JP4523893B2 - Steel for welded structure having a tensile strength of 590 N / mm2 and excellent in toughness of base metal and weld heat-affected zone, and method for producing the same - Google Patents

Description

The present invention relates to a welded structural steel having a tensile strength of 590 N / mm ² and excellent in toughness of a base material and a weld heat-affected zone, and a method for producing the same. The welded structural steel of the present invention can be widely applied to fields such as construction, shipbuilding and construction machinery as well as high strength thick steel plates for offshore structures and bridges.

  Conventionally, it has been considered that grain refinement is effective as a means to increase the toughness of welds in thick steel plates. As a method for this, the inclusions such as oxides during solidification are controlled and reheating such as welding is performed. At times, technology has been developed in which inclusions act as a pinning effect for grain growth and as a nucleation site for ferrite.

  As described above, steel that promotes the formation of intragranular ferrite (IGF) using Ti oxide (hereinafter sometimes abbreviated as TiO) as a nucleus by adding Ti to the steel material. Has been proposed (for example, Patent Document 1).

  Further, by dispersing Ti nitride (hereinafter sometimes abbreviated as “TiN”) in the matrix, grain growth during reheating is suppressed by a pinning effect, and the heat affected zone (HAZ) of the weld heat affected zone (HAZ) Steels that ensure toughness have been proposed (for example, Patent Documents 2 and 3).

  In addition, Ti-Mg oxide dispersed in the matrix suppresses grain growth during reheating by the pinning effect, refines the ferrite by the effect of promoting the formation of IGF, and proposes a steel that secures the toughness of HAZ (For example, Patent Document 4).

  However, the steels described in Patent Documents 1 to 4 have a problem that they require a very complicated process for producing a steel having excellent HAZ toughness, and are expensive. Since it is difficult to determine the amount of addition and control of precipitates, the effect of fine graining is limited.

  On the other hand, the use of MnS is also effective for miniaturization of HAZ, so MnS is generated and IGF formation is promoted using MnS as a nucleus, and toughness is ensured by effectively reducing the crystal grain size. Steel has been proposed (for example, Patent Document 5).

  However, in the steel described in Patent Document 5, it is difficult to determine the amount of each element added at the time of production and to control the precipitates, and thus there is a limit to the effect of refinement.

  In addition, the strength and toughness are improved by adding a large amount of Mn of about 3.0% to the thick steel plate. By adding Mn up to 3.5%, the high strength and the high toughness are improved. Steel has been proposed (for example, Patent Document 6).

However, the steel described in Patent Document 6 has a problem that center segregation of Mn tends to occur when molten steel is used as a slab by continuous casting. One of the adverse effects is the generation of coarse MnS that can cause cracks, and the amount of coarse MnS increases as the amount of Mn added increases, causing cracks.
For the above-mentioned reasons, it has been avoided to add a large amount of Mn in a thick steel plate having a large thickness and it is difficult to completely eliminate segregation by rolling or the like.
JP-A-5-171341 Japanese Patent Publication No.55-26164 JP 2001-164333 A JP 11-279684 A Japanese Patent Laid-Open No. 7-252586 JP-A-9-3595

The present invention has been made in view of the above circumstances, and is excellent in weldability, and can be manufactured at a low cost without using complicated processes and methods, and a tensile strength of 590 N / mm excellent in the toughness of the weld heat affected zone. ^An object of the present invention is to provide a ^second grade welded structural steel and a method for producing the same.

The gist of the present invention is as follows.
(1) By mass%, C: 0.03 to 0.12%, Si: 0.05 to 0.30%, Mn: 1.6 to 3.00%, P: 0.015% or less, S: 0.001 to 0.015%, Cu + Ni: 0.10% or less, Al: 0.005% or less, Ti: 0.005 to 0.030%, Nb: 0.005 to 0.100%, N: 0 .0025 to 0.0060%, O: 0.0035% or less, REM: 0.001 to 0.020%, the balance is made of iron and inevitable impurities, and the bainite structure fraction of the base material is 80% The relationship between S, O, and REM is as follows:
0.007 ≦ 2.7 × ([S] (%) + [O] (%)) − 0.6 × [REM] (%) ≦ 0.015 (1)
A welded structural steel having a tensile strength of 590 N / mm ² and excellent in toughness of a base material and a weld heat-affected zone, characterized by:
(2) Further, in mass%, Mo: 0.2% or less, V: 0.1% or less, Cr: 0.5% or less, Ca: 0.0035% or less, Mg: 0.0050% or less, B : Containing at least one selected from the group of 0.002% or less or two or more, tensile strength of 590 N / mm ² excellent in toughness of base material and weld heat affected zone according to (1) Grade welded structural steel.
(3) A steel slab having a component composition of a tensile strength of 590 N / mm grade ² welded structural steel excellent in the toughness of the base material and welding heat-affected zone described in (1) or (2) is 1200 ° C. or less. After heating to a temperature and then performing hot rolling at a cumulative reduction ratio of 40% or more in the non-recrystallization temperature range and completing the hot rolling at a temperature of 850 ° C. or higher, the temperature is increased from 800 ° C. or higher to 5 ° C. / A method for producing a welded structural steel having a tensile strength of 590 N / mm ² and excellent in toughness of a base material and a weld heat-affected zone, characterized by cooling to 400 ° C. or lower at a cooling rate of at least sec.
(4) Reheating the steel obtained by the method for producing a welded structural steel having a tensile strength of 590 N / mm ² class excellent in toughness of the base material and the weld heat-affected zone described in (3), and a temperature of 400 to 650 ° C. A method for producing a steel for welded structure having a tensile strength of 590 N / mm ² and excellent in toughness of a base material and a weld heat-affected zone, characterized by subjecting to tempering.

According to the welded structural steel having a tensile strength of 590 N / mm ² and excellent in toughness of the base material and the weld heat-affected zone of the present invention, C: 0.03 to 0.12%, Si: 0.00. 05 to 0.30%, Mn: 1.6 to 3.00%, P: 0.015% or less, S: 0.001 to 0.015%, Cu + Ni: 0.10% or less, Al: 0.005 %: Ti: 0.005-0.030%, Nb: 0.005-0.100%, N: 0.0025-0.0060%, O: 0.0035% or less, REM: 0.001- 0.020% is contained, the balance is made of iron and inevitable impurities, the base material has a bainite structure fraction of 80% or more, and the relationship between the S, O, and REM is expressed by the formula (0.007 ≦ 2 0.7 × ([S] (%) + [O] (%)) − 0.6 × [REM] (%) ≦ 0.015) Have a relationship.
By making the component composition within the above range and the relationship between S, O, and REM within the range represented by the above formula, the effect of central segregation, which is a problem when adding a large amount of Mn, is reduced. It can be mitigated and HAZ crystal grain coarsening due to welding can be suppressed.
As a result, the toughness of the steel material is stabilized and the weldability is improved. The tensile strength of 590 N / mm ² class, which is excellent in the toughness of the base material and the weld heat affected zone, is low-cost without using complicated processes and methods. A welded structural steel is obtained, which is extremely useful industrially.

Hereinafter, an embodiment of a welded structural steel having a tensile strength of 590 N / mm ² and excellent in toughness of the base material and the weld heat affected zone according to the present invention will be described.
Note that this embodiment is described in detail for better understanding of the gist of the invention, and thus does not limit the present invention unless otherwise specified.

In the steel for welded structure having a tensile strength of 590 N / mm ² and excellent in toughness of the base material and the weld heat-affected zone of the present embodiment, C: 0.03 to 0.12%, Si: 0.05 ˜0.30%, Mn: 1.6 to 3.00%, P: 0.015% or less, S: 0.001 to 0.015%, Cu + Ni: 0.10% or less, Al: 0.005% Ti: 0.005 to 0.030%, Nb: 0.005 to 0.100%, N: 0.0025 to 0.0060%, O: 0.0035% or less, REM: 0.001 to 0 0.020%, the balance is made of iron and inevitable impurities, the base material has a bainite structure fraction of 80% or more, and the relationship between S, O, and REM is expressed by the following formula (1):
0.007 ≦ 2.7 × ([S] (%) + [O] (%)) − 0.6 × [REM] (%) ≦ 0.015 (1)
It is set as the component composition represented by these.

Further, in the welded structural steel having a tensile strength of 590 N / mm ² which is excellent in the toughness of the base material and the weld heat affected zone of the present embodiment, Mo: 0.2% or less, V: 0.00%. 1% or less, Cr: 0.5% or less, Ca: 0.0035% or less, Mg: 0.0050% or less, B: 0.002% or less It can be set as the component composition to contain.

In order to solve the above-mentioned problems, a large amount of Mn having a relatively low alloy cost is added to the welded structural steel having a tensile strength of 590 N / mm ² that is excellent in the toughness of the base material and the weld heat affected zone of the present embodiment. This makes it possible to reduce the crystal grain size of the base material by finely dispersing the REM oxide and sulfide produced by the addition of REM while ensuring the strength and toughness at a low cost. By reducing the resulting MnS, a steel having a uniform material and excellent base material toughness can be obtained. In addition, by combining the IGF generation promoting action by TiO and MnS, excellent HAZ toughness can be obtained.

  In order to increase the toughness of the welded portion, it is effective to refine the microstructure of the welded portion. As means for refining the microstructure, it is possible to suppress the coarsening of crystal grains due to welding heat, increase the number of transformation nucleation sites during cooling, and reduce the effective crystal grain size of the finally obtained microstructure. Can be mentioned. In order to suppress the coarsening of crystal grains due to welding heat, it is effective to finely disperse as many inclusions and precipitates as possible in the molten steel stage. For this reason, it is important to add an element capable of fine precipitation in the molten steel.

One of the compounds that can be finely precipitated at high temperature is a REM (rare earth element) compound. REM mainly combines with O and S to form fine precipitates. REM has a very strong bonding force with O and S, and generates oxides and sulfides at high temperatures. Therefore, REM has a sufficient amount of MnS generation of harmful sulfides that are sufficiently dispersed in molten steel and cause cracks. To reduce. Also, since REM oxides and sulfides have a high melting point, once they are produced as oxides and sulfides, they do not dissolve even at high temperatures such as hot rolling and high temperature heating such as welding, and in steel. Exists.
As described above, the addition of REM can avoid the generation of cracks due to coarse MnS generation due to the central segregation of Mn, and can obtain the effect of refining the crystal grains of the base material.

  Since MnS serves as a nucleation of IGF in HAZ and has an effect of reducing the effective crystal grain size of HAZ, it is not preferable that MnS is not completely generated. However, when about 1.6 to 3.00% of Mn is added, if about 0.001 to 0.020% of REM is added, the conventional steel can be obtained without actively reducing the amount of S. The same amount of MnS can be obtained.

  As described above, it is possible to avoid cracking due to MnS, which is concerned by adding Mn up to about 3.00%. In addition, since sufficient strength and toughness can be secured by adding Mn, it is possible to replace Ni and Cu that have been added to high-strength steel and the like. It is effective for.

  Conventionally, REM addition has been used for shape control of inclusions, and from this point, REM addition is effective for improving toughness.

There are various methods for producing a thick plate having high strength and high toughness. One of them is a DQT method in which tempering (T) treatment is performed after direct quenching (DQ) after hot rolling.
However, the T treatment increases the cost because it is once cooled and then reheated and held at that temperature for a certain period of time. From the viewpoint of cost reduction, the T process should be avoided as much as possible. In this embodiment, since excellent toughness can be ensured without performing the T treatment, a high-quality steel material can be manufactured without increasing costs. Moreover, when toughness is especially requested | required, the steel material which has the more superior toughness can be obtained by performing T process.

[Component composition of steel materials]
Hereinafter, the reasons for limiting the component composition of the steel for welded structure having a tensile strength of 590 N / mm ² and excellent in the toughness of the base material and the weld heat affected zone of the present embodiment will be described. In the following description, “%” for the composition means mass%.

"C: Carbon" 0.03-0.12%
C is an element necessary for ensuring strength.
In order to obtain the effect of improving the strength, it is necessary to add 0.03% or more of C. However, since excessive addition may cause a reduction in the toughness of HAZ, the upper limit is set to 0.12%. .

"Si: silicon" 0.05-0.30%
Si is effective as a deoxidizer and is an element effective for improving the strength of steel by solid solution strengthening.
When the Si content is less than 0.05%, the above-described effects are reduced, and when the Si content exceeds 0.30%, the toughness of the HAZ is deteriorated.
For this reason, the Si content is limited to a range of 0.05 to 0.30%.

“Mn: Manganese” 1.6 to 3.00%
Mn is an element effective for increasing the strength because it increases the strength of the steel. Mn combines with S to form MnS, which serves as a nucleation of IGF and promotes refinement of the effective crystal grain size of HAZ, thereby suppressing degradation of HAZ toughness.
In order to secure the toughness of the weld heat affected zone while maintaining high strength, the Mn content needs to be 1.6% or more. However, if Mn is added in excess of 3.00%, the toughness deteriorates.
For this reason, content of Mn was limited to 1.6 to 3.00% of range.

“P: Phosphorus” 0.015% or less P is segregated at the grain boundary and deteriorates the toughness of the steel. Therefore, it is preferable to reduce its content as much as possible, but up to 0.015% is acceptable. The amount was set to 0.015% or less.

“S: Sulfur” 0.001 to 0.015%
S forms sulfides with REM and has the effect of refining the crystal grains of the base material. Moreover, since it exists in steel as MnS and has the effect | action which refines | miniaturizes the structure of HAZ, content of 0.001% or more is required. On the other hand, if the S content exceeds 0.015%, the toughness and ductility in the thickness direction are reduced, so S needs to be limited to 0.015% or less.
For this reason, content of S was limited to 0.001 to 0.015% of range.

“Cu + Ni: Copper + Nickel” 0.10% or less Conventionally, Cu is an element effective for securing strength, but in order to avoid a decrease in hot workability due to the addition of Cu, It is essential to add the same amount of Ni.
However, since Ni is an extremely expensive element, adding a large amount of Ni may hinder cost reduction, which is one of the purposes of the steel of the present invention. For this reason, it is preferable not to add Cu and Ni as much as possible. However, when manufacturing slabs using scrap, Cu and Ni may inevitably be mixed by about 0.05%, so the upper limit of the total amount of Cu and Ni is 0.10%. did.

“Al: aluminum” 0.005% or less In order to generate TiO, it is preferable that the content of Al is small, and it is necessary to substantially prevent Al from being contained.
However, the component composition not containing Al is limited in industrial production, and the content of about 0.005% by mass is an allowable range that does not hinder the generation of TiO. Was 0.005% or less.

"Ti: Titanium" 0.005-0.030%
Ti combines with O to form TiO, promotes the formation of IGF, and combines with N to form TiN in the steel.
In order to obtain the above-mentioned action, it is necessary to add 0.005% or more of Ti.
If the Ti content exceeds 0.030%, the base material toughness may be deteriorated, so the Ti content is limited to a range of 0.005 to 0.030%.

“Nb: Niobium” 0.005 to 0.100%
Nb is an element that has the effect of enlarging the non-recrystallized region of austenite and promoting the refinement of ferrite, and can generate Nb carbide to ensure the strength.
In order to obtain the above-described action, Nb needs to be added in an amount of 0.005% or more.
If the Nb content exceeds 0.100%, HAZ embrittlement is likely to occur due to Nb carbide, so the Nb content is limited to a range of 0.005 to 0.100%.

“N: Nitrogen” 0.0025 to 0.0060%
N needs to be added in an amount of 0.0025% or more in order to combine with Ti to form TiN in the steel. However, since N has a very large effect as a solid solution strengthening element, adding a large amount of N may possibly deteriorate the HAZ toughness. Therefore, the upper limit of N is set to 0.0060% so that the effect of TiN can be maximized without significantly affecting the HAZ toughness.

"O: Oxygen" 0.0035% or less O is necessary to form TiO, but if the content exceeds 0.0035%, coarse TiO may be generated and toughness may be deteriorated. .
For this reason, the upper limit of O was made 0.0035%.

“REM: rare earth element” 0.001 to 0.020%
REM has the effect of finely dispersing and crystallization of oxides and sulfides in molten steel, preventing the formation of coarse MnS due to the central segregation of Mn, and the effect of making the solidified structure fine by pinning the crystallized matter. There is.
In order to acquire the above-mentioned effect, it is necessary to add REM 0.001% or more.
If the content of REM exceeds 0.020%, the amount of crystallized product increases, and panning occurs during casting. Therefore, the upper limit value is limited to 0.020%. A more preferable upper limit value is 0.010%.

The tensile strength 590 N / mm grade ² welded structural steel with excellent toughness of the base material and weld heat affected zone of the present embodiment is based on the above-mentioned components, but further has strength, toughness, ductility, etc. For the purpose of improving the mechanical properties, a component composition may be used that actively contains one or more selectively permissible additive elements described below.

“Mo: molybdenum, V: vanadium, Cr: chromium, Ca: calcium, Mg: magnesium, B: boron”
Mo, V, and Cr are all effective elements for improving the hardenability, and one or more kinds can be selected and contained as required. In this, V can produce | generate VN and can optimize the structure refinement | miniaturization effect, and has the effect of promoting the precipitation strengthening by VN.
Moreover, since the Ar ₃ point is lowered due to the inclusion of Mo, V, and Cr, the effect of refining ferrite grains is further increased.
Further, by adding Ca, the form of MnS can be controlled and the low-temperature toughness can be further improved. Therefore, when severe HAZ characteristics are required, it can be selectively added.
In addition, Mg has the effect of suppressing austenite grain growth in HAZ and making it finer, and this improves HAZ toughness. Therefore, when severe HAZ toughness is required, it should be selected and added. Can do.
B is an element that greatly improves the hardenability by adding a small amount, and can be selected and added when it is difficult to secure a cooling rate as in the case of an extremely thick steel plate.

In order to obtain the effects described above, the content of each element is, in mass%, Mo: 0.2% or less, V: 0.1% or less, Cr: 0.5% or less, Ca: 0.0035% or less, Mg: 0.0050% or less and B: 0.002% or less are preferable.
In addition, when Mo exceeding 0.2% or Cr exceeding 0.5% is added, the weldability and toughness may be impaired, and the cost may increase.
Moreover, when V exceeding 0.03% is added, there exists a possibility that weldability and toughness may be impaired.
Further, when Ca exceeding 0.0035% is added, the cleanliness of the steel is impaired, and there is a possibility that the toughness is deteriorated and the sensitivity to hydrogen-induced cracking is increased.
Further, when Mg exceeding 0.0050% is added, the austenite refining effect margin is small, and this is not a good measure in terms of cost.
Moreover, when adding B exceeding 0.0002%, there exists a possibility of impairing the toughness of steel.

[Steel structure]
The reason for limiting the bainite structure fraction of the base material will be described below.
In order to obtain a tensile strength of 590 N / mm ² while securing the toughness of the base material and HAZ with the low alloy steel, the bainite structure fraction needs to be 80% or more.
Moreover, in order to obtain the said intensity | strength, the bainite structure fraction is preferably 85% or more, and more preferably 90% or more.
The bainite structure described here refers to a structure composed of acicular ferrite in addition to the upper bainite structure and the lower bainite structure.

[Reason for defining the relationship between S, O, and REM]
The reason why the relationship between S, O, and REM is defined by equation (1) will be described.
By making S, O, and REM into the relationship represented by Formula (1), it is possible to suppress the generation of excessive MnS.
MnS is effective because it acts as a nucleus for IGF formation, but excessive production of coarse MnS causes cracking and is harmful. For this reason, the optimum range of MnS must be considered. However, since S has a stronger binding force with REM than Mn, it must be considered that REM sulfide is generated earlier than MnS. Moreover, since REM has a stronger bonding force with O, it is necessary to consider that REM sulfide is generated by REM remaining after REM oxide formation.
Considering these actions, the relationship between effective MnS (eff · MnS), S, O, and REM is expressed by equation (2).

0.007 ≦ 2.7 × ([S] (%) + [O] (%)) − 0.6 × [REM] (%) ≦ 0.015 (1)

eff · MnS = 2.7 × ([S] (%) + [O] (%)) − 0.6 × [REM] (%) (2)

When the effective MnS content (eff · MnS) calculated by the above formula (2) is less than 0.007%, the effect of improving toughness is small because the amount of MnS produced is small. Has a problem that toughness deteriorates due to excessive formation of REM oxides and sulfides.
For this reason, it was made into conditions that content of S, O, and REM satisfy | fills the relationship represented by (1) Formula.

[Production conditions for steel]
Below, the manufacturing method of the steel for structural welds of the tensile strength 590N / mm ² excellent in the toughness of the base material of this embodiment and a welding heat affected zone is demonstrated.
In this embodiment, the steel slab having the above-described component composition is heated to a temperature of 1200 ° C. or lower, and then hot-rolled at a cumulative reduction ratio of 40% or higher in an unrecrystallized temperature range, and 850 ° C. or higher. After completing the hot rolling at a temperature, the tensile strength of 590 N excellent in the toughness of the base material and the weld heat affected zone is obtained by cooling from a temperature of 800 ° C. or more to 400 ° C. or less at a cooling rate of 5 ° C./sec or more. / Mm A method for obtaining a grade ² welded structural steel.
Furthermore, it is good also as the method of reheating the obtained steel and performing a tempering process at 400-650 degreeC.
Below, the reason for limitation of each manufacturing condition is demonstrated.

About heating temperature, it is necessary to set it as the temperature of 1200 degrees C or less.
The reason for setting the heating temperature to 1200 ° C. or less is that, when heated at a high temperature, the precipitate formed by controlling the cooling rate during solidification may be redissolved.
Moreover, in order to complete the phase transformation, a heating temperature of 1200 ° C. is sufficient, and it is possible to prevent the coarsening of crystal grains considered to occur during heating in advance.
For the above reason, the heating temperature is limited to 1200 ° C. or less.

Next, it is necessary to perform hot rolling with a cumulative rolling reduction of 40% or more in the non-recrystallization temperature range.
The increase in the amount of reduction in the non-recrystallization temperature region contributes to the refinement of austenite grains during rolling, and as a result, has the effect of refining the ferrite grains and improving the mechanical properties. Such an effect becomes remarkable when the cumulative rolling reduction in the non-recrystallized region is 40% or more.
For the above reasons, the cumulative reduction amount in the non-recrystallized region is limited to 40% or more.

The steel slab needs to be hot-rolled at 850 ° C. or higher and then cooled from a temperature of 800 ° C. or higher to 400 ° C. or lower at a cooling rate of 5 ° C./sec or higher.
The reason for cooling from a temperature of 800 ° C. or higher is that starting from a temperature of less than 800 ° C. is disadvantageous from the viewpoint of hardenability, and the required strength may not be obtained.
If the cooling rate is less than 5 ° C./sec, it cannot be expected to obtain a steel having a uniform microstructure, and as a result, the effect of accelerated cooling is reduced.
In general, if the steel is cooled to a temperature of 400 ° C. or lower, the transformation is fully completed. In this embodiment, since sufficient toughness can be ensured by continuously cooling the steel to 400 ° C. or lower at a cooling rate of 5 ° C./sec or higher, it can be used as a steel material without performing T treatment.
For the above reasons, the steel slab is limited to the conditions for cooling the steel slab at a temperature of 850 ° C. or higher to a temperature of 800 ° C. or higher to a temperature of 400 ° C. or lower at a cooling rate of 5 ° C./sec or higher .

Further, particularly excellent toughness values are required, and when tempering is performed after hot rolling and accelerated cooling, the tempering temperature needs to be within a range of 400 to 650 ° C.
When performing the tempering treatment, the driving force for crystal grain growth increases as the tempering treatment temperature increases, but this becomes significant when the tempering treatment temperature exceeds 650 ° C.
Further, at the tempering temperature of less than 400 ° C., there is a possibility that the above-mentioned effect cannot be obtained sufficiently.
For the above reasons, when tempering after hot rolling, it is limited to tempering at 400 to 650 ° C.

As described above, according to the welded structural steel having a tensile strength of 590 N / mm ² that is excellent in the toughness of the base material and the welding heat-affected zone according to the present embodiment, C: 0.03 to 0 in mass%. .12%, Si: 0.05 to 0.30%, Mn: 1.6 to 3.00%, P: 0.015% or less, S: 0.001 to 0.015%, Cu + Ni: 0.10 %: Al: 0.005% or less, Ti: 0.005-0.030%, Nb: 0.005-0.100%, N: 0.0025-0.0060%, O: 0.0035% Hereinafter, REM: 0.001 to 0.020% is contained, the balance is made of iron and inevitable impurities, the bainite structure fraction of the base material is 80% or more, and the relationship between S, O, and REM is , Formula (0.007 ≦ 2.7 × ([S] (%) + [O] (%)) − 0.6 × [REM] ( ) Is a relationship expressed by ≦ 0.015).
The component composition is within the above range, and the relationship between S, O, and REM is within the range represented by the above formula, thereby mitigating the influence of central segregation which becomes a problem when Mn is added. In addition, the HAZ crystal grain coarsening due to welding can be suppressed.
As a result, the toughness of the steel material is stabilized and the weldability is improved. The tensile strength of 590 N / mm ² class, which is excellent in the toughness of the base material and the weld heat affected zone, is low-cost without using complicated processes and methods. A welded structural steel is obtained.

Hereinafter, the present invention will be described in more detail by giving examples of welded structural steels having a tensile strength of 590 N / mm class ² excellent in the toughness of the base material and the weld heat affected zone according to the present invention. However, the present invention is not limited to the following examples, and can be implemented with appropriate modifications within a range that can meet the gist of the preceding and following descriptions, all of which are included in the technical scope of the present invention. It is what

[Sample preparation]
By using a slab having each component composition as shown in Table 1 and Table 2 and performing hot rolling treatment under each condition shown in Table 3, the base material of the present invention and the toughness of the weld heat affected zone are excellent. Further, a welded structural steel having a tensile strength of 590 N / mm ² (invention steels 1 to 16 in Table 1) and a conventional welded structural steel (comparative steels 17 to 31 in Table 2) were obtained.

[Evaluation test]
In order to evaluate the mechanical properties of the samples, the following evaluation tests were performed.
First, a JIS No. 4 test piece was sampled from ¼ part of the thickness of each steel sheet, and using this test piece, tensile strength TS (N / mm ² ), yield strength YS (0.2% proof stress) ( N / mm ² ) and total elongation El (%) were measured.
Also, as a base material toughness evaluation test, a 2 mm V notch test piece was taken from 1/4 t of the thickness of each steel plate, and a Charpy impact test was performed at a temperature of −5 ° C. using this test piece. The obtained impact absorption energy value was measured.
In addition, as a HAZ toughness evaluation test, a steel material subjected to a reproducible thermal cycle test corresponding to a welding heat input of 10 (kJ / mm) is subjected to a Charpy impact test at a temperature of -5 ° C., and the impact absorption energy obtained thereby. The value was measured.
The bainite structure fraction was evaluated by observing the structure of a steel material etched with nital with an optical microscope.
The evaluation results are shown in Table 4.

[Evaluation test results]
In Tables 1 to 4, the numbers described in the columns of “Invention Steel” and “Comparative Steel” are shown in the same manner.
In each table, 1 to 16 (invention steel) are examples of the welded structural steel of the present invention, and 17 to 31 (comparative steel) are comparative examples of conventional welded structural steel.

In the steel for welded structure (steel according to the present invention) of the present invention, the steel material has the component composition shown in Table 1, and the hot rolling treatment is performed under the conditions shown in Table 3, thereby satisfying each of the component composition and production conditions. is doing.
As a result, as shown in Table 4, in all samples, the expression (1) (0.007 ≦ 2.7 × ([S] (%) + [O] (%)) − 0.6 × [REM] ] (%) ≦ 0.015) is satisfied, and the bainite structure fraction is 80% or more.
Moreover, as shown in the column of base material characteristics, the tensile strength TS of all the samples is 590 (N / mm ² ) or more, and the Charpy impact energy value at −5 ° C. is 100 J or more even in high heat input welding. It is clear that the characteristics as a base material are very excellent.
Moreover, if it is in a regulation range, even if it adds Mo, V, Cr, Ca, Mg, it turns out that favorable toughness is acquired even if it performs a tempering process.

  On the other hand, in the conventional welded structural steels (comparative steels) shown in comparative steels 17 to 31, the amount of Mn (comparative steel 19), the amount of C (comparative steel 17), the amount of Nb (comparative steel 22), the amount of Ti, respectively. (Comparative Steel 21), Si amount (Comparative Steel 18), Al amount (Comparative Steel 20), N amount (Comparative Steel 27), Mo, V amount (Comparative Steel 23), Cr amount (Comparative Steel 26), Ca, Mg amount (Comparative steel 25), O amount (Comparative steel 24), REM amount (Comparative steels 28 and 29), Cu + Ni amount (Comparative steel 30), B amount (Comparative steel 31), Cooling after hot rolling Conditions of speed (comparative steel 27), tempering treatment (comparative steel 24), cumulative rolling reduction (comparative steel 26), reheating temperature (comparative steel 25), and cooling start temperature after rolling (comparative steel 30) are the present invention. Therefore, the HAZ toughness is inferior. Moreover, since REM was not added regarding the comparative steel 29, the sulfide became coarse, and as a result, the toughness was inferior.

From the above results, it became clear that the steel for welded structure having a tensile strength of 590 N / mm ² and excellent in toughness of the base material and the weld heat affected zone according to the present invention has high mechanical properties.

Claims (4)

In mass%, C: 0.03 to 0.12%, Si: 0.05 to 0.30%, Mn: 1.6 to 3.00%, P: 0.015% or less, S: 0.001 -0.015%, Cu + Ni: 0.10% or less, Al: 0.005% or less, Ti: 0.005-0.030%, Nb: 0.005-0.100%, N: 0.0025 0.0060%, O: 0.0035% or less, REM: 0.001 to 0.020%, the balance is made of iron and inevitable impurities, and the bainite structure fraction of the base material is 80% or more ,
The relationship between S, O, and REM is expressed by the following formula (1).
0.007 ≦ 2.7 × ([S] (%) + [O] (%)) − 0.6 × [REM] (%) ≦ 0.015 (1)
A welded structural steel having a tensile strength of 590 N / mm ² and excellent in toughness of a base material and a weld heat-affected zone, characterized by: Further, in terms of mass%, Mo: 0.2% or less, V: 0.1% or less, Cr: 0.5% or less, Ca: 0.0035% or less, Mg: 0.0050% or less, B: 0.0. It contains at least one kind selected from the group of 002% or less, or two or more kinds, and has a tensile strength of 590 N / mm ² class excellent in toughness of the base material and the weld heat-affected zone according to claim 1. Structural steel. The steel slab having the component composition according to claim 1 or 2 is heated to a temperature of 1200 ° C or lower, and then hot-rolled at a cumulative reduction ratio of 40% or higher in a non-recrystallization temperature range, and a temperature of 850 ° C or higher. After completing the hot rolling in step 1, the tensile strength is excellent in the toughness of the base material and the weld heat affected zone, characterized by cooling from a temperature of 800 ° C. to 400 ° C. at a cooling rate of 5 ° C./sec or more. 590 N / mm Grade ² welded structural steel manufacturing method. The steel obtained by the method for producing a welded structural steel having a tensile strength of 590 N / mm ² with excellent toughness of the base material and the weld heat-affected zone according to claim 3 is reheated and tempered at 400 to 650 ° C. A method for producing a welded structural steel having a tensile strength of 590 N / mm ² and excellent in toughness of a base material and a weld heat-affected zone.