JPH093591A - Extra-thick high-tensile steel plate and method for manufacturing the same - Google Patents

Abstract

(57) [Summary] (Modified) [Purpose] A TS590MPa grade extra-thick high-strength steel sheet used for large steel structures, in which the toughness of the base metal and weld heat affected zone is high Providing steel materials that can be relaxed and methods for manufacturing the steel materials. [Structure] C, Si, Mn, Ni, Cr, Mo, Nb,
B, Ti, sol AL, N, Cu and V are specified steels, which are extra-thick high-strength steel sheets having an average lath length of bainite of 15 μm or less and a ferrite volume ratio of less than 30%. (2) After heating to a temperature of 1000 to 1250 ° C., rolling is performed at a cumulative rolling reduction of 30% or more in the recrystallized austenite region and 50% or more in the unrecrystallized austenite region to obtain a recrystallized austenite. From the state of less than 5% in volume ratio of accelerating to 580 ° C or less (1)
For manufacturing extra-thick high-strength steel sheet.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an extra-thick high-strength steel plate used for large steel structures such as offshore structures, penstocks, bridges, etc., which have extremely strict safety requirements.
The steel sheet of the present invention is suitable for increasing the welding work efficiency.

[0002]

2. Description of the Related Art Ultra-thick high-strength steel sheets used for large steel structures usually contain a high concentration of alloying elements in order to secure strength up to the center. When steel has a high alloy content, the susceptibility to welding cracks increases, and cold cracking tends to occur during welding. Preheating to prevent weld cracking has reduced efficiency and workability, and has been an obstacle to reducing the construction cost of extra-thick high-strength steel sheets.

On the other hand, the extra-thick high-strength steel having a low alloy concentration for preventing weld cracking does not have a completely hardened structure even by the quenching treatment, and has a sufficient base metal performance (strength / toughness balance). Absent.

In order to solve this problem, a method for thermomechanical treatment of high-strength steel of 590 MPa class having both weldability and base material performance is disclosed in JP-A-2-205627 and JP-A-1-963.
No. 29 and Japanese Patent Laid-Open No. 1-149923.

The method disclosed in Japanese Patent Application Laid-Open No. 2-205627 relies on the effect of improving the hardenability of boron (B) to secure the strength. In general, when strong working is performed, the segregation concentration of B at the grain boundaries decreases, and the effect of improving the hardenability of B decreases. Therefore, in order to secure hardenability by this method, the rolling reduction is 30%.
It is necessary to limit the rolling to the following. Such rolling cannot be expected to improve the toughness due to the refinement of the structure.

[0006] The method disclosed in Japanese Patent Laid-Open No. 1-96329 is based on obtaining fine bainite from fine recrystallized austenite. Therefore, this method is limited to the transformation from recrystallized austenite having a small lattice defect density. As a result, since there is no nucleation in the austenite grains, there is a limit to the refinement necessary for improving the toughness.

The method disclosed in JP-A-1-149923 is
The unrecrystallized austenite that has been rolled is transformed to obtain a fine structure. However, a compositional measure to ensure a uniform and fine structure up to the center of the extra-thick high-strength steel sheet and to secure the strength of the center without degrading toughness.
2 points are not enough. Therefore, it is insufficient to secure extremely high toughness at the central portion of the extra-thick high-strength steel sheet. According to the example of Japanese Patent Application Laid-Open No. 1-149923, the sheet thickness actually subjected to the rolling experiment is as thin as 15 mm, and only the cooling rate is cooled according to the extra-thick high-strength steel sheet. Applying this result to the actual ultra-thick high-strength steel sheet as it is,
The effect of rolling will be overestimated. In order to obtain high toughness with an actual ultra-thick high-strength steel sheet, the effect of rolling must be utilized, but in combination with it, means for improving the composition are essential.

All of the above-mentioned prior arts are common in that they use thermo-mechanical treatment in order to achieve both weldability and base metal performance, but try to obtain good performance up to the center of extra-thick high-strength steel sheet. At times, both are insufficient. Since the toughness of extra-thick high-strength steel plate is governed by the toughness of the central part of the plate thickness,
Increasing the toughness of the central portion means that the ultra-thick high-strength steel sheet can be used at lower temperatures or under more severe load conditions.

[0009]

DISCLOSURE OF THE INVENTION The present invention is directed to a high-toughness ultra-thickness high-strength steel sheet having a plate thickness of 50 mm or more, which has low weld cracking susceptibility, using existing equipment and without using an expensive alloy, and a method for producing the same. It was made as an issue to provide. A specific object of the present invention is to provide an extra-thick high-strength steel sheet having the following characteristics (1), which have been difficult to achieve by conventional techniques, and a method for producing the same.

Tensile strength of 590 MPa or more, high toughness at all plate thickness positions of the extremely thick steel plate, for example, -80
V notch Charpy absorbed energy at ℃ -50 J or more,
Ensuring high toughness at the weld. For example, in the case of the weld reproduction heat cycle method (equivalent to heat input 35 kJ / cm), -40 ° C Charpy absorbed energy -50 J or more and reduction of weld cracking susceptibility, for example, when securing the same tensile strength (TS), preheating temperature 25 ° C. Above (compared to conventional steel) decreased.

[0011]

[Means for Solving the Problems] As a result of intensive studies conducted by the present inventors on TS590 MPa class extra-thick high-strength steel having a plate thickness of 50 mm or more, which has both weldability and balance of strength and toughness, the following findings were obtained. It was

Conventionally, in a mixed structure of bainite and ferrite, the bainite lath length may become fine, but in the case of a bainite-based structure, it is difficult to make the bainite lath length fine. Met. Particularly in the case of extra-thick high-strength steel, it is difficult to make bainite fine because the effect of rolling does not easily reach the central portion of the plate thickness. The present inventors have found that even in an extremely thick structure mainly composed of bainite, the lath length of bainite can be made fine by using Nb and Ti at the same time and utilizing the rolling process, and Cu which causes precipitation hardening can be obtained. It was also found that if at least one or both of V and V are contained according to the amount of C, the strength of the central portion can be secured without degrading the toughness.

Only by combining these and
It can satisfy the high requirements for weldability and balance of strength and toughness mentioned above.

Here, the gist of the present invention resides in the following ultra-thick high-strength steel sheet and a method for producing the same.

(1) By weight ratio, C: 0.02 to 0.1
5%, Si: 0.30% or less, Mn: 0.6 to 2.0
%, Ni: 1.50% or less, Cr: 1.0% or less, M
o: 1.0% or less, Nb: 0.01 to 0.07%, B:
0.002% or less, Ti: 0.005 to 0.030%,
sol Al: 0.08% or less, N: 0.006% or less, C
u and V are in the range of 0.02 to 0.20 as C (%) × (Cu (%) + 5V (%)), and the balance is F.
An extremely thick high-strength steel sheet having a composition of e and unavoidable impurities, wherein the average lath length of bainite in the structure is 15 μm or less and the volume fraction of ferrite is less than 30%.

(2) A steel slab having the composition according to claim 1 is heated to a temperature of 1000 to 1250 ° C., and then 30% or more in the recrystallized austenite region and 50 in the unrecrystallized austenite region. Rolling with a cumulative reduction of at least%,
From the state where the volume ratio of recrystallized austenite is less than 5%, 5
The average lath length of bainite in the structure was 15 μm by accelerated cooling to a temperature of 80 ° C. or lower and transformation.
The method for producing an ultra-thick high-strength steel sheet, characterized in that the volume ratio of ferrite is less than 30%.

[0017]

First, the reasons for limiting the composition of steel will be described.

The C concentration is 0.02% or more and 0.15% or less. C is an element effective for increasing the strength, and for that purpose 0.02% or more is necessary. However, the upper limit must be 0.15% from the viewpoint of ensuring toughness and preventing weld cracking.

The Si concentration is limited to 0.30% or less. S
i is an element effective for deoxidation, but its content is 0.30
%, The low temperature toughness of the heat-affected zone of the weld is deteriorated, so the upper limit must be 0.30%. The Si content may be substantially 0, but in order to reduce Si to 0
Therefore, a small amount of Si that remains after deoxidation, for example, about 0.01% is preferable as the lower limit.

The Mn concentration must be 0.6% or more and 2.0% or less. Mn is an element effective for increasing strength,
For that purpose, 0.6% or more is required. However, if the content exceeds 2.0%, the toughness deteriorates, so it is necessary to set the upper limit to 2.0%.

Ni may not be intentionally contained. However, it is desirable to insert it when the plate thickness is extremely thick. When added, the content is preferably 0.25 to 1.50%. Ni has the effect of improving toughness, but
If it does not contain 0.25% or more, the effect will not appear,
This is because if it exceeds 1.50%, the increase in strength and the improvement in toughness commensurate with the cost increase cannot be obtained.

No Cr may be added. When it is necessary to add it for strengthening, its content should be 0.25.
It is good to be set to 1.0%. The lower limit is preferably 0.25% because if it is not contained any more, it does not work for increasing the strength, and if it is 1.0% or less, the weldability is not deteriorated.

Mo does not have to be contained as long as the plate thickness is up to about 50 mm. However, when the plate thickness exceeds 50 mm and becomes thick, it is preferable to contain 0.08 to 1.0% in order to secure the strength. However, if contained too much, the toughness is lowered and the weldability is lowered, so the upper limit is 1.0%.
It is necessary to When adding, the content is 0.08
The reason why it is preferable to set the ratio to be not less than% is that the effect is not remarkable unless it is more than this.

The Nb concentration is in the range of 0.01% to 0.07%. Nb is an essential element for increasing the hardenability and strength of steel in a solid solution state and for refining the structure by the effect of rolling, and for that purpose, 0.01% or more is necessary. But 0.07
If it exceeds 0.1%, cracks frequently occur in the continuously cast slab, so it is necessary to set the upper limit to 0.07%.

The Ti concentration is adjusted within the range of 0.005% to 0.03%. Ti not only increases the strength of the steel by the solid solution state and precipitation, but also suppresses the austenite grain growth at the time of heating the slab. In the case of ultra-thick high-strength steel in which crystal grains cannot be refined only by rolling, it is an effective element for refining the lath length of bainite. For that purpose, 0.005% or more is required. Further, since the hardness is lowered by preventing the crystal grain coarsening of the weld heat affected zone, it is effective for improving the toughness of the weld heat affected zone and lowering the preheating temperature. In Nb-containing steel, a small amount of Ti is effective in suppressing cracking of the surface of the continuously cast slab promoted by Nb. These effects are exhibited at 0.005% or more. However, the Ti content is 0.03
%, The toughness deteriorates, so the upper limit is 0.03%.
It is necessary to

The sol Al concentration is 0.08% or less. A
Although l is an element effective as a deoxidizing agent, excessive addition of Al causes surface scratches, so the upper limit of the content as sol Al must be 0.08%. It is preferably 0.001% or more in order to prevent the generation of pinholes after solidification.

B may not be added. However, since B is an element that improves the hardenability and the accompanying increase in strength, when the plate thickness is large, 0.0003 to 0.0020%
It is desirable to include it. This is because the minimum amount of 0.0003% is necessary to fully exert the effect of B. However, if it exceeds 0.0020%, the toughness deteriorates, so it is necessary to set the upper limit to 0.0020%.

The N concentration is limited to 0.006% or less. N
Is effective in refining the crystal grains by combining with Al to form a nitride, but an excessive amount of N impairs the toughness of the welded portion.
The upper limit needs to be 0.006%. When it is expected that the crystal grains become finer, 0.0015% or more is contained.

The Cu and V concentrations are C (%) × (Cu
(%) + 5V (%)) 0.02 or more, and 0.2
Adjust within the following range. At this time, either one of Cu and V or the simultaneous inclusion of both may be used. Cu
And V are elements effective for increasing the strength of the central portion of the ultra-thick high-tensile steel plate having a plate thickness of 50 mm or more, which is the object of the present invention. C
If the appropriate amount is added according to the concentration, the deterioration of toughness is small. C (%) × (Cu (%) + 5V (%)) as 0.
02 or more is required. However, a content exceeding 0.2 must be avoided because the deterioration of toughness is remarkable. In this case, in addition to the restriction that Cu and V are within the range of the above formula, it is preferable to set each of Cu and V within the following ranges. Cu may not be added in some cases, but when it is added, the content is preferably 0.20 to 2.0%. This is because if it is not contained in an amount of 0.25% or more, remarkable precipitation hardening of Cu cannot be obtained, and if it exceeds 2.0%, surface scratches often occur. V may not be added. However, when it is added, its content is preferably 0.01 to 0.4%. This is because if it is not more than 0.01%, a large effect of increasing the strength cannot be obtained, and if it exceeds 0.4%, the toughness may be significantly deteriorated.

Next, the reason for limiting the organization is as follows.

When steel having a plate thickness of 50 mm or more and having the above-mentioned composition is accelerated and cooled, a structure mainly composed of bainite can be obtained although there is a change depending on rolling conditions. The average lath length of the bainite is set to 15 μm or less because it is necessary to set the average lath length to 15 μm or less in order to suppress the adverse effect of bainite on the toughness of the base material. In order to obtain even better toughness, it is desirable that the average lath length of bainite be 10 μm or less. Here, the average lath length of bainite is the average of the longitudinal lengths of the individual laths of the bainite structure that appear when an arbitrary cross section of steel is observed with a microscope or the like. By limiting the average lath length of bainite, the adverse effect on toughness can be reduced while receiving the effect of bainite, which has a strong strength improving effect.

The reason why the ferrite volume ratio is less than 30% is that if the ferrite volume ratio is higher than this, it is difficult to secure the target strength. In the case of high strength, the ferrite volume ratio may be zero,
When increasing strength while obtaining toughness, it is desirable to contain about 10% of ferrite.

With such a structure, it is possible to obtain an extra-thick high-strength steel sheet having both tensile strength and toughness of 590 MPa or more while keeping the alloy concentration low, that is, the welding cold cracking susceptibility low.

The structure of bainite and the like described above is
It may be tempered bainite or the like, or may be structure such as bainite not tempered.

The reason why each condition of the manufacturing method is limited will be described below.

The billet heating temperature has an upper limit of 1250 ° C. in order to prevent coarsening of austenite crystal grains, and a lower limit in order to form a solid solution of Nb which is effective in refining crystal grains during rolling and precipitation strengthening. The temperature is 1000 ° C.

Fine recrystallized austenite grains must be obtained together with the effect of Ti by rolling at a cumulative rolling reduction of 30% or more in the austenite recrystallization temperature range. If the rolling reduction is smaller than this, fine bainite cannot be finally obtained.

Here, the accumulation of the cumulative rolling reduction means that the recrystallization temperature region or the unrecrystallized temperature region described later is independently accumulated. The reason why rolling with a cumulative reduction of 50% or more is performed even in the non-recrystallization temperature range and accelerated cooling is performed from the state where recrystallized austenite is less than 5% is as follows.

Due to the lattice defects introduced by rolling,
(A) Bainite transformation often occurs also in the grains, and (b) growth thereof is suppressed by some of the lattice defects.
Bainite becomes fine for two reasons. Fine bainite cannot be obtained under any of the conditions that the cumulative rolling reduction in the unrecrystallized temperature range is 50% or more and the recrystallized austenite is less than 5%. Nb described above expands the unrecrystallized temperature range of austenite to a high temperature, facilitates rolling in the unrecrystallized temperature range, and promotes miniaturization of bainite. In the steel of the composition of the present invention, recrystallized austenite is added to 5
In order to cool from a state of less than%, the time until cooling after rolling needs to be 100 seconds or less.

Here, recrystallization means that new austenite grains having a low dislocation density are newly generated in the austenite grain boundaries which have been deformed by rolling or in the deformation zone introduced during the deformation. The higher the temperature, the easier it will recrystallize. The non-recrystallization temperature range refers to a temperature range in which recrystallization does not occur after rolling. Immediately after rolling, recrystallization progresses with time even in an unrecrystallized state, and recrystallization occurs as the cumulative rolling strain increases. In the present invention, the rolling temperature range is referred to as a non-recrystallization temperature range unless it is recrystallized within about 15 seconds (approximately the time until the next rolling) after rolling.

Accelerated cooling after rolling is necessary to secure strength up to the center of the extra-thick high-strength steel sheet. The cooling rate is preferably 10 ° C./s or more. 580 ° C
The reason for cooling to below is that if the cooling is stopped at a temperature higher than that, the volume ratio of ferrite increases due to recuperation and the required strength cannot be obtained. After the temperature becomes 580 ° C or lower, it can be cooled to room temperature as it is, but
Desirably, the cooling is stopped at a temperature of 350 [deg.] C. or higher, and then the cooling is performed. This is because when the concentration of hydrogen contained in steel is high, hydrogen accumulates on inclusions and the like to generate hydrogen defects, so that the release of hydrogen is promoted during cooling.

The steel of the present invention can obtain sufficient strength and toughness without tempering. However, in order to obtain higher yield strength and toughness, tempering is performed with Ac ₁ or less.

By tempering, precipitation hardening elements such as Cu or V which are effective for securing the strength can be precipitated, and transformation strain can be removed to further improve the toughness.

[0044]

EXAMPLES Table 1 is a listing of steels having compositions within the scope of the present invention and steels having compositions outside the ranges for comparison. Steel pieces obtained by melting and casting these steels and subjected to thermomechanical treatment were used as test steel sheets. Table 2 is a list showing the heat treatment conditions, the plate thickness and the metal structure obtained thereby. Board thickness is 50 and 70 m
There are two types, m.

[0045]

[Table 1]

[0046]

[Table 2]

The plate thickness of these steel plates is 1/4 t and 1/2
A test piece was sampled from the t portion and subjected to a tensile test and a 2 mmV notch Charpy impact test. In order to evaluate the weldability, a welding heat cycle (maximum heating temperature 1350 ° C.) corresponding to a heat input of 35 kJ / cm was applied to the Charpy test. The preheating temperature for stopping weld cracking is the y-type weld cracking test (J
It was evaluated according to IS Z 3158). Table 3 is a list showing the performances obtained by those tests. The base metal performance of the steel of the present invention is 59 at TS at the plate thickness position 1 / 2t.
Absorbed energy in Charpy impact test at 0 MPa or more and -80 ° C is 215 J or more, and plate thickness is 50 mm.
It has extremely excellent strength and toughness as the above ultra-thick high-strength steel. FIG. 1 is a drawing showing the relationship between TS and vE- ₈₀ at 1/2 t part of the present invention steel and comparative steel. The number attached to each plot in the figure represents the test No. in Table 3.
It is clear that the steel of the present invention is superior to the comparative steel.

As shown in Table 3, the weld thermal cycle reproduction part of the steel of the present invention shows absorbed energy of 200 J or more in the Charpy impact test at -40 ° C, which is toughness superior to that of the comparative steel. FIG. 2 is a drawing showing the relationship between the crack stop preheating temperature and the 1 / 2t part TS in the y-type weld cracking test. In the figure, it is clear that the preheating temperature of the steel of the present invention is low when comparing the same TS. The number given to each plot in the figure is the test number in Table 3. The performance of the comparative example is inferior to that of the example of the present invention because the comparative example lacks any condition of the composition and structure or the composition and the manufacturing method within the scope of the present invention.

[0049]

[Table 3]

[0050]

EFFECTS OF THE INVENTION The steel sheet of the present invention has a tensile strength of 590 MPa or more and a sheet thickness of 50 by having the above-mentioned composition and structure.
Excellent in toughness up to the center of the plate thickness at mm or more, and low susceptibility to weld cracking. By using the steel of the present invention, preheating upon welding can be reduced, so that construction cost can be reduced. The steel of the present invention can be manufactured relatively easily and inexpensively by the above-mentioned method.

[Brief description of drawings]

FIG. 1 is a drawing showing a relationship between TS and vE-80 in a 1 / 2t portion of a steel sheet of the present invention and a comparative steel sheet having a plate thickness of 50 mm.

FIG. 2 is a drawing showing a relationship between a TS (1 / 2t part) having a plate thickness of 50 mm and a weld crack stop preheating temperature of the steels of the present invention and comparative steels.

Claims (2)

[Claims] 1. A weight ratio of C: 0.02 to 0.15%,
Si: 0.30% or less, Mn: 0.6 to 2.0%, N
i: 1.50% or less, Cr: 1.0% or less, Mo: 1.
0% or less, Nb: 0.01 to 0.07%, B: 0.00
2% or less, Ti: 0.005 to 0.030%, sol A
1: 0.08% or less, N: 0.006% or less, Cu and V are in the range of 0.02 to 0.20 as C (%) × (Cu (%) + 5V (%)), and the balance Is a steel composed of Fe and unavoidable impurities, the average lath length of bainite in the structure is 15 μm or less, and
A very high-strength steel sheet having a ferrite volume ratio of less than 30%. 2. A steel slab having the composition according to claim 1
After heating to a temperature of 000 to 1250 ° C., rolling at a cumulative rolling reduction of 30% or more in the recrystallized austenite region and 50% or more in the unrecrystallized austenite region is performed to obtain a volume ratio of the recrystallized austenite. From less than 5% to 580 ° C
A method for producing an extremely thick high-strength steel sheet, characterized in that the average lath length of bainite in the structure is 15 μm or less and the volume fraction of ferrite is less than 30% by accelerated cooling to the following temperature and transformation.