JPH108193A - High-tensile steel excellent in large heat input weldability, susceptibility to weld cracking and weather resistance and method for producing the same - Google Patents

Abstract

(57) Abstract: large heat input welding and provides excellent 600N / mm ² class tempered type high strength steel and a manufacturing method thereof weld crack susceptibility and weather resistance. SOLUTION: In weight%, C: 0.06-0.1%, Si: 0.01-0.4%,
Mn: 0.5-1.4%, Cu: 0.2-0.5%, Ni: 0.05-0.3%, Cr: 0.3-
0.8%, Nb: 0.005-0.05%, Al: 0.005-0.1%, N: 0.0005-0.
005%, Ti <0.005%, B <0.0003%, Pcm =
C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60
The Pcm value defined by + Cr / 20 + Mo / 15 + V / 10 + 5B is 0.21 or less, and Ceq = C + Mn /
6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V
/ 14 has a Ceq value of 0.40 or less and a balance of iron and unavoidable impurities, a tensile strength of 570 N / mm ²
A high-strength steel excellent in high heat input weldability, weld cracking susceptibility and weather resistance having the above.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention bridges, warehouses, buildings of manufacturing large heat input welding properties, weld crack susceptibility and weather resistance excellent 600N / mm ² class high strength steel and used for a steel structure such as It is about the method.

[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. Although there is no description about the applicable plate thickness in this gazette, 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 application to gas tanks and line pipes. There is no technology at all.

Japanese Unexamined Patent Publication (Kokai) No. 53-119219 discloses a thick plate having a thickness of 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.

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

On the other hand, a high heat input weldability 600 is improved.
As prior arts of N / mm ² grade tempered high-strength steel, Japanese Patent Application Laid-Open Nos. 3-162522, 4-22837, 63-149354, and 60-1
No. 74820. In each of these technologies, T
Utilization of i, B, Ca, Zr, and REM is used to improve large heat input weldability, and there is no consideration regarding weather resistance.

Further, as steel materials having excellent weather resistance, Japanese Patent Application Laid-Open Nos. 5-117745 and 6-316723 are mentioned. JP-A-5-117745 is a technique relating to a method for producing a high-strength steel for building structures of 490 N / mm ² class, and is characterized by having excellent fire resistance performance.
Is a technology that imparts more excellent gas cutting properties.

The present invention has been made in view of the above circumstances, and has as its object to provide 600 N / mm ^{2 having} excellent heat input weldability, weld cracking susceptibility and weather resistance.
It is an object of the present invention to provide a grade tempered high-strength steel and a method for producing 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 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.

In order to obtain excellent high heat input weldability, it is necessary to ensure high heat input weld 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. However, an increase in the Ceq value is accompanied by deterioration in joint toughness, and it is difficult to achieve both.

In order to solve these problems which have prevented industrial supply of 600 N / mm class ² high-strength steel having a thickness of 100 mm, which is excellent in large heat input weldability, weld cracking susceptibility and weather resistance. As a result of intensive studies by the present inventors,
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 as a chemical component and employing a direct quenching method, it is possible to utilize the effect of improving the hardenability of Nb solid-dissolved during rolling and heating. As a result, the amount of alloying elements added for securing other hardenability can be reduced. Nb has the effect of finely dispersing carbides and has a function of 1 / th of thick materials.
This is extremely effective in ensuring the toughness of the 2t portion.

(2) Tempering after 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.

(3) Cu, C necessary for securing weather resistance, including the contribution of precipitation hardening elements such as Nb, Mo, and V
The required amount of alloying elements according to the thickness of the steel sheet to secure the strength was ascertained on the premise of the addition of r and Ni, and the 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: 0.5-1.4%, 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 to 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 0.21 or less, and Ceq = C
+ Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo
The Ceq value defined by / 4 + V / 14 is 0.40 or less, and the balance is iron and inevitable impurities, and the tensile strength is 570 N.
/ Mm ² or higher tensile strength steel with excellent heat input weldability, susceptibility to weld cracking and weather resistance.

(2) Mo: 0.15% or less in the above composition;
V: High tensile steel excellent in large heat input weldability, weld crack susceptibility and weather resistance further containing 0.1% or less. (3) By weight%, C: 0.06-0.1%, Si: 0.
01-0.4%, Mn: 0.5-1.4%, 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 to 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 0.21 or less, and Ceq = C
+ Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo
When hot rolling a steel having a Ceq value of 0.40 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. A direct quenching, and further a tempering treatment at a temperature not higher than the Ac1 transformation point.
A method for producing a high-strength steel having excellent heat input weldability of 0 N / mm ² or more, susceptibility to weld cracking, and excellent weatherability.

(4) Mo: 0.15% or less in the above composition
V: A method for producing a high-tensile steel excellent in large heat input weldability, weld cracking susceptibility and weather resistance further containing 0.1% or less. (5) Ceq = C + Mn / 6 + Si / 24 + Ni / 40
Using a Ceq value defined by + 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 + 27)
3.15) The amount of solid solution Nb calculated from the relationship of
Effective Nb, V content, target steel sheet thickness t
(Mm), 800 (effective Nb) +320 V + 270 Ceq ≧ t + 6
4. The tensile strength 57 described above, characterized by satisfying the following relationship:
A method for producing a high-strength steel having excellent heat input weldability of 0 N / mm ² or more, susceptibility to weld cracking, and excellent weatherability.

(6) The above-mentioned large heat input weldability having a tensile strength of 570 N / mm ² or more, wherein hot rolling is performed at a cumulative rolling reduction of 20% or more at 1050 ° C. or less during hot rolling. This is a method for producing a high-strength steel excellent in weld cracking sensitivity and weather resistance.

[0025]

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 C content is 0.1% in order to secure susceptibility to weld cracking.

<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 ensuring the strength of the base material and the strength of the welded joint, it is added in an amount of 0.5% or more. However, the addition exceeding 1.4% deteriorates the susceptibility to weld cracking, causes more hardenability than necessary, and deteriorates the base material toughness and the joint toughness.

<Cu, Ni, Cr> Cu, Ni, Cr have the effect of improving the strength of the base metal and the weld joint. N
i has the function of further improving toughness. Further, in order to secure weather resistance, these alloy elements are Cu:
0.2-0.5%, Ni: 0.3% or less, Cr: 0.3
Must be added in the range of ~ 0.8%. However, the upper limit was set for each in consideration of not securing the hardenability more than necessary.

<Nb> Nb is added in an amount of 0.005% or more in order to secure the base material strength and the welded joint. However, 0.0
Since the addition exceeding 5% 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 toughness of the base material, and reacts with Nb, V, etc. during tempering to cause precipitation hardening. Add to ensure strength.

<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 a concern about the instability of the base material performance due to the addition of Ti, and the content is restricted to less than 0.005% as an impurity element, but it is desirable that the content be less than 3.4 times the N content described later.

B must be restricted to less than 0.0003% as an impurity element in order to reduce the hardness of the heat-affected zone. If the addition is less than 0.0005%, precipitates necessary for refining the microstructure and securing the strength are not formed.
Additions exceeding 0.05% rather impair the toughness of the base metal and large heat input welded joints.

<Mo> Mo can be selectively added because it has the effect of improving the base metal strength, the joint strength, and the weather resistance. However, the upper limit is set to 0.15% because there is a tendency to deteriorate the weld cracking susceptibility and the weld joint toughness. However, it is preferable to set the upper limit to 0.1% in consideration of not securing the hardenability more than necessary. Also,
The lower limit of the addition is preferably set to 0.02% in order to exert the above effects.

<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 metal toughness. <P, S> P and S are all impurity elements. P has an effect of improving weather resistance. However, to obtain a sound base metal and a welded joint,
It is desirably regulated to 15% or less, preferably 0.01% or less.

<Pcm> Pcm is an index indicating the susceptibility to weld cracking, and is regulated to 0.21 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, but rather the Ceq value is limited to 0.40 or less to ensure high heat input weld joint toughness.

Calculation formula: 800Nb + 320V + 270C
eq ≧ t + 64 Calculation formula: 800Nb + 320V + 270Ceq is an index representing the strength at a plate thickness of 1 / 2t of the base material, and the contribution of Nb and V, which are the key of the present invention, to the carbon equivalent formula (Ceq) generally known to those skilled in the art. 25 to 100
It is a numerical formula arranged in consideration of the thickness effect in the thickness range of 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 inevitably changes according to the sheet thickness, and therefore the base metal strength changes. Refers to 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 JI classified as 600N / mm ² class weathering high strength steel also in 1 / 2t is
A steel sheet conforming to SG3114 SMA570 is to be obtained, and in the calculation formula, the term of the sheet thickness is treated as a linear correlation with the strength of 1 / 2t of the sheet thickness of 25 to 75 mm for the purpose of industrial simplicity, and the calculation formula: 800Nb + 320V + 270C
eq was set to be larger than the value obtained by adding 64 to the plate thickness t (mm). Therefore, in the region where the plate thickness exceeds 50 mm, the relationship of the calculation formula generally required is strict, and in reality, the plate thickness is 75 mm.
When the distance exceeds mm, the calculation formula is: 800Nb + 320V + 2
When 70 Ceq exceeds the value obtained by adding 64 to the plate thickness t (mm) / 2, the target strength of 1 / 2t can be secured.

In this equation, 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-
Assuming that the amount of solute Nb calculated from the relationship of 6770 / (T (° C.) + 273.15) is the effective Nb amount, the above formula is calculated as 800 (effective Nb) + 320V + 270Ceq ≧ t + 6.
4 shall be applied.

It should be noted that the range of the plate thickness targeted by the present invention is approximately 2
It is in the range of 5 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 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 quenching. 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 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.

[0045]

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.

A tensile test and a Charpy impact test were taken from the center of the thickness of all the test steels in a direction perpendicular to the rolling direction, and the mechanical properties of the base metal as a 600 N / mm class ² steel were evaluated. Table 3 shows the results.

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. All of these tests were carried out using an ultra-low hydrogen type welding material for 60 kg class steel under the conditions of an atmosphere of 20 ° C. to 60% and a test piece initial temperature of 25 ° C. Further, large heat input weldability was evaluated using a test steel having a thickness of 25 mm. Heat input 10 to 12 kJ by one layer on one side by electrogas arc welding
/ Mm, a joint tensile test and a Charpy impact test using a notch position as a bond portion were performed to evaluate joint strength and joint toughness.

Example No. 1 and 2 are examples of the present invention using steel A. Calculated value of steel A (800Nb + 320V + 27
0Ceq = 120) is 64 for the test steel sheet thickness (25, 50).
Was exceeded (89, 114), so that the tensile strength of the base material at the center of the plate thickness exceeded 570 N / mm ² and the toughness was good. The Pcm value was as low as 0.19,
No welding crack occurred in the crack test. Further, in Example No. having a plate thickness of 25 mm. The large heat input weldability examined in 1 is good in both joint strength and joint toughness.

Example No. Nos. 3 and 4 are examples of the present invention and comparative examples using steel B. At a plate thickness of 50 mm, the calculated value exceeds the plate thickness t + 64, and the mechanical properties of the base material in the center of the plate thickness are good. However, at a plate thickness of 75 mm, the calculated value is below the plate thickness t + 64, and 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 test steel plate thickness (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
In Comparative Example, the effective Nb was reduced to 0.012%, so that the calculated value was 132, which was less than the plate thickness t + 64, and the strength at the central portion of the plate thickness was less than 570 N / mm ² .

However, under the same conditions, a sheet thickness of 50 mm was examined. 7 (Example of the present invention), the calculated value is 6
Exceeding the value obtained by adding 4 gives good strength. Example No. Nos. 8 to 11 are inventive examples and comparative examples using steels D and E.

In the case of steel E, which has a lower chemical composition than steel D, the strength is insufficient with a material having a thickness of 50 mm as the calculated value decreases. Example No. 12 is a comparative example using steel F. In order to obtain a steel plate having a thickness of 75 mm without using Nb, which is a feature of the present invention, the maximum hardness is high with Pcm exceeding 0.21%,
In addition, cracking occurred in the Y-type cracking test.

In the embodiment No. In Nos. 8 and 10, evaluations of the large heat input weldability of these steels having small Ceq values of 0.32 and 0.31 were performed, and good performance was confirmed. 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.

No. 14 and 15 are Examples and Comparative Examples using steel H. No. As shown in Fig. 14, the steel H satisfying the scope of the present invention is combined with the DQT process which is a requirement of the present invention, so that a good base metal strength is secured.
o. In No. 15, since the QT process by heating at 900 ° C. was applied, the effective Nb amount was drastically reduced to 0.004%, and the target strength could not be secured.

No. Reference numeral 16 is a comparative example in which a weather-resistant 600 N / mm ² steel was manufactured using a conventional chemical composition that deviates from the scope of the present invention. Although the base material had good mechanical properties, the weld cracking susceptibility was 25 mm, so no crack was generated in the Y-type crack test, but the maximum hardness was 324 and the heat input was large. The joint toughness is not good.

The embodiments shown here are all based on JIS.
C required by SMA570 specified in G3114
Since it contains u, Cr, and Ni, it forms a stable oxide film during use and exhibits good weather resistance.

[0057]

[Table 1]

[0058]

[Table 2]

[0059]

[Table 3]

[0060]

As described above, according to the present invention, 600 N / d is excellent in large heat input weldability, weld crack susceptibility and weather resistance.
mm ² class high strength steel and a method for producing the same can be provided.

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
0.005%, 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 0.21 or less, and Ceq = C
+ Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo
The Ceq value defined by / 4 + V / 14 is 0.40 or less, and the balance is iron and inevitable impurities, and the tensile strength is 570 N.
/ Mm ² or higher tensile strength steel with excellent heat input weldability, susceptibility to weld cracking and weather resistance. 2. The high-strength steel according to claim 1, further comprising 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
0.005%, 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 0.21 or less, and Ceq = C
+ Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo
When hot rolling a steel having a Ceq value of 0.40 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. A direct quenching, and further a tempering treatment at a temperature not higher than the Ac1 transformation point.
A method for producing a high-strength steel having excellent heat input weldability of 0 N / mm ² or more, susceptibility to weld cracking, and excellent weatherability. 4. The method for producing a high tensile strength steel excellent in high heat input weldability, weld cracking susceptibility 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 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), using the amount of solute Nb calculated as the effective Nb amount as the effective Nb amount, using the effective Nb and V contents and the target steel sheet thickness t (mm), 800 (effective Nb) + 320V + 270Ceq ≧ t + 6
5. The method for producing a high-tensile steel according to claim 3 or 4, wherein the high-strength steel having a large heat input weldability of 570 N / mm ² or more, excellent in weld cracking susceptibility and weather resistance is satisfied. 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 heat input weldability of N / mm ² or more, susceptibility to weld cracking, and excellent weatherability.