welding crack sensitivity composition P _CM is defined by the following equation.

P _CM = C + Si / 30 + Mn / 20 + Cu
/ 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 1
0 + 5B

Next, the manufacturing conditions defined in claim 4 of the present invention and the reasons for limiting them will be described.

As described above, various production methods can be employed depending on the purpose and use of the limited components.

First, a method for producing the as-rolled product according to claim 4 of the present invention will be described. The reason for limiting the heating temperature prior to rolling to 1000 to 1250 ° C. is to keep the austenite grains small during heating and to achieve a finer rolling structure. 1250 ° C. is the upper limit temperature at which austenite during heating does not become extremely coarse. If the heating temperature exceeds this temperature, austenite grains are coarsely mixed and the structure after transformation is also coarse, so that the toughness of steel is significantly deteriorated.

On the other hand, if the heating temperature is too low, not only is it difficult to secure the rolling end temperature (750 ° C. or higher) described later, but also if Nb is added, the recrystallization temperature of austenite is increased and hot rolling is performed. The lower limit was limited to 1000 ° C. from the viewpoint of solutionizing Nb for maximizing the effect of controlled rolling at the time and for exhibiting precipitation hardening.

When Nb is not added, since it is not necessary to consider the solution, it is preferable to heat the austenite at a temperature of 1150 ° C. or less from the viewpoint of preventing the austenite from being excessively coarsened.

In the rolling of a slab or a slab reheated to the above temperature range, it is necessary to end the hot rolling at 750 ° C. or higher with the cumulative reduction at 1000 ° C. or lower being 30% or more. If the cumulative rolling reduction below 1000 ° C is small, Mo
This is because, even in the component of the present invention to which a relatively large amount of is added, grain refinement of rolled austenite becomes insufficient, and it is difficult to secure toughness.

If the temperature at the end of rolling is lower than 750 ° C., there is a high possibility that transformation will partially start, and there is a possibility that a worked (rolled) structure may be left in the final structure, which is not only unfavorable in toughness but also yield ratio. When a low yield ratio is required for building applications or the like, it becomes difficult to manufacture the as-rolled product, so the rolling end temperature is limited to 750 ° C. or higher.

After the rolling, the steel sheet is cooled or accelerated from a temperature of 700 ° C. or more to an arbitrary temperature of 600 ° C. or less at a cooling speed equivalent to the cooling. Cooling conditions such as cooling or accelerated cooling are properties that should be changed naturally depending on the intended strength and toughness level.For the purpose of simultaneously improving strength and toughness and obtaining higher strength and high toughness, cooling Preferred is the application of accelerated cooling to obtain a fine structure.

The accelerated cooling stop temperature is set to 600 ° C. or less because the effect of accelerated cooling at the initial stage of transformation progress cannot be sufficiently obtained at a temperature exceeding 600 ° C. If the temperature is 600 ° C. or lower, the accelerated cooling stop temperature can be set to any temperature. However, if the temperature is stopped at a relatively high temperature (for example, 400 ° C. or higher), the subsequent cooling is substantially tempering, Tempering for the purpose of adjusting the strength, improving the toughness, or removing the residual stress of the steel plate can be omitted.

In the case of a low-grade steel material, which does not require a high level of material, a sufficient material can be obtained even when it is left to cool.
It is also preferable from the viewpoint of manufacturability and cost.

The cooling speed during the accelerated cooling cannot be unconditionally determined because it varies depending on the steel composition and the intended material (strength, toughness) level. It is desirable that the average cooling rate up to the temperature be at least 3 ° C./sec or more.

Next, a method for manufacturing by normalizing or quenching according to claims 5 to 6 of the present invention will be described.

Even if normalizing or quenching is performed after the hot rolling of the steel having the components defined by the present invention due to the use and the restrictions on the specifications of the steel, the excellent properties of the steel of the present invention are not impaired. Absent. Rather, it is a preferable method depending on the purpose because the structure of the steel material and the resulting material are homogenized.

However, even in this case, since the refinement of the structure is one of the points for simultaneously improving the strength and toughness of the steel material, the normalizing or quenching temperature is set to Ac _3.
It is necessary to set the temperature to at least 950 ° C. The lower limit is that heating to the austenite single phase region is essential in the definition of normalizing or quenching, and the upper limit is that the austenite grain size during reheating is not unnecessarily increased.

[0059] steel sheet is manufactured by various manufacturing methods described above, then, even if tempering at a temperature of less than Ac _1, excellent properties of the present invention is not intended to somewhat also be impaired.
Rather, in some cases, it is preferable to perform tempering for the purpose of adjusting the strength, improving the toughness by decomposing a low-temperature transformation generation structure such as martensite, which is an embrittlement structure, or removing residual stress from a steel sheet. When an element having a precipitation hardening effect, such as Nb, V, or Cu, is added, fine precipitation of precipitates is promoted by the tempering treatment, and the precipitation hardening phenomenon can be further exhibited.

Finally, a description will be given of a manufacturing method for applying a heat treatment in the austenite + ferrite two-phase coexistence region according to claim 8 of the present invention.

The heat treatment in the austenite + ferrite two-phase coexistence region is applied when a low yield ratio is required from the viewpoint of seismic resistance, for example, when the steel of the present invention is applied to the field of construction. The metallurgical implications of heat treatment in the coexistence region of austenite + ferrite are that the untransformed ferrite that discharges C and the reverse transformed austenite that is enriched in C are separated, and the latter is re-transformed in the cooling process to reduce the hardened structure. In addition, a low yield ratio is achieved by the former softened structure by substantially high-temperature tempering.

In the present invention, the amount of C is Mo, Nb,
V and Ti are stoichiometrically equal to or less than the added amount of carbide forming elements such as Ti, and the calculation shows that there is almost no substantial C amount contributing to transformation, but actually, cementite is also precipitated, Due to their solid solution and the like, a C enrichment phenomenon to reverse transformed austenite is observed.

The heating temperature at the time of the heat treatment is related to the composition ratio of austenite and ferrite, and has a property that should be changed according to the steel component and the target level of the yield ratio.

The cooling speed at the time of cooling can be naturally cooled or a higher cooling speed depending on the steel composition and the desired strength level. The cooling speed exceeding the cooling, that is, the accelerated cooling, may be performed up to a temperature of 600 ° C. or less, for the same reason as the accelerated cooling after the above-described rolling. These may be tempered at a temperature lower than Ac ₁ if necessary, for the same reason as described above.

The structure before the heat treatment in the two-phase coexistence zone slightly affects the mechanical properties after the heat treatment.
In the present invention, it may be arbitrarily selected according to the purpose such as strength, toughness level and application, and is not particularly limited.

[0066]

EXAMPLE A steel plate (thickness: 20 to 100 mm) of various steel components was manufactured in a converter-continuous casting-thick plate process, and its strength, yield ratio (YR), toughness, yield strength at 600 ° C, and welding were performed. Properties (oblique y-shaped weld cracking test) were investigated.

Table 1 shows the steel composition of the steel of the present invention together with the comparative steel, and Table 2 shows the results of investigations on the manufacturing conditions and various properties of the steel sheet.

[0068]

[Table 1]

[0069]

[Table 2]

The steel sheets (the steels of the present invention) having the components, structures and manufacturing methods according to the present invention all have good properties. In contrast, the comparative steels departing from the limiting scope of the present invention is inferior in toughness and high-temperature YS, root cracking is generated by y crack test at room temperature with P _CM high steel. Particularly, in Comparative Example 24, since the amount of Ni added was lower than the amount of Cu added, cracks occurred during hot rolling, making production difficult. In Comparative Example 26, since the added amount of Mo is high, P _CM While it is within the limited range of the present invention, y at room temperature
A root crack was generated by the crack test.

[0071]

According to the present invention, it has become possible to provide a steel having excellent high-temperature strength, which simultaneously achieves a low yield ratio depending on the weldability and toughness and the production method. As a result, it can be supplied in large quantities and inexpensively as high-strength steel with excellent high-temperature strength, weldability and toughness, as well as fire-resistant building steel with excellent seismic performance, in addition to high-temperature strength for various uses as welded steel structures. It became so. By using such steel materials,
Maintains strength at high temperatures such as during a fire, and has excellent weldability and toughness, as well as a low yield ratio for building steel, which further improves the safety of various types of welded steel structures. Became possible.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C21D 8/02 C21D 8/02 A C22C 38/14 C22C 38/14 38/58 38/58 (72) Invention Person Rikio Chiziwa 1 Kimitsu, Kimitsu-shi Nippon Steel Corporation Kimitsu Works F-term (reference) 4K032 AA01 AA02 AA04 AA05 AA08 AA11 AA14 AA16 AA19 AA20 AA21 AA22 AA23 AA27 AA29 AA31 CD40 CF01 CF02 CF03

Claims (8)

[Claims] 1. The steel component in mass%, C: 0.03-0.
15%, Si: 0.6% or less, Mn: 1.6% or less,
P: 0.02% or less, S: 0.01% or less, Mo: 0.
7 to 1.5%, Al: 0.06% or less, N: 0.006
% Or less, and [C-0.13Nb-0.24V-0.25 (Ti-
Nb: 0.005 to 0.1%, V: 0.01 to 0 so that the amount defined as 3.4N)] / (0.063Mo) satisfies the range of 0.5 to 1.0. .
2%, Ti: at least one or more in the range of 0.005 to 0.1%, and P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni
/ 60 + Cr / 20 + Mo / 15 + V / 10 + 5B and definition to weld cracking susceptibility composition P _CM is below 0.25%, a high tensile steel balance and excellent high-temperature strength, characterized in that it consists of iron and inevitable impurities. 2. In addition to the above steel components, Cu:
0.05 to 1.0%, Ni: 0.05 to 1.0%, and 以上 or more of the added amount of Cu, Cr: 0.05 to 1.0
%, B: 0.0002 to 0.003%, Mg: 0.00
The high-tensile steel excellent in high-temperature strength according to claim 1, comprising one or more kinds in the range of 02 to 0.005%. 3. Ca: 0.0005 to 0.0% by mass
The high-tensile steel excellent in high-temperature strength according to claim 1 or 2, further comprising any one of 0.4% and REM: 0.0005 to 0.004%. 4. After reheating a steel slab or a slab made of the steel component according to any one of claims 1 to 3 to a temperature range of 1000 to 1250 ° C., the cumulative rolling reduction at 1000 ° C. or less is 30. % And finish the rolling at a temperature of 750 ° C or more,
A method for producing a high-strength steel excellent in high-temperature strength, wherein the steel is then cooled or accelerated from a temperature of 700 ° C. or higher to an arbitrary temperature of 600 ° C. or lower at a cooling speed equivalent to cooling. 5. A steel slab or a slab made of the steel component according to claim 1 is hot-rolled and then normalized at a temperature of not less than Ac _{3 and not} more than 950 ° C. To produce high-strength steel with excellent high-temperature strength. 6. A slab or a slab made of the steel component according to any one of claims 1 to 3, after hot rolling, reheating to a temperature of Ac _{3 to} 950 ° C. and then quenching. A method for producing a high-tensile steel excellent in high-temperature strength, characterized by the following. In 7. intensity adjustment and improving toughness, or residual stress relief purposes of the steel sheet, according to any one of claims 4-6, characterized in that tempering the steel plate at a temperature of less than Ac ₁ A method for producing high-strength steel with excellent high-temperature strength. 8. For the purpose of lowering the yield ratio, the steel sheet should be made of more than Ac ₁
After reheating to the two phase coexistence region of ferrite and austenite less than c ₃ , cool it to a temperature of 600 ° C. or less at a cooling rate of not less than that, and then temper at a temperature less than Ac _{1 as} necessary. The method for producing a high-strength steel excellent in high-temperature strength according to any one of claims 4 to 6, characterized in that: