JP3374688B2 - Method for producing tempered 600 N / mm2 class high strength steel excellent in weld cracking sensitivity and low-temperature toughness - Google Patents

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 600 N / mm ² class high strength steel having excellent weld crack susceptibility and low temperature toughness.

[0002]

^2. Description of the Related Art Conventionally, there have been many demands for improving the performance of 600 N / mm ² class high strength steel, and many studies have been made so far. Among these, Japanese Patent Laid-Open No. 49-37814 and Japanese Patent Publication No. 4-13406 are known as techniques characterized by lowering C and adding Ti-B for the purpose of improving susceptibility to welding cracks. Technologies such as these can provide 600N / mm ² class high-strength steel with improved weld cracking susceptibility, but the tensile strength required for 600N / mm ² class high-strength steel is achieved by utilizing B. Therefore, there is a concern that the base material characteristics may become unstable due to changes in chemical composition and manufacturing conditions, and the hardness of the weld heat affected zone may increase significantly. This increase in hardness of the heat-affected zone of welding generally leads to deterioration of the toughness of the bond portion, which is the most concerned in the welded joint portion. Japanese Unexamined Patent Application Publication No. 2-205627 discloses an excellent toughness of 600 N / mm using a direct quenching method.
We provide a manufacturing method for ^second- class high-strength steel. This technology is N
Since the combined addition of b and B is indispensable, there is a concern that the same adverse effect as the addition of B may occur.

As a technique in which B is not added, JP-A-5-3315
38, Japanese Patent Publication No. 60-9086, Japanese Patent Laid-Open No. 2-254119, Japanese Patent Publication No. 59-113120, Japanese Patent Publication No. 61-12970,
JP-A-53-119219 has been proposed.

Of these, the technique disclosed in Japanese Unexamined Patent Publication No. 5-331538 relates to 500 N / mm ² grade non-heat treated high strength steel. Also, Japanese Patent Publication No. 60-9086 and Japanese Patent Laid-Open No. 2-25
The techniques disclosed in Japanese Patent No. 4119 and Japanese Patent Laid-Open No. 59-113120 are all related to 600 N / mm ² class non-heat treated high-strength steel, and from the examples, the upper limit of the plate thickness to which these techniques are applied is Is also known to be about 20 mm. Tokusho
No. 61-12970 discloses 600N which has excellent weld crack susceptibility by combining low carbon content, V addition and direct quenching.
/ Mm ² class high-strength steel
In this case as well, the applicable plate thickness remains 30 mm.

Further, Japanese Patent Laid-Open No. 53-119219 discloses a thick plate of 500 N / mm ² by a reheating quenching and tempering process.
It is intended to provide high-strength steel of a grade or higher. According to this technique, an undissolved Nb carbonitride is left at the time of reheating by adding a relatively large amount of Nb exceeding 0.02% to prevent coarsening of crystal grains and mainly to improve the toughness of the base material. To do. Therefore, the effect of improving the hardenability of solid solution Nb and the precipitation hardening cannot be fully utilized in quenching. Therefore, in order to secure strength as seen in the examples, Nb,
In addition to V, it is essentially essential to add Ni and Mo, and 600 N / mm at the position where the plate thickness of the thick material is 1 / 4t.
Pcm for the invention example (test steel J) that can secure the ^second grade strength
The value reaches 0.22 and the susceptibility to weld cracking is poor.

As described above, 60 is excellent in susceptibility to welding cracks.
Most of the prior art of 0N / mm ² grade tempered high-strength steel has been achieved by securing the hardenability by adding B, and when B is not added, it is limited to a thin material.

On the other hand, Japanese Patent Laid-Open No. 5-271760, Japanese Patent Laid-Open No. 6-128638, and Japanese Patent Laid-Open No. 6-93332 disclose conventional methods for rolling high-strength steel with improved low-temperature toughness. In JP-A-6-93332, C, Si, Mn, Nb,
Ti, B, sol. Immediately after controlled rolling of a material steel in which Al and N are regulated, accelerated cooling to a predetermined temperature range,
Then, a method for producing a fine bainitic steel excellent in weld cracking susceptibility and low temperature toughness is disclosed by holding the temperature range isothermally for a certain period of time, or by gradually cooling the temperature range for a certain period of time. The time is extremely long, the rolling efficiency is lowered, the isothermal holding and the gradual cooling increase the cost, and the productivity is remarkably reduced.

[0008]

In order to solve such a problem of waiting for temperature adjustment, Japanese Patent Laid-Open No. 6-128638 discloses that a slab containing V and Nb is added to Ac.
_It is characterized by heating to ₃ points or more and hot rolling while performing cooling at ₃ points or more of Ar, and then allowing it to cool or accelerate cooling to a temperature of 650 ° C or less at a cooling rate of 5 ° C / sec or more. A method for manufacturing a structural steel plate having high strength and high toughness is disclosed. However, according to these conventional techniques, as a means for improving the low temperature toughness, it is disclosed that the temperature at which the hot rolling is finished is set to a low temperature near the Ar ₃ point (around 700 to 800 ° C.). As a result, the deformation resistance of the steel to be rolled increases, and a great load is applied to the rolling mill. No consideration is given to the weld crack susceptibility of the rolled steel.
From the above, a method for producing a tempered 600 N / mm ² class high-strength steel excellent in weld crack susceptibility and low temperature toughness without performing controlled rolling at a low temperature of around the Ar ₃ point has not been provided to date. .

The present invention has been made in view of the above circumstances, and a controlled rolling of tempered 600 N / mm ² grade steel up to a plate thickness of 50 mm, which is excellent in both characteristics of weld cracking susceptibility and low temperature toughness, is carried out at a low temperature. It is an object of the present invention to provide a manufacturing method without carrying out.

[0010]

In order to improve the weld crack susceptibility, Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni
It is effective to reduce the Pcm value defined by / 30 + Cr / 20 + Mo / 15 + V / 10 + 5B. Although addition of B is considered as an effective means for securing the strength of the base material while securing the susceptibility to weld cracking, deterioration of joint toughness due to a significant increase in hardness of the heat-affected zone of the weld is a concern especially for large heat input welded joints described below. To be done. It is preferable not to add Ti, which is often added conventionally in order to effectively utilize B, in order to stably obtain the base material performance. Therefore, in order to obtain 600 N / mm ² class high-strength steel while simultaneously improving the weld crack susceptibility and ensuring the soundness of the welded joint without adding Ti and B, it is necessary to apply the conventional reheating quenching and tempering process. There is a concern that the applicable thickness range will be restricted.

The present invention, which is aimed at solving these problems, has prevented the industrial supply of 600 N / mm ² grade high-strength steel up to a plate thickness of 50 mm which is excellent in weld cracking susceptibility and low temperature toughness. It was proposed based on the following knowledge on the premise that the direct quenching and tempering process is applied.

(1) By adopting Nb as a chemical component and adopting a direct quenching method, it is possible to utilize the effect of improving hardenability by Nb dissolved in a solid solution at the time of rolling and heating. This makes it possible to reduce the amount of addition of alloying elements for securing other hardenability. Nb also has the function of finely dispersing carbides,
It is extremely effective in securing the toughness of the 2t part.

(2) N is obtained by the tempering treatment after direct quenching.
b Precipitation hardening of carbonitride can be utilized. This is effective in securing the strength of the central portion of the plate thickness where the cooling rate during quenching is necessarily slower than that on the surface side. That is, this makes it possible to secure the strength of the central portion of the plate thickness without securing the hardenability more than necessary.

(3) Rolling in the austenite unrecrystallized region, which is effective for improving the low temperature toughness of the steel material, can be realized by adding Nb which raises the austenite recrystallization temperature, without performing reduction at an extremely low temperature.

(4) As the reduction ratio in the unrecrystallized austenite region increases the sound velocity ratio (acoustic anisotropy) between the direction parallel to the rolling direction (L direction) and the direction perpendicular to the rolling direction (C direction). , Fracture transition temperature of Charpy impact test (vTr
The low temperature toughness represented by s) is improved without applying reduction at extremely low temperatures. However, the acoustic anisotropy is 1.
If it exceeds 035, the low temperature toughness deteriorates. Figure 1
The results are shown in.

(5) From the above, the addition of B becomes unnecessary, and B
From the viewpoint of effectively utilizing
Rather, it is preferable not to add Ti in order to stably obtain good base material performance.

As described in (6) and (4), in the present invention, the acoustic anisotropy of the steel material is limited to a predetermined range.
At the time of hot rolling, in order to improve the low temperature toughness by refining the γ grains, and to increase the value of the acoustic anisotropy within the range not exceeding 1.035, the temperature is less than 1000 ° C and 800 ° C or less.
A cumulative reduction of 0% or more is applied, and direct quenching is performed from at least 3 Ar points.

That is, according to the present invention, (1) C by weight%:
0.04 to 0.1%, Si: 0.01 to 0.4%, M
n: 0.5 to 1.6%, P: 0.015% or less, S:
0.01% or less, Nb: 0.005 to 0.05%, V:
0.1% or less, Al: 0.005 to 0.1%, N: 0.
0005-0.008%, Ti <0.005%, B <
Including 0.0003%, Pcm = C + Si / 30 + Mn
/ 20 + Cu / 20 + Ni / 30 + Cr / 20 + Mo /
A steel material having a Pcm value defined by 15 + V / 10 + 5B of 0.2 or less and a balance of iron and unavoidable impurities is heated to a temperature range of 1000 to 1250 ° C. during hot rolling, and then 20 at 1000 ° C. to 1050 ° C. % or more following the cumulative reduction, non-recrystallization temperature of 800 ° C. or higher than 1000 ° C.
The rolling reduction is 5% or more in each rolling pass, and
A rolling finish temperature of 800 ° C. or higher, a cumulative reduction of 20% or higher, direct quenching at least from the Ar ₃ transformation point or higher, and then tempering at the Ac ₁ transformation point or lower, tensile strength of 570 N / A method for producing a tempered type 600 N / mm ² class high-strength steel excellent in weld cracking susceptibility of mm ² or more and low temperature toughness, (2) steel material is Mo: 0.3% or less,
Cu: 0.5% or less, Ni: 1.5% or less, Cr: 0.
A method for producing a tempered type 600 N / mm 2 class high-strength steel excellent in weld cracking susceptibility with a tensile strength of 570 N / mm ² or more and excellent in low temperature toughness as described in (1) further containing one or more of 5% or less, (3) Using the heating temperature T set in the temperature range of 1000 to 1250 ° C., the steel material having the composition described in (1) or (2) is log {(Nb) × (C + 12 /
14N)} = 2.26-6770 / (T + 273.1)
Ceq = C + Mn / 6 + Si / 24 + Ni / 40 +, where the solid solution Nb amount calculated from the relationship of 5) is the effective Nb amount.
Ceq value defined by Cr / 5 + Mo / 4 + V / 14, effective Nb amount, and V content X = 625 (effective N
b) + 250V + 210Ceq satisfies the relation of X ≧ 40 + t (where t indicates the plate thickness (mm) of the steel plate), and has a tensile strength of 570 N / mm ² or more, which is excellent in weld cracking susceptibility and low temperature toughness. This is a method for producing a mold 600 N / mm ² class high strength steel.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the constituent features of the present invention will be described below. If the amount of <C> C is less than 0.04%, it is necessary to add a large amount of other hardenability-improving elements, resulting in high cost, toughness deterioration, and deterioration of weld cracking susceptibility. Further, particularly when subjecting the steel of the present invention to high heat input welding, if the C content is less than 0.04%, the dilution of C into the weld metal becomes small and it is difficult to secure the joint strength with general welding materials. Becomes The upper limit of the amount of C is 0.1% in order to secure the susceptibility to welding cracks. <Si> Si effectively acts to secure the strength of the base metal and the strength of the welded joint, so 0.01% or more is added. However, 0.
Addition of more than 4% deteriorates weld crack susceptibility and weld joint toughness. <Mn> Mn works effectively in securing the strength of the base metal and the strength of the welded joint, so 0.5% or more is added. But 1.6
%, The weld cracking susceptibility is deteriorated, the hardenability is increased more than necessary, and the base metal toughness and the joint toughness are deteriorated. <P, S> P and S are both impurity elements, and P is 0.015% or less, preferably 0.01% or less, and S is 0.01% in order to obtain a sound base metal and a welded joint. It is desirable to be regulated to%. <Nb> As described above, Nb is a weld crack susceptibility index according to the effective Nb amount described in claim 3 when the direct quenching method is adopted: Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + N
The hardenability is improved without increasing i / 30 + Cr / 20 + Mo / 15 + V / 10 + 5B, and the base metal strength and the weld joint strength are improved by the precipitation effect of Nb carbonitride by the tempering treatment of the direct quenching iron. As a result, the amount of addition of other alloying elements such as Cu, Ni, Cr, and Mo for securing the hardenability can be reduced, so 0.00
Add more than 5% positively. However, since the addition of more than 0.05% tends to deteriorate the weld joint toughness, the upper limit of the Nb addition amount is set to 0.05%, preferably 0.03%. <V> V not only improves the hardenability but also improves the base metal strength and the welded joint strength due to the precipitation effect of V carbonitrides by the tempering treatment, so it is added in an amount not exceeding 0.1%. <Al> Al is added for deoxidizing steel, and is usually 0.00
5% or more is contained. Further, 0.01% is added in order to secure the toughness of the base material due to the refinement of the microstructure. But,
Addition of Al in excess of 0.1% impairs base material toughness. <Ti, B> Ti has the effect of improving the toughness of the base material and the welded joint through grain refinement of the microstructure. Also, B
Additive steel is often added positively to secure B that works effectively for hardenability.

On the other hand, in the present invention, as described above, the strength of the base metal is secured without adding B, which may cause hardening of the weld heat affected zone, and the hardness of the coarse grained zone of the heat affected zone is reduced. There is no need to add Ti to achieve weld joint toughness. Rather, there is a concern about instability in the base material performance due to the addition of Ti, and the content of the impurity element is regulated to less than 0.005%, but it is desirable to be less than 3.4 times the N content described later.

B must be regulated to less than 0.0003% as an impurity element in order to reduce the hardness of the heat affected zone. <N> N reacts with Al, Nb, etc. to form a precipitate, thereby making the microstructure finer and improving the base material toughness, and reacting with Nb, V, etc. during tempering to secure the strength by precipitation hardening. To add.

If the addition amount is less than 0.0005%, precipitates necessary for refining the microstructure and ensuring strength are not formed,
Addition of more than 0.008% rather impairs the toughness of the base metal and the welded joint. <Mo> Mo has the effect of improving the base material strength and joint strength, so it is selectively added within a range not exceeding 0.3%. <Cu, Ni, Cr> Cu, Ni, Cr are not always essential elements. However, Cu and Ni are for improving the base metal strength and weld joint strength, and Ni is Cu ≦ 0.5%, N for improving both the base metal strength, toughness and joint strength.
There is no problem even if i ≦ 1.5% and Cr ≦ 0.5% are added. In particular, by replacing a part of Mn with these elements, improvement of toughness and reduction of segregation can be achieved. <Pcm> Pcm is an index representing the susceptibility to welding cracks,
It is regulated to 0.2% or less in order to eliminate the need for preheating during welding in a normal environment. <Calculation formula: X = 625Nb + 250V + 210Ceq,
X ≧ t + 40> Calculation formula: 625Nb + 250V + 210Ceq is an index representing the strength of the base material at a plate thickness of 1 / 2t, and the carbon equivalent formula (Ceq) generally known to those skilled in the art is the key to Nb and V of the present invention. This is a mathematical formula in which the contribution is taken into consideration and the thickness effect in the thickness range up to approximately 50 mm is taken into consideration and arranged. Note that the plate thickness effect means that when the steel plate is forcibly cooled from the Ar ₃ transformation point or higher by direct quenching after hot rolling, the cooling rate inevitably changes depending on the plate thickness, and therefore the base metal strength changes. It means to do. Here, even at 1 / 2t, JIS G3 is classified as 600N / mm ² class high-strength steel.
A steel plate suitable for 106SM570Q is to be obtained, and the plate thickness term in the calculation formula is 50 m for industrial simplicity.
The correlation with the strength of 1 / 2t up to m was treated as linear, and the calculation formula: 625Nb + 250V + 210Ceq was determined to exceed the value obtained by adding 40 to the plate thickness t (mm). In the case of V, the strength increasing effect of Nb and V in this formula is
The contribution due to the precipitation hardening of V carbonitride is shown. In the case of Nb, the contribution due to the increase in the hardenability is considered in addition to the precipitation hardening of Nb carbonitride. The effect of these elements expected by the direct quenching and tempering process is that they must be solid-solved in the heating stage before hot rolling. In the range of the present invention, all of V can be solid-dissolved, but in the case of Nb, not all S are necessarily solid-dissolved. Therefore, Nb
When it is not possible to secure the solid solution for the entire amount, log {(Nb) ×
(C + 12N / 14)} = 2.26-6770 / (T +
273.15), the amount of solid solution Nb calculated from the relationship of 273.15) is used as the effective Nb amount, and the above calculation formula is calculated as 625 (effective Nb) + 250V + 210Ceq ≧ t + 4.
Must be applied as 0.

The plate thickness range targeted by the present invention is approximately 2
It is in the range of 5 mm to 50 mm. <Heating temperature before hot rolling> In order to homogenize alloy elements and form a solid solution of Nb, it is necessary to set the heating temperature to 1000 ° C or higher. However, if the heating temperature exceeds 1250 ° C, the toughness of the base material cannot be ensured due to the coarsening of the microstructure, so the upper limit is 1250 ° C, preferably 1200 ° C, and more preferably 1150 ° C. <Rolling conditions> The step of hot rolling the uniformly heated steel of the present invention to a predetermined plate thickness is in the austenite single crystal region, and it is necessary to give a cumulative reduction of 20% or more at 1050 ° C or lower and 1000 ° C or higher. This makes the γ grains finer and improves the low temperature toughness. Furthermore, in order to stably secure and improve the toughness of the base material, the acoustic anisotropy of the steel material should be 1.01 to 1.03.
From the viewpoint of being limited to 5, it is necessary to secure a rolling reduction of 5% or more, more preferably 10% or more, and a cumulative rolling reduction of 20% or more in each rolling pass in a temperature range of less than 1000 ° C. and 800 ° C. or more.

That is, as shown in FIG. 1, the low temperature toughness is improved as the acoustic anisotropy of the steel sheet changes from 1.01 to 1.035. After hot rolling, if sufficient time has passed for recrystallization at the temperature before direct quenching is performed, the steel sheet recrystallizes. As a result, the acoustic anisotropy is almost 1.00.
It will be in the range of 0 to 1.005. On the other hand, when the rolling is finished at a low temperature or is directly quenched immediately after the rolling even at a relatively high temperature, the steel sheet is not sufficiently recrystallized and the acoustic anisotropy is 1.005 or more.

The Nb-added steel according to the present invention has a high non-recrystallization temperature. As a result, in the case of practical thick plate hot rolling,
Due to the unavoidable temperature decrease of the steel to be rolled, acoustic anisotropy of more than 1.01 is achieved without rolling at a low temperature that impairs rolling efficiency, and the low temperature toughness is improved.
On the other hand, when rolling is carried out at a low temperature of less than 800 ° C., the acoustic anisotropy increases to 1.035 or more, which may rather hinder the low temperature toughness. Therefore, by setting the rolling finishing temperature to 800 ° C. or higher, preferably 850 ° C. or higher, the acoustic anisotropy is 1.035 or lower, preferably 1.01 or less.
It is set to 5 to 1.025. <Direct Quenching> After completion of hot rolling, it is necessary to forcibly cool and quench the steel sheet having a temperature at least higher than the Ar ₃ transformation point. For forced cooling, a cooling medium such as water must be uniformly applied to the steel sheet to achieve a cooling rate of at least 1 ° C./second or more at a sheet thickness of 1 / 2t. It is also possible to achieve a low yield ratio by providing a temperature difference between the rolling finish temperature and the quenching start temperature (at least the Ar ₃ transformation point or higher). It is also possible to adjust the low yield ratio and acoustic anisotropy by providing a temperature difference between the rolling finish temperature and the quenching start temperature (at least the Ar3 transformation point or higher). <Temperature Temperature> Tempering is carried out below the Ac ₁ transformation point in order to eliminate concerns about performance changes due to welding and SR. In the present invention, tempering has an important meaning of securing the strength of the base metal by precipitation hardening of Nb carbonitride. The reason for limiting the temperature to the Ac ₁ transformation point or lower is that when tempering is performed at a temperature higher than the Ac ₁ transformation point, the strength is remarkably lowered, and the strength as 600 N / mm ² class high tensile steel cannot be secured. The preferable lower limit of the tempering temperature is 570 ° C., particularly 600 ° C.

[0026]

EXAMPLES Tables 1 and 2 show chemical compositions of steels used in Examples of the present invention. In the rightmost column of Table 2, the heating temperature is sufficiently high and X = 625 (effective Nb) + 250V + 210Ceq.
The calculated value in the case where the effective Nb amount in the calculation formula is equal to the total Nb amount added is shown. Steel of the chemical composition shown in Tables 1 and 2 is melted to form a steel ingot, hot rolled to a predetermined plate thickness under the manufacturing conditions shown in Tables 3 to 6, then directly quenched, and further tempered. Treatment was performed to obtain a test steel. The rolling finishing temperatures are all 800 ° C. or higher, rolling is not carried out at an extremely low temperature, and temperature adjustment or the like at the time of rolling is unnecessary. The tempering temperature was in the range of 580 to 680 ° C. Tables 3 to 6
Medium, t: plate thickness, SL: slab heating temperature, FT: rolling finish
Temperature, UST: Acoustic anisotropy, vTs: Charpy impact test
It is the fracture surface transition temperature of the test.

Tensile tests and Charpy impact tests were taken in the direction perpendicular to the rolling direction from the center part of the plate thickness of all the sample steels, and the mechanical properties of the base material as 600 N / mm ² class steel were evaluated.

Further, an oblique y-type weld cracking test was carried out according to JIS Z3158 and a maximum hardness test was carried out according to JIS Z3101 to evaluate the weld cracking susceptibility. In all of these tests, an ultra low hydrogen type welding material for 600 N / mm ² grade steel was used and the atmosphere was 20 ° C.-60.
%, The test piece initial temperature was 25 ° C.

Calculated value for steel No. 1: X = 625 (effective Nb)
+ 250V + 210Ceq = 98 is the sample steel plate thickness: 38
(Mm) and the tensile strength at the center of the thickness of the test steel plate is 57.
Exceeding 0 N / mm ² and good toughness. Further, the Pcm value was as low as 0.18, and no weld crack occurred in the y crack test. No. 1A has a relatively high temperature of 1000 ° C, although the γ grains become finer due to the cumulative reduction below 1050 ° C.
Since the steel sheet was sufficiently recrystallized after the completion of rolling in 1., the value of acoustic anisotropy was 1.003, and the fracture surface transition temperature in the Charpy impact test remained at vTs = -57 (° C). No. 1
In B, the acoustic anisotropy increased to 1.029 due to the cumulative reduction at 1000 ° C or lower (900 ° C at the end of rolling), so that the vTs = -80 (° C) and the toughness are further improved.

The calculated value of steel No. 2 exceeds the thickness of the sample steel to which 40 is added, and shows good strength and toughness. No. 1 whose acoustic anisotropy increased to 1.023. Even if rolling is performed at 1000C or lower at 2B, the acoustic anisotropy becomes 1.001 due to the progress of recrystallization from the end of rolling at 960 ° C, which is a relatively high temperature, to the direct quenching treatment. Fracture transition temperature: stays at vTs = −52 (° C.). The acoustic anisotropy increased due to the decrease in rolling finishing temperature. Two
For B, 2C and 2D, vTs = -70, -63, -70.
(° C) and low temperature toughness are further improved.

The calculated value of steel No. 3 is 83 and the thickness of the test steel plate is 38
It exceeds that obtained by adding 40 to (mm) and shows good strength and toughness. However, No. At 3A, the temperature is relatively high 10
The acoustic anisotropy is 1.003 because the steel sheet was sufficiently recrystallized after the rolling at 00 ° C., and the fracture surface transition temperature in the Charpy impact test: vTs = −67 ° C. 1000 ° C
No. 1 whose acoustic anisotropy increased due to the following rolling.
In 3B and 3C, vTs = -81 (° C), -85 (° C)
And toughness is improved. Reduction of rolling finish temperature (800
C.), the acoustic anisotropy exceeds 1.035. 3D
Has a toughness of vTs = -76 (° C.), and the base metal strength is less than 570 N / mm ² .

No. 1 having an acoustic anisotropy of 1.011. 4A is
Although it has excellent strength and toughness, In 4B, the acoustic anisotropy increases to 1.03, and further improvement in toughness is achieved. Steel Nos. 5 to 12 and 18 are examples containing Mo. Calculated value of each steel number: X = 625 (effective Nb) +250
V + 210 Ceq exceeds the thickness of the test steel sheet, and the tensile strength of the center portion of the thickness of the test steel sheet exceeds 570 N / mm ² and the toughness is also good. However, No. In case of 8K, heating temperature is 1050 ℃
The calculation formula: X = 675 (effective Nb) + 250V +
The value of 210 Ceq is 92. As a result, No. 1 whose acoustic anisotropy is almost equal to 1.034. The strength is lower than 8I and 8J. Similarly, No. Calculation formula in 9D: X = 6
The value of 75 (effective Nb) + 250V + 210Ceq is 87
And the acoustic anisotropy is almost the same. The strength is lower than that of 9B. However, Nb which is not solid-solved during heating of the slab suppresses the coarsening of the heated γ grains. 8
K, No. The low temperature toughness of 9D is dramatically improved.

The Pcm values of steel Nos. 5 to 12 and 18 are 0.
It is as low as 2 or less, and no weld crack occurs in the y crack test. Also, improvement in toughness is achieved as the acoustic anisotropy increases from 1.01 to 1.035. However, when the acoustic anisotropy exceeds 1.035, vTs shifts to the high temperature side and the toughness deteriorates rather.

Steel Nos. 13 to 17 and 20, 21 are examples containing Mo and Cu, Ni, Cr. Calculated value of each steel number: X = 625 (effective Nb) + 250V + 210Ceq
Exceeds the thickness of the test steel sheet, the tensile strength of the center portion of the thickness of the test steel sheet exceeds 570 N / mm ^2, and the toughness is also good. Also Pcm
The value is as low as 0.2 or less, and no weld crack occurs in the y crack test. Toughness has an acoustic anisotropy of 1.01 to 1.0
It is improved as it increases to 35.

Steel Nos. 21 to 25 have chemical compositions outside the scope of the present invention. Steel Nos. 21 and 22 do not contain Nb, and Steel No. 23 has a low C content. Steel No. 24 has a C content of 0.
131 and steel No. 25 have a high Cr content of 0.7% Pc
The m value exceeds 0.2, cracks are confirmed in the y crack test, and the maximum hardness value also exceeds 290.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

[Table 3]

[0039]

[Table 4]

[0040]

[Table 5]

[0041]

[Table 6]

[0042]

INDUSTRIAL APPLICABILITY As described above, the present invention exerts a remarkable effect of providing a tempered type 600 N / mm ² class high strength steel excellent in weld crack susceptibility and low temperature toughness and a method for producing the same.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between low temperature toughness and acoustic anisotropy.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-8-176731 (JP, A) JP-A-8-176727 (JP, A) JP-A-8-209238 (JP, A) JP-A-8- 209242 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21D 8/00-8/10 C22C 38/00-38/60

Claims (3)

(57) [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, Nb:
0.005-0.05%, V: 0.1% or less, Al:
0.005-0.1%, N: 0.0005-0.008
%, Ti <0.005%, B <0.0003%,
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + N
A steel material having a Pcm value defined by i / 30 + Cr / 20 + Mo / 15 + V / 10 + 5B of 0.2 or less, and the balance of iron and unavoidable impurities was 1000 at the time of hot rolling.
After heating to a temperature range of -1250 ° C, 1000 ° C-105
Continued under 20% or more cumulative pressure at 0 ℃, then 1000 ℃
Less than 800 ℃ or more, in the non-recrystallization temperature range , every 1 pass of rolling
The rolling reduction is 5% or more, and the rolling finishing temperature is 800 ° C.
As described above, a tensile strength of 57 is characterized by performing a cumulative reduction of 20% or more, directly quenching at least from the Ar ₃ transformation point, and then tempering at the Ac ₁ transformation point or less.
A method for producing a tempered type 600 N / mm ² class high-strength steel excellent in weld cracking susceptibility of 0 N / mm ² or more and low temperature toughness. 2. Steel: Mo: 0.3% or less, Cu:
Weld cracking susceptibility with a tensile strength of 570 N / mm ² or more according to claim 1, further comprising one or more of 0.5% or less, Ni: 1.5% or less, and Cr: 0.5% or less. A method for producing a tempered 600N / mm ² class high strength steel with excellent low temperature toughness. 3. A steel material having the composition according to claim 1 or 2, using a heating temperature T set in a temperature range of 1000 to 1250 ° C., log { (Nb) × (C + 12/1
4N) } = 2.26-6770 / (T + 273.15)
The amount of solid solution Nb calculated from the relationship of
Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr
X = 625 (effective Nb) consisting of Ceq value defined by / 5 + Mo / 4 + V / 14, effective Nb amount, and V content
+ 250V + 210Ceq satisfies the relation of X ≧ 40 + t (where t indicates the plate thickness (mm) of the steel plate) tensile strength 570 N /
A method for producing a tempered type 600 N / mm ² class high-strength steel excellent in weld cracking susceptibility of mm ² or more and low temperature toughness.