JP2002173734A - Steel having excellent weldability and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method capable of stably and industrially supplying steel applicable to wide range of use, by which control of strength, toughness and yield ratio can be made so as to meet with the use. SOLUTION: In the steel, the content of C is controlled to <=0.1 mass%, further, the contents of Si, Mn, P, S, Al, N or the like are controlled, and, also, Nb and Ti are incorporated with limit in a prescribed range. Further, in its production method, the steel is reheated at 1,000 to 1,300 deg.C, is thereafter rolled so as to control the cumulative draft at <=1,000 deg.C to >=30%, and to be finished at >=720 deg.C, is subsequently allowed to cool, or is subjected to accelerated cooling from >=700 deg.C to an optional temperature of <=600 deg.C, and is, if required, tempered.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel having excellent weldability and a method for producing the same, and can be applied to a thick plate, a section steel, a hot strip mill and the like in the steel industry.
The steel can be applied to a wide range of uses as welded structural steel for construction, civil engineering, marine structures, shipbuilding, various storage tanks, construction and industrial machinery, and the like.

[0002]

2. Description of the Related Art Numerous published publications and patent publications have been disclosed in the past without reference to steel materials that claim to improve weldability and methods for producing the same. In any case, basically, the main point is to reduce the carbon equivalent (Ceq) and the weld crack susceptibility composition (P _CM ) by adjusting the composition of the steel material. It claims patentability by a combination of. For example, TMCP (thermo-mechani
It is well known that the weldability has been dramatically improved by precisely controlling the manufacturing process of the steel material called heating, rolling (controlled rolling), and cooling (controlled cooling), which is called cal control process, together with the steel components. It is.

[0003] A microalloy represented by Nb plays an important role in the TMCP technology. The added amount was not so large from the viewpoint of effect on metallurgical phenomena in terms of merit and cost and from the viewpoint of cost. In particular, Nb has a large atomic weight ratio between Nb and C (Nb
b / C ≒ 7.8), the extremely low C IF (Int
Other than steel, it is not added in excess of the stoichiometric excess to C, and the metallurgical effects such as the weldability of Nb in such a range are not necessarily required. There was not enough knowledge.

[0004] In addition, the metallurgical effect of "solid solution Nb" can be utilized by temporarily forming Nb into a solution by heating at a high temperature. After being precipitated as carbonitride, Nb in a solid solution state cannot be used.

[0005]

DISCLOSURE OF THE INVENTION The present invention aims to provide a steel having excellent weldability without deteriorating its basic properties such as strength and toughness as a steel material for a welded structure, rather than an IF steel having an extremely low C and almost no alloy addition. To obtain Nb stoichiometrically, C, N
In excess, so that it can be used as "solid solution Nb" in all aspects. Furthermore, by limiting the manufacturing method, it is possible to adjust the strength and toughness according to the application, and to adjust the yield ratio, and to provide a method that can industrially supply steel suitable for a wide range of applications. is there.

[0006]

The most important point of the present invention is that Nb is added in stoichiometric excess with respect to C and N to secure "solid solution Nb" in all aspects. In this way, solid-solution Nb can be secured without the need to perform high-temperature heating, so that metallurgical effects due to solid-solution Nb such as an effect of suppressing recrystallization of austenite during hot rolling and an effect of suppressing transformation during cooling can be enjoyed. As a result, toughness can be achieved with a relatively low carbon equivalent (Ceq) and a weld cracking susceptibility composition (P _CM ) having excellent weldability.

For this purpose, the manufacturing method including the steel component is limited according to the present invention, and the gist is as follows.

(1) Steel component in mass%, C: 0.1% or less, Si: 0.6% or less, Mn: 0.2 to 1.6%,
P: 0.02% or less, S: 0.01% or less, Al: 0.
06% or less, N: 0.006% or less, and Access Nb = Nb−7.8 × [C− (Ti−
3.4N) / 4] Nb: 0.01 to 0.5%, Ti: 0.0 so that Excess Nb satisfying + 0.01% or more is satisfied.
Nb alone or Nb and Ti within the range of 0.05 to 0.1%.
Characterized by excellent weldability, characterized by containing both of the above, with the balance being iron and unavoidable impurities.

(2) In addition to the above steel components, C
u: 0.05 to 2.0%, Ni: 0.05 to 1.0%, 1/2 or more of the added amount of Cu, Cr: 0.05 to 1.0.
(1) The steel having excellent weldability according to (1), wherein the steel contains one or more kinds in the range of 0% and Mo: 0.05 to 1.0%.

(3) In mass%, V: 0.005 to 0.1
% Ta: The steel having excellent weldability according to (1) or (2), wherein one or two types are contained in a range of 0.005 to 0.1%.

(4) B: 0.0002 to 0.1% by mass.
(1)-characterized by further containing 005%.
The steel excellent in weldability according to any one of (3).

(5) Ca: 0.0005 to 5% by mass
0.001%; REM: 0.0005% to 0.004%.
The steel excellent in weldability according to any one of-(4).

(6) Mg: 0.0002-% by mass
(1) characterized by further containing 0.005%.
A steel excellent in weldability according to any one of-to (5).

(7) A slab or a slab made of the steel component according to any one of (1) to (6) is prepared in the range of 1000 to 130.
After reheating to a temperature range of 0 ° C., rolling is completed at a temperature of 720 ° C. or more with the cumulative rolling reduction at 1000 ° C. or less being 30% or more, and then cooling is performed or the temperature is reduced from a temperature of 700 ° C. or more to 600 ° C.
A method for producing steel having excellent weldability, characterized in that the steel is accelerated to an arbitrary temperature of not more than ° C.

(8) After hot rolling a slab or a slab made of the steel component according to any one of (1) to (6),
_A method for producing steel having excellent weldability, characterized in that normalizing is performed at a temperature of _{3 to} 1000 ° C.

(9) After hot rolling a slab or a slab comprising the steel component according to any one of (1) to (3),
_A method for producing steel having excellent weldability, comprising quenching after reheating to a temperature of _{3 to} 1000 ° C.

(10) The steel sheet according to any one of (7) to (9), wherein the steel sheet is tempered at a temperature lower than Ac ₁ for the purpose of adjusting strength, improving toughness, or removing residual stress of the steel sheet. 3. A method for producing steel having excellent weldability according to item 1.

(11) For the purpose of lowering the yield ratio,
After reheating to the two-phase coexistence region of ferrite and austenite of more than ₁ Ac _{3 and} less, it is allowed to cool to a temperature of 600 ° C. or less at a cooling rate of not less than ₁ and then tempered at a temperature of less than Ac ₁ if necessary (7)-
(9) The method for producing a high-tensile steel excellent in high-temperature strength according to any one of (9).

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.

The reason why the present invention limits the steel composition and the production method as described in the claims will be described.

First of all, C has the greatest effect on the weldability of steel, and as the amount of addition increases, the weldability deteriorates. Therefore, the lower the amount of addition, the more preferable. Secondly, it is necessary to add Nb, which is a feature of the present invention, in a stoichiometric excess with respect to the amounts of C and N to secure solid solution Nb.
At this time, in order to reduce the absolute amount of Nb as much as possible, the C amount is preferably as low as possible. Therefore, the lower limit is not particularly limited, but is naturally limited by the strength level to be obtained, the steelmaking capacity for decarburization, the cost, and the like. . On the other hand, the upper limit is 0.1 from the viewpoint described above.
%. Note that this upper limit is C in terms of weldability.
It is not determined solely by the amount, and also in terms of the amount of Nb added, mainly because of cost, and does not have a critical meaning in characteristics. In other words, the present invention is merely limited to clarify the features of the present invention.

Although Si is an element contained in the deoxidized upper steel, if added in a large amount, the weldability and the HAZ toughness deteriorate, so the upper limit was set to 0.6%. Deoxidation of steel is Ti, Al
Alone is sufficiently possible, and the lower the better, from the viewpoints of HAZ toughness, hardenability, etc., and it is not always necessary to add.

Mn is a useful element for securing the strength and toughness of the base material. Since it is a relatively inexpensive element, the addition of 0.2% or more is essential from the viewpoint of securing strength. The upper limit is limited to 1.6% because excessive addition promotes central segregation of the continuously cast slab and deteriorates weldability.

P is an impurity in the steel of the present invention,
Since a reduction in the amount of P tends to reduce grain boundary fracture in HAZ, a smaller amount is more preferable. If the content is large, the low-temperature toughness of the base material and the welded portion is deteriorated, so the upper limit is 0.02.
%.

S, like P, is an impurity in the steel of the present invention, and is preferably as small as possible from the viewpoint of the low-temperature toughness of the base material. If the content is large, the low-temperature toughness of the base material and the welded portion is deteriorated, so the upper limit was made 0.01%.

Al is an element generally contained in the deoxidized upper steel, but the deoxidation is sufficient with only Si or Ti, and in the steel of the present invention, the lower limit is not limited (including 0%). However, when the amount of Al increases, not only the cleanliness of the steel deteriorates, but also the toughness of the weld metal deteriorates, so the upper limit was made 0.06%.

N is contained in steel as an unavoidable impurity, but when Ti, which will be described later, is added, T
iN is formed to enhance the properties of steel, or combined with Nb, V, and Ta to form carbonitrides, thereby increasing strength. For this purpose, it is desirable that the content of N be at least 0.001%. However, an increase in the amount of N is extremely detrimental to HAZ toughness and weldability, and is preferably as small as possible from the viewpoint of securing solid solution Nb. In the present invention, the upper limit is 0.006%.

Nb is an indispensable element that forms the basis of the constitution in the present invention, and is characterized by the stoichiometrically excessive addition of Nb to C and N. The degree of excess is in mass%,
Access Nb = Nb−7.8 × [C− (Ti−
3.4N) / 4] is defined as +
0.01% or more. If it is less than + 0.01%, the so-called "solid solution Nb" is not sufficiently secured, and the effects of solid solution Nb in all aspects aimed at by the present invention cannot be enjoyed. In order for Excess Nb to satisfy + 0.01% or more, at least 0.01% or more of Nb is necessary even if the amounts of C and N are zero. Although the inventors have not grasped the upper limit of the amount of Nb, they are limited to 0.5% in consideration of the range confirmed by the laboratory and the alloy cost. Therefore,
This upper limit has no critical effect on the effect.

As a general effect of Nb, first, solid solution Nb raises the recrystallization temperature of austenite and exerts the effect of controlled rolling at the time of hot rolling to the maximum. In addition, a relatively large amount of solute Nb segregates at austenite grain boundaries and has an effect of delaying and suppressing ferrite transformation. As a result,
It contributes to the refinement and toughening of the structure. In addition, when Excess Nb, which is a feature of the present invention, is + 0.01% or more, it contributes to an increase in strength due to solid solution strengthening of Nb and an improvement in high-temperature strength due to interaction with dislocations at high temperatures. Further, Nb carbonitride contributes to reheating prior to rolling and grain refinement of heated austenite during heat treatment after rolling. Also,
The finely precipitated Nb carbonitride has a strength-improving effect as precipitation hardening, and also contributes to high-temperature strength improvement.

It is preferable to add Ti when the requirements for base metal and weld toughness are severe. The reason for this is that when the amount of Al is small (for example, 0.003%
The following is combined with O to form a precipitate mainly composed of Ti ₂ O _3, which serves as a nucleus for the formation of intragranular transformed ferrite and improves weld toughness. Further, Ti combines with N to form fine precipitates in the slab as TiN, which suppresses coarsening of γ grains during heating and is effective for reducing the rolling structure. Fine TiN present in the steel sheet is welded. This is because the structure of the heat affected zone is sometimes refined. In order to obtain these effects, Ti should be at least 0.1.
005% is required. Ti, C and N before Nb
Is preferably added from the viewpoint of securing Excess Nb. However, if it is too large, a large amount of TiC is formed, and the low-temperature toughness and weldability are deteriorated. Therefore, the upper limit is limited to 0.1%.

Next, Cu, Ni, Cr, Mo, V, Ta, B, Ca, RE which can be contained as required.
The reason for adding M and Mg will be described.

The main purpose of adding these elements to the basic components is to improve properties such as strength and toughness without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount added is of a nature that should be naturally restricted.

If Cu is not excessively added, weldability,
Improves the strength and toughness of the base material without adversely affecting HAZ toughness. In order to exert these effects, it is essential to add at least 0.05% or more. In particular, 0.
If it exceeds 5%, the strength is remarkably improved by the aging precipitation treatment. However, excessive addition causes deterioration of weldability and Cu-cracks during hot rolling, which makes production difficult. Therefore, the upper limit is limited to 2.0%.

Ni, like Cu, is not added unless excessively added.
Improves the strength and toughness of the base material without adversely affecting weldability and HAZ toughness. In order to exert these effects, it is essential to add at least 0.05% or more. On the other hand, excessive addition is not only expensive but also unfavorable for weldability, so the upper limit was made 1.0%. When Cu is added, in order to prevent Cu-cracks during hot rolling,
While satisfying the above addition range, at the same time, the amount of Cu added was １ ／.
It is necessary to do above.

Cr and Mo both improve the strength and toughness of the base material. The minimum amount for surely enjoying the effect is 0.05%. In particular, the addition of Mo also contributes to the improvement of the high-temperature strength, and the effect becomes remarkable at 0.4% or more. However, if the addition amount of both elements is too large, the toughness and weldability of the base metal and the welded portion are deteriorated.
0%.

The addition of Cu, Ni, Cr and Mo is as follows.
It also has an advantageous effect on weather resistance.

V and Ta have almost the same action as Nb, but their effects are smaller than Nb.
V also affects hardenability, and V, Ta
Contributes to the improvement of high-temperature strength. The effect similar to Nb is 0.
If it is less than 005%, the effect is small, and the upper limit can be allowed up to 0.1%.

B segregates at austenite grain boundaries and suppresses the formation of ferrite, thereby improving hardenability and contributing to strength improvement. To enjoy this effect, at least 0.0002% is required. However, too much addition not only saturates the hardenability improvement effect, but also may form B precipitates that are harmful to toughness, so the upper limit was made 0.005%. In cases where stress corrosion cracking is a concern for steel for tanks and the like, reduction of the hardness of the base metal and the heat affected zone is often a key point (for example, to prevent sulfide stress corrosion cracking (SSC)). HR for
C ≦ 22 (HV ≦ 248) is essential. In such a case, the addition of B which increases the hardenability is not preferable.

Ca and REM control the morphology of MnS, improve the low-temperature toughness of the base material, and cause hydrogen-induced cracking (HIC, SSC, SOHIC) in a wet hydrogen sulfide environment.
Reduces sensitivity. To achieve these effects, a minimum of 0.0005% is required. However, too much addition conversely increases the cleanliness of the steel, and increases the base metal toughness and hydrogen-induced cracking (HIC, SSC, SOHI) in a wet hydrogen sulfide environment.
C) To increase the sensitivity, the upper limit of the amount added is 0.004%
Limited to. Since Ca and REM have almost the same effect, any one of them may be added in the above range.

Mg has the effect of suppressing the growth of austenite grains in the heat affected zone of welding and reducing the size of the grains, thereby toughening the welded portion. In order to enjoy such effects, Mg needs to be 0.0002% or more. On the other hand, as the addition amount increases, the effect cost on the addition amount decreases,
The upper limit is set to 0.005% because it is not advantageous in terms of cost.

Next, the manufacturing conditions defined in claims 7 to 11 of the present invention and the reasons for limiting them will be described.

First, the heating temperature prior to the rolling according to claim 7 of the present invention is limited to 1000 to 1300 ° C. In structural steels, balancing the strength and toughness in a well-balanced manner is often one of the biggest issues, and microstructural refinement is one of the effective solutions. Reducing the austenite grains during heating is also effective in miniaturizing the rolling structure, and 1300 ° C., which is defined as the upper limit of the heating temperature by the present invention, is a temperature at which austenite during heating does not become extremely coarse. If the heating temperature exceeds this, austenite grains are coarsely mixed and the structure after transformation is also coarse, so that the toughness of the steel is deteriorated. On the other hand, a low heating temperature is advantageous in terms of refining the heated austenite grains, but not only increases the rolling load but also makes it difficult to secure a rolling end temperature (720 ° C. or higher) described later. Further, in the present invention, Nb is added in excess with respect to the amounts of C and N. However, the recrystallization temperature of austenite is increased by further forming a solution of the precipitated Nb during heating more or less. In order to maximize the effect of controlled rolling at the time and to exhibit the precipitation effect, the lower limit of heating is 100
Limited to 0 ° C.

In the rolling of a slab or a steel slab reheated to the above-mentioned temperature range, it is necessary to end the hot rolling at 720 ° C. or higher with the cumulative reduction at 1000 ° C. or lower being 30% or more. This is because if the cumulative rolling reduction at a temperature of 1000 ° C. or less is small, the grain size of the rolled austenite becomes insufficient, and it is difficult to secure toughness. Further, when the rolling end temperature is lower than 720 ° C., in the steel of the present invention having a relatively low C content, the possibility that transformation is partially started increases, and there is a possibility that a processed (rolled) structure may be left in the final structure, and the toughness may be reduced. If the yield ratio is not only unfavorable but also increases the yield ratio and a low yield ratio is required for architectural use, etc., the production becomes difficult. Therefore, the rolling end temperature is limited to 720 ° C. or higher.

After the rolling, the steel sheet is allowed to cool or accelerated cooling from a temperature of 700 ° C. or more to an arbitrary temperature of 600 ° C. or less. The present invention is characterized in that Nb is added in excess with respect to the amounts of C and N, and Nb in a solid solution state is always present in a relatively large amount. As described above, this relatively large amount of solute Nb segregates at austenite grain boundaries and has the effect of delaying and suppressing ferrite transformation. Therefore, by setting the pressure heating-rolling conditions as in the present invention,
After rolling, the structure can be refined even if it is left to cool. In addition, accelerated cooling from a temperature of 700 ° C. or more to an arbitrary temperature of 600 ° C. or less after rolling is preferably applied for the purpose of further increasing the toughness in order to further refine the structure. At this time, since the Nb content is stoichiometrically added in excess of C and N by + 0.01% or more, even if accelerated cooling to room temperature, no excessive hardening occurs, and the accelerated cooling stop temperature Is not limited. Also, 300-6
Stopping the accelerated cooling at a moderate temperature of 00 ° C. is a preferable method depending on the purpose because the subsequent cooling step has a tempering effect.

The cooling rate during the accelerated cooling cannot be unconditionally determined because it varies depending on the steel composition and the intended material (strength, toughness) level. It is desirable that the average cooling rate up to the temperature be at least 3 ° C./sec or more.

Next, a manufacturing method according to claims 8 and 9 of the present invention will be described.

Even if the steel having the components defined by the present invention is hot-rolled and then subjected to the heat treatment defined by the present invention, the excellent properties of the steel material of the present invention are not impaired. Rather, it may be preferable depending on the purpose, because the structure of the steel material and the resulting material are homogenized. However, even in this case, since the refining of the structure is one of the points for simultaneously improving the strength and toughness of the steel material, the reheating temperature during the heat treatment is not less than Ac _3.
It is necessary to keep the temperature below 000 ° C. Generally, in the case of Nb-added steel, at the normalizing or quenching temperature of the degree specified in the present invention, Nb does not sufficiently turn into a solution,
Although the metallurgical effect of the above cannot be sufficiently enjoyed, such a heat treatment can be used because the present invention secures a specific amount of Excess Nb or more. The lower limit of the heat treatment temperature is for homogenization of the structure, and the upper limit is for not increasing the austenite grain size during reheating more than necessary. The cooling after the heat treatment may be either normalizing or quenching for the same reason as the cooling after rolling (cooling or accelerated cooling) according to claim 7.

The reason for the tempering at a temperature lower than Ac _{1 according to the} tenth aspect is to adjust the strength, improve the toughness, or to remove the residual stress of the steel sheet. It can be applied to any of the heat-treated materials according to Examples 9 and 9 as needed, and does not impair the excellent features of the present invention at all.

The reheating heat treatment in the two-phase coexistence region of ferrite less than Ac ₁ and less than Ac ₃ and austenite according to the eleventh aspect is performed as necessary to reduce the yield ratio required for building steel and the like. . This is for obtaining a two-phase mixed structure of a soft structure, ferrite and a hard structure, and can be applied to any of the rolled material according to claim 7 and the heat-treated material according to claims 8 and 9. . At the time of reheating, since two-phase separation is achieved, the cooling after reheating may be allowed to cool to a temperature of 600 ° C. or less at a cooling rate higher than that, but depending on the intended strength, the cooling rate may be reduced. In some cases it is preferable to increase. Then, if necessary, Ac ₁
Tempering at a temperature lower than the above is the same as the explanation and the reason according to claim 10.

[0050]

EXAMPLES Steel plates (thickness: 20 to 100 mm) of various steel components were produced in a converter-continuous casting-thick plate process, and their mechanical properties were investigated.

Table 1 shows the steel composition of the steel according to the present invention together with the comparative steel, and Table 2 shows the results of examination of the manufacturing conditions and various properties of the steel sheet.

The steel sheet (steel according to the present invention) having the components, structure and production method according to the present invention all have good properties. On the other hand, the comparative steels whose steel composition and production conditions deviate from the limited range of the present invention are clearly inferior in strength, toughness or high-temperature strength.

That is, in Comparative Example 21, Nb was 0.1
%, But because of the high C content,
Insufficient ss Nb content results in poor toughness and high-temperature strength.
Comparative Example 22 is also inferior in toughness and high-temperature strength due to the low amount of Excess Nb and the small amount of cumulative reduction at 1000 ° C. or lower in the rolling step. Also, since the amount of Ni added is lower than the amount of Cu added,
Cracks occurred during hot rolling, making production difficult. In Comparative Example 23, although the amount of Excess Nb is stoichiometrically positive, it is lower than + 0.01% specified by the present invention. Not as good, but somewhat inferior. Comparative Example 24
Then, because the amount of Nb added is small, the calculation is Excess
Although the Nb content is within the range of the present invention, the strength is low and the high temperature strength is poor.

[0054] Incidentally, weldability, the present invention examples, both Comparative Examples Ceq, since the design low P _CM, It may be noted that one no problem even.

[0055]

[Table 1]

[0056]

[Table 2]

[0057]

According to the present invention, it has become possible to provide steel having excellent weldability and excellent basic performance such as strength and toughness. The steel is also excellent in high-temperature strength incidentally, and it has become possible to supply a large amount and inexpensively of a steel material exhibiting excellent performance for various uses as a welded structural steel. By using such a steel material, it has become possible to further improve the safety of various welded steel structures.

Continued on the front page (72) Inventor Hirosaku Shioda 1 Kimitsu, Kimitsu-shi Nippon Steel Corporation Kimitsu Works F-term (reference) 4K032 AA00 AA01 AA02 AA04 AA08 AA11 AA14 AA15 AA16 AA19 AA21 AA22 AA23 AA27 AA29 AA31 A33 AA35 AA36 AA40 BA01 CA02 CA03 CC02 CC03 CD05 CF02 CF03

Claims (11)

[Claims] 1. A steel component in mass%, C: 0.1% or less,
Si: 0.6% or less, Mn: 0.2 to 1.6%, P:
0.02% or less, S: 0.01% or less, Al: 0.06
% Or less, N: 0.006% or less, and Access Nb = Nb−7.8 × [C− (Ti−
3.4N) / 4] Nb: 0.01 to 0.5%, Ti: 0.0 so that Excess Nb satisfying + 0.01% or more is satisfied.
Nb alone or Nb and Ti within the range of 0.05 to 0.1%.
Characterized by excellent weldability, characterized by containing both of the above, with the balance being iron and unavoidable impurities. 2. In addition to the above steel components, Cu:
0.05 to 2.0%, Ni: in the range of 0.05 to 1.0%, 1/2 or more of the added amount of Cu, Cr: 0.05 to 1.0
%, Mo: 0.05 to 1.0%, 1 or 2 types
The steel with excellent weldability according to claim 1, comprising at least one kind. 3. The composition according to claim 1, wherein one or two kinds of V are contained in the range of 0.005 to 0.1% Ta: 0.005 to 0.1% by mass%. The steel with excellent weldability described in 1. 4. B: 0.0002 to 0.00% by mass
The steel excellent in weldability according to any one of claims 1 to 3, further containing 5%. 5. Ca: 0.0005 to 0.0% by mass
5. The composition of claim 1, further comprising one of the following: 0.4%, REM: 0.0005 to 0.004%.
The steel excellent in weldability according to any one of the above. 6. Mg: 0.0002 to 0.0% by mass.
The composition further contains 0.05%.
The steel excellent in weldability according to any one of the above. 7. A steel slab or a slab made of the steel component according to any one of claims 1 to 6 is reheated to a temperature range of 1000 to 1300 ° C., and the cumulative rolling reduction at 1000 ° C. or lower is 30. % And finish the rolling at a temperature of 720 ° C or higher,
Thereafter, the method for producing steel having excellent weldability is characterized in that it is left to cool or accelerated cooling from a temperature of 700 ° C. or more to an arbitrary temperature of 600 ° C. or less. 8. A steel slab or a slab made of the steel component according to any one of claims 1 to 6, which is hot-rolled and then normalized at a temperature of Ac ₃ or more and 1000 ° C. or less. To produce steel with excellent weldability. 9. A slab or a slab made of the steel component according to any one of claims 1 to 6, after hot rolling, reheating to a temperature of _{3 to} 1,000 ° C. and then quenching. A method for producing steel having excellent weldability, characterized by the following. In 10. strength adjustment and improving toughness, or residual stress relief purposes of the steel sheet, according to any one of claims 7-9, characterized in that tempering the steel plate at a temperature of less than Ac ₁ A method for producing steel with excellent weldability. 11. For the purpose of lowering the yield ratio, after reheating a steel sheet to a dual phase coexistence region of a ferrite and an austenite of more than Ac ₁ and less than Ac ₃ , the steel sheet is allowed to cool to a temperature of 600 ° C. or lower at a cooling rate of more than that. cooled, any one method of manufacturing steel excellent in weldability according to claim 7-9, characterized in that the tempering thereafter further optionally Ac ₁ temperature below.