JPH07233438A - High tensile strength steel and its production - Google Patents

Abstract

PURPOSE:To produce a high tensile strength steel excellent in SR cracking resistance or the like by preparing a high tensile strength steel in which the componental compsn. is specified and the weld cracking sensitivity index and carbon equivalent value are prescribed. CONSTITUTION:A high tensile strength steel having a compsn. contg., by weight, 0.075 to 0.094% C, 0.01 to 0.4% Si, 0.5 to 1.5% Mn, <=0.01% P, <=0.005% S, 0.5 to 2% Ni, 0.1 to 0.9% Cr, 0.2 to 1% Mo, 0.003 to 0.03% Nb, 0.01 to 0.08% Al and 0.0005 to 0.008% N, and in which PcM value defined by PcM=C+Si/30+ Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B (wt.%) is regulated to <=0.24 and Ceq value defined by Ceq=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14 (wt.%) is regulated to >=0.45, and the balance iron with inevitable impurities and substantially contg. no B is prepd. Thus, the 780N/mm<2> class high tensile strength steel excellent in weldability, acoustic anisotropy and SR cracking properties can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a 780 N / mm ² class high-strength steel excellent in weldability, acoustic anisotropy and SR resistance and a method for producing the same.

[0002]

^2. Description of the Related Art 780 N / mm ² high-strength steel (hereinafter referred to as H
Various steel grades have been proposed so far, but most of them are produced by adding B to improve hardenability and performing quenching and tempering treatment. These steels have excellent base material strength and toughness, but since they contain B, the hardenability of the welded portion is high (the weld temperature is highly sensitive to cracking), so it is necessary to take measures to prevent weld cracking during welding. . Generally, as a measure for preventing weld cracks, preheating the work piece to 100 ° C or higher is performed, but the working environment heated to a high temperature is not preferable from the viewpoint of safety and hygiene, and under such environment, construction efficiency is high. Is significantly reduced.

In order to solve the problem of the B-containing steel HT780, according to Japanese Patent Publication No. Sho 49-42568 and Japanese Patent Laid-Open No. 4-314825, a B-free high-strength steel containing no B is put to practical use. Has been done.

The former is C: 0.07 to 0.09%, S
i: 0.46 to 0.50%, Mn: 1.30 to 1.38
%, Cu: 0.10 to 0.23%, Ni: 0.43 to
0.92%, Cr: 0.95 to 1.20%, Mo: 0.
48 to 0.59%, Ti: 0.008 to 0.015%,
Disclosed is a high-strength steel containing Al: 0.08 to 0.11% (all by weight) and having a high toughness weld heat affected zone.

The latter is C: 0.038 to 0.053%,
Cu: 1.02 to 1.72%, Nb: 0.013 to 0.
032%, V: 0.021 to 0.057%, Ti: 0.
011 to 0.019% and 59.3C + Cu ≦
4.2 In order to utilize precipitation strengthening by containing C and Cu so as to satisfy the requirement, performing direct quenching after hot rolling, 55
H with excellent weldability that is tempered at 0-600 ℃
A method of manufacturing T780 is disclosed.

On the other hand, in welded structures such as bridges,
From the viewpoint of ensuring safety, it is necessary to strictly detect welding defects by ultrasonic flaw detection using oblique angles. In ultrasonic flaw detection, if there is a difference in sound velocity between the final rolling direction (L direction) of the steel sheet and the direction (C direction) orthogonal to the final rolling direction, it becomes difficult to accurately detect the defect. In this case, since there is a technical limit to the evaluation judgment by distinguishing the inspection in the L direction from the inspection in the C direction, it is necessary to repair all the welded spots where a suspected defect echo is found. However, repairing more defects than necessary will result in enormous construction costs.

In order to solve such a problem relating to acoustic anisotropy, Japanese Patent Laid-Open No. 63-235431 discloses a manufacturing method for obtaining a steel sheet having a small acoustic anisotropy. This has a (C + Mn / 6) value of 0.36% or less and a carbon equivalent value Ceq (= C + Mn / 6 + (Cr + Mo + V) /
Steel having a composition of 5+ (Ni + Cu) / 15) of 0.40% or less is heated to 1000 ° C or more and 1200 ° C or less, the total rolling reduction is 50% or more, and the rolling finishing temperature is 850 ° C or more in the recrystallization region of austenite. At a cooling rate of 5 ° C. or more and less than 15 ° C. per second from a temperature range of 50 ° C. below the Ar ₃ transformation point.
A manufacturing method for obtaining a steel sheet having a small acoustic anisotropy by cooling to a temperature range of 00 ° C to 680 ° C is described.

Further, in various tanks and pressure vessels, stress relief annealing (hereinafter referred to as SR) treatment is performed in order to improve performance and reduce welding residual stress in welded joints of thick steel plates. However, since SR treatment is generally performed in a temperature range slightly lower than the tempering temperature range, the base metal and the welded portion are embrittled, and the so-called SR treatment is performed.
May cause cracks. In order to prevent the occurrence of SR cracks, the base material and the toe of the weld are usually grinded to a smooth shape without stress concentration.

In order to improve such SR cracking characteristics, JP-A-51-17112 and JP-A-62-19.
No. 2564, JP-A-62-37342, and JP-A-5-51696 describe steels having excellent SR cracking characteristics in which the amount of S that increases SR cracking sensitivity is reduced.

Further, JP-A-55-24966, JP-A-57-92125, and JP-A-3-150335.
In Japanese Patent Laid-Open No. 5-13551 and Japanese Patent Laid-Open No. 5-1351, Ca and T
It describes a steel that suppresses the amount of solid solution S by adding a sulfide-forming element such as i, and has excellent SR cracking properties, and a manufacturing method.

[0011]

However, the B-free high-strength steel described in JP-B-49-42568 is
Since Si is contained in 0.46% or more, the weldability and weld joint toughness are insufficient. The B-free high-strength steel described in JP-A-4-314825 has excellent weldability, but in order to secure the strength of the base metal by utilizing precipitation strengthening, the tempering treatment is performed at a low temperature of 550 to 600 ° C. Implementation in the region becomes essential. Therefore, the strength of the welded joint becomes insufficient. As a countermeasure against this, rolling in a low temperature region may be considered in order to secure the strength of the base material. However, a steel sheet hot-rolled in a low temperature region causes an increase in acoustic anisotropy in which the propagation speed of ultrasonic waves is greatly different in the L direction and the C direction.

The steel sheet having a small acoustic anisotropy described in JP-A-63-235431 has a carbon equivalent value Ceq of 0.40.
% Or less, the base metal strength is insufficient, and even if the base metal strength reaches about HT780, Ceq is 0.40% or less and the weld joint strength is insufficient.

Further, the above-mentioned JP-A-51-17112, JP-A-55-24966 and JP-A-62-37.
342, JP-A-57-92125, JP-A-62-192564, JP-A-3-150335, JP-A-51351, and JP-A-5-5169.
All of the steels described in Japanese Patent No. 6 are B-containing steels, and thus have poor weldability.

Despite the high demands of consumers for HT780 as described above, a 780 N / mm ² class high strength steel excellent in all of weldability, acoustic anisotropy and SR cracking properties has not yet been realized. .

[0015]

SUMMARY OF THE INVENTION The present invention solves these problems, including weldability, acoustic anisotropy and SR.
It is an object of the present invention to provide a 780 N / mm ² class high strength steel having excellent cracking properties and a method for producing the same.

The high-strength steel according to the present invention has a C0.
075-0.094%, Si0.01-0.4%, Mn
0.5-1.5%, P0.01% or less, S0.005%
Hereinafter, Ni 0.5 to 2%, Cr 0.1 to 0.9%, Mo
0.2-1%, Nb0.003-0.03%, Al0.
01 to 0.08%, N 0.0005 to 0.008% included, P _CM = C + Si / 30 + Mn / 20 + Cu / 20 +
Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
The P _CM value defined by (wt%) is 0.24 or less, and Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr /
The Ceq value defined by 5 + Mo / 4 + V / 14 (wt%) is 0.45 or more, and the balance is substantially B-free weldability, acoustic anisotropy and SR resistance, which consist of iron and inevitable impurities. It is characterized by excellent characteristics.

Further, Cu 0.01-1.5%, V0.
It is desirable to contain at least one of 005 to 0.1% and Ti 0.003 to 0.02%. Furthermore, C
a 0.0005-0.01%, REM 0.0005
It is desirable to contain at least one of 0.01%.

The method for producing high-strength steel according to the present invention uses C0.075 to 0.094% by weight and Si 0.01 to 0.
4%, Mn 0.50 to 1.5%, P 0.01% or less, S
0.005% or less, Ni 0.5-2%, Cr 0.1-
0.9%, Mo 0.2 to 1%, Nb 0.003 to 0.0
3%, Al 0.01 to 0.08%, N 0.0005 to 5%
Including 0.008%, P _CM = C + Si / 30 + Mn / 2
0 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15
The P _CM value defined by + V / 10 + 5B (% by weight) is 0.
24 or less and Ceq = C + Mn / 6 + Si / 24 + N
A steel having a Ceq value defined by i / 40 + Cr / 5 + Mo / 4 + V / 14 (% by weight) of 0.45 or more and containing substantially no B is heated to a temperature range of 1000 to 1250 ° C. and 1050 ° C. 20% cumulative reduction in the following temperature range
Above, and T = 85Mn + 40Ni + 95Cr + 1
20Mo + 3100Nb + 40Cu + 500V + 100
0 (Ti-3.42N) + 580, after rolling so that the rolling finishing temperature is in the range of T ° C to 1050 ° C, it is directly quenched from the Ar ₃ transformation point or higher, and then Ac ₁
It is characterized in that it is reheated to a temperature range below the transformation point and tempered to obtain excellent weldability, acoustic anisotropy and SR resistance.

[0019]

The reason for limiting the constituent elements of the present invention is as follows. 1) C: 0.075 to 0.094% C is added to improve both the base metal strength and the weld joint strength. If it is less than 0.075%, the strength will be insufficient,
If it exceeds 0.094%, the weldability is significantly deteriorated. 2) Si: 0.01 to 0.4% Si is added to improve both the base metal strength and the weld joint strength. If it is less than 0.01%, the strength will be insufficient,
If it exceeds 0.4%, the weldability and the weld joint toughness are significantly deteriorated. 3) Mn: 0.5 to 1.5% Mn is added to improve both the base metal strength and the weld joint strength. If it is less than 0.5%, the strength will be insufficient,
If it exceeds 1.5%, the weldability deteriorates. 4) P: 0.01% or less If P, which is an impurity element, exceeds 0.01%, the welded joint performance is significantly deteriorated, and SR susceptibility is also increased, so it is desirable to reduce it as much as possible. 5) S: 0.005% or less If S, which is an impurity element, exceeds 0.005%, SR cracking sensitivity becomes high and the welded joint performance is significantly deteriorated. Therefore, it is desirable to reduce S as much as possible. 6) Ni: 0.5 to 2% Ni is added to improve the base material strength and toughness, and the weld joint strength. If it is less than 0.5%, the strength and toughness are insufficient, and if it exceeds 2%, the economy is impaired. 7) Cr: 0.1 to 0.9% Cr is added to improve both the base metal strength and the weld joint strength. If it is less than 0.1%, the strength will be insufficient,
Addition of more than 0.9% impairs weldability. 8) Mo: 0.2 to 1% Mo is added to improve both the base metal strength and the weld joint strength. If it is less than 0.2%, the strength is insufficient and 1
If it is added in excess of%, the weldability is impaired. 9) Nb: 0.003 to 0.03% Nb is added to improve both the base metal strength and the weld joint strength. If it is less than 0.003%, the strength becomes insufficient, and if it exceeds 0.03%, the toughness of the weld metal is impaired. 10) Al: 0.01 to 0.08% Al is added for deoxidation and ensuring the toughness of the base material by refining the microstructure. If it is less than 0.01%, it becomes insufficient to secure the base material toughness due to the refinement of the structure, and 0.08%
If the addition exceeds the above range, the toughness of the base material is deteriorated. 11) N: 0.0005 to 0.008% N reacts with Al to form a precipitate, thereby making the microstructure fine and improving the base material toughness. If it is less than 0.0005%, the amount of precipitates is insufficient, so that
Addition of more than 008% will rather deteriorate the toughness of the base metal and the toughness of the welded joint.

In the present invention, preferable results can be obtained even if one or more of the following elements such as Cu, V and Ti are contained in addition to the above alloy elements. 12) Cu: 0.01 to 1.5% Cu is added to improve both the base metal strength and the weld joint strength. If it is less than 0.01%, the strength will be insufficient,
Addition of more than 1.50% impairs weldability. 13)
V: 0.005-0.1% V is added to improve both the base metal strength and the weld joint strength. If it is less than 0.005%, the strength will be insufficient,
Addition of more than 0.100% impairs base material toughness and weldability. 14) Ti: 0.003 to 0.02% Ti is added to improve both the base metal toughness and the weld joint toughness through the refinement of the microstructure. Furthermore T
i combines with S to form a stable sulfide, thereby reducing the amount of solid solution S in the crystal grain boundaries, and thus improves the SR resistance property. If it is less than 0.003%, the effect of improving the toughness due to the refinement of the structure is insufficient, and the effect of improving the SR cracking property cannot be obtained. On the other hand, addition of more than 0.020% impairs both the base metal toughness and the weld joint toughness. 15) Ca: 0.0005 to 0.01% Ca refines the crystal grains and forms sulfides to reduce the amount of solid solution S in the crystal grain boundaries.
Further enhances the radius characteristics. To obtain this effect effectively, it is necessary to add an amount of 0.0005% or more.
Addition of 0.0100% or more is not preferable because the amount of non-metallic inclusions increases and the ductility decreases. 16) REM: 0.0005 to 0.01% REM forms a sulfide as REM (O, S) in steel, thereby reducing the amount of solid solution S in the crystal grain boundaries and enhancing the SR resistance property. It is necessary to add 0.0005% or more, but addition of 0.0100% or more is not preferable because the cleanliness of the steel decreases and the toughness deteriorates. 17) P _CM (welding crack susceptibility index): 0.24 or less P _CM (= C + Si / 30 + Mn / 2) which is an index of weldability
0 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15
+ V / 10 + 5B (%)) is suppressed to 0.24% or less in order to reduce the preheating temperature during welding. 18) Ceq (carbon equivalent value): 0.45 or more Ceq, which is an index of hardenability, is set to 0.45% or more in order to secure both weld joint strength and weld joint toughness. 19) Slab heating temperature during direct quenching: 1000-12
50 ° C. In order to form a solid solution of alloying elements to ensure sufficient hardenability and to achieve predetermined conditions during rolling, the heating temperature must be 1000 ° C. or higher. However, the heating temperature exceeding 1250 ° C impairs the toughness of the base material due to the coarsening of the microstructure, so the upper limit is made 1250 ° C. 20) Rolling finishing temperature during direct quenching: T to 1050 ° C., where T = 85Mn + 40Ni + 95Cr + 120Mo
+ 3100Nb + 40Cu + 500V + 1000 (Ti
-3.42 N) +580 In the present invention, the rolling finishing temperature is a factor that greatly affects the base material strength, the base material toughness, and the acoustic anisotropy, and needs to be strictly limited according to the amount of the added element. When the rolling finishing temperature becomes lower than T ° C. calculated by the above formula, the strength of the base material and the toughness of the base material suddenly decrease, and the acoustic anisotropy increases. Therefore, the lower limit temperature is limited to T ° C. On the other hand, when the rolling finishing temperature exceeds 1050, the microstructure becomes coarse and the toughness of the base material deteriorates significantly. Therefore, the upper limit temperature is limited to 1050 ° C. 21) Tempering temperature: Ac ₁ transformation point or lower (600 to 700)
The tempering temperature must be 600 ° C or higher to secure the strength of the welded joint. However, if tempering is carried out at a temperature higher than 700 ° C., the strength of the base material is rapidly lowered. 22) Impurity element B: 0.0002% or less Since the impurity element B significantly deteriorates the weldability even in a small amount, its content in the steel of the present invention is 0.0
It is desirable to suppress it to 002% or less.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Various embodiments of the present invention will be described below with reference to the accompanying drawings and Tables 1 and 2. Steel types 1 to 20 in Table 1 correspond to steel types of Examples that fall within the scope of the present invention in terms of component composition, and steel types 21 to 30 correspond to steel types of Comparative Examples that fall outside the scope of the present invention in terms of component composition. T (° C.) in the rightmost column of Table 1 is T = 85Mn + 40Ni + 95Cr + 1
20Mo + 3100Nb + 40Cu + 500V + 100
The value obtained from the formula represented by 0 (Ti-3.42N) +580 is shown. P _{CM in the} next column is P _CM = C + Si /
30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 2
The values obtained from the weld crack sensitivity index formula represented by 0 + Mo / 15 + V / 10 + 5B are shown. Ceq in the next column
Is Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + C
The values obtained from the carbon equivalent formula represented by r / 5 + Mo / 4 + V / 14 (wt%) are shown.

Steel types 5, 6, 8, 9, 11, among the examples
13, 14, 16, 18, 19, 20 are Cu-containing steels, steel types 5, 6, 7, 10, 11, 13, 15, 16 are V-containing steels, and steel types 12, 13, 14, 15 are Ti-containing steels, steel types 4, 5, 15 and 20 are Ca-containing steels, and steel types 5, 7, and 20 are REM-containing steels. The additive components of these steel types are Cu 0.01 to 1.50, respectively.
%, V 0.005 to 0.100%, Ti 0.003 to
0.020%, Ca 0.0005 to 0.0100%, R
It is in the range of EM 0.0005 to 0.0100%.

Steel types 1 to 3 among the examples are C0.075.
.About.0.094%, Si0.01 to 0.40%, Mn0.
50-1.50%, P0.01% or less, S0.005%
Hereinafter, Ni 0.50 to 2.00%, Cr 0.10 to 0.
90%, Mo 0.20 to 1.00%, Nb 0.003 to
0.030%, Al0.01-0.08%, N0.00
Including 05 to 0.0080%, P _CM = C + Si / 30 +
Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + M
P _CM value defined by o / 15 + V / 10 + 5B (%) is 0.24% or less, and Ceq = C + Mn / 6 + Si / 2
It is satisfied that the Ceq value defined by 4 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 is 0.45% or more.

Steel grades 1, 5 to 13, 16, 18, 18, 19, 2
At 0, the carbon equivalent value Ceq is 0.51, 0.49, 0.4.
8, 0.50, 0.51, 0.45, 0.51, 0.4
8, 0.51, 0.50, 0.51, 0.49, 0.4
6 and 0.47 respectively. Especially for steel type 1, C
The carbon equivalent value Ceq is about 0.51 because the addition amount of other elements is suppressed even though the content is close to the upper limit value of 0.094%.

Steel type 2 has a C content of 0.075%,
The contents of Cr and Mo are 0.86% and 0.64, respectively.
%, The carbon equivalent value Ceq is 0.55.
Steel type 3 had a C content of 0.076% and Mn, Ni and Mo contents of 1.37%, 1.80%,
Since it is 0.94%, the carbon equivalent value Ceq is 0.61.
And the highest among the examples.

The steel grades 1 to 20 of the examples all have a weld crack sensitivity index P _CM in the range of 0.20 to 0.24,
The temperature T is in the range of 846 to 936 ° C. Among the comparative examples, the steel types 21 and 29 had a C content of 0.09 each.
8% and 0.110% are too high, and steel type 22 has a C content of 0.071%, which is too low.

Among the comparative examples, the steel type 23 had a too high Nb content of 0.034%, and the steel type 24 had a Nb content of 0.034%.
It is too low at 02%. Steel types 25, 26, 28 among comparative examples
Has a B content of 0.0009%, 0.0010%, 0.
0004% and B exceeding the specified value of 0.0002%, respectively
It is contained steel.

Among the comparative examples, the steel type 27 has a carbon equivalent value Ceq.
Is 0.43, which is below the specified value of 0.45. Among the comparative examples, the steel type 30 has an S content of 0.007%, which is too high.

Next, the effect of the embodiment will be described with reference to Table 2 and FIGS. Table 2 is a summary of the results obtained by examining various properties of each steel type having the composition shown in Table 1. Specifically, when manufactured under various conditions such as plate thickness, slab heating temperature, rolling finishing temperature, tempering temperature, etc., acoustic anisotropy of each steel type, fracture surface transition temperature (toughness), mechanical strength (tensile strength) The influence of strength, yield strength), weldability (maximum hardness), SR breaking property, etc. was investigated. Hereinafter, various properties will be described in order. 1) Acoustic Anisotropy The acoustic anisotropy was evaluated according to the ultrasonic test prescribed in JIS Z3060, and the one having a sound velocity ratio of 1.02 or less was judged to be acceptable. Steel type 1.1 (Example) and steel type 1.2 (Comparative example) correspond to steels having the same composition as Steel type 1 above, but manufactured at different rolling finish temperatures. With steel type 1.1, the rolling finishing temperature is 970 ° C, which is within the range of T to 1050 ° C.
Although the acoustic anisotropy is good, the steel type 1.2 of the comparative example has a rolling finishing temperature of 830 ° C., which is lower than T ° C. (853 ° C.), and the acoustic anisotropy is increased. Normal,
It is considered good if the acoustic anisotropy is 1.02 or less.
It can be said that 1.1 is excellent in acoustic anisotropy, and steel type 1.2 is inferior in acoustic anisotropy.

FIG. 1 is a characteristic diagram showing the results of investigating the correlation between rolling finish temperatures (° C.) on the horizontal axis and acoustic anisotropy on the vertical axis for steel materials having composition of steel type 1. is there. As is clear from the figure, the acoustic anisotropy changes greatly when the rolling finishing temperature is about 853 ° C., and when the hot rolling is finished below this temperature, the acoustic anisotropy greatly exceeds 1.02. It was 2) Fracture surface transition temperature The fracture surface transition temperature vTs is the JIS No. 4 test piece (JISZ22
02) using 2mmV notch Charpy impact test (load 1
0 kgf). Fracture transition temperature vTs of base material
Those having a temperature of -40 ° C or less were judged to be acceptable. The steel types 27.1 and 27.2 of the comparative examples are steel materials having the same composition as the above-mentioned steel type 27, respectively, with tempering temperatures of 580 ° C. and 610 ° C.
It corresponds to the one manufactured by changing. The vTs of steel type 27.1 was −21 ° C. and the vTs of steel type 27.1 was −27 ° C. Further, in the steel type 1.2 of the comparative example, the result that the fracture surface transition temperature was −35 ° C. was obtained. In addition, in the comparative example, in steel type 23, poor toughness of the weld metal part was recognized, and steel type 27.1
With steel type 27.2, poor toughness of the welded joint was observed. On the other hand, all of the steel types 1.1 and 2 to 20 of the examples obtained the results exceeding the predetermined passing level.

In FIG. 2, the horizontal axis represents the rolling finish temperature (° C.) and the vertical axis represents the fracture surface transition temperature vTs (° C.).
It is a characteristic diagram which shows the result of having investigated the correlation of both with respect to the steel material of composition. As is clear from the figure, the fracture surface transition temperature greatly changes around a rolling finish temperature of about 853 ° C., and when hot rolling is completed below this temperature, the fracture surface transition temperature vTs greatly exceeds −40 ° C. Is significantly degraded. 3) Mechanical strength The tensile strength and the yield strength of the base material of each steel type were measured using JIS No. 4 test piece or No. 4 test piece (JIS Z2201). A product having a yield strength of 685 N / mm ² or more and a tensile strength of 780 N / mm ² or more was judged to be acceptable.
The steel grades 1.1 and 2 to 20 of the examples all obtained the results satisfying the requirements of yield strength and tensile strength. On the other hand, the yield strengths of the steel types 1.2, 22, and 24 of the comparative examples are insufficient.

In FIG. 2, the horizontal axis represents the rolling finish temperature (° C.) and the vertical axis represents the tensile strength (N / mm ² ). It is a characteristic diagram shown. As is clear from the figure, the rolling finishing temperature is about 85
The tensile strength greatly changes around 3 ° C., and when the hot rolling is finished at this temperature or less, the tensile strength greatly falls below 780 N / mm ² , and the strength is remarkably reduced. 4) Weldability Weldability was evaluated by the maximum hardness of the weld. Welded plate materials of each steel type under the following conditions and JIS No. 1 test piece (JIS
Z3101) was sampled and the maximum hardness Hv of the welded portion was determined by a Vickers hardness meter. A sample having a maximum hardness Hv value of 350 or less was judged to be acceptable and used as an index for evaluating weldability.

Welding method: Coated arc welding Welding current: 170 A Welding voltage: 25 V Welding speed: 150 mm / min Welding atmosphere: 20 ° C. Test piece temperature: 20 ° C. All the steel grades 21, 25, 26, 28 and 29 of the comparative examples. The maximum hardness Hv value exceeded 350. Although the steel types 23, 24, 27.1, and 27.2 of the comparative examples had no problem in terms of the maximum hardness, but poor toughness was observed in the weld metal part and HAZ, so it can be said that the weldability is good. Absent. on the other hand,
The steel types 1.1 and 2 to 20 of the examples have the highest hardness Hv.
A value of 350 or less was obtained, and it was found that the weldability was also excellent. 5) SR cracking characteristics SR cracking characteristics were evaluated by an oblique y-type SR cracking test based on JIS Z3158. Those having an SR cross-section crack ratio of 75% or less were judged to be acceptable. The SR holding temperature was about 595 ° C and the SR holding time was 4.5 to 7.4 hours.

In each of the steel types 1.1 and 2 to 20 of the examples, the SR cross-section cracking ratio was less than 75%. On the other hand, in the steel types 21, 29 and 39 of the comparative examples, the SR cross-section crack ratios are 87%, 100% and 94%, respectively.
Highly susceptible to cracking.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

According to the present invention, it is possible to provide a 780 N / mm ² class high strength steel excellent in weldability, acoustic anisotropy and SR cracking properties, and a method for producing the same.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing a correlation between acoustic anisotropy and rolling finish temperature.

FIG. 2 is a characteristic diagram showing a correlation between a fracture surface transition temperature and a rolling finish temperature.

FIG. 3 is a characteristic diagram showing a correlation between tensile strength and rolling finish temperature.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Toru Kawanaka Toru Kawanaka 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan KK (72) Koshiro Tsukada, Inventor 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside the Steel Pipe Co., Ltd. (72) Inventor Tatsuya Shimoda 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan Steel Pipe Ltd. (72) Inventor Saburo Tani, 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Within the corporation

Claims (8)

[Claims] 1. C0.075-0.094% by weight,
Si 0.01-0.4%, Mn 0.5-1.5%, P
0.01% or less, S0.005% or less, Ni 0.5 to 2
%, Cr 0.1 to 0.9%, Mo 0.2 to 1%, Nb
0.003-0.03%, Al0.01-0.08%,
N 0.0005 to 0.008% included, P _CM = C + Si
/ 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr /
P _CM value defined by 20 + Mo / 15 + V / 10 + 5B (% by weight) is 0.24 or less, and Ceq = C + Mn / 6
+ Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V /
Ceq value defined by 14 (wt%) is 0.45 or more, and the balance is essentially B and consists of iron and inevitable impurities.
A high-strength steel characterized by having excellent weldability, acoustic anisotropy, and resistance to SR cracking, which does not contain iron. 2. C0.075 to 0.094% by weight,
Si 0.01-0.4%, Mn 0.50-1.5%, P
0.01% or less, S0.005% or less, Ni 0.5 to 2
%, Cr 0.1 to 0.9%, Mo 0.2 to 1%, Nb
0.003-0.03%, Al0.01-0.08%,
N 0.0005 to 0.008% included, P _CM = C + Si
/ 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr /
P _CM value defined by 20 + Mo / 15 + V / 10 + 5B (% by weight) is 0.24 or less, and Ceq = C + Mn / 6
+ Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V /
A steel having a Ceq value defined by 14 (% by weight) of 0.45 or more and containing substantially no B is heated to a temperature range of 1000 to 1250 ° C., and 1050 ° C.
The cumulative rolling reduction in the temperature range below is 20% or more, and T
= 85Mn + 40Ni + 95Cr + 120Mo + 310
0Nb + 40Cu + 500V + 1000 (Ti-3.4
2N) +580, the rolling finishing temperature is T ° C to 10 ° C.
After rolling so as to be in the range of 50 ° C., it is directly quenched from the Ar ₃ transformation point or higher, then reheated to a temperature range of the Ac ₁ transformation point or lower and tempered to obtain weldability, acoustic anisotropy and A method for producing high-strength steel, which is characterized by excellent SR resistance. 3. C0.075 to 0.094% by weight,
Si 0.01-0.4%, Mn 0.5-1.5%, P
0.01% or less, S0.005% or less, Ni 0.5 to 2
%, Cr 0.1 to 0.9%, Mo 0.2 to 1%, Nb
0.003-0.03%, Al0.01-0.08%,
N 0.0005 to 0.008%, and Ca 0.
0005-0.01%, REM 0.0005-0.01
% Of at least one, P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni
/ 60 + Cr / 20 + Mo / 15 + V / 10 + 5B P CM value defined by (% by weight) is 0.24 or less, and Ceq
= C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 +
The Ceq value defined by Mo / 4 + V / 14 (wt%) is 0.45 or more, and the balance is substantially B-free weldability, acoustic anisotropy and SR resistance, which consist of iron and inevitable impurities. A high-strength steel characterized by excellent properties. 4. C0.075-0.094% by weight,
Si 0.01-0.4%, Mn 0.5-1.5%, P
0.01% or less, S0.005% or less, Ni 0.5 to 2
%, Cr 0.1 to 0.9%, Mo 0.2 to 1%, Nb
0.003-0.03%, Al0.01-0.08%,
N 0.0005 to 0.008%, and Ca 0.
0005-0.01%, REM 0.0005-0.01
% Of at least one, P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni
/ 60 + Cr / 20 + Mo / 15 + V / 10 + 5B P CM value defined by (% by weight) is 0.24 or less, and Ceq
= C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 +
Steel having a Ceq value defined by Mo / 4 + V / 14 (wt%) of 0.45 or more and containing substantially no B is heated to a temperature range of 1000 to 1250 ° C., and 1050 ° C.
The cumulative rolling reduction in the temperature range below is 20% or more, and T
= 85Mn + 40Ni + 95Cr + 120Mo + 310
0Nb + 40Cu + 500V + 1000 (Ti-3.4
2N) +580, the rolling finishing temperature is T ° C to 10 ° C.
After rolling so as to be in the range of 50 ° C., it is directly quenched from the Ar ₃ transformation point or higher, then reheated to a temperature range of the Ac ₁ transformation point or lower and tempered to obtain weldability, acoustic anisotropy and A method for producing high-strength steel, which is characterized by excellent SR resistance. 5. C0.075-0.094% by weight,
Si 0.01-0.4%, Mn 0.5-1.5%, P
0.01% or less, S0.005% or less, Ni 0.5 to 2
%, Cr 0.1 to 0.9%, Mo 0.2 to 1%, Nb
0.003-0.03%, Al0.01-0.08%,
N0.0005 to 0.008%, and Cu0.
01 to 1.50%, V 0.005 to 0.1%, Ti 0.
Containing at least one of 003 to 0.02%, P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni
/ 60 + Cr / 20 + Mo / 15 + V / 10 + 5B P CM value defined by (% by weight) is 0.24 or less, and Ceq
= C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 +
The Ceq value defined by Mo / 4 + V / 14 (wt%) is 0.45 or more, and the balance is substantially B-free weldability, acoustic anisotropy and SR resistance, which consist of iron and inevitable impurities. A high-strength steel characterized by excellent properties. 6. C0.075 to 0.094% by weight,
Si 0.01-0.4%, Mn 0.5-1.5%, P
0.01% or less, S0.005% or less, Ni 0.5 to 2
%, Cr 0.1 to 0.9%, Mo 0.2 to 1%, Nb
0.003-0.03%, Al0.01-0.08%,
N0.0005 to 0.008%, and Cu0.
01-1.5%, V0.005-0.1%, Ti0.0
03-0.02% of at least one, P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni
/ 60 + Cr / 20 + Mo / 15 + V / 10 + 5B P CM value defined by (% by weight) is 0.24 or less, and Ceq
= C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 +
Steel having a Ceq value defined by Mo / 4 + V / 14 (wt%) of 0.45 or more and containing substantially no B is heated to a temperature range of 1000 to 1250 ° C., and 1050 ° C.
The cumulative rolling reduction in the temperature range below is 20% or more, and T
= 85Mn + 40Ni + 95Cr + 120Mo + 310
0Nb + 40Cu + 500V + 1000 (Ti-3.4
2N) +580, the rolling finishing temperature is T ° C to 10 ° C.
After rolling so as to be in the range of 50 ° C., it is directly quenched from the Ar ₃ transformation point or higher, then reheated to a temperature range of the Ac ₁ transformation point or lower and tempered to obtain weldability, acoustic anisotropy and A method for producing high-strength steel, which is characterized by excellent SR resistance. 7. A weight percentage of C 0.075 to 0.094%,
Si 0.01-0.4%, Mn 0.5-1.5%, P
0.01% or less, S0.005% or less, Ni 0.5 to 2
%, Cr 0.1 to 0.9%, Mo 0.2 to 1%, Nb
0.003-0.03%, Al0.01-0.08%,
N0.0005 to 0.008%, and Cu0.
01-1.5%, V0.005-0.1%, Ti0.0
03-0.02% of at least one, further Ca
0.0005-0.01%, REM 0.0005-0.
01% of at least one of P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni
/ 60 + Cr / 20 + Mo / 15 + V / 10 + 5B P CM value defined by (% by weight) is 0.24 or less, and Ceq
= C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 +
The Ceq value defined by Mo / 4 + V / 14 (wt%) is 0.45 or more, and the balance is substantially B-free weldability, acoustic anisotropy and SR resistance, which consist of iron and inevitable impurities. A high-strength steel characterized by excellent properties. 8. C0.075-0.094% by weight,
Si 0.01-0.4%, Mn 0.5-1.5%, P
0.01% or less, S0.005% or less, Ni 0.5 to 2
%, Cr 0.1 to 0.9%, Mo 0.2 to 1%, Nb
0.003-0.03%, Al0.01-0.08%,
N0.0005 to 0.008%, and Cu0.
01-1.5%, V0.005-0.1%, Ti0.0
03-0.02% of at least one, further Ca
0.0005-0.01%, REM 0.0005-0.
01% of at least one of P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni
/ 60 + Cr / 20 + Mo / 15 + V / 10 + 5B P CM value defined by (% by weight) is 0.24 or less, and Ceq
= C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 +
Steel having a Ceq value defined by Mo / 4 + V / 14 (wt%) of 0.45 or more and containing substantially no B is heated to a temperature range of 1000 to 1250 ° C., and 1050 ° C.
The cumulative rolling reduction in the temperature range below is 20% or more, and T
= 85Mn + 40Ni + 95Cr + 120Mo + 310
0Nb + 40Cu + 500V + 1000 (Ti-3.4
2N) +580, the rolling finishing temperature is T ° C to 10 ° C.
After rolling so as to be in the range of 50 ° C., it is directly quenched from the Ar ₃ transformation point or higher, then reheated to a temperature range of the Ac ₁ transformation point or lower and tempered to obtain weldability, acoustic anisotropy and A method for producing high-strength steel, which is characterized by excellent SR resistance.