JPH1068045A - 600n/mm2 class high tensile strength steel excellent in weld cracking sensitivity and large heat input welded joint toughness and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a 600N/mm<2> class high tensile strength steel excellent in large heat input weldability and weld cracking sensitivity and used for steel structures such as bridges, warehouses and buildings and to provide a method for producing the same. SOLUTION: This steel is the one having a compsn. contg., by weight, 0.06 to 0.1% C, 0.01 to 0.4% Si, 0.5 to 1.6% Mn, 0.005 to 0.05% Nb, 0.005 to 0.1% Al, 0.0005 to 0.005% N, <0.005% Ti and <0.0003% B, in which Pcm value defined by Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B is regulated to <=0.20, also, Ceq value defied by Ceq=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14 is regulated to <=0.37, and the balance iron with inevitable impurities. Then, the hardness of the coarse-grained region in the heat-affected zone formed by large heat input welding is regulated to <=188Hv, and it has >=570N/mm<2> tensile strength.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a 600N / mm class ² high-strength steel excellent in weld cracking susceptibility and large heat input welded joint toughness used for steel structures such as bridges, warehouses and buildings. Things.

[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 Application Laid-Open No. 49-37814 and Japanese Patent Publication No. 4-13406 disclose a technique characterized by low C and addition of Ti-B for the purpose of improving the susceptibility to weld cracking.
Publications are known. 600N / mm with improved weld cracking susceptibility by techniques represented by these
Class ² high strength steel is obtained, but the tensile strength required for 600N / mm class ² high strength steel is achieved by utilizing B, and base material properties are unstable due to fluctuations in chemical components and manufacturing conditions. And the hardness of the heat affected zone is significantly increased. This increase in the hardness of the weld heat-affected zone generally causes the toughness of the bond portion to be deteriorated most at the weld joint. In particular, in the case of single-pass welding with large heat input, in which the microstructure of the heat-affected zone is remarkably coarsened and is not subjected to reheating by a subsequent pass, deterioration of toughness is extremely undesirable.

Japanese Patent Application Laid-Open No. Hei 2-205627 provides a method for producing a 600 N / mm ² class high-strength steel excellent in toughness 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.
31538, JP-B-60-9086, JP-A-2-254119, JP-A-59-113120
JP, JP-B-61-12970, JP-B 2-8
No. 322 and Japanese Patent Application Laid-Open No. Sho 53-119219 have been proposed.

[0005] Among these techniques disclosed in JP-A-5-331538 relates to 500 N / mm ² Kyuhi tempered high tensile steel. Further, Japanese Patent Publication No. 60-9086, Japanese Patent Application Laid-Open No. 2-254119, Japanese Patent Application Laid-Open No.
The technology disclosed in Japanese Patent No. 3120 is 600 N / m.
It is related to m ² class non-finished high tensile strength, and it is known from the examples and the like that the upper limit of the applied plate thickness of each of these techniques is 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 susceptibility to weld cracking by combining low C with addition of V and direct quenching. There is no technology regarding large heat input weldability.

[0007] Japanese Patent Application Laid-Open No. 2-8322 discloses low 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.

[0008] Although there is no description about the applicable plate thickness in this publication, it is presumed that it is intended to be applied to steel materials having a plate thickness of approximately 50 mm or less because it is intended for use in gas tanks and line pipes. There is no technology relating to heat weldability.

Japanese Unexamined Patent Publication No. Sho 53-119219 discloses a reheating, quenching and tempering process, in which a thick 500 N /
It is intended to provide high-strength steel of mm ² class or more. 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, in addition to Nb and V, addition of Ni and Mo is substantially essential in order to secure the strength as shown in the embodiment, and 600N at a position of 1 / 4t of the thick material.
In the invention example (test steel J) which can secure the strength of / mm ² class, the Pcm value reaches 0.22 and the weldability is poor.

[0010] As described above, 60 having excellent weld cracking susceptibility.
Most of the prior art of 0N / mm ² grade tempered high-strength steel is achieved by ensuring the hardenability by adding B, and no consideration is given to the large heat input weldability.

On the other hand, as prior arts of tempered high-strength steels having improved high heat input weldability, Japanese Patent Application Laid-Open Nos. Hei 3-162522, Hei 4-22837, and Hei 63-1 are disclosed.
49354 and JP-A-60-174820. All of these technologies are intended to improve the large heat input weldability of 500 N / mm ² class high strength steel by utilizing Ti, B, Ca, Zr and REM. For example, JP-A-3-162522 is directed to a steel plate having a thickness of 15 to 20 mm.
Some examples satisfying the mechanical properties of the N / mm ² class tempered type high strength steel has no margin in both strength, application to more steel plate thickness 20mm is estimated difficult. In addition, Japanese Unexamined Patent Publication
The example in which the grade of 600 N / mm ² was obtained in JP 3-149354 A is based on tempering at a low temperature of 300 to 400 ° C. Furthermore, none of these shows examples relating to weld cracking susceptibility.

[0012] The present invention has been made in view of the above circumstances, it is an object of large heat input welding, excellent both in weld crack susceptibility was 600N / mm ² class tempered type high strength steel And a method for manufacturing the same.

[0013]

That is, in order to improve the susceptibility to weld cracking, 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. B addition is considered as an effective means to secure the strength of the base material while securing the weld cracking susceptibility. Is done. In order to effectively utilize B, it is preferable that Ti, which is conventionally frequently added, is not added in order to stably obtain the performance of the base material. Therefore, Ti, while both ensuring the soundness of improving the welded joint weld crack susceptibility without addition of B in order to obtain a 600N / mm ² class high strength steel in the application of conventional reheating quenching and tempering process There is a concern that the range of applicable thickness may be restricted.

In order to obtain excellent large heat input weldability, it is necessary to ensure high heat input welded joint strength and joint toughness. Large heat input welded joint strength is 600 N / mm ^2nd class conventional high strength steel Ceq = C + Mn / 6 + Si / 24 + N
It has been ensured by setting the Ceq value defined by i / 40 + Cr / 5 + Mo / 4 + V / 14 to approximately 0.40 or more. On the other hand, the large heat input welded joint toughness was examined by applying a heat cycle to reproduce the coarse grained area in the heat-affected zone where the deterioration is most concerned. As a result, it was found that the reduction of the Ceq value was effective in improving the joint toughness. Revealed. Therefore, it is difficult to achieve both the strength and the toughness of the large heat input welded joint in the 600 N / mm ² class high strength steel by the conventional technology.

The inventor of the present invention has conducted intensive studies to solve these problems that have prevented the industrial supply of a 600 N / mm ² class high-tensile steel excellent in high heat input weldability and weld cracking susceptibility. The following findings were found on the assumption that the direct quenching and tempering process was applied.

(1) By using a Nb-added chemical component and employing a direct quenching method, the effect of improving the hardenability of Nb solid-dissolved during rolling and heating can be utilized. As a result, the amount of alloying elements added for securing other hardenability can be reduced.

(2) The tempering treatment after the direct quenching
b The precipitation hardening of carbonitride can be utilized. This is effective for securing the strength at the center of the sheet thickness where the cooling rate during quenching is necessarily slower than the surface layer 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. Further, an effect of suppressing a decrease in hardness in a softened region generated from the heat-affected zone to the base metal portion by large heat input welding is exhibited. This effect is equivalent to reducing the Ceq value by 0.04 by adding 0.015% of Nb, thereby securing the joint strength required for 600 N / mm ² class high-strength steel even when the Ceq value is reduced to 0.40 or less. it can.

Further, if the amount of Nb added exceeds 0.01%, large heat input welding contributes not less to the suppression of coarse graining of the bond portion and the reduction of the area of the coarse grain region, and the amount of C added is 0.1%. %, The toughness of the large heat input welded joint due to the addition or increase of Nb can be prevented.

(3) The required amount of alloying element in accordance with the thickness of the steel sheet to secure the strength was grasped, and conditions for preventing the large heat input welded joint toughness and susceptibility to weld cracking were clarified.

(4) The above makes the addition of B unnecessary, and it is necessary to actively control the incorporation of B, especially in order to ensure the toughness of the high heat input 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 present invention has been made based on these findings. The gist of the present invention is as follows: (1) C: 0.06 to 0.1% by weight;
01-0.4%, Mn: 0.5-1.6%, Nb: 0.
005-0.05%, Al: 0.005-0.1%,
N: 0.0005 to 0.005%, Ti <0.005
%, B <0.0003%, Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + N
The Pcm value defined by i / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B is 0.20 or less, and Ceq = C
+ Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo
When the Ceq value defined by / 4 + V / 14 is 0.37 or less, the balance consists of iron and unavoidable impurities, and the hardness of the heat-affected zone coarse-grained region generated by large heat input welding is 188 Hv or less and the tensile strength is 570 N / High-strength steel with excellent weld cracking susceptibility and high heat input weld joint toughness of at least ² mm2.

(2) Cu: 0.5% or less, Ni: 0.3% or less, Cr: 0.6% or less, Mo:
A high-strength steel excellent in weld cracking susceptibility and high heat input weld joint toughness further containing one or more of 0.3% or less and V: 0.1% or less.

(3) C: 0.06-0.1% by weight
%, Si: 0.01 to 0.4%, Mn: 0.5 to 1.6
%, Nb: 0.005 to 0.05%, Al: 0.005
-0.1%, N: 0.0005-0.005%, Ti <
0.005%, B <0.0003%, Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + N
The Pcm value defined by i / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B is 0.20 or less, and Ceq = C
+ Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo
When hot rolling a steel having a Ceq value of 0.37 or less defined as / 4 + V / 14, the steel is heated to a temperature of 1000 ° C. or more and 1250 ° C. or less, and then hot-rolled. Direct quenching, and further tempering at a temperature not higher than the Ac1 transformation point, and the hardness of the heat-affected zone coarse-grained region generated by large heat input welding is 188 Hv or less, a tensile strength of 570 N / mm ^2. A method for producing a high-strength steel having excellent weld cracking susceptibility and high heat input weld joint toughness.

(4) Cu: 0.5% or less, Ni: 0.3% or less, Cr: 0.6% or less, Mo:
A method for producing a high-tensile steel excellent in weld cracking susceptibility and large heat input weld joint toughness further containing one or more of 0.3% or less and V: 0.1% or less.

(5) Ceq = C + Mn / 6 + Si / 24
Using a Ceq value defined by + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 and a heating temperature T (° C.) set in a temperature range of 1000 to 1250 ° C., log ｛(N
b) × (C + 12N / 14)｝ = 2.26-6770 /
(T + 273.15) The smaller of the amount of solid solution Nb calculated from the relationship of (T + 273.15) and the amount of added Nb added to the steel is defined as the effective Nb amount. Using t (mm), 625 (effective Nb) +250 V + 210 Ceq ≧ t + 4
0, wherein the tensile strength 570 is satisfied.
A method for producing a high-strength steel having excellent weld cracking susceptibility of N / mm ² or more and high heat input weld toughness.

[0026] (6) In the hot rolling, the aforementioned tensile strength of 570N / mm ² or more weld crack susceptibility and rafters, characterized by subjecting a hot-rolled at a cumulative reduction rate of 20% or more at 1050 ° C. or less This is a method for producing high-strength steel excellent in toughness of heat-welded joints.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the constituent elements in the present invention will be described below. <C> If the C content is less than 0.06%, it is necessary to add a large amount of other hardenability improving elements, resulting in high cost, deterioration in toughness, and deterioration in weldability. In particular, when large heat input welding is performed on the steel of the present invention, if the C content is less than 0.06%, the C content in the weld metal is reduced.
And it becomes difficult to secure joint strength with general welding materials. The upper limit of the amount of C was set to 0.1% in order to ensure weld cracking susceptibility and to ensure high heat input weld joint toughness accompanying the addition of Nb.

<Si> Since Si works effectively in securing the base metal strength and the strength of the welded joint, it is added in an amount of 0.01% or more.
However, the addition exceeding 0.4% deteriorates weld cracking susceptibility and weld joint toughness.

<Mn> Since Mn works effectively in securing the base metal strength and the strength of the weld joint, Mn is added in an amount of 0.5% or more. However, the addition exceeding 1.6% deteriorates the susceptibility to weld cracking, causes more hardenability than necessary, and deteriorates the base metal toughness and the joint toughness.

<Nb> Nb is added in an amount of 0.005% or more to secure the strength of the base material and the strength of the welded joint. But,
Since the addition exceeding 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%.

<Al> Al is added for deoxidizing steel,
Usually, 0.005% 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. However, the addition of Al exceeding 0.1% impairs the base material toughness.

<N> N reacts with Al, Nb, etc. to form precipitates, thereby refining the microstructure and improving the base material toughness, and reacts with Nb, V, etc. during tempering to cause precipitation hardening. Add to ensure strength.

If the addition is less than 0.0005%, precipitates required for refining the microstructure and ensuring the strength are not formed,
Additions exceeding 0.005% rather impair the toughness of the base metal and the high heat input welded joint.

<Ti, B> Ti has the effect of improving the toughness of the base metal and the welded joint through refinement of the microstructure. In addition, in the B-added steel, it is added positively in order to secure B effectively acting on hardenability. However, in the present invention, Ti is added in order to secure the base metal strength without adding B, which is likely to harden the weld heat affected zone, and to achieve weld joint toughness particularly by reducing the hardness of the heat affected zone coarse grain region. You don't have to.
Rather, there is concern about the instability of the base metal performance due to the addition of Ti, and the impurity element is regulated to less than 0.005%.
Desirably, it is less than 3.4 times the content.

B must be restricted to less than 0.0003% as an impurity element in order to reduce the hardness of the heat-affected zone. <Cu, Ni, Cr> Cu, Ni, Cr have the effect of improving the strength of the base material and the weld joint. Ni further functions to improve toughness. However, more than necessary addition Ce
Since q was increased and the toughness of the large heat input welded joint and the productivity were deteriorated, the upper limit was set for each. In particular, excessive addition of Cu causes a reduction in hot rolling, so that the content is set to 0.5% or less.

<Mo> Mo can be selectively added because it has the effect of improving the base metal strength and the joint strength. However, the upper limit is set to 0.3% because there is a tendency to deteriorate the weld cracking sensitivity and the weld joint toughness. However, it is preferable to set the upper limit to 0.2% in consideration of not securing the hardenability more than necessary. In addition, the lower limit in the case of adding is preferably set to 0.02% in order to exert the above effect.

<V> Since V works effectively in securing the base metal strength and the strength of the welded joint, it is possible to selectively add 0.01% or more of V.

However, the addition exceeding 0.1% deteriorates the susceptibility to weld cracking and impairs the base material toughness. <P, S> P and S are all impurity elements. In order to obtain a sound base metal and a welded joint,
It is desirable that the content be regulated to 5% or less, preferably 0.01% or less.

<Pcm> Pcm is an index indicating the susceptibility to weld cracking, and is regulated to 0.20 or less in order to eliminate the need for preheating during welding in a normal environment.

<Ceq> Ceq is a carbon equivalent value well known conventionally. Conventional 600N / including thick material
Although the Ceq value of the mm ² class high-strength steel generally exceeded 0.40, in the present invention, since the base material strength and the large heat input joint strength were secured by the addition of Nb as described above, the Ceq value exceeded 0.40.
The eq value is not necessary. Rather, the Ceq value is set to 0.37 or less, preferably 0.3 to secure high heat input weld joint toughness.
Limit to 5 or less.

Formula: 625 (effective Nb) +250 V +
210Ceq ≧ t + 40 Formula: 625 (effective Nb) + 250V + 210Ceq
Is an index representing the strength at a plate thickness of 1 / 2t of the base material,
The carbon equivalent equation (Ceq) generally known among the traders is added to the contribution of Nb and V, which is the key of the present invention, and further about 15%.
It is a numerical formula arranged in consideration of the thickness effect in the thickness range of up to 60 mm. The thickness effect means that when the steel sheet is forcibly cooled from the Ar3 transformation point or higher by direct quenching after hot rolling, the cooling rate is inevitably changed according to the sheet thickness,
This means that the base material strength changes. Since the relationship between the plate thickness and the cooling rate is a linear relationship in log-log representation, the plate thickness and the strength can be approximated by the same relationship. Here, 1/2
JIS G3114SMA570 and JIS classified as 600 N / mm ² class weather resistant high tensile steel at t
To obtain a steel plate conforming to G3106SM570,
The term of the sheet thickness in the calculation formula is set to a sheet thickness of 15 to facilitate industrial simplicity.
The correlation with the intensity of ｔt of ６０60 mm is treated as linear, and the calculation formula is: 625 (effective Nb) +250 V + 210
Ceq was determined to be greater than the value obtained by adding 40 to the plate thickness t (mm).

In this formula, the effect of increasing the strength of Nb and V indicates the contribution of precipitation hardening of V carbonitride in the case of V, and the contribution of increasing hardenability in addition to precipitation hardening of Nb carbonitride in the case of Nb. Is considered. 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,
og {(Nb) × (C + 12N / 14)} = 2.26-
The smaller of the amount of solid solution Nb calculated from the relationship of 6770 / (T (° C.) + 273.15) and the amount of added Nb added to the steel, whichever is smaller, is defined as the effective Nb amount. (Effective Nb) + 250V + 210Ceq ≧ t + 4
Must be applied as 0.

Incidentally, the range of the plate thickness targeted by the present invention is approximately 1
The range is 5 mm to 60 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 secured due to coarsening of the microstructure, so the upper limit is set to 1250 ° C., preferably 1200 ° C., and more preferably 1150 ° C.

<Rolling Conditions> The step of hot rolling the uniformly heated steel of the present invention 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 draft of 5% or more, more preferably 10% or more for each pass of rolling.

<Direct Quenching> After the completion of hot rolling, it is necessary to forcibly cool a steel sheet having a temperature higher than the Ar3 transformation point to perform a 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 performed in order to eliminate concerns about performance changes 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 Ac1 transformation point, the strength is significantly reduced, and 600 N / mm.
The strength as a ^second grade high strength steel is not secured.

<Acoustic Anisotropy> Acoustic anisotropy refers to a direction (C direction) perpendicular to a direction parallel to the rolling direction of steel (L direction).
Is defined by the sound speed ratio. If sufficient time has passed at that temperature after hot rolling, the steel sheet will recrystallize and the acoustic anisotropy will be in the approximate range of 1.00 to 1.005. On the other hand, when the rolling is completed at a low temperature or directly quenched immediately after the completion of the rolling even at a relatively high temperature, the steel sheet does not sufficiently recrystallize, and the acoustic anisotropy becomes a value of 1.005 or more. In the case of the Nb-added steel as in the present invention, the recrystallization temperature is high. For example, when a cumulative reduction of 20% or more is applied at 1000 ° C. or lower, rolling at a low temperature that impairs the rolling efficiency is performed by 1.01%.
Is achieved, and the low-temperature toughness is improved. As shown in FIG. 4, when the acoustic anisotropy of the steel sheet exceeds 1.01, the low-temperature toughness is improved.

[0049]

EXAMPLES Table 1 shows chemical components of steel used in 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 In addition, the rolling finish temperature is 850 ° C. or more in each case,
The tempering temperature was in the range of 580 to 680 ° C. Further, a part of the test steel was subjected to a reheating quenching and tempering treatment and used for a comparative example.

A tensile test and a Charpy impact test were taken from the center of the thickness of all test steels in a direction perpendicular to the rolling direction, and the mechanical properties of the base material as a 600 N / mm ² class steel were evaluated. The results are shown in Table 3. Further, an oblique Y-type welding crack 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 welding crack susceptibility. In each of these tests, an ultra-low hydrogen type welding material for 60 kg steel was used.
The test was performed at 5 ° C. The large heat input welded joint toughness was evaluated by reproducing the coarse grain area of the heat-affected zone where deterioration is most concerned by the heat cycle shown in FIG. 1 and conducting a Charpy impact test on this. In addition, a large heat input welded joint was manufactured by electrogas arc welding using several test steels, and a Charpy impact test was performed using the notch position where the joint strength and toughness were most concerned as a bond, and the present invention was performed. Was validated.

Embodiment No. 1 is a comparative example using steel A. Calculated value of steel A not containing Nb, which is a requirement of the present invention (625 (effective Nb) +250 V + 210 Ceq = 7
8) exceeds the value (60) obtained by adding 40 to the test steel plate thickness (20), so that the tensile strength of the base metal at the center of the plate thickness is 57.
It exceeded 0 N / mm ² and the toughness was good. Pcm
The value was as low as 0.17, and no welding crack occurred in the Y crack test. However, since it does not contain Nb, the strength of the large heat input welded joint is less than 570 N / mm ² .

Example No. 2 to 10 are examples of the present invention using BI steel. The toughness of the base metal and the reproduced heat-affected zone of the large heat input welded joint is good. In addition, it was confirmed that the performance of the large heat input welded joint actually produced was good in both strength and toughness. In addition, in Example No. No. 6,7
Is an example of the present invention in which the applied plate thickness is changed.
5 (effective Nb) +250 V + 210 Ceq). Also, in Example No. 1 using E steel. 5 and F
No. by steel From the comparison between Nos. 6 and 7, it is understood that it is more preferable to suppress P to 0.01 or less from the viewpoint of securing the toughness of the coarse grain area in the heat-affected zone of the large heat input welded joint.

Example No. 11 to 14 are comparative examples using J and K steels. In these examples, since Ceq exceeds 0.37, the toughness of the heat-affected zone of the large heat input welded joint is inferior to that of the present invention. In addition, N heated at 1000 ° C.
o. Example No. 12 is No. 12 heated at 1100 ° C. 11
, The effective Nb amount decreases, and as a result, the calculated value (625)
(Effective Nb) + 250V + 210Ceq) was reduced to 100, and a decrease in base metal strength was observed. No. Reference numeral 13 is an example in which a reheating quenching and tempering (QT) process is applied instead of the direct quenching and tempering (DQT) process, and a target base material strength is not achieved. These are all calculated formulas (625 (effective Nb)) which give the necessity of the direct quenching and tempering process which is a requirement of the present invention and the condition for securing the base material strength.
+ 250V + 210Ceq).

Example No. 15 to 17 are comparative examples using L, M, and N steels. Although the strength and toughness of the base materials of these examples are good, Pcm exceeds 0.20 and Ceq is 0.3 in order to obtain base material performance without adding Nb, B, and Ti at all.
Over eight. Therefore, the maximum hardness exceeds 300Hv and Y
Although cracking did not occur in the mold welding cracking test, cracking was a concern when the welding material was made of a low hydrogen type, and the welding cracking sensitivity was not good. Further, the toughness of the heat-affected zone coarse-grained area of the large heat input welded joint is inferior to that of the present invention.

Example No. 18 is a feature of the present invention, N
This is a comparative example using O steel in which P is reduced and Ceq is reduced by utilizing B without using b. Although the weld cracking susceptibility was good, the toughness of the heat-affected zone coarse-grained region of the large heat input welded joint was inferior to that of the present invention despite the Ceq of 0.37 or less.

[0056]

[Table 1]

[0057]

[Table 2]

[0058]

[Table 3]

Next, the relationship between acoustic anisotropy and vTs for steel numbers 5E and 8G was examined. The results are shown in Table 4 and FIG. From this result, the acoustic anisotropy was 1.01 to
It can be seen that those having a range of 1.035 have low vTs.

[0060]

[Table 4]

[0061]

As described above, according to the present invention, 600 N / mm ² excellent in weld cracking susceptibility and high heat input weld joint toughness.
Grade high strength steel and a method for producing the same.

[Brief description of the drawings]

FIG. 1 is a diagram showing a heat cycle used for a simulation of toughness evaluation in a coarse grained area of a heat-affected zone of a large heat input welded joint.

FIG. 2 is a diagram showing the relationship between the reproduced hardness and toughness of a coarse grain area in the heat-affected zone.

FIG. 3 is a diagram showing a relationship between a carbon equivalent (Ceq) value and reproduced hardness of a heat-affected zone coarse grain region.

FIG. 4 is a diagram showing a relationship between vTs (° C.) and acoustic anisotropy.

Claims (6)

[Claims] 1. C .: 0.06 to 0.1% by weight, S
i: 0.01 to 0.4%, Mn: 0.5 to 1.6%, N
b: 0.005 to 0.05%, Al: 0.005 to 0.
1%, N: 0.0005 to 0.005%, Ti <0.0
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + N
The Pcm value defined by i / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B is 0.20 or less, and Ceq = C
+ Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo
When the Ceq value defined by / 4 + V / 14 is 0.37 or less, the balance consists of iron and unavoidable impurities, and the hardness of the heat-affected zone coarse-grained region generated by large heat input welding is 188 Hv or less and the tensile strength is 570 N / High-strength steel with excellent weld cracking susceptibility and high heat input weld joint toughness of at least ² mm2. 2. Cu: 0.5% or less, Ni: 0.3% or less, Cr: 0.6% or less, Mo: 0.3% or less, V:
2. The composition according to claim 1, further comprising one or more of 0.1% or less.
High tensile strength steel with excellent weld cracking susceptibility and high heat input weld joint toughness described in 1. 3. C: 0.06 to 0.1% by weight, S
i: 0.01 to 0.4%, Mn: 0.5 to 1.6%, N
b: 0.005 to 0.05%, Al: 0.005 to 0.
1%, N: 0.0005 to 0.005%, Ti <0.0
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + N
The Pcm value defined by i / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B is 0.20 or less, and Ceq = C
+ Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo
When hot rolling a steel having a Ceq value of 0.37 or less defined as / 4 + V / 14, the steel is heated to a temperature of 1000 ° C. or more and 1250 ° C. or less, and then hot-rolled. Direct quenching, and further tempering at a temperature not higher than the Ac1 transformation point, and the hardness of the heat-affected zone coarse-grained region generated by large heat input welding is 188 Hv or less, a tensile strength of 570 N / mm ^2. A method for producing a high-strength steel having excellent weld cracking susceptibility and high heat input weld joint toughness. 4. Cu: 0.5% or less, Ni: 0.3% or less, Cr: 0.6% or less, Mo: 0.3% or less, V:
4. The composition according to claim 3, further comprising one or more of 0.1% or less.
2. A method for producing a high-strength steel having excellent weld cracking susceptibility and high heat input weld joint toughness described in 1). 5. Ceq = C + Mn / 6 + Si / 24 + N
Using a Ceq value defined by i / 40 + Cr / 5 + Mo / 4 + V / 14 and a heating temperature T (° C.) set in a temperature range of 1000 to 1250 ° C., log ｛(Nb)
× (C + 12N / 14)｝ = 2.26-6770 / (T
+273.15), the smaller of the amount of solid solution Nb and the amount of added Nb added to the steel calculated as the effective Nb amount, the effective Nb and V content, and the target steel sheet thickness t (Mm), 625 (effective Nb) +250 V + 210 Ceq ≧ t + 4
5. The method for producing a high-strength steel according to claim 3, wherein the high-strength steel has excellent weld cracking sensitivity of 570 N / mm ² or more and high heat input weld toughness. 6. A hot rolling process at a temperature of 1050 ° C. or less
The tensile strength 570 according to any one of claims 3 to 5, wherein the hot rolling is performed at a cumulative rolling reduction of 0% or more.
A method for producing a high-strength steel having excellent weld cracking susceptibility of N / mm ² or more and high heat input weld toughness.