JP7364906B2 - Steel materials and their manufacturing methods - Google Patents

Description

The present invention relates to a steel material and a method for manufacturing the same.

Applications 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. BACKGROUND ART In recent years, welded structures have become larger due to the increase in the carrying weight of container ships. Along with this, steel materials are required to have thicker plates and higher strength. In addition, in the above-mentioned welded structures, it is necessary to ensure even greater safety and reliability for the welded parts, and the toughness of the weld heat affected zone (hereinafter sometimes referred to as "HAZ toughness"). Improving this has become an issue.

Furthermore, welding construction costs account for a large portion of the overall construction costs of welded structures, and in order to reduce this cost, highly efficient welding is required. Specifically, it is effective to perform welding with a large heat input and reduce the number of welding passes. However, when welding with a large heat input is performed, the HAZ structure of the steel material generally becomes coarse, and deterioration of toughness is unavoidable.

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

To refine the HAZ structure using inclusions, there is a method of suppressing the growth of crystal grains by the pinning effect of inclusions, and a method of adding ferrite using inclusions as nuclei within austenite grains that have become coarsened due to the heat effect during welding. There is a method to refine the structure by generating (intragranular transformation). Regarding microstructure refinement by intragranular transformation, techniques have been proposed so far that utilize nitrides such as TiN, sulfides such as MnS, or oxides that are chemically stable even at high temperatures as ferrite generation sites.

Patent Document 1 proposes a method of improving HAZ toughness using inclusions containing REM and Zr.

Patent Document 2 states that 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 plate having an S content of more than 0% and less than 20% is described.

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

Patent Document 4 states that out of all the inclusions contained in the steel material, including oxides containing Zr, REM, and Ca, the inclusions with a circular equivalent diameter of 0.1 to 2 μm are 120 inclusions per 1 mm ² of observation field area. A steel material in which the number of oxides with a circular equivalent diameter of more than 3 μm is 5.0 or less per ^mm2 of observation field area, and the composition of inclusions in the steel material satisfies the relationship of formula (1) below. is listed.
(Insol.Ti-3.4×Insol.N)/Insol.Al=1.0~8...(1)

Patent Document 5 describes an inclusion that satisfies the average composition of ZrO ₂ : 5 to 50%, REM oxide: 5 to 50%, CaO: 50% or less, and has an equivalent circle diameter of 0.1 to 50%. 120 or more inclusions of 2 μm per 1 mm ² of observation field area, oxides with circle equivalent diameter of more than 3 μm per 1 mm ² of observation field area, 5.0 or less inclusions per 1 mm of circle equivalent diameter of oxides of more than 5 μm of circle equivalent diameter of observation field of view area REM and Zr-containing inclusions I with a number of 5.0 or less per 1 mm ² and a molar ratio of REM and Zr (REM/Zr) of 0.6 to 1.4 with respect to the total number of inclusions. 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 with respect to the number of all inclusions [(REM+Zr)/( A steel material is described in which the number ratio of inclusions II containing REM, Zr, Al, Ca, and Ti is 40% or more.

In addition, steel materials used in structures such as ships, high-rise buildings, other buildings, bridges, offshore structures, LNG storage tanks and other large tanks, and line pipes are designed to suppress brittle fracture of structures. In addition, arrestability (brittle crack propagation stopping function), which is the ability to suppress the propagation of brittle crack fractures, is also required.

Japanese Patent Application Publication No. 2008-291347 Japanese Patent Application Publication No. 2014-214371 Japanese Patent Application Publication No. 2014-185364 JP 2014-1432 Publication Japanese Patent Application Publication No. 2012-162797

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

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

(A) Sol. in steel. The HAZ toughness tends to improve as the Zr content decreases, and the content 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 electrolytic extraction residue analysis.

(B) Due to the inclusion of Zr and B, B nitrides are precipitated in steel with Zr-containing oxides as nuclei. The (Zr, B)-containing oxide particles in which such B nitrides are precipitated function more effectively as intragranular ferrite generation sites. To obtain 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, thereby increasing the amount of B _F dissolved in the steel. The content is preferably 0.0030% 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 generation sites. do. In order to obtain this effect, it is preferable to add Zr after the dissolved oxygen concentration of the molten steel becomes 0.0050% or less in the refining process, and then perform continuous casting.

(D) Among the (Zr, B)-containing oxide particles, the (Zr, B)-containing oxide particles have 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 pieces/mm ² , fine and large amounts of intragranular ferrite are generated in the HAZ, improving HAZ toughness.

(E) When steel contains an excessive amount of Al, which acts as a strong deoxidizing element, the formation of Zr-containing oxides is inhibited. In order to ensure the amount of dissolved oxygen in the molten steel and to generate Zr-containing oxides in the steel, the Al content is preferably 0.010% by mass or less. Further, it is preferable to limit the total amount 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 discovered that by controlling the microstructure and the texture in the plate thickness direction, we can improve the It was discovered that directional arrestability can be improved.

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

[1] In mass%,
C: 0.040-0.160%,
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 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 remainder 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%,
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%. has a microstructure,
At a position 1 to 5 mm from the plate surface, the area ratio of {110} planes forming an angle of 15° or less with respect to a vertical plane perpendicular to the main rolling direction is 30 to 60%,
In 1/4 part of the plate thickness, the area ratio of the {100} plane that forms an angle of 15° or less with respect to the vertical plane that is perpendicular to the main rolling direction is 10 to 40%,
In 1/2 part of the plate thickness, the area ratio of the {110} plane that forms an angle of 15° or less with respect to the vertical plane that is perpendicular to the main rolling direction is 40 to 70%,
Among (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 pieces/mm ² .
B _F '=B-[N-{Ti-(O-Insol.Zr×(32/91.224))×(95.734/48)}×(14/47.867)]×(10.811 /14) …(1)
When B _F '>B, B _F = B; when 0≦B _F '≦B, B _F = B F '; when B _{F '} <0, B _F =0...(2)
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15...(3)
However, N, Ti, O, and B in 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.
In addition, C, Mn, Cr, Mo, V, Cu, and Ni in formula (3) are the contents (mass%) of each element contained in the steel, and are set to 0 if the element is not contained. substitute.
[2] 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 [1] above, containing one or more selected from the group consisting of V: 0.150% or less.
[3] Furthermore, in mass%,
W: 1.00% or less,
The steel material according to [1] or [2] above, characterized in that it contains one or two selected from the group consisting of Sn: 0.50% or less.
[4] The method for manufacturing a steel material according to any one of [1] to [3] above, in which molten steel is vacuum degassed and the dissolved oxygen concentration of the molten steel is 0.0050%. a refining step in which Zr is added after the Zr is added, and B is added after 1.0 to 5.0 minutes have elapsed from the Zr addition;
When continuous casting is performed on the molten steel after the refining process to produce a slab, the average cooling rate until the surface temperature of the slab reaches from 1200°C to 900°C is 0.5°C/second or less. continuous casting process;
a heating step of heating the slab after the continuous casting to 950 to 1200°C;
a hot rolling step of hot rolling the heated slab into a steel material,
The hot rolling process is a process of sequentially performing a rough rolling process, a primary cooling process, a finish rolling process, and a secondary cooling process,
In the rough rolling step, the slab heated in the heating step is rolled at a rolling temperature of not less than a recrystallization temperature Trex (° C.) and not more than 1050° C., as shown in the following formula (4), at a cumulative reduction rate of 10 to 75%. It is a process of
In the primary cooling step, the cooling start temperature is in the range of Ar ₃ °C or more and 1050 °C or less as shown in the following formula (5), and the cooling stop temperature is in the range of 500 °C or more and (Ar ₃ -30) °C or less (however, _Ar3 is a range expressed by the following formula (5)), and the average cooling rate from the start of cooling to the stop of cooling is a process of cooling under conditions of 35 to 100 ° C / sec,
In the finish rolling process, rolling is performed at a finish rolling temperature of 750 to 850°C, a rolling pass number of 4 to 15, an average rolling shape ratio of 0.5 to 1.0, and a cumulative reduction rate of 45 to 75%. It is a process,
In the secondary cooling step, the cooling start temperature is set to (Ar ₃ -100)°C or higher (Ar ₃ is represented by the following formula (5)), and the recrystallization temperature Trex (°C) shown in the following formula (4) is set. 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. A method of manufacturing steel materials.
Trex=-91900[Nb*] ² +9400[Nb*]+770...(4)
Ar ₃ (℃)=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 [Sol. Nb] and the Nb content (mass%) in the steel is Nb≧[Sol. Nb], then [Nb*]=[Sol. Nb] and Nb<[Sol. Nb], set [Nb*]=Nb,
The element symbols in formulas (5) to (6) are the content (mass%) of each element contained in the steel, and if the element is not contained, 0 is substituted,
T in formula (6) is the temperature (° C.) of the slab at the time of extracting the slab in the heating step.
[5] The method for producing a steel material according to [4] above, characterized in that, after the secondary cooling step, a tempering step of heating to a temperature in the range of 350 to 650° C. is performed.

According to the present invention, it is possible to provide a steel material having excellent HAZ toughness, particularly in the HAZ of high heat input welding with a heat input of 35 kJ/mm or more, and further excellent arrestability, and a method for manufacturing the same.

It is known that Ti oxides and B nitrides are dispersed in weld metals and HAZs and have the effect of refining their structures. On the other hand, Zr is not an element generally added to steel materials, and research conducted in the past on the effects of adding Zr has been extremely limited.
In particular, there has been no study to date on how B nitride, which is further precipitated in a composite manner with the Zr-containing oxide, affects the refinement of the HAZ structure and the improvement of HAZ toughness of steel materials. Further, the relationship between the composition of Zr-containing oxide and B nitride has not been studied.

The present inventors focused on Zr-containing oxides and B nitrides as intragranular ferrite generation sites for refining the HAZ structure, and conducted extensive studies. As a result, we mainly obtained the following new findings (a) to (e).

(a) In order to obtain a predetermined number density or more of Zr-containing oxides that contribute to refinement of the HAZ structure, the Zr content needs to be 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. Zr (Sol.Zr) that does not form oxides significantly deteriorates the toughness of not only the HAZ but also the steel material itself. Therefore, in order to ensure the toughness not only of HAZ but also of the steel material itself, Sol. It is necessary to reduce Zr. Sol. Toughness tends to improve as Zr content decreases, and in order to obtain a steel material with excellent HAZ toughness, Sol. It is preferable to limit Zr to 0.0010% by mass or less. For further improvement of HAZ toughness, Sol. It is desirable to limit Zr to 0.0003% by mass or less.

(b) It was found that in steel in which Zr-containing oxides are dispersed, even if the number of inclusions increases, there are inclusions that function as ferrite generation sites and inclusions that do not function as ferrite generation sites.
In addition, the present inventors investigated various elements in order to promote ferrite formation more effectively. As a result, by containing a certain amount or more of B, B nitride is precipitated with Zr-containing oxide as the core during casting, hot rolling, or welding, and this composite precipitate (Zr, B)-containing oxide It has been found that the particles function more effectively as intragranular ferrite generation sites.
That is, due to the B nitride, the Zr-containing oxide, which alone cannot easily function as an intragranular ferrite generation site, becomes a ferrite generation site, and contributes to the refinement of the HAZ structure more efficiently. In order to obtain these effects, Zr is added after the dissolved oxygen concentration of molten steel becomes 0.0050% or less in the refining process, and then B is added, so that the amount of B dissolved in solid solution in the steel is reduced. It is preferable that B _F be 0.0020% by mass or less.

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

In molten steel containing Ti, Zr, and B, Zr, which has stronger deoxidizing power than Ti, preferentially becomes an oxide, and the excess oxygen and Ti combine to form a composite oxide of Zr and Ti. . Next, the remaining Ti that does not form an oxide combines with nitrogen to form a nitride. Next, it is thought that the remaining nitrogen that does not combine with Ti forms B nitride.

It is thought that Zr forms ZrO ₂ , Ti forms Ti ₂ O ₃ and TiN, and B forms BN, respectively. Based on these atomic weights or molecular weights, the following formula (A1) is used to form B ( The content (mass%) of BasBN) can be determined. Then, as shown in the following formula (A2), the difference obtained by subtracting the B that becomes B nitride from the B contained in the steel is set as the calculated value (B _F ') of B dissolved in the steel. If the relationship between the calculated value B _F ' calculated using the following formula (A2) and the amount of B in the steel is B _F '> B, the amount of B in the steel is calculated as the amount of B dissolved in the steel (B _F ) (B _F =B). In addition, in the case of 0≦B _F ′≦B, the calculated value B _F ′ obtained by the following formula (A2) is taken as the amount of B (B _F ) dissolved in the steel (B _F =B _{F ′} ). Furthermore, in the case of B _F ′<0, the amount of B dissolved in the steel (B _F ) is set to 0% by mass (B _F =0). By setting the B _F determined in this way to 0.0020% by mass or less, the effect of improving HAZ toughness due to B nitride can be obtained, and by controlling the texture in the thickness direction, the arrestability can be effectively improved. can be improved.

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

B _F '=B-BasBN...(A2)

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

A steel material obtained by hot rolling a steel slab having a composition with a B _F of 0.0020% by mass or less contains fine Zr-containing oxides (composite oxides mainly containing Zr and Ti). Spread. In addition, B nitride is further precipitated in some Zr-containing oxides.
B nitride re-dissolves when heated to a temperature range exceeding 1200°C during welding, but Zr-containing oxide remains stable even when heated to 1400°C. Therefore, during heating during welding, the B nitride is dissolved in solid solution, and the solid solution B is unevenly distributed around the Zr-containing oxide. It is thought that this solid solution B re-precipitates as B nitrides with oxides as nuclei during the cooling process after welding.

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

(e) Furthermore, since Al acts as a strong deoxidizing element in steel, if it is contained in a large amount in steel, it inhibits the formation of oxides of Zr and Ti. In order to ensure the amount of dissolved oxygen in the molten steel and to generate Zr-containing oxides in the steel, the Al content is preferably 0.010% by mass or less. More desirably, the Al content is 0.005% by mass or less. The total amount 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 to satisfy a predetermined number. In addition, most of these (Zr, B)-containing oxide particles are composite oxides containing Zr and Ti, and B nitride is precipitated with the oxide as a nucleus, and furthermore, the Al ₂ O ₃ composition is 50% by mass. % or less. When high heat input welding was actually performed on this steel material, it was found that the oxide particles effectively functioned as intragranular ferrite generation sites in the HAZ, improving HAZ toughness through refinement of the HAZ structure. has become clear.

Furthermore, by controlling the microstructure and texture in the plate thickness direction in addition to the chemical composition and Zr-containing oxide of the steel material, the present inventors have discovered that by controlling the microstructure and the texture in the thickness direction, It has been found that the arrestability in the parallel direction can be improved.

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

The steel material of this embodiment has C: 0.040 to 0.160%, Si: 0.01 to 0.50%, Mn: 0.70 to 2.50%, and P: 0.030% in mass %. 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: 0.003-0.024%, Zr: 0.0007-0.0050%, B: 0.0003-0.0040%, Total content of Ca, Mg and REM: 0.0005% or less , the remainder consists of Fe and impurity elements, and Insol. Zr: 0.0007-0.0040%, Sol. Zr: 0.0010% or less, B _F 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% or less. 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 pearlite with an area ratio of 0 to 5%. It has a microstructure containing a martensite-austenite mixed structure, and forms an angle of 15° or less with respect to a vertical plane that is perpendicular to the main rolling direction at a position 1 to 5 mm from the plate surface. The area ratio of the {110} plane is 30 to 60%, and in 1/4 part of the plate thickness, the {100} {110} plane whose area ratio is 10 to 40% and which forms an angle within 15° with respect to the vertical plane, which is a plane perpendicular to the main rolling direction, in 1/2 part of the plate thickness Of the (Zr, B)-containing oxide particles, which have an area ratio of 40 to 70% and contain 5% by mass or more of Zr, 0.1% by mass or more of B, and 1% by mass or more of O, The number density of (Zr, B)-containing oxide particles having a diameter of 0.5 μm or more and having an Al ₂ O ₃ composition of 50% by mass or less is 5 to 300 It is a steel material with a diameter of ^2.0 mm/mm2.

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

If B _F '>B, B _F = B, if 0≦B _F '≦B, B _F = B _F ', if B _F '<0, B _F = 0...(B2)

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

However, N, Ti, O, and B in formula (B1) and formula (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 formula (B3) are the contents (mass%) of each element contained in the steel, and are set to 0 if the element is not contained. substitute.

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

C: 0.040-0.160%
C is contained in an amount of 0.040% or more in order to ensure the strength and toughness of the steel material. If the C content exceeds 0.160%, it will be difficult to ensure good HAZ toughness, so the C content should be 0.160% or less. Therefore, the C content is 0.040% or more, preferably 0.050% or more, and more preferably 0.060% or more. Further, 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 reinforcing element, it is contained in an amount of 0.01% or more. If the Si content exceeds 0.50%, the HAZ toughness will be significantly degraded, so the Si content should be 0.50% or less. Therefore, the Si content is 0.01% or more, preferably 0.03% or more, and more preferably 0.05% or more. Further, 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 noticeable and the toughness of the steel material and HAZ in the area where center segregation occurs deteriorates, so the Mn content is set to 2.50% or less. Therefore, the Mn content is 0.70% or more, preferably 0.90% or more, and more preferably 1.20% or more. Further, 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 that exists in steel as an impurity. In order to stably ensure HAZ toughness, the P content is set to 0.030% or less. Preferably it is 0.020% or less, more preferably 0.015% or less. The lower limit is 0%, but considering the cost of reducing the P content, it may be set to 0.0001% or more.

S: 0.008% or less S is an element that exists in steel as an impurity. If the S content exceeds 0.008%, a large amount of stretched MnS will be generated in the central segregation area, and the toughness and ductility of the steel material and HAZ will deteriorate. Therefore, the S content is set to 0.008% or less. Preferably it is 0.005% or less. The lower the S content is, the more preferable it is, so the lower limit is not particularly defined, but from the viewpoint of manufacturing costs, it may be 0.0001% or more.

Al: 0.010% or less Al is generally an element that is actively 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 production sites in the HAZ will be lost. 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: 0.0010-0.0080%
N is an important element in the present invention. In order to suppress the austenite grain size from increasing during heating of the steel billet, it is necessary to form Ti nitrides, so the content must be 0.0010% or more. However, if the N content exceeds 0.0080%, the steel material becomes brittle, so the N content is set to 0.0080% or less. Therefore, the N content is 0.0010% or more, preferably 0.0015% or more, and more preferably 0.0020% or more. Further, the N content is 0.0080% or less, preferably 0.0065% or less, and more preferably 0.0060% or less.

O: 0.0005-0.0040%
O is an element contained in steel, and exists as a dissolved or 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.), which is the sum of both. When the oxygen concentration in the thick steel plate is less than 0.0005%, the oxide dispersion number required to ensure toughness cannot be obtained. On the other hand, if it is contained in the steel material in an amount exceeding 0.0040%, the cleanliness of molten steel will deteriorate, and it may become a factor that reduces productivity such as nozzle clogging in the molten steel stage. Therefore, the appropriate range of O content in steel is 0.0005 to 0.0040%.

In addition, if dissolved oxygen exceeds 0.0050% in the molten steel before Zr is added in the steel refining process, the amount of _ZrO2 generated by Zr addition will increase, causing problems when continuously casting the molten steel. There is an increased risk of blockage of the injection nozzle into the tundish. Furthermore, if the dissolved oxygen in the 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 in the molten steel stage before adding Zr.

Nb: 0.003-0.050%
Nb can improve the strength and toughness of steel materials. In addition, in order to obtain a predetermined texture, rolling is required in the unrecrystallized austenite region, and Nb is an effective element for expanding the unrecrystallized temperature range, increasing the rolling temperature, It also contributes to improving productivity. In order to obtain this effect, it is necessary to contain 0.003% or more. However, if the Nb content exceeds 0.050%, HAZ toughness and weldability will deteriorate, so the Nb content should be 0.050% or less. Therefore, the Nb content is set to 0.003% or more, preferably 0.005% or more, and more preferably 0.008% or more. Further, the Nb content is 0.050% or less, preferably 0.025% or less, and 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 generation 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 produces nitrides. If a large amount of Ti nitride is produced, the amount of B nitride produced is suppressed, making it impossible to obtain the desired effect in this embodiment. Furthermore, excessive Ti forms TiC, which deteriorates the toughness of the steel material and HAZ. Therefore, the Ti content is set to 0.024% or less. Preferably it is 0.020% or less.

Zr: 0.0007-0.0050%
The Zr content contained in the steel material is determined by Sol. Zr and Insol. This is the total with Zr. The Zr content is 0.0007% or more, preferably 0.0010% or more. Moreover, the Zr content is Insol. The upper limit of Zr and Sol. The total amount including 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 the steel. Sol. When the content of Zr increases, HAZ toughness deteriorates significantly. Therefore, its content is set to 0.0010% or less. Sol. Since the smaller the Zr content is, the more preferable it is, so the lower limit is not particularly defined and may be 0%.

In addition, at the molten steel stage, Sol. Zr and Insol. There are no particular restrictions on Zr, but if Zr is added in excess relative to dissolved oxygen, even steel materials will have Sol. In addition to a large amount of Zr remaining, the dissolved oxygen concentration decreases, resulting in a decrease in the number density of (Zr, B)-containing oxide particles. Therefore, Sol. It is desirable that the Zr concentration is 0.0020% or less. In addition, in order to prevent nozzle clogging, Insol. It is desirable that the Zr concentration is 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 the core of intragranular transformation. However, Insol. If Zr is less than 0.0007%, the oxide composition required to ensure toughness will not be achieved. On the other hand, Insol. When Zr is contained in an amount exceeding 0.0040%, most of it is ZrO ₂ generated in the molten steel stage, and nozzle clogging occurs more frequently. For this reason, Insol. The appropriate 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, the parent phase of steel is dissolved by electrolysis in a nonaqueous solvent (such as an acetylacetone-methanol solution), and the residue (precipitates and inclusions) is extracted using a filter with a pore size of 0.2 μm. This is a method of separation. After separation, the amount of Zr contained in the solution is Sol. This is the Zr content, and the amount of Zr contained in the residue is Insol. This is the content of Zr.

B: 0.0003-0.0040%
B improves the hardenability of steel materials, and also precipitates as BN around Zr-containing oxides to form (Zr, B)-containing oxide particles, and forms inside the grains of (Zr, B)-containing oxide particles. It is an element that improves metamorphosis ability. In order to precipitate BN around the Zr-containing oxide, B needs to be contained at least 0.0003% or more. On the other hand, the effect is saturated even if the B content exceeds 0.0040%, so the appropriate range of B content is set to 0.0003 to 0.0040%. In order to keep the B concentration in the steel material within the range shown on the left, it is desirable that B 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 more preferentially react with oxygen than Al. In order to form the desired Zr-containing oxide, the total content of Ca, Mg, and REM is set to 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 this embodiment basically contains each of the above-mentioned elements, with the remainder consisting of Fe and impurities. Impurities refer to components that are mixed in from raw materials such as ores and scraps or due to other factors during the industrial production of steel materials, and are tolerable as long as they do not adversely affect properties.

In place of a part of Fe, the steel material according to this embodiment includes one or more selected from the group consisting of Cu, Ni, Cr, Mo, and V, as described below, for the purpose of further increasing the strength. It may be contained within a range. Further, for the purpose of improving 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 this 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%. It may contain one or more selected from the group consisting of: The lower limit of these elements is 0%.

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, performance improvement commensurate with the increase in alloy cost will not be observed, and it may actually cause surface cracking of the steel material, so it is set to 1.00% or less. In order to stably obtain the effect of containing Cu, the Cu content may be set to 0.10% or more. In order to improve the strength and toughness of the steel material, the Cu content may be set to 0.20% or more. 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 included. Further, Ni is an element that has the effect of increasing the toughness of the steel matrix (dough) in a solid solution state. However, if the Ni content is too large, HAZ toughness and weldability will deteriorate, so the Ni content is set to 2.50% or less. In order to stably obtain the effect of containing Ni, the Ni content may be set to 0.10% or more. In order to improve the strength and toughness of the steel material, the Ni content may be set to 0.20% or more. The Ni content may be 1.00% or less, 0.50% or less, or 0.30% or less, if necessary.

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, HAZ toughness and weldability will deteriorate, so the content is set to 1.00% or less. In order to stably obtain the effect of containing Cr, the Cr content may be set to 0.10% or more or 0.20% or more. The Cr content may be 0.80% or less, 0.50% or less, or 0.30% or less, if necessary.

Mo: 0.50% or less By containing Mo, the strength and toughness of the steel material can be improved, so it may be included. However, if the Mo content is too large, HAZ toughness and weldability will deteriorate, so the Mo content is set to 0.50% or less. In order to stably obtain the effect of Mo content, the Mo content may be set to 0.01% or more or 0.02% or more. The Mo content may be 0.30% or less, 0.20% or less, or 0.10% or less, if necessary.

V: 0.150% or less By containing V, the strength and toughness of the steel material can be improved, so it may be included. However, if the V content is too large, HAZ toughness and weldability will deteriorate, so the V content should be 0.150% or less. In order to stably obtain the effect of containing V, the V content may be set to 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, if necessary.

Further, the steel material of the present embodiment may further contain one or two of W: 1.00% or less and Sn: 0.50% or less in mass %. The lower limit of these elements is 0%.

W: 1.00% or less W is an element that dissolves and adsorbs to rust in the form of oxygen acid ions WO ₄ ^- , suppresses the permeation of chloride ions in the rust layer, and improves corrosion resistance, so it should not be included. It's okay. In order to obtain this effect, it is preferable that the W content is 0.01% or more. On the other hand, when the W content exceeds 1.00%, not only the above effects are saturated, but also the toughness of the steel material and HAZ may decrease. Therefore, even when W is included, 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 the effect of inhibiting corrosion by acting as an inhibitor in an acidic chloride solution, so it may be included. Furthermore, Sn has the effect of suppressing the anode dissolution reaction of steel and improving corrosion resistance. In order to obtain these effects, the Sn content is preferably 0.03% or more. On the other hand, when Sn is contained in excess of 0.50%, not only the effect is saturated, but also rolling cracks in the steel material are likely to occur. Therefore, even when Sn is contained, the content is set to 0.50% or less.

Ceq: 0.30% to 0.55%
Further, in the steel material of this embodiment, the carbon equivalent Ceq expressed by the following formula (C) must be in the range 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 formula (C) are the content (mass%) of each element contained in the steel material, and if the relevant element is not contained, 0 is substituted. do.

If the carbon equivalent Ceq is 0.30% or more, the strength and arrestability required of the steel material can be ensured. Further, if the carbon equivalent Ceq is 0.55% or less, better HAZ toughness can be ensured. 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. Further, 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.

BF': 0.0020% or less After controlling the content of each element as described above, the steel material according to the present embodiment has a _BF derived from the following formulas (D1) and (D2) of 0.0020% or less. It is necessary that it is 0.020% or less. _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)

When B _F '>B, B _F = B; when 0≦B _F '≦B, B _F = B F '; when B _{F '} <0, B _F =0...(D2)

However, N, Ti, O, and B in formula (D1) and formula (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 contained in the (Zr, B)-containing oxide particles. Compared to those in which nitrides are precipitated, the formation of intragranular ferrite during cooling after welding can be more effectively promoted, the structure can be refined, and HAZ toughness can be improved. In order to obtain this effect and improve the arrestability at the same time, the B content present as solid solution B, that is, the B _F derived from the above formulas (D1) and (D2) must be 0.0020% or less. It is necessary to More preferably, it is 0.0010% or less. When B _F exceeds 0.0020%, the hardenability of the steel material becomes excessive, coarsening of bainite and excessive increase in hardness occur, resulting in a decrease in arrestability. Therefore, a more preferable upper limit of B _F is 0.0010% or less.

Next, the microstructure of the steel material according to this embodiment will be explained.
The steel material according to this embodiment has a mixed structure of ferrite and bainite, or a mixed structure of ferrite, bainite, and pearlite, or a mixed structure of ferrite, bainite, and martensite/austenite, or a mixed structure of ferrite, bainite, pearlite, and It has a mixed structure of martensite and austenite, 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 produce a steel material with a thick plate thickness and high strength. Furthermore, if the area ratio of ferrite is less than 5%, the area ratio of texture becomes excessive and arrestability cannot be ensured. If the remaining structure excluding ferrite can be a predetermined bainite, or bainite and pearlite, or bainite and martensite/austenite mixed structure, or bainite, pearlite, and martensite/austenite mixed structure, the desired plate thickness can be achieved. It is possible to obtain steel materials with high strength and texture. This embodiment is aimed at thick-walled 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 thick steel plate with high strength. The bainite area ratio is 95% or less in order to ensure the ferrite area ratio and increase the texture that becomes an obstacle to brittle crack propagation. This embodiment is aimed at thick-walled high-strength steel, and the bainite area ratio may be set to 50% or more, 60% or more, 70% or more, or 80% or more. The area ratio of bainite is preferably 90% or less.

Pearlite may be included in order to obtain a steel material with a desired thickness and strength, but if it is present in excess, it becomes a brittle phase and reduces the arrestability, so the pearlite area ratio is set to 15% or less. The pearlite area ratio may be 10% or less, 5% or less, or 3% or less, and the lower limit is 0%.

In addition to ferrite, pearlite, and bainite, a martensite/austenite mixed structure may be present, but if it is present in excess, it becomes a brittle phase that significantly reduces arrestability, so the area ratio of the martensite/austenite mixed structure is 5% or less. The area ratio of the martensite-austenite mixed structure may be 3% or less, 2% or less, or 1% or less, and 0% is most desirable.

The phase fraction of the microstructure was determined by photographing the microstructure of 1/2 part of the plate thickness at 500x magnification using an optical microscope, and by image analysis, the area of each of ferrite, bainite, pearlite, and martensite-austenite mixed structure was determined. Calculate by dividing by the measured area.

Next, the texture of the steel material according to this embodiment will be explained.
In order to stably improve arrestability, it is important to control the direction of crack propagation by utilizing texture. A brittle crack that occurs in a steel material when the steel material is subjected to external stress propagates along the cleavage plane of the {100} plane. For this reason, if a {100} plane texture developed in a plane perpendicular to the external stress is formed over the entire thickness of the steel material, cracks throughout the thickness will easily propagate in the same direction, resulting in improved arrestability. was found to decrease.

Therefore, in this embodiment, as explained below, the vertical plane, which is the plane perpendicular to the main rolling direction, is , the area ratio of the {110} plane that forms an angle within 15°, and the area ratio of the {110} plane that forms an angle within 15° with respect to the vertical plane that is perpendicular to the main rolling direction at 1/4 part of the plate thickness. By limiting the area ratio of the {100} plane, arrestability is stably improved.

Note that external stress refers to stress that is externally applied to a steel structure. Brittle cracks often initiate and propagate in the direction perpendicular to the highest external stress. Therefore, here, the highest stress externally applied to a steel structure is defined as external stress. Generally, external stress is applied approximately parallel to the main rolling direction of the steel material. Therefore, a plane perpendicular to the external stress can be treated as a plane perpendicular to the main rolling direction of the steel material.

The main rolling direction of the steel material can be determined, for example, by corroding the surface of the steel material with picric acid and measuring the aspect ratio of prior austenite. That is, the direction in which the aspect ratio of prior austenite is large can be specified as the main rolling direction of the steel material.

An angle of 15° or less with respect to a plane perpendicular to the main rolling direction of the steel material (hereinafter, a "plane perpendicular to the main rolling direction of the steel material" may be referred to as a "vertical surface") {110} If the area ratio of the surface is set to 40 to 70% at 1/2 part of the plate thickness, brittle cracks near the 1/2 part will not propagate straight in the direction perpendicular to external stress. It was found that the driving force for crack propagation can be reduced by propagating the crack at an angle. However, if a similar texture is developed in areas other than 1/2 of the thickness of the plate, cracks will propagate in an inclined manner and a sufficient effect of improving arrestability cannot be exhibited.

Therefore, in order to propagate the crack straight in the direction perpendicular to the external stress, in the 1/4th part of the board thickness, an angle of 15° or less is formed with respect to the vertical plane. It was found that by setting the area ratio of the {100} plane to 10 to 40%, propagation of an inclined crack in the 1/2 part to parts of the plate thickness other than the 1/2 part can be suppressed. Ta.

Furthermore, in this embodiment, in order to propagate the crack obliquely rather than straight in the direction perpendicular to the external stress near the surface, a It has been found that by setting the area ratio of {110} planes forming an angle of 15° or less to 30 to 60%, it is possible to suppress the propagation of straight cracks in the 1/4 portion to the vicinity of the surface layer.

Based on the above findings, it is preferable that the texture of the steel material according to the present embodiment satisfies the following conditions (E) to (G).
(E) At a position 1 to 5 mm from the surface, the area ratio of {110} planes forming an angle of 15° or less with respect to the vertical plane is 30 to 60%.
(F) In 1/4 part of the plate thickness, the area ratio of {100} planes forming an angle of 15° or less with respect to the vertical plane is 10 to 40%.
(G) In 1/2 part of the plate thickness, the area ratio of {110} planes forming an angle of 15° or less with respect to the vertical plane is 40 to 70%.

By satisfying (E) to (G) above, the crack in the 1/2 part will propagate obliquely, and the crack in the 1/4 part will propagate straight, and the crack will be 1 to 5 mm from the surface. The crack propagates obliquely, further increasing the crack propagation resistance. Thereby, the arrestability can exhibit a sufficient value.

The reason why the area ratio of {110} planes that form an angle of 15° or less with respect to the vertical plane at a position 1 to 5 mm from the surface is set to 30% or more is that if it is less than 30%, the crack propagates at an angle. This is because it cannot be obtained.

In addition, the reason why the area ratio of the {110} plane that forms an angle of 15° or less with the vertical plane at a position 1 to 5 mm from the surface is set to 60% or less is that if it exceeds 60%, the resistance of 1/4 part This is because the arrestability deteriorates if the beam propagates in an inclined manner without being affected by the force.

At a position 1 to 5 mm from the surface, the area ratio of {110} planes forming an angle of 15° or less with respect to the vertical plane is preferably 35 to 55%, more preferably 40 to 50%.

The reason why the area ratio of {100} planes that form an angle of 15 degrees or less with the vertical plane in 1/4 part of the plate thickness is set to 10% or more is that if it is less than 10%, the effect of propagating the crack straight is not effective. This is because they cannot be obtained.

In addition, the reason why the area ratio of the {100} plane that forms an angle of 15 degrees or less with the vertical plane in 1/4 part of the plate thickness is set to 40% or less is that if it exceeds 40%, it will be less than 1/2 part. This is because the crack propagation at the /4 portion becomes dominant and the crack propagates straight, resulting in a decrease in arrestability.

The area ratio of {100} planes forming an angle of 15° or less with respect to the vertical plane in 1/4 part of the plate thickness is preferably 13 to 37%, more preferably 15 to 35%.

The reason why the area ratio of {110} planes forming an angle of 15° or less with respect to the vertical plane in 1/2 part of the plate thickness is set to 40% or more is that if it is less than 40%, cracks propagate at an angle. This is because no effect can be obtained.

In addition, the reason why the area ratio of {110} planes that form an angle of 15 degrees or less with the vertical plane in 1/2 part of the plate thickness is 70% or less is that if it exceeds 70%, the resistance of 1/4 part is This is because the arrestability deteriorates if the light propagates in an inclined manner without being received.

In 1/2 part of the plate thickness, the area ratio of {110} planes forming an angle of 15° or less with respect to the vertical plane is preferably 45 to 65%, more preferably 50 to 60%. .

Texture is measured by the EBSD method.
More specifically, using the EBSD method, the {110} plane forms an angle of 15° or less with respect to the vertical plane at 1 to 5 mm from the surface, and the angle of 15° or less with respect to the vertical plane at 1/4 of the plate thickness. By creating a map of the {100} plane that forms a plane, and the {110} plane that forms an angle of 15° or less with respect to the vertical plane at 1/2 part of the plate thickness, and dividing the total area by the measured area, , their area ratios can be found.

Next, the (Zr, B)-containing oxide particles included in the steel material according to this embodiment will be explained.
The steel material according to the present embodiment includes (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. Among these, the number density of (Zr, B)-containing oxide particles with 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, B nitride is precipitated using the Zr-containing oxide as a core, and (Zr, B)-containing oxide particles, which are composite inclusions, are formed. These composite inclusions become intragranular ferrite generation sites during cooling after welding. Zr-containing oxides are mainly oxides containing Zr and Ti, but when used as precipitation nuclei for B nitrides, the Zr concentration in the oxide must be equal to or higher than the Ti concentration. preferable.

Furthermore, in the present embodiment, among the (Zr, B)-containing oxide particles, the (Zr, B)-containing oxide particles contain 5% by mass or more of Zr, 0.1% by mass or more of B, and 1% by mass or more of O. Targets oxide particles. The (Zr, B)-containing oxide particles having such a composition can function as a production site for intragranular ferrite, and can form more intragranular ferrite. Oxide particles in which the content of Zr, B, or O deviates from the preferred range cannot sufficiently function as intragranular ferrite generation sites. Note that in this embodiment, the amount of Ti in the (Zr, B)-containing oxide particles does not need to be particularly specified, but 1% by mass or more of Ti may be included.

Furthermore, in this embodiment, among the (Zr, B)-containing oxide particles, the number density is defined 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 generation site for intragranular ferrite, forming a large amount of intragranular ferrite. can be done.

Furthermore, 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, The 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 generation sites, the equivalent circle diameter is preferably large, so there is no upper limit. However, as the equivalent circle diameter becomes larger, the number density of the (Zr, B)-containing oxide particles becomes relatively smaller, and the possibility that the coarse oxide particles themselves act as a starting point for destruction increases. Therefore, the equivalent circle diameter of the (Zr, B)-containing oxide particles is preferably 10.0 μm or less.

Further, as a condition for acting as a generation site of intragranular ferrite, it is preferable that one or more (Zr, B)-containing oxide particles are dispersed within the austenite grains when heated during welding. Therefore, it has a circular equivalent diameter of 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 has a composition of Al ₂ O _3. 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 number density of such (Zr, B)-containing oxide particles is desirable because the number density increases as the ^number of ferrite generation sites increases. 300 pieces/ ^mm2 or less. In particular, the (Zr, B)-containing oxide particles according to this embodiment in which the composition of Al ₂ O ₃ is 50% by mass or less 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 having an Al ₂ O ₃ composition of more than 50% by mass. However, it can be very effective.

The equivalent circle diameter and number density of the (Zr, B)-containing oxide particles can be measured by observing the mirror-polished surface of the steel material using a scanning electron microscope (SEM). Specifically, the number of (Zr, B)-containing oxide particles with an equivalent circular diameter of 0.5 μm or more was measured in an area of 10 mm x 10 mm (100 mm ² ) using SEM, and the number was divided by the area of the observed visual field. and measure the number density. A photograph taken by SEM may also be used. The particles to be measured for number density have an equivalent circle diameter of 0.5 μm or more, and quantitative analysis using an energy dispersive Particles containing at least 1% by mass of B and 1% by mass or more of O, and having an Al ₂ O ₃ composition of 50% by mass or less are confirmed.

The 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 to 720 MPa, and the yield stress YP is preferably in the range of 390 to 650 MPa. Evaluation of tensile strength TS and yield stress YP is performed according to JIS Z 2241:2011. The test piece shall be a No. 1B test piece. The test method is the permanent elongation method.

The steel material of this embodiment has excellent toughness in the weld heat affected zone when welded under conditions where the welding heat input is 35 kJ/mm or more. In particular, the Charpy absorbed energy at -40°C can be improved.

More specifically, a reproduced thermal cycle test simulating high heat input welding is applied to a sample taken from the steel material of this embodiment assuming that electrogas welding is applied. Specifically, the simulated heat cycle conditions simulate welding a 50 mm thick plate in one pass by electrogas welding, heating from room temperature to 1400°C, holding at 1400°C for 5 seconds, and then Cooling is performed by controlling the temperature range from 800° C. to 500° C., which is the temperature range related to internal transformation, at a rate of 1.0° C./sec. After applying a thermal cycle to thick steel plates, they are processed into V-notch test pieces, and three pieces of each steel plate 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 of the three test pieces is 50 J or more, it can be said that the toughness of the weld heat affected zone is excellent. Note that the V-notch test piece may be created according to the V-notch test piece described in JIS Z 2242:2005. Further, the Charpy impact test is preferably conducted according to JIS Z 2242:2005.

Further, the steel material of this 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-ductility transition temperature (NDT temperature) is -60°C or less, and the fracture surface transition temperature (vTrs) is -60°C. If all of the following are satisfied, it can be said that the arrestability is excellent.

Arrest toughness value Kca _-10℃ evaluation is as per NK Classification Society Steel Ship Regulations Inspection Guidelines Part K Annex K3.12.2-1. (2018), “Inspection Guidelines for Brittle Crack Arrest Toughness Value Kca Test Method”. The arrest toughness value Kca at -10°C is determined by the test.

In addition, the evaluation of the Nil-Ductility-Transition Temperature (NDT temperature) is determined by conducting a test in accordance with the NRL (Naval Research Laboratory) drop weight test method specified by ASTM E208-06. . The test piece is a P-3 type (T: 16 mm, L: 130 mm, W: 50 mm), and the specimen is taken up to a position of 16 mm in the thickness direction, including 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 as a crack starter in the direction perpendicular to the rolling direction (C direction).

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 was set to include t/4 part of the steel plate thickness t. Collect.

Next, a method for manufacturing a steel material according to this embodiment will be explained.
In the method for manufacturing steel materials of this embodiment, molten steel is vacuum degassed, and Zr is added after the dissolved oxygen concentration of the molten steel becomes 0.0050% or less. During the refining process in which B is added after a minute has elapsed, and when the molten steel after the refining process is continuously cast to produce slabs, the average cooling rate from 1200°C to 900°C in the surface temperature of the slabs is as follows: A continuous casting step at 0.5° C./second or less and a hot rolling step of hot rolling the slab after continuous casting are performed in sequence.

In this embodiment, molten steel is tapped into a ladle from a steelmaking furnace and then subjected to depressurization treatment in a vacuum degassing device. After being tapped into a ladle, alloys or the like may be added to adjust the composition while the steel is being transported to a vacuum degassing device.

In the refining process, degassing is performed in a vacuum degassing device, and after adjusting the molten steel components excluding Zr and B, Zr is added. It is desirable to control dissolved oxygen in molten steel to 0.0050% or less before adding Zr. If Zr is added before dissolved oxygen reaches 0.0050% or less, it becomes difficult to refine the (Zr, B)-containing oxide particles, and the Al ₂ O ₃ of the (Zr, B)-containing oxide particles There is a possibility that the composition cannot be controlled to 50% by mass or less.

Next, 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 nitride is contained in the Zr-containing oxide, and B nitride is precipitated on the surface layer of the (Zr, B)-containing oxide particles. be able to. If the timing of B addition is less than 1.0 minutes or more than 5.0 minutes after Zr addition, desired (Zr, B)-containing oxide particles cannot be obtained.

The molten steel after the refining process is made into slabs in a continuous casting process. In the continuous casting process, the average cooling rate until the surface temperature of the slab reaches 900°C from 1200°C is 0.5°C/second or less. As a result, separation of ZrO ₂ and Al ₂ O ₃ in the Zr-containing oxide progresses, and the Al ₂ O ₃ composition of the (Zr,B)-containing oxide particles can be made 50% by mass or less.

The slab obtained by the continuous casting process is heated to 950 to 1200°C in a heating process, and then hot rolled into a steel material in a hot rolling process. The conditions for the heating process and hot rolling process are as follows. In the hot rolling process, rolling conditions are preferably set so that the thickness of the steel material is in the range of 50 to 100 mm.

The heating process is a process that contributes to controlling the structure of the austenite phase by heating the slab. In the heating step, the slab after continuous casting is heated to 950 to 1200°C. The heating step is preferably performed in a heating furnace. In addition, heating the slab after continuous casting to 950 to 1200 ° C. means heating it so that the average temperature of the entire thickness of the slab when extracted from the heating furnace is in the range of 950 to 1200 ° C. In this specification, the average temperature of the entire thickness of the slab is referred to as the heating temperature of the slab. If the heating temperature is less than 950° C., austenitization will be insufficient and the austenite grains will become finer, resulting in a decrease in hardenability, making it difficult to produce a steel material with a thick plate and high strength. Furthermore, if the heating temperature exceeds 1200° C., the austenite grains will become coarser and it will take time to wait for the temperature to drop before starting rolling, resulting in lower productivity. The preferred heating temperature range is 1000 to 1150°C.

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

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

Note that 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 processing heat generation in the steel material due to rough rolling, 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 the [Sol. Nb] and the Nb content (mass%) in the steel is Nb≧[Sol. Nb], then [Nb*]=[Sol. Nb] and Nb<[Sol. Nb], then [Nb*]=Nb. C and N in formula (I) are the contents (mass%) of C and N contained in the steel. T in formula (I) is the average temperature (° C.) of the entire thickness of the slab when extracted from the heating furnace in the heating process.

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

Next, the steel material after rough rolling is subjected to primary cooling. In the primary cooling step, the cooling start temperature is set to a range of Ar ₃ °C or higher and 1050 °C or lower as shown in the following formula (J), and the cooling stop temperature is set to a range of 500 °C or higher and (Ar ₃ -30) °C or lower (however, Ar ₃ is expressed by the following formula (J)), and the average cooling rate from the start of cooling to the stop of cooling is 35 to 100° C./sec. By performing primary cooling under these conditions, the area of the {110} plane that forms an angle within 15° with respect to the vertical plane that is perpendicular to the main rolling direction at a position 1 to 5 mm from the surface of the steel material. The percentage can range from 30 to 60%. The cooling start temperature, cooling stop temperature, and average cooling rate are the temperatures at a depth of 1 mm from the surface of the steel material.

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

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

Next, finish rolling is performed on the steel material after the primary cooling process. In the finish rolling process, rolling is performed at a finish rolling temperature of 750 to 850° C., 4 to 15 rolling passes, an average rolling shape ratio of 0.5 to 1.0, and a cumulative reduction rate of 45 to 75%. Here, rolling the steel material at a finish rolling temperature of 750 to 850°C means that finish rolling is started with the surface temperature of the steel material being 750 to 850°C, and the surface temperature of the steel material at the end of finish rolling is 750 to 850°C. The temperature is 850°C. Also, rolling with a cumulative reduction ratio (finish rolling) in the range of 45 to 75% means that the thickness of the steel material rolled in rough rolling is calculated by subtracting the thickness of the steel material after finish rolling from the thickness of the steel material rolled in rough rolling. Rolling is performed so that the cumulative reduction ratio (finish rolling) divided by the plate thickness is in the range of 45 to 75%.

When the finish rolling temperature exceeds 850° C., the steel does not enter the unrecrystallized region sufficiently, suppressing the increase in dislocations, and making it impossible to obtain a predetermined texture. If the finishing rolling temperature is less than 750°C, productivity will decrease and since processed ferrite will be included, the finishing rolling temperature will be lower than 750°C. It may be difficult to reduce the area ratio of {110} planes forming an angle to 60% or less. The preferred finish rolling temperature is 760 to 840°C.

When the number of rolling passes in finish rolling is less than 4 passes, it is difficult to make the rolling shape ratio m _j less than 1, and when it exceeds 15 passes, productivity decreases. The preferred number of passes is 5 to 13 passes.

The rolled shape ratio m _j is determined by the following equation (K). Moreover, the average value of the rolling shape ratio m _j is the average value of the rolling shape ratio m _j in all rolling passes.

m _j =2 {R (H _j-1 - H _j )} ^1/2 / (H _j-1 + H _j )...(K)

In formula (K), j is the number of rolling passes, _mj is the shape ratio of the jth pass, R is the roll radius (mm), and _Hj represents the plate thickness (mm) after the jth pass. .

The rolling shape ratio m _j is an index representing what kind of strain component is imparted to the steel material by rolling. When the shape ratio is small, a large amount of shear strain component is applied, and when it is large, a large amount of compressive strain component is applied. Since the change in the strain component due to this change in shape ratio has a large effect on the formation of the texture in the 1/4 part of the plate thickness, the range thereof is set as described above.

The reason why the average value of the rolled shape ratio m _j is set to 0.5 to 1.0 is as follows. When the average value of the rolling shape ratio m _j is less than 0.5, the shear strain of rolling becomes dominant, resulting in the development of a {100} texture, and in a quarter of the thickness of the steel sheet, the shear strain in the main rolling direction of the steel sheet This is because it is difficult to reduce the area ratio of the {100} plane, which forms an angle of 15° or less to a plane perpendicular to the other plane, to 40% or less. In addition, when the average value of the rolling shape ratio m _j exceeds 1.0, the compressive strain of rolling becomes dominant, resulting in the development of {110} texture. This is because it is difficult to make the surface area ratio 10% or more. The preferred average value of the shape ratio m _j is in the range of 0.6 to 0.9.

If the cumulative rolling reduction rate is less than 45%, it is difficult to develop a specified texture due to the accumulation of strain, and if it exceeds 75%, productivity decreases, so it is set to 45 to 75%. The preferred range of cumulative rolling reduction is 50 to 70%.

Next, the steel material after finish rolling is subjected to secondary cooling. In the secondary cooling step, the cooling start temperature is set at (Ar ₃ -100)°C or higher (Ar ₃ is represented by the above formula (J)) and below the recrystallization temperature Trex (°C) shown in the above formula (H). Cooling is carried out under the conditions that 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 a quarter position in the thickness direction of the steel material. By setting the conditions of the secondary cooling process within the above range, the transformation of the microstructure due to quenching is promoted and the desired microstructure is obtained, which increases the tensile strength TS and yield stress YP and improves the arrestability. can.

In the steel manufacturing method of this embodiment, a tempering step of heating to a temperature in the range of 350 to 650° C. may be performed after the secondary cooling step. By performing the tempering process, it is possible to reduce the dislocation density that has become excessively high due to rolling.

Next, an example of the present invention will be described. The conditions in the example are examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is based on this example of conditions. It is not limited. The present invention may adopt various conditions as long as the objectives of the present invention are achieved without departing from the gist of the present invention.

The hot metal tapped from the blast furnace was subjected to desulfurization treatment in the hot metal pretreatment, and after being subjected to deP and deC treatment in a converter type refining vessel, it was received in a ladle. During tapping, alloying elements were added and cover slag for heat retention was added.

In the refining process, the molten steel in the ladle was subjected to depressurization treatment using an RH vacuum degassing device. During melting, samples of molten steel were taken as appropriate and analyzed to obtain molten steel components. The molten steel temperature changed from 1560°C to 1610°C. In the first half of the RH treatment, alloys excluding Zr and B were 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. Thereafter, Zr was added, and after another 0.8 to 5.3 minutes, B was added. Then, reflux treatment was performed to mix uniformly. In addition, in No. 51, B was added 2.2 minutes before the addition of Zr. Therefore, in Table 2C, No. The difference in addition time between Zr and B in No. 51 was described as "-2.2".

After treatment with the RH vacuum degassing device, the continuous casting method is used.In continuous casting, the average cooling rate from 1200°C to 900°C is set at an average cooling rate of 0.1 to 0.7°C/sec. And so. Then, a slab with a thickness of 246 to 380 mm was obtained as a semi-finished product. Thereafter, it was processed through a hot rolling process to a thickness of 50 to 100 mm to produce a thick steel plate.

Tables 1A to 1D show steel 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 from 1200° C. to 900° C. during continuous casting. Further, Tables 2A to 2C show the conditions of the heating step, rough rolling step, primary cooling step, finish rolling step, secondary cooling step, and tempering step. Furthermore, Tables 3A to 3C show that Insol. Zr amount, Sol. Zr content, BF _content , microstructure evaluation results, texture evaluation results, number density of (Zr, B)-containing oxide particles, Charpy absorbed energy, tensile strength TS, yield stress YP, arrest at -10°C The toughness value Kca, NDT temperature and vTrs are shown. Note that the underlined portions in Tables 1A to 3C indicate that the properties are outside the scope of the present invention or outside the range of preferred characteristics.

The equivalent circle diameter and number density of the (Zr, B)-containing oxide particles were measured by observing the mirror-polished surface of the steel material using a scanning electron microscope (SEM). Specifically, the number of (Zr, B)-containing oxide particles with an equivalent circular diameter of 0.5 μm or more was measured in an area of 10 mm x 10 mm (100 mm ² ) using SEM, and the number was divided by the area of the observed visual field. The number density was measured. The particles to be measured for number density have an equivalent circle diameter of 0.5 μm or more, and quantitative analysis using an energy dispersive It was confirmed that the particles contained at least 1 mass% of B and 1 mass% or more of O, and had an Al ₂ O ₃ composition of 50 mass% or less.

Insol. Zr and Sol. Zr was measured by electrolytic extraction residue analysis method. In the electrolytic extraction residue analysis method, the parent phase of steel is dissolved by electrolysis in a nonaqueous solvent (acetylacetone-methanol solution), 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 is determined as Sol. The Zr content is defined as Insol. It was taken as the content of Zr.

The amount of _BF was determined by the above formula (B1) and formula (B2).

Next, a test piece for a thermal cycle test was taken from the steel material. This test piece was subjected to a thermal cycle that reproduced welding with a heat input of 35 kJ/mm (high heat input welding). Specific thermal cycle conditions include heating from room temperature to 1400°C, holding at 1400°C for 5 seconds, and then heating the temperature range from 800°C to 500°C, which is the temperature range related to intragranular transformation, to 1.0°C. Cooling was controlled at a rate of °C/sec.
Three V-notch test pieces were taken from the steel material after being subjected to thermal cycling, and a Charpy impact test was conducted at -40°C to measure the absorbed energy (vE _-40 ). Note that the V-notch test piece was created according to the V-notch test piece described in JIS Z 2242:2005. Further, the Charpy impact test was conducted in accordance with JIS Z 2242:2005.
The test was passed if the average value of the absorbed energy (vE _-40 ) of the three test pieces was 100 J or more, and the minimum value of the absorbed energy (vE _-40 ) of the three test pieces was 50 J or more.

The evaluation of arrestability is carried out in accordance with NK Classification Society Steel Ship Regulations Inspection Guidelines Part K Annex K3.12.2-1. (2018), “Inspection Guidelines for Brittle Crack Arrest Toughness Value Kca Test Method”. Through the test, the arrest toughness value Kca at -10°C was determined.

In addition, as an evaluation of arrestability, the non-ductility-transition temperature (NDT temperature) was determined. The NDT temperature was determined by conducting a test in accordance with the NRL (Naval Research Laboratory) drop weight test method specified by ASTM E208-06. The test piece was a P-3 type (T: 16 mm, L: 130 mm, W: 50 mm), and was sampled up to a position of 16 mm in the thickness direction, including 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 as a crack starter in the direction perpendicular to the rolling direction (C direction).

Furthermore, as an evaluation of arrestability, the fracture surface transition temperature (vTrs) was determined. The evaluation of vTrs was based on JIS Z 2242:2005, and the test piece was a V-notch test piece, and the test piece was taken at a position that included the t/4 part of the plate thickness t of the steel material.

Arrest toughness value Kca at -10°C: When _-10°C is 6000N/mm ^1.5 or more, non-ductility transition temperature (NDT temperature) is -60°C or less, and fracture surface transition temperature (vTrs) is -60°C or less. was passed.

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

As shown in Tables 1A to 3C, No. 1, which is an example of the present invention. Nos. 1 to 25 all had excellent toughness and arrestability, and also had excellent mechanical properties.

On the other hand, as shown in Tables 1A to 3C, comparative example No. Samples Nos. 26 to 37 and 40 to 47 had chemical compositions outside the range specified by the present invention, and therefore either toughness, arrestability, or mechanical properties were deteriorated.

Also, No. Samples Nos. 38, 39, and 48 to 77 had chemical components that met the component range of the present invention, but manufacturing conditions did not satisfy the conditions of the present invention. Therefore, No. 38 is Sol. Zr did not satisfy the present invention, and No. In No. 39, _BF did not satisfy the present invention. No. In samples No. 48 to No. 52, the number density of (Zr, B)-containing oxide particles did not satisfy the range of the present invention. No. 53, 65 to 70, 72, 74, and 77 are {110} planes that form an angle within 15° with respect to a vertical plane that is a plane perpendicular to the main rolling direction at 1/2 part of the plate thickness. The area ratio did not meet the scope of the present invention. No. Nos. 54 and 76 had microstructures that did not meet the scope of the present invention. No. 55 to 64 and 71 have the area ratio of the {110} plane that forms an angle of 15° or less with respect to the vertical plane that is a plane perpendicular to the main rolling direction at a position of 1 to 5 mm from the plate surface. The range was not satisfied. No. Sample No. 73 had a microstructure that did not meet the scope of the present invention because the average cooling rate in secondary cooling was too high. Also, No. Sample No. 75 had a microstructure that did not meet the scope of the present invention because the cooling start temperature in secondary cooling was too high.

Claims (5)

In mass%,
C: 0.040-0.160%,
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 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 remainder 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%,
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%. has a microstructure,
At a position 1 to 5 mm from the plate surface, the area ratio of {110} planes forming an angle of 15° or less with respect to a vertical plane perpendicular to the main rolling direction is 30 to 60%,
In 1/4 part of the plate thickness, the area ratio of the {100} plane that forms an angle of 15° or less with respect to the vertical plane that is perpendicular to the main rolling direction is 10 to 40%,
In 1/2 part of the plate thickness, the area ratio of the {110} plane that forms an angle of 15° or less with respect to the vertical plane that is perpendicular to the main rolling direction is 40 to 70%,
Among (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 pieces/mm ² .
B _F '=B-[N-{Ti-(O-Insol.Zr×(32/91.224))×(95.734/48)}×(14/47.867)]×(10.811 /14) …(1)
When B _F '>B, B _F = B; when 0≦B _F '≦B, B _F = B F '; when B _{F '} <0, B _F =0...(2)
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15...(3)
However, N, Ti, O, and B in 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.
In addition, C, Mn, Cr, Mo, V, Cu, and Ni in formula (3) are the contents (mass%) of each element contained in the steel, and are set to 0 if 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 in that it contains 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 in that it contains 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, comprising:
A refining process in which molten steel is vacuum degassed, Zr is added after the dissolved oxygen concentration of the molten steel becomes 0.0050% or less, and B is added 1.0 to 5.0 minutes after Zr addition. and,
When continuous casting is performed on the molten steel after the refining process to produce a slab, the average cooling rate until the surface temperature of the slab reaches from 1200°C to 900°C is 0.5°C/second or less. continuous casting process;
a heating step of heating the slab after the continuous casting to 950 to 1200°C;
a hot rolling step of hot rolling the heated slab into a steel material,
The hot rolling process is a process of sequentially performing a rough rolling process, a primary cooling process, a finish rolling process, and a secondary cooling process,
In the rough rolling step, the slab heated in the heating step is rolled at a rolling temperature of not less than a recrystallization temperature Trex (° C.) and not more than 1050° C., as shown in the following formula (4), at a cumulative reduction rate of 10 to 75%. It is a process of
In the primary cooling step, the cooling start temperature is in the range of Ar ₃ °C or more and 1050 °C or less as shown in the following formula (5), and the cooling stop temperature is in the range of 500 °C or more and (Ar ₃ -30) °C or less (however, _Ar3 is a range expressed by the following formula (5)), and the average cooling rate from the start of cooling to the stop of cooling is a process of cooling under conditions of 35 to 100 ° C / sec,
In the finish rolling process, rolling is performed at a finish rolling temperature of 750 to 850°C, a rolling pass number of 4 to 15, an average rolling shape ratio of 0.5 to 1.0, and a cumulative reduction rate of 45 to 75%. It is a process,
In the secondary cooling step, the cooling start temperature is set to (Ar ₃ -100)°C or higher (Ar ₃ is represented by the following formula (5)), and the recrystallization temperature Trex (°C) shown in the following formula (4) is set. 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. A method of manufacturing steel materials.
Trex=-91900[Nb*] ² +9400[Nb*]+770...(4)
Ar ₃ (℃)=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 [Sol. Nb] and the Nb content (mass%) in the steel is Nb≧[Sol. Nb], then [Nb*]=[Sol. Nb] and Nb<[Sol. Nb], set [Nb*]=Nb,
The element symbols in formulas (5) to (6) are the content (mass%) of each element contained in the steel, and if the element is not contained, 0 is substituted,
T in formula (6) is the temperature (° C.) of the slab at the time of extracting the slab in the heating step. 5. The method for manufacturing a steel material according to claim 4, further comprising performing a tempering step of heating to a temperature in the range of 350 to 650° C. after the secondary cooling step.