JP7207250B2 - Steel material and its manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel material and its manufacturing method.

Uses of steel materials include ships, high-rise buildings, other buildings, bridges, offshore structures, LNG storage tanks and other large tanks, and welded structures such as line pipes. 2. Description of the Related Art In recent years, due to an increase in the loading weight of container ships, etc., the size of welded structures is being increased. Along with this, steel materials are required to be thicker and stronger. In addition, in the welded structure as described above, it is necessary to ensure the safety and reliability of the weld zone, and the toughness of the weld heat affected zone (hereinafter sometimes referred to as "HAZ toughness"). improvement is an issue.

In addition, welding costs account for a large part of the overall construction costs of welded structures, and high-efficiency welding is required in order to reduce this cost. Specifically, it is effective to perform welding with a large heat input to reduce the number of welding passes. However, when welding is performed with a large heat input, the structure of the HAZ of the steel generally coarsens, and deterioration of toughness is inevitable.

Conventionally, it is known that the grain size of austenite (γ), the transformation structure, the hardness of the HAZ, the coarse hard phase, etc. have a large effect on the HAZ toughness of high-strength steel sheets. Various countermeasures have been proposed. Of these, refinement of the HAZ structure is most effective for improving the HAZ toughness, and many methods of refining the HAZ structure by utilizing inclusions have been proposed.

Refinement of the HAZ structure using inclusions includes a method of suppressing the growth of crystal grains by the pinning effect of inclusions, and a method of forming ferrite grains with inclusions as nuclei in austenite grains coarsened by the heat effect during welding. There is a method of refining the structure by generating (intragranular transformation). With respect to microstructural refinement by intragranular transformation, there have been proposed techniques that use nitrides such as TiN, sulfides such as MnS, or oxides that are chemically stable even at high temperatures as ferrite generation sites.

Patent Literature 1 proposes a method of improving HAZ toughness with inclusions containing REM and Zr.

In Patent Document 2, in the composition of inclusions with a width of 1 μm or more contained in steel, the amount of Zr in the inclusions is 5 to 60%, the amount of REM is 5 to 50%, and the amount of Al is 5 to 30%. , a steel sheet having an S content of more than 0% and less than 20%.

Patent Document 3 discloses a composite oxide containing REM, Zr, Ti, Al, Ca, and S. Regarding the composite oxide in the steel material, the oxide having an equivalent circle diameter of more than 3 μm is 5.0 per 1 mm ² of 0.1 to 3 μm in equivalent circle diameter, the number of composite oxides satisfying a predetermined formula is 100/mm ² or more, and 0.0. The average composition of the composite oxide of 1 to 3 μm is Al ₂ O ₃ : 20% or less, TiO ₂ : 3 to 20%, ZrO ₂ : 5 to 50%, REM oxide: 5 to 50%, CaO: 5 to 50% S: 1-15% steel is described.

In Patent Document 4, oxides containing Zr, REM, and Ca are included, and among all the inclusions contained in the steel material, there are 120 inclusions with an equivalent circle diameter of 0.1 to 2 μm per 1 mm ² of the observation field area. 5.0 or less per 1 mm ² of observation field area, and the chemical composition of inclusions contained in the steel satisfies the relationship of the following formula (1): is described.
(Insol.Ti−3.4×Insol.N)/Insol.Al=1.0 to 8 (1)

Patent Document 5 describes inclusions satisfying the average composition of ZrO ₂ : 5 to 50%, REM oxide: 5 to 50%, CaO: 50% or less, and having an equivalent circle diameter of 0.1 to 50%. There are 120 or more inclusions of 2 μm per 1 mm ² of the observation field area, 5.0 or less oxides with an equivalent circle diameter of more than 3 μm per 1 mm ² of the observation field area, and oxides with an equivalent circle diameter of more than 5 μm in the observation field area. REM- and Zr-containing inclusions I whose number is 5.0 or less per 1 ^mm2 and whose molar ratio of REM and Zr (REM/Zr) to the total number of inclusions satisfies 0.6 to 1.4 The number ratio is 30% or more, and/or the ratio of the total number of moles of REM and Zr to the total number of moles of Al, Ca, and Ti [(REM+Zr)/( Al+Ca+Ti)] satisfies 0.5 to 1.2.

In addition, steel materials used for structures such as ships, high-rise buildings, other buildings, bridges, offshore structures, LNG storage tanks and other large tanks, line pipes, etc. should be protected against brittle fracture. In addition, arrestability (brittle fracture propagation stopping function), which is the ability to suppress the propagation of brittle fracture, is also required.

JP 2008-291347 A JP 2014-214371 A JP 2014-185364 A JP 2014-1432 A JP 2012-162797 A

The present invention provides a steel material that has excellent HAZ toughness, particularly in the HAZ of high heat input welding of 35 kJ / mm or more, has excellent arrestability, and has excellent strength, and its manufacture. The challenge is to provide a method.

The present inventors focused on Zr-containing oxides and B nitrides as intragranular ferrite formation sites for microstructural refinement in the HAZ and conducted earnest studies. As a result, the following new findings (A) to (E) were mainly obtained.

(A) Sol. The smaller the Zr content, the more the HAZ toughness tends to be improved, and it is preferably 0.0010% by mass or less. Here, Sol. Zr is acid-soluble Zr and corresponds to Zr dissolved in steel, which can be measured by an electrolytic extraction residue analysis method or the like.

(B) Due to the inclusion of Zr and B, B nitrides precipitate in the steel with the Zr-containing oxides serving as nuclei. Such (Zr, B)-containing oxide particles with precipitated B nitrides function more effectively as intragranular ferrite formation sites. When obtaining this effect, Zr is added after the dissolved oxygen concentration of the molten steel becomes 0.0050% or less in the refining process, and then B is added to reduce _BF , which is the amount of B dissolved in the steel. It is preferable to make it 0.0020% by mass or less.

(C) When the Al ₂ O ₃ composition contained in the (Zr, B)-containing oxide particles is 50% by mass or less, the (Zr, B)-containing oxide particles function more effectively as intragranular ferrite formation sites. do. In order to obtain this effect, it is preferable to add Zr after the dissolved oxygen concentration of the molten steel reaches 0.0050% or less in the refining process, and then carry out continuous casting.

(D) Among the (Zr, B)-containing oxide particles, (Zr, B)-containing oxide particles having an equivalent circle diameter of 0.5 μm or more and an Al ₂ O ₃ composition of 50% by mass or less. When the number density is 5 to 300/mm ² , a large amount of fine intragranular ferrite is generated in the HAZ, improving the HAZ toughness.

(E) Excessive content of Al, which acts as a strong deoxidizing element, in steel inhibits the formation of Zr-containing oxides. The Al content is preferably 0.010% by mass or less in order to ensure the dissolved oxygen content in the molten steel and generate Zr-containing oxides in the steel. Further, it is preferable to limit the total content of elements such as Ca, Mg, and REM, which have stronger deoxidizing power than Al, to 0.0005% by mass or less.

Furthermore, in addition to the above (A) to (E), the present inventors have found that by controlling the microstructure and the grain boundary density in the plate thickness direction, the direction parallel to the steel material surface, for example, the rolling direction It was found that arrestability in the parallel direction can be improved.

The present invention has been completed based on the above findings, and the gist thereof is as follows.

[1] in % by mass,
C: 0.040 to 0.160%,
Si: 0.01 to 0.50%,
Mn: 0.70-2.50%,
P: 0.030% or less,
S: 0.008% or less,
Al: 0.010% or less,
N: 0.0010 to 0.0080%,
O: 0.0005 to 0.0040%,
Nb: 0.003 to 0.050%,
Ti: 0.003-0.024%,
Zr: 0.0007 to 0.0050%,
B: 0.0003 to 0.0040%,
total content of Ca, Mg and REM: 0.0005% or less;
The balance consists of Fe and impurity elements,
Insol. Zr: 0.0007 to 0.0040%,
Sol. Zr: 0.0010% or less,
B _F represented by the following formulas (1) and (2) is 0.0020% or less,
The carbon equivalent Ceq represented by the following formula (3) is 0.30% to 0.55%,
It contains ferrite with an area ratio of 5 to 70%, bainite with an area ratio of 30% or more, pearlite with an area ratio of 0 to 15%, and martensite-austenite mixed structure with an area ratio of 0 to 5%. having a microstructure,
The grain boundary density at a position 1 to 5 mm from the plate surface is 500 to 1100 mm / mm ² ,
The grain boundary density at the position of 1/4 part of the plate thickness is 400 to 1000 mm / mm ² ,
The grain boundary density at the position of 1/2 part of the plate thickness is 300 to 900 mm / mm ² ,
Of the (Zr, B)-containing oxide particles containing 5% by mass or more of Zr, 0.1% by mass or more of B, and 1% by mass or more of O, the equivalent circle diameter is 0.5 μm or more (Zr, B) A steel material in which the number density of (Zr, B)-containing oxide particles having an Al ₂ O ₃ composition of 50% by mass or less is 5 to 300/mm ² .
B _F ′=B−[N−{Ti−(O−Insol.Zr×(32/91.224))×(95.734/48)}×(14/47.867)]×(10.811 /14) …(1)
_BF =B if _BF '>B, _{BF=BF' if 0≦BF'≦B, BF = 0 if BF} '<0 (2)
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15 (3)
However, N, Ti, O and B in the formulas (1) and (2) are the contents in mass % of N, Ti, O and B contained in the steel, and Insol. Zr is the content in mass % of acid-insoluble Zr.
Further, C, Mn, Cr, Mo, V, Cu, and Ni in the formula (3) are the content (% by mass) of each element contained in the steel, and 0 is set when the element is not contained. substitute.
[2] Furthermore, in % by mass,
Cu: 1.00% or less,
Ni: 2.50% or less,
Cr: 1.00% or less,
Mo: 0.50% or less,
V: The steel material according to [1] above, characterized by containing one or more selected from the group consisting of 0.150% or less.
[3] Furthermore, in % by mass,
W: 1.00% or less,
The steel material according to [1] or [2] above, characterized by containing one or two selected from the group consisting of Sn: 0.50% or less.
[4] The steel manufacturing method according to any one of [1] to [3] above, wherein the molten steel is subjected to vacuum degassing so that the dissolved oxygen concentration of the molten steel is 0.0050%. A refining step in which Zr is added after the following, and B is added after 1.0 to 5.0 minutes from the addition of Zr;
When the molten steel after the refining process is continuously cast to form a slab, the average cooling rate for the surface temperature of the slab from 1200° C. to 900° C. is 0.5° C./second or less. a continuous casting process;
A heating step of heating the slab after the continuous casting to 950 to 1150 ° C.;
a hot rolling step of hot-rolling the heated slab into a steel material,
The hot rolling step is a step of sequentially performing a rough rolling step, a finish rolling step, and a cooling step,
In the rough rolling step, the cast slab heated in the heating step is rolled at a rolling temperature of not less than the recrystallization temperature Trex (° C.) shown in the following formula (4) and not more than 1050° C. at a cumulative reduction rate of 10 to 75%. is a process to
In the finish rolling step, the finish rolling temperature is (Ar ₃ -50) ° C. or higher (where Ar ₃ is represented by the following formula (5)) and is lower than the recrystallization temperature Trex (° C.) shown in the following formula (4). It is a step of rolling under the condition of a cumulative reduction rate of 45 to 75%,
In the cooling step, the cooling start temperature is (Ar ₃ -100) ° C. or higher (where Ar ₃ is represented by the following formula (5)) and is lower than the recrystallization temperature Trex (° C.) shown in the following formula (4). range, the cooling stop temperature is in the range of 0 ° C. or higher and 600 ° C. or lower, and the average cooling rate from the start of cooling to the end of cooling is 2 to 15 ° C./sec. , steel manufacturing method.
Trex=−91900 [Nb*] ² +9400 [Nb*]+770 (4)
Ar ₃ (° C.)=910-310C+65Si-80Mn-20Cu-55Ni-15Cr-80Mo (5)
[Sol. Nb]=(10 ^{(−6770/(T+273)+2.26)} )/(C+12/14×N) (6)
However, [Nb*] in formula (4) is equivalent to [Sol. Nb] and the Nb content (% by mass) in the steel, Nb≧[Sol. Nb], [Nb*]=[Sol. Nb] and Nb<[Sol. Nb], let [Nb*]=Nb,
The element symbols in formulas (5) to (6) are the content (% by mass) of each element contained in the steel, and 0 is substituted when the element is not contained,
T in the formula (6) is the temperature (° C.) of the slab at the time of the slab extraction in the heating step.
[5] The method for producing a steel material according to [4] above, characterized in that a tempering step of heating to a temperature in the range of 350 to 650° C. is performed after the cooling step.

According to the present invention, a steel material having excellent HAZ toughness, particularly excellent toughness in the HAZ of high heat input welding of 35 kJ / mm or more, excellent arrestability, and excellent strength, and its A manufacturing method can be provided.

It is known that Ti oxides and B nitrides are dispersed in the weld metal and HAZ and have the effect of refining the structure. In contrast, Zr is not an element commonly added to steel materials, and research done in the past on the effects of adding Zr has been very limited.
In particular, until now, there has been no study on how B nitrides further precipitated in Zr-containing oxides affect the refinement of the HAZ structure of steel materials and the improvement of HAZ toughness. Also, the relationship between the composition of the Zr-containing oxide and the B nitride has not been studied.

The present inventors paid attention to Zr-containing oxides and B nitrides as intragranular ferrite formation sites for refining the HAZ structure, and conducted earnest studies. As a result, the following new findings (a) to (e) were mainly obtained.

(a) In order to obtain a predetermined number density or more of Zr-containing oxides that contribute to refinement of the HAZ structure, it is necessary to set the Zr content to a certain amount or more. On the other hand, not all Zr in steel forms oxides, and some Zr remains in steel without forming oxides. This oxide-free Zr (Sol. Zr) significantly degrades not only the HAZ but also the toughness of the steel material itself. Therefore, in order to ensure the toughness of not only the HAZ but also the steel material itself, Sol. Zr should be reduced. Sol. Toughness tends to improve as the Zr content decreases. It is preferable to limit Zr to 0.0010 mass % or less. Sol. It is desirable to limit Zr to 0.0003% by mass or less.

(b) In the steel in which the Zr-containing oxide is dispersed, even if the number of inclusions increases, it was found that there are inclusions that function as ferrite formation sites and inclusions that do not function as ferrite formation sites.
In addition, the present inventors investigated various elements in order to promote ferrite formation more effectively. As a result, when B is contained in a certain amount or more, B nitrides precipitate with Zr-containing oxides as nuclei during casting, hot rolling, or welding, and these composite precipitates (Zr, B)-containing oxides It was found that grains function more effectively as intragranular ferrite formation sites.
That is, the B nitride makes the Zr-containing oxide, which is difficult to function as an intragranular ferrite formation site by itself, also become a ferrite formation site, and more efficiently contributes to the refinement of the HAZ structure. In order to obtain these effects, Zr is added after the dissolved oxygen concentration of the molten steel becomes 0.0050% or less in the refining process, and then B is added. B _F is preferably 0.0020% by mass or less.

(c) In addition to B, Ti acts as a nitride-forming element in steel. Therefore, in order to precipitate B nitrides efficiently, it is necessary to suppress the formation of Ti nitrides. The present inventors have investigated to clarify the formation mechanism of inclusions including oxides and nitrides, and to clarify the conditions for forming B nitrides.

In molten steel containing Ti, Zr, and B, Zr, which has a stronger deoxidizing power than Ti, preferentially becomes an oxide, and excess oxygen and Ti combine to form a composite oxide of Zr and Ti. . Excess Ti that did not form oxides then combines with nitrogen to form nitrides. Next, it is believed that excess nitrogen that does not combine with Ti forms B nitrides.

It is believed that Zr forms ZrO ₂ , Ti forms Ti ₂ O ₃ and TiN, and B forms BN, respectively. BasBN) content (% by mass) can be determined. Then, as shown in the following formula (A2), the difference obtained by subtracting the B that forms the B nitride from the B that is contained in the steel is taken as the calculated value (B _F ') of B dissolved in the steel. Then, when the relationship between the calculated value B _F ' obtained by the following formula (A2) and the amount of B in the steel is B _F '>B, the amount of B in the steel is changed to the amount of B dissolved in the steel (B _F ) (B _F =B). If 0≤B _F '≤B, the calculated value B _F ' obtained by the following formula (A2) is taken as the B amount (B _F ) dissolved in the steel (B _F =B _F '). Furthermore, when B _F ′<0, the amount of B (B _F ) dissolved in steel is set to 0% by mass (B _F =0). By setting the B 2 _F content to 0.0020% by mass or less as described above, the effect of improving the HAZ toughness by the B nitride can be obtained, and the crack arrestability can be effectively improved by refining the structure.

BasBN = (N-(Ti-(O-Insol.Zr x (32/91.224)) x (95.734/48)) x (14/47.867)) x (10.811/14) ... (A1)

_BF '=B-BasBN (A2)

Here, N, Ti and O in the formula (A1) are the contents (% by mass) of each element (N, Ti, O) contained in the steel, and Insol. Zr is the content (% by mass) of acid-insoluble Zr.
Further, B in the formula (A2) is the content (% by mass) of B contained in the steel, and BasBN is a value obtained from the formula (A1).

A steel material obtained by hot-rolling a steel slab having a composition in which _BF is 0.0020% by mass or less contains fine Zr-containing oxides (complex oxides mainly containing Zr and Ti). Spread. In addition, B nitrides are precipitated in a complex manner in some Zr-containing oxides.
B nitrides redissolve when heated to a temperature range exceeding 1200°C during welding, but Zr-containing oxides exist stably even when heated to 1400°C. Therefore, the B nitride is solid-dissolved during welding heating, and the solid-solution B is unevenly distributed around the Zr-containing oxide. It is believed that this solid solution B reprecipitates as B nitrides with oxides as nuclei during the cooling process after welding.

(d) Furthermore, it is necessary to control the composition of the (Zr, B)-containing oxide particles in order to facilitate efficient deposition of B nitrides on the Zr-containing oxide. Specifically, when the Al ₂ O ₃ composition contained in the (Zr, B)-containing oxide particles is 50% by mass or less, the B nitride precipitates more efficiently and becomes more effective as an intragranular ferrite formation site. become functional.

(e) In addition, since Al acts as a strong deoxidizing element in steel, if it is contained in a large amount, it inhibits the formation of oxides of Zr and Ti. The Al content is preferably 0.010% by mass or less in order to ensure the dissolved oxygen content in the molten steel and generate Zr-containing oxides in the steel. More desirably, the Al content is 0.005% by mass or less. The total content of deoxidizing elements stronger than Al, such as Ca, Mg, and REM, is preferably 0.0005% by mass or less.

In a steel material that satisfies these conditions, (Zr, B)-containing oxide particles of a predetermined size are generated in a predetermined number. In addition, many of these (Zr, B)-containing oxide particles are composite oxides containing Zr and Ti, B nitrides precipitate with the oxide as a nucleus, and the Al ₂ O ₃ composition is 50 mass. % or less. Then, when this steel material is actually subjected to high heat input welding, the oxide particles effectively function as intragranular ferrite formation sites in the HAZ, and the HAZ structure is refined to improve the HAZ toughness. became clear.

Furthermore, in addition to the chemical composition and Zr-containing oxides of the steel material, the present inventors have found that by controlling the microstructure and the grain boundary density in the plate thickness direction, It has been found that arrestability in the vertical or parallel direction can be improved.

Hereinafter, a steel material according to one embodiment of the present invention (steel material according to this embodiment) will be described in detail.

The steel material of this embodiment is, in mass%, C: 0.040 to 0.160%, Si: 0.01 to 0.50%, Mn: 0.70 to 2.50%, P: 0.030%. Below, S: 0.008% or less, Al: 0.010% or less, N: 0.0010 to 0.0080%, O: 0.0005 to 0.0040%, Nb: 0.003 to 0.050% , Ti: 0.003 to 0.024%, Zr: 0.0007 to 0.0050%, B: 0.0003 to 0.0040%, total content of Ca, Mg and REM: 0.0005% or less , the balance being Fe and impurity elements, and Insol. Zr: 0.0007-0.0040%, Sol. Zr: 0.0010% or less, _BF represented by the following formulas (B1) and (B2) is 0.0020% or less, and the carbon equivalent Ceq represented by the following formula (B3) is 0.0020%. 30% to 0.55%, ferrite with an area ratio of 5 to 70%, bainite with an area ratio of 30% or more, pearlite with an area ratio of 0 to 15%, and area ratio of 0 to 5%. It has a microstructure containing a martensite-austenite mixed structure, the grain boundary density at a position of 1 to 5 mm from the plate surface is 500 to 1100 mm / mm ² , and the position of 1/4 part of the plate thickness The grain boundary density is 400 to 1000 mm/mm ² , the grain boundary density at the position of 1/2 of the plate thickness is 300 to 900 mm/mm ² , 5% by mass or more of Zr and 0.1 Among (Zr, B)-containing oxide particles containing not less than 1% by mass of B and not less than 1% by mass of O, (Zr, B)-containing oxide particles having an equivalent circle diameter of 0.5 μm or more, The steel material has a number density of (Zr, B)-containing oxide particles having an Al ₂ O ₃ composition of 50% by mass or less and a number density of 5 to 300 particles/mm ² .

B _F ′=B−[N−{Ti−(O−Insol.Zr×(32/91.224))×(95.734/48)}×(14/47.867)]×(10.811 /14) ... (B1)

_BF =B if _BF '>B, _{BF=BF' if 0≦BF'≦B, BF = 0 if BF} '<0 (B2)

Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15 (B3)

However, N, Ti, O and B in the formulas (B1) and (B2) are the contents in mass % of N, Ti, O and B contained in the steel, and Insol. Zr is the content in mass % of acid-insoluble Zr.

In addition, C, Mn, Cr, Mo, V, Cu, and Ni in the formula (B3) are the contents (% by mass) of each element contained in the steel, and 0 is used when the element is not contained. substitute.

First, the chemical composition of the steel material of this embodiment will be described. In the following explanation of chemical components, "% by mass" is expressed as "%".

C: 0.040-0.160%
0.040% or more of C is contained in order to ensure the strength and toughness of the steel material. If the C content exceeds 0.160%, it becomes difficult to ensure good HAZ toughness, so the C content is made 0.160% or less. Therefore, the C content is 0.040% or more, preferably 0.050% or more, more preferably 0.060% or more. Also, the C content is 0.160% or less, preferably 0.140% or less, more preferably 0.120% or less.

Si: 0.01-0.50%
Since Si is effective as a deoxidizing element and a strengthening element, it is contained in an amount of 0.01% or more. If the Si content exceeds 0.50%, the HAZ toughness deteriorates significantly, so the Si content is made 0.50% or less. Therefore, the Si content is 0.01% or more, preferably 0.03% or more, more preferably 0.05% or more. Also, the Si content is 0.50% or less, preferably 0.40% or less, more preferably 0.35% or less or 0.30% or less.

Mn: 0.70-2.50%
Mn is contained in an amount of 0.70% or more in order to economically ensure the strength and toughness of the steel material. If the Mn content exceeds 2.50%, center segregation becomes significant, and the toughness of the steel material and HAZ at the portion where center segregation occurs deteriorates, so the Mn content is made 2.50% or less. Therefore, the Mn content is 0.70% or more, preferably 0.90% or more, more preferably 1.20% or more. Also, the Mn content is 2.50% or less, preferably 2.00% or less, more preferably 1.80% or less or 1.60% or less.

P: 0.030% or less P is an element present in steel as an impurity. In order to stably ensure HAZ toughness, the P content is set to 0.030% or less. It is preferably 0.020% or less, more preferably 0.015% or less. Although the lower limit is 0%, the P content may be 0.0001% or more in consideration of the cost for reducing the P content.

S: 0.008% or less S is an element present in steel as an impurity. If the S content exceeds 0.008%, a large amount of elongated MnS is generated in the central segregation part, and the toughness and ductility of the steel material and HAZ deteriorate. Therefore, the S content is set to 0.008% or less. Preferably, it is 0.005% or less. Since the lower the S content, the better, so the lower limit is not particularly defined, but from the viewpoint of production cost, the S content may be 0.0001% or more.

Al: 0.010% or less Al is generally an element that is positively added as a deoxidizing element. However, since Al tends to preferentially react with oxygen, if its content is excessive, the formation of the desired (Zr, B)-containing oxide particles will be insufficient, and effective ferrite formation sites in the HAZ will not be formed. Decrease. Furthermore, when the Al content becomes excessive, the formation of coarse cluster-like alumina (Al ₂ O ₃ )-based inclusions is promoted, and the toughness of the steel material and HAZ deteriorates. Therefore, it is preferable to reduce the Al content as much as possible. The allowable Al content is 0.010% or less, preferably 0.005% or less.

N: 0.0010 to 0.0080%
N is an important element in the present invention. In order to prevent the austenite grain size from increasing when the billet is heated, it is necessary to form Ti nitrides, so the Ti content should be 0.0010% or more. However, if the N content exceeds 0.0080%, the steel becomes embrittled, so the N content is made 0.0080% or less. Therefore, the N content is 0.0010% or more, preferably 0.0015% or more, more preferably 0.0020% or more. Also, the N content is 0.0080% or less, preferably 0.0065% or less, more preferably 0.0060% or less.

O: 0.0005 to 0.0040%
O is an element contained in steel and is dissolved or exists as an oxide. Since it is difficult to clearly separate the two, the O concentration in the present invention is the total oxygen concentration (also referred to as T.O) that combines the two. If the oxygen concentration in the steel plate is less than 0.0005%, the oxide dispersion number required to ensure toughness cannot be obtained. On the other hand, when it is contained in the steel material in excess of 0.0040%, the cleanliness of the molten steel deteriorates, and it may cause nozzle clogging in the molten steel stage, resulting in a decrease in productivity. Therefore, the appropriate range of O content in the steel material is 0.0005 to 0.0040%.

In addition, if the molten steel before adding Zr in the steel refining process contains more than _0.0050 % dissolved oxygen, the amount of ZrO2 generated by adding Zr increases, and when the molten steel is continuously cast increased risk of clogging of the injection nozzle into the tundish. Also, if the dissolved oxygen in molten steel before adding Zr is high, the composition of Al ₂ O ₃ in the (Zr, B)-containing oxide particles may increase. Therefore, it is desirable to reduce dissolved oxygen to 0.0050% or less at the stage of molten steel before adding Zr.

Nb: 0.003-0.050%
Nb can improve the strength and toughness of steel. In addition, in order to obtain a predetermined grain boundary density, rolling in the non-recrystallized austenite region is required, and Nb is an element effective for expanding the non-recrystallized temperature region, and raises the rolling temperature. It also contributes to productivity improvement. In order to obtain this effect, the content should be 0.003% or more. However, if the Nb content exceeds 0.050%, the HAZ toughness and weldability deteriorate, so the Nb content is made 0.050% or less. Therefore, the Nb content is 0.003% or more, preferably 0.005% or more, more preferably 0.008% or more. Also, the Nb content is 0.050% or less, preferably 0.025% or less, more preferably 0.018% or less.

Ti: 0.003-0.024%
Ti is an element that forms (Zr, B)-containing oxide particles together with Zr. These (Zr, B)-containing oxide particles function as intragranular ferrite formation sites in the HAZ and contribute to refinement of the HAZ structure. In order to obtain this effect, the Ti content should be 0.003% or more. The Ti content is preferably 0.005% or more. On the other hand, Ti forms nitrides. If a large amount of Ti nitrides is produced, the amount of B nitrides produced is suppressed, and the desired effects of this embodiment cannot be obtained. Furthermore, excess Ti forms TiC, which degrades the toughness of the steel and HAZ. Therefore, the Ti content should be 0.024% or less. A preferable Ti content is 0.020% or less.

Zr: 0.0007-0.0050%
The Zr content contained in the steel is determined by Sol. Zr and Insol. It is the sum with Zr. The Zr content is 0.0007% or more, preferably 0.0010% or more. Also, the Zr content is determined according to Insol. Zr upper limit and Sol. The sum with the upper limit of Zr, that is, 0.0050% or less, preferably 0.0040% or less.

Sol. Zr: 0.0010% or less Sol. Zr represents acid-soluble Zr, that is, Zr dissolved in steel. Sol. When the Zr content increases, the HAZ toughness deteriorates significantly. Therefore, the content is made 0.0010% or less. Sol. Since Zr is preferably as small as possible, the lower limit is not particularly defined, and may be 0%.

At the molten steel stage, Sol. Zr and Insol. Although there is no particular limitation on Zr, if Zr is added excessively with respect to dissolved oxygen, even the steel material becomes Sol. In addition to a large amount of Zr remaining, the concentration of dissolved oxygen decreases and the number density of (Zr, B)-containing oxide particles decreases. Therefore, the Sol. The Zr concentration is desirably 0.0020% or less. Also, in order to prevent nozzle clogging, Insol. The Zr concentration is desirably 0.0020% or less.

Insol. Zr: 0.0007-0.0040%
Insol. Zr is acid-insoluble Zr and is Zr contained in inclusions such as (Zr, B)-containing oxide particles. Zr is an important element that forms oxides that serve as nuclei for intragranular transformation. However, Insol. If the Zr content is less than 0.0007%, the oxide composition required for ensuring toughness cannot be obtained. On the other hand, Insol. When the Zr content exceeds 0.0040%, most of it is ZrO ₂ generated in the molten steel stage, and the frequency of nozzle clogging increases. Therefore, Insol. The proper range of Zr is 0.0007 to 0.0040%.

Insol. Zr and Sol. Zr can be measured by an electrolytic extraction residue analysis method. In the electrolytic extraction residue analysis method, steel is electrolyzed in a non-aqueous solvent (acetylacetone-methanol solution, etc.) to dissolve the mother phase, and the residue (precipitates and inclusions) is extracted with a filter with a pore size of 0.2 μm. It is a method of separation. After separation, the amount of Zr contained in the solution was determined by Sol. is the content of Zr, and the amount of Zr contained in the residue is Insol. Zr content.

B: 0.0003 to 0.0040%
B improves the hardenability of the steel material, precipitates as BN around the Zr-containing oxide to form (Zr, B)-containing oxide particles, and forms (Zr, B)-containing oxide particles. It is an element that improves transformation ability. In order to deposit BN around the Zr-containing oxide, B must be contained at least 0.0003% or more. On the other hand, even if the B content exceeds 0.0040%, the effect is saturated, so the proper range of the B content is set to 0.0003 to 0.0040%. In order to set the B concentration in the steel material to the range shown on the left, it is desirable that the B content be in the range of 0.0003 to 0.0040% even in the molten steel stage.

Total of Ca, Mg and REM: 0.0005% or less Ca, Mg and REM are elements that preferentially react with oxygen more than Al. In order to form the desired Zr-containing oxide, the total content of Ca, Mg and REM should be 0.0005% or less. More preferably, the Ca content is less than 0.0003%, the Mg content is less than 0.0003%, and the REM content is less than 0.0003%, and the total content is 0.0005% or less.

The steel material according to the present embodiment basically contains each of the above elements, with the balance being Fe and impurities. The term "impurities" refers to components that are mixed from raw materials such as ores, scraps, or other factors during the industrial production of steel materials, and that are allowed within a range that does not adversely affect the properties.

In the steel material according to the present embodiment, one or two or more selected from the group consisting of Cu, Ni, Cr, Mo, and V are added in place of part of Fe for the purpose of further increasing strength, which will be described later. It may be contained within the range. For the purpose of enhancing corrosion resistance, one or two selected from the group consisting of W and Sn may be contained within the range described below.

The steel material of the present embodiment further has Cu: 1.00% or less, Ni: 2.50% or less, Cr: 1.00% or less, Mo: 0.50% or less, and V: 0.150% by mass%. It may contain one or more selected from the group consisting of the following.

Cu: 1.00% or less By containing Cu, the strength and toughness of the steel material can be improved. However, if the Cu content is too large, the improvement in performance commensurate with the increase in alloy cost cannot be seen, and rather, it may cause surface cracks in the steel material, so the upper limit is made 1.00%. In order to stably obtain the effect of containing Cu, the Cu content may be 0.10% or more. The Cu content may be 0.20% or more to improve the strength and toughness of the steel material. In order to improve HAZ toughness and weldability, the Cu content may be 0.80% or less, 0.50% or less, or 0.30% or less as necessary.

Ni: 2.50% or less Ni is an element that has the effect of improving the strength of steel, so it may be contained. In addition, Ni is an element that has the effect of increasing the toughness of the steel matrix (material) in a solid solution state. However, if the Ni content is too large, the HAZ toughness and weldability deteriorate, so the upper limit is made 2.50%. In order to stably obtain the effect of Ni content, the Ni content may be 0.10% or more. In order to improve the strength and toughness of the steel material, the Ni content may be 0.20% or more. The Ni content may be 1.00% or less, 0.50% or less, or 0.30% or less as required.

Cr: 1.00% or less By containing Cr, the strength and toughness of the steel material can be improved. However, if the Cr content is too large, the HAZ toughness and weldability deteriorate, so the upper limit is made 1.00%. The Cr content may be 0.10% or more or 0.20% or more in order to stably obtain the Cr containing effect. The Cr content may be 0.80% or less, 0.50% or less, or 0.30% or less as required.

Mo: 0.50% or less By containing Mo, the strength and toughness of the steel material can be improved, so Mo may be contained. However, if the Mo content is too high, the HAZ toughness and weldability deteriorate, so the upper limit is made 0.50%. In order to stably obtain the effect of containing Mo, the Mo content may be 0.01% or more or 0.02% or more. Mo content is good also as 0.30% or less, 0.20% or less, or 0.10% or less as needed.

V: 0.150% or less By containing V, the strength and toughness of the steel material can be improved, so V may be contained. However, if the V content is too large, the HAZ toughness and weldability deteriorate, so the upper limit is made 0.150%. In order to stably obtain the effect of containing V, the V content may be 0.010% or more or 0.020% or more. The V content may be 0.100% or less, 0.070% or less, or 0.050% or less as required.

In addition, the steel material of the present embodiment may further contain, by mass %, one or two of W: 1.00% or less and Sn: 0.50% or less.

W: 1.00% or less W is an element that dissolves and adsorbs to rust in the form of oxyacid ions _WO4- ^, suppresses permeation of chloride ions in the rust layer, and improves corrosion resistance. may In order to obtain this effect, the W content is preferably 0.01% or more. On the other hand, when the W content exceeds 1.00%, not only the above effect is saturated, but also the toughness of the steel material and HAZ may be lowered. Therefore, even when W is contained, the W content is set to 1.00% or less. Preferably, the W content is 0.75% or less.

Sn: 0.50% or less Sn is an element that dissolves as Sn ²⁺ and has an inhibitory effect on corrosion in an acidic chloride solution, so it may be contained. In addition, Sn has the effect of suppressing the anodic dissolution reaction of steel and improving the corrosion resistance. In order to obtain these effects, the Sn content is preferably 0.03% or more. On the other hand, when the Sn content exceeds 0.50%, not only does the effect saturate, but also rolling cracks are likely to occur in the steel material. Therefore, even when Sn is contained, the content is made 0.50% or less.

In addition, the steel material of this embodiment must have a carbon equivalent Ceq represented by the following formula (C) of 0.30% to 0.55%.

Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15 (C)

However, C, Mn, Cr, Mo, V, Cu, and Ni in the formula (C) are the contents (mass%) of each element contained in the steel material, and 0 is substituted when the element is not contained. do.

If the carbon equivalent Ceq is 0.30% or more, the strength and arrestability required for the steel material can be secured. Further, if the carbon equivalent Ceq is 0.55% or less, it is possible to ensure better HAZ toughness. The carbon equivalent Ceq is 0.30% or more, preferably 0.32% or more, more preferably 0.34% or more, still more preferably 0.36% or more. Also, the carbon equivalent Ceq is 0.55% or less, preferably 0.50% or less, more preferably 0.45% or less, still more preferably 0.40% or less.

In the steel material according to the present embodiment, after controlling the content of each element as described above, B _F derived from the following formulas (D1) and (D2) must be 0.0020% or less. is. _BF is the B content present as solid solution B in the steel. The reason will be explained below.

B _F ′=B−[N−{Ti−(O−Insol.Zr×(32/91.224))×(95.734/48)}×(14/47.867)]×(10.811 /14) … (D1)

_BF =B if _BF '>B, _BF = _BF ' if _{0≤BF'≤B, BF =0 if BF '} <0 (D2)

However, N, Ti, O and B in the formulas (D1) and (D2) are the contents in mass % of N, Ti, O and B contained in the steel, and Insol. Zr is the content in mass % of acid-insoluble Zr.

As described above, in the steel material according to the present embodiment, B nitride is precipitated on the surface layer of the (Zr, B)-containing oxide particles, so that B is uniformly distributed in the (Zr, B)-containing oxide particles. It can more effectively promote the formation of intragranular ferrite during cooling after welding, refine the structure, and improve the HAZ toughness, compared to nitride precipitates. In order to obtain this effect and simultaneously improve the arrestability, the content of B present as solid solution B, that is, B _F derived from the above formulas (D1) and (D2), should be 0.0020% or less. need to be More preferably, it is 0.0010% or less. When _BF exceeds 0.0020%, the hardenability of the steel material becomes excessive, causing coarsening of bainite and an excessive increase in hardness, thereby deteriorating arrestability. Therefore, the upper limit of _BF is more preferably 0.0010% or less.

Next, the microstructure of the steel material according to this embodiment will be described.
The steel material according to the present embodiment has a mixed structure of ferrite and bainite, a mixed structure of ferrite, bainite and pearlite, a mixed structure of ferrite, bainite and martensite/austenite mixed structure, or a mixed structure of ferrite, bainite, pearlite and It is a mixed structure of martensite/austenite mixed structure, and has a microstructure with a ferrite area ratio of 5 to 70% and a bainite area ratio of 30% or more.

If the ferrite area ratio exceeds 70%, it is difficult to obtain a steel material having a large plate thickness and high strength. Moreover, if the area ratio of ferrite is less than 5%, a sufficient grain boundary density cannot be ensured. Desired plate thickness, strength, and grain boundary density if a predetermined bainite, or bainite and pearlite, or bainite and martensite-austenite mixed structure, or bainite, pearlite and martensite-austenite mixed structure can be obtained of steel can be obtained. This embodiment targets thick high-strength steel, and the upper limit of the ferrite area ratio may be less than 50%, less than 30%, less than 20%, or less than 10%.

If the bainite area ratio is less than 30%, it is difficult to obtain a steel sheet having a large thickness and high strength. The upper limit of the bainite area ratio may be 95% in order to secure the ferrite area ratio and increase the grain boundaries that hinder brittle crack propagation. This embodiment targets thick high-strength steel, and the lower limit of the bainite area ratio may be limited to 50% or more, 60% or more, 70% or more, or 80% or more. The upper limit of the bainite area ratio is preferably 90% or less.

Pearlite may be contained if a steel material having a desired thickness and strength can be obtained. Therefore, the pearlite area ratio may be 15% or less, 10% or less, 5% or less, or 3% or less. The lower limit of the pearlite area ratio is 0%.

A martensite/austenite mixed structure may be present in addition to ferrite, pearlite, and bainite. 5% or less. The area ratio of the martensite-austenite mixed structure may be limited to 3% or less, 2% or less, or 1% or less, and is most preferably 0%.

The phase fraction of the microstructure is obtained by photographing the microstructure with an optical microscope at a magnification of 500 times at 1/2 part of the plate thickness, and by image analysis, the total area of ferrite, bainite, pearlite, and martensite/austenite mixed structure. and divided by the measured area.

Next, the grain boundary density in the steel material according to this embodiment will be described.

The dominant factor in the improvement of arrestability is the contribution of grain boundaries. This is because the grain boundaries hinder brittle crack propagation. That is, since the crystal orientation differs between adjacent crystal grains at the grain boundary, the direction in which the crack propagates changes at this portion. As a result, an unbroken region is generated, and the stress is dispersed by the unbroken region, resulting in crack closure stress. Therefore, the driving force for crack propagation is reduced, and arrestability is improved. In addition, since the unbroken region finally undergoes ductile fracture, the energy required for brittle fracture is absorbed. Therefore, arrestability is improved.

Conventionally, it was thought that it was necessary to reduce the grain size in order to increase the grain boundaries. This is true for ferrite-based structures, but the use of bainite is essential for thick, high-strength steels. Unlike ferrite, this bainite has a complicated substructure shape, so it is extremely difficult to define grains. For this reason, even if the relationship between the grain size and arrestability is obtained by converting the equivalent circle diameter, the variation is large, and it is difficult to determine the grain size necessary for improving the arrestability. Therefore, we return to the basic principle that grain boundaries are obstacles to crack propagation, define the total length of grain boundaries per unit area (hereafter referred to as grain boundary density), and use this to determine the relationship with arrestability. It was found that the correlation was the best when arranging

Therefore, in the steel plate according to the present embodiment,
(A) The grain boundary density at a position 1 to 5 mm from the surface is 500 to 1100 mm / mm ² ,
(B) the grain boundary density at 1/4 part of the plate thickness is 400 to 1000 mm / mm ² ,
(C) The grain boundary density at 1/2 of the plate thickness is 300 to 900 mm/mm ² .

Here, the "grain boundary density" means "total length of grain boundaries having a crystal orientation difference of 15° or more per measurement area where the crystal orientation is measured". The reason why the crystal orientation difference is set to 15° or more is that if the crystal orientation difference is less than 15°, the crystal grain boundary hardly becomes an obstacle to brittle crack propagation, and the effect of improving arrestability is reduced.

That is, when the grain boundary density satisfies the requirements of 500, 400, and 300 mm/ ^mm2 or more at 1 to 5 mm from the surface and 1/4 part and 1/2 part of the plate thickness, respectively, -10 C. C. arrest toughness value (Kca-10.degree. _C. ) is 6000 ^N.mm.sup.1.5 or higher, exhibiting high arrestability. In order to further stably improve the arrestability, the grain boundary density should be 600, 500, 400 mm/ ^mm2 or more at a position 1 to 5 mm from the surface and at 1/4 part and 1/2 part of the plate thickness, respectively. or more preferably 700, 600, 500 mm/mm ² or more.

The higher the grain boundary density, the better the crack arrestability. However, if the grain boundary density is increased excessively, the rolling load will increase and the productivity will decrease. 1100, 1000, and 900 mm/ ^mm2 or less at the position and 1/4 part and 1/2 part of the plate thickness, respectively. Each upper limit may be 1000, 900, 800 mm/mm ² or less, or 900, 800, 700 mm/mm ² or less.

The reason why the grain boundary density is specified at a position 1 to 5 mm from the surface and 1/4 part and 1/2 part of the plate thickness is that in order to improve the arrestability of extra-thick materials, the grain boundaries throughout the plate thickness It is necessary to increase the density, and by controlling the position of 1 to 5 mm from the surface and 1/4 part and 1/2 part of the plate thickness, it is possible to make it a representative value of the average grain boundary density of the plate thickness. It is from. In addition, according to the manufacturing method described later in which the grain boundary density of 1/2 part of the plate thickness is mainly controlled, the temperature at other plate thickness positions is inevitably low, the cooling rate is high, and the crystal grain Since the field density tends to increase, there is no particular need to limit the numerical value. However, depending on the heating method, a large temperature gradient may occur in the plate thickness direction, and the grain boundary density may be reversed between 1/4 and 1/2 of the plate thickness. ing.

It is preferable to use an EBSD (Electron Back Scatter Diffraction Pattern) method for measuring the crystal grain boundaries, which enables accurate measurement of crystal orientation information over a wide field of view. Using the EBSD method, it is also possible to identify grain boundaries in complex structures such as bainite.

More specifically, the grain boundary density is measured by the EBSD method at a position 1 to 5 mm from the surface, 1/4 part of the plate thickness, and a cross section perpendicular to the rolling direction of the steel plate at 1/2 part (so-called C cross section) A 500 μm × 500 μm region of is measured at a pitch of 1 μm, and a boundary with a crystal orientation difference of 15° or more with an adjacent grain is defined as a grain boundary, and the total length, which is the total length of the grain boundary at that time, It can be obtained by dividing by the measurement area (the area of the measurement region of 500 μm×500 μm described above).

Next, the (Zr, B)-containing oxide particles contained in the steel material according to this embodiment will be described.
The steel material according to the present embodiment contains (Zr, B)-containing oxide particles containing 5% by mass or more of Zr, 0.1% by mass or more of B, and 1% by mass or more of O. Of these, the number density of (Zr, B)-containing oxide particles having an equivalent circle diameter of 0.5 μm or more and an Al ₂ O ₃ composition of 50% by mass or less must be 5 to 300 particles/mm ² There is

In the steel material according to the present embodiment, (Zr, B)-containing oxide particles, which are composite inclusions, are formed by precipitation of B nitrides with Zr-containing oxides as nuclei. These composite inclusions become intragranular ferrite formation sites during cooling after welding. The Zr-containing oxide is mainly an oxide containing Zr and Ti, but when it is used as a precipitation nucleus for B nitride, the Zr concentration in the oxide is equal to or higher than the Ti concentration. preferable.

Furthermore, in the present embodiment, among the (Zr, B)-containing oxide particles, (Zr, B) containing 5% by mass or more of Zr, 0.1% by mass or more of B, and 1% by mass or more of O It is intended for oxide particles. The (Zr, B)-containing oxide particles having such a composition can function as intragranular ferrite formation sites, and can form more intragranular ferrite. Oxide particles having a Zr, B or O content outside the preferred range cannot sufficiently function as sites for the formation of intragranular ferrite. In this embodiment, the amount of Ti in the (Zr, B)-containing oxide particles does not need to be specified, but Ti may be contained in an amount of 1% by mass or more.

Furthermore, in the present embodiment, among the (Zr, B)-containing oxide particles, the number density is specified for those having an Al ₂ O ₃ composition of 50% by mass or less. When the composition of Al ₂ O ₃ in the (Zr, B)-containing oxide particles is 50% by mass or less, it can function more effectively as a site for generating intragranular ferrite, and a large amount of intragranular ferrite is formed. can be made

In addition, when the equivalent circle diameter of the (Zr, B)-containing oxide particles [the diameter of a circle having the same area as the observed cross-sectional area of the (Zr, B)-containing oxide particles] is 0.5 μm or more, An effect of precipitating a large amount of intragranular ferrite can be obtained. In order for the (Zr, B)-containing oxide particles to function as intragranular ferrite formation sites, the equivalent circle diameter is preferably as large as possible, so there is no upper limit. However, when the equivalent circle diameter increases, the number density of the (Zr, B)-containing oxide particles decreases, and the possibility that the coarse oxide particles themselves act as starting points for fracture increases. Therefore, the equivalent circle diameter of the (Zr, B)-containing oxide particles is preferably 10.0 μm or less.

Moreover, as a condition for acting as intragranular ferrite generation sites, it is preferable that one or more (Zr, B)-containing oxide particles are dispersed in the austenite grains when heated during welding. Therefore, the equivalent circle diameter is 0.5 μm or more, contains 5% by mass or more of Zr, 0.1% by mass or more of B, and 1% by mass or more of O, and the composition of Al ₂ O ₃ is 50% by mass or less of (Zr, B)-containing oxide particles are dispersed at a number density of 5 particles/mm ² or more. The higher the number density of such (Zr, B)-containing oxide particles, the ^more the number of ferrite formation sites increases. 300 pieces/mm ² or less. In particular, the (Zr, B)-containing oxide particles having an Al ₂ O ₃ composition of 50% by mass or less according to the present embodiment have a high ability to form intragranular ferrite. Therefore, the number density of the (Zr, B)-containing oxide particles according to the present embodiment is lower than that of the (Zr, B)-containing oxide particles in which the Al ₂ O ₃ composition exceeds 50% by mass. However, it can be effective enough.

The circle-equivalent diameter and number density of (Zr, B)-containing oxide particles can be measured by observing a mirror-polished steel surface with a scanning electron microscope (SEM). Specifically, the number of (Zr, B)-containing oxide particles with an equivalent circle diameter of 0.5 μm or more is measured by SEM in a range of 10 mm × 10 mm (100 mm ² ), and divided by the area of the observed field of view. and measure the number density. A photograph taken by SEM may be used. The particles whose number density is to be measured have an equivalent circle diameter of 0.5 μm or more, and are quantitatively analyzed by an energy dispersive X-ray spectrometer (EDX) attached to the SEM, and are found to contain 5% by mass or more of Zr and 0.1% by mass. Particles containing not less than 1% by mass of B and not less than 1% by mass of O and having an Al ₂ O ₃ composition of not more than 50% by mass are confirmed.

The plate thickness of the steel material of this embodiment is not particularly limited, but is preferably in the range of 50 to 100 mm.

Further, the tensile strength TS of the steel material of this embodiment is preferably in the range of 510-720 MPa, and the yield stress YP is preferably in the range of 390-650 MPa. Evaluation of tensile strength TS and yield stress YP is performed according to JIS Z 2241:2011. The test piece shall be No. 1B test piece. The test method shall be the elongation set method.

The steel material of the present embodiment exhibits excellent toughness in the weld heat affected zone when welded under the condition that the welding heat input is 35 kJ/mm or more. In particular, the Charpy absorption energy at -40°C can be improved.

More specifically, a simulated heat cycle test simulating high heat input welding is applied to samples taken from the steel material of the present embodiment, assuming application of electrogas welding. As a specific reproducible thermal cycle condition, one-pass electrogas welding of a plate with a thickness of 50 mm is simulated, heated from room temperature to 1400 ° C., held at 1400 ° C. The temperature range from 800° C. to 500° C., which is the temperature range related to internal transformation, is controlled at a rate of 1.0° C./second for cooling. After subjecting the thick plate steel to a thermal cycle, it is processed into V-notch test pieces, and three pieces of each steel are subjected to a Charpy impact test at a test temperature of −40° C. to measure the absorbed energy. When the average absorbed energy of the three test pieces is 100 J or more and the minimum absorbed energy among the three test pieces is 50 J or more, it can be said that the toughness of the weld heat affected zone is excellent. The V-notch test piece may be prepared according to the V-notch test piece described in JIS Z 2242:2005. Also, the Charpy impact test should be performed according to JIS Z 2242:2005.

In addition, the steel material of the present embodiment has excellent arrestability. In particular, the arrest toughness value Kca at -10°C can be increased. In this embodiment, the arrest toughness value Kca at _{-10 ° C.} is 6000 N / mm ^1.5 or more, the non-ductile transition temperature (NDT temperature) is -60 ° C. or less, and the fracture surface transition temperature (vTrs) is -60 ° C. It can be said that the arrestability is excellent when all of the following conditions are satisfied.

The arrest toughness value Kca _-10°C is evaluated according to NK Classification Society Steel Ship Rules and Inspections, Part K, Appendix K3.12.2-1. (2018) in accordance with the "Inspection Guidelines for Test Method for Brittle Crack Arrest Toughness Value Kca". The arrest toughness value Kca at -10°C is determined by the test.

In addition, the evaluation of the non-ductile transition temperature (NDT temperature; Nil-Ductility-Transition Temperature) is determined by conducting a test in accordance with the NRL (Naval Research Laboratory) drop weight test method specified in ASTM E208-06. . The test piece is P-3 type (T: 16 mm, L: 130 mm, W: 50 mm), and is sampled up to a position of 16 mm in the plate thickness direction so as to include the outermost surface of the steel plate. A test piece is taken in the rolling direction (L direction), a weld bead is provided on the outermost surface of the test piece in the L direction, and a notch is provided in a direction perpendicular to the rolling direction (C direction) as a crack starter.

Furthermore, the fracture surface transition temperature (vTrs) was evaluated in accordance with JIS Z 2242: 2005, the test piece was a V notch test piece, and the test piece sampling position included t / 4 parts of the plate thickness t of the steel material. Collect.

Next, a method for manufacturing the steel material of this embodiment will be described.
In the steel manufacturing method of the present embodiment, the molten steel is vacuum degassed, Zr is added after the dissolved oxygen concentration of the molten steel is 0.0050% or less, and the Zr addition is 1.0 to 5.0%. The average cooling rate from 1200 ° C to 900 ° C in the refining process in which B is added after the elapse of minutes and the continuous casting of the molten steel after the refining process to form a slab, A continuous casting process with a temperature of 0.5° C./sec or less and a hot rolling process of hot rolling the slab after continuous casting are sequentially performed.

In the present embodiment, molten steel is tapped from a steelmaking furnace into a ladle and then depressurized by a vacuum degassing device. After the steel is tapped into the ladle, the composition may be adjusted by adding an alloy or the like while the steel is conveyed to the vacuum degassing device.

In the refining process, degassing is performed in a vacuum degassing device, and after adjusting molten steel components other than Zr and B, Zr is added. It is desirable to control dissolved oxygen in the molten steel to 0.0050% or less before adding Zr. If Zr is added before the dissolved oxygen reaches 0.0050% or less, it becomes difficult to refine the (Zr, B)-containing oxide particles, and Al ₂ O ₃ of the (Zr, B)-containing oxide particles It may become impossible to control the composition to 50% by mass or less.

Then B is added 1.0 to 5.0 minutes after the addition of Zr. As a result, B is segregated around the Zr-containing oxide, B nitrides are contained in the Zr-containing oxide, and B nitrides are precipitated on the surface layer of the (Zr, B)-containing oxide particles. be able to. If the B addition timing is less than 1.0 minute or more than 5.0 minutes after the addition of Zr, the desired (Zr, B)-containing oxide particles cannot be obtained.

The molten steel after the refining process is used as a slab in the continuous casting process. In the continuous casting process, the average cooling rate for the surface temperature of the slab from 1200°C to 900°C is set to 0.5°C/sec or less. As a result, separation of ZrO ₂ and Al ₂ O ₃ in the Zr-containing oxide proceeds, and the Al ₂ O ₃ composition of the (Zr, B)-containing oxide particles can be reduced to 50% by mass or less.

A cast slab obtained by the continuous casting process is heated to 950 to 1150° C. in a heating process, and then hot-rolled in a hot-rolling process to obtain a steel material. The conditions of the heating process and the hot rolling process are as follows. In the hot rolling process, it is preferable to set the rolling conditions so that the plate thickness of the steel material is in the range of 50 to 100 mm.

A heating process is a process which contributes to structure control of an austenite phase by heating a cast piece. In the heating step, the cast slab after continuous casting is heated to 950 to 1150°C. The heating step is preferably performed in a heating furnace. Heating the slab after continuous casting to 950 to 1150 ° C. means heating the slab in the heating furnace so that the maximum surface temperature of the slab in the furnace is in the range of 950 to 1150 ° C. In this specification, the maximum surface temperature of the slab is referred to as the heating temperature of the slab. If the heating temperature is less than 950° C., the austenitization becomes insufficient and the austenite grains become finer, which reduces the hardenability. On the other hand, if the heating temperature exceeds 1150° C., the austenite grains become coarse and there is time to wait for the temperature to drop before the start of rolling, resulting in low productivity. A preferable heating temperature range is 1000 to 1100°C.

In the hot rolling process, a rough rolling process, a finish rolling process, and a cooling process are sequentially performed.

In the rough rolling step, the slab heated in the heating step is rolled at a recrystallization temperature Trex (° C.) or more shown in the following formula (H) and 1050° C. or less, and the cumulative reduction rate (rough rolling) is 10 to 75%. It is a step of rolling as a range of. Here, rolling the slab heated in the heating step at a rolling temperature of 1050° C. or higher than the recrystallization temperature Trex (° C.) shown in the following formula (H) means that the surface temperature of the slab heated in the heating step is , Rough rolling is started at a recrystallization temperature Trex (°C) or higher and 1050°C or lower, and the surface temperature of the steel material at the end of rough rolling is set to Trex (°C) or higher and 1050°C or lower. Rolling with a cumulative rolling reduction (rough rolling) in the range of 10 to 75% means that the thickness of the cast slab heated in the heating step minus the plate thickness after rough rolling is the cast heated in the heating step. Rolling is carried out so that the cumulative rolling reduction ratio (rough rolling) divided by the plate thickness of the piece is in the range of 10 to 75%. If the rolling temperature of the rough rolling exceeds 1050°C, the recrystallized austenite grains cannot be made fine even in the subsequent finish rolling. Further, when the temperature of rough rolling is lower than the recrystallization temperature Trex (°C), the productivity decreases. A preferred rolling temperature is 900-1000°C.

The surface temperature of the steel material at the end of rough rolling may be higher than the surface temperature of the steel material at the start of rough rolling. This is thought to be due to the effect of heat generated during rough rolling in the steel material, and the effect of heat transfer in the thickness direction of the steel material due to the fact that the internal temperature of the steel material is higher than the surface temperature of the steel material.

Trex=−91900 [Nb*] ² +9400 [Nb*]+770 (H)
[Sol. Nb]=(10 ^{(−6770/(T+273)+2.26)} )/(C+12/14×N) (I)

However, [Nb*] in formula (H) is represented by formula (I) [Sol. Nb] and the Nb content (% by mass) in the steel, Nb≧[Sol. Nb], [Nb*]=[Sol. Nb] and Nb<[Sol. Nb], [Nb*]=Nb. C and N in the formula (I) are the contents (% by mass) of C and N contained in the steel. T in the formula (I) is the maximum temperature (° C.) of the cast slab surface in the heating furnace in the heating step.

In addition, if the cumulative rolling reduction during rough rolling is less than 10%, it is difficult to refine the steel by recrystallization of austenite, and porosity remains, which may cause internal cracks and deterioration of ductility and toughness. Moreover, when the cumulative rolling reduction exceeds 75%, the number of passes increases and the productivity decreases. A preferred cumulative rolling reduction is 30 to 60%.

Next, finish rolling is performed on the steel material after the rough rolling. In the finish rolling process, the steel material rolled in the rough rolling process is heated to (Ar ₃ -50) ° C. or higher (where Ar ₃ is represented by the following formula (J)), and the recrystallization temperature Trex shown in the above formula (H) (° C.) at a rolling temperature of less than 45 to 75%. Here, the steel material after rough rolling is (Ar ₃ −50) ° C. or higher (where Ar ₃ is represented by the following formula (J)) and less than the recrystallization temperature Trex (° C.) shown in the above formula (H) Rolling as means that finish rolling is started with the surface temperature of the steel material after rough rolling set to (Ar ₃ -50) ° C. or more and less than the recrystallization temperature Trex (° C.), and the surface temperature of the steel material when finish rolling is completed. is (Ar ₃ −50)° C. or more and less than the recrystallization temperature Trex (° C.). In addition, rolling with a cumulative reduction rate (finish rolling) in the range of 45 to 75% means that the thickness of the steel material rolled by rough rolling minus the thickness after finish rolling is the thickness of the steel material rolled by rough rolling. Rolling is carried out so that the cumulative rolling reduction ratio (finish rolling) divided by the plate thickness is in the range of 45 to 75%.

If the finish rolling temperature is equal to or higher than the recrystallization temperature Trex (° C.), the steel does not sufficiently enter the non-recrystallized region, suppressing an increase in dislocations and making it impossible to obtain a predetermined grain boundary density. If the finish rolling temperature is lower than (Ar ₃ −50)° C., the productivity is lowered, and it is difficult to set the grain boundary density within the desired range because the steel contains a part of deformed ferrite. A preferred finish rolling temperature is 760 to 840°C.

The surface temperature of the steel material at the end of finish rolling may be higher than the surface temperature of the steel material at the start of finish rolling. This is thought to be due to the effect of work heat generation in the steel material due to finish rolling and the effect of heat transfer in the thickness direction of the steel material due to the internal temperature of the steel material being higher than the surface temperature of the steel material.

Ar ₃ (° C.)=910-310C+65Si-80Mn-20Cu-55Ni-15Cr-80Mo (J)

The symbol of element in the formula (J) is the content (% by mass) of each element contained in the steel, and 0 is substituted when the element is not contained.

If the cumulative rolling reduction during finish rolling is less than 45%, it is difficult to obtain the specified grain boundary density due to the accumulation of dislocations, and if it exceeds 75%, the productivity decreases. A preferable range of cumulative rolling reduction is 50 to 70%.

Next, the steel material after finish rolling is cooled. In the cooling step, the cooling start temperature is (Ar ₃ -100) ° C. or higher (where Ar ₃ is represented by the above formula (J)), and the range is below the recrystallization temperature Trex (° C.) shown in the above formula (H). The cooling stop temperature is in the range of 0° C. or more and 600° C. or less, and the average cooling rate from the start of cooling to the end of cooling is 2 to 15° C./sec. The cooling start temperature, cooling stop temperature, and average cooling rate are the temperatures at the 1/4 position in the thickness direction of the steel material. By setting the conditions of the cooling process within the above range, the transformation of the microstructure by quenching is promoted, and the desired microstructure is obtained, thereby increasing the tensile strength TS and the yield stress YP and improving the arrestability.

In the steel manufacturing method of the present embodiment, a tempering step of heating to a temperature in the range of 350 to 650° C. may be performed after the cooling step. By performing the tempering process, it is possible to reduce the dislocation density excessively increased by rolling.

Next, examples of the present invention will be described. The conditions in the examples are one example of conditions adopted for confirming the feasibility and effect of the present invention, and the present invention is based on this one example of conditions. It is not limited. Various conditions can be adopted in the present invention as long as the object of the present invention is achieved without departing from the gist of the present invention.

Hot metal tapped from a blast furnace was desulfurized by hot metal pretreatment, dephosphorized and decarburized in a converter-type refining vessel, and then received in a ladle. At the time of tapping, alloying elements were added, and cover slag for heat retention was added.

In the refining process, the molten steel in the ladle was decompressed by the RH vacuum degassing device. During the melting process, molten steel samples were taken as needed and subjected to analysis to obtain molten steel components. The molten steel temperature changed from 1560°C to 1610°C. In the first half of the RH treatment, an alloy other than Zr and B was added to adjust the components, and vacuum degassing was performed to adjust the dissolved oxygen concentration. Dissolved oxygen concentration was measured using an oxygen concentration probe. After that, Zr was added, and after 0.8 to 5.3 minutes, B was added. Then, a reflux treatment was carried out for uniform mixing. For No. 51, B was added 2.2 minutes before Zr was added. Therefore, in Table 2C, No. The addition time difference between Zr and B in No. 51 was described as "-2.2".

After treatment with the RH vacuum degassing device, the average cooling rate for the surface temperature of the slab from 1200 ° C. to 900 ° C. in continuous casting is 0.1 to 0.7 ° C./sec. and A slab having a thickness of 251 to 372 mm was obtained as a semi-finished product. After that, it was processed to a thickness of 50 to 100 mm by a hot rolling process to produce a thick steel plate.

Tables 1A to 1D show steel material components and carbon equivalents. Tables 2A to 2C show the dissolved oxygen concentration at the time of Zr addition, the time from Zr addition to B addition, and the average cooling rate for the surface temperature of the slab during continuous casting from 1200°C to 900°C. Tables 2A to 2C show the conditions of the heating process and hot rolling process. In Tables 2D-2F, Insol. Zr amount, Sol. Zr content, _BF content, number density of (Zr, B)-containing oxide particles, microstructure evaluation results, grain boundary density evaluation results, tensile strength TS, yield stress YP, Charpy absorbed energy, and -10°C 2 shows the arrest toughness value Kca, NDT temperature and vTrs at . The underlined parts in Tables 1C, 1D, 2B, 2C, 2E, and 2F are outside the scope of the present invention.

The circle-equivalent diameter and number density of the (Zr, B)-containing oxide particles were measured by observing the surface of the mirror-polished steel material with a scanning electron microscope (SEM). Specifically, the number of (Zr, B)-containing oxide particles with an equivalent circle diameter of 0.5 μm or more is measured by SEM in a range of 10 mm × 10 mm (100 mm ² ), and divided by the area of the observed field of view. Then, the number density was measured. The particles whose number density is to be measured have an equivalent circle diameter of 0.5 μm or more, and are quantitatively analyzed by an energy dispersive X-ray spectrometer (EDX) attached to the SEM, and are found to contain 5% by mass or more of Zr and 0.1% by mass. It was confirmed that the particles contained B of 1% by mass or more and O of 1% by mass or more, and the composition of Al ₂ O ₃ was 50% by mass or less.

Insol. Zr and Sol. Zr was measured by electrolytic extraction residue analysis method. In the electrolytic extraction residue analysis method, steel is electrolyzed in a non-aqueous solvent (acetylacetone-methanol solution) to dissolve the mother phase, and the residue (precipitates and inclusions) is extracted and separated using a filter with a pore size of 0.2 μm. . After separation, the amount of Zr contained in the solution was determined by Sol. Zr content, and the amount of Zr contained in the residue is Insol. Zr content.

The amount of _BF was determined by the above formulas (C1) and (C2).

Next, a test piece for a thermal cycle test was taken from the steel material. A heat cycle that reproduces welding with a heat input of 35 kJ/mm (high heat input welding) was applied to this test piece. As a specific heat cycle condition, after heating from room temperature to 1400° C., holding at 1400° C. for 5 seconds, after that, the temperature range from 800° C. to 500° C., which is the temperature range related to intragranular transformation, is set to 1.0. Cooling was controlled at a rate of °C/sec.

Three V-notch test pieces were taken from the steel material after the heat cycle was applied, and a Charpy impact test was performed at -40°C to measure the absorbed energy (vE _-40 ). The V-notch test piece was prepared according to the V-notch test piece described in JIS Z 2242:2005. Moreover, the Charpy impact test was performed based on JISZ2242:2005.
When the average value of the absorbed energy (vE _-40 ) of the three test pieces was 100J or more and the minimum value of the absorbed energy (vE _-40 ) of the three test pieces was 50J or more, it was judged as passing.

Evaluation of arrestability is based on NK Classification Society Steel Ship Rules and Surveys, Part K, Annex K3.12.2-1. (2018) in accordance with "Inspection Guidelines for Test Method for Brittle Crack Arrest Toughness Value Kca". The arrest toughness value Kca at -10°C was determined by the test.

Also, as an evaluation of arrestability, a non-ductile transition temperature (NDT temperature; Nil-Ductility-Transition Temperature) was obtained. The NDT temperature was obtained by conducting a test according to the NRL (Naval Research Laboratory) drop weight test method specified in ASTM E208-06. The test piece was P-3 type (T: 16 mm, L: 130 mm, W: 50 mm), and was sampled up to a position of 16 mm in the plate thickness direction so as to include the outermost surface of the steel plate. A test piece was taken in the rolling direction (L direction), a weld bead was provided on the outermost surface of the test piece in the L direction, and a notch was provided in the direction perpendicular to the rolling direction (C direction) as a crack starter.

Furthermore, fracture surface transition temperature (vTrs) was determined as evaluation of arrestability. The evaluation of vTrs conforms to JIS Z 2242:2005, the test piece is a V-notch test piece, and the test piece sampling position includes t/4 part of the plate thickness t of the steel material.

Crest toughness value Kca at _-10°C is 6000 N/mm ^1.5 or higher, non-ductile transition temperature (NDT temperature) is -60°C or lower, and fracture surface transition temperature (vTrs) is -60°C or lower. was passed.

Evaluation of tensile strength TS and yield stress YP was performed according to JIS Z 2241:2011. No. 1B test piece was used as the test piece. The test method was the permanent elongation method. Those having a tensile strength TS of 510 to 720 MPa and a yield stress YP of 390 to 650 MPa were judged to pass.

As shown in Tables 1A to 1D, Tables 2A and 2D, no. All of Nos. 1 to 25 had excellent toughness and arrestability, and also excellent mechanical properties.

On the other hand, as shown in Table 1C, Table 1D, Table 2B, Table 2C, Table 2E and Table 2F, Comparative Example No. In Nos. 26-37 and 40-47, the chemical compositions were out of the range defined by the present invention, so any of the toughness, arrestability or mechanical properties deteriorated.

Also, No. 38, 39, and 48 to 67 satisfied the chemical composition range of the present invention, but the manufacturing conditions did not satisfy the conditions of the present invention. Therefore, No. 38 is Sol. Zr does not satisfy the present invention, and No. 39 is Insol. Zr and _BF did not satisfy the present invention. No. In Nos. 48 to 52, the number density of (Zr, B)-containing oxide particles did not satisfy the range of the present invention. No. Nos. 53, 55 to 59, 62, 63 and 66 did not satisfy the range of the present invention in grain boundary density. No. Nos. 54, 60, 61, 64, 65 and 67 did not satisfy the scope of the present invention in microstructure. As a result, either toughness, arrestability or mechanical properties deteriorated.

Claims (5)

in % by mass,
C: 0.040 to 0.160%,
Si: 0.01 to 0.50%,
Mn: 0.70-2.50%,
P: 0.030% or less,
S: 0.008% or less,
Al: 0.010% or less,
N: 0.0010 to 0.0080%,
O: 0.0005 to 0.0040%,
Nb: 0.003 to 0.050%,
Ti: 0.003-0.024%,
Zr: 0.0007 to 0.0050%,
B: 0.0003 to 0.0040%,
total content of Ca, Mg and REM: 0.0005% or less;
The balance consists of Fe and impurity elements,
Insol. Zr: 0.0007 to 0.0040%,
Sol. Zr: 0.0010% or less,
B _F represented by the following formulas (1) and (2) is 0.0020% or less,
The carbon equivalent Ceq represented by the following formula (3) is 0.30% to 0.55%,
It contains ferrite with an area ratio of 5 to 70%, bainite with an area ratio of 30% or more, pearlite with an area ratio of 0 to 15%, and martensite-austenite mixed structure with an area ratio of 0 to 5%. having a microstructure,
The grain boundary density at a position 1 to 5 mm from the plate surface is 500 to 1100 mm / mm ² ,
The grain boundary density at the position of 1/4 part of the plate thickness is 400 to 1000 mm / mm ² ,
The grain boundary density at the position of 1/2 part of the plate thickness is 300 to 900 mm / mm ² ,
Of the (Zr, B)-containing oxide particles containing 5% by mass or more of Zr, 0.1% by mass or more of B, and 1% by mass or more of O, the equivalent circle diameter is 0.5 μm or more (Zr, B) A steel material in which the number density of (Zr, B)-containing oxide particles having an Al ₂ O ₃ composition of 50% by mass or less is 5 to 300/mm ² .
B _F ′=B−[N−{Ti−(O−Insol.Zr×(32/91.224))×(95.734/48)}×(14/47.867)]×(10.811 /14) …(1)
_BF =B if _BF '>B, _{BF=BF' if 0≦BF'≦B, BF = 0 if BF} '<0 (2)
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15 (3)
However, N, Ti, O and B in the formulas (1) and (2) are the contents in mass % of N, Ti, O and B contained in the steel, and Insol. Zr is the content in mass % of acid-insoluble Zr.
Further, C, Mn, Cr, Mo, V, Cu, and Ni in the formula (3) are the content (% by mass) of each element contained in the steel, and 0 is set when the element is not contained. substitute. Furthermore, in mass %,
Cu: 1.00% or less,
Ni: 2.50% or less,
Cr: 1.00% or less,
Mo: 0.50% or less,
The steel material according to claim 1, characterized by containing one or more selected from the group consisting of V: 0.150% or less. Furthermore, in mass %,
W: 1.00% or less,
The steel material according to claim 1 or 2, characterized by containing one or two selected from the group consisting of Sn: 0.50% or less. A method for manufacturing a steel material according to any one of claims 1 to 3,
A refining process in which vacuum degassing is performed on the molten steel, Zr is added after the dissolved oxygen concentration of the molten steel is 0.0050% or less, and B is added 1.0 to 5.0 minutes after the addition of Zr. When,
When the molten steel after the refining process is continuously cast to form a slab, the average cooling rate for the surface temperature of the slab from 1200° C. to 900° C. is 0.5° C./second or less. a continuous casting process;
A heating step of heating the slab after the continuous casting to 950 to 1150 ° C.;
a hot rolling step of hot-rolling the heated slab into a steel material,
The hot rolling step is a step of sequentially performing a rough rolling step, a finish rolling step, and a cooling step,
In the rough rolling step, the cast slab heated in the heating step is rolled at a rolling temperature of not less than the recrystallization temperature Trex (° C.) shown in the following formula (4) and not more than 1050° C. at a cumulative reduction rate of 10 to 75%. is a process to
In the finish rolling step, the finish rolling temperature is (Ar ₃ -50) ° C. or higher (where Ar ₃ is represented by the following formula (5)) and is lower than the recrystallization temperature Trex (° C.) shown in the following formula (4). It is a step of rolling under the condition of a cumulative reduction rate of 45 to 75%,
In the cooling step, the cooling start temperature is (Ar ₃ -100) ° C. or higher (where Ar ₃ is represented by the following formula (5)) and is lower than the recrystallization temperature Trex (° C.) shown in the following formula (4). range, the cooling stop temperature is in the range of 0 ° C. or higher and 600 ° C. or lower, and the average cooling rate from the start of cooling to the end of cooling is 2 to 15 ° C./sec. , steel manufacturing method.
Trex=−91900 [Nb*] ² +9400 [Nb*]+770 (4)
Ar ₃ (° C.)=910-310C+65Si-80Mn-20Cu-55Ni-15Cr-80Mo (5)
[Sol. Nb]=(10 ^{(−6770/(T+273)+2.26)} )/(C+12/14×N) (6)
However, [Nb*] in formula (4) is equivalent to [Sol. Nb] and the Nb content (% by mass) in the steel, Nb≧[Sol. Nb], [Nb*]=[Sol. Nb] and Nb<[Sol. Nb], let [Nb*]=Nb,
The element symbols in formulas (5) to (6) are the content (% by mass) of each element contained in the steel, and 0 is substituted when the element is not contained,
T in the formula (6) is the temperature (° C.) of the slab at the time of the slab extraction in the heating step. 5. The method of manufacturing a steel material according to claim 4, wherein a tempering step of heating to a temperature in the range of 350 to 650° C. is performed after the cooling step.