JP3468168B2 - High-strength steel sheet with excellent economy and 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 high-strength steel sheet excellent in economy and toughness. More specifically, the present invention relates to a high-tensile steel sheet having excellent economical efficiency and toughness, which is suitable for being applied to a structural member of a structure machined by welding such as arc welding or beam welding.

[0002]

2. Description of the Related Art Up to now, various steel materials have been used as structural members for welded structures such as line pipes. Among these steel materials, high-strength steel materials, in which a small amount of alloying elements are added to low carbon steel to improve toughness and weldability, aim to reduce welding man-hours and transportation costs by reducing the plate thickness and diameter. Therefore, we have expanded the scope of its application.

However, in this high-strength steel material, as the amount of carbon, the amount of alloying elements added, and the variation of heat input increase, in addition to the toughness of the base metal itself, the weld metal and weld heat-affected zone Toughness is also becoming more important. Therefore, many inventions have been proposed for improving the toughness of the weld metal and the heat-affected zone of high-strength steel.

For example, Japanese Patent Laid-Open No. 7-278736 discloses
C: 0.01 to 0.25% (in this specification, "%" means " mass %" unless otherwise specified), Si:
0.6% or less, Mn: 0.3 to 3.0%, N: 0.0005 to 0.0100
%, O: 0.0010 to 0.0070%, Al: 0.02% or less, and further C
r: 0 to 1.5%, Mo: 0 to 1.5%, Cu: 0 to 1.5%, N
i: 0 to 3.0%, Nb: 0 to 0.5%, V: 0 to 0.5%, 1
Or more, and B: 0 to 0.0020%, P in impurities: 0.03% or less and S: 0.01% or less, and Al in steel
-Mn oxide dispersed particles are 0.2 to 20 μm, the average density is also 4 to less than 1000 per 1 mm ² , the relation between Al and Mn in dispersed particles is (Al + Mn) ≧ 40 mol%, and Al / Mn = 1.0. ~
A steel material having excellent toughness in the heat-affected zone of welding which satisfies both of less than 5.0 has been proposed. That is, the invention according to this proposal, by specifying the number of Al-Mn oxide dispersed particles that are the nuclei for the formation of acicular ferrite to an appropriate value,
To improve the toughness of steel materials.

However, the invention according to this proposal is
As a matter of course, it could not be applied to a steel material having a high hardenability and not forming acicular ferrite and having a strength higher than that of bainite. In addition, since it is difficult to control the inclusions delicately in the actual melting process, the amount of inclusions varies greatly from lot to lot, and stable characteristics cannot be obtained on an industrial scale.

On the other hand, Japanese Patent Laid-Open Publication No. 54-132421 discloses that a steel having a very small carbon equivalent and having B and Ti added is rolled under specific conditions so that both the weldability and the low temperature toughness are improved. A method for producing an excellent bainite high-strength thick steel plate is disclosed in Japanese Patent Laid-Open No. 9-249934 and C: 0.001.
~ 0.02%, Mn: 1.0-3.0%, Ti: 0.005-0.20%,
B: 0.0003 to 0.0050%, Cu: more than 2.0% and 3.0% or less, and
High-strength steel materials containing Al: 0.10% or less have been proposed. Each of the inventions according to these proposals secures strength by reducing the amount of C to about 0.005 to 0.03% and adding a trace amount of B, whereby the weld metal and welding heat of steel materials having a bainite structure are obtained. The toughness of the affected area is improved.

[0007]

SUMMARY OF THE INVENTION
In the inventions proposed by Japanese Patent Laid-Open No. 54-132421 and Japanese Patent Laid-Open No. 9-249934, B added to secure the toughness segregates alone at the grain boundaries, which hinders the improvement of the toughness, resulting in high tensile strength. It is not possible to satisfy the above-mentioned requirements for the toughness of steel materials.

Further, in order to secure a desired strength with a C content of about 0.005 to 0.03%, it is necessary to add a large amount of alloying elements. For this reason, the manufacturing cost increases, which is a serious problem especially in the case of high-strength steel materials for which cost reduction is strongly demanded.

A number of inventions have been proposed so far for a high-strength steel material having a C content of about 0.005 to 0.03% and excellent in toughness of the weld metal and the weld heat affected zone. For example, JP-A-52-131918 discloses that the C content is
An invention has been disclosed in which high toughness is obtained by performing unrecrystallized region rolling using a 0.02 to 0.10% slab. But,
According to these inventions, the toughness of the heat-affected zone of the joint section welded under severe conditions is, for example, the shock absorption energy vE.
_-40 (atFL) ≈ 10 to 50 (J) may be obtained, which is insufficient.

An object of the present invention is to provide a high-strength steel sheet excellent in economy and toughness and a method for producing the same, specifically, a structure of a structure to be worked by welding such as arc welding or beam welding. It is an object of the present invention to provide a high-strength steel sheet excellent in economic efficiency and toughness suitable for being applied to a member.

More specifically, the present invention relates to the FL of the base metal part.
The impact absorption energy vE- ₄₀ (atFL) at -40 ° C of the joint part of the notch Charpy impact test is more than 100 (J), and it is economically suitable for use as a structural member such as a pressure vessel or offshore structure. And to provide a high-tensile steel plate excellent in toughness.

[0012]

Means for Solving the Problems As a result of intensive studies to solve the above problems, the present inventor can obtain the novel findings (1) to (6) listed below. Knowledge
Based on (1) to (6), it is possible to obtain a high-strength steel sheet that obtains good weld toughness without significantly reducing the C content, and thus sufficiently increase the manufacturing cost of the high-strength steel sheet. The present invention has been completed based on the finding that the above can be suppressed.

(1) When the same bainite-based structure is formed, it is effective to improve the toughness by suppressing the C content as low as possible. That is, the toughness improvement is affected by the production amount of bainite, particularly MA (island martensite) existing between laths of the upper bainite, and the toughness is improved by reducing the production amount of MA.

The term "upper bainite" means cementite or MA constituent (carbon-enriched retained austenite or martensite, or a mixture of both) at the interface of lath bainitic ferrite.
Alternatively, it means a structure in which both are present, and means all bainite structures except lower bainite in which cementite is arranged in a point sequence inside bainitic ferrite. It also includes the structure after tempering. When the plate thickness is thick and the cooling rate is low, or when the water cooling stop temperature is high and the air cooling time after the water cooling is long is long, the apparent morphology changes from lath-like to granular due to coalescence of bainitic ferrites, This case is also included.

(2) If the contents of Si and Al, which have the effect of suppressing the precipitation of cementite during the bainite transformation, are suppressed as low as possible, the toughness can be improved by suppressing the amount of MA produced. In particular, since the reduction of the Al content is an important point for suppressing the MA production amount, Al is not added. However, during refining, addition of Si or Al is indispensable for deoxidation, so the minimum required amount of Si contained for deoxidation is allowed.

(3) Since MA is mainly formed in the upper bainite in a large amount, the ratio F of the upper bainite in the base metal is F.
By setting a specific relationship between the _BU and the C content and the Si content, it is possible to reliably reduce the MA production amount and improve the toughness.

In order to obtain the volume ratio of the upper bainite, observation with a scanning electron microscope or observation with a transmission electron microscope is performed. In particular, the scanning electron microscope is useful because it can observe not only locally but over a relatively wide area.
To obtain the mixing ratio of upper bainite in the entire metallographic structure using a scanning electron microscope, 10 to 30 times 1000 to 2000 times is required.
It is desirable to take an average over the field of view. Although a transmission electron microscope enables precise measurement, it is inevitable to increase the magnification. Therefore, 50 to 100 times 10,000 times
It is desirable to average the field of view.

(4) B segregates at the grain boundaries to improve hardenability, but deteriorates toughness, so that the above-mentioned JP-A-54-13 is used.
Unlike the inventions proposed by JP-A-2421 and JP-A-9-249934, no addition is made.

(5) The alloying element to be added is economical
Alloying elements other than Cr, with Cr at the center, are appropriately added to adjust the hardenability.

(6) The carbon equivalent Ceq, which is known as an index of hardenability, is adjusted between 0.35 and 0.45% so as to have sufficient strength of, for example, 50 to 60 kgf / mm ² .

In the present invention, C: 0.03 to 0.08 %, S
i: 0.10 to 0.40%, Mn: 0.6 to 2.0%, Cr: 0.05 to 1.6
%, Nb: 0.005-0.1%, Ti: 0.005-0.1%, if necessary Cu: 1.0% or less, Ni: 2.0% or less, M
o: 1.0% or less, V: 0.1% or less, W: 1.0% or less, REM
: 0.02% or less and Ca: 0.02% or less, and one or more selected from the group consisting essentially of Al and B and having a carbon equivalent Ceq defined by the following formula (1): in the range of from 0.30 to 0.45%, having a steel composition comprising the balance Fe and unavoidable impurities, ferrite bainite structure caused a with C amount and amount of Si satisfies the following formula (2), FL notch Charpy impact By test −
It is a high-strength steel sheet with excellent economic efficiency and toughness, characterized by having an impact absorption energy at 40 ° C of more than 100 (J) .

[0022]

[Equation 2]

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of a high-strength steel sheet excellent in economy and toughness according to the present invention will be specifically described below with reference to the accompanying drawings . First, the reason for limiting the composition of the high-strength steel sheet excellent in economical efficiency and toughness according to the present invention will be described.

(C: 0.03 to 0.10%) C is the most effective element for improving strength and is an inexpensive element, but the C content is 0.1.
If it is less than 03%, it becomes necessary to increase the content of elements other than C to compensate for the lack of strength, and as a result, the economical efficiency is impaired. On the other hand, if the C content exceeds 0.10%, the amount of MA increases and the toughness of the joint is significantly impaired. Therefore, in the present invention, the C content is limited to 0.03% or more and 0.10% or less. From the same viewpoint, it is desirable that the upper limit of the C content be 0.08% and the lower limit thereof be 0.03%.

(Si: 0.10 to 0.40%) Si has the effect of forming MA and therefore needs to be controlled as low as possible, but at least it is necessary for deoxidation during refining, so at least
Contains 0.10% or more. However, if the Si content exceeds 0.40%, the toughness is significantly deteriorated through the formation of MA. Therefore, in the present invention, the Si content is limited to 0.10% or more and 0.40% or less. From the same viewpoint, it is desirable that the upper limit of the Si content be 0.25% and the lower limit thereof be 0.10%.

(Mn: 0.6 to 2.0%) Mn is contained in an amount of 0.6% or more to secure the strength. However, if the Mn content exceeds 2.0%, the toughness and arrestability are significantly deteriorated. Therefore, in the present invention, the Mn content is limited to 0.6% or more and 2.0% or less. From the same viewpoint, the upper limit of Mn content is 1.4%,
The lower limit is preferably 0.7%.

(Cr: 0.05 to 1.6%) Cr has a function of enhancing hardenability and is inexpensive, so it is added for the purpose of securing strength. When the Cr content is 0.05% or more, it is effective in increasing strength. On the other hand, if the Cr content exceeds 1.6%, the toughness and weldability are deteriorated. Therefore, in the present invention, the Cr content is limited to 0.05% or more and 1.6% or less. From a similar perspective,
It is desirable that the upper limit of the Cr content be 0.9% and the lower limit thereof be 0.2%.

(Nb: 0.005 to 0.1%) By containing 0.005% or more of Nb, crystal grain coarsening is suppressed during slab heating, and the unrecrystallized region is enlarged by the pinning effect of Nb (C, N). In particular, the toughness of the base material is improved. On the other hand, Nb
If the content exceeds 0.1%, the toughness is significantly impaired. Therefore, in the present invention, the Nb content is limited to 0.005% or more and 0.1% or less. From the same viewpoint, the upper limit of Nb content is 0.04%,
The lower limit is preferably 0.008%.

(Ti: 0.005 to 0.1%) By containing 0.005% or more of Ti, grain coarsening is suppressed during heating of the slab, and it contributes to the increase in strength through the formation of precipitates and also the formation of AlN. Improve the surface quality of the slab through suppression. On the other hand, if the Ti content exceeds 0.1%, the toughness is significantly impaired. Therefore, in the present invention, the Ti content is 0.005% or more.
Limited to 0.1% or less. From the same viewpoint, it is desirable that the upper limit of the Ti content be 0.02% and the lower limit thereof be 0.008%.

Further, in the present invention, in order to further improve the strength, at least one of Cu, Ni, Mo, V, W, REM and Ca may be contained as an optional additional element. Hereinafter, these optional additional elements will be described.

(Cu: 1.0% or less) Cu is a precipitation strengthening element and is effective in increasing strength. However, the Cu content is 1.0
If it exceeds%, the surface properties of the steel sheet are significantly deteriorated due to the generation of scale, and further the toughness is deteriorated. So Cu
When added, the content is preferably limited to 1.0% or less.

(Ni: 2.0% or less) Ni is an element effective for improving both strength and toughness. But Ni
If the content exceeds 2.0%, the cost will increase. Therefore, when Ni is added, its content is preferably limited to 2.0% or less.

(Mo: 1.0% or less) Mo is an element effective in improving the strength, but if the Mo content exceeds 1.0%, the toughness is impaired. Therefore, when adding Mo, the content is 1.
It is desirable to limit it to 0% or less.

(V: 0.1% or less) V is an element effective for improving strength, but if the V content exceeds 0.1%, toughness is impaired. Therefore, when V is added, its content is 0.
It is desirable to limit it to 1% or less.

(W: 1.0% or less) W is an element effective for improving the strength, but if the W content exceeds 1.0%, the toughness is impaired. Therefore, when W is added, its content is 1.
It is desirable to limit it to 0% or less.

(REM: 0.02% or less) REM is effective for improving the strength, but if the REM content exceeds 0.02%, the economical efficiency is impaired. Therefore, when REM is added, its content is preferably limited to 0.02% or less.

(Ca: 0.02% or less) Ca is effective in improving the toughness through the effect of controlling the morphology of inclusions, but the Ca content is 0.
If it exceeds 02%, the toughness is impaired. Therefore, when Ca is added, its content is preferably limited to 0.02% or less.

In the present invention, neither Al nor B is substantially contained. That is, Al has the effect of delaying the precipitation of cementite, and consequently increases the amount of MA produced. In ordinary steel, it is added to achieve deoxidation and refinement of crystal grains during heating, but in the present invention, Al is not added in order to reduce the amount of MA as much as possible. On the other hand, B segregates at the austenite grain boundaries during quenching, delays ferrite transformation and improves hardenability. However, B, which is a solid solution in the matrix, significantly impairs the toughness of the matrix, so B is not added.

However, although Al and B may be contained in the raw material iron ore in an extremely small amount, they are inevitably contained, and the phrase "substantially not contained" as used herein means that this inevitable small amount is contained. The purpose is to do.

(Carbon equivalent Ceq: 0.30 to 0.45%) Carbon equivalent C
eq has been widely used so far as an index showing hardenability. By adjusting the carbon equivalent Ceq to 0.30 or more and 0.45 or less, the strength level targeted by the present invention: 50 to 60 kgf /
mm ² can be realized. Therefore, in the present invention, the carbon equivalent Ceq
Is limited to 0.30 to 0.45%.

Compositions other than the above are Fe and inevitable impurities. The high-strength steel sheet having excellent toughness according to the present invention has a C content and a Si content satisfying the expression (2) and exhibits a ferrite-bainite structure.

That is, the high-strength steel sheet according to the present invention is
Since Al is not added, the relationship defined by the formula (2) between the C content and the Si content and the upper bainite ratio F _BU (%), that is, C ≦ 3.2 / (Si ^0.32 × F _BU By satisfying the condition (1), it is possible to reduce the amount of MA produced and surely improve the toughness.

1 (a) to 1 (c), the plate thickness is 24
When a FL notch Charpy impact test was performed on a welded part consisting of mm, X groove and SAW 3.0 kJ / mm, C
FIG. 1A is a graph showing the influence of the content and Si content on the FL notch Charpy absorbed energy. FIG. 1A shows the case where the upper bainite ratio F _BU is 100%.
(b) shows the case where the upper bainite ratio F _BU is 90%, and further, FIG. 1 (c) shows that the upper bainite ratio F _BU is 80%.
Is shown.

As shown in the graphs of FIGS. 1 (a) to 1 (c), C: 0.03 to 0.10%, Si: 0.10 to 0.40% and C ≦ 3.
By satisfying 2 / (Si ^0.32 × F _BU )
It is possible to secure the impact absorption energy vE _-80 (atFL) at 40 ° C of 100 (J) or more.

The high-strength steel sheet according to the present invention has a shock absorption energy vE _-40 (atFL) at −40 ° C. in the FL notch Charpy impact test of the joint portion of more than 100 (J), and a sufficient weld metal or weld. Has the toughness of the heat-affected zone. Therefore, this high-tensile steel plate can be sufficiently used as a structural member such as a pressure vessel or a marine structure. Further, since the high-tensile steel sheet according to the present invention suppresses the content of expensive alloy elements as much as possible, it is possible to suppress an increase in cost.

In order to manufacture the high-strength steel sheet according to the present invention, it is necessary to reliably control the upper bainite ratio. To this end, (i) when securing strength and toughness by water cooling, the slab heating temperature, rolling finishing temperature, water cooling start temperature and stop temperature, and (ii)
When strength and toughness are secured by off-line heat treatment, the slab heating temperature, the rolling finishing temperature, the heat treatment temperature, the holding time, etc. may be appropriately controlled.

[0047]

EXAMPLE A slab having a steel composition and a carbon equivalent Ceq shown in Table 1 was subjected to slab heating, hot rolling and water cooling under appropriate conditions to obtain a plate thickness having an upper bainite ratio F _BU shown in Table 1. 24 mm thick steel plates 1 to 30 were obtained. In addition, the manufacturing conditions of the thick steel plate 1 are collectively shown in Table 3 and illustrated.

[0048]

[Table 1]

[0049]

[Table 2]

[0050]

[Table 3]

Thick steel plate No. 1 , 2, 4 to thick steel plate in Table 1
No. 18 is a steel type that satisfies the composition specified in the present invention, and thick steel plate No. 3 and thick steel plate No. 19 to thick steel plate No. 30 are steel types that do not satisfy the composition specified in the present invention. In addition, Table 1
The * mark in indicates that the scope of the present invention is not satisfied.

Sample No. 1 to Sample No. 30 were cut out from these thick steel plate No. 1 to thick steel plate No. 30 and the structure was observed using a scanning electron microscope or a transmission electron microscope to obtain the upper bainite. Measure the ratio F _BU to 3.2 / (Si ^0.32 × F
_BU )) and FL notch Charpy impact test to determine the impact absorption energy vE of the joint at −40 ° C.
_-40 (atFL) was measured. The welding conditions for sample No. 1 to sample No. 30 are groove shape: X groove, welding method: SA
W, heat input: 3.0 kJ / mm. 3.2 / (Si ^0.32 × F _BU ) is shown in Table 1, and shock absorption energy vE _-40 (atFL) is shown in Table 2. In addition, ** in Table 2 indicates that the data is unsatisfactory.

Sample No. 1 , 2, 4 to Sample No. 1 in Table 2
No. 8 is an example of the present invention in which the composition of the steel type, the carbon equivalent Ceq, and the structure all satisfy the scope of the present invention. Sample No. 1 ,
In Nos. 2 and 4 to Sample No. 18, the shock absorption energy vE _-40 (atFL) at -40 ° C of the joint was more than 100 (J),
Satisfy the toughness target of weld metal and heat affected zone. Therefore, Sample Nos . 1 , 2, 4 to Sample No. 18 can be suitably used as structural members such as pressure vessels and marine structures.

On the other hand, in sample No. 19, both the C content and the carbon equivalent Ceq were below the range of the present invention, so the target value of the weld zone toughness could not be satisfied. Sample No. 20 is
Since the Si content exceeds the range of the present invention, the target value of weld toughness could not be satisfied.

Sample No. 21 has Mn content and carbon equivalent C
Since both eq are below the range of the present invention, the target value of weld zone toughness could not be satisfied. Since the Cr content and the carbon equivalent Ceq of Sample No. 22 both exceeded the ranges of the present invention, the target value of weld toughness could not be satisfied.

In sample No. 23, the Nb content was below the range of the present invention, so the target value of the weld zone toughness could not be satisfied.
Sample No. 24 has a Ti content below the range of the present invention,
The target value of weld toughness could not be satisfied.

In sample No. 25, the Cu content exceeded the range of the present invention, and therefore the target value of weld zone toughness could not be satisfied.
Sample No. 26 has a Ni content exceeding the range of the present invention,
The target value of weld toughness could not be satisfied.

Sample No. 27 had a Mo content and a carbon equivalent C
Since both eq exceed the range of the present invention, the target value of weld zone toughness could not be satisfied. In sample No. 28, the V content exceeded the range of the present invention, so the target value of weld zone toughness could not be satisfied.

In sample No. 29, the W content exceeded the range of the present invention, and therefore the target value of weld zone toughness could not be satisfied.
Further, in Sample No. 30, 3.2 / (Si ^0.32 × F _BU ) was below the range of the present invention, so the target value of weld zone toughness could not be satisfied.

[0060]

As described in detail above, according to the present invention, it is applied to a high-tensile steel plate having excellent toughness, specifically, to a structural member of a structure machined by welding such as arc welding or beam welding. To provide a high-strength steel sheet with excellent economic efficiency and toughness suitable for, and more specifically, the shock absorption energy vE- ₄₀ (atFL) at -40 ° C of the joint part.
It is possible to provide a high-strength steel sheet having excellent economic efficiency and toughness, which is more than 100 (J) and is suitable for use as a structural member such as a pressure vessel or an offshore structure. The significance of the present invention having such an effect is extremely remarkable.

[Brief description of drawings]

FIG. 1 (a) to FIG. 1 (c) are each a plate thickness of 24 mm,
FIG. 3 is a graph showing the influence of C content and Si content on the FL notch Charpy absorbed energy when a FL notch Charpy impact test is performed on a welded portion under the conditions of X groove and SAW 3.0 kJ / mm, 1 (a) shows the case where the upper bainite ratio F _BU is 100%, FIG. 1 (b) shows the case where the upper bainite ratio F _BU is 90%, and FIG. 1 (c) shows the upper bainite. The case where the ratio F _BU is 80% is shown.

Claims (2)

(57) [Claims] 1. C: 0.03 to 0.08 % by mass %, Si: 0.10.
~ 0.40%, Mn: 0.6-2.0%, Cr: 0.05-1.6%, Nb:
0.005 to 0.1%, Ti: 0.005 to 0.1%, substantially no Al and B, the carbon equivalent Ceq defined by the following formula (1) is in the range of 0.30 to 0.45%, and the balance Fe and has a steel composition consisting of unavoidable impurities, caused a ferrite bainite structure with the C content and the Si content satisfies the following expression (2), FL notch Charpy impact trial
According to the test, the impact absorption energy at -40 ° C exceeds 100 (J).
High tension excellent economy and toughness, characterized in that
Steel plate . [Equation 1] Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 ・ ・ ・ ・ ・ ・ ・ (1) C ≦ 3.2 / (Si ^0.32 × F _BU ) ・ ・ ・ ・ ・ ・ ( 2) However, the symbol F _BU indicates the upper bainite ratio (%). 2. Further, in mass% , Cu: 1.0% or less, N
i: 2.0% or less, Mo: 1.0% or less, V: 0.1% or less,
W: 1.0% or less, REM: 0.02% or less, and Ca: 0.02% or less, and one or more kinds selected from the group are contained, and the economical efficiency and toughness according to claim 1 are contained. Excellent high-tensile steel plate .