JPH08209237A - Production of high tensile strength steel excellent in weldability and acoustic anisotropy - Google Patents

Abstract

PURPOSE: To provide a method for producing a heat treated 60kgf/mm<2> class steel in which the prevention of low temp. cracking or the like at the time of the execution of welding and the acoustic anisotropy are simultaneously improved. CONSTITUTION: At the time of subjecting a steel having a compsn. contg., by weight, 0.04 to 0.1% C, 0.01 to 0.4% Si, 0.5 to 1.6% Mn, <=0.015% P, <=0.01% S, 0.01 to 0.3% Mo, 0.005 to 0.05% Nb, <0.005% Ti, <0.0003% B, and in which Pcm=C+Si/30+Mn/20+Cu/20+Ni/30+Cr/20+Mo/15+V/10+5B is regulated to <=0.2 to hot rolling, as for a steel in which, the solid solution Nb content obtd. by log(Nb)×(C+12/14N)=2.26-6770/(Tsl+273.15) deg.C (Tsl: heating temp.) is defined as the effective Nb content, Ceq=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14, X (constituted of the effective Nb and V content)=625 (the effective Nb)+250V+210Ceq satisfies x>=40+t ((t) denotes plate thickness mm), after heating to 1000-1250 deg.C, hot rolling is finished at the temp. of Tft=-530C+100 Mn+15Mo+2250Nb+100V+770( deg.C) or above, and it is directly hardened from the Ar3 transformation point or above and is tempered at the Ac1 transformation point or below.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a tempered 60 kgf / mm ² grade steel excellent in weldability and acoustic anisotropy.

[0002]

2. Description of the Related Art When high-strength steel is used for welded structures such as bridges and tanks, preheating during welding is required from the viewpoint of preventing brittle fracture, resulting in a decrease in work efficiency. In order to solve this problem, JP-A-3-219012 and JP-A-621902
According to JP-A-214124, TMCP (Thermo Mech
A high-strength 60 kg steel with excellent weldability has been proposed by utilizing an anical controlled process) technology. The former is C: 0.03 to 0.10% in weight ratio,
Si: 0.05 to 0.60%, Mn: 0.60 to 2.0
0%, P: 0.030% or less, S: 0.020% or less,
Mo: 0.10 to 0.50% inclusive, C + Si / 30 +
Mn / 20 + Ni / 50 + Cr / 20 + Cu / 20 + M
Pcm defined by o / 15 + V / 10 + 5B is 0.1
6-0.21% steel is directly quenched or reheat-quenched, further heated and held at the two-phase region temperature of the AC3 to AC1 transformation point, then re-quenched at a cooling rate of air cooling or higher, then 45
Disclosed is a steel having a tensile strength (TS) of 60 kgf / mm ² or more and requiring no preheating during welding by tempering in a temperature range of 0 ° C to 600 ° C. The latter is C: 0.
02-0.15%, Si: 0.40-0.80%, M
n: 0.80 to 1.60%, V: 0.005 to 0.10.
%, Nb: 0.005 to 0.10%, Al: 0.05%
Hereinafter, steel containing P: 0.020% or less and S: 0.015% or less is heated to 950 to 1200 ° C., and rolling is performed at a reduction rate of 30% or more in an unrecrystallized region subsequent to the austenite recrystallized region. After that, it is forcibly cooled at a cooling rate of 5 ° C. or more and further tempered in a temperature range of 200 to 400 ° C. to obtain excellent weldability and a tensile strength of 60 kgf / m.
Steels of m ² or more are disclosed.

On the other hand, in the joint portion of the welded structure,
Ultrasonic flaw detection is generally performed by the oblique angle flaw detection method. However, in a TMCP steel sheet having a large acoustic anisotropy, the formation of a rolling texture causes a perpendicular (L direction) direction parallel to the rolling direction. There may be a difference in sound velocity in the direction (C direction). For this reason, there are cases in which a defect is actually detected in a defect-free portion, and it is difficult to accurately detect a welding defect, and repair is performed in an unnecessary range, resulting in a huge construction cost. In order to solve this problem, JP-A-63-235431 and JP-A-2-3
Japanese Patent No. 05918 discloses a method of manufacturing a steel sheet having small acoustic anisotropy. In the former, C: 0.05% or more and less than 0.18%, Si: 0.05% or more and less than 0.5%,
Mn: 0.70% or more and less than 1.8%, Al: 0.005
% To less than 0.1%, Nb: 0.003% to 0.03
Less than 0%, N: less than 0.006% contained, C + Mn /
6 is 0.36% or less steel is heated to 100 ℃ or more and 1200 ℃ or less, 50% of the total reduction in the austenite recrystallization region
By adding the above-mentioned rolling to a rolling finishing temperature of 850 ° C. or higher and cooling to a temperature range of 400 ° C. or higher and 680 ° C. or lower at a cooling rate of 5 ° C./s or higher and lower than 15 ° C./s from Ar3-50 ° C. or higher. A method of manufacturing a steel sheet having a small acoustic anisotropy has been proposed. In the latter case, C: 0.01 to 0.20
%, Si: 0.05-0.60%, Mn: 0.5-2.
Steel containing 5%, sol Al: 0.005 to 0.10%, or Cu: 0.01 to 0.20 in addition to the above components.
%, Ni: 0.10 to 5.0%, Cr: 0.05 to 1.
0%, Mo: 0.08 to 1.0%, Nb: 0.005
0.1%, V: 0.01 to 0.1%, Ti: 0.007
~ 0.15%, B: 0.0005 to 0.010%, at the time of hot rolling of steel containing at least one kind or two or more kinds, the cumulative rolling reduction ratio at (Ar3 point + 200 ° C) to Ar3 point is 4
A method for producing a TMCP steel sheet having a small acoustic anisotropy is described by setting the rolling reduction temperature to 0% or more, the final pass rolling reduction of 5% or more, and the rolling finishing temperature in the range of (Ar3 point + 100 ° C.) to Ar3 point. .

[0004]

As described above, the present invention relates to a 60 kg class high-strength steel capable of preventing cold cracking during welding and a method of manufacturing a steel material for improving the acoustic anisotropy of the base material. Although proposals have been made individually, a method for producing a steel material having both properties, particularly a 60 kg class high-strength steel, has not yet been proposed despite strong demands in the field of construction and the like. An object of the present invention is to provide a method for producing a tempered 60 kgf / mm ² grade steel which is excellent in both properties of weldability and acoustic anisotropy.

[0005]

The present invention, in weight percent, comprises:
C: 0.04 to 0.1%, Si: 0.01 to 0.4%,
Mn: 0.5 to 1.6%, P: 0.015% or less, S:
0.01% or less, Mo: 0.01 to 0.3%, Nb:
0.005-0.05%, V: 0.1% or less, Al:
0.01-0.08%, N: 0.0005-0.008
%, Ti <0.005%, B <0.0003%,
If necessary, Cu: 0.5% or less, Ni: 1.5%
Hereinafter, Cr: 0.5% or less, including one or more,
A steel material having a Pcm value of 0.2 or less and X ≧ 40 + t, the balance of which is iron and inevitable impurities, is 10
After heating to a temperature of 00 ° C to 1250 ° C, Tft = -5
30C + 100Mn + 15Mo + 2250 (effective Nb)
Weldability and acoustic characteristics, characterized in that after hot rolling is completed above the temperature given by + 100V + 770 (° C), it is directly quenched from at least the Ar3 transformation point and then tempered at a temperature below the Ac1 transformation point. This is a method for producing high-strength steel having excellent anisotropy.

However, Pcm = C + Si / 30 + Mn /
20 + Cu / 20 + Ni / 30 + Cr / 20 + Mo / 1
5 + V / 10 + 5B, effective Nb amount is 100
Using the heating temperature Tsl set in the temperature range of 0 to 1250 ° C., log (Nb) × (C + 12 / 14N) = 2.2
6-6770 / (Tsl + 273.15) shows the amount of solid solution Nb calculated from the relationship, Ceq = C + Mn / 6 + Si
/ 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14, X = 625 (effective Nb) + 250V + 210
Ceq, t represents the plate thickness (mm) of the steel plate.

[0007]

The reasons for limiting the constituent features of the present invention will be described below. C: 0.04 to 0.1% C is an element effective in increasing the strength, but if it exceeds 0.1%, the weldability is impaired, and if it is less than 0.04%, the strength becomes insufficient.

Si: 0.01 to 0.4% Si is an element effective in deoxidizing and improving strength, but
If it exceeds 4%, the weldability is impaired, and if it is less than 0.01%, the strength becomes insufficient.

Mn: 0.5 to 1.6% Mn is added for the strength and toughness of the base material. However, if it exceeds 1.6%, the weldability and toughness deteriorate,
If it is less than 0.5%, the strength is insufficient.

P, S: P ≦ 0.015%, S ≦ 0.01
% P and S are both impurity elements, and P is regulated to 0.015% or less, preferably 0.01% or less, and S is regulated to 0.01% or less in order to obtain a sound base metal and a welded joint. Is desirable.

Mo: 0.01-0.3% Mo contributes to the improvement of the base metal strength by adding 0.01% or more. However, if it exceeds 0.3%, the weldability and toughness are impaired.

Nb: 0.005 to 0.05% Nb is added to improve the strength of the base metal and the strength of the welded joint. If less than 0.005%, the effect is lost, 0.05
%, The weld joint toughness is impaired.

The solid solution temperature of Nb carbonitride in steel is Nb,
Log (Nb) × (C + 12 /
14N) = 2.26-6770 / (T + 273.15)
Although determined more, the solid solution temperature of Nb carbonitride may be higher than the heating temperature Tsl in the above Nb addition range. Calculation formula described later: X = 625 (effective Nb) + 250V + 21
At 0 Ceq, the term of effective Nb indicates the amount of increase in the base metal strength due to the precipitation hardening of Nb after tempering, and the effective Nb refers to Nb dissolved in the heating stage before hot rolling.
Therefore, in using the limitation by this calculation formula, Nb
Considering the relationship between the solid solution temperature of carbonitride and the rolling heating temperature Tsl, when the solid solution temperature of Nb carbonitride is higher than the heating temperature Tsl, the amount of the dissolved Nb obtained from the C and N contents is determined as the effective Nb.
The quantity will be used in the calculation formula.

When the solid solution temperature of Nb carbonitride is lower than the heating temperature Tsl, all the Nb contained in the steel dissolves, so the Nb content and the solid solution Nb amount (effective Nb amount) are synonymous. Becomes

V: 0.1% or less V is an element effective for improving the strength of the base material due to the precipitation effect, but if the content exceeds 0.1%, the weldability is impaired.

The solid solution temperature of V carbonitrides within the scope of the present invention is lower than the heating temperature adopted in ordinary hot rolling, and therefore the consideration described in the item of Nb above is unnecessary. . Cu, Ni, Cr: Cu ≦ 0.5%, Ni ≦ 1.5%,
Cr ≦ 0.5% Cu, Ni and Cr are not necessarily essential elements. However, Cu and Cr may be added within the above ranges in order to improve the base material strength and weld joint strength, and Ni in order to improve both the base material strength, toughness and joint strength. Especially M
By replacing a part of n with these elements, it is possible to improve toughness and reduce segregation.

Ti, B: Ti <0.005%, B <0.
0003% Ti has the effect of improving the toughness of the base material and the welded joint through the refinement of the microstructure. In addition, in the B-added steel,
It is often added positively in order to secure B that works effectively for hardenability. However, in the present invention, B, which may cause hardening of the weld heat affected zone, is not added to secure the base metal strength, and in particular to achieve weld joint toughness by reducing the hardness in the coarse grained zone of the heat affected zone, Ti is added. There is no need to add it. In the present invention, it is rather concerned about the instability of the base material performance due to the addition of Ti, so the content of the impurity element is limited to less than 0.005%. In particular, it is desirable that the N content is less than 3.4 times the content.

B is regulated to less than 0.0003% as an impurity element in order to reduce the hardness of the above-mentioned welding heat affected zone. Al: 0.01 to 0.08% Al reacts with deoxidation and N to generate a precipitate, thereby refining the microstructure and contributing to the improvement of the base metal toughness and joint toughness. If it is added in an amount of less than 0.01%, the microstructure becomes insufficiently fine, and if it exceeds 0.08%, the toughness of the base material is impaired.

N: 0.0005 to 0.008% N reacts with Al to form a precipitate, thereby refining the microstructure to improve the base material toughness, weld joint toughness, Nb, V, etc. during tempering. Reacts with and contributes to precipitation hardening. If the addition amount is less than 0.0005%, the amount of precipitates becomes insufficient, and if the addition amount exceeds 0.008%, the base material toughness and the weld joint toughness are impaired.

Pcm (welding crack susceptibility index): 0.2%
Hereinafter, Pcm is an index of weldability in order to reduce the preheating temperature during welding. Pcm (= C + Si / 30 + Mn / 20 + Cu / 20 +
Ni / 30 + Cr / 20 + Mo / 15 + V / 10 + 5
B) is suppressed to 0.2% or less.

X = 625 (effective Nb) + 250V + 21
0Ceq: X ≧ plate thickness (mm) +40 60 kgf / mm2 In order to secure a tensile strength (TS) of class ² , Ceq, which is an index of hardenability, and effective Nb, V content that contributes to precipitation hardening during tempering treatment. X consisting of quantity
The value of = 625 (effective Nb) + 250V + 210Ceq is equal to or more than the plate thickness (mm) +40. Incidentally, the tensile strength of 60 kgf / mm ² grade here means JIS SM57.
0Q and high-strength steel represented by JIS SPV490Q, and the plate thickness is in the range of 15 mm to 60 mm.

Slab heating temperature during hot rolling Tsl: 100
The slab heating temperature Tsl in the 0 to 1250 ° C hot rolling needs to be 1000 ° C or more in order to form a solid solution of Nb. However, if the heating temperature Tsl exceeds 1250 ° C, the toughness of the base material deteriorates due to the coarsening of the microstructure, so the upper limit is 1250 ° C.
The temperature is preferably 1200 ° C.

Rolling finish temperature during direct quenching: Tft ° C or higher, where Tft = −530C + 100Mn + 15Mo + 2
250 (effective Nb) + 100V + 770 In the present invention, the rolling finishing temperature has a great influence on the acoustic anisotropy. The acoustic anisotropy increases when the temperature becomes lower than Tft ° C calculated from the above equation. Therefore, the lower limit temperature for rolling is T
ft ℃

Direct quenching: Ar3 transformation point or higher After hot rolling is completed, the steel sheet must be quenched by forced cooling from a temperature at least above the Ar3 transformation point. It is also possible to reduce the yield ratio by providing a temperature difference between the rolling finish temperature (Tft ° C.) and the quenching start temperature (at least the Ar3 transformation point or higher).

[0025]

EXAMPLES Various examples of the present invention will be described with reference to Tables 1 and 2. Steel types 1 to 23 in Table 1 are steel types of the present invention, and steel types 24 to 28 are comparative steels which are out of the scope of the present invention in terms of composition. In addition, Tft in the rightmost column in the table
(° C) is Tft = -530C + 100Mn + 15Mo +
The value obtained from the equation represented by 2250Nb + 100V + 770 is shown. The next column is X = 625 (valid Nb)
The value calculated from the equation represented by + 250V + 210Ceq is shown. The next column shows t: plate thickness (mm), and the next column shows Ceq = C + Mn / 6 + Si / 24 + Ni / 40.
The value calculated from the formula expressed by + Cr / 5 + Mo / 4 + V / 14 is shown. The next column is Pcm = C + Si / 30 +
Mn / 20 + Cu / 20 + Ni / 30 + Cr / 20 + M
The values obtained from the weld crack susceptibility index formula represented by o / 15 + V / 10 + 5B are shown.

In the examples, the steel types 1 to 18 are represented by weight%.
C: 0.04 to 0.1%, Si: 0.01 to 0.4
%, Mn: 0.5 to 1.6%, P: 0.01% or less,
S: 0.01% or less, Mo: 0.01 to 0.3%, N
b: 0.005-0.05%, V: 0.1% or less, A
1: 0.01 to 0.08%, N: 0.0005 to 0.0
08%, Ti <0.005%, B <0.0003% included, Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + N
Pcm value defined by i / 30 + Cr / 20 + Mo / 15 + V / 10 + 5B is 0.2 or less, and Ceq = C
+ Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo
Ceq value defined by / 4 + V / 14 and effective N
b = V content X = 625 (effective Nb) +250
V + 210Ceq satisfies the relationship of X ≧ 40 + t (where t represents the plate thickness (mm) of the steel plate).

In Table 1, all of the added Nb is in solid solution,
It shall be effective Nb. Steel type 1 has a C content of 0.
Although the content of Mb and Mo is 1.49%,
Since it is as high as 0.19%, X = 625 (effective Nb) +2
The value of 50 V + 210 Ceq is 88.

Steel types 2 and 3 have a fairly low C content, but since Cu and Cr are added, X = 625.
(Effective Nb) + 250V + 210Ceq is 93,1
It becomes 03.

Steel type 4 has a slightly low C content of 0.066% and a low Nb addition amount of 0.007%.
And Mo contents are as high as 1.53% and 0.20%, respectively, and the Nb content becomes 0.
Since it increases to 013% and 0.019%, X = 625
(Effective Nb) + 250V + 210Ceq value is 84,8
It becomes 8,92.

Steel type 7 has a low C content, so X = 6.
The value of 25 (effective Nb) + 250V + 210Ceq is 81
Becomes Steel types 8 and 9 have a C content of 0.089%, 0.
Although it is as high as 086%, the amount of Nb added is 0.01%,
X = 625 (effective N
b) The value of + 250V + 210Ceq becomes 88.

Steel types 10 and 11 have C and Nb contents of 0.091%, 0.086% and 0.020, respectively.
%, 0.034%, so X = 625 (effective N
b) The value of + 250V + 210Ceq is 93,1 respectively.
It becomes 01.

Steel types 12 to 16 have a high Mn content, and X = 625 (effective Nb) + 250V + 210 Ceq.
The values of are 98, 100, 105, 103, 99. Steel type 17 has a low C content of 0.070%, but N
Since the b content is as high as 0.034%, the value of X = 625 (effective Nb) + 250V + 210Ceq is 98.

Steel type 18 has a C content of 0.099%, which is close to the upper limit, but has a Mn content of 1.29%, so that the value of X = 625 (effective Nb) + 250V + 210 Ceq is 102.

Steel grades 19 to 23 are cases containing Cu, Ni, and Cr, and since steel grade 19 has a large Cr content, X = 625 (effective Nb) + 250V + 210.
The value of Ceq is as high as 104.

Among the comparative examples, the steel types 24 and 26 do not contain Nb, and the steel type 25 has a C content of 0.033%, which is too small.
Steel type 27 has a C content of 0.131%, and steel type 28 has a Cr content of 0.70%, which is too high, so the Pcm value exceeds 0.2.

Next, the effect of the embodiment will be described with reference to Table 2. Table 2 shows the properties of the manufactured steel types of Table 1, and 60 kgf / m for all the steel types.
It exhibits excellent strength and toughness as an m ² grade.

Next, the acoustic anisotropy defined by JIS Z 3060 will be described. The letters a and b at the end of the reference numerals of the steel types 1 to 23 show the results obtained by changing the rolling finishing temperature in the above steel types.

For steel type 1a, the rolling finishing temperature is Tft = -5.
30C + 100Mn + 15Mo + 2250 (effective Nb)
The value of L / C, which is the ratio of the sound velocity (L) in the direction parallel to the rolling direction and the sound velocity (C) in the direction perpendicular to the rolling direction, is 1.0 because the temperature is 990 ° C., which is + 100V + 770 ° C. or higher.
It becomes 00. In steel type 1b, the rolling finish temperature is lower than Tft ° C., so the L / C value is as large as 1.023. Since it is generally considered that the acoustic anisotropy is 1.02 or less, the steel type 1a is excellent in the acoustic anisotropy and the steel type 1b is
Is inferior in acoustic anisotropy. Similarly, for the steel types 2 to 18, the rolling finish temperature is Tft ° C. or more at the end of the symbol a and thus the acoustic anisotropy is excellent, while the rolling finish temperature at the end of the symbol b is less than Tft ° C. Inferior in anisotropy. Regarding the steel type 11c, the case where the rolling finish temperature is further lowered than that of the steel type 11b is shown. This gives L /
The value of C is further increased and the acoustic anisotropy is further deteriorated.

The steel type 1x has a heating temperature of 100 during hot rolling.
Since it was set at 0 ° C, 0.015% of Nb contained,
All do not form a solid solution, and the effective Nb amount remains at 0.011%. As a result, X = 625 (effective Nb) + 250V + 2
10 Ceq and Tft = -530C + 100Mn + 15
The values of Mo + 2250 (effective Nb) + 100V + 770 are also low at 86 and 907, respectively. 1 for steel type 11x
Similar to x, since the heating temperature during hot rolling is set to 1000 ° C., the X value and Tft value are as low as 87 and 884, respectively.
Further, with steel type 18b, the heating temperature during hot rolling is 1150.
When the temperature is set to ℃, the amount of solute Nb (effective Nb) is 0.031
%, The X value and the Tft value are as low as 101 and 921, respectively.

On the other hand, regarding weldability, in all of the steel types 1 to 23, since the Pcm value is 0.2 or less, JIS
No crack was observed in the oblique y-type weld cracking test (preheating temperature 25 ° C.) specified in 3158, and JIS 310
The maximum hardness value of the weld heat affected zone specified in 1 is also 290H
It is as good as v or less.

Among the comparative examples, the steel types 24 to 26 have a C or Nb content outside the range of the present invention, and therefore have insufficient strength. In steel type 27, the C content is out of the range of the present invention, so the Pcm value is larger than 0.2, and cracks are observed in the oblique y-type weld cracking test (preheating temperature 25 ° C.) specified in JIS3158, and The maximum hardness value of the weld heat affected zone specified by JIS3101 is also large at 314 Hv.
In steel type 28, since the Cr content is outside the range of the present invention,
The Pcm value was larger than 0.2, and cracks were observed in the oblique y-type weld cracking test (preheating temperature 25 ° C.) specified in JIS3158.

[0042]

[Table 1]

[0043]

[Table 2]

[0044]

[Table 3]

[0045]

[Table 4]

[0046]

Effect of the Invention] According to the present invention method, welding improved possible weldability prevention of cold cracking and the like during construction, better tempering type characteristics of the acoustic anisotropy 60 kgf / mm ² class You can get steel.

Claims (2)

[Claims] 1. C: 0.04 to 0.1% by weight, S
i: 0.01 to 0.4%, Mn: 0.5 to 1.6%,
P: 0.015% or less, S: 0.01% or less, Mo:
0.01-0.3%, Nb: 0.005-0.05%,
V: 0.1% or less, Al: 0.01 to 0.08%, N:
0.0005-0.008%, Ti <0.005%, B
<0.0003% included, Pcm value is 0.2 or less, X ≧
After heating a steel material satisfying the relation of 40 + t and the balance being iron and unavoidable impurities to a temperature of 1000 ° C. or higher and 1250 ° C. or lower, Tft = −530C + 100Mn + 15Mo
+2250 (effective Nb) + 100V +770 (° C) After the hot rolling is finished at a temperature above the temperature, it is directly quenched from at least the Ar3 transformation point, and then tempered at a temperature below the Ac1 transformation point. Method for producing high-strength steel with excellent weldability and acoustic anisotropy. However, Pcm = C + Si / 30 + Mn / 20 + Cu / 20
+ Ni / 30 + Cr / 20 + Mo / 15 + V / 10 + 5
The effective Nb amount is defined as B and the heating temperature Tsl set in the temperature range of 1000 to 1250 ° C. is used, and log (Nb) × (C + 1
2 / 14N) = 2.26-6770 / (Tsl + 273.
15) shows the amount of solid solution Nb calculated from the relationship of 15), Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr
/ 5 + Mo / 4 + V / 14, X = 625 (effective Nb) + 250V + 210Ceq, t
Indicates the plate thickness (mm) of the steel plate. 2. C: 0.04 to 0.1% by weight, S
i: 0.01 to 0.4%, Mn: 0.5 to 1.6%,
P: 0.015% or less, S: 0.01% or less, Mo:
0.01-0.3%, Nb: 0.005-0.05%,
V: 0.1% or less, Al: 0.01 to 0.08%, N:
0.0005-0.008%, Ti <0.005%, B
<Including 0.0003%, further Cu: 0.5% or less,
Ni: 1.5% or less, Cr: 0.5% or less, one or two
After heating a steel material containing at least 1000 kinds of materials and having a Pcm value of 0.2 or less and X ≧ 40 + t and the balance of iron and unavoidable impurities to a temperature of 1000 ° C. or more and 1250 ° C. or less, Tft = −530C + 100Mn + 15Mo + 225.
It is characterized in that after hot rolling is completed at a temperature higher than 0 (effective Nb) + 100V + 770 (° C.), it is directly quenched from at least the Ar3 transformation point and then tempered at a temperature below the Ac1 transformation point. Method for producing high-strength steel with excellent weldability and acoustic anisotropy. However, Pc
m = C + Si / 30 + Mn / 20 + Cu / 20 + Ni /
It is defined by 30 + Cr / 20 + Mo / 15 + V / 10 + 5B, and the effective Nb amount is log (Nb) × (C + 1) using the heating temperature Tsl set in the temperature range of 1000 to 1250 ° C.
2 / 14N) = 2.26-6770 / (Tsl + 273.
15) shows the amount of solid solution Nb calculated from the relationship of 15), Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr
/ 5 + Mo / 4 + V / 14, X = 625 (effective Nb) + 250V + 210Ceq, t
Indicates the plate thickness (mm) of the steel plate.