JPH08176731A - High tensile steel and production thereof - Google Patents

Abstract

PURPOSE: To obtain a high tensile steel having excellent weld cracking sensitivity and large heat input weldability by specifying the chemical compsn. and PCM value of steel and controlling the temp. of hot rolling. CONSTITUTION: The compsn. of the steel is composed, by weight%, of 0.06 to 1.0% C, 0.01 to O.4% Si, 0.5 to 1.6% Mn, <=0.3% Mo, 0.005 to 0.05% Nb, <=0.1% V, 0.005 to 0.1% Al, 0.0005 to 0.008% N, <0.005% Ti, <0.0003% B, and the balance Fe and inevitable impurities. The PCM value defined by equation I is specified to <=0.2. The relation defined by equation IV is satisfied by using the Ceq value defined by equation III and the effective Nb content, V content and thickness t(mm) of the steel sheet with the solid soln. Nb quantity calculated from the relation expressed by equation II using the heating temp. T set at 100 to 1250 deg.C as an effective Nb quantity at the time of hot rolling the steel having such chemical compsn. The steel sheet is directly quench- hardened from the Ar3 transformation point after rolling and is subjected to a tempering treatment at a temp. below the Ac1 transformation point.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a 600 N / mm ² class high strength steel excellent in weld cracking susceptibility and large penetration weldability, which is used for steel structures such as bridges, tanks, iron pipes, warehouses and buildings, and its production. Regarding the method.

[0002]

^2. Description of the Related Art Conventionally, there have been many demands for improving the performance of 600 N / mm ² class high strength steel, and many studies have been made so far. Among these, as a technique characterized by lowering C and adding Ti-B for the purpose of improving susceptibility to welding cracks, JP-A-49-37814 and Japanese Patent Publication No. 4-13406.
The gazette is known. 600N / mm with improved weld crack susceptibility by the technologies represented by these
^Although class ² high-strength steel can be obtained, the tensile strength required for 600 N / mm ² class high-strength steel is achieved by utilizing B, so the base material properties are unstable due to changes in chemical composition and manufacturing conditions. Is a concern, and the hardness of the weld heat affected zone increases significantly. This increase in the hardness of the weld heat affected zone is not preferable because it causes deterioration of the toughness of the bond portion, which is most feared particularly in the high heat input welded joint.

Japanese Unexamined Patent Publication (Kokai) No. 2-8322 discloses a combination of low carbon content, utilization of precipitation hardening elements, and direct quenching method to improve SS resistance.
This is a technique aimed at improving the C property and the weld cracking susceptibility.
Steel plates produced by this technology have been thoroughly reduced in carbon content to ensure SSC resistance. Therefore, when joints are made by general large penetration welding, the performance of the weld metal is not secured and sufficient joint strength is obtained. It's hard to be beaten. Further, since the strength required for high-strength steel of 600 N / mm ² class has been achieved by lowering C, it is necessary to add a large amount of hardenability improving element in addition to a large amount of precipitation hardening element. As a result, the hardness of the coarse-grained zone of the heat-affected zone is increased during high heat input welding, and there is concern about joint toughness.

Japanese Examined Patent Publication No. 61-12970 is intended to provide a 600 N / mm ² class high-strength steel excellent in susceptibility to welding cracks by combining reduction of C, addition of V and direct quenching. According to the description of this publication, 600N / mm ² class high-strength steel having excellent weld crack susceptibility can be obtained at least up to a plate thickness of 30 mm, but there is no description relating to weld joint performance. It can be said that the large heat input weldability is unsolved.

Japanese Unexamined Patent Publication No. 55-69241 is a proposal relating to a 600 N / mm ² class high-strength steel excellent in high heat input weldability. The characteristics of this technique are low Si content and high Al content, and the weld crack susceptibility is not always solved.

As other conventional techniques, Watanabe et al., "Development of steel NK-HIWEL for large heat input welding", Nippon Steel Pipe Technical Report N
o. 97 (1983) and Japanese Patent Laid-Open No. 174820/1985, but these are mainly 500 N.
/ Mm ² It is a technology for ² class steel, 600N / mm
Grade ² high-strength steel is out of the scope.

[0007]

The present invention has been made in view of the above circumstances, and an object thereof is to provide a 600 N / mm ² class high-strength steel excellent in weld cracking susceptibility and high heat input weldability. .

[0008]

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. From the comparison of the coefficients, it is easily known that the reduction of C is most effective, but extremely low C makes it difficult to secure the joint strength.
Furthermore, in order to secure the strength of the base metal, it is necessary to add a large amount of alloying elements, which not only leads to an increase in cost, but there is a concern about joint toughness associated with hardening of the coarse-grained zone of the heat-affected zone of the welding heat affected zone.

As a means of securing the strength of the base material instead of adding a large amount of alloying elements, B addition is considered, but there is a concern that the joint toughness may deteriorate due to a marked increase in hardness of the weld heat affected zone.

Therefore, Ti from the viewpoint of effectively utilizing B
Addition is not essential, but it is preferable not to add Ti in order to stably obtain good base material toughness. So 60
In order to improve the weld crack susceptibility of 0 N / mm ² class high-strength steel and to secure high heat input weldability, optimization of C content and additive alloy element content was investigated and the following findings were obtained to complete the present invention.

(1) In order to obtain a sound weld metal, obtain sufficient joint strength, and avoid adding a large amount of alloying elements, it is necessary to contain 0.06% or more of C in the base material.

(2) However, the addition of C exceeding 0.1% increases the weld crack susceptibility and deteriorates the joint toughness.

(3) By adopting the direct quenching method, it is possible to utilize the precipitation hardening by the effective Nb solid-soluted during heating to secure the strength of the base material.

(4) The addition of Nb makes it possible to stably secure the high heat input welded joint strength.

(5) There is a range of optimum addition amounts of alloying elements depending on the steel plate thickness.

That is, in the present invention, C: 0.0% by weight.
6 to 0.1%, Si: 0.01 to 0.4%, Mn: 0.
5 to 1.6%, Mo: 0.3% or less, Nb: 0.005
~ 0.05%, V: 0.1% or less, Al: 0.005-
0.1%, N: 0.0005 to 0.008%, Ti <
0.005%, B <0.0003% included, and if necessary, Cu: 0.5% or less, Ni: 1.5% or less, C
r: contains 0.5% or less of one kind or two or more kinds, and Pcm =
C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60
Pcr value defined by + Cr / 20 + Mo / 15 + V / 10 + 5B is 0.2 or less, and Ceq = C + Mn /
6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V
Ceq value defined by / 14 and Nb, V content,
It is a high-strength steel excellent in weld cracking susceptibility and large heat input weldability, characterized by satisfying a relation of 625Nb + 250V + 210Ceq ≧ 40 + t using a steel plate thickness t (mm), and the balance consisting of iron and inevitable impurities.

A steel having this chemical composition is 100
After heating to a temperature range of 0 to 1250 ° C., hot rolling to a predetermined plate thickness, and then directly quenching from the Ar3 transformation point or higher,
A method for producing a high-strength steel excellent in weld crack susceptibility and high heat input meltability, characterized by performing tempering treatment at a temperature not higher than the Acl transformation point.

In the present invention, when the steel having the above-mentioned chemical composition is produced, the steel is heated to a temperature range of 1000 to 1250 ° C., hot rolled at a cumulative reduction of 20% or more at 1050 ° C. or less, and then continued. Quench directly from the Ar3 transformation point or higher,
A method for producing high-strength steel excellent in weld crack sensitivity and high heat input weldability, characterized by performing tempering at a temperature not higher than the Acl transformation point.

[0019]

The reasons for limiting the constituent features of the present invention will be described below.

<C> C is added in an amount of 0.06% or more in order to secure joint strength and avoid adding a large amount of alloying elements, as described above. If the C content is less than 0.06%, it is necessary to add 0.2% or more of Mo, which leads to high cost, deterioration of the toughness of the large heat input joint, and the strength of the large heat input welded joint cannot be secured. C amount is 0.1%
If it exceeds, the susceptibility to welding cracks deteriorates, and the toughness of large heat input joints may deteriorate.

<Si> Si is effective in ensuring the strength of the base metal and the high heat input welding strength, so 0.01% or more is added. However, the addition of more than 0.4% deteriorates the weld crack susceptibility and the toughness of large welded joints.

<Mn> Mn works effectively in securing the strength of the base metal and the high heat input welding strength, so it is added in an amount of 0.5% or more.
However, addition of more than 1.6% deteriorates the weld crack susceptibility.

<Mo> Mo has a tendency to deteriorate weld crack susceptibility and toughness of large-entry welded joints, so the upper limit is made 0.3%. However, since Mo works effectively in securing the base material strength and the high heat input welding strength, 0.0
It is preferable to add 1% or more and 0.2% or less.

<Nb> Nb works effectively in securing the strength of the base metal and the high heat input welding strength, so 0.005% or more is added. However, the addition of more than 0.05% deteriorates the toughness of the weld metal.

In the above Nb addition range, the solution temperature of Nb carbonitride may be higher than the heating temperature of the present invention. Calculation formula described later: 625 (effective Nb) + 250V +
In 210 Ceq, the term of effective Nb is Nb after tempering.
It shows the amount of increase in base metal strength due to precipitation hardening of
Effective Nb refers to Nb which is solid-solved in the heating stage before hot rolling. Therefore, in using the limitation by this formula,
In consideration of the relationship between the solid solution temperature of Nb carbonitride and the rolling heating temperature, when the solid solution temperature of Nb carbonitride is higher than the heating temperature, the solid solution Nb amount obtained from the C and N contents is calculated as the effective Nb amount. It will be used in the formula.

<V> If V exceeds 0.1%, the weld crack susceptibility is deteriorated and the base metal toughness is impaired.
% Or less. However, 0.01% or more is added if necessary because it works effectively in securing the base material strength and the high heat input welding strength.

Incidentally, the solid solution temperature of V carbonitride within the scope of the present invention is lower than the heating temperature adopted in ordinary hot rolling, and therefore the consideration described in the item of Nb above is unnecessary.

<Al> Al is added for deoxidizing steel,
Usually, 0.005% or more is contained. Further, 0.01% or more is preferably added to secure the toughness of the base metal due to the refinement of the microstructure. However, addition of Al in excess of 0.1% impairs base material toughness.

<Ti, B> Ti has the effect of improving the toughness of the base material and the welded joint through grain refinement of the microstructure. In addition, B-added steel is often positively added in order to secure B that works effectively for hardenability. However, in the present invention, Ti is added in order to secure the base metal strength without adding B, which may cause hardening of the heat-affected zone of the weld, and to achieve the weld joint toughness by reducing the hardness of the coarse-grained zone of the heat-affected zone. There is no need to add it. Rather, there is concern about instability of the base material performance due to the addition of Ti, so the content of the impurity element is limited to less than 0.005%. In particular, it is desirable that the N content is less than 3.4 times the content.

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 refining the microstructure and improving the toughness of the base metal, and reacting with Nb, V, etc. during tempering to cause precipitation hardening. Add to secure strength.

If added in an amount of less than 0.0005%, the precipitates necessary for refining the microstructure and ensuring the strength are not formed,
Addition of more than 0.008% rather impairs the toughness of the base metal and the welded joint.

<P, S> P and S are both impurity elements, and are 0.0 to obtain a sound base metal and weld joint.
It is desired to be regulated to 15% or less, preferably 0.01% or less.

<Cu, Ni, Cr> Cu, Cr has the effect of improving the strength of the base material and the welded joint. Ni has a function of further improving toughness. In the present invention, these alloying elements may be selectively added within the range of the limited formula of chemical components. In particular, by replacing a part of Mn with these elements, improvement of toughness and reduction of segregation can be expected.

<Pcm> Pcm is an index showing the susceptibility to welding cracks, and is regulated to 0.2 or less in order to eliminate the need for preheating during welding in a normal environment.

<Calculation formula: 625 (effective Nb) + 250V
+210 Ceq> Calculation formula: 625 (effective Nb) +250
V + 210Ceq is an index representing the strength of the base material at a plate thickness of 1 / 2t, and the plate thickness effect is taken into consideration in the carbon equivalent formula (Ceq) generally known by those skilled in the art. In addition to the contribution of 15 to 60 mm
It is a mathematical formula arranged in consideration of the plate thickness effect in the plate thickness range.

The plate thickness effect means that when the steel plate is forcibly cooled from the Ar3 transformation point or higher by direct quenching after hot rolling,
It means that the cooling rate inevitably changes depending on the plate thickness, and therefore the base metal strength changes.

In order to obtain a steel sheet that complies with JIS G3106 SM570Q, which is classified as 600 N / mm ² class high-strength steel, the calculation formula is: 625 (effective Nb) + 250V + 2
It is necessary that 10 Ceq exceeds the value obtained by adding 40 to the plate thickness (mm).

<Heating temperature before hot rolling> In order to homogenize alloy elements and form a solid solution of Nb, it is necessary to set the heating temperature to 1000 ° C. or higher. However, the heating temperature is 1250 ° C
If it exceeds, the toughness of the base material cannot be ensured due to the coarsening of the microstructure, so the upper limit is 1250 ° C., preferably 120.
Set to 0 ° 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. By applying a cumulative reduction of 20% or more in a temperature range of 1050 ° C. or less, grain refinement due to recrystallization of γ grains can be achieved, and the toughness of the base material can be secured and improved more stably. For the same reason, it is desirable to secure a rolling reduction of 5% or more, more preferably 10% or more, for each pass of rolling.

The rolling finishing temperature must be above the Ar3 transformation point. If the rolling is completed at a temperature below the Ar3 transformation point, the subsequent direct quenching will result in incomplete quenching, and good base material properties cannot be secured. From the viewpoint of ensuring the acoustic anisotropy of the base material, the rolling finish temperature is preferably higher than 900 ° C. and higher.

<Direct Quenching> After completion of hot rolling, it is necessary to quench the steel sheet having a temperature higher than the Ar3 transformation point by forced cooling. For forced cooling, it is necessary to uniformly apply a cooling medium such as water to the steel sheet so as to achieve a cooling rate of approximately 3 ° C./sec or more near the transformation temperature of the sheet thickness 1 / 2t.

<Tempering Temperature> In tempering, if the tempering is carried out at a temperature exceeding the Acl transformation point, the strength is remarkably lowered.
Since the strength as 0 N / mm ² class high-strength steel cannot be ensured, it is performed below the Ac1 transformation point. However, in order to secure the strength of the welded joint and to secure the strength of the base metal due to the precipitation effect of Nb, it is preferable to carry out at 570 ° C, preferably 600 ° C or higher.

[0044]

EXAMPLES Table 1 shows the chemical composition of the steel used in the examples of the present invention. Steel with the chemical composition shown in Table 1 was melted to form a steel ingot, hot rolled to a prescribed plate thickness under the manufacturing conditions shown in Table 2, directly quenched, and further tempered. Got The finishing temperature of hot rolling is 900 to 10 in all cases.
The temperature was set to 00 ° C and the tempering temperature was set to 580 to 680 ° C. In addition, a part of the test steel was subjected to reheating quenching and tempering treatment and used for comparison with the examples.

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.

In addition, an oblique Y-type weld cracking test was carried out according to JIS Z3158 and a maximum hardness test was carried out according to JIS Z3101 to evaluate the weld cracking susceptibility. These test atmospheres were controlled at 20 ° C and -60%, and the initial temperature of the test piece was 25 ° C.

The large heat input meltability was evaluated by making a joint by ordinary electrogas arc welding, measuring the strength of the joint, and conducting a Charpy impact test on the bond portion where the toughness is most concerned.

The electrogas arc welded joint was prepared by welding one side of one layer only to the sample steel having a plate thickness of 25 mm, and for the sample steel having a plate thickness larger than that, it was divided into one layer on both sides at the center of the plate thickness. Welded and manufactured. The welding heat input on the final side is specified in Table 2, but the heat input on the backing side is 7 to 8 kJ / mm in all cases.

Example No. 1 shown in Table 2 Steel No. A used for 1
The steel No. 1 is a comparative steel as is clear from Table 1. Steel number A
Calculated value of steel (625 (effective Nb) + 250V + 210
Ceq = 82) exceeds the value (78) obtained by adding 40 to the test steel plate thickness (38), and therefore the tensile strength of the base metal is 570.
N / mm ² was exceeded. Further, the Pcm value was as low as 0.17, and no welding cracks occurred in the Y crack test. However, the high heat input welded joint strength was less than 570 N / mm ² because Nb was not added.

Example No. 2 to 4 are examples of the present invention using steel No. B. Plate thickness 25 mm and 38 mm
The mechanical properties and weld crack susceptibility of the test steel base metal of No. 3 were good, and a sound large heat input welded joint was realized.

Example No. 5-11 are Mn, Mo, N
It is an Example of this invention by the steel No. C-I steel which changed the content of b. In all cases, the mechanical properties of the sound base metal and the high heat input welded joint were confirmed, and the weld crack susceptibility was also good. Further, even if the Mn, Mo, and Nb contents are changed as in these examples, the correlation between the calculated value and the base material strength is good, which shows that this calculation formula is industrially useful.

Example No. Reference numerals 12 and 13 are examples of the present invention using steel No. J steel in which V is not added. No. No. 12 is the case where the heating temperature is 1000 ° C., and all Nb is not solid-solved before rolling. Even in such a case, the accuracy of the calculation formula can be maintained by setting the Nb amount substituted for the calculated value as the effective Nb amount that works for precipitation hardening.

Example No. No. 14 is a comparative example in which the reheating quenching and tempering process is applied to the J steel, which is 600 N / m.
The strength of the base metal as m ² class high-strength steel is not secured.

Example No. No. 15 is a comparative example with steel No. K steel whose C content is out of the range of the present invention. Even if an appropriate amount of alloying element is found from the calculation formula, a sound weld metal cannot be obtained in the high heat input welded joint, and in this case, the joint is fractured from the weld metal part in the joint tensile test and the joint strength cannot be secured. It was

Example No. No. 16 is a comparative example of steel No. L steel having a high C content, and welding cracks occurred in the oblique Y-type welding crack test.

Example No. 17 has a high C content, and N
It is a comparative example by the steel number M steel which does not contain b. No weld cracks were found in the oblique Y-type weld crack test, but the maximum hardness exceeds 290 Hv and cannot be said to be good. Further, the toughness of the high heat input welded joint is inferior to any of the examples of the present invention.

Example No. 18 to 21 and 23 are Cu,
It is an example of the present invention by the steel of steel numbers N, O, P, Q, and S when it contains Ni and Cr.

No. 22 is a comparative steel made of steel No. R steel. Contains Cr in an amount exceeding the range specified in the present invention, and has Pc
Since the m value exceeded 0.2, weld cracking occurred in the oblique Y-type weld cracking test.

No. 24 and 25 are comparative examples of steels with steel numbers N and O containing B. In all cases, good base metal strength was obtained, but the base metal toughness was no. 25 is better. Both have good weld crack susceptibility, but the toughness of the high heat input welded joint is poor.

No. 26 is 10 for steel No. D steel.
It is an example of the present invention manufactured by heating at 00 ° C. Although the calculation formula defined by the effective Nb amount is satisfied, No. A decrease in the base metal strength corresponding to the decrease in the amount of effective Nb was recognized as compared with the example of No. 6.

[0061]

[Table 1]

[0062]

[Table 2]

[0063]

[Table 3]

[0064]

[Table 4]

[0065]

As described above, according to the present invention, 600 N / m, which has low weld crack susceptibility and excellent large heat input weldability.
An m ² class high-strength steel and a manufacturing method thereof can be provided.

[Brief description of drawings]

1 is an example No. 1; The figure which showed the hardness distribution of the large heat input welding joint part of this invention example corresponding to 6. The arrow in the figure represents the bond portion, and the hardness of the base material side heat affected zone in the immediate vicinity thereof shows a low value.

2 is an example No. The figure which showed the hardness distribution of the large heat input welding joint part of the example of this invention corresponding to 7. The arrow in the figure represents the bond portion, and the hardness of the base material side heat affected zone in the immediate vicinity thereof shows a low value.

3 is an example No. The figure which showed the hardness distribution of the large heat input welding joint part of the example of this invention corresponding to 9. The arrow in the figure represents the bond portion, and the hardness of the base material side heat affected zone in the immediate vicinity thereof shows a low value.

4 is an example No. The figure which showed the hardness distribution of the large heat input welding joint part of the example of this invention corresponding to 10. The arrow in the figure represents the bond portion, and the hardness of the base material side heat affected zone in the immediate vicinity thereof shows a low value.

5: Example No. The figure which showed the hardness distribution of the large heat input welding joint part of the comparative example corresponding to 24. A marked increase in hardness is recognized in the vicinity of the bond portion in the heat-affected zone as compared with the inventive examples.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Toru Kawanaka, Toru Kawanaka, 1-2, Marunouchi, Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (72) Kazuyuki Matsui, 1-2, Marunouchi, Chiyoda-ku, Tokyo Date Main Steel Pipe Co., Ltd.

Claims (4)

[Claims] 1. C: 0.06 to 0.1% by weight, S
i: 0.01 to 0.4%, Mn: 0.5 to 1.6%, M
o: 0.3% or less, Nb: 0.005 to 0.05%,
V: 0.1% or less, Al: 0.005-0.1%, N:
0.0005-0.008%, Ti <0.005%, B
<Including 0.0003%, Pcm = C + Si / 30 + M
n / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo
Pcm value defined by / 15 + V / 10 + 5B is 0.2
When hot rolling a steel having the following chemical composition:
Using the heating temperature T set in the temperature range of 0 to 1250 ° C., log (Nb) × (C + 12N / 14) = 2.2
Ceq = C + Mn, where the amount of solid solution Nb calculated from the relationship of 6-6770 / (T + 273.15) is the effective Nb amount.
/ 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 +
Ceq value defined by V / 14, effective Nb amount, V
Using the content and the steel plate thickness t (mm), 625 (effective Nb) + 250V + 210Ceq ≧ 40 +
A high-strength steel excellent in weld cracking susceptibility and high heat input weldability, characterized by satisfying the relationship of t 3 with the balance consisting of iron and unavoidable impurities. 2. C: 0.06 to 0.1% by weight, S
i: 0.01 to 0.4%, Mn: 0.5 to 1.6%, M
o: 0.3% or less, Nb: 0.005 to 0.05%,
V: 0.1% or less, Al: 0.005-0.1%, N:
0.0005-0.008%, Ti <0.005%, B
<Including 0.0003%, further Cu: 0.5% or less,
Ni: 1.5% or less, Cr: 0.5% or less, one or two
Including more than one species, Pcm = C + Si / 30 + Mn / 20 +
Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V
When a steel having a chemical composition with a Pcm value defined by / 10 + 5B of 0.2 or less is hot-rolled, 1000 to 125
Using the heating temperature T set in the temperature range of 0 ° C.,
g (Nb) × (C + 12N / 14) = 2.26−677
Solid solution N calculated from the relationship of 0 / (T + 273.15)
Ceq = C + Mn / 6 + Si with b amount as effective Nb amount
Using the Ceq value defined by / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 and the effective Nb amount, V content, and steel plate thickness t (mm), 625 (effective Nb) + 250V + 210Ceq ≧ 40 +
A high-strength steel excellent in weld cracking susceptibility and high heat input weldability, characterized by satisfying the relationship of t 3 with the balance consisting of iron and unavoidable impurities. 3. When producing the steel according to claim 1 or 2, after hot rolling to a predetermined plate thickness, subsequently quenching directly from the Ar3 transformation point or higher and tempering at a temperature not higher than the Ac1 transformation point. A method for producing high-strength steel excellent in weld cracking susceptibility and high heat input weldability. 4. When producing the steel according to claim 1 or 2, heating to a temperature range of 1000 to 1250 ° C.
After hot rolling at a cumulative reduction of 20% or more at 50 ° C or less,
A method for producing a high-strength steel having excellent weld crack sensitivity and high heat input weldability, which comprises directly quenching from an Ar3 transformation point or higher and tempering at a temperature not higher than the Ac transformation point.