JP2002047532A - High tensile strength steel sheet excellent in weldability and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel sheet being in a class of 590 to <780 MPa and having excellent weldability (large heat input HAZ toughness and weld cracking resistance) and to provide its production method. SOLUTION: This high tensile strength steel sheet having excellent weldability has a chemical composition containing 0.010 to 0.06% C, 1.25 to 2.5% Mn, 0.1 to 2.0% Cr, <=1.5% Mo, <=0.04% V, <=0.04% Nb, 0.005 to 0.03% Ti, 0.0006 to 0.005% B and 0.0020 to 0.0010% N, satisfying 2.4<=KP<=4.5 and KV<=0.040 and having tensile strength of 590 to <780 MPa; wherein, KP =[Mn]+1.5×[Cr]+2×[Mo] and KV=[V]+[Nb] (figures in brackets denotes the content of each element by mass%). Further, in this production method, if required, high tensile strength steel sheets having a low yield ratio or a high yield ratio are separately produced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to weldability (high heat input H
590 MPa or more and less than 780 MPa steel sheet (hereinafter simply referred to as “590 MPa”) having excellent AZ toughness and weld crack resistance.
Grade steel sheet ") and a method for producing the same. The high-tensile steel sheet of the present invention is suitably used particularly for large structures such as building structures and bridges.

[0002]

2. Description of the Related Art In a 590 MPa class steel sheet, since a large amount of alloy components are added from the viewpoint of securing base metal strength, HAZ (welding heat affected zone) is used under small heat input welding conditions with a high cooling rate.
Is hardened to cause welding cracks (low-temperature cracking), and it is necessary to preheat at about 75 ° C. during welding for the purpose of preventing such welding cracks. Therefore, if this preheating step can be omitted, the construction efficiency will be greatly improved and the cost will be reduced. Therefore, the provision of a 590 MPa class steel sheet having excellent weld cracking resistance has been desired.

Incidentally, a parameter called Pcm (%) defined by the following equation is generally used as an index of weld cracking resistance. Pcm = [C] + [Si] / 30 + [Mn] / 20 +
[Cu] / 20 + [Ni] / 60 + [Cr] / 20 +
[Mo] / 15 + [V] / 10 + 5 × [B] << where [] indicates the content (% by mass) of each element >> For example, Japanese Patent Application Laid-Open No. 10-68045 discloses this Pcm
Is improved to 0.25 or less to improve weld cracking resistance.

On the other hand, in the same 590 MPa grade steel sheet,
There is a problem that HAZ toughness is deteriorated during large heat input welding. This is based on the fact that as the heat input increases, the cooling rate of the HAZ decreases, and accordingly, the hardenability of the HAZ decreases, and coarse island martensite is generated. This problem occurred for both thick and thin objects, and heat input was restricted during actual welding work, resulting in poor welding efficiency.

In order to improve the HAZ toughness at the time of large heat input welding, the carbon equivalent (Ceq) represented by the following formula is set to 0 in Japanese Unexamined Patent Publication No. 10-121191 in addition to Japanese Unexamined Patent Publication No. 10-68045. It is disclosed that the restriction is as low as 35 to 0.40. Ceq = [C] + [Mn] / 6 + [Si] / 24 + [N
i] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 1
4 << In the formula, [] indicates the content (% by mass) of each element >>.

As described above, conventionally, by controlling Pcm to a low value, welding crack resistance during small heat input welding is improved, or large heat input HAZ is controlled by controlling Ceq.
In addition to improving toughness, the reduction in base metal strength due to the limitation of the content of alloy components was compensated for by improving the manufacturing process. As a result, in a 590 MPa class steel sheet, preheating free during welding can be achieved for a thin material having a relatively high cooling rate during quenching during base metal production, but for a thick material having a slow cooling rate, preheating free during welding and base metal strength are achieved. It was difficult to achieve both. Further, a method of securing the base material strength by utilizing the precipitation of Cu is also disclosed, but sufficient base material strength cannot be obtained with a thick material having a slow cooling rate.

[0007] As described above, in the small heat input welding, the HAZ portion has a high cooling rate after being heated to a high temperature, so that it hardens and easily causes welding cracks (low-temperature cracking). On the other hand, as the base material becomes thicker, the cooling rate becomes slower, so that it is difficult to secure the strength by the quenching effect after rolling. Therefore, 590
In the case of a thick material of MPa class steel plate, in order to prevent welding cracks during small heat input welding, it is not hardened even if the cooling speed is increased, and then the cooling speed during steel plate manufacturing is slow, and it is difficult to obtain the quenching effect. Even so, how to secure the strength is an important issue.

[0008] In addition, in the case of large heat input welding, the cooling rate of the HAZ portion becomes slow, and the hardenability of the HAZ portion is reduced, and a coarse island-like martensite structure is formed. Although the toughness decreases, this HA
In order to improve the Z toughness, an important issue is how to suppress the formation of the island-like martensite structure even when the cooling rate is low.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a 590M having excellent weldability (high heat input HAZ toughness and weld crack resistance).
Providing a Pa-grade steel sheet, and using such a 590 MPa-grade steel sheet having excellent weldability, if necessary, at a low yield ratio (hereinafter, referred to as “low YR”) or a high yield ratio (hereinafter, “high YR”) ) Is provided.

[0010]

The high-tensile steel sheet excellent in weldability according to the present invention, which can solve the above-mentioned problems, has a C: 0.0
10 to 0.06% (meaning by mass%, the same applies hereinafter), Mn:
1.25 to 2.5%, Cr: 0.1 to 2.0%, Mo:
1.5% or less (including 0%), V: 0.04% or less (0
%, Nb: 0.04% or less (including 0%), T
i: 0.005 to 0.03%, B: 0.0006 to 0.
005%, N: made of steel satisfying 0.0020 to 0.010%, and KP represented by the following formula (1) is 2.4 ≦ KP ≦
4.5, KV represented by the following equation (2) is KV ≦ 0.04.
0 and each has a gist where the tensile strength is 590 MPa or more and less than 780 MPa. KP = [Mn] + 1.5 × [Cr] + 2 × [Mo] (1) KV = [V] + [Nb] (2) << where [] is the content of each element (% By mass) >>.

In the present invention, when the high tensile strength steel sheet further contains 5% or less of Ni and / or 1.2% or less of Cu, or when the above-mentioned Mn content is 1.25 to 1.8%, High-strength steel sheets containing Cu: more than 1.2% and 2.0% or less, and high-strength steel sheets satisfying the Mn content and the Cu content and further containing Ni: 5% or less have further improved weldability. It is preferable because it can be increased. Further, a high-strength steel sheet further containing Ca: 0.005% or less,
Further, Si: 1% or less, P: 0.020% or less, S:
A high-strength steel sheet, which is suppressed to 0.010% or less and Al: 0.2% or less, is a preferable embodiment because the weldability is further enhanced.

The high-strength steel sheet of the present invention has a thickness of 80%.
Even those having a thickness of not less than mm have good weldability and base material strength.

The above-mentioned high-strength steel sheet can be manufactured by the following manufacturing method. (1) A step of heating to a temperature of A _{c3 to} 1300 ° C., performing hot rolling at a rolling finish temperature of 700 ° C. or higher, and then cooling.
A method comprising the step of reheating to 0 to 900 ° C and cooling. (2) heating the A _c3 point to 1300 ° C., and cooled after hot rolling at a finish rolling temperature of 700 ° C. or higher, then A _c1
Tempering at a temperature below the point. (3) Heating from A _c3 point to 1300 ° C, rolling finish temperature 7
After hot rolling at a temperature of 00 ° C. or more, cooling was performed, and then A _c3
A method of reheating to a temperature of from about 1000 ° C. to 1000 ° C., followed by cooling, and then, if necessary, tempering at a temperature not higher than the A _c1 point. (4) Heating from A _c3 point to 1300 ° C, rolling finish temperature 7
A method in which hot rolling is performed at a temperature of 00 ° C. or higher and then cooling is performed.

The production method (1) includes a heat treatment at (α + γ) two-phase temperature of 720 to 900 ° C. after hot rolling → cooling, whereby a low YR (YR ≦ 80) is obtained.
%) Of a high-strength steel sheet. Also,
The production methods (2) to (4) do not include the heat treatment at the (α + γ) two-phase region temperature. Among them, the production methods (2) and (3) have high YR (YR> 80%). )) Can be manufactured. In the manufacturing method (4),
Although a high-strength steel sheet having a low YR or a high YR can be obtained due to the influence of the chemical composition and the like, this method can manufacture the high-strength steel sheet of the present invention in fewer steps than the manufacturing methods (1) to (3).

The chemical composition of the high-tensile steel sheet according to the present invention typically comprises the balance of Fe and unavoidable impurities in addition to the above elements, but other chemical components do not impair the effects of the present invention. It may be contained within the range.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, in the case of a 490 MPa steel sheet, it was possible to achieve both improvement in welding crack resistance and securing of base metal strength by controlling Pcm.
Even if the component control by Pcm is performed on a grade steel sheet, it is difficult to satisfy both characteristics especially in a thick material.

In general, when upper bainite is generated during large heat input welding, island martensite is generated, and the HAZ toughness of the steel is deteriorated. Therefore, in a 490 MPa class steel sheet, ferrite is actively generated in the HAZ. In order to improve the large heat input HAZ toughness, attempts have been made to improve the large heat input HAZ toughness by controlling the Ceq, but this is contrary to increasing the strength and increasing the wall thickness. It was also difficult to achieve both thickness and thickness.

Therefore, in the present invention, there is no Pcm which has been used as an index for the resistance to weld cracking and Ceq which has been used as an index for securing high heat input HAZ toughness.
It was studied whether welding crack resistance and large heat input HAZ toughness could be controlled by completely different parameters. as a result,
Using KP represented by the above equation (1) and KV represented by the above equation (2) in consideration of the steel structure, further reducing the amount of C and adding B to provide excellent welding crack resistance, They have found that high heat input HAZ toughness and base metal strength can be achieved, and have completed the present invention.

First, a technique for improving the weld crack resistance and the large heat input HAZ toughness in the present invention will be described. As described above, in the present invention, after limiting C to extremely low C, Mn and Cr, which are quenchability-improving elements, and in some cases, Mo are further positively added, and are determined by the content of the quench-enhancing element. Control the appropriate KP value,
B is further added, and V, which is a high heat input HAZ toughness reducing element, is added.
The point is that the addition of Nb and Nb was suppressed by appropriately controlling the KV value. By appropriately adding these components, the continuous cooling curve of bainite (see the CCT diagram in FIG. 4) moves to a short time side and a low temperature side, and the CCT line of ferrite moves to a long time side. (Moved from solid line to dashed line).

Therefore, conventionally, since a martensite is formed at a high cooling rate and ferrite or upper bainite is formed at a low cooling rate, the cooling rate sensitivity of the hardness is large,
Although it was not possible to achieve both a reduction in the hardness of the HAZ portion during welding with a small heat input (improved weld cracking resistance) and an assurance of the base material strength, it was difficult to achieve preheating-free. Generates low-temperature transformation bainite at both low and high cooling rates, lowers the cooling rate sensitivity of hardness, reduces HAZ hardness during welding (improves weld cracking resistance) and ensures base metal strength It was a break.

On the other hand, in the case of large heat input welding, the cooling rate of the HAZ becomes slow, so that conventionally, ferrite or upper bainite is generated, and a coarse and massive island-like martensite structure is generated, thereby deteriorating the HAZ toughness. However, in the present invention, even when the cooling rate is low, the low-temperature transformation bainite is formed, so that a martensite structure is formed instead of a lump, instead of a lump. HAZ toughness could be secured.

The above-described welding crack resistance and high heat input HA
An application for an approach to improving the Z toughness has already been filed (Japanese Patent Application Nos. 10-336268, 11).
-356606). These prior inventions are 780 MPa
Cracking Resistance and High Heat Input HA in High Strength Steel Sheets
It was filed for the purpose of improving Z toughness. Therefore, both the above-mentioned prior invention and the present invention are common in that they aim to improve weld cracking resistance and high heat input HAZ toughness.
The invention of the present application is directed to a “590MP
This is different from these prior inventions in that the present invention is directed to a high-tensile steel sheet of “a or more and less than 780 MPa”.

The components that contribute to the improvement of the weld cracking resistance and the high heat input HAZ toughness, and the KP and KV values will be described below.

C: 0.010% to 0.06% C is an important element for achieving both welding crack resistance of the HAZ portion at the time of welding and the strength of the base material, and improving high heat input HAZ toughness. . When C exceeds 0.06%, martensite is generated instead of low-temperature transformed bainite on the high cooling rate side, and welding crack resistance and large heat input HAZ toughness are not improved. Preferably it is 0.055% or less.
If it is less than 0.010%, the necessary minimum base material strength cannot be obtained. Preferably it is 0.020% or more.

Mn: 1.25 to 2.5% Cr: 0.1 to 2.0% Mo: 1.5% or less (including 0%) These elements have an effect of improving hardenability. At a high cooling rate to a low cooling rate, low-temperature transformed bainite is easily generated, and as described above, the extremely low C is set, and at the same time, a predetermined B
By adding the amount, it is useful in that the welding crack resistance of the HAZ portion at the time of small heat input welding and the securing of the base material strength can both be achieved, and the large heat input HAZ toughness can be improved.

First, the contents of Mn and Cr must be 1.25% or more and 0.1% or more, respectively. If the content is less than these, the desired hardenability improving effect is not exhibited, and the base material strength is insufficient. Preferably M
n: 1.3% or more, Cr: 0.3% or more. Cr:
More preferably, it is more than 0.5%. However, the contents of Mn, Cr and Mo are 2.5%, 2.0% and 1.5%, respectively.
%, The toughness of the base material decreases. Preferably Mn:
2.2% or less, Cr: 1.5% or less, Mo: 1.3% or less.

Further, the KP value determined by these elements needs to be 2.4 or more and 4.5 or less. KP
When the value is less than 2.4, the above effect cannot be effectively exerted, and upper bainite or ferrite is generated, and a base material strength of 590 MPa or more cannot be obtained (see FIG. 1 described later). Preferably it is 2.7 or more. However, when the KP value exceeds 4.5, the high heat input HAZ toughness decreases. Preferably it is 4.3 or less.

V: 0.04% or less (including 0%) Nb: 0.04% or less (including 0%) V has an effect of increasing hardenability and temper softening resistance by adding a small amount. However, when added in excess of 0.04%, the high heat input HAZ toughness decreases. Preferably V: 0.0
3% or less. Nb refines the γ grain size and thereby refines the bainite block size after transformation, thereby contributing to an improvement in base material toughness. However, when the added amount of Nb is 0.1.
If it exceeds 04%, the high heat input HAZ toughness decreases. Preferably, Nb is 0.03% or less.

Further, the KV value determined by these elements needs to be 0.040 or less. As above,
If both of these elements are added in too large amounts, large heat input HA
This is because the Z toughness is reduced. Preferably 0.035
It is as follows.

B: 0.0006 to 0.005% B is a hardenability improving element, which facilitates the formation of low-temperature transformed bainite at a low cooling rate, and has an extremely low C as described above, and at the same time, an appropriate amount of Mn and Cr. By adding Mo, Mo, it is useful in that the welding crack resistance of the HAZ at the time of small heat input welding and the securing of the base metal strength can both be achieved. B
If less than 0.0006%, the effect of improving hardenability cannot be expected, and the base material strength will be insufficient. Preferably 0.000
7% or more, more preferably 0.0010% or more. However, if B exceeds 0.005%, the hardenability is rather reduced and the base material strength is insufficient. Preferably 0.00
3% or less.

Ti: 0.005 to 0.03% Ti forms nitride with N to form HA at the time of high heat input welding.
It is useful in that it refines the γ grains in the Z portion and contributes to the improvement in HAZ toughness. However, when Ti exceeds 0.03%, H
AZ toughness decreases. Preferably it is 0.02% or less. If the content is less than 0.005%, the effect of improving the large heat input HAZ toughness is not sufficient. Preferably it is 0.007% or more.

N: 0.0020% to 0.010% As described above, N is useful in that it forms a nitride with Ti and contributes to the improvement of HAZ toughness during large heat input welding. However, N combines with B to reduce solid solution B, impairs the effect of improving the hardenability of B, and also has the effect of reducing the toughness of the base metal and the high heat input HAZ toughness. Is 0.01
If it exceeds 0%, the effect becomes significant. Preferably 0.
008% or less. In addition, if less than 0.0020%, T
The effect of improving the large heat input HAZ toughness by forming a nitride with i is not sufficient. Preferably it is 0.0030% or more.

In the present invention, in order to further improve the weldability, it is recommended to actively add the following elements or to suppress the contents thereof.

Ni: 5% or less Ni is an element useful for improving the base material toughness. However, if it exceeds 5%, scale flaws are likely to occur, so the upper limit is preferably set to 5%. More preferably 4%
It is as follows.

Cu: 1.2% or less Cu is an element which improves the strength of the base material by solid solution strengthening and precipitation strengthening and also has an effect of improving hardenability. However, if added in excess of 1.2%, the high heat input HAZ toughness decreases, so the upper limit is preferably set to 1.2%. More preferably, it is 1.0% or less.

However, when the Mn content is in the range of 1.25 to 1.8%, it is possible to suppress a decrease in the high heat input HAZ toughness due to Cu. Even if added in excess, high heat input HAZ toughness can be ensured. However, even in this case, if the Cu content exceeds 2.0%, the large heat input H
Since the AZ toughness decreases, the upper limit is preferably set to 2.0%. More preferably, it is 1.5% or less.

Ca: 0.005% or less Ca is an element that has the effect of reducing the anisotropy of inclusions because it spheroidizes MnS. In order to exert such an effect, it is preferable to add 0.0005% or more. More preferably, it is 0.0010% or more. However,
If it is added in excess of 0.005%, the base material toughness is reduced, so the upper limit is preferably made 0.005%. More preferably, it is 0.004% or less.

Si: 1% or less Si is an element useful as a deoxidizing agent, but if added in excess of 1%, the weldability and the base metal toughness are reduced, so the upper limit is preferably set to 1%. More preferably 0.6
% Or less.

P: 0.020% or less, S: 0.010%
Hereinafter, P and S are impurity elements. Therefore 0.020 each
% Or less, preferably 0.010% or less.

Al: not more than 0.2% Al is a deoxidizing element and an element which fixes N and increases solid solution B to enhance the quenching property improving action based on B. If added in excess of 0.1%, the toughness of the base material decreases, so the upper limit is preferably set to 0.2%. It is more preferably at most 0.1%.

Next, a method for producing the steel sheet of the present invention will be described. The steel sheet of the present invention can be manufactured by using a steel satisfying the above-mentioned chemical composition and appropriately employing the manufacturing process and conditions (temperature, time, etc.) of a commonly used high-tensile steel sheet.

As described above, by adopting the manufacturing methods (1) to (3), it is possible to selectively produce high-strength steel sheets having a yield ratio according to the purpose of use. For example, when it is used for a structural material that is particularly required to have earthquake resistance, it is preferable to use a high-strength steel plate having a low YR, and it may be manufactured by the method (1). On the other hand, when it is used for a structural material that emphasizes proof stress or strength (such as for a bridge), a high-tensile steel plate with a high YR may be used, and the production method of (2) or (3) may be appropriately determined according to the chemical composition and required characteristics. Just choose.

As described above, the high-strength steel sheet of the present invention comprises (1)
In addition to the method of (3), low YR and high YR can be produced separately. In addition, if the manufacturing method of (4) that only cools after hot rolling is used, the influence of chemical composition, etc. May be low YR or high YR,
The high-tensile steel sheet of the present invention can be manufactured with a small number of steps.

In the production method (1), a step of cooling after hot rolling and a step of performing a heat treatment at a temperature of (α + γ) two-phase region (a step of cooling by reheating to 720 to 900 ° C.) In the meantime, if necessary, a step of cooling after reheating to the Ac ₃ point to 1000 ° C. can be included. This step makes it possible to increase the toughness of the steel sheet by refining the structure. Also,
After the step of performing the heat treatment at the (α + γ) two-phase region temperature, a step of tempering at a temperature equal to or lower than the A _c1 point may be included if necessary. This step removes the stress remaining in the steel sheet and stabilizes the steel sheet. be able to. These steps included as necessary may be appropriately performed according to the chemical composition of the steel, the required characteristics of the obtained steel sheet, and the like.

In the production method of the present invention, after hot rolling, after reheating from the Ac ₃ point to 1000 ° C., and (α +
γ) Each cooling method after the heat treatment at the two-phase region temperature is not particularly limited, and a known cooling method such as air cooling or water cooling can be adopted, and may be appropriately selected according to the chemical composition, required characteristics, and the like.

As a specific example, 950 to 1300
After heating at ℃ for 2 hours or more, hot rolling is performed, and
The rolling is completed at 950 ° C. and then cooled. Then 72
After maintaining at (α + γ) two-phase region temperature of 0 to 900 ° C. for 30 minutes or more, water cooling is performed to obtain the high-tensile steel sheet of the present invention. In the case of performing the tempering step, it is recommended that the tempering step be performed by holding at 450 to 650 ° C. for 10 to 40 minutes.

[0047]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on embodiments. However, the following embodiments do not limit the present invention, and all modifications and implementations without departing from the spirit of the preceding and following embodiments are included in the technical scope of the present invention.

Experiment 1 Steel having the chemical composition shown in Tables 1 and 2 was smelted by a normal smelting method to form a slab, then subjected to normal heating and hot rolling, and then directly quenched from 850 ° C. Thereafter, quenching and tempering were performed under the conditions shown in Tables 3 and 4, to produce a high-strength steel sheet having a predetermined thickness.

For each of the steel sheets thus obtained,
Base material properties [strength and toughness (vE _-40 )]
Was evaluated, and the base material level (590 MP
Those that satisfied a ≦ tensile strength <780 MPa, vE ₋₄₀ ≧ 47 J) were further evaluated for weldability (weld cracking resistance and large heat input HAZ toughness).

[Base material property test] Tensile test: A JIS No. 4 test piece was sampled from a quarter of the thickness of each steel sheet, and a 0.2% proof stress and a tensile strength were measured by performing a tensile test. 590 MPa ≦ tensile strength <
780 MPa was regarded as a pass. Impact test: A JIS No. 4 test piece was sampled from a quarter of the thickness of each steel sheet, and subjected to a Charpy impact test to obtain an absorbed energy (vE _-40 ). vE _-40 ≧ 47J
Was passed.

[Weldability test] HAZ toughness: heat input 100 or 120 kJ / mm
(Electro-slag welding method), a JIS No. 4 test piece was sampled from the site shown in FIG. 3, and a Charpy impact test was performed to determine the absorbed energy (vE ₋₁₀ ) of the bond portion. vE _-10 ≧ 100J was accepted. Weld crack resistance: Covered arc welding was performed at a heat input of 1.7 kJ / mm based on the y-type weld crack test method described in JIS Z 3158, and the root crack prevention preheat temperature was measured. 25 ° C or less was regarded as acceptable.

The results are shown in Tables 3 and 4.

[0053]

[Table 1]

[0054]

[Table 2]

[0055]

[Table 3]

[0056]

[Table 4]

From Tables 3 and 4, the following can be considered.

First, the steel sheets shown in Table 1 are examples satisfying the requirements of the present invention. As shown in Table 3, all of the steel sheets were excellent in base metal properties and weldability.

On the other hand, the steel sheets shown in Table 2 are comparative examples that do not satisfy the requirements of the present invention or are reference examples that do not partially satisfy the requirements of the present invention.

First, No. In No. 25, the C content was below the lower limit of the present invention, and the desired base material strength could not be obtained.
In addition, No. 26 is an example in which the C content exceeds the upper limit of the present invention, and the welding crack resistance was reduced.

No. 27 and no. No. 28 is an example in which the KP value was lower than the lower limit of the present invention, and the desired base material strength could not be obtained.

No. No. 29 is an example in which the KP value exceeds the upper limit of the present invention, and the high heat input HAZ toughness was reduced.

No. Sample No. 30 is an example in which the amount of Mn is lower than the lower limit of the present invention, and the desired base material strength was not obtained. In addition, No. 31 is an example in which the amount of Mn exceeds the upper limit of the present invention, and the desired base material toughness was not obtained.

No. No. 32 is an example in which the Cr content was lower than the lower limit of the present invention, and the desired base material strength could not be obtained. In addition, No. No. 33 is an example in which the Cr content exceeds the upper limit of the present invention, and the desired base material toughness was not obtained.

No. Sample No. 34 is an example in which the Mo content exceeds the upper limit of the present invention, and the desired base material toughness was not obtained.

No. No. 35 is an example in which the V amount and the KV value exceeded the upper limits of the present invention, and the large heat input HAZ toughness was reduced.

No. No. 36 is an example in which the Nb amount and the KV value exceeded the upper limits of the present invention, and the high heat input HAZ toughness was reduced.

No. 37 is an example in which the KV value exceeds the upper limit of the present invention, and the high heat input HAZ toughness is reduced.

No. 38 is an example in which the B value is lower than the lower limit of the present invention, and the desired base material strength was not obtained. Also,
No. 39 is an example where the B value exceeds the upper limit of the present invention,
The desired base material strength was not obtained.

No. In the example No. 40, the amount of Ti was lower than the lower limit of the present invention, and the high heat input HAZ toughness was lowered. Also, N
o. 41 is an example in which the amount of Ti exceeds the upper limit of the present invention,
Large heat input HAZ toughness decreased.

No. 42 is an example in which the Ca amount exceeds the upper limit of the present invention, and the desired base material toughness was not obtained.

No. No. 43 is an example in which the amount of N was lower than the lower limit of the present invention, and the large heat input HAZ toughness was reduced. Also, N
o. Sample No. 44 is an example in which the amount of N exceeds the upper limit of the present invention, and the large heat input HAZ toughness was reduced.

No. No. 45 is an example in which the balance between the Mn amount and the Cu amount is poor and exceeds the upper limit of the present invention.
Z toughness decreased.

FIG. 1 is a graph showing the relationship between the base material strength (tensile strength) and the KP value based on the above results.
590MP by controlling KP value greater than 2.4
It turns out that the tensile strength more than a is obtained.

FIG. 2 shows the HAZ toughness (v) when welding with a heat input of 100 or 120 kJ / mm based on the above results.
It is a graph of the relationship between E- ₁₀ ) and the KV value.
By controlling the KV value to 0.040 or less, 100J
It can be seen that the above HAZ toughness can be obtained.

Experiment 2 Steel (No. 46) having the chemical composition shown in Table 5 was smelted by a usual smelting method to form a slab, and then hot-rolled under the conditions shown in Table 6. Heat treatment was performed under the conditions described to produce a high-tensile steel sheet having a predetermined thickness. The heat treatment under the “heat treatment condition 2” was performed after the heat treatment under the “heat treatment condition 1”.

Each of the obtained steel sheets was evaluated in the same manner as in Experiment 1. The “yield ratio” was measured during a tensile test. Table 6 shows the results.

[0078]

[Table 5]

[0079]

[Table 6]

As can be seen from Table 6, after hot rolling → cooling, heat treatment was performed at 720-900 ° C., which is the (α + γ) two-phase region temperature. The steel sheets Nos. 46-1 to 46-12 have a low yield ratio (YR ≦ 80%). Steel plates 46-13 to 16 have a high yield ratio (YR> 80%). In addition, No. No heat treatment was performed after hot rolling → cooling. 46
In the steel sheet of −17, although the base material properties were slightly lower than those of the other steel sheets, all the properties were acceptable values.

[0081]

The present invention is configured as described above.
A steel plate having excellent weldability (weld crack resistance and high heat input HAZ toughness) of 590 MPa or more and less than 780 MPa could be provided. According to the present invention, it is possible to provide a high-strength steel sheet having the above characteristics even if the thickness is 80 mm or more.

Further, by employing the above-mentioned production methods (1) to (3), the high-strength steel sheet of the present invention having a yield ratio according to the intended use can be produced, and the above-mentioned (4) having a small number of steps. , The high-tensile steel sheet of the present invention can be produced.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between base material strength and KP value.

FIG. 2 is a graph showing the relationship between high heat input HAZ toughness and KV value.

FIG. 3 is a schematic explanatory view showing a test specimen collection position for bond toughness during electroslag welding.

FIG. 4 is a schematic CCT diagram for explaining the concept of component design of the present invention.

 ────────────────────────────────────────────────── ─── Continued on the front page F term (reference) 4K032 AA01 AA02 AA04 AA08 AA11 AA12 AA14 AA15 AA16 AA17 AA19 AA20 AA21 AA22 AA23 AA24 AA27 AA29 AA31 AA35 AA36 BA01 CA02 CA03 CC02 CC03 CC04 CF02 CF03

Claims (11)

[Claims] 1. C: 0.010 to 0.06% (meaning by mass%, the same applies hereinafter), Mn: 1.25 to 2.5%, Cr:
0.1 to 2.0%, Mo: 1.5% or less (including 0%), V: 0.04% or less (including 0%), Nb:
0.04% or less (including 0%), Ti: 0.005 to
0.03%, B: 0.0006 to 0.005%, N
: Made of steel satisfying 0.0020 to 0.010%, satisfying 2.4 ≦ KP ≦ 4.5 KV ≦ 0.040, respectively, and having a tensile strength of 590 MPa or more 7
A high-tensile steel sheet excellent in weldability, characterized by being less than 80 MPa. However, KP = [Mn] + 1.5 × [Cr] + 2 × [Mo] KV = [V] + [Nb] << In the formula, [] means the content (% by mass) of each element. 》 2. Ni and 5% or less and / or C
The high-tensile steel sheet according to claim 1, wherein u: 1.2% or less is contained. 3. The high-tensile steel sheet according to claim 1, wherein
When the Mn content is 1.25 to 1.8%,
u: High-tensile steel sheet containing more than 1.2% and not more than 2.0%. 4. The high-strength steel sheet according to claim 3, further comprising Ni: 5% or less. 5. The high-strength steel sheet according to claim 1, further comprising Ca: 0.005% or less. 6. The method according to claim 1, wherein Si: 1% or less, P: 0.020% or less, S: 0.010% or less, and Al: 0.2% or less. High strength steel sheet. 7. The method according to claim 1, wherein the thickness is 80 mm or more.
A high-tensile steel sheet according to any one of the above. 8. The method for producing a high-strength steel sheet according to claim 1, wherein the steel sheet is heated to a point of Ac _{3 to} 1300 ° C., hot-rolled at a rolling finish temperature of 700 ° C. or higher, and then cooled. And a step of reheating to 720 to 900 ° C. and cooling to produce a low-yield-ratio high-tensile steel sheet having excellent weldability. 9. The method for producing a high-strength steel sheet according to claim 1, wherein the steel sheet is heated to an A _c point of 1 to 1300 ° C. and a rolling finish temperature is 700.
A method for producing a high-yield-ratio high-strength steel sheet excellent in weldability, characterized in that after hot rolling at a temperature of not less than ° C, it is cooled and then tempered at a temperature of not more than A _c1 point. 10. The method for producing a high-strength steel sheet according to claim 1, wherein the steel sheet is heated to an A _c point of _{3 to} 1300 ° C. and a rolling finish temperature is 700.
℃ cooling after hot rolling at above, it continued A _c3 point -
After reheating to 1000 ° C., it is cooled, and then A
_A method for producing a high-yield-ratio high-strength steel sheet excellent in weldability, characterized by tempering at a temperature of _c1 point or less. 11. The method for producing a high-strength steel sheet according to claim 1, wherein the steel sheet is heated to an A _c point of 1 to 1300 ° C. and a rolling finish temperature of 700.
A method for producing a high-strength steel sheet having excellent weldability, wherein hot-rolling is performed at a temperature of at least 100 ° C. and then cooling is performed.