JP4778779B2 - High-tensile steel plate with excellent low-temperature toughness in heat affected zone - Google Patents

Description

  The present invention relates to a high-tensile steel sheet having excellent low-temperature toughness in the heat-affected zone of the weld, and particularly excellent in toughness of the weld-heat-affected zone and the toughness of the base metal even when used in applications exposed to low temperatures. The present invention relates to a high-strength steel sheet. The present invention is not limited to the above-described high-strength steel plate welding method, and can be applied to submerged arc welding, electrogas arc welding, etc., but in the following, it is said that securing the toughness of the weld heat affected zone is particularly difficult. The case where the single-sided submerged arc welding with large heat input is applied will be described as an example.

  In recent years, single-sided submerged arc welding with large heat input, for example, with a heat input of 50 to 200 kJ / cm, has been widely used to produce marine structures and low-temperature tanks for storing liquefied gas such as LPG in a short period of time. It has been adopted. However, while the welding can realize high efficiency of construction, it is difficult to stably secure the toughness of the weld heat affected zone (hereinafter referred to as “HAZ”) formed by welding, and it is due to low heat input. In many cases, it is necessary to apply multilayer welding. Accordingly, a steel sheet having high HAZ toughness is required for manufacturing the low-temperature tank or the like, in which the high heat input welding method capable of high-efficiency construction is employed and even at a low temperature of about −60 ° C. ing.

  Until now, various methods have been proposed to improve the low temperature toughness of the HAZ. For example, Patent Document 1 and Patent Document 2 propose a method for improving HAZ toughness by suppressing the austenite grain coarsening by pinning particles such as TiN and Al oxide. Patent Document 3 and Patent Document 4 disclose a technique for miniaturizing crystal grains by making many ferrite transformation nuclei exist in austenite grains. Specifically, the use of TiN, MnS, BN, Ti oxide or the like as ferrite transformation nuclei achieves refinement of crystal grains and improves the low temperature toughness of HAZ.

However, in any of the above methods, when performing single-sided submerged arc welding with high heat input, precipitates such as TiN are considerably dissolved, and it is difficult to suppress subsequent grain coarsening, In order to secure excellent HAZ toughness (hereinafter sometimes referred to as “HAZ toughness” or simply “HAZ toughness”) at a low temperature of about −60 ° C., further improvement is considered necessary. .
Japanese Patent Publication No. 55-026164 Japanese Patent No. 2950076 Japanese Patent Publication No. 07-068577 Japanese Patent Publication No. 05-017300

  The present invention has been made in view of such circumstances, and its purpose is to provide high HAZ low-temperature toughness and excellent low-temperature toughness of the base metal even when welding is performed with high heat input. It is to provide a tension steel sheet.

The high-tensile steel plate excellent in low temperature toughness of the weld heat affected zone according to the present invention is
% By mass (the same applies below)
C: 0.03-0.07%,
Si: 0.01 to 0.25%,
Mn: 1.20 to 1.60%,
P: 0.010% or less (excluding 0%),
S: 0.003% or less (excluding 0%),
Al: 0.02 to 0.04%,
Nb: 0.005 to 0.016%,
B: 0.0006 to 0.0020%,
N: 0.0045 to 0.0090%,
Ti: 0.008 to 0.020%
And the following formula (1) is satisfied, and the remaining iron and inevitable impurities are characteristic.
−20 ≦ (B-NT / 1.3) ≦ 10 (1)
{In formula, B shows B content (mass ppm).
NT is
The relationship between N (N content, unit: mass ppm) and Ti (Ti content, unit: mass ppm) is
When (N-Ti / 3.4) ≧ 0, NT = (N-Ti / 3.4),
(N-Ti / 3.4) <0 indicates NT = 0}

The high-tensile steel plate is further
Cu: 0.5% or less,
One or more selected from the group consisting of Ni: 0.8% or less and V: 0.05% or less may be included so as to satisfy the following formula (2), and further Ca: 0.003% The following (not including 0%) may be included.
(Cu + Ni + 60Nb + 20V) ≦ 1.4 (2)
{Wherein Cu, Ni, Nb, and V represent the content (% by mass) of each element}

  According to the present invention, even when high heat input welding is performed on a steel plate, HAZ exhibits excellent toughness at about −60 ° C., and therefore, a low temperature tank for storing liquefied gas such as offshore structures and LPG, etc. For example, a large-heat-input single-sided submerged arc welding method can be employed to manufacture the marine structure and the like in a shorter period of time.

  The present inventor has intensively studied to obtain a high-strength steel sheet that is particularly excellent in low temperature toughness of HAZ when welding with high heat input is performed.

as a result,
(A) After setting C to be 0.07% or less and Si to be 0.25% or less, the balance between the prescribed amounts of B, N and Ti is optimized, and a certain amount of Nb is added. For example, the generation of coarse ferrite from the austenite grain boundary (hereinafter sometimes simply referred to as “grain boundary ferrite”) is sufficiently suppressed, and the grain refinement within the austenite grain can be achieved.
(B) Furthermore, when Cu, Ni, and V are added to further increase the strength, if the contents of Cu, Ni, V, and Nb are comprehensively controlled, deterioration of the HAZ toughness can be suppressed.
The specific method was found based on the idea.

  First, in the present invention, by optimizing the balance of individual prescribed amounts of B, N, and Ti and rigorously optimizing the amount of dissolved B, the crystal grains in the austenite grains can be refined, and as a result, HAZ It is characterized in that the low-temperature toughness of can be greatly improved.

FIG. 1 shows that 0.06% C-0.20% Si-1.4% Mn-0.03% Al-0.010% Nb is a basic component, and B, N, and Ti are within the specified ranges described later. (B-NT / 1.3) {B is B content (mass ppm), NT is N (N content, unit: mass ppm) and Ti (Ti content, unit: mass ppm) Relationship
When (N-Ti / 3.4) ≧ 0, NT = (N-Ti / 3.4),
When (N-Ti / 3.4) <0, NT = 0 is indicated.
The same applies to equation (1) below}
A heat cycle test was performed using steel sheets having various values of and the low temperature toughness (vE- ₆₀ ) of HAZ was measured as described in the examples described later, and these results were arranged. In the thermal cycle test, welding heat input: 60 kJ / cm (plate thickness 12 mm) was assumed, and after heating and holding at 1400 ° C. × 5 sec, cooling from 800 ° C. to 500 ° C. was performed in 150 sec.

From FIG. 1, in order to achieve vE ₋₆₀ : 100 J or more as the low temperature toughness of HAZ, as shown in the following formula (1), the value of (B-NT / 1.3) is −20 ppm or more and 10 ppm or less. It turns out that it needs to be within the range.
−20 ≦ (B-NT / 1.3) ≦ 10 (1)

  By optimizing the balance of B, N and Ti as in the above formula (1), the coarsening of the grain boundary ferrite due to the solid solution B existing at the grain boundary in the austenite grain is suppressed, and from the grain boundary. It is considered that the effect of suppressing the formation of the ferrite side plate and the effect of BN as the ferrite transformation nucleus could be maximized.

  As described above, to optimize the balance of B, N and Ti to reliably increase the low temperature toughness of HAZ and to ensure the strength of the base material, the contents of B, N and Ti are within the following ranges, respectively. There is a need to.

<B: 0.0006 to 0.0020%>
B fixes solute N which is harmful to HAZ toughness by generating BN, and also has an effect of promoting the formation of intragranular ferrite. Solid solution B also has the effect of suppressing the coarsening of grain boundary ferrite and the formation of ferrite side plates, and making the crystal grains in the austenite grains finer. In order to fully exhibit this effect, it is necessary to contain B 0.0006% or more. On the other hand, when there is too much B, a crystal | crystallization is formed in a fixed direction by the effect | action of excess solute B, and HAZ toughness deteriorates on the contrary. Therefore, the amount of B is suppressed to 0.0020% or less.

<N: 0.0045 to 0.0090%>
N is an element that forms a nitride with an element such as Ti or Al to improve the HAZ toughness, and therefore may be contained in an amount of 0.0045% or more, preferably 0.0060% or more. In addition, the solid solution N causes the HAZ toughness to deteriorate. By increasing the total nitrogen amount, the above-mentioned nitride increases, but the solid solution N also becomes excessive. Therefore, in the present invention, the N amount is suppressed to 0.0090% or less.

<Ti: 0.008 to 0.020%>
Ti is an element that generates TiN-based precipitates and promotes the formation of intragranular ferrite, and is also effective in suppressing austenite grain coarsening. It is also an element that contributes to high yield strength. In order to exert such an action effectively, it is necessary to contain Ti by 0.008% or more, and preferably 0.012% or more. However, if Ti is excessively contained, the HAZ toughness is reduced instead, so the content is made 0.020% or less.

  In the present invention, as described above, the balance of individual prescribed amounts of B, Ti, and N is optimized, and a certain amount of Nb is added. Nb is an element useful for sufficiently suppressing the formation of coarse grain boundary ferrite and achieving crystal grain refinement in the austenite grains. In the present invention, Nb is contained in an amount of 0.005% or more in order to sufficiently exhibit such an effect. However, if it is contained excessively, hard phase MA (Martensite-Austenite constituent) is likely to be formed, and crystals are formed in a certain direction, leading to deterioration of HAZ toughness.

  In order to increase the low temperature toughness of HAZ more reliably, it is effective to further reduce C and Si. In the present invention, the amount of C is suppressed to 0.07% or less in order to suppress the formation of MA, which is a hard phase, and to ensure the HAZ toughness at about −60 ° C. On the other hand, C is an element essential for securing the strength of the steel sheet, so 0.03% or more is contained.

  Furthermore, by reducing Si to 0.25% or less, the formation of MA can be sufficiently suppressed, and the low temperature toughness of HAZ can be easily ensured. On the other hand, since Si is an element that is used for deoxidation of molten steel and effectively acts to improve the strength, it may be contained in an amount of 0.01% or more, preferably 0.05% or more.

  In addition, as described above, in order to reliably increase the HAZ toughness and to provide other characteristics such as strength and toughness of the steel plate (base material), it is necessary to set the content of components other than the above within the following ranges.

<Mn: 1.20 to 1.60%>
Mn is an element useful for capturing S as MnS and suppressing degradation of HAZ toughness due to S. It is also an element that contributes to increasing the strength (higher TS and higher YS) of the steel sheet by increasing the hardenability. In order to exhibit such an action effectively, it is necessary to contain 1.20% or more of Mn. However, if the amount of Mn becomes excessive, the HAZ toughness deteriorates on the contrary, so it is suppressed to 1.60% or less.

<P: 0.010% or less (excluding 0%)>
P is an element that deteriorates the HAZ toughness, so it is necessary to reduce it as much as possible. In the present invention, P is suppressed to 0.010% or less.

<S: 0.003% or less (excluding 0%)>
S is an element that generates coarse sulfides and degrades the HAZ toughness. Therefore, it is necessary to reduce as much as possible, and in the present invention, it is suppressed to 0.003% or less.

<Al: 0.02-0.04%>
Al is an element that is used as a deoxidizer and that also generates AlN-based precipitates to improve the HAZ toughness during high heat input welding. In the present invention, Al is contained in an amount of 0.02% or more. However, when the amount of Al becomes excessive, oxide inclusions such as alumina increase, and the formation of MA is promoted and the HAZ toughness is deteriorated.

  The contained elements defined in the present invention are as described above, and the balance is iron and unavoidable impurities. As the unavoidable impurities, mixing of elements brought in depending on the situation of raw materials, materials, manufacturing facilities, etc. can be allowed. Further, it is possible to further contain the following elements.

<Cu: 0.5% or less,
One or more selected from the group consisting of Ni: 0.8% or less and V: 0.05% or less (however, within the range of the following formula (2)).
(Cu + Ni + 60Nb + 20V) ≦ 1.4 (2)
{Wherein Cu, Ni, Nb, and V represent the content (mass%) of each element} >>
Cu, Ni, and V are all elements useful for ensuring strength. Cu is an element effective for increasing the strength (TS and YS) by solid solution strengthening and precipitation strengthening. However, if excessively contained, the hot workability is inhibited, so the content is suppressed to 0.5% or less.

  Ni is an element that simultaneously improves the strength and toughness of the base material. In order to exhibit such an action effectively, it is preferable to contain 0.2% or more. However, excessive addition increases the cost, so it is suppressed to 0.8% or less.

  V is an element useful for increasing the hardenability and ensuring high strength, and for increasing the temper softening resistance. However, if excessively contained, the HAZ toughness deteriorates, so the content is suppressed to 0.05% or less.

  In the present invention, as described above, Nb is suppressed to 0.016% or less, and the contents of Cu, Ni, Nb, and V are limited as shown in the following formula (2), thereby comprising Cu, Ni, and V. Even when one or more selected from the group is contained, excellent HAZ toughness can be ensured.

FIG. 2 shows that 0.06% C-0.20% Si-1.4% Mn-0.03% Al-0.014Ti-0.0014% B-0.0065% N as a basic component, Cu: One or more selected from the group consisting of 0.5% or less, Ni: 0.8% or less, and V: 0.05% or less, and a specified amount of Nb, including (Cu + Ni + 60Nb + 20V) having various values. A heat cycle test was conducted using a steel plate, and the low temperature toughness (vE- ₆₀ ) of HAZ was measured as in the examples described later, and these results were organized. In the thermal cycle test, welding heat input: 60 kJ / cm (plate thickness 12 mm) was assumed, and after heating and holding at 1400 ° C. × 5 sec, cooling from 800 ° C. to 500 ° C. was performed in 150 sec.

From FIG. 2, when one or more selected from the group consisting of Cu: 0.5% or less, Ni: 0.8% or less, and V: 0.05% or less is contained, vE ₋ as the low temperature toughness of HAZ It can be seen that in order to achieve ₆₀ : 100 J or more, the value of (Cu + Ni + 60Nb + 20V) needs to be 1.4% or less as shown in the following formula (2). While suppressing Nb to 0.016% or less and comprehensively limiting the contents of Cu, Ni, Nb, and V as described above, the formation of MA, which is a hard phase, is suppressed and excellent HAZ toughness is achieved. Can be secured.
(Cu + Ni + 60Nb + 20V) ≦ 1.4 (%) (2)
{Wherein Cu, Ni, Nb, and V represent the content (% by mass) of each element}

<Ca: 0.003% or less (excluding 0%)>
Ca is an element effective for fixing S, which adversely affects HAZ toughness, as CaS, and for improving the toughness by controlling the form of nonmetallic inclusions in a granular form. In order to exert such effects sufficiently, it is preferable to contain 0.0010% or more of Ca, but even if Ca is contained excessively, these effects are saturated and the HAZ toughness deteriorates. Therefore, the Ca content is preferably 0.003% or less.

  The present invention can be advantageously applied to so-called thick plates. The plate thickness is about 7 mm or more and the upper limit is not particularly limited, but is usually about 40 mm or less.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

  Steel slabs having the composition shown in Tables 1 and 2 are heated to 1100 ° C. and rolled to a predetermined thickness (12 mm or 30 mm) (rolling end temperature: 700 to 780 ° C.), cooling rate: 3 to 8 ° C./second. To 550 ° C. and then air cooled. The manufacturing conditions are not limited to the above conditions because they do not adversely affect the characteristics of the HAZ even if known direct quenching / tempering techniques are applied.

  About the obtained steel plate, a base material characteristic and HAZ toughness were evaluated as follows.

[Evaluation of base material properties]
From the total thickness of each steel plate, take a JIS Z 2201 1B test piece in a direction perpendicular to the rolling direction, conduct a tensile test in accordance with JIS Z 2241, and determine the yield strength (YS) and tensile strength (TS). It was measured. And the thing whose tensile strength is 440 Mpa or more was evaluated as high tension.

In addition, a V-notch test piece of JISZ 2202 was taken in the rolling direction from a portion cut by 1 mm from the surface side of each steel plate, and subjected to a Charpy impact test according to the procedure of JISZ 2242. vE- ₆₀ ) was measured. And the thing with this absorbed energy (vE- ₆₀ ) of 100J or more was evaluated as having excellent base material toughness.

[Evaluation of HAZ toughness]
Single-sided submerged arc welding using the steel plate was performed by the FCB method. The FCB method is a method of laying a backing flux on a copper plate, pressing it against the back of the groove, and completing the welding while forming a back bead from one side of the surface. Yes. The groove shape is shown in FIG. 3 [(a) when the plate thickness is 12 mm, (b) when the plate thickness is 30 mm]. As the welding material, the following low-temperature steel welding material (manufactured by Kobe Steel) was used, and welded joints were produced under the welding conditions shown in FIG. 4 and Table 3.
<Welding material>
・ Wire; US-255
・ Front flux; PFI-50LT
・ Backing flux; MF-1R

  Then, three V-notch test pieces of JISZ 2202 each cut by 1 mm from the surface side and notched perpendicularly to the plate surface at the position of HAZ (bond part, bond part + 1 mm [HAZ1 mm]) were collected, respectively. The Charpy impact test was conducted as described above. And the thing whose average value of absorbed energy is 100J or more was evaluated as being excellent in low temperature toughness of HAZ. These results are shown in Tables 4 and 5.

  The following can be considered from Tables 1, 2, 4 and 5 (note that the following No. indicates the experiment No. in the table).

  No. satisfying the requirements defined in the present invention. Steel plates 4, 7, and 15 to 17 are high-tensile steel plates that are excellent in low-temperature toughness of HAZ and excellent in base material properties, and are welded by a high heat input single-sided submerged arc welding method. Excellent characteristics when used in low temperature applications.

  On the other hand, No. which does not satisfy the provisions of the present invention. Each of 19 to 38 has the following problems.

  That is, no. No. 19 has a C amount exceeding the upper limit, and No. 20 is inferior in HAZ toughness because the Si amount exceeds the upper limit.

  No. No. 21 is inferior in HAZ toughness because the amount of Mn is insufficient. On the other hand, no. No. 22 cannot secure excellent HAZ toughness because the amount of Mn is excessive.

  No. No. 23 has an excessive amount of P. No. 24 is inferior in HAZ toughness because the amount of S is excessive.

  No. No. 25 lacks the amount of Al. No. 26 is inferior in HAZ toughness because the amount of Al is excessive. No. No. 27 has an insufficient amount of Nb. No. 28 is inferior in HAZ toughness because the amount of Nb is excessive.

  No. No. 29 has insufficient Ti amount. No. 30 is inferior in HAZ toughness because the Ti amount is excessive. No. No. 31 has insufficient B amount. No. 32 is inferior in HAZ toughness because the amount of B is excessive. No. No. 33 has an insufficient amount of N. No. 34 is inferior in HAZ toughness because the amount of N is excessive.

  No. No. 35 (B-NT / 1.3) is below the lower limit of formula (1). No. 36 is inferior in HAZ toughness because (B-NT / 1.3) exceeds the upper limit of formula (1).

  No. Although 37 and 38 contain 1 or more types selected from the group which consists of Cu, Ni, and V, since it exceeds the upper limit of Formula (2), it is inferior to HAZ toughness.

It is a graph which shows the relationship between (B-NT / 1.3) and vE- _{60 of} HAZ. It is a graph which shows the relationship between (Cu + Ni + 60Nb + 20V) and vE- _{60 of} HAZ. Sectional drawing of the groove shape in the welding in an Example is shown. The schematic diagram of the electrode arrangement | positioning at the time of FCB welding is shown.

Claims (2)

% By mass (the same applies below)
C: 0.03-0.06%,
Si: 0.01 to 0.25%,
Mn: 1.20 to 1.60%,
P: 0.010% or less (excluding 0%),
S: 0.003% or less (excluding 0%),
Al: 0.02 to 0.04%,
Nb: 0.005~0.0 08%,
B: 0.0006 to 0.0020%,
N: 0.0045 to 0.0090%,
Ti: 0.008 to 0.020% ,
Ca: 0.003% or less (excluding 0%)
A high-tensile steel sheet that satisfies the following formula (1) and is excellent in low-temperature toughness of the weld heat-affected zone, characterized by being the remaining iron and inevitable impurities.
−20 ≦ (B-NT / 1.3) ≦ 10 (1)
{In formula, B shows B content (mass ppm).
NT is
When the relationship between N (N content, unit: mass ppm) and Ti (Ti content, unit: mass ppm) is (N—Ti / 3.4) ≧ 0, NT = (N—Ti /3.4),
(N-Ti / 3.4) <0 indicates NT = 0} Furthermore,
Cu: 0.5% or less,
The steel plate according to claim 1, comprising at least one selected from the group consisting of Ni: 0.8% or less and V: 0.05% or less so as to satisfy the following formula (2).
(Cu + Ni + 60Nb + 20V) ≦ 1.4 (2)
{Wherein Cu, Ni, Nb, and V represent the content (% by mass) of each element}

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