JP5966907B2 - Steel material for large heat input welding - Google Patents

Description

  The present invention is suitable for steel materials used in various welded structures such as shipbuilding, construction, civil engineering, etc., and low temperature toughness of the heat affected zone when high heat input with a heat input exceeding 300 kJ / cm is applied. The present invention relates to a steel material for large heat input welding which is excellent in resistance.

  Steel materials used in the fields of shipbuilding, construction, civil engineering and the like are generally finished into a structure having a desired shape by welding. In these structures, from the viewpoint of safety, not only the base material toughness of the steel material used but also the toughness of the welded portion is required to be excellent. On the other hand, these structures and ships are becoming larger and more efficient, such as submerged arc welding, electrogas welding, and electroslag welding, as the steel materials used become stronger and thicker. Heat input welding is applied. For this reason, when welding is performed by high heat input welding, a steel material excellent in toughness of the welded portion is required.

  However, it is generally known that as the welding heat input increases, the structure of the weld heat affected zone (HAZ) becomes coarse, and the toughness of the weld heat affected zone decreases. Many countermeasures have been proposed for the reduction of toughness due to such high heat input welding.

  For example, Patent Document 1 discloses a technique that uses a function of suppressing the coarsening of austenite grains by fine dispersion of TiN and acts as a ferrite transformation nucleus, and a technique that disperses a Ti oxide.

  In these conventional techniques mainly using TiN, the above-mentioned action of Ti is eliminated in the welding heat affected zone heated to a temperature range where TiN dissolves, and further, the structure of the ground becomes solid solution Ti and solid solution. There is a problem that the toughness is significantly lowered due to embrittlement by N. Further, the technology using Ti oxide has a problem that it is difficult to disperse the oxide uniformly and finely.

  On the other hand, for example, in Patent Document 2, in order to improve the toughness of the weld heat-affected zone welded with a large heat input exceeding 300 kJ / cm, Ca necessary for the morphology control of sulfide is appropriately contained, Utilizing CaS is disclosed. By utilizing CaS that crystallizes at a lower temperature than oxides, this is finely dispersed, and precipitates that become ferrite transformation formation nuclei such as MnS, TiN, and BN, which are precipitated as a core, are finely cooled during cooling. Dispersed. As a result, the structure of the weld heat-affected zone was changed to a fine ferrite pearlite structure, and the toughness of the weld heat-affected zone could be increased.

JP 57-51243 A Patent 3546308

  However, the use environment of the welded structure tends to be more severe. For example, a ship that sails in Sakhalin needs a high-strength low-temperature toughness specification with improved low-temperature toughness than ever before. In order to satisfy such strict requirements, it is necessary to cope with further enhancement of the high heat input welding heat-affected zone toughness improving technology.

  In the present invention, for high strength steel having a yield strength of 390 MPa or more, the low temperature toughness of the weld heat affected zone when high heat input with a heat input exceeding 300 kJ / cm is required in the field of thick steel plates for shipbuilding. Combined with the characteristics of the F class steel with the strictest characteristics (Japan Maritime Association Steel Ship Rules K and M, Class F steel: base metal toughness: -60 ° C specification, high heat input welded joint toughness: -40 ° C specification) It aims at providing the steel material for large heat input welding.

In order to solve the above-mentioned problems, the present inventors have intensively studied and obtained the following knowledge.
1. In order to improve the toughness of the heat-affected zone with high heat input welding, the coarsening of austenite grains in the high temperature range is suppressed, and in the subsequent cooling process, ferrite (hereinafter referred to as grains) is generated from within the grains rather than the grain boundaries of the austenite grains. It is important to promote the nucleation of the inner ferrite) and to reduce the grain size of the intragranular ferrite.
2. As a specific component design guideline, to secure the desired Ti and N amounts to suppress the coarsening of austenite grains, and to control the Ca, S and O amounts appropriately and to increase the amount of each production site of intragranular ferrite sufficiently large As a result, it is effective to reduce the grain size of the intragranular ferrite. The present invention has been made based on the above findings and further studies. That is, the present invention is as follows.
[1] By mass%, C: 0.030 to 0.120%, Si: 0.50% or less, Mn: 1.40 to 2.00%, P: 0.020% or less, S: 0.0005 -0.0040%, Al: 0.005-0.100%, Nb: 0.003-0.030%, Ti: 0.004-0.030%, B: 0.0003-0.0030%, N: 0.0035 to 0.0070%, Ca: 0.0005 to 0.0030%, O: 0.0040% or less, and satisfying the following formula (1), the balance from Fe and inevitable impurities In the heat-affected zone structure near the bond when high heat input welding with a heat input of 300 kJ / cm or more is applied, the grain size of the intragranular ferrite is 50 μm or less. Steel material.
0 <(Ca− (0.18 + 130 × Ca) × O) /1.25/S≦0.40 (1)
However, Ca, O, and S in the above formulas represent the content (% by mass) of each component.
[2] The steel material for high heat input welding according to [1], further satisfying the following expression (2).
Mn + 0.8 × (Cr + Mo + V) ≦ 1.78 (2)
However, Mn, Cr, Mo, V in the above formula represents the content (% by mass) of each component, and 0 when not contained.
[3] Further, in mass%, V: 0.20% or less, Ni: 1.00% or less, Cu: 1.00% or less, Cr: 0.40% or less, Mo: 0.40% or less, W : The steel material for large heat input welding according to [1] or [2], which contains one or more selected from 0.05 to 0.40%.
[4] Further, by mass%, one or more selected from Mg: 0.0005-0.0050%, Zr: 0.0010-0.0200%, REM: 0.0010-0.0200% The steel material for high heat input welding according to any one of [1] to [3], which is contained.

  According to the present invention, a steel material excellent in the strength and toughness of the weld heat-affected zone can be obtained at a low cost even when high heat input welding exceeding 300 kJ / cm is performed. Therefore, the steel material of the present invention is suitably used for ships and large steel structures constructed by high heat input welding such as submerged arc welding, electrogas welding, and electroslag welding.

  The form for implementing this invention is demonstrated below. First, the significance of limiting chemical components in the present invention will be described. In the present invention, “%” regarding chemical components means “% by mass”.

C: 0.030 to 0.120%
C has a lower limit of 0.030% in order to obtain the strength required for structural steel, and an upper limit of 0.120% in order to suppress the amount of island martensite produced.

Si: 0.50% or less Si is an element that is necessarily contained in steel, and has the effect of improving the strength of the base material. In order to generate island martensite and deteriorate toughness, the upper limit is made 0.50%.

Mn: 1.40 to 2.00%
Mn needs to be 1.40% or more in order to secure the strength of the base material, and if it exceeds 2.00%, the toughness of the welded portion is deteriorated. Therefore, the range of Mn is 1.40 to 2.00%, preferably 1.40% to 1.78%, and more preferably 1.40% to 1.60%. It is a range.

P: 0.020% or less P is an impurity that is inevitably mixed. If it exceeds 0.020%, the toughness of the base metal and the welded portion is lowered, so the upper limit is made 0.020%.

S: 0.0005 to 0.0040%
S is required to be 0.0005% or more in order to produce required CaS or MnS, and if it exceeds 0.0040%, the toughness of the base material is deteriorated. Therefore, the S content is in the range of 0.0005 to 0.0040%.

Al: 0.005 to 0.100%
Al is required to be 0.005% or more in terms of deoxidation of steel, and is preferably contained in an amount of 0.01% or more. Degradation of toughness. Therefore, the Al content is in the range of 0.005 to 0.100%.

Nb: 0.003 to 0.030%
Nb is an element effective for ensuring the strength and toughness of the base material and the strength of the joint, but its effect is small when it is less than 0.003%. If the content exceeds 0.030%, the toughness deteriorates by forming island martensite in the weld heat affected zone. Therefore, the Nb content is in the range of 0.003 to 0.030%.

Ti: 0.004 to 0.030%
Ti precipitates as TiN during solidification and contributes to the suppression of austenite coarsening in the weld heat-affected zone and to high toughness as a ferrite transformation nucleus in the grain boundaries and grains. If less than 0.004%, the effect is small, and if it exceeds 0.030%, the expected effect cannot be obtained due to the coarsening of TiN particles. Therefore, the Ti content is in the range of 0.004 to 0.030%. Preferably, it is 0.006% to 0.028% of range.

B: 0.0003 to 0.0030%
B is an element that generates BN at the weld heat affected zone to reduce the solid solution N and to act as a ferrite transformation nucleus in the austenite grain boundary or in the grains. In order to exhibit such an effect, the content of 0.0003% or more is necessary. However, when the content exceeds 0.0030%, the hardenability is excessively increased and the toughness is deteriorated. Therefore, the B content is in the range of 0.0003 to 0.0030%.

N: 0.0035 to 0.0070%
N is required to secure an amount commensurate with Ti for forming TiN, and if it is less than 0.0035%, a sufficient amount of TiN cannot be obtained, and austenite coarsening is suppressed in the weld heat affected zone or ferrite. Effects such as contributing to high toughness as a transformation nucleus cannot be obtained. If it exceeds 0.0070%, the toughness is significantly lowered due to an increase in the amount of solute N in the region where TiN is dissolved by the welding heat cycle. Therefore, the N content is in the range of 0.0035 to 0.0070%. Preferably, it is set as 0.0037 to 0.0070% of range.

Ca: 0.0005 to 0.0030%
Ca is an element having a toughness improving effect by chemically fixing S by forming CaS. In order to exhibit such an effect, it is necessary to contain at least 0.0005% or more, but even if it exceeds 0.0030%, the effect is saturated. For this reason, in this invention, it limits to the range of 0.0005%-0.0030%.

O: 0.0040% or less O is always contained, but precipitates as an oxide at the time of solidification. Therefore, if it exceeds 0.0040%, the toughness of the base material and the weld heat-affected zone decreases. For this reason, content of O shall be 0.0040% or less.

0 <(Ca− (0.18 + 130 × Ca) × O) /1.25/S≦0.40 (1) where Ca, O, and S are the contents of each component (mass%). Represent.
Ca, O, and S must be contained so as to satisfy the relationship of 0 <(Ca− (0.18 + 130 × Ca) × O) /1.25/S≦0.40. In this case, it becomes the form of the composite sulfide in which MnS is deposited on CaS. If the value of Ca− (0.18 + 130 × Ca) × O) /1.25/S exceeds 0.40, the number of nucleation of intragranular ferrite decreases and the ferrite grains cannot be made fine. On the contrary, if the value of Ca− (0.18 + 130 × Ca) × O) /1.25/S exceeds 0 and is 0.40 or less, the number of nucleation of intragranular ferrite is remarkable. As will be described later, the average grain size of the intragranular ferrite can be increased to 50 μm.

In the case of (Ca− (0.18 + 130 × Ca) × O) /1.25/S≦0, since CaS does not crystallize, S precipitates in the form of MnS alone. Since it is elongated by rolling and is not uniformly and finely dispersed, it causes a decrease in the toughness of the base material and an increase in toughness at the weld heat affected zone is not achieved. Therefore, 0 <(Ca− (0.18 + 130 × Ca) × O) /1.25/S≦0.40. Preferably, the range is 0.10 ≦ (Ca− (0.18 + 130 × Ca) × O) /1.25/S <0.30.
Mn + 0.8 × (Cr + Mo + V) ≦ 1.78 (2)
However, Mn, Cr, Mo, V in the above formula represents the content (% by mass) of each component, and 0 when not contained.
When Mn + 0.8 × (Cr + Mo + V) exceeds 1.78, the effect of suppressing the formation of island martensite in the high heat input welding heat-affected zone structure is insufficient, and the strength of the iron matrix phase is excessively increased. It becomes difficult to ensure the stability of the joint HAZ toughness at a temperature of −40 ° C. Therefore, it is preferable that Mn + 0.8 × (Cr + Mo + V) ≦ 1.78.

  In the present invention, one or more selected from V, Ni, Cu, Cr, Mo, and W can be selectively contained within the following ranges. When V, Cr, and Mo are contained, the content is defined in relation to the amount of Mn.

V: 0.20% or less V works as an improvement in the strength and toughness of the base metal and as a ferrite formation nucleus as VN. This effect is exhibited by containing 0.05% or more, but 0.20% On the other hand, the toughness may be reduced. Therefore, when adding V, it is preferable to make it 0.20% or less.

Ni: 1.00% or less Ni increases the strength while maintaining the high toughness of the base material. This effect is exhibited by containing 0.10% or more, but the effect may be saturated even if it exceeds 1.00%, so when adding Ni, the effect may be 1.00% or less. preferable.

Cu: 1.00% or less Cu increases the strength while maintaining the high toughness of the base material, similar to Ni. This effect is exhibited by containing 0.10% or more, but if it exceeds 1.00%, hot brittleness may occur, and the surface properties of the steel sheet may be deteriorated. Therefore, when adding Cu, it is preferable to set it as 1.00% or less.

Cr: 0.40% or less Cr is an element effective for increasing the strength of the base material. This effect is exhibited when contained in an amount of 0.03% or more, but if added in excess, it adversely affects toughness. Therefore, the upper limit is preferably set to 0.40%.

Mo: 0.40% or less Mo is an element effective for increasing the strength of the base material, and this effect is exhibited by containing 0.05% or more, but if added in excess, it adversely affects toughness There is. Therefore, when adding Mo, it is preferable to set it as 0.40% or less.

W: 0.05-0.40%
W is an element effective for increasing the strength of the base material, and this effect is exhibited when it is contained in an amount of 0.05% or more, but if added excessively, the toughness may be adversely affected. Therefore, when adding W, it is preferable to set it as 0.05 to 0.40%.

  In the present invention, one or more selected from Mg, Zr, and REM can be further contained within the following range.

Mg: 0.0005 to 0.0050%
Mg is an element having an effect of improving toughness due to dispersion of oxides. In order to exhibit such an effect, it is preferable to contain 0.0005% or more, but even if it contains more than 0.0050%, the effect may be saturated. Therefore, when adding Mg, it is preferable to set it as 0.0005 to 0.0050%.

Zr: 0.0010 to 0.0200%
Zr is an element having an effect of improving toughness due to dispersion of oxides. In order to exert such an effect, it is preferable to contain 0.0010% or more, but even if it exceeds 0.0200%, the effect may be saturated. Therefore, when adding Zr, it is preferable to set it as 0.0010 to 0.0200%.

REM: 0.0010 to 0.0200%
REM is an element having an effect of improving toughness due to dispersion of oxides. In order to exhibit such an effect, it is preferable to contain 0.0010% or more, but even if it contains over 0.0200%, the effect may be saturated. Therefore, when adding REM, it is preferable to set it as 0.0010 to 0.0200%.

In the heat-affected zone structure near the bond when large heat input welding with a welding heat input exceeding 300 kJ / cm is performed, the grain size of the intragranular ferrite is 50 μm or less. The large heat input with a welding heat input exceeding 300 kJ / cm The purpose of heat welding is to prevent generation of island martensite in the case of such high heat input welding.

  The heat-affected zone structure in the vicinity of the bond refers to a region from the boundary between the weld metal and the base steel plate to a position on the steel plate side of the base metal of about 0.5 mm.

  The particle size of the intragranular ferrite can be measured using an image analyzer as will be described later. The reason why the grain size of the intragranular ferrite is 50 μm or less is that if it is 50 μm or less, the toughness can be stably secured. When it exceeds 50 μm, the variation in toughness value increases.

  In the present invention, as described above, by limiting the value of Ca− (0.18 + 130 × Ca) × O) /1.25/S to a range larger than 0 and 0.40 or less, Combined with the austenite coarsening suppression and toughening effect in the weld heat affected zone by controlling the precipitation behavior of TiN, the grain size of the intragranular ferrite in the weld heat affected zone can be controlled to a fine grain of 50 μm or less. As a result, excellent toughness is achieved in the weld heat affected zone.

  In addition, the steel material of this invention is manufactured as follows, for example. First, the hot metal is smelted in a converter to obtain steel, and then RH degassing is performed to obtain a steel slab through a continuous casting or ingot-bundling process. This is reheated and allowed to cool after hot rolling, or, after the hot rolling, manufactured through accelerated cooling, direct quenching-tempering, reheating quenching-tempering, reheating normalization-tempering, etc. can do.

Next, this invention is demonstrated based on an Example.
In a 150 kg high-frequency melting furnace, steel having the composition shown in Table 1 was melted to form a slab having a thickness of 170 mm. The slab was heated to 1150 ° C. for 1 hour, and then the hot rolling with a finish rolling temperature of 830 ° C. at the plate thickness center temperature was finished to a plate thickness of 50 mm, and then a cooling rate of 10 ° C./s (plate thickness center portion) ) Accelerated cooling. A round bar tensile test piece having a parallel portion of 14 mmφ × 85 mm and a distance between gauge points of 70 mm from the C direction (direction perpendicular to the rolling direction) at a position of 1/4 t (t: plate thickness) of the obtained thick steel plate is set to 1/4 t. A 2 mm V notch Charpy test piece was taken from the L direction (rolling direction) at the position of, and the strength (YS, TS) and toughness of the base material were evaluated.

  Furthermore, in order to evaluate the characteristics of the heat affected zone of the welded joint, after producing the joint by electrogas welding (EGW) of high heat input welding (400 to 500 kJ / cm), the HAZ toughness is measured on the front and back surfaces in the plate thickness direction. Using a Charpy test piece with a notch in the bond part at the 1 mm position, the absorption energy at a test temperature of −40 ° C. (the average value of three test pieces, described as “vE-40 ° C.”) was evaluated. . The target characteristic was a grade F steel with a high heat input welded joint Charpy toughness of −40 ° C., with a welded joint HAZ toughness of absorbed energy vE−40 ° C. ≧ 50 J.

  The grain size of the intragranular ferrite in the weld heat affected zone was evaluated using a cutting method using five optical micrographs (magnification: 50 to 200 times). The short piece was measured about the flat crystal grain. As the intragranular ferrite, the austenite grain boundary is first corroded with a corrosive liquid that preferentially corrodes, so that the former austenite grain boundary appears and traced, and then the ferrite structure generated inside is targeted. . Table 2 shows the grain size of the intragranular ferrite and the HAZ toughness together with the mechanical properties of the base material.

From Table 2, Ru Inventive Example der No. 1 to 4 and 6 , both have excellent base material properties such as yield stress YS of 390 N / mm ² or more, tensile strength TS of 510 N / mm ² or more, and brittle fracture surface transition temperature (vTrs) of −80 ° C. or less. It was confirmed that Further, the steel of the present invention has an absorption energy vE-40 ° C. of 50 J or more in the weld heat affected zone, and is excellent in weld heat affected zone toughness. On the other hand, at least one of the chemical component, the value of (Ca− (0.18 + 130 × Ca) × O) /1.25/S, and the particle size of the intragranular ferrite is a comparative example No. 9 to 20 are inferior in any one or more of the above characteristics.

Claims (3)

% By mass
C: 0.030 to 0.093%
Si: 0.50% or less Mn: 1.40 to 2.00%
P: 0.020% or less S: 0.0005 to 0.0040%
Al: 0.005 to 0.050%
Nb: 0.003 to 0.030%
Ti: 0.004 to 0.030%
B: 0.0003 to 0.0030%
N: 0.0035 to 0.0070%
Ca: 0.0005 to 0.0030%
O: 0.0040% or less is satisfied, the following formula (1) and the following formula (2) are satisfied, the balance is composed of Fe and unavoidable impurities, and in the base material, vTrs ≦ −80 ° C. in the heat affected zone tissue of the bond near when the heat is subjected to high heat input welding exceeding 300 kJ / cm, the particle size of the intragranular ferrite Ri der less 50 [mu] m, and characterized by a vE-40 ℃ ≧ 177J be that large heat input welding steel materials.
0 <(Ca− (0.18 + 130 × Ca) × O) /1.25/S≦0.40 (1)
Mn + 0.8 × (Cr + Mo + V) ≦ 1.78 (2)
However, Ca, O, S, Mn, Cr, Mo, and V in the above formulas represent the content (% by mass) of each component, and 0 when not contained.   Further, in terms of mass%, V: 0.20% or less, Ni: 1.00% or less, Cu: 1.00% or less, Cr: 0.40% or less, Mo: 0.40% or less, W: 0.0. The steel material for high heat input welding according to claim 1, comprising one or more selected from 05 to 0.40%.   Furthermore, it contains at least one selected from Mg: 0.0005 to 0.0050%, Zr: 0.0010 to 0.0200%, and REM: 0.0010 to 0.0200% by mass%. The steel material for high heat input welding according to claim 1 or 2.