JPH09176783A - High tensile strength steel excellent in weldability and atmospheric corrosion resistance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a 600N/mm<2> class high tensile strength steel used for iron and steel structures, such as bridge, warehouse, and building, and excellent in weldability and atmospheric corrosion resistance and to provide its production method. SOLUTION: This steel has a composition which consists of, by weight, 0.06-0.1% C, 0.01-0.4% Si, 0.5-1.4% Mn, 0.2-0.5% Cu, 0.05-0.3% Ni, 0.3-0.8% Cr, 0.005-0.05% Nb, 0.005-0.1% Al, 0.0005-0.008% N, <0.005% Ti, <0.0003% B, and the balance iron with inevitable impurities and in which the value of Pcm, represented by Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B, is regulated to <=0.21. This steel has >=570N/mm<2> tensile strength.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention is used for steel structures such as bridges, warehouses and buildings, and has excellent weldability and weather resistance.
The present invention relates to a 0 N / mm ² class high-strength steel and a manufacturing method thereof.

[0002]

^2. Description of the Related Art There have been many requests for improving the performance of 600 N / mm ² class high-tensile steel, and many studies have been made so far. Among these, Japanese Patent Laid-Open No. 49-37814 and Japanese Patent Publication No. 4-13406 are known as techniques characterized by lowering C and adding Ti-B for the purpose of improving susceptibility to welding cracks. By these typified technologies, a 600 N / mm ² class high strength steel with improved weld cracking susceptibility can be obtained, but the tensile strength required for 600 N / mm ² class high strength steel is achieved by utilizing B. Therefore, there is a concern that the properties of the base metal may be unstable due to changes in the chemical composition and manufacturing conditions, and the hardness of the heat affected zone is significantly increased. This increase in the hardness of the heat-affected zone of the weld generally causes the deterioration of the toughness of the bond portion, which is the most concerned in the welded joint portion, and is not preferable.

Japanese Unexamined Patent Publication No. 2-205627 provides a method for producing a 600 N / mm ² class high strength steel having excellent toughness by using a direct quenching method. Since this technique requires the complex addition of Nb and B, there is a concern that the same harmful effects as described above may be caused by the addition of B.

As a technique without adding B, Japanese Patent Application Laid-Open No.
No. 31538, Japanese Patent Publication No. 60-9086, and Japanese Patent Laid-Open No. 2-25
4119, JP 59-113120, JP 61-12970
Japanese Patent Publication No. 2-8322 and Japanese Patent Publication No. 53-119219 have been proposed.

Of these, the technique disclosed in Japanese Patent Laid-Open No. 5-331538 relates to a 500 N / mm ² grade non-heat treated high strength steel. In addition, Japanese Patent Publication No. 60-9086,
The techniques disclosed in Japanese Patent No. 4119 and Japanese Patent Laid-Open No. 59-113120 are all related to 600 N / mm ² class non-heat treated high tension, and the upper limits of the plate thickness to which these techniques are applied are all from the examples. It is known to be about 20 mm.

Japanese Patent Publication No. 61-12970 discloses an attempt to provide a 600 N / mm ² class high-strength steel excellent in weld cracking susceptibility by combining reduction of C, addition of V and direct quenching. There is no description regarding provision for plate thicknesses exceeding 30 mm.

Japanese Unexamined Patent Publication (Kokai) No. 2-8322 discloses reduction of C and Mo, N
This is a technique relating to a 600 N / mm ² class high-strength steel, which requires the combined addition of b and Ti, combines a direct quenching method, and aims to improve SSC resistance and susceptibility to weld cracking. Although there is no description about the applicable plate thickness in the specification, it is generally 5 because it is intended for application to gas tanks and line pipes.
It is presumed that the purpose is to apply to steel materials having a plate thickness of 0 mm or less.

Further, JP-A-53-119219 is intended to provide a high-strength steel having a thick plate thickness of 500 N / mm ² grade or more by a reheating quenching and tempering process. According to this technique, by adding a relatively large amount of Nb exceeding 0.02%, undissolved Nb carbonitride is left at the time of reheating to prevent coarsening of crystal grains and mainly improve the toughness of the base material. Things. Therefore, at the time of quenching, the effect of improving the hardenability of solid solution Nb and precipitation hardening cannot be fully utilized. Therefore, as seen in the examples, in order to secure the strength, it is essentially necessary to add Ni and Mo in addition to Nb and V, and to secure the strength of 600 N / mm ² class at the position where the plate thickness is 1/4 t. In a possible invention example (test steel J), the Pcm value exceeds 0.21 and the weldability is poor. In addition to the above, JP-A-60-1748
In the examples of JP-A-20, only one example is an invention example suggesting a 600 N / mm ² class high-strength steel, but it is possible to make it by utilizing B as is clear from the chemical composition. is there.

[0009] As described above, 60 having excellent weld cracking susceptibility.
Most of the prior art of 0N / mm ² grade tempered high-strength steel was achieved by ensuring the hardenability by adding B, and no consideration was given to the weather resistance.

On the other hand, as a steel material having excellent weather resistance, Japanese Patent
Examples include 5-117745 and JP-A-6-316723.
Japanese Unexamined Patent Publication No. Hei 5-117745 is a technology relating to a method for producing 490 N / mm ² class high-strength steel for building structures, which is characterized by having excellent fire resistance performance, and Japanese Unexamined Patent Publication No. Hei 6-316723 is more excellent in gas cutting. It is a technology that imparts sex.

That is, 600 N / which is excellent in weldability and weather resistance.
mm ² grade tempered high strength steel has not been offered to date. The present invention is 600N which is excellent in both weldability and weather resistance.
/ Mm ² class tempered high-strength steel and its manufacturing method.

[0012]

First, in order to improve the weld crack susceptibility, Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + N
It is effective to reduce the Pcm value defined by i / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B. Although addition of B can be considered as an effective means for securing the strength of the base material while securing the susceptibility to welding cracks, there is a concern that the joint toughness may deteriorate due to a significant increase in the hardness of the weld heat affected zone. It is preferable not to add Ti, which has been conventionally added for a long time to effectively utilize B, in order to stably obtain the base material performance. Therefore, in order to obtain 600 N / mm ² class high-strength steel while simultaneously improving the weld crack susceptibility and ensuring the soundness of the welded joint without adding Ti and B, it is necessary to apply the conventional reheating quenching and tempering process. There is a concern that the plate thickness may be restricted.

In order to ensure weather resistance, JIS
As shown in G3114, it is necessary to add Cu, Cr, and Ni. However, the addition of these alloys causes an increase in the Pcm value and impairs the weld cracking susceptibility. If other added elements are reduced, it is possible not to impair the weld cracking susceptibility, but it becomes difficult to secure a tensile strength of 570 N / mm ² or more.

Therefore, the present invention has been completed based on the knowledge of the following (1) to (4) on the assumption that the direct quenching and tempering process is applied. (1) By adopting the direct quenching method, it is possible to utilize the effect of improving the hardenability by the Nb dissolved during rolling and heating. As a result, the amount of alloying elements added for securing other hardenability can be reduced. Further, Nb has a function of finely dispersing the carbide, and is extremely effective in securing the toughness of the 1 / 2t part of the thick material.

(2) Nb is obtained by tempering treatment after direct quenching.
The precipitation hardening of carbonitrides can be utilized. This is particularly effective for securing the strength of the central portion of the plate thickness where the cooling rate during quenching is necessarily slower than that on the surface side. That is, it is possible to secure the strength at the central portion of the sheet thickness without securing the hardenability more than necessary.

(3) Including the contribution of precipitation hardening elements such as Nb and V, Cu, Cr and Ni necessary for ensuring weather resistance.
Assuming the addition, the necessary addition amount of alloying elements according to the steel plate thickness to secure the strength was grasped, and the conditions for not inhibiting weld crack susceptibility were clarified.

(4) From the above, the addition of B is not necessary, but rather it is necessary to positively regulate the mixture thereof in order to ensure the soundness of the welded joint. Further, from the viewpoint of effectively utilizing B, addition of Ti is not essential, but rather, it is preferable not to add Ti in order to stably obtain good base material performance.

The first invention based on these findings is weight%
C: 0.06 to 0.1%, Si: 0.01 to 0.4
%, Mn: 0.5 to 1.4%, Cu: 0.2 to 0.5
%, Ni: 0.05 to 0.3%, Cr: 0.3 to 0.8
%, Nb: 0.005 to 0.05%, Al: 0.005
~ 0.1%, N: 0.0005 to 0.008%, Ti <
0.005%, B <0.0003%, selectively added Mo: 0.15% or less, V: 0.1% or less, Pc
m = C + Si / 30 + Mn / 20 + Cu / 20 + Ni /
60 + Cr / 20 + Mo / 15 + V / 10 + 5B with a Pcm value of 0.21 or less and the balance being iron and unavoidable impurities with a tensile strength of 570 N / mm ² or more, which is a high-strength steel excellent in weldability and weather resistance. is there.

The second invention is, in% by weight, C: 0.06 to.
0.1%, Si: 0.01 to 0.4%, Mn: 0.5 to
1.4%, Cu: 0.2 to 0.5%, Ni: 0.05 to
0.3%, Cr: 0.3 to 0.8%, Nb: 0.005
-0.05%, Al: 0.005-0.1%, N: 0.
0005-0.008%, Ti <0.005%, B <
0.0003%, Mo: 0.15% or less selectively added, V: 0.1% or less further included, Pcm = C + Si
/ 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr /
Pcm defined by 20 + Mo / 15 + V / 10 + 5B
When steel with a value of 0.21 or less is hot-rolled, it is heated to a temperature of 1000 ° C. or higher and 1250 ° C. or lower, then hot-rolled with a predetermined plate thickness, and then directly quenched from a temperature of Ar ₃ transformation point or higher. Further, the present invention is a method for producing a high-strength steel excellent in weldability and tensile strength of 570 N / mm ² or more, which is characterized by performing tempering treatment at a temperature of Ac ₁ transformation point or lower.

A third aspect of the present invention is the method for producing high-strength steel described above, wherein Ceq = C + Mn / 6 + Si / 24 + Ni / 4.
Ceq defined by 0 + Cr / 5 + Mo / 4 + V / 14
Value and the heating temperature T set in the temperature range of 1000 to 1250 ° C., log {(Nb) × (C + 12
N / 14)} = 2.26-6770 / (T + 273.1)
The amount of solid solution Nb calculated from the relationship of 5) is taken as the effective Nb amount, the effective Nb and V contents, and the target steel plate thickness t (mm)
800 (effective Nb) + 320V + 270Ceq ≧ t + 6
4 tensile strength 570 N / m characterized by satisfying the relationship
It is a method for producing a high-strength steel excellent in weldability and weather resistance of m ² or more.

In a fourth aspect of the present invention, in the method for producing high-strength steel, hot rolling is performed at a cumulative rolling reduction of 20% or more at 1050 ° C. or less and a tensile strength of 570 N / It is a method for producing high-strength steel having excellent weldability of mm ² or more and excellent weather resistance. In addition, when it is possible to secure the total solid solution of Nb, it is not necessary to calculate the effective Nb amount, and the effective Nb amount = Nb amount.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for adding and limiting elements such as additive elements of the steel of the present invention will be described below. If the amount of <C> C is less than 0.06%, it is necessary to add a large amount of a hardenability improving element, resulting in high cost, deterioration of toughness, and deterioration of weldability. Further, particularly when the steel of the present invention is subjected to high heat input welding, C
If the content is less than 0.06%, the dilution of C into the weld metal will be small, and it will be difficult to secure the joint strength with general welding materials. The upper limit of the C content is 0.1% in order to secure susceptibility to weld cracking.

<Si> Si effectively acts to secure the base metal strength and the weld joint strength, so 0.01% or more is added.
However, the addition exceeding 0.4% deteriorates weld cracking susceptibility and weld joint toughness.

<Mn> Mn works effectively to secure the strength of the base metal and the strength of the welded joint, so 0.5% or more is added. However, addition of more than 1.4% deteriorates the weld cracking susceptibility, causes excessive hardenability, and deteriorates the base material toughness.

<Mo> Mo is the base material strength and weld joint strength,
Further, since it works effectively in securing weather resistance, it can be selectively added. However, the upper limit is set to 0.15% because the tendency to deteriorate the weld cracking susceptibility and the toughness of the welded joint is recognized, and the toughness is deteriorated by unnecessarily increasing the hardenability of the base metal. It is preferable to set the upper limit to 0.1% in consideration of not ensuring hardenability. Further, the lower limit thereof is preferably 0.02% in order to exert the above effects.

<Nb> Nb is added in an amount of 0.005% or more in order to obtain the above effects. However, since the addition of more than 0.05% tends to deteriorate the weld joint toughness, the upper limit of the Nb addition amount is set to 0.05%, preferably 0.03%.

<V> V works effectively to secure the strength of the base metal and the strength of the welded joint, so that 0.01% or more can be selectively added.

However, the addition of more than 0.1% deteriorates the weld cracking susceptibility and impairs the base metal toughness. <Al> Al is added for deoxidation of steel, usually 0.00
5% or more is contained. In addition, 0.01% is added in order to secure the base material toughness due to the refinement of the microstructure. But,
Addition of Al in excess of 0.1% impairs base material toughness.

<Ti, B> In general, Ti has the effect of improving the toughness of the base material and the welded joint by refining the microstructure. Further, in the B-added steel, it is often added positively in order to secure B that works effectively for hardenability.

However, in the present invention, the strength of the base metal is ensured without adding B, which may cause the hardening of the weld heat affected zone, and in particular the weld joint toughness is achieved by reducing the hardness in the coarse grained zone of the heat affected zone. , Ti need not be added. Rather, there is a concern about instability of the base material performance due to the addition of Ti, and the content of N is regulated to less than 0.005% as an impurity element, but it is preferably less than 3.4 times the N content described below.

B must be regulated to less than 0.0003% as an impurity element to reduce the hardness of the heat-affected zone. <N> N reacts with Al, Nb, etc. to form a precipitate, thereby making the microstructure finer and improving the base material toughness, and reacting with Nb, V, etc. during tempering to secure the strength by precipitation hardening. To add.

If the addition is less than 0.0005%, precipitates required for refining the microstructure and ensuring the strength are not formed,
Addition in excess of 0.008% rather impairs the toughness of the base metal and the welded joint.

<P, S> P and S are all impurity elements, but P has the effect of improving weather resistance. However, in order to obtain a sound base metal and a welded joint, it is desirable that both are controlled to 0.015% or less, preferably 0.01% or less.

<Cu, Ni, Cr> Cu, Ni, Cr have the effect of improving the strength of the base material and the welded joint. N
i has the function of further improving toughness. Further, in order to secure weather resistance, these alloy elements are Cu in the present invention.
0.2-0.5%, Ni0.05-0.3%, Cr0.
It must be added in the range of 3 to 0.8%. However, the upper limit was set for each in consideration of not securing the hardenability more than necessary.

<Pcm> Pcm is an index showing the susceptibility to welding cracks, and is regulated to 0.21 or less in order to eliminate the need for preheating during welding in a normal environment.

<Calculation formula: 800 (effective Nb) + 320V
+ 270Ceq ≧ t + 64> First, the calculation formula: 800 Nb
+ 320V + 270Ceq is an index representing the strength of the base material at a plate thickness of 1 / 2t, and N is an additive element of the present invention in the carbon equivalent formula (Ceq) generally known by those skilled in the art.
This is a mathematical formula in which the contributions of b and V are taken into consideration and the thickness effect in a thickness range of approximately 25 to 100 mm is taken into consideration. The thickness effect means that when the steel sheet is forcibly cooled from the Ar ₃ transformation point or higher by direct quenching after hot rolling, the cooling rate inevitably changes according to the sheet thickness, and therefore the base metal strength changes. To do.

In order to obtain a steel plate which is classified as 600 N / mm ² class corrosion resistant high strength steel and conforms to JIS G3114 SMA570, a calculation formula: 800 Nb + 320V + 27
0Ceq needs to exceed the value obtained by adding 64 to the plate thickness t (mm).

In the case of V, the strength increasing effect of Nb and V in this calculation formula represents the contribution of precipitation hardening of V carbonitride, and in the case of Nb, the contribution of increasing hardenability in addition to precipitation hardening of Nb carbonitride. Is taken into consideration. The effect of these elements expected by the tempering process after direct quenching,
It is necessary that a solid solution be formed in a heating step before hot rolling. Although all of the added amount of V can form a solid solution within the scope of the present invention, in the case of Nb, the entire amount does not always form a solid solution. Therefore, when it is not possible to secure the total solid solution of Nb, lo
g {(Nb) × (C + 12N / 14)} = 2.26-6
Using the solid solution Nb amount calculated from the relationship of 770 / (T + 273.15) as the effective Nb amount, the above calculation formula is 800 (effective Nb) + 320V + 270Ceq ≧ t + 6
4 shall be applied.

The plate thickness range targeted by the present invention is approximately 25 mm to 100 mm. <Heating temperature before hot rolling> The heating temperature needs to be set to 1000 ° C. or higher in order to homogenize the alloying elements and achieve a solid solution of Nb. However, if the heating temperature exceeds 1250 ° C, the toughness of the base material cannot be ensured due to the coarsening of the microstructure, so the upper limit is 1250 ° C, preferably 1200 ° C.

<Rolling Conditions> The step of hot rolling the uniformly heated steel of the present invention to a predetermined plate thickness may be performed under ordinary conditions.

From the viewpoint of more stably securing and improving the toughness of the base material, it is desirable to apply a cumulative reduction of 20% or more in a temperature range of 1050 ° C. or less. By setting the cumulative reduction to 20% or more, fine graining accompanying the recrystallization of γ grains can be achieved, and the toughness of the base material can be more stably secured and improved. For the same reason, it is desirable to secure a rolling reduction of 5% or more, more preferably 10% or more, for each rolling pass.

<Direct Quenching> After the hot rolling is finished, it is necessary to forcibly cool the steel sheet having a temperature higher than the Ar ₃ transformation point to carry out the quenching treatment. In the forced cooling, a cooling medium such as water is uniformly applied to the steel sheet, and at least 1 ° C./sec at a thickness of 1/2 t.
A cooling rate of c or higher must be achieved.

<Tempering Temperature> Tempering is carried out in order to eliminate the concern about the performance change due to welding and SR.
In the present invention, it has an important meaning of securing base material strength by precipitation hardening of Nb carbonitride. The above object cannot be achieved unless tempering is performed at 570 ° C. or higher.
Perform at or above ° C. However, when tempering is performed at a temperature exceeding the Ac ₁ transformation point, the strength is remarkably reduced, and 600 N / mm
The strength as a ^second grade high strength steel is not secured. The high-strength steel thus obtained has excellent weldability and weather resistance.

[0044]

Embodiments of the present invention will be described below. Table 1 shows the chemical composition of the steel used in the examples of the present invention. A steel having the chemical composition shown in Table 1 was melted and made into an ingot, hot-rolled to a predetermined thickness under the manufacturing conditions shown in Table 2, directly quenched, and further tempered. I got The rolling finishing temperature is 850 ° C. or higher, and the tempering temperature is 580 to 68.
The range was 0 ° C.

Tensile tests and Charpy impact tests were taken in the direction perpendicular to the rolling direction from the center part of the plate thickness of all the test steels to evaluate the mechanical properties of the base metal as 600N / mm ² class steel.

A diagonal Y-type weld cracking test was carried out in accordance with JIS Z3158 and a maximum hardness test was carried out in accordance with JIS Z3101 to evaluate the weld cracking susceptibility. All of these tests were performed using a low hydrogen type welding material for 60 kg class steel in an atmosphere of 20 ° C-60% and an initial temperature of the test piece of 25 ° C.

Example No. Reference numerals 1 and 2 are examples of the present invention made of A steel. Calculated value of steel A (800Nb + 320V + 27
0Ceq = 120) is 64 for the thickness of the steel sheet under test (25, 50)
(89, 114), the tensile strength of the base material in the central part of the plate thickness exceeds 570 N / mm ^2, and the toughness was also good. The Pcm value was as low as 0.19,
No welding crack occurred in the crack test.

Example No. Reference numerals 3 and 4 are examples of the present invention and comparative examples made of B steel. When the plate thickness is 50 mm, the calculated value is the plate thickness t
Above +64, the mechanical properties of the base material of the plate thickness center part are good, but at a plate thickness of 75 mm, the strength does not reach 570 N / mm ² .

Example No. Nos. 5 to 7 are the present invention examples and comparative examples using steel C. No. 5, the calculated value of steel C heated at 1150 ° C. (800 Nb + 320 V + 270 C
eq = 141) exceeds the value (139) obtained by adding 64 to the steel plate thickness under test (75), and the strength at the center of the plate thickness is 570 N / m.
m ² , but the heating temperature was 1000 ° C. 6
Then, the effective Nb was reduced to 0.012%, so the calculated value was 132 and the strength at the center of the plate thickness was less than 570 N / mm ² . However, under the same conditions, No. In No. 7, the calculated value exceeds the value obtained by adding 64 to the plate thickness of 50 mm, and good strength is obtained.

Example No. Nos. 8 to 11 are inventive examples and comparative examples using steels D and E. In Steel E, which has a lower chemical composition than Steel D, the strength becomes insufficient with a plate thickness of 50 mm as the calculated value decreases.

Embodiment No. 12 is a comparative example using steel F. Since it was attempted to obtain a steel plate having a plate thickness of 75 mm without using Nb, which is a feature of the present invention, Pcm exceeded 0.21% and the maximum hardness was high, and cracks occurred in the Y-type crack test.

No. 13 is an example of the present invention in which the C content is the lower limit of the present invention. Good mechanical properties as well as weldability were confirmed for a 50 mm thick material. Note that the examples shown here are all SMA57 specified in JIS G3114.
Since Cu, Cr, and Ni required for 0 are contained, a stable oxide film is formed during use and good weather resistance is exhibited.

[0053]

[Table 1]

[0054]

[Table 2]

[0055]

As described above, according to the present invention, in particular,
To provide a 600 N / mm ² class high-strength steel having improved weldability and weather resistance by improving the Nb addition effect by a direct quenching method and limiting the addition of Ti and B, and a manufacturing method thereof. You can

Claims (6)

[Claims] 1. C: 0.06 to 0.1% by weight, S
i: 0.01 to 0.4%, Mn: 0.5 to 1.4%, C
u: 0.2-0.5%, Ni: 0.05-0.3%, C
r: 0.3 to 0.8%, Nb: 0.005 to 0.05
%, Al: 0.005 to 0.1%, N: 0.0005 to
0.008%, Ti <0.005%, B <0.0003
% Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + N
i / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B, Pcm value is 0.21 or less, the balance is iron and unavoidable impurities, and the tensile strength is 570 N / mm ² or more. High tensile strength with excellent weldability and weather resistance. steel. 2. The high-strength steel excellent in weldability and weather resistance according to claim 1, further containing Mo: 0.15% or less and V: 0.1% or less. 3. C: 0.06 to 0.1% by weight, S
i: 0.01 to 0.4%, Mn: 0.5 to 1.4%, C
u: 0.2-0.5%, Ni: 0.05-0.3%, C
r: 0.3 to 0.8%, Nb: 0.005 to 0.05
%, Al: 0.005 to 0.1%, N: 0.0005 to
0.008%, Ti <0.005%, B <0.0003
% Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + N
When the Pcm value defined by i / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B is 0.21 or less and the balance is iron and steel with unavoidable impurities, 1000 ° C. or more 12
After heating to a temperature of 50 ° C. or lower, hot rolling is performed with a predetermined plate thickness, followed by direct quenching from a temperature of Ar ₃ transformation point or higher,
Furthermore, a method for producing a high-strength steel excellent in weldability and weather resistance with a tensile strength of 570 N / mm ² or more, characterized by performing tempering treatment at a temperature of Ac ₁ transformation point or lower. 4. The method for producing a high-strength steel excellent in weldability and weather resistance according to claim 3, further comprising Mo: 0.15% or less and V: 0.1% or less. 5. Ceq = C + Mn / 6 + Si / 24 + N
Using the Ceq value defined by i / 40 + Cr / 5 + Mo / 4 + V / 14 and the heating temperature T set in the temperature range of 1000 to 1250 ° C., log {(Nb) × (C
+ 12N / 14)} = 2.26-6770 / (T + 27)
3.15) The amount of solid solution Nb calculated from the relationship of
As the amount, effective Nb, V content, target steel plate thickness t
(Mm), 800 (effective Nb) +320 V + 270 Ceq ≧ t + 6
4. The method for producing a high-strength steel excellent in weldability and weather resistance having a tensile strength of 570 N / mm ² or more according to claim 3 or 4, characterized in that the relationship of 4 is satisfied. 6. A hot rolling process at a temperature of 1050 ° C. or less
The tensile strength 570N according to any one of claims 3 to 5, wherein hot rolling is performed at a cumulative reduction of 0% or more.
/ Mm ² or more weldability and high-strength steel with excellent weather resistance.