JP4652952B2 - High-tensile steel plate with excellent toughness of heat affected zone - Google Patents

Description

  The present invention relates to a high-tensile steel plate excellent in toughness of a heat input zone affected by high heat input, and in particular, tensile strength excellent in toughness of the heat affected zone when subjected to high heat input exceeding 500 kJ / cm. The present invention relates to a high-tensile steel plate having a strength of 590 MPa or more.

  For example, in submerged arc welding, electroslag welding, etc. applied to box column assembly of building structures, large heat input welding exceeding 500 kJ / cm may be performed in order to further improve the efficiency of construction. . However, in general, it is known that when the heat input of welding increases, the structure of the heat affected zone (HAZ) becomes coarse and the toughness tends to decrease, and so far, the safety of building structures and the like is further improved. From the viewpoint, various methods have been proposed to improve the toughness of the HAZ.

  As the above-mentioned method, for example, by using the pinning effect due to fine dispersion of TiN or composite oxide (for example, a composite made of Ti-containing oxide and MnS in Patent Document 1), the austenite grains are prevented from coarsening. Methods for improving the toughness of HAZ have been proposed.

  However, in high heat input welding construction, if the heat affected zone near the weld metal is exposed to a high temperature for a long time, there is a problem that much of TiN is dissolved and the pinning effect is not sufficiently exhibited. In addition, since the latter complex oxide is difficult to uniformly finely disperse, there is a problem that the effect is not sufficient although many studies have been made.

  In addition, in order to improve the toughness of HAZ, a technique for refining the transformation structure in γ grains has also been proposed. For example, a technique using TiN or BN (see Patent Document 2) as a ferrite transformation nucleus has been proposed. Yes. However, even in this case, in the heat-affected zone exposed to a high temperature for a long time, there is a problem that many of the precipitates are dissolved and a sufficient effect cannot be obtained. Further, a technique for promoting intragranular ferrite formation in which Ti oxide is dispersed has been developed (see, for example, Patent Document 3). However, as in the case of the composite oxide, there is a problem that it is difficult to finely disperse uniformly. is there.

Furthermore, from the viewpoint of suppressing the occurrence of MA (Martensite-Austenite constituent), which is the starting point of fracture, and refining the structure in the γ grains, C is made extremely low C, and the hardenability improving elements such as Mn and Cr are added. A technique for reducing the bainite block size by adding B and adding B (for example, Patent Document 4) has also been proposed. However, in this technique, it is difficult to say that the block size is sufficiently reduced under high heat input welding conditions.
Japanese Patent No. 3256118 Japanese Patent No. 1824290 Japanese Patent Publication No. 05-017300 Japanese Patent No. 3606021

  The present invention has been made in view of such circumstances, and its purpose is to provide a large heat input exceeding 500 kJ / cm while high safety is required in parallel with the increase in size of building structures and the like. An object of the present invention is to provide a high-tensile steel plate of 590 MPa or more that can ensure excellent HAZ toughness when submerged arc welding or electroslag welding is performed.

The high-tensile steel plate excellent in the toughness of the high heat input welding heat-affected zone according to the present invention,
% By mass (the same applies below)
C: 0.02 to 0.05%,
Si: 0.05-0.20%,
Mn: 1.0 to 2.5%
P: 0.02% or less (excluding 0%),
S: 0.005% or less (excluding 0%),
Al: 0.01 to 0.05%,
Ni: 0.2 to 2.0%,
Cr: 0.5 to 2.0%,
Ti: 0.005 to 0.025%,
N: 0.004 to 0.010%
And satisfies the following formulas (1) and (2), and is characterized in that it consists of the remaining iron and inevitable impurities.
2.3 ≦ (Mn + 0.7 × Ni + Cr) ≦ 3.7 (1)
[Cr / (Mn + 0.7 × Ni)] ≧ 0.3 (2)
{Wherein Mn, Ni, and Cr indicate the content (% by mass) of each element}

The steel sheet is further
(A) Mo: 0.2% or less (excluding 0%),
(B) one or more selected from the group consisting of V: 0.05% or less, Nb: 0.01% or less, and B: 0.0020% or less [provided that the following formula (3) is satisfied) And]
(V + 2Nb + 10B) ≦ 0.03 (3)
{Wherein V, Nb, and B represent the content (% by mass) of each element}
May be included.

  According to the present invention, even when submerged arc welding or electroslag welding with a large heat input exceeding 500 kJ / cm is performed, excellent HAZ toughness can be ensured. Can be manufactured between.

The present inventor conducted intensive research to obtain a high-tensile steel sheet having excellent HAZ toughness (hereinafter sometimes simply referred to as “HAZ toughness”) when high heat input welding exceeding 500 kJ / cm is performed. as a result,
(I) To stably secure the high toughness of the heat affected zone,
-Control of the form of MA as well as MA suppression by extremely low C (specifically, suppression of the formation of acicular MA),
-Suppressing a coarse structure [grain boundary ferrite (GBF) + ferrite side plate (FSP)] that grows at the γ grain boundary,
・ Reducing the block size of the transformation structure in the γ grains,
In the present invention, this can be realized by properly containing hardenability improving elements Mn, Ni and Cr,
(Ii) Furthermore, when adding one or more selected from the group consisting of Nb, V and B which are hardenability improving elements, the contents of these elements are comprehensively limited, and MA and If the generation of a coarse lath structure is suppressed, the degradation of HAZ toughness can be suppressed,
As a result, the present invention has been conceived.

  First, the above (i) will be described. The inventor has (Mn + 0.7 × Ni + Cr), the amount (area%) of a coarse structure [grain boundary ferrite (GBF) + ferrite side plate (FSP)] that grows at the γ grain boundary in HAZ, and the block size. I found out that there is a correlation between them, and arranged these relationships. The result is shown in FIG.

From FIG. 1, in order to suppress the amount of (GBF + FSP) to 50 area% or less, it is necessary to set (Mn + 0.7 × Ni + Cr) to 2.3% or more as shown in the following formula (1), In order to reduce the size to 40 μm or less, it is found that (Mn + 0.7 × Ni + Cr) needs to be 3.7% or less as shown in the following formula (1). In order to further reduce the block size to 20 μm or less, (Mn + 0.7 × Ni + Cr) may be set to 3.3% or less.
2.3 ≦ (Mn + 0.7 × Ni + Cr) ≦ 3.7 (1)
{Wherein Mn, Ni, and Cr indicate the content (% by mass) of each element}

FIG. 2 shows the relationship between (Mn + 0.7 × Ni + Cr) and HAZ toughness (vE ₀ measured by the method shown in the examples described later), and (Mn + 0.7 × Ni + Cr) is 2.3. If the amount of (GBF + FSP) is suppressed within the range of ~ 3.7% and the block size is miniaturized, even if high heat input welding is performed, excellent HAZ toughness of vE ₀ : 100 J or more can be secured. I understand.

  In addition, the present inventor has [Cr / (Mn + 0.7 × Ni)] and the form of MA [specifically, the area ratio of acicular MA having an aspect ratio (major axis / minor axis) of 2.5 or more] I found out that there is a correlation between them, and arranged these relationships. The result is shown in FIG.

From FIG. 3, it is necessary to set [Cr / (Mn + 0.7 × Ni)] to 0.3 or more as shown in the following formula (2) in order to suppress the area ratio of the needle-like MA to 4% or less. I understood.
[Cr / (Mn + 0.7 × Ni)] ≧ 0.3 (2)
{Wherein Mn, Ni, and Cr indicate the content (% by mass) of each element}

FIG. 4 shows the relationship between [Cr / (Mn + 0.7 × Ni)] and HAZ toughness (vE ₀ measured by the method shown in Examples described later). 7 × Ni)] is set to 0.3 or more to suppress the formation of acicular MA, it can be seen that excellent HAZ toughness of vE ₀ : 100 J or more can be secured even when high heat input welding is performed.

  As described above, in order to optimize the balance of Mn, Ni and Cr to reliably increase the HAZ toughness, and to ensure the strength and toughness of the base material, the contents of Mn, Ni and Cr are within the following ranges, respectively. There is a need to.

<Mn: 1.0 to 2.5%>
Mn is an element useful for improving the hardenability and ensuring the strength and toughness of the base material. It is also an element useful for reducing the block size of HAZ. In order to exhibit these effects, 1.0% or more is contained. However, when Mn becomes excessive, MA increases and HAZ toughness deteriorates. Therefore, the amount of Mn is suppressed to 2.5% or less.

<Ni: 0.2-2.0%>
Ni is an element that improves the hardenability to ensure the strength and toughness of the base material and is effective in preventing hot cracking. Further, it is an element useful for reducing the block size of HAZ, and in order to exert these effects, it is contained in an amount of 0.2% or more. On the other hand, when Ni is excessive, scale wrinkles are likely to occur, so the amount is suppressed to 2.0% or less.

<Cr: 0.5 to 2.0%>
Cr is an element useful for improving the hardenability and ensuring the strength and toughness of the base material. It is also an element useful for reducing the block size of HAZ. In order to exhibit these effects, 0.5% or more is contained. However, if Cr is present in excess, MA increases and HAZ toughness deteriorates. Therefore, Cr is suppressed to 2.0% or less.

  FIG. 5 is a graph showing the relationship between the Cr amount and (Mn + 0.7 × Ni). In the graphs (1) and (2), the upper limit of the Cr amount and the lower limit of (Mn + 0.7 × Ni). The range surrounded by (hatched portion) is shown, but as shown in FIG. 5, if the relationship between Cr, Mn and Ni is within the range of the shaded portion, large heat input welding is performed. The HAZ toughness when applied can be reliably increased as compared with the prior art.

  As described above, in order to reliably increase the HAZ toughness and to ensure that other properties such as strength and toughness of the steel plate (base material) are provided, it is necessary to set the content of components other than the above within the following ranges.

<C: 0.02 to 0.05%>
C is an element necessary for ensuring the strength of the base material and suppressing the coarsening of the γ grains to ensure the HAZ toughness. In order to exert the effect, it is necessary to contain 0.02% or more. . On the other hand, if the amount of C is excessive, MA increases and the high-temperature stability of TiN formed during casting decreases, so the HAZ toughness deteriorates. Moreover, it becomes a cause of cold cracking deterioration. Therefore, the C content is limited to 0.05% or less.

<Si: 0.05-0.20%>
Si is an element necessary for deoxidation at the time of steel making, and is contained at 0.05% or more. However, if the amount of Si becomes excessive, MA increases and the HAZ toughness deteriorates, so it is suppressed to 0.20% or less.

<P: 0.02% or less (excluding 0%)>
P causes the deterioration of the base material toughness and the destruction of the γ grain boundary due to the segregation of P, so is suppressed to 0.02% or less.

<S: 0.005% or less (excluding 0%)>
Similarly to P, S is also suppressed to 0.005% or less because it causes deterioration of the base material toughness and breakage of the γ grain boundary due to segregation of MnS.

<Al: 0.01 to 0.05%>
Al is an element necessary for deoxidation during steel making, and is contained in an amount of 0.01% or more. However, when Al is excessive, coarse inclusions such as alumina increase and the base material toughness deteriorates. In addition, since MA increases and HAZ toughness deteriorates, it is suppressed to 0.05% or less.

<Ti: 0.005-0.025%>
Ti is an element that combines with N to form TiN, and this TiN is effective for improving HAZ toughness by increasing the high-temperature stability in the steel of the present invention, which has been reduced in C, making HAZ γ grains finer. Contribute. In order to exert such an effect, 0.005% or more (preferably 0.010% or more) Ti is contained. On the other hand, when Ti becomes excessive, TiN becomes coarse, and both the base metal toughness and the HAZ toughness deteriorate, so it is suppressed to 0.025% or less.

<N: 0.004 to 0.010%>
N is an element that combines with the Ti to form TiN, refines HAZ γ grains by the TiN, and contributes to improvement of toughness. In order to exert this effect, the N content is made 0.004% or more. On the other hand, if N is present excessively, solid solution N increases, and both the base metal toughness and the HAZ toughness deteriorate. Therefore, the N content is suppressed to 0.010% or less.

  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.

<Mo: 0.2% or less (excluding 0%)>
Mo is an element effective for increasing the hardenability and temper softening resistance and ensuring the strength and toughness of the base material. In order to exert the effects, it is preferable to contain Mo by 0.05% or more. However, if Mo is excessive, the γ grains become coarse after rolling due to the recrystallization inhibiting action, and the block size of the transformation structure becomes coarse. Therefore, in the present invention, the Mo amount is suppressed to 0.2% or less.

<V: 0.05% or less,
1 or more types selected from the group consisting of Nb: 0.01% or less, and B: 0.0020% or less [provided that it is within the range of the following formula (3)]
(V + 2Nb + 10B) ≦ 0.03 (3)
{In the formula, V, Nb, and B indicate the content (% by mass) of each element} >>
These V, Nb, and B are effective elements for securing the strength and toughness of the base material. V is an element useful for improving the hardenability and resistance to temper softening and ensuring the strength and toughness of the base material. However, if V is contained excessively, the HAZ toughness deteriorates, so the V content is preferably suppressed to 0.05% or less.

  Nb is an element effective for refining γ grains and ensuring the strength and toughness of the base material. However, if Nb is contained excessively, the HAZ toughness deteriorates, so it is better to keep it at 0.01% or less.

  B is an element useful for improving the hardenability and ensuring the strength and toughness of the base material. However, if B is excessively contained, ferroboride precipitates and the toughness of the base metal deteriorates, so it is preferable to keep it to 0.0020% or less.

  Moreover, when it contains 1 or more types selected from the group which consists of said V, Nb, and B, it is necessary to satisfy the said Formula (3). By suppressing (V + 2Nb + 10B) to 0.03% or less, as shown in the above (ii), the generation of MA and a coarse lath structure can be suppressed, and the degradation of HAZ toughness can be suppressed.

<Cu: 0.1 to 1.0%>
Cu is an element effective for improving the hardenability, and is preferably contained in an amount of 0.1% or more for exhibiting the effect. However, if Cu is excessive, hot cracking is likely to occur during rolling, so it is preferable to keep it to 1.0% or less.

<Ca: 0.0005 to 0.0030%>
Ca is an element effective for improving the HAZ toughness by controlling the form of nonmetallic inclusions in a granular form. In order to sufficiently exhibit such an effect, it is preferable to contain Ca in an amount of 0.0005% or more. However, if it is excessively contained, Ca inclusions are coarsened and the ductility of the base material is deteriorated. Therefore, the Ca content is preferably 0.0030% 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 materials having the composition shown in Table 1 were melted to form slabs, heated to 1100 ° C., and hot rolled to a plate thickness of 50 mm. Then, it re-heated to 930 degreeC, quenched, and tempered at 500 degreeC and manufactured. In addition, you may quench and temper immediately after hot rolling.

  And using the obtained steel plate, the base material strength was measured and the HAZ toughness was evaluated as follows.

[Measurement of base material strength]
From t / 4 (t is the plate thickness) of each steel plate, specimen No. 4 of JISZ 2201 was sampled in a direction perpendicular to the rolling direction, and subjected to a tensile test in accordance with the procedure of JISZ 2241. Tensile strength (TS) Was measured. And the thing whose tensile strength is 590 Mpa or more was evaluated as high tension.

[Evaluation of HAZ toughness]
Test specimen of 12.5 mm thickness x 32 mm width x 55 mm length taken from t / 4, simulating the heat history of the heat affected zone near the bond when electroslag welding with welding heat input of 800 kJ / cm was performed Heating: 1400 ° C. for 30 seconds, 800-500 ° C. cooling time (Tc): 700 seconds, heat treatment was performed with a high-frequency induction heating apparatus [the test piece after heat treatment is a skin plate material ( 50 mm thickness) and a diaphragm material (50 mm thickness), corresponding to a heat-affected zone when electroslag welding with a welding heat input of 800 kJ / cm is performed.

Then, a V-notch test piece of JIS Z 2202 is sampled and subjected to a Charpy impact test according to the procedure of JISZ 2242, and the absorbed energy (vE ₀ ) at a test temperature of 0 ° C. is measured. Was evaluated as having excellent HAZ toughness.

  These results are shown in Table 2.

  Table 1 and Table 2 can be considered as follows (note that the following No. indicates the experiment No. in the table).

  No. 1 that satisfies the requirements defined in the present invention and falls within the hatched area (the scope of the present invention) of FIG. It can be seen that the steel plates 1 to 7, 9, and 10 are high-tensile steel plates that exhibit good HAZ toughness.

  On the other hand, No. which does not satisfy the provisions of the present invention. Each of 13 to 28 has the following problems. That is, no. No. 13 is inferior in HAZ toughness because the amount of C exceeds the upper limit.

  No. No. 14 is inferior in HAZ toughness because the amount of Mn and the amount of Ni are insufficient.

  No. 15 and No. No. 17 has an upper limit of (Mn + 0.7 × Ni + Cr). No. 16 has a low HAZ toughness because (Mn + 0.7 × Ni + Cr) is below the lower limit.

  No. No. 18 has an excessive amount of Cr. No. 19 is inferior in HAZ toughness because the amount of Al is excessive.

  No. No. 20 is inferior in HAZ toughness because the N amount is insufficient.

  No. No. 21 is inferior in HAZ toughness because the Ti amount and the N amount exceed the upper limit.

  No. In No. 22, [Cr / (Mn + 0.7 × Ni)] is below the lower limit, so the HAZ toughness is degraded.

  No. No. 23 is inferior in HAZ toughness because it contains excessive Mo.

  No. No. 24, the individual contents of V and B are within the specified range, but (V + 2Nb + 10B) exceeds 0.03%, so the HAZ toughness is deteriorated.

  No. No. 25 has an excessive amount of Mn. No. 26 has an excessive amount of Ni. No. 27 has an excessive amount of Ti. No. 28 is inferior in HAZ toughness because the N amount is excessive.

It is a graph which shows the relationship between (Mn + 0.7 * Ni + Cr) and (GBF + FSP) amount or block size. Is a graph showing the relationship (Mn + 0.7 × Ni + Cr ) and HAZ toughness (vE _0). It is a graph which shows the relationship between [Cr / (Mn + 0.7 * Ni)] and the area ratio of acicular MA [MA whose aspect ratio (major axis / minor axis) is 2.5 or more]. Is a graph showing the relationship between [Cr / (Mn + 0.7 × Ni)] and HAZ toughness (vE _0). It is the figure which showed the range prescribed | regulated by this invention in the relationship between Cr amount and (Mn + 0.7 * Ni).

Claims (3)

% By mass (the same applies below)
C: 0.02 to 0.05%,
Si: 0.05-0.20%,
Mn: 1.0 to 2.5%
P: 0.02% or less (excluding 0%),
S: 0.005% or less (excluding 0%),
Al: 0.01 to 0.05%,
Ni: 0.2 to 2.0%,
Cr: 0.5 to 2.0%,
Ti: 0.005 to 0.025%,
N: 0.004 to 0.010%
A high-tensile steel sheet that satisfies the following formulas (1) and (2) and has the toughness of the heat-affected zone with high heat input welding, characterized by comprising the remaining iron and inevitable impurities.
2.3 ≦ (Mn + 0.7 × Ni + Cr) ≦ 3.7 (1)
[Cr / (Mn + 0.7 × Ni)] ≧ 0.3 (2)
{Wherein Mn, Ni, and Cr indicate the content (% by mass) of each element}   Furthermore, the steel plate of Claim 1 containing Mo: 0.2% or less (it does not contain 0%). Furthermore,
V: 0.05% or less,
The steel plate according to claim 1 or 2, comprising at least one selected from the group consisting of Nb: 0.01% or less and B: 0.0020% or less so as to satisfy the following formula (3).
(V + 2Nb + 10B) ≦ 0.03 (3)
{Wherein V, Nb, and B represent the content (% by mass) of each element}

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