JPH1161249A - Production of steel having tensile strength of 780 n/mm2 and excellent in toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the steel excellent in weldability, sound anisotropy, large input heat weld joint property as well as toughness at a plate thickness center by subjecting a steel, in which a content of C, Ni, Nb and a value of Ceq are specified, to direct quenching and tempering treatments immediately after hot rolling at a specified temp. SOLUTION: The steel, which contains, by weight, >=0.06% C, 0.5-3.0% Ni, 0.02-0.04% Nb as an indispensable component and satisfies >=0.48% Ceq, is heated at a temp., at which a Nb quantity not entering into solid solution exists >=0.005%, or lower to >=950 deg.C and then is subjected to hot rolling. The Ceq is defined as C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14. The steel, after hot rolling, is subjected to direct quenching and then tempering at a temp. of the Ac transformation point or lower to >=600 deg.C. Further the steel contains 0.15-1.5% Mn, <=0.01% P, <=0.01% S, 0.1-0.8% Mo, 0.01-0.08% Al, 0.001-0.008% N and <=0.0002% B.

Description

DETAILED DESCRIPTION OF THE INVENTION

TECHNICAL FIELD The present invention relates to a steel structure such as a bridge or a floodgate, which has excellent characteristics of stopping brittle fracture propagation at low temperatures, and has weldability, acoustic anisotropy and heat input of 5 k.
High heat input submerged arc welding exceeding J / mm (SA
W) Excellent tensile strength of 780 N /
a method of manufacturing a mm ² class steel.

[0002]

^2. Description of the Related Art In recent years, high-strength steels having a tensile strength of 780 N / mm ² or more (hereinafter referred to as “HT780 steels”) have been developed.
It is used for various steel structures such as spherical tanks, bridges, and floodgates. This is because HT780 steel is about 3 times less than ordinary steel.
This is because it has twice the yield strength and about twice the tensile strength, and thus has the advantages of reducing the thickness of the steel material and reducing the weight of the steel structure in order to satisfy the required design strength as a structure.

[0003] However, in order to realize the high strength of HT780 steel, it is inevitable to increase the amount of alloying elements contained in the steel. Since the content is extremely high, improvement of weldability when constructing a steel structure has been a major issue. Naturally, toughness was also required in terms of safety of the steel structure.

[0004] To solve these problems, various types of steel have been proposed. Most of them have been produced by quenching and tempering, containing a trace amount of B which significantly improves hardenability. HT containing B
In 780 steel, a sufficiently quenched martensitic structure can be obtained, and excellent base metal strength and toughness can be obtained.

On the other hand, since the hardening of the welded portion is increased due to the B content and the susceptibility of the welded portion to low-temperature cracking is impaired, it is necessary to take sufficient measures for preventing weld cracking during welding. Generally, in order to prevent welding cracks, the work to be welded is preheated to 100 ° C. or higher. However, a work environment heated to a high temperature is not preferable in terms of safety and health, and the work efficiency is significantly reduced. Is the actual situation.

[0006] In order to solve such a problem of the B-containing HT780 steel, Japanese Patent Application Laid-Open No. 4-333516 discloses an HT780 steel containing no B. This is because the amount of C that enhances the weld hardenability is 0.043 to 0.07.
It is limited to 8% and contains a large amount of Cu, which does not adversely affect the weld hardenability, as much as 0.9 to 1.8%, and utilizes precipitation hardening of Cu.

However, the HT780 steel containing no Cu and containing B has excellent weld hardening properties,
In order to secure the strength of the base material by utilizing the precipitation strengthening of the steel, it is necessary to limit the tempering temperature to a relatively low temperature of 550 ° C. to 600 ° C. Therefore, when welding is performed, the vicinity of the welded portion is reheated to the temperature or higher, so that the hardness of the portion is reduced and there is a concern that the strength of the weld joint is insufficient. 600 ° C
When the above-mentioned high-temperature tempering is performed, the strength increase due to precipitation strengthening is reduced, so that the base material strength may be insufficient.

On the other hand, in addition to such a problem of weld hardening,
Recently, from the viewpoint of rationalization of construction of steel structures, HT7 has two new requirements that the acoustic anisotropy of the steel sheet is small and that the toughness of the welded portion is good even when large heat input welding is performed.
Given to 80 steel.

The requirement for acoustic anisotropy stems from the fact that the detection of welding defects is performed by ultrasonic flaw detection at oblique angles in order to ensure the safety of welded structures such as bridges. In ultrasonic testing, if there is a difference in the directionality of crystal grains in the final rolling direction (L direction) of the steel sheet and a direction (C direction) orthogonal to the final rolling direction, that is, the rolling texture, ultrasonic waves are incident. There is a difference in sound speed depending on the direction, which makes it difficult to detect a defect accurately.

In addition, there is a technical limit in evaluating and judging the inspection in the L direction and the inspection in the C direction separately. Therefore, if an echo that is suspected of a welding defect is found, it must be repaired at all locations, and repair of the defect more than necessary is unavoidable, and the construction cost becomes enormous.

In order to solve the problem concerning acoustic anisotropy, for example, Japanese Patent Application Laid-Open No. 63-235431 discloses a method for producing a steel sheet having small acoustic anisotropy. This is intended to reduce acoustic anisotropy by using a steel having a carbon equivalent value equal to or less than a certain amount and controlling the rolling reduction in hot rolling, the rolling temperature, and the cooling rate after rolling. However, this method can be realized with a tensile strength of 490 N / mm ² grade steel, and satisfies all strict requirements such as strength, toughness, weldability, etc. in steels having a high alloy content such as HT780 steel. If necessary, it is not easy to reduce acoustic anisotropy.

On the other hand, in the case of high heat input welding of HT780 steel, the strength of the welded joint portion is greatly reduced due to the large heat input during welding, and the toughness of the weld heat affected zone (HAZ) is significantly deteriorated. There are various problems. Therefore, generally, the welding heat input amount that does not cause such a problem has to be limited to, for example, 5 kJ / mm or less. However, if a large heat input submerged arc welding (hereinafter, referred to as "large heat input SAW") method can be adopted so that the heat input amount exceeds 5 kJ / mm in the construction of a welded structure such as a water gate iron pipe, the productivity of the construction will be increased. Is significantly improved, and the cost can be significantly reduced.

JP-A-61-044161 discloses that H
A method for solving the problems associated with such a large heat input of T780 steel is disclosed. The present invention seeks to optimize the alloy element content of HT780 steel and to reduce the carbon content (C: 0.0
6% or more and 0.12% or less) and high carbon equivalent (Ceq:
0.50 or more).

The present invention is based on the premise that welding is performed using an electrogas welding method with a heat input of 9 kJ / mm, and has excellent toughness of a weld bond portion. It has been found that when steel is welded with the above-described high heat input SAW, the toughness of the weld is insufficient. That is, the magnitude of the welding heat input greatly affects the coarsening of austenite crystal grains in the vicinity of the welded portion, but at the same time, the cooling rate after reheating differs due to the difference in welding method, and the generated structure differs. The large heat input SAW has a lower cooling rate after welding with the same heat input as compared with the electrogas welding method, and thus a structure disadvantageous to the toughness of the HAZ is likely to be generated.

[0015] Therefore, it can be seen that in the case of large heat input SAW, it is more difficult to secure the strength of the weld joint and the toughness of the weld heat affected zone than by electrogas welding. As a countermeasure against this, it is conceivable to further increase the carbon equivalent value, but this causes problems such as deterioration of weldability and an increase in cost. In particular, it is not easy to realize HT780 steel having a thickness of 50 mm or more.

Japanese Unexamined Patent Application Publication No. 8-33, filed by a part of the present inventors.
No. 3626 discloses an HT780 steel having a plate thickness of 50 mm or more in which these various properties are improved. The present invention aims at further optimizing the alloy element content of the HT780 steel, and has a low carbon content (C: 0.055 to 0.084%).
By using a steel not containing B having a Ceq of 0.48 or more, and after slab heating, rolling is completed at a temperature at which no rolling texture is generated, thereby improving weldability, large heat input joint characteristics, and acoustic anisotropy. This is a method for producing steel with excellent properties.

From the examples of the present invention, it can be seen that all the properties required for HT780 steel are satisfied. But,
According to the subsequent studies by the inventors, even with this method, the toughness at the center of the sheet thickness cannot be said to be sufficient depending on the manufacturing conditions. For example, the propagation arrest property (arrest property) of brittle fracture at low temperature is as follows: There is room for improvement.

The demand for a thick material in the HT780 steel is particularly taken into consideration for application to a long bridge or a high-fall floodgate, and the steel plate used has a thickness of 50 mm or more, and in some cases, 100 mm or more. May be required. In general, continuous cast slabs are often used in the production of thick steel sheets. For this reason, the amount of reduction from the rolling start thickness to the finished product thickness is restricted, and it is difficult to obtain a large reduction ratio per pass.

Further, in order to adjust the shape of the steel sheet, it is necessary to apply a light reduction of 5% or less per pass. Therefore, in hot rolling, grain refinement by recrystallization of austenite cannot be sufficiently performed, and it is difficult to expect improvement in toughness due to refinement of the crystal structure.

The cooling rate at the time of quenching the steel sheet is naturally lower at the center of the sheet thickness than at other positions, and in the case of a thick HT780 steel, martensite is present in the portion from the surface layer to the sheet thickness of 1 / 4t in the thickness direction. Even when a structure is obtained, an upper bainite structure that is not favorable in toughness is likely to be generated in the central portion of the plate thickness.

For these reasons, it is not easy to secure toughness at the center of the thickness of the thick HT780 steel. In addition, an impact test generally conforms to JIS standards (for example, JIS
G3106, rolled steel sheet for welded structure: sampling position of 9.2.1 (4) impact test specimen), which is made at a position 1/4 of the thickness from the surface. The excellent toughness (Charpy fracture surface transition temperature) shown in the above-mentioned prior art is also the 1 / 4t position in the example where the sampling position is specified, and the one not specified is presumed to be the position.

In addition, rope materials used for bridges and sluice gates are required to have arrest characteristics from the viewpoint of ensuring safety, and a high fracture toughness value is required even in the center of the plate thickness. Therefore, the Charpy absorbed energy or the fracture surface transition temperature alone in the impact test is not always sufficient, and a toughness by a test method capable of judging the propagation stopping characteristic of brittle fracture is required. Such fracture toughness values are not disclosed in the prior art.

[0023]

As described above, although there are prior arts relating to steels having excellent weldability, acoustic anisotropy, and high heat input welding characteristics, in addition to these, the toughness at the center of the sheet thickness is also reduced. At present, a method for stably producing excellent HT780 steel has not yet been realized, despite high demands from consumers. The present invention is intended to solve these problems. In addition to the weldability, acoustic anisotropy, and high heat input welded joint properties, a thick material having excellent toughness in the central portion of the sheet thickness is obtained.
An object of the present invention is to provide a method for producing T780 steel.

[0024]

In order to solve the above-mentioned problems, the present inventors comprehensively investigated the chemical composition of HT780 steel, the slab heating temperature during hot rolling, the rolling finishing temperature, and the heat treatment conditions after rolling. The following invention was made. That is, in the first invention, C: 0.06% or more and Ni: 0.5 to
A steel containing 3.0%, Nb: 0.02 to 0.04% and satisfying Ceq: 0.48% or more is converted into undissolved Nb
After heating at 950 ° C. or higher at a temperature of 0.005% or higher, hot rolling is performed, and the steel is directly quenched immediately after the completion of rolling, and then tempered at a temperature of 600 ° C. or lower at an Ac1 transformation point or lower. HT780 steel with excellent toughness. Here, Ceq (%) is defined by the following equation. Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr
/ 5 + Mo / 4 + V / 14 According to the present invention, HT having good toughness even in the center of the plate thickness
780 steel can be obtained.

According to a second aspect of the present invention, C: 0.06 to
0.09%, Mn: 0.5 to 1.5%, P: 0.01%
Hereinafter, S: 0.01% or less, Ni: 0.5 to 3.0%,
Mo: 0.1 to 0.8%, Nb: 0.02 to 0.04
%, Al: 0.01 to 0.08%, N: 0.001 to
Steel containing 0.008% or less, B: 0.0002% or less, and satisfying Ceq: 0.48% or more, with the balance being iron and unavoidable impurities.
After heating at a temperature not lower than the existing temperature of 950 ° C. or higher, the hot rolling is completed within the range of _{TF to} 1050 ° C., and the steel sheet is directly quenched immediately after the end of the rolling.
This is a method for producing a 780 N / mm ² grade steel having excellent toughness, characterized by tempering at a temperature of not less than ℃. Here, T _F = 20Mn + 10Ni + 15Cr + 1
00Mo + 1500Nb + 15Cu + 150V + 500
(Ti-3.42N) -8√Thickness (mm) +830
(° C.) Further, “√plate thickness (mm)” means 0.5% of the plate thickness (mm).
Means square. According to the present invention, HT780 steel having excellent weldability, large heat input joint characteristics, and acoustic anisotropy and having good toughness even in the center of the thickness can be obtained.

According to a third aspect of the present invention, the steel is contained in a percentage by weight of Si: 0. O1-0.5%, Cu: 0.01-0.
5%, Cr: 0.01-1.0%, V: 0.01-0.
1%, Ti: 0.005 to 0.02% or more, characterized by having a toughness and excellent tensile strength of 780N
/ Mm This is a method for producing grade ² steel. According to the present invention, weldability, large heat input joint characteristics, and toughness at the center of the sheet thickness can be further improved.

Hereinafter, the basic concept of the present invention will be described. As described above, a continuous cast slab is often used even for an extremely thick HT780 steel having a plate thickness of 50 mm or more, and in some cases, 100 mm or more. Therefore, in hot rolling, the total rolling reduction is small, and a large rolling reduction per pass cannot be taken, and improvement in toughness by refining the structure through refining of austenite cannot be expected.

Therefore, the present inventors have made the Nb contained for the purpose of increasing the strength to remain partially undissolved in the slab before hot rolling, whereby the austenite crystal grains during the slab heating are reduced. The problem was solved by suppressing the coarsening of austenite, and in particular, by giving the austenite grains recrystallized by high-temperature rolling to have the function of suppressing the growth of crystal grains generated in the subsequent rolling pass under light pressure.

FIG. 1 shows the amount of undissolved Nb before hot rolling,
It is a figure which shows the relationship of the no-ductility transition temperature (NDTT) of the NRL type drop weight test taken from the board thickness center part. Here, the amount of undissolved Nb was determined from the following equation. log @ (Nb-undissolved Nb). (C + 12N / 1
4)} = 2.26-6770 / (T H +273.15) T H: Slab heating temperature (℃)

In the NRL type drop test, the thickness was 25 m.
m, a weld bead was placed on one side of a test piece having a length of 360 mm and a width of 90 mm, and a test piece having a notch of 1.5 mm or less was cooled to various temperatures. The weight was dropped on the back surface to break the test piece. The ductile transition temperature (NDTT) is the maximum temperature at which a specimen will break.

As is clear from FIG. 1, an increase in the amount of undissolved Nb is effective in improving the toughness in the center of the sheet thickness. HT78
Considering the environment where 0 steel is used, it is considered that NDTT is required to be at least 130 ° C. or less, but it is understood that the amount of undissolved Nb needs to remain 0.005% or more to satisfy this. . FIG. 1 also shows that steel having a chemical composition outside the range of the present invention has insufficient toughness at the center of the plate thickness.

The reasons for limiting the chemical components and production conditions of the present invention are described below. C: 0.06 to 0.09%. C is included to ensure base material strength, but at the same time, to obtain a martensite structure at the center of the sheet thickness and secure toughness,
The content is required to be 0.06% or more, but if it exceeds 0.09%, the weldability and the large heat input joint toughness are significantly deteriorated.

Mn: 0.5 to 1.5%. Mn is added to improve the base metal strength and the welded joint strength, but if it is less than 0.5%, it is insufficient to improve the strength.
If it exceeds 5%, the weldability deteriorates.

P: 0.01% or less. P, which is an impurity element, promotes tempering embrittlement of the base material and degrades the toughness of a large heat input welded joint, so the upper limit is made 0.01%.

S: 0.01% or less. If the content of S, which is an impurity element, exceeds 0.01%, the toughness of a high heat input weld joint is significantly deteriorated. Therefore, the content of S is set to 0.01% or less, preferably 0.005% or less.

Ni: 0.5 to 3.0%. Ni is
Improves base metal strength and toughness, as well as the strength and toughness of large heat input welded joints. However, if the content is less than 0.5%, it does not contribute to toughness, and if it exceeds 3.0%, economic efficiency is impaired.

Mo: 0.1 to 0.8%. Mo is
Although it is contained in order to improve the base metal strength and the strength of the large heat input welded joint, if it is less than 0.1%, it is insufficient to improve the strength, and if it exceeds 0.8%, the weldability is impaired.

Nb: 0.02 to 0.04%. The undissolved Nb at the time of slab heating suppresses the formation of giant crystal grains that occur at the time of light reduction rolling, refines the microstructure after rolling, and improves toughness. Nb also has the effect of improving the strength of the base metal and the strength of the large heat input welded joint. However, 0.0
If the content is less than 2%, the effect of suppressing large grains due to undissolved Nb cannot be sufficiently obtained, and the effect of improving toughness is small. 0.0
If the addition exceeds 4%, the toughness of the weld metal part is particularly impaired, so the range is limited to the above.

Al: 0.01% to 0.08%. Al
Is contained mainly for deoxidation in the direct quenched material, but if its content is less than 0.01%, its effect is insufficient, and if it exceeds 0.08%, it impairs the cleanliness of the steel. Deteriorates material toughness.

N: 0.001 to 0.008%. N
Is generally contained in combination with Al to form a nitride, suppresses coarsening of austenite grains, and improves base metal toughness. A favorable effect of dissolved Nb is produced. If the content is less than 0.001%, the amount of the precipitate is insufficient. If the content exceeds 0.008%, the base material toughness and the large heat input welded joint toughness are rather deteriorated.

B: 0.0002% or less. B contains a very small amount, significantly improves hardenability, increases the hardening of the weld, and increases the low-temperature cracking susceptibility of the weld.
The upper limit is set to 0.0002%.

Ceq (carbon equivalent value): not less than 0.48%. Ceq is an index of hardenability as well as an index of weld hardening, and contributes to base metal strength and toughness, large heat input welded joint strength and toughness. In particular, in order to secure a sufficiently hardened structure at the center of the sheet thickness, 0.48%
The above is necessary.

Slab heating temperature: undissolved Nb content is 0.00
5% or more and the remaining temperature is 950 or more. The heating temperature needs to be 950 ° C. or higher in order to achieve a solid solution of the alloy element and secure sufficient hardenability. However, when the slab heating temperature is high and the amount of undissolved Nb is less than 0.005%, the effect of suppressing the formation of giant grains generated during light rolling is not sufficiently obtained, and the base material toughness decreases. The above temperature range is set.

The amount of undissolved Nb depends on the temperature, the amount of C, and the amount of N. The following equation is an example to show this, but it can also be directly obtained by chemical analysis. log ｛(Nb-undissolved Nb) · (C + 12N / 1
4)} = 2.26-6770 / (T H +273.15) T H: Slab heating temperature (℃)

Rolling finishing temperature: T _{F １1} at the surface temperature of the steel sheet
The temperature is in the range of 050 ° C. _{Here, T H = 20Mn + 10Ni +} 15Cr + 100Mo + 1
500Nb + 15Cu + 150V + 500 (Ti-3.
42N) -8√plate thickness (mm) +830 (℃)

In the present invention, the rolling finishing temperature is a factor that affects not only the strength and toughness of the base material but also particularly the acoustic anisotropy, and must be strictly limited according to the content of the alloying element. The above equation empirically determines the effect of alloying elements on the formation of the rolling texture of HT780 steel. If the rolling finish temperature is lower than _TF ° C., the base material toughness decreases and the acoustic anisotropy increases. On the other hand, when the rolling finish temperature is 1
If the temperature exceeds 050 ° C., the microstructure becomes coarse, and the deterioration of the base material toughness becomes significant.

Tempering temperature: lower than the Acl transformation point and higher than 600 ° C. The tempering temperature needs to be 600 ° C. or more to optimize strength, secure toughness and high heat input joint strength.
However, if the tempering temperature exceeds the Ac1 transformation point, an excessive decrease in strength is caused.

In the present invention, preferable results can be obtained by further containing one or more of the alloy elements of Si, Cu, Cr, V and Ti in addition to the above alloy elements. Si: 0.01 to 0.3%. Si is contained to improve the strength of the base material and the strength of the weld joint. 0.01
If it is less than 0.3%, the effect is small, and if it exceeds 0.3%, the weldability and the large heat input weld joint toughness are significantly deteriorated.

Cu: 0.01 to 0.5%. Cu is contained in order to improve the strength of the base material, the toughness, and the strength of the large heat input welded joint. If the content is less than 0.01%, the effect is small, and if the content exceeds 0.5%, hot cracking of the weld metal during large heat input welding tends to occur.

Cr: 0.01 to 1.0%. Cr is contained to improve the base metal strength, toughness, and high heat input weld strength. If it is less than 0.01%, the effect is small, and if it exceeds 1.0%, the weldability is impaired.

V: 0.01 to 0.1%. V is contained to improve the strength of the base material and the strength of the high heat input welded joint. Less than 0.01%, the effect is small, 0.1%
If the content exceeds 3, the base metal toughness and weldability are impaired.

Ti: 0.003 to 0.02%. T
i is contained in order to improve both the base metal toughness and the weld joint toughness through refinement of the microstructure. 0.00
If the content is less than 3%, the effect is small, and if it exceeds 0.02%, the base material toughness and the large heat input welded joint toughness are rather deteriorated.

[0053]

EXAMPLES Various examples obtained by the present invention will be described below. Tables 1-1 and 1-2 shown as FIGS.
Indicates the chemical composition and plate thickness of the test steel. Steel No. in the table. 1
Nos. To 18 are steels of the present invention, Reference numerals 19 to 27 indicate comparative steels whose component compositions fall outside the range of the present invention.

Tables 2-1 and 2-2 shown as FIGS.
It is a summary of the results of investigations on various properties of steel having the chemical compositions shown in Tables 1-1 and 1-2. Mechanical properties (yield strength, tensile strength), toughness (fracture surface transition temperature vTs and non-ductile transition temperature NDTT), acoustic anisotropy of steel produced at the slab heating temperature, rolling finish temperature, and tempering temperature shown in the table , Weldability (maximum hardness), and high heat input weld joint toughness (vTs of the HAZ at the time of high heat input welding).

Further, steel No. 1.1 and 1.2, 2.1 and 2.2, 3.1 and 3.2 are shown in Tables 1-1 and 1-2, respectively.
Steel No. Steel Nos. 1, 2, and 3 were manufactured by changing the heating temperature. 4.1 and 4.2 are steel No. in the table. No. 4 was manufactured by changing the rolling finishing temperature. In addition, at the time of rolling the steel sheet shown in the present embodiment, three or more passes of light rolling with a rolling reduction of 5% or less are included.

Hereinafter, various properties will be described in order. 1) Mechanical properties Yield strength and tensile strength are measured according to JIS4 from the center of the thickness of each steel.
No. specimens were collected and measured by a tensile test. HT780 steel having a yield strength of 685 N / mm ² or more and a tensile strength of 780 N / mm ² or more was accepted. In the steel of the present invention, both the yield strength and the tensile strength passed,
Steel No. of the comparative example. No. 4.2, since the rolling finish temperature is lower than T _F , steel No. Steel No. 19 has a low C content of 0.054%. 21 has a low Ni content of 0.45%, and thus has insufficient strength.

2) Fracture surface transition temperature Fracture surface transition temperature (vTs) is determined by JI from the center of the thickness of each steel.
S4 test piece (JISZ2202) was collected and 2 mmV
It was measured by a notch-shearpy impact test. Pass / fail was determined based on the base material having a vTs of -40 ° C or lower. The steels of the present invention all had good vTs, but steel No. 1.
Steel Nos. 2, 2.2 and 3.2 were heated at a temperature at which the amount of undissolved Nb did not remain. 4.2: Rolling finish temperature is T
_{Since the} temperature is lower than _F , 19: 0.054% of C content
And steel No. Steel No. 21 has a low Ni content of 0.45%. Steel No. 25 has a low Nb content of 0.017%. Sample No. 26 has a low Ceq of 0.47, and thus has poor toughness.

3) Non-ductile transition temperature The non-ductile transition temperature (NDTT) was determined by taking a test piece from the center of the thickness of each steel and performing an NRL drop test. The test piece is a test piece having a thickness of 25 mm, a length of 360 mm, and a width of 90 mm, in which a weld bead is provided on one side, and a cut of 1.5 mm or less is formed. The pass / fail of the NDTT of the base material was determined based on -30 ° C or lower. The steels of the present invention are all NDT
T was good, but steel No. 1.2, 2.
Steel Nos. 2 and 3.2 were heated at a temperature at which the amount of undissolved Nb did not remain. No. 4.2, since the rolling finish temperature is lower than _TF , steel No. Steel No. 19 has a low C content of 0.054%. Steel No. 21 has a low Ni content of 0.45%. No. 25 has a low Nb content of 0.017%.
o. 26 has a low Ceq of 0.47, so each NDTT
Is not good.

4) Acoustic Anisotropy The acoustic anisotropy was evaluated according to the ultrasonic test specified in JIS Z 3060, and the quality was judged based on the sound speed ratio of 1.02 or less. Each of the steels of the present invention had a small acoustic anisotropy and was good. In 4.2, since the rolling finish temperature was lower than _TF , the acoustic anisotropy was poor.

5) Weldability The weldability was evaluated by a maximum hardness test of the welded portion. A JIS No. test piece (JIS Z3101) was sampled from each steel, welded under the following conditions, and the maximum hardness Hv (98N) of the welded portion was determined by a Vickers hardness tester. Maximum hardness is 35
The quality was judged based on 0 or less. Welding method: coated arc welding Heat input: 1.7 kJ / mm Welding atmosphere temperature: 20 ° C. Welding humidity: 60% The steels of the present invention are all excellent in weldability, but steel No. of Comparative Example was used. Steel No. 20 has a high C content of 0.096%.
Steel No. 22 has a high Cr content of 1.08%. Steel No. 23 has a high Mo content of 0.87%. Since No. 27 contained B, it was insufficient in terms of weldability (weld hardenability).

6) Toughness of High Heat Input Welded Joint The characteristics of the large heat input welded joint were evaluated by a Charpy impact test of the bond portion of the large heat input SAW. SAW heat input is 1
The groove was 2 kJ / mm and the groove was X. In the Charpy impact test, a JIS No. 4 test piece (JIS Z 2) in which a notch position was formed at a bond portion (a position at which the ratio of weld metal to HAZ is 1: 1) was taken from the final side 1 / 4t of welding.
202), and the pass / fail was determined based on vTs of 0 ° C. or lower. Each of the steels of the present invention is excellent in large heat input welded joint toughness. 20 has a C content of 0.
096%, steel No. No. 26 has a low Ceq of 047, and therefore steel No. 26 has a low Ceq of 047. Since No. 27 contains B, the large heat input welded joint toughness is not good. In addition, steel No. 24
Since Nb content was as high as 0.051%, poor toughness of the weld metal part of large heat input welding was recognized.

[0062]

As described above, according to the present invention, H is excellent in weldability, acoustic anisotropy, and toughness at the center of the thickness.
T780 steel can be manufactured. This suggests that the present invention can be applied to various steel structures that require high-tensile steel, and at the same time, greatly contributes to the reduction of the construction cost in their construction. Further, the present invention is directed to a steel plate, but it is needless to say that the idea of the present invention can be applied to other steel material manufacturing processes, for example, manufacturing of a shaped steel.

[Brief description of the drawings]

FIG. 1 shows the amount of undissolved Nb before hot rolling and the non-ductile transition temperature (NDT) of the NRL drop test taken from the center of the sheet thickness.
It is a figure which shows the relationship of T).

FIG. 2 is a diagram showing a chemical composition, a plate thickness, and the like of a test steel as Table 1-1.

FIG. 3 is a diagram showing a chemical composition, a plate thickness, and the like of a test steel as Table 1-2.

FIG. 4 summarizes the results of investigations on various properties of steels having the chemical compositions shown in Tables 1-1 and 1-2.
It is a figure shown as -1.

FIG. 5 summarizes the results of investigations on various properties of steels having the chemical compositions shown in Tables 1-1 and 1-2.
It is a figure shown as -2.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Toru Kawanaka 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd.

Claims (3)

[Claims] (1) In terms of% by weight, C: 0.06% or more, Ni:
Steel containing 0.5 to 3.0% and Nb: 0.02 to 0.04% and satisfying Ceq: 0.48% or more has an undissolved Nb content of 0.005% or more. Below temperature 950 ℃
After the above heating, hot rolling is performed, and the steel is directly quenched immediately after the rolling is completed, and then tempered at a temperature not higher than the Ac1 transformation point and not lower than 600 ° C., and has a tensile strength of 780 N / mm ² with excellent toughness. Method of manufacturing grade steel. Here, Ceq = C
+ Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo
/ 4 + V / 14 (%) 2. C: 0.06 to 0.09% by weight,
Mn: 0.5 to 1.5%, P: 0.01% or less, S:
0.01% or less, Ni: 0.5 to 3.0%, Mo: 0.
1-0.8%, Nb: 0.02-0.04%, Al:
0.01-0.08%, N: 0.001-0.008
%, B: 0.0002% or less, and Ceq:
After heating the steel which satisfies 0.48% or more and the balance is iron and inevitable impurities at a temperature of 950 ° C. or higher at a temperature at which the amount of undissolved Nb is 0.005% or higher, hot rolling is performed at _TF to
Terminate in a range of 1050 ° C., after completion of rolling immediately direct quenching, then characterized by tempering at Ac1 transformation point 600 ° C. or higher, toughness excellent tensile strength 780N / mm ² class steel Manufacturing method. Here, T _F = 2
0Mn + 10Ni + 15Cr + 100Mo + 1500N
b + 15Cu + 150V + 500 (Ti-3.42N)
-8 mm thickness (mm) + 830 (° C) 3. The steel according to claim 1, further comprising:
O1 to 0.5%, Cu: 0.01 to 0.5%, Cr:
0.01 to 1.0%, V: 0.01 to 0.1%, Ti:
The tensile strength 78 having excellent toughness according to claim 2, characterized by containing at least one of 0.005 to 0.02%.
0N / mm Method for producing grade ² steel. [0001]