JPH09194990A - High-strength steel with excellent toughness - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To provide a high-strength steel having good low-temperature HAZ toughness and capable of welding under a high heat input condition. SOLUTION: In% by weight, C: 0.03 to 0.15%, S
i: 0.05 to 0.5%, Mn: 0.6 to 2.0%, A
1: 0.001-0.009%, Ti: 0.005-
0.02%, N: 0.005% or less and O: 0.00
2 to 0.006% and satisfy the following formula, with the balance being Fe and impurities (P: 0.030% or less, S:
High tensile steel consisting of 0.030% or less and B: 0.0002% or less). 0.3 ≦ Al (%) / O (%) ≦ 1.5 Further, Cu: 0.2 to 1.5%, Ni: 0.2 to 3.
0%, Cr: 0.05 to 1.0%, Mo: 0.05 to
1.0%, V: 0.03-0.2% and Nb: 0.0
One or more of 2 to 0.2% may be contained.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a pressure vessel, a ship,
TECHNICAL FIELD The present invention relates to a high-strength steel having excellent weld heat-affected zone toughness used for welded structures such as bridges, buildings, offshore structures and line pipes.

[0002]

2. Description of the Related Art In recent years, in high-strength steel for welding used in offshore structures installed in ice seas, line pipes for cold regions, or large steel structures such as ships and LNG tanks, material properties have been improved. The demand for is becoming stricter. Needless to say, sufficient strength is required according to the purpose of use, and particularly the heat-affected zone of the base metal in contact with the weld metal (hereinafter,
There is a strong demand for improvement in toughness (referred to as HAZ).

Conventionally, with respect to the toughness of a welded portion of a high-strength steel sheet, (a) austenite (hereinafter referred to as "γ") crystal grain size, (b) transformation structure, and (c) fine hardened phase precipitation state , And (d) the amount of solute nitrogen in the steel sheet has a great influence, and various measures for improving toughness have been proposed.

For example, with respect to the above (a) and (b), a method of suppressing the coarsening of γ crystal grains by adding Ti in a small amount and finely precipitating TiN in the steel (“Iron and Steel” No. 65)
Year (1979) No. 8, pages 1232-1241) and Ca
A minute amount of CaO to generate CaO and CaS, and to refine the structure by refining γ crystal grains and precipitation of intragranular ferrite with CaO and CaS as nuclei (hereinafter, ferrite is referred to as “α”). ("Journal of the Welding Society" Vol. 52 (19
83) No. 2, pp. 117-124), a method of refining crystal grains in the same manner as in the case of adding Ca by an oxide of a rare earth element (JP-A-64-15320), and nucleating Ti oxide particles. A method in which intragranular α is generated as a site to make the structure finer (JP-A-57-51243 and JP-A-61-79745), and V and Ti are added in combination to precipitate in the cooling process. A method of using VN as a transformation nucleus of α (Japanese Patent Laid-Open No. 5-186868) has been proposed.

With regard to the above (c), a method of suppressing precipitation of a hardened phase by lowering the carbon equivalent or reducing the contents of Si and Al (aluminum) (Japanese Patent Laid-Open Publication No. HEI-2-
No. 190423) has been proposed. Also,
Regarding (d), a method of reducing the amount of N (nitrogen) contained in steel and a method of fixing N as AlN by adding excess Al have been proposed.

However, in the above measures, in the method of finely depositing TiN to suppress the coarsening of the γ crystal grains, T
In the part where iN is heated to 1400 ° C. or higher, most of it melts in the base metal, so that γ crystal grains are unavoidably coarsened particularly near the fusion line of HAZ during high heat input welding. further,
The TiN dissolved in the heating process does not reprecipitate in the cooling process. That is, in the portion where TiN is melted, there is a disadvantage that α-transformation does not occur in the grains during the cooling process, the amount of solute N increases, and deterioration of HAZ toughness cannot be avoided.

According to the method of utilizing Ti oxide particles as α nucleation sites, it is very difficult to uniformly disperse the Ti oxide particles in the steel in a finely stable state during melting. Problem remains. Similar problems occur in the method of refining the structure by adding a trace amount of Ca or adding an oxide of a rare earth element. Further, Japanese Unexamined Patent Publication No.
In Japanese Patent No. 78740, the steel in which Ti oxide is finely dispersed has a HAZ region (coarse grain region H in which γ grains near the melting line are coarsened).
The effect of refining the HAZ structure in the AZ: region heated to 1400 ° C. or higher) is large, but in the region in which the γ grain is slightly large (sub-coarse grain region HAZ: region heated to 1200 to 1350 ° C.) It was pointed out that
It has been proposed to accelerate the precipitation of α in the grain by using an oxide. However, it is difficult to uniformly disperse the Ce oxide in the steel in a finely stable state during smelting, as with the Ti oxide, and there is still a problem in terms of actual operation.

Techniques such as controlling the precipitation morphology of the fine hardened phase by lowering carbon equivalent, lowering Si and lowering Al, and reducing the amount of solute N are synergistic effects with the toughness improvement by the above-mentioned nitride or oxide. However, the HAZ toughness improving effect by itself had its own limit.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and an object thereof is from a coarse grain region in which γ grains are coarsened to a sub coarse grain region in which γ grains are slightly large. It is intended to provide a high-strength steel that stably exhibits good low-temperature HAZ toughness even in a region and can be welded under a high heat input condition.

[0010]

In order to achieve the above object, the present inventors have paid attention to the balance between the amount of Al and the amount of O (oxygen) which is expected to significantly affect the deoxidation phenomenon.
The composition, dispersion state, and γ of the deoxidation product using steel types in which the ratio of the Al amount (wt%) to the amount (wt%) (Al (wt%) / O (wt%)) was changed to various values. → We investigated the function of α generation in α transformation. as a result,
The facts described in (a) to (d) below were found.

(A) A formed in a steel type satisfying the relationship of 0.3≤Al (wt%) / O (wt%) ≤1.5
The complex oxide consisting of 1, Mn and Si is easily dispersed finely in steel, and has a coarse grain area HAZ and a sub-coarse grain area HA.
In Z, it functions as an excellent intragranular α precipitation nucleus. Al
In a steel in which (wt%) / O (wt%) (hereinafter, simply referred to as “Al / O”) is less than 0.5, the ratio of Al component contained in the oxide in the steel is small, and the oxide is a grain. Loss of the function of α in the nucleus. On the other hand, in the steel having Al / O exceeding 1.5, the ratio of Mn and Si components contained in the oxide in the steel is small, and also does not have a function as a precipitation nucleus of intragranular α.

The reason for the loss of the α-producing function of the oxide is not clear, but it is due to the deterioration of the crystal conformity with α due to the change of the crystal structure of the oxide caused by the change of the content ratio of the Al component. It is supposed to be. Note that 0.3
As a result of identifying the structure of the oxide in the extraction residue collected from the steel type satisfying the relation of ≦ Al / O ≦ 1.5 by X-ray diffraction, a diffraction peak very close to MnAl ₂ O ₄ was obtained. MnAl ₂ O ₄ has a good crystal matching with α and is an oxide attracting attention as a nucleation site of acicular ferrite of the weld metal.

(B) The total amount of the complex oxide depends on the O content in the steel, and in order to obtain a sufficient microstructure improving effect, the O content must be 0.002% by weight or more. .

(C) Even if the steel satisfying the above conditions (a) and (b) is not added or contains Ti, if the amount is less than 0.005% by weight, Deterioration is observed in the toughness of the material, which poses a practical problem. this is,
In steel satisfying the relation of 0.3 ≦ Al / O ≦ 1.5,
If only Al, Mn and Si are contained, O in the steel
It is considered that the reaction does not sufficiently react with, and unstable oxides such as FeO are generated, and the toughness of the base material deteriorates. In addition, the relationship of 0.3 ≦ Al / O ≦ 1.5 is satisfied,
In addition, the X-ray diffraction result of the oxide in the extraction residue collected from the steel containing a predetermined amount of Ti, which will be described later, also shows MnA.
A diffraction peak very close to l ₂ O ₄ was obtained, TiO ₂ ,
No peak of Ti oxide such as Ti ₂ O ₃ was observed.

From these results, the above-mentioned composite oxide functioning as an excellent precipitation nucleus of intragranular α in the coarse grain region HAZ and the sub-coarse grain region HAZ is mainly composed of MnAl ₂ O ₄ , and Mn or It is presumed that Al and Ti exist in a state of being replaced with each other.

(D) When the oxygen content is 0.002% by weight or more,
Regarding the steel satisfying the relation of 0.3 ≦ Al / O ≦ 1.5 and containing Ti, the base material properties and HAZ toughness were examined, and it was found that the oxides having almost the same composition and dispersibility were contained. It was found that even in steels having excellent material toughness and HAZ toughness in the coarse grain region, differences in the toughness in the HAZ in the sub-grain region may be observed. When the HAZ in the sub-coarse grain region exhibits high toughness, precipitation of grain boundaries α is observed, whereas
When the HAZ shows a low toughness in the sub-coarse grain region, the old γ grain boundary was clearly observed. Therefore, the influence of B (boron), which is considered to have inevitably penetrated, was examined. It has been confirmed that steel containing HAZ having a high toughness contains almost no B, whereas steel containing a low toughness contains a few ppm when B is contained in a large amount.

The present invention was made on the basis of the above findings, and its gist resides in the high-strength steels of the following (1) to (3). In addition, "%" of an alloy element means "weight%."

(1) C: 0.03 to 0.15%, Si:
0.05-0.5%, Mn: 0.6-2.0%, Al:
0.001-0.009%, Ti: 0.005-0.0
2% and O: 0.002 to 0.006%, Al / O is 0.3 to 1.5, the balance is Fe and inevitable impurities, and P in the impurities is 0. 030
%, S is 0.030% or less, N is 0.005% or less, and B is 0.0002% or less, a high-strength steel having excellent toughness in the weld heat-affected zone.

(2) In addition to the components described in (1) above,
Furthermore, Cu: 0.2-1.5% and Ni: 0.2-
It contains at least one of 3.0%, Al / O is 0.3 to 1.5, the balance is Fe and inevitable impurities, and P in the impurities is 0.030% or less. , S is 0.030% or less, N is 0.005% or less, B is 0.0
A high-strength steel having excellent toughness in the weld heat-affected zone, which is characterized by being 002% or less.

(3) In addition to the components described in (1) or (2) above, Cr: 0.05 to 1.0%, M
o: 0.05 to 1.0%, V: 0.03 to 0.2%, and Nb: 0.02 to 0.2%, and one or more kinds are contained,
Moreover, Al / O is 0.3 to 1.5, the balance is Fe and inevitable impurities, and P in the impurities is 0.03.
A high-strength steel having excellent toughness in the weld heat-affected zone, which is 0% or less, S is 0.030% or less, N is 0.005% or less, and B is 0.0002% or less.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION The effects of the components constituting the high-strength steels (inventive steels) of (1) to (3) and the reasons for limiting their contents will be described below.

C: C is added to secure the strength of the steel, but if the content is less than 0.03%, its effect is not sufficient, while if it exceeds 0.15%, it is welded. Martensite (α ') or similar pearlite (α / F)
e ₃ C) is generated to deteriorate the HAZ toughness, and
It also adversely affects the toughness and weldability of the base material. Therefore,
The C content is 0.03 to 0.15%.

Si: Si is an element effective in deoxidizing molten steel, and in the steel of the present invention, it is an important element constituting a complex oxide that serves as a nucleus for intragranular α precipitation. For this reason,
It is necessary to contain 0.05% or more in steel. On the other hand, if the content exceeds 0.5%, since Si does not form a solid solution in cementite, the decomposition of untransformed γ into α grains and cementite is inhibited, and the formation of island-like martensite, which is a fine hardening phase, is generated. Is promoted to deteriorate the HAZ toughness. Therefore, the content is set to 0.05 to 0.5%.

Mn: Mn is an element effective for deoxidizing molten steel, and is an element necessary for ensuring the strength and toughness of steel. Further, in the steel of the present invention, it is an important element that constitutes the complex oxide that becomes the nucleus of intra-grain α precipitation. For this reason,
Mn needs to be contained by 0.6% or more. However, if the content exceeds 2.0%, the hardenability increases and the weldability and HAZ toughness deteriorate, so the content is made 0.6 to 2.0%. Al: Al is an element necessary as a deoxidizing agent, and in the steel of the present invention, it is also an essential element as a constituent element of the complex oxide that serves as a precipitation nucleus of intragranular α. Therefore, it is necessary to contain Al in an amount of 0.001% or more. However, the upper limit value is naturally determined by the upper limit value of the oxygen amount and the regulation of Al / O described later, and is 0.009%.

Therefore, the content of Al is 0.001 to
It is set to 0.009%.

Ti: Ti is added to suppress the formation of unstable oxides such as FeO. For this purpose, it is necessary to contain 0.005% or more in steel. However, if the content exceeds 0.02%, coarse TiC precipitates, which adversely affects the HAZ and the toughness of the base material. Therefore, the content is set to 0.005 to 0.02%.

O: O is required to be contained in an amount of at least 0.002% in order to precipitate a complex oxide which serves as a precipitation nucleus of intragranular α. However, if it is present in excess of 0.006%, the toughness of the base material deteriorates. Therefore,
The content of O is 0.002 to 0.006%.

Al / O: Al is a very strong deoxidizer, so when Al is present in excess of O (Al / O
Of more than 1.5), most of the oxides in the steel are α
The Al-based oxide does not function as a precipitation nucleus. On the other hand, when Al is insufficient with respect to O (when Al / O is smaller than 0.3), Mn, Si and Ti which are deoxidizing agents other than Al are contained within the above range. However, the toughness of the base material, which is presumed to be caused by the formation of an unstable oxide such as FeO, occurs. Therefore, the upper and lower limits of Al / O are set to 1.5 and 0.3, respectively.

The steel of the above (1) is a high-strength steel containing balance of Fe and unavoidable impurities in addition to the above components. It is necessary to suppress the upper limits of P, S, N and B as impurities.

P: P is an impurity element inevitably contained in steel. Since P is a grain boundary segregation element and causes grain boundary cracking in the HAZ, the lower the content, the better. However, the reduction of P causes a cost increase. Therefore, considering economic efficiency, the upper limit is set. It is set to 0.030%. In order to further improve the base material toughness and HAZ toughness and reduce the slab center segregation, its content is preferably 0.01% or less.

S: S is M when a large amount is present in steel.
Precipitates such as nS, which are the starting points of welding cracks, are generated. Therefore, the lower the content, the more preferable, but in consideration of economic efficiency, the upper limit is set to 0.030%. Base material toughness, HAZ
In order to further improve the toughness and reduce the slab center segregation, its content is preferably 0.01% or less.

N: N has the effect of refining the γ grain size through the formation of nitrides. However, in the steel of the present invention,
If the content exceeds 0.005%, the toughness deteriorates due to an increase in solid solution N, so the upper limit is made 0.005%.

B: B has the effect of enhancing the hardenability of grain boundaries and preventing the precipitation of grain boundaries α. However, in the steel of the present invention, if B is contained, the toughness of the HAZ in the sub-coarse grain region is significantly deteriorated, so the lower the content, the better. Therefore, the upper limit is set to 0.0002%.

In addition to the components of the steel of the above (1), the steel of the above (2) further contains Cu: 0.2 to 1.5% and Ni: 0.1% as elements for improving strength and toughness. 2 to
It is a high-strength steel containing at least one of 3.0%.
The reasons for limiting the contents of Cu and Ni as described above are as follows.

Cu and Ni: Cu and Ni are effective elements for increasing the strength and toughness of steel, but the effects of both Cu and Ni are sufficiently exhibited when the content is less than 0.2%. On the other hand, if the Cu content exceeds 1.5% and the Ni content exceeds 3.0%, the toughness of the base material and the HAZ decreases. Therefore, when these elements are added, the Cu content is 0.2 to 1.5% and the Ni content is 0.2 to 3.0.
%.

The steel of the above (3) further contains at least one of Cr, Mo, V and Nb as a strength improving element in addition to the components of the steel of the above (1) or (2). High-strength steel. The reasons for limiting the contents of Cr, Mo, V and Nb as described above are as follows.

Cr, Mo, V and Nb: Cr, Mo,
V and Nb are effective elements for increasing hardenability and strength of steel, but their contents are Cr and Mo.
Is less than 0.05%, V is less than 0.03%, and Nb is less than 0.02%, the effect is not sufficiently exhibited. On the other hand, for Cr and Mo, 1.0
%, And if V and Nb are excessively contained in excess of 0.2%, the toughness of the base material and HAZ decreases, and it is not preferable from the viewpoint of hardening of HAZ and suppression of weld cold cracking. . Therefore, the content when adding these elements is 0.0 for Cr and Mo.
5 to 1.0%, V is 0.03 to 0.2%, and Nb is 0.02 to 0.2%.

In producing a steel sheet using the above-mentioned steel of the present invention, for example, a steel of the above component system is melted in a converter, an electric furnace or the like, and a steel slab is cast by a continuous casting method or an ingot agglomeration method. . In the case of casting a steel slab, a faster cooling rate is preferable from the viewpoint of fine dispersion of the complex oxide. Therefore, the continuous casting method is preferable to the ingot-agglomeration method as the casting method. Also,
For the same reason, the thinner slab is more preferable.

Thereafter, the steel slab is subjected to thick plate heating and hot rolling to manufacture a steel plate having a predetermined thickness. Regarding the manufacturing conditions after heating the thick plate, various techniques such as known controlled rolling and controlled cooling can be applied, and the toughness of the HAZ is not affected thereby. In addition, after hot rolling, even if heat treatment is applied to improve the mechanical properties of the base material, H
It has no adverse effect on the toughness of AZ. In addition,
It is not always necessary to reheat the billet, and the characteristics of the steel of the present invention are not impaired even if hot charge rolling or direct rolling is performed.

The steel of the present invention has high base metal toughness and HAZ toughness for welding under high heat input conditions and is suitable as a welding steel.

[0041]

EXAMPLES Steels having the compositions shown in Tables 1 to 3 were melted in a converter, and steel pieces obtained by the ingot-agglomeration method or the continuous casting method were used.
After heating in a temperature range of 50 to 1250 ° C., the heat treatment shown in Tables 4 to 6 was performed to manufacture a steel plate having a plate thickness of 25 to 60 mm.

For these steel sheets, the yield point (YP) and tensile strength (TS) of the base material were measured, and a Charpy impact test was conducted to determine the transition temperature (vTs
). In addition, the HAZ of the welded part
The toughness was investigated.

<Reproduced HAZ1> The welding heat input to the steel sheet is 1
00kJ / cm, maximum heating temperature is 1450 ° C, 800
Thermal cycle (coarse grain region H
(Corresponding to the welding heat cycle in AZ), and then subjected to a Charpy impact test to determine the transition temperature.

<Reproduced HAZ2> The welding heat input to the steel plate is 1
00kJ / cm, maximum heating temperature is 1300 ℃, 800
After applying a heat cycle (corresponding to a welding heat cycle in the sub-coarse grain region HAZ) having a cooling time of 60 seconds at ˜500 ° C., a Charpy impact test is performed to determine the transition temperature.

The examination results are also shown in Tables 4 to 6. In these tables, the numerical values in the columns of reproduced HAZ1 and reproduced HAZ2 represent the transition temperatures obtained by the above method.

Steels 1 to 40 are an example corresponding to the invention of (1) above and a comparative example thereof.

In Steels 1 and 3, the desired composite oxide was not obtained because Al / O was out of the range defined by the present invention, and the transition temperature was high in both the reproduced HAZ1 and the reproduced HAZ2 methods, which was excellent. HAZ toughness was not obtained. In Steel 2, since the O content was insufficient, a sufficient amount of complex oxide was not formed, and the toughness of HAZ was also insufficient.

In Steels 4, 9, 14 and 19, Al / O deviates from the range defined by the present invention to the lower side. For this reason, the toughness of the base material was considered to be decreased due to the formation of unstable oxides such as FeO, and the toughness of the HAZ was low because a complex oxide having a desired composition could not be obtained. In steels 8, 12, 13, 18 and 22, Al / O
Is out of the range defined by the present invention on the higher side. Therefore, although the toughness of the base material was secured, the toughness of HAZ was insufficient.

In the steels 23 to 26, the O content is high, exceeding the range specified in the present invention. Therefore, deterioration of the toughness of the base material was recognized, and the toughness of the HAZ was not always good.

The steels 27 and 28 do not contain Ti or the content thereof is insufficient. Therefore, deterioration of the toughness of the base material was recognized, and the toughness of the HAZ was not sufficient. On the other hand, in Steel 31, Ti was excessively contained, so that in the HAZ, the hardenability was excessively increased and the toughness was low.

In the steels 34 and 35, the content of B exceeds the range specified in the present invention. Therefore, the sub-coarse grain area HAZ
The hardenability of grain boundaries in the
Had low toughness.

In Steels 36 to 40, C, Mn or Si was out of the range defined by the present invention, and remarkable deterioration of HAZ toughness was observed.

On the other hand, steel 5-7, 10, 11, 1
5-17, 20, 21, 29, 30, 32 and 33
In the above (the steel of the present invention), both the base material toughness and the HAZ toughness were good.

Steels 41 to 44 are embodiments corresponding to the invention of (2), and steels 45 to 57 are embodiments corresponding to the invention of (3). Even when the alloying element is added to improve the strength and toughness of the base material as described above, there is no problem as long as the addition is within the range defined by the present invention.

Steels 58 to 60 are the results of investigations when the heat treatment method or rolling method was changed for steel 5 (inventive steel), but HAZ was obtained even if the heat treatment or rolling method was changed.
It is clear that the toughness of is not adversely affected.

[0056]

[Table 1]

[0057]

[Table 2]

[0058]

[Table 3]

[0059]

[Table 4]

[0060]

[Table 5]

[0061]

[Table 6]

[0062]

The high tensile strength steel of the present invention has high base metal toughness and H
It has AZ toughness and can significantly improve the low temperature toughness of the welded structure. In addition, welding can be performed under conditions of high heat input. Therefore, by using the steel of the present invention, it is possible to significantly improve the safety of the welded structure and the welding workability at the time of manufacturing the welded structure.

Claims (4)

[Claims] (1) C: 0.03 to 0.15% by weight, S
i: 0.05 to 0.5%, Mn: 0.6 to 2.0%, A
1: 0.001-0.009%, Ti: 0.005-
0.02% and O: 0.002 to 0.006% are satisfied, and the following formula is satisfied, the balance consists of Fe and unavoidable impurities, and P in the impurities is 0.030% or less,
A high-strength steel having excellent toughness in the weld heat-affected zone, characterized in that S is 0.030% or less, N is 0.005% or less, and B is 0.0002% or less. 0.3 ≦ Al (%) / O (%) ≦ 1.5 2. The composition according to claim 1, further comprising:
% By weight, Cu: 0.2-1.5% and Ni: 0.2
To 3.0%, at least one of them is satisfied, and the following formula is satisfied, the balance consists of Fe and unavoidable impurities, P in the impurities is 0.030% or less, and S is 0.030%. Hereinafter, N is 0.005% or less, and B is 0.0002% or less, a high-strength steel having excellent toughness in the weld heat-affected zone. 0.3 ≦ Al (%) / O (%) ≦ 1.5 3. The composition according to claim 1, further comprising:
% By weight, Cr: 0.05-1.0%, Mo: 0.05
.About.1.0%, V: 0.03 to 0.2% and Nb: 0.
It contains at least one of 02 to 0.2% and satisfies the following formula, the balance consists of Fe and unavoidable impurities, P in the impurities is 0.030% or less, and S is 0.030.
%, N is 0.005% or less, and B is 0.0002% or less, a high-strength steel excellent in toughness in the weld heat affected zone. 0.3 ≦ Al (%) / O (%) ≦ 1.5 4. In addition to the component according to claim 2, further
% By weight, Cr: 0.05-1.0%, Mo: 0.05
.About.1.0%, V: 0.03 to 0.2% and Nb: 0.
It contains at least one of 02 to 0.2% and satisfies the following formula, the balance consists of Fe and unavoidable impurities, P in the impurities is 0.030% or less, and S is 0.030.
%, N is 0.005% or less, and B is 0.0002% or less, a high-strength steel excellent in toughness in the weld heat affected zone. 0.3 ≦ Al (%) / O (%) ≦ 1.5