KR20210127736A - Steel and its manufacturing method - Google Patents

Abstract

This steel material has a predetermined chemical composition, Insol.Zr: 0.0007 to 0.0040%, Sol.Zr: 0.0010% or less, B _F represented by the following formulas (1) and (2) is 0.0030% or less, Among the (Zr, B)-containing oxide particles containing 5.0 mass% or more of Zr, 0.1 mass% or more of B, and 1.0 mass% or more of O, (Zr, B) containing oxide particles having a circle equivalent diameter of 0.5 µm or more, Al ₂ O ₃ The number density of the (Zr, B) containing oxide particle whose composition is 50 mass % or less is 5-300 pieces/mm<2>.

Description

Steel and its manufacturing method

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

This application is the Japanese Patent Application No. 2019-119789, filed in Japan on June 27, 2019, Japanese Patent Application No. 2019-184528, filed in Japan on October 7, 2019, April 1, 2020 Priority is claimed based on Japanese Patent Application No. 2020-065648 for which it applied to Japan and Japan, and the content is used here.

As a use of steel materials, welded structures, such as ships, high-rise buildings and other buildings, bridges, offshore structures, LNG storage tanks, other large tanks, and line pipes, are mentioned. In recent years, due to the increase in the loading weight of a container ship, etc., the enlargement of a welded structure is progressing. In connection with this, thickness increase and high strength increase are calculated|required by steel materials. In addition, in the above-mentioned welded structure, it is necessary to secure further safety and reliability also for the welded portion, and improvement of the toughness of the heat-affected zone of the weld (hereinafter referred to as "HAZ toughness" in some cases) is a problem.

Moreover, the welding construction cost which occupies for the whole construction cost of a welded structure is large, and in order to reduce this cost, it is calculated|required to perform welding with high efficiency. Specifically, it is effective to perform welding with a large heat input and to reduce the number of welding passes. However, when high heat input welding is performed, generally, the structure of HAZ of steel materials coarsens, and deterioration of toughness (HAZ toughness) cannot be avoided.

Conventionally, it is known that the grain size of austenite (γ), the transformation structure, the hardness of the HAZ, the coarse hard phase, etc. have a large influence on the HAZ toughness of the high-tensile steel sheet, and various countermeasures for improving the HAZ toughness have been proposed. . Among these, refining of the HAZ structure is most effective for improving the HAZ toughness, and numerous methods for refining the HAZ structure using inclusions have been proposed.

In refining the HAZ structure using inclusions, there is a method of suppressing the growth of crystal grains by the pinning effect of inclusions, and the generation of ferrite using the inclusions as nuclei in austenite grains coarsened by the heat effect during welding (grains). There is a way to refine the tissue by using my metamorphosis). Regarding the refining of the structure by intragranular transformation, a technique has been proposed in which a nitride such as TiN, a sulfide such as MnS, or an oxide chemically stable even at a high temperature is used as a ferrite formation site (nucleus).

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

In Patent Document 2, in the composition of inclusions having 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%, the amount of Al is 5 to 30%, and the amount of S is 0. A steel sheet with more than 20% % and less is described.

Patent Document 3 includes complex oxides containing REM, Zr, Ti, Al, Ca and S, and with respect to the complex oxides in steel, the number of oxides having an equivalent circle diameter of more than 3 µm is 5.0 or less per 1 mm2, In addition, with respect to the composite oxide having an equivalent circle diameter of 0.1 to 3 µm, the number of complex oxides satisfying the predetermined formula is 100 pieces/mm 2 or more, and the average composition of the composite oxide of 0.1 to 3 µm satisfying the predetermined formula This, Al ₂ O ₃ : 20% or less, TiO ₂ : 3 to 20%, ZrO ₂ : 5 to 50%, REM oxide: 5 to 50%, CaO: 5 to 50%, S: 1 to 15% of steel is described.

In Patent Document 4, oxides containing Zr, REM, and Ca are included, and among all inclusions contained in steel, 120 or more inclusions with a circle equivalent diameter of 0.1 to 2 µm per mm2 of observation field area, circle equivalent diameter There is described a steel material in which the number of oxides exceeding 3 µm is 5.0 or less per 1 mm 2 of the observation field area, and the component composition of the inclusions contained in the steel material satisfies the relationship of the following formula (1).

(Insol.Ti-3.4×Insol.N)/Insol.Al=1.0 to 8… (One)

In Patent Document 5, _{inclusions satisfying ZrO 2} : 5 to 50%, REM oxide: 5 to 50%, and CaO: 50% or less as an average composition, inclusions having an equivalent circle diameter of 0.1 to 2 µm are observation fields Oxides having an area of 120 or more per mm2 and an equivalent circle diameter of more than 3 µm are 5.0 or less per 1 mm2 of the observation field area, and oxides having an equivalent circle diameter of more than 5 µm are 5.0 or less per mm of an observation field area, all With respect to the number of inclusions, the number ratio of REM and Zr-containing inclusions I that satisfy the molar ratio of REM and Zr (REM/Zr) of 0.6 to 1.4 is 30% or more, and/or with respect to the total number of inclusions, REM and REM, Zr, Al, Ca and Ti-containing inclusions II in which the ratio of the total number of moles of Zr to the total number of moles of Al, Ca and Ti [(REM+Zr)/(Al+Ca+Ti)] satisfies 0.5 to 1.2 Steel materials having a number ratio of 40% or more are described.

However, although a certain HAZ toughness improvement effect is obtained with these techniques, it cannot be said that sufficient toughness is necessarily obtained in the HAZ of high heat input welding.

In addition, in steel materials used for structures such as ships, high-rise buildings, other buildings, bridges, offshore structures, LNG storage tanks, other large tanks, and line pipes, in order to suppress brittle fracture of structures, brittle fracture is propagated. In some cases, the arresting property (brittle fracture propagation stop function), which is the ability to suppress things, is required. In particular, in a high-strength thick steel sheet, it is desired to improve the arrestability.

However, in Patent Documents 1 to 5, arrestability was not considered.

Japanese Patent Laid-Open No. 2008-291347 Japanese Patent Laid-Open No. 2014-214371 Japanese Patent Laid-Open No. 2014-185364 Japanese Patent Laid-Open No. 2014-1432 Japanese Patent Laid-Open No. 2012-162797

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

In addition, the present invention has excellent HAZ toughness, in particular, has excellent toughness in HAZ of high heat input welding of 35 kJ/mm or more of heat input, has more excellent arresting properties, and provides a high-strength steel material and a method for manufacturing the same. do.

The inventors of the present invention earnestly studied, paying attention to Zr-containing oxides and B nitrides as intra-grain ferrite production sites for refining the structure in HAZ. As a result, mainly new knowledge of the following (A)-(E) was acquired.

(A) HAZ toughness tends to be improved, so that there is little Sol.Zr in steel, and it is preferable that Sol.Zr in steel shall be 0.0010 mass % or less. Here, Sol.Zr is acid-soluble Zr, and corresponds to Zr dissolved in steel, which can be measured by an electrolytic extraction residue analysis method or the like.

(B) By containing Zr and B, in steel, B nitride is precipitated with a Zr containing oxide as a nucleus. The (Zr, B)-containing oxide particles in which such B nitrides are deposited function more effectively as intra-particle ferrite formation sites, compared to the Zr-containing oxide. The (Zr, B) when obtaining the free oxide particles, the concentration of dissolved oxygen in the molten steel is less than 0.0050%, then the addition of Zr in the refining process, and then, by adding B, the B amount B _F employing a steel It is preferable to set it as 0.0030 mass % or less.

_{(C) When Al 2} O ₃ contained in the (Zr, B)-containing oxide particles is 50% by mass or less in the composition of the inclusion particles, the (Zr, B)-containing oxide particles are more effectively used as intra-particle ferrite formation sites function When the Al ₂ O ₃ higher than 50% by mass, to the concentration of dissolved oxygen in the molten steel is less than 0.0050%, then the addition of Zr in the refining process, and thereafter, continuous casting is preferable.

(D) Among the (Zr, B)-containing oxide particles, the number density of (Zr, B)-containing oxide particles having an equivalent circle diameter of 0.5 µm or more and an Al ₂ O _{3 composition of 50 mass% or less is 5 to 300 pieces} In the case of /mm 2 , fine and large amounts of intra-grain ferrite are generated in the HAZ, and the HAZ toughness is improved.

(E) When Al which acts as a strong deoxidizing element is contained in steel excessively, the production|generation of a Zr containing oxide will be inhibited. In order to ensure the amount of dissolved oxygen in molten steel and generate|occur|produce a Zr containing oxide in steel, it is preferable to make Al content into 0.010 mass % or less. Moreover, it is preferable to limit the element with a stronger deoxidation power than Al, such as Ca, Mg, and REM, to 0.0005 mass % or less in total.

In addition to the above (A) to (E), the present inventors control the microstructure and the grain boundary density in the plate thickness direction or the texture in the plate thickness direction in a direction parallel to the steel material surface, for example, in the rolling direction and It has been found that the arrestability in the vertical or parallel direction is improved.

This invention was completed based on the said knowledge, The summary is as follows.

[1] The steel material according to an aspect of the present invention has a chemical composition, in mass%, C: 0.040 to 0.160%, Si: 0.01 to 0.50%, Mn: 0.70 to 2.50%, P: 0.030% or less, S: 0.008% or less, Al: 0.010% or less, total of Ca, Mg and REM content: 0.0005% or less, N: 0.0010 to 0.0080%, O: 0.0005 to 0.0040%, Ti: 0.003 to 0.024%, Zr: 0.0007 to 0.0050 %, B: 0.0003 to 0.0040%, Cu: 0 to 1.00%, Ni: 0 to 2.50%, Cr: 0 to 1.00%, Mo: 0 to 0.50%, Nb: 0 to 0.050%, V: 0 to 0.150% , W: 0 to 1.00%, Sn: 0 to 0.50%, balance: Fe and impurity elements, Insol.Zr: 0.0007 to 0.0040%, Sol.Zr: 0.0010% or less, the following formulas (1) and ( _{B F} represented by 2) is 0.0030% or less, and (Zr, B) containing oxide particles containing 5.0 mass% or more Zr, 0.1 mass% or more B, and 1.0 mass% or more O, the equivalent circle diameter is 0.5 µm or more (Zr, B), and containing oxide particles, Al ₂ O ₃ composition is the number density of oxide particles containing 50 mass% or less (Zr, B) is from 5 to 300 / ㎟.

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

For B _F '>B, B _F =B, for 0≤B _F ' _{≤B B F} =B _F ', for B _F '<0 B _F =0 … (2)

However, in Formula (1) and Formula (2), N, Ti, O, and B are content by mass % of N, Ti, O, and B contained in steel, Insol.Zr is acid-insoluble Zr It is content by mass %.

[2] In the steel material according to [1], the chemical composition is, in mass%, Cu: 0.10 to 1.00%, Ni: 0.10 to 2.50%, Cr: 0.10 to 1.00%, Mo: 0.01 to 0.50%, Nb : 0.003 to 0.050%, V: 0.010 to 0.150%, W: 0.01 to 1.00%, and Sn: 0.03 to 0.50%, You may contain 1 type or 2 or more types selected from the group which consists of.

[3] The above-mentioned [1], the steel material as set forth in [2], wherein the chemical composition, in mass%, Nb: by containing 0.003 to 0.050%, and is the B _F is less than 0.0020%, the following formula (3) The indicated carbon equivalent Ceq is 0.30% to 0.55%, with ferrite of 5 to 70% by area ratio, bainite of 30% or more by area ratio, pearlite of 0 to 15% by area ratio, and 0 by area ratio It has a microstructure containing a martensite-austenite mixed structure of 5% to 5%, and a grain boundary density at a position of 1 to 5 mm from the surface is 500 to 1100 mm/mm 2 , and at a position of 1/4 part of the plate thickness. The grain boundary density in this is 400-1000 mm/mm<2>, and 300-900 mm/mm<2> may be sufficient as the grain boundary density in the position of 1/2 part of plate|board thickness.

Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15 … (3)

C, Mn, Cr, Mo, V, Cu, and Ni in Formula (3) are content (mass %) of each element contained in steel, When the said element is not contained, 0 is substituted.

[4] The steel material according to [1] or [2], wherein the chemical composition is in mass%, Nb: 0.003 to 0.050%, the B _F is 0.0020% or less, by the following formula (4) The indicated carbon equivalent Ceq is 0.30% to 0.55%, with ferrite of 5 to 70% by area ratio, bainite of 30% or more by area ratio, pearlite of 0 to 15% by area ratio, and 0 by area ratio It has a microstructure containing a martensite-austenite mixed structure of from 5% to 5%, and at a position of 1 to 5 mm from the surface of a vertical plane that is a plane perpendicular to the main rolling direction, the {110} plane is relative to the vertical plane The area ratio of the region forming an angle within 15° is 30 to 60%, and in 1/4 part of the plate thickness of the vertical plane, the area of the region where the {100} plane forms an angle within 15° with respect to the vertical plane The ratio is 10 to 40%, and in 1/2 part of the thickness of the vertical plane, the area ratio of the region in which the {110} plane forms an angle within 15° with respect to the vertical plane may be 40 to 70%.

Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15 … (4)

C, Mn, Cr, Mo, V, Cu, and Ni in Formula (4) are content (mass %) of each element contained in steel, When the said element is not contained, 0 is substituted.

[5] A method for manufacturing a steel material according to another aspect of the present invention is the method for manufacturing a steel material according to the above [1] or [2], wherein vacuum degassing is performed on the molten steel, and the dissolved oxygen concentration of the molten steel is 0.0050% A refining step of adding Zr after the following, and adding B after 1.0 to 5.0 minutes from Zr addition, and continuous casting to the molten steel after the refining step to obtain a cast steel, the surface temperature of the cast steel is 1200 ° C. A continuous casting process in which the average cooling rate until it becomes 900 ° C. is 0.5 ° C./sec or less, a heating process of heating the slab after the continuous casting process, and hot rolling the slab after the heating process to obtain a steel material A hot rolling step of

[6] The method for producing a steel material according to [5] is the method for producing a steel material according to [3], and in the heating step, the maximum temperature of the surface temperature of the slab in the ash furnace in the heating furnace is 950 to 1150 ° C. is heated to a range of, and the hot rolling step is a step of sequentially performing a rough rolling step, a finish rolling step, and a cooling step. 5) at the recrystallization temperature Trex (°C) or higher and 1050°C or lower, rolling at a cumulative reduction ratio of 10 to 75%, and when Ar ₃ is represented by the following formula (6), in the finish rolling process, The finish rolling temperature is set to (Ar ₃ -50)° C. or higher and the recrystallization temperature Trex (° C.) is less than the recrystallization temperature Trex (° C.), and rolling is performed under the condition of a cumulative reduction ratio of 45 to 75%, and in the cooling step, the cooling start temperature is (Ar ₃ - 100) ° C. or higher and lower than the recrystallization temperature Trex ( ° C ), the cooling stop temperature is set to 0 ° C or higher and 600 ° C or lower, and the average cooling rate from the start of cooling to the stop of cooling is 2 to 15 ° C / You may cool under the conditions used as a candle.

Trex(℃)=-91900×[Nb*] ² +9400×[Nb*]+770 … (5)

Ar ₃ (℃)=910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo … (6)

[Sol.Nb]=(10 ^{(-6770/(T+273)+2.26)} )/(C+12/14×N) … (7)

However, [Nb*] in Formula (5) is [Sol.Nb] represented by Formula (7), and when the relationship between Nb content (mass %) in steel is Nb≥[Sol.Nb], [ Let Nb*]=[Sol.Nb], let [Nb*]=Nb when Nb<[Sol.Nb],

The element symbols in formulas (6) to (7) are the content by mass% of each element contained in steel, and when the element is not contained, 0 is substituted;

T in the formula (7) is the temperature in unit °C of the slab at the time of extraction of the slab in the heating step.

[7] The method for producing a steel material according to [5] is the method for producing a steel material according to [4], and in the heating step, the total thickness average temperature of the cast steel when extracted with a heating furnace is 950 to 1200 ℃ in the range, and the hot rolling step is a step of sequentially performing a rough rolling step, a primary cooling step, a finish rolling step, and a secondary cooling step, in the rough rolling step, in the heating step The heated slab is rolled at a rolling temperature of not less than the recrystallization temperature Trex (° C.) and not more than 1050° C. shown in the following formula (8) at a cumulative reduction ratio of 10 to 75%, and Ar ₃ is represented by the following formula (9) In the primary cooling process, the cooling start temperature is in _{the range of Ar 3} ℃ or more and 1050 ℃ or less, and the cooling stop temperature is 500 ℃ or more and (Ar ₃ -30) ℃ or less in the range, Cooling is performed under the condition that the average cooling rate from the start of cooling to the stop of cooling is 35 to 100° C./sec. In the finish rolling process, the finish rolling temperature is 750 to 850° C., the number of rolling passes is 4 to 15 passes, and the rolling aspect ratio is Rolling is carried out under conditions such that the average value is 0.5 to 1.0 and the cumulative reduction ratio is 45 to 75%, and in the secondary cooling step, the cooling start temperature is set to (Ar ₃ -100)° C. or higher, represented by the following formula (8) The recrystallization temperature Trex (°C) is less than the range, the cooling stop temperature is in the range of 0°C or more and 600°C or less, and the cooling rate is 2 to 15°C/sec. do.

Trex=-91900×[Nb*] ² +9400×[Nb*]+770 … (8)

Ar ₃ (℃)=910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo … (9)

[Sol.Nb]=(10 ^{(-6770/(T+273)+2.26)} )/(C+12/14×N) … (10)

However, [Nb*] in Formula (8) is [Nb] when the relationship between [Sol.Nb] represented by Formula (10) and Nb content (mass %) in steel is Nb≥[Sol.Nb] Let *]=[Sol.Nb], and when Nb<[Sol.Nb], let [Nb*]=Nb,

The element symbols in formulas (9) to (10) are the content by mass% of each element contained in steel, and when the element is not contained, 0 is substituted;

T in the formula (10) is the temperature in unit °C of the slab at the time of extraction of the slab in the heating step.

[8] The method for manufacturing a steel material according to [6] or [7] may include a tempering step of heating the steel material to a range of 350 to 650°C after the hot rolling step.

According to the above aspect of the present invention, it is possible to provide a steel material having excellent HAZ toughness, particularly, excellent toughness in HAZ of high heat input welding of 35 kJ/mm or more of heat input, and a method for manufacturing the same. In addition, according to a preferred aspect of the present invention, it is possible to provide a steel material having excellent HAZ toughness, in particular, having excellent toughness in HAZ of high heat input welding of 35 kJ/mm or more of heat input, excellent arresting properties, and high strength, and a method for manufacturing the same. have.

It is known that Ti oxide and B nitride disperse|distribute to a weld metal or HAZ, and have the effect of refining|miniaturizing the structure. On the other hand, Zr is not an element that is generally added to steel materials, but studies conducted in the past have been very limited with respect to the effect by the inclusion of Zr.

In particular, until now, there has been no study on how the B nitride compound precipitated in addition to the Zr-containing oxide affects the refinement of the HAZ structure of the steel and the improvement of the HAZ toughness thereby. Further, the relationship between the composition of the Zr-containing oxide and the B nitride has not been studied either.

The present inventors focused on Zr-containing oxides and B nitrides as intra-grain ferrite production sites for HAZ structure refining, and conducted intensive studies. As a result, mainly new knowledge of the following (a)-(e) was acquired.

(a) In order to obtain a Zr-containing oxide contributing to the miniaturization of the HAZ structure at a predetermined number density or more, it is necessary to make the Zr content a certain amount or more. On the other hand, not all of Zr in the steel forms an oxide, but a part of Zr remains in the steel without forming an oxide. Zr (Sol.Zr) which does not form this oxide deteriorates not only HAZ but also the toughness of steel material itself remarkably. Therefore, in order to ensure HAZ toughness and toughness of steel material itself, it is necessary to reduce Sol.Zr in steel. The less Sol.Zr, the better the toughness tends to be. In order to obtain a steel material having excellent HAZ toughness, it is preferable to limit the Sol.Zr content to 0.0010% by mass or less. In order to further improve the HAZ toughness, it is preferable to limit the Sol.Zr content to 0.0003 mass% or less.

(b) In the steel in which the Zr-containing oxide is dispersed, even if the number of inclusions increases, not all inclusions function as ferrite formation sites, but there are inclusions functioning as ferrite formation sites and inclusions not functioning as formation sites it can be seen that

The present inventors studied various elements in order to more effectively promote ferrite production|generation. As a result, by containing B in a certain amount or more, during casting, hot rolling, or welding, B nitride is precipitated with the Zr-containing oxide as the nucleus, and (Zr, B)-containing oxide particles that are the composite precipitates are produced as ferrite in the particles. It was found that it functions more effectively as a site.

That is, the Zr-containing oxide, which cannot function as an intra-grain ferrite formation site by itself, also becomes a ferrite formation site by B nitride, and contributes to the miniaturization of the HAZ structure more efficiently. As a result of investigation by the present inventors, in order to obtain (Zr, B) containing oxide particles, Zr is added after the dissolved oxygen concentration of the molten steel becomes 0.0050% or less in the refining step, and then B is added to dissolve it in steel. the amount B _F B which it can be seen that it is preferred that less than 0.0030% by mass.

(c) In steel, Ti acts as a nitride-forming element in addition to B. Therefore, in order to efficiently precipitate B nitride, it is necessary to suppress the formation of Ti nitride. The present inventors investigated in order to clarify the production|generation mechanism of the inclusion containing an oxide and a nitride, and to clarify the conditions for producing|generating B nitride.

In molten steel containing Ti, Zr, and B, Zr, which has a stronger deoxidizing power than Ti, preferentially becomes an oxide, and the remaining oxygen (O) and Ti combine to form a composite oxide of Zr and Ti. Next, Ti remaining without forming an oxide is combined with nitrogen (N) to form a nitride. Subsequently, it is thought that nitrogen remaining without bonding with Ti forms B nitride.

Zr is ZrO ₂ , Ti is Ti ₂ O ₃ and TiN, and B is thought to form BN, respectively, so based on their atomic weight or molecular weight, using the following formula (A1), B (BasBN) as a B nitride content (mass %) of can be calculated|required. Then, as shown in the following formula (A2), the difference obtained by subtracting B used as B nitride from B contained in steel is taken as the calculated value (B _F ') of B dissolved in steel. _{Then, when the relationship between the calculated value B F} ' obtained by the following formula (A2) and the amount of B in the steel is B _F '>B, the amount of B in the steel is taken as the amount of B dissolved in the steel (B _F ) ( B _F =B). In addition, when 0≤B _{F '≤B, the calculated value B F} ' calculated by the following formula (A2) is defined as the amount of B _{dissolved in the steel (B F} ) (B _F =B _F '). In addition, in _{the case of B F} '<0, the amount of B _{dissolved in} steel (B F ) is set to 0 mass % (B _F =0). By the B _F obtained in this way to less than 0.0030% by mass, it is considered the HAZ toughness improvement effect by the B nitride be obtained.

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 steel, and Insol.Zr is the content (mass %) of acid-insoluble Zr .

In addition, B in formula (A2) is B content (mass %) contained in steel, and BasBN is a value calculated|required by formula (A1).

B _F is steel obtained by hot-rolling a billet having a component that is less than 0.0030 mass%, the dispersion containing the fine Zr oxide (mainly a composite oxide containing Zr and Ti). In addition, B nitride is further precipitated in some Zr-containing oxides.

The B nitride is re-dissolved when heated to a temperature range of over 1200°C during welding, but the Zr-containing oxide is stably present even when heated to 1400°C. Therefore, during heating of welding, B nitride is dissolved in solid solution, and solid solution B is localized around the Zr-containing oxide. It is thought that this solid solution B is re-precipitated as B nitride which has an oxide as a nucleus in the cooling process after welding.

(d) In addition, in order to facilitate the efficient precipitation of B nitride on 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 50 mass % or less, the B nitride is more efficiently precipitated, and it functions even more effectively as an intra-grain ferrite formation site.

(e) Moreover, since Al acts as a strong deoxidizing element in steel, when it contains in steel abundantly, it inhibits the oxide production|generation of Zr and Ti. In order to ensure the amount of dissolved oxygen in molten steel and generate|occur|produce a Zr containing oxide in steel, it is preferable to make Al content into 0.010 mass % or less. More preferably, Al content shall be 0.005 mass % or less. The amount of deoxidation elements stronger than Al, such as Ca, Mg, and REM, is preferably 0.0005 mass% or less in total.

In the steel material satisfying these conditions, (Zr, B) containing oxide particles of a predetermined size are produced so as 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 the nucleus, and the Al ₂ O ₃ composition (Al _{2 in the composition)} proportion of O ₃₎ may be less than 50% by mass. And, when high heat input welding was actually performed on this steel, it was found that the oxide particles effectively function as intra-grain ferrite production sites in the HAZ, and improve the HAZ toughness through miniaturization of the HAZ structure.

Moreover, in the steel materials which produced|generated the above-mentioned (Zr, B) containing oxide particle, in order to improve the restraint property of steel materials, the present inventors earnestly examined. As a result, mainly the following (f)-(g) new knowledge was acquired.

(f) When B _F shall be 0.0020 mass % or less, arrest property can be improved by refinement|miniaturization of the structure|tissue of steel materials.

(g) in addition to the chemical composition and Zr-containing oxide of the steel material, by controlling the microstructure and the grain boundary density in the sheet thickness direction or the texture in the sheet thickness direction, in a direction parallel to the steel material surface, for example perpendicular to the rolling direction, or The arrestability of the parallel direction can be improved.

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

The steel material according to the present embodiment, in mass%, C: 0.040 to 0.160%, Si: 0.01 to 0.50%, Mn: 0.70 to 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%, Ti: 0.003 to 0.024%, Zr: 0.0007 to 0.0050%, B: 0.0003 to 0.0040%, sum of Ca, Mg and REM content: 0.0005% or less If necessary, Cu: 1.00% or less, Ni: 2.50% or less, Cr: 1.00% or less, Mo: 0.50% or less, Nb: 0.050% or less, V: 0.150% or less, W: 1.00% or less, and Sn : contains at least one selected from the group consisting of 0.50% or less, the balance: consists of Fe and impurity elements, Insol.Zr: 0.0007 to 0.0040%, Sol.Zr: 0.0010% or less, the following formula (B1) _{And B F} represented by (B2) is 0.0030% or less, and (Zr, B) containing oxide particles containing 5.0 mass% or more Zr, 0.1 mass% or more B, and 1.0 mass% or more O, the equivalent circle diameter is 0.5 ㎛ or more and containing oxide particles _{(Zr, B), Al 2} O 3 composition is the number density of oxide particles containing (Zr, B) 50% by mass or less is from 5 to 300 / ㎟.

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

For B _F '>B, B _F =B, for 0≤B _F ' _{≤B B F} =B _F ', for B _F '<0 B _F =0 … (B2)

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

First, the chemical composition of the steel which concerns on this embodiment is demonstrated. In the description of the chemical composition below, "mass %" regarding the content of each element is expressed as "%".

C: 0.040 to 0.160%

C is an effective element in order to improve the intensity|strength and toughness of steel materials. In order to acquire this effect, C content shall be 0.040 % or more. C content becomes like this. Preferably it is 0.050 % or more, More preferably, it is 0.060 % or more. On the other hand, when the C content exceeds 0.160%, it becomes difficult to ensure good HAZ toughness, so the C content is made 0.160% or less. C content becomes like this. Preferably it is 0.140 % or less, More preferably, it is 0.120 % or less.

Si: 0.01 to 0.50%

Si is an element effective as a deoxidation element and a strengthening element. In order to acquire this effect, Si content shall be 0.01 % or more. Si content becomes like this. Preferably it is 0.03 % or more, More preferably, it is 0.05 % or more.

On the other hand, when the Si content exceeds 0.50%, the HAZ toughness greatly deteriorates, so the Si content is made 0.50% or less. Si content becomes like this. Preferably it is 0.40 % or less, More preferably, it is 0.35 % or less or 0.30 % or less.

Mn: 0.70 to 2.50%

Mn is an effective element in order to economically improve the strength and toughness of steel materials. In order to acquire this effect, Mn content is made into 0.70 % or more. Mn content becomes like this. Preferably it is 0.90 % or more, More preferably, it is 1.20 % or more.

On the other hand, when Mn content exceeds 2.50 %, center segregation becomes remarkable, and the toughness of the steel material and HAZ in the part which center segregation generate|occur|produced deteriorates. Therefore, the Mn content is set to 2.50% or less. Mn content becomes like this. Preferably it is 2.00 % or less, More preferably, it is 1.80 % or less or 1.60 % or less.

P: 0.030% or less

P is an element which exists in steel as an impurity. In order to ensure HAZ toughness stably, the P content is made into 0.030% or less. Preferably, it is 0.020 % or less, More preferably, it is 0.015 % or less. Although the lower limit is 0%, in consideration of the cost for reducing the P content, the P content may be 0.0001% or more.

S: 0.008% or less

S is an element present in steel as an impurity. When the S content exceeds 0.008%, a large amount of stretched MnS is generated in the central segregation portion, and the toughness and ductility of the steel and HAZ deteriorate. For this reason, the S content is made 0.008% or less. Preferably it is 0.005 % or less. Since it is so preferable that the S content is small, the lower limit is not particularly specified and may be 0%. However, from the viewpoint of manufacturing cost, the S content may be 0.0001% or more.

Al: 0.010% or less

Al is generally an element actively added as a deoxidation element. However, since Al preferentially reacts with oxygen, the formation of desired (Zr, B)-containing oxide particles becomes insufficient when the content thereof is excessive. In this case, the effective ferrite production site in HAZ decreases. In addition, when the Al content is in excess, are promoting the formation of inclusion-based alumina (Al ₂ O ₃₎ in the cluster coarse, the steel material and the HAZ toughness is deteriorated. Therefore, it is preferable to reduce the Al content as much as possible. The allowable Al content is 0.010% or less. The Al content is preferably 0.005% or less. The lower limit of the Al content is not particularly limited, but may be 0.0005% or more or 0.001% or more.

Sum of Ca, Mg and REM: 0.0005% or less

Ca, Mg, and REM are elements more likely to react with oxygen more preferentially than Al. In order to form a desired (Zr, B)-containing oxide, the total content of Ca, Mg and REM is made 0.0005% or less. 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 thereof is 0.0005% or less.

N: 0.0010 to 0.0080%

N is an important element in the steel material which concerns on this embodiment. In the steel material according to the present embodiment, in the manufacturing process, Ti nitride is formed in order to suppress an increase in the austenite grain size at the time of heating the steel piece. In the steel materials according to the present embodiment, in order to form this Ti nitride, the N content is made 0.0010% or more. N content becomes like this. Preferably it is 0.0015 % or more, More preferably, it is 0.0020 % or more.

On the other hand, when the N content exceeds 0.0080%, the steel materials become brittle. Therefore, the N content is made 0.0080% or less. N content becomes like this. Preferably it is 0.0065 % or less, More preferably, it is 0.0060 % or less.

O: 0.0005 to 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 content in the present invention is the total oxygen content (also referred to as T. O) in the present invention. When the O content in the steel is less than 0.0005%, the number of oxide dispersions necessary for securing toughness cannot be obtained. Therefore, the O content is made 0.0005% or more.

On the other hand, when the O content exceeds 0.0040%, the cleanliness of the molten steel is deteriorated and productivity such as nozzle clogging in the molten steel step may be reduced. For this reason, O content in steel materials shall be 0.0040 % or less.

In addition, when dissolved oxygen is contained in the molten steel before adding Zr in the steel refining step in excess of 0.0050%, the _{amount of ZrO 2} generated by the addition of Zr increases, and the tundish at the time of continuous casting of molten steel The risk of blockage of the injection nozzle for In addition, there is a case that molten steel is high, the dissolved oxygen prior to addition of Zr, the increase ratio of (Zr, B) containing Al ₂ O ₃ in the oxide particles. Therefore, it is preferable to reduce dissolved oxygen to 0.0050% or less before adding Zr in the molten steel step.

Ti: 0.003 to 0.024%

Ti is an element that forms (Zr, B)-containing oxide particles together with Zr. These (Zr, B)-containing oxide particles function as intra-grain ferrite production sites in the HAZ, and contribute to refinement of the HAZ structure. In order to acquire this effect, Ti content shall be 0.003 % or more. The Ti content is preferably 0.005% or more.

On the other hand, Ti forms nitride. When Ti nitride is produced in a large amount, the amount of B nitride produced is suppressed, and the desired effect cannot be obtained in the present embodiment. In addition, excess Ti forms TiC and deteriorates the toughness of steel materials and HAZ. Therefore, the Ti content is made 0.024% or less. Ti content becomes like this. Preferably it is 0.020 % or less.

Zr: 0.0007 to 0.0050%

Zr content contained in steel materials is the sum total of content of Sol.Zr and Insol.Zr mentioned later. Zr content is 0.0007 % or more, Preferably it is 0.0010 % or more. The Zr content is the sum of the upper limit of Insol.Zr and the upper limit of Sol.Zr, that is, 0.0050% or less, and preferably 0.0040% or less.

Sol.Zr: 0.0010% or less

Sol.Zr represents acid-soluble Zr, ie, Zr dissolved in steel. As the Sol.Zr content increases, the HAZ toughness deteriorates remarkably. Therefore, the content is made 0.0010% or less. Since it is so preferable that there is little Sol.Zr content, a minimum is not specifically prescribed|regulated, and 0 % may be sufficient.

Insol.Zr: 0.0007 to 0.0040%

Insol.Zr is acid-insoluble Zr, and is Zr contained in inclusions, such as (Zr, B) containing oxide particle. Zr is an important element for forming oxides that become nuclei of intragranular transformation. However, when Insol.Zr is less than 0.0007%, the oxide composition necessary for securing toughness is not obtained. Therefore, the Insol.Zr content is made 0.0007% or more.

On the other hand, when the Insol.Zr content exceeds 0.0040%, most of it is ZrO ₂ generated in the molten steel step, and the frequency of nozzle clogging increases. For this reason, Insol.Zr in steel materials shall be 0.0040 % or less.

In the molten steel stage, there is no particular limitation on the content of Sol.Zr and Insol.Zr, but when Zr is added in excess with respect to dissolved oxygen, in addition to a large amount of Sol.Zr remaining until the steel, the dissolved oxygen concentration is lowered (Zr , B) The number density of the containing oxide particles is lowered. For this reason, it is preferable that the Sol.Zr content in the molten steel step is 0.0020% or less. Moreover, in order not to generate|occur|produce nozzle blockage, it is preferable that Insol.Zr content in a molten steel step is 0.0020 % or less.

The content of Insol.Zr and Sol.Zr described above can be measured by an electrolytic extraction residue analysis method. In the electrolytic extraction residue analysis method, the mother phase is dissolved by electrolysis of steel in a non-aqueous solvent (acetylacetone-methanol solution, etc.), and the residue (precipitates and inclusions) is extracted and separated with a filter having a pore diameter of 0.2 µm. After separation, the amount of Zr contained in the solution is the Sol.Zr content, and the amount of Zr contained in the residue is the Insol.Zr content.

B: 0.0003 to 0.0040%

B, while improving the hardenability of the steel, precipitates as BN around the Zr-containing oxide to form (Zr, B)-containing oxide particles, and the intra-particle transformation ability of the (Zr, B)-containing oxide particles element to improve. In order to precipitate as BN around the Zr-containing oxide, it is necessary to contain at least 0.0003% of B.

On the other hand, since the effect is saturated even if the B content exceeds 0.0040%, the B content is made 0.0040% or less.

In order to make B content in steel materials into the said range, it is preferable that B content is also in a molten steel step that it is the range of 0.0003 to 0.0040%.

The steel material according to the present embodiment contains each of the above elements, and the balance is based on Fe and impurities. An impurity is a component mixed from raw materials, such as ore, scrap, or other factors when manufacturing steel materials industrially, and means that it is permissible in the range which does not adversely affect a characteristic.

However, in the steel material according to the present embodiment, instead of a part of Fe, for the purpose of further increasing the strength, one or two or more selected from the group consisting of Cu, Ni, Cr, Mo, Nb, and V are described later. may be included in Moreover, in order to improve corrosion resistance, you may contain 1 type or 2 types selected from the group which consists of W and Sn in the range mentioned later. Since Cu, Ni, Cr, Mo, Nb, V, W and Sn are not essential elements, the lower limit of these elements is 0%. Hereinafter, preferable content of these elements is demonstrated.

The steel material according to the present embodiment, in mass%, Cu: 1.00% or less, Ni: 2.50% or less, Cr: 1.00% or less, Mo: 0.50% or less, Nb: 0.050% or less, V: a group consisting of 0.150% or less You may further contain 1 type(s) or 2 or more types selected from.

Cu: 0 to 1.00%

By containing Cu, since the intensity|strength and toughness of steel materials can be improved, you may contain it. In order to obtain the effect of containing Cu stably, it is good also considering Cu content as 0.10 % or more. In order to improve the strength and toughness of steel materials, the Cu content may be 0.10% or more or 0.20% or more.

On the other hand, when there is too much Cu content, the improvement of the performance suitable for an alloy cost increase is not seen but it may rather become a cause of steel material surface cracks. Therefore, Cu content shall be 1.00 % or less. In addition, for the improvement of HAZ toughness and weldability, Cu content is good also as 0.90 % or less, 0.80 % or less, 0.50 % or less, or 0.30 % or less as needed.

Ni: 0 to 2.50%

Since Ni is an element which has the effect of improving the intensity|strength of steel, you may contain it. Moreover, Ni is an element which has the effect of raising the toughness of the matrix (dough) of steel in a solid solution state. In order to acquire these effects, it is preferable to make Ni content into 0.10 % or more. In order to improve the strength and toughness of steel materials, the Ni content may be 0.20% or more.

On the other hand, when there is too much Ni content, HAZ toughness and weldability will deteriorate. Therefore, the Ni content is made 2.50% or less. Ni content is good also as 2.00 % or less, 1.00 % or less, 0.50 % or less, or 0.30 % or less as needed.

Cr: 0 to 1.00%

By containing Cr, since the intensity|strength and toughness of steel materials can be improved, you may contain it. In order to acquire the effect of containing Cr stably, it is good also considering Cr content as 0.10 % or more or 0.20 % or more.

On the other hand, when there is too much Cr content, HAZ toughness and weldability will deteriorate. Therefore, the Cr content is made 1.00% or less. Cr content is good also as 1.00 % or less, 0.80 % or less, 0.50 % or less, or 0.30 % or less as needed.

Mo: 0 to 0.50%

By containing Mo, since the intensity|strength and toughness of steel materials can be improved, you may contain it. In order to obtain the effect of containing Mo stably, it is good also considering Mo content as 0.01 % or more or 0.02 % or more.

On the other hand, when there is too much Mo content, HAZ toughness and weldability will deteriorate. Therefore, Mo content shall be 0.50 % or less. Mo content is good also as 0.40 % or less, 0.30 % or less, 0.20 % or less, or 0.10 % or less as needed.

Nb: 0 to 0.050%

Nb is an element capable of improving the strength and toughness of steel materials. In addition, Nb is an effective element in order to expand a non-recrystallization temperature range when rolling in a non-recrystallization austenite region is required in order to form a predetermined grain boundary density and texture. Moreover, Nb raises a rolling temperature and contributes also to productivity improvement. Therefore, you may make it contain. In order to acquire these effects, it is preferable to make Nb content into 0.003 % or more. Nb content becomes like this. Preferably it is 0.005 % or more, More preferably, it is 0.008 % or more.

On the other hand, when Nb content exceeds 0.050 %, HAZ toughness and weldability will fall. Therefore, the Nb content is made 0.050% or less. Nb content becomes like this. Preferably it is 0.025 % or less, More preferably, it is 0.018 % or less.

V: 0 to 0.150%

V is an element capable of improving the strength and toughness of steel materials. Therefore, you may make it contain. In order to obtain the effect of containing V stably, it is good also considering V content as 0.010 % or more or 0.020 % or more.

On the other hand, when there is too much V content, HAZ toughness and weldability will deteriorate. Therefore, the V content is made 0.150% or less. V content is good also as 0.100 % or less, 0.070 % or less, or 0.050 % or less as needed.

The steel materials concerning this embodiment may contain 1 type or 2 types further among W: 1.00 % or less and Sn: 0.50 % or less in mass %.

W: 0 to 1.00%

W is an element which dissolves and is _{adsorbed to rust in the form of oxygenate ions WO 4} ⁻ , suppresses permeation of chloride ions in the rust layer, and improves corrosion resistance. Therefore, you may make it contain. In order to acquire this effect, it is preferable to make W content into 0.01 % or more.

On the other hand, when W content exceeds 1.00 %, not only the said effect is saturated, but the toughness of steel materials and HAZ may fall. Therefore, even when it is made to contain, W content shall be 1.00 % or less. Preferably, the W content is set to 0.75% or less.

Sn: 0 to 0.50%

Sn is dissolved to become Sn ²⁺ and is an element having an action of suppressing corrosion by the inhibitor action in the acid chloride solution. In addition, Sn has the effect of suppressing the anode dissolution reaction of the steel and improving the corrosion resistance. Therefore, you may make it contain. In order to acquire these effects, it is preferable to make Sn content into 0.03 % or more.

On the other hand, when Sn content exceeds 0.50 %, the effect will not only be saturated, but it will become easy to generate|occur|produce the rolling crack of steel materials. For this reason, even when containing Sn, the content is made into 0.50 % or less.

Moreover, after making content of each element into the said range, it is preferable that the carbon equivalent Ceq. represented by following formula (D) is 0.30 % - 0.55 % of steel materials concerning this embodiment.

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

However, C, Mn, Cr, Mo, V, Cu, and Ni in Formula (D) are content (mass %) of each element contained in steel materials, and when the said element is not contained, 0 is substituted.

When carbon equivalent Ceq. is 0.30 % or more, the intensity|strength and arrestability calculated|required of steel materials can be improved. Therefore, the carbon equivalent Ceq. is preferably 0.30% or more. Carbon equivalent Ceq. becomes like this. More preferably, it is 0.32 % or more, More preferably, it is 0.34 % or more, More preferably, it is 0.36 % or more.

Moreover, more excellent HAZ toughness can be ensured as carbon equivalent Ceq. is 0.55 % or less. Therefore, the carbon equivalent Ceq. is preferably 0.55% or less. Carbon equivalent Ceq. becomes like this. More preferably, it is 0.50 % or less, More preferably, it is 0.45 % or less, More preferably, it is 0.40 % or less.

Steels according to the present embodiment, in the case of improving the HAZ toughness, which controls the contents of the individual elements as described above, and then the following formula (C1) and the B _F, derived from (C2), 0.0030% or less is necessary do. Further, in the case where with the HAZ toughness, improve eoreseuteu sex, _F B is preferably not more than 0.0020%. Wherein, B is _F, a B content which is present as B employed in the steel. Hereinafter, the reason is demonstrated.

B _F '=B-[N-{Ti-(O-Insol.Zr×(32/91.224))×(95.734/48)}×(14/47.867)]×(10.811/14) … (C1)

For B _F '>B, B _F =B, for 0≤B _F ' _{≤B B F} =B _F ', for B _F '<0 B _F =0 … (C2)

However, in formulas (C1) and (C2), N, Ti, O, and B are the contents by mass% of N, Ti, O, and B contained in steel, and Insol.Zr is the amount of acid-insoluble Zr. It is content by mass %.

As described above, in the steel material according to the present embodiment, by depositing the B nitride on the surface layer of the (Zr, B)-containing oxide particles, the generation of intragranular ferrite during cooling after welding can be promoted more effectively, and the structure can be refined Thus, the HAZ toughness can be improved. In order to acquire this effect, the B content present as solid solution B, ie, B _F derived from the above formulas (C1) and (C2), needs to be 0.0030% or less. When B _F exceeds 0.0030%, the B nitride precipitated on the surface layer of the (Zr, B)-containing oxide particles decreases, the generation of ferrite in the particles becomes insufficient, the structure is not refined, and the HAZ toughness decreases. Moreover, the quenching property of steel materials becomes excessive, and it becomes a cause of low-temperature crack generation in a welding part. Therefore, more preferable B _F is 0.0020% or less.

In addition, there are cases when it is more than 0.0020% B is _F, ?? chingseong of the steel is excessive, since coarse mad excessive hardness of the bainite occur, it does not get sufficient eoreseuteu sex. Therefore, in the viewpoint eoreseuteu, _F B is preferably 0.0020% or less, more preferably 0.0010% or less.

Next, the (Zr, B) containing oxide particle which the steel material which concerns on this embodiment has is demonstrated.

The (Zr, B) containing oxide particle containing 5.0 mass % or more of Zr, 0.1 mass % or more, and 1.0 mass % or more of O is contained in the steel material which concerns on this embodiment. Among these, a circle equivalent diameter of more than 0.5㎛, Al ₂ O ₃ composition is not more than 50% by mass, the number density of oxide particles containing (Zr, B) needs to be 5 to 300 / ㎟.

In the steel material according to the present embodiment, (Zr, B)-containing oxide particles, which are composite inclusions in which B nitrides are precipitated, are formed by using the Zr-containing oxide as the nucleus. This composite inclusion becomes an intragranular ferrite formation site at the time of cooling after welding. Zr-containing oxides are mainly oxides containing Zr and Ti. However, in the case of B nitride precipitation nuclei, it is preferable that the Zr concentration by mass% in the oxide is equal to or higher than the Ti concentration.

In the present embodiment, (Zr, B)-containing oxide particles containing 5.0 mass% or more of Zr, 0.1 mass% or more of B, and 1.0 mass% or more of O among the (Zr, B)-containing oxide particles, Al ₂ O ₃ The number density is prescribed|regulated with a composition making object of 50 mass % or less.

The (Zr, B)-containing oxide particles having such a composition can function as a production site of intra-particle ferrite, and can form more intra-particle ferrite. Oxide particles in which the concentration of Zr, B, or O deviates from the preferred range cannot sufficiently achieve the function as a ferrite production site in the particle. In this embodiment, although it is not necessary to prescribe|regulate in particular the amount of Ti in (Zr, B) containing oxide particle, 1.0 mass % or more of Ti may be contained.

Further, in the (Zr, B)-containing oxide particles, when the Al ₂ O ₃ composition in the (Zr, B)-containing oxide particles is 50 mass% or less, it can function more effectively as a ferrite production site in the particles, and many particles I can form my ferrite.

Further, 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, more intra-grain ferrite is precipitated effect is obtained. In order for the (Zr, B)-containing oxide particles to function as intra-particle ferrite formation sites, the larger equivalent circle diameter is preferable, and therefore the upper limit is not limited. However, when the equivalent circle diameter is increased, the number density of the (Zr, B)-containing oxide particles is relatively decreased, and the possibility that the coarse oxide particles themselves act as a starting point of destruction increases. Therefore, the equivalent circle diameter of the (Zr, B)-containing oxide particles is preferably 10.0 µm or less.

In addition, as a condition in which (Zr, B)-containing oxide particles act as intra-grain ferrite production sites, one or more (Zr, B)-containing oxide particles are present (dispersed) in the austenite particles when heated during welding. ) is preferable. For this reason, the equivalent circle diameter is 0.5 µm or more, contains 5.0 mass% or more of Zr, 0.1 mass% or more of B, and 1.0 mass% or more of O, and _{the composition of Al 2} O ₃ is 50 mass% or less (Zr, B ) containing oxide particles are dispersed at a number density of 5 pieces/mm 2 or more. The number density of such (Zr, B)-containing oxide particles is preferable because the number of ferrite formation sites increases as the number density increases, but the effect is saturated even when dispersed in excess of 300 particles/mm 2 . Therefore, the upper limit is 300 pieces/mm 2 . In particular, (Zr, B) containing oxide particles having a composition of _{Al 2} O ₃ of 50% by mass or less according to the present embodiment have a high ability to form ferrite in the particles. Therefore, the number density of the (Zr, B)-containing oxide particles according to the present embodiment is sufficient even if the number density is small compared to the (Zr, B)-containing oxide particles in which the _{Al 2} O _{3 composition exceeds 50 mass%.} effect can be exerted.

The circle equivalent diameter and number density of the (Zr, B) containing oxide particles containing a predetermined element can be measured by observing the mirror-polished steel surface with a scanning electron microscope (SEM). Specifically, the number of oxide particles containing (Zr, B) having a circle equivalent diameter of 0.5 µm or more in a range of 10 mm × 10 mm (100 mm 2 ) is measured by SEM, and divided by the area of the observed field of view. Measure the number density. A photograph taken by SEM may be used. Particles to be measured for number density have an equivalent circle diameter of 0.5 µm or more, and by quantitative analysis by an energy dispersive X-ray analyzer (EDX) attached to SEM, Zr of 5.0 mass% or more and 0.1 mass% or more comprises a B or more and 1.0 mass% of O, Further, the content of Al ₂ O ₃ in the composition shall be not more than 50 mass% based on the particles.

Next, the microstructure of the steel material which concerns on this embodiment is demonstrated.

The steel material according to the present embodiment has a structure composed of ferrite and bainite, or a structure composed of ferrite, bainite and pearlite, or a structure composed of a mixed structure of ferrite, bainite and martensite and austenite, or ferrite, bainite, It is a structure which consists of a pearlite and a martensite-austenite mixed structure, and it is preferable to have a microstructure with a ferrite area ratio of 5-70 % and a bainite area ratio of 30 % or more.

When the ferrite area ratio in the microstructure is more than 70%, it is difficult to obtain a steel material having a thick plate thickness and high strength. Moreover, when the area ratio of ferrite is less than 5 %, sufficient grain boundary density cannot be ensured. As for the structure other than ferrite, if a predetermined bainite, bainite and pearlite, or a bainite and martensite/austenite mixed structure, or a bainite, pearlite, and martensite/austenite mixed structure can be formed, a desired plate It is possible to obtain steel materials of thickness, strength, and grain boundary density. When making thick high-strength steel as object, the ferrite area ratio may be less than 50%, less than 30%, less than 20%, or less than 10%.

When the bainite area ratio is less than 30%, it is difficult to obtain a steel material having a thick sheet thickness and high strength. In order to ensure the ferrite area ratio and to increase the grain boundary which becomes an obstacle to brittle crack propagation, it is preferable that the bainite area ratio shall be 95 % or less. As for the area ratio of bainite, 90 % or less is more preferable. Moreover, when making thick high strength steel into object, it is preferable to make the bainite area ratio into 50 % or more, 60 % or more, 70 % or more, or 80 % or more.

You may contain pearlite, if the steel materials of a desired plate|board thickness and intensity|strength can be obtained. When the pearlite area ratio exceeds 15%, sufficient strength may not be obtained. Therefore, it is good also considering the pearlite area ratio as 15 % or less, 10 % or less, 5 % or less, or 3 % or less. The lower limit of the pearlite area ratio is 0%.

In addition to ferrite, pearlite, and bainite, a martensite-austenite mixed structure (MA) may be present. However, when the martensite-austenite mixed structure exists excessively, the arrestability is remarkably reduced as a brittle phase. Therefore, the area ratio of the martensite-austenite mixed structure is set to 5% or less. The area ratio of the martensite-austenite mixed structure may be limited to 3% or less, 2% or less, or 1% or less, and 0% is most preferable.

The phase fraction (area ratio) of the microstructure is a microstructure at a magnification of 500 times the 1/2 part of the plate thickness (the position of 1/2 of the plate thickness in the plate thickness direction from the surface of the steel material) by an optical microscope. It image|photographs, and calculates|requires each area of a ferrite, bainite, a pearlite, and a martensite-austenite mixed structure by image analysis, and divides|requires by a measurement area.

Moreover, in the steel material which concerns on this embodiment, the crystal grain boundary density in the position of 1-5 mm from the surface shall be 500-1100 mm/mm<2>, and the grain boundary density in 1/4 part of plate thickness shall be 400-1000 mm. It is preferable to set it as /mm<2> and to make the grain boundary density in 1/2 part of plate|board thickness into 300-900 mm/mm<2>.

The dominant factor in the improvement of the arresting property is a large contribution of the grain boundary. This is because the grain boundary becomes an obstacle to brittle crack propagation. That is, in the grain boundary, since the crystal orientation is different between adjacent grains, the direction in which the crack propagates in this portion changes. For this reason, an unbreakable area|region generate|occur|produces, the stress is disperse|distributed by an unbreakable area|region, and it becomes a crack closure stress. Therefore, the driving force of crack propagation falls, and arrestability improves. Moreover, since an unbroken area|region finally ductile fractures, the energy required for a brittle fracture is absorbed. For this reason, arrestability improves.

Conventionally, it was thought that it was necessary to make a crystal grain size fine in order to increase a crystal grain boundary. In the structure where ferrite is the main component, that is true, but in high-strength steel with a thick plate, the use of bainite is indispensable. Unlike ferrite, this bainite has a complicated underlying structure, so it is very difficult to define grains. For this reason, even if the relationship between the crystal grain size and the arresting property was calculated in terms of the equivalent circle diameter, the fluctuation was large, and it was difficult to determine the crystal grain size required for improving the arresting property. So, returning to the basic principle that grain boundaries are an obstacle to crack propagation, defining the total length of grain boundaries per unit area (hereinafter referred to as 'grain boundary density'), and using it to organize the relationship between arrestability, the best correlation is found out it was good

Then, in the steel materials concerning this embodiment, when improving arrestability,

(A) the grain boundary density at a position of 1 to 5 mm (a position of 1 to 5 mm from the surface) on the plate surface in the plate thickness direction is 500 to 1100 mm/mm 2 ,

(B) the grain boundary density in 1/4 part of the plate thickness is 400 to 1000 mm / mm 2 ,

(C) It is preferable that the grain boundary density in 1/2 part of plate|board thickness shall be 300-900 mm/mm<2>.

Here, the "crystal grain boundary density" means "the total length of the total length of the grain boundaries per measurement area in which the crystal orientation is measured, when a boundary with a crystal orientation difference of 15° or more is defined as a grain boundary." The reason that the grain boundary is a boundary with a grain orientation difference of 15° or more is that at a boundary of less than 15°, it is difficult to become an obstacle to brittle crack propagation, and even if a boundary with a crystal orientation difference of less than 15° is increased, sufficient restraint improvement effect is not obtained. am.

When the grain boundary density satisfies the requirements of 500, 400, and 300 mm/mm 2 or more at a position of 1 to 5 mm from the surface and at 1/4 part and 1/2 part of the plate thickness, respectively, the arrestor at -10°C A toughness value (Kca _-10°C ) of ^{6000 N·mm 1.5} or more exhibits high arresting properties. In addition, in order to stably improve the arresting property, it is preferable that the grain boundary density be 600, 500, 400 mm/mm 2 or more at a position of 1 to 5 mm from the surface and at 1/4 part and 1/2 part of the plate thickness, respectively. or more preferably 700, 600, or 500 mm/mm 2 or more, respectively.

As the grain boundary density increases, the arresting property is improved, but excessively increasing the grain boundary density increases the rolling load and reduces the productivity. Therefore, it is preferable that a grain boundary density sets it as 1100, 1000, and 900 mm/mm<2> or less, respectively at the position of 1-5 mm from the surface, and 1/4 part and 1/2 part of plate|board thickness. It is good also as 1000, 900, 800 mm/mm<2> or less, respectively, or 900, 800, and 700 mm/mm<2> or less, respectively.

The reason why the grain boundary density is defined at a position of 1 to 5 mm from the surface and at 1/4 part and 1/2 part of the plate thickness is that it is necessary to increase the grain boundary density of the entire plate thickness in order to improve the arrestability of the ultra-thick material, It is because it can be set as the representative value of the grain boundary density of an average plate|board thickness by controlling the position of 1-5 mm from the surface, and 1/4 part and 1/2 part of plate|board thickness. According to the manufacturing method described later, which mainly controls the grain boundary density of 1/2 part of the plate thickness, in other plate thickness positions, the temperature is inevitably low, the cooling rate becomes large, and the grain boundary density tends to increase, so a special value I don't think it's necessary to limit it. However, depending on the heating method, a large temperature gradient is generated in the plate thickness direction, and the grain boundary density of 1/4 part and 1/2 part of the plate thickness is reversed in some cases, so the numerical value is daringly prescribed.

For measurement of grain boundary density, it is preferable to use the EBSD (Electron Back Scatter Diffraction pattern) method which can measure crystal orientation information with high precision in a wide field of view. Using the EBSD method, it is also possible to identify grain boundaries of complex structures such as bainite.

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

Moreover, in the steel materials which concern on this embodiment, also by having a predetermined texture instead of the grain boundary density mentioned above, arrestability can be improved. Specifically, at a position of 1 to 5 mm (a position of 1 to 5 mm from the surface) of the vertical plane in the plate thickness direction from the plate surface, the {110} plane of the area forming an angle within 15° with respect to the vertical plane The area ratio is 30 to 60%, and in 1/4 part of the thickness of the vertical plane, the area ratio of the region where the {100} plane forms an angle within 15° with respect to the vertical plane is 10 to 40%, the thickness of the vertical plane is 10 to 40% In 1/2 part, the restraint property can be improved by having a texture whose area ratio of the area|region which makes an angle within 15 degrees with respect to a {110} plane with respect to a vertical plane is 40 to 70%.

In order to stably improve the arresting property, it is important to control the crack propagation direction using the texture. When a steel material is subjected to an external stress, a brittle crack that occurs in the steel material propagates along the cleavage plane of the {100} plane. For this reason, when the {100} plane texture developed in the plane perpendicular to the external stress is formed over the entire thickness of the steel material, the cracks in the entire thickness propagate easily in the same direction, further lowering the arresting properties.

Therefore, in the present embodiment, as will be described below, {110} with respect to a vertical plane that is a plane perpendicular to the main rolling direction in each of a position of 1 to 5 mm from the surface and a 1/2 part of the plate thickness. The area ratio of the region where the plane makes an angle within 15° and the {100} plane forms an angle within 15° with respect to the vertical plane that is the plane perpendicular to the main rolling direction in 1/4 part of the plate thickness By limiting the area ratio of the region, the arrestability is stably improved.

External stress is a stress applied externally to a steel structure. Brittle cracks often occur and propagate in a direction perpendicular to the highest external stress. Therefore, here, the highest stress externally imparted to a steel structure is defined as an external stress. In general, external stress is applied substantially parallel to the main rolling direction of the steel material. For this reason, the plane perpendicular|vertical with respect to an external stress can be handled as a plane perpendicular|vertical with respect to the main rolling direction of steel materials.

About the main rolling direction of steel materials, it can specify, for example by corroding the steel materials surface with picric acid, and measuring the aspect-ratio (whole body direction) of old austenite. That is, the direction in which the aspect ratio of prior austenite is large can be specified as the main rolling direction of steel materials.

As a result of the investigation by the present inventors, with respect to a surface perpendicular to the main rolling direction of steel (hereinafter, "a surface perpendicular to the main rolling direction of steel" may be referred to as a "vertical surface"), the {110} plane is When the area ratio of the region forming an angle within 15° is 40 to 70% in 1/2 part of the plate thickness, brittle cracks near 1/2 part propagate straight in the direction perpendicular to the external stress. It has been found that the driving force of crack propagation can be reduced by the crack propagating at an inclination without being cracked. However, at the same time, the present inventors also found that if the same texture was developed in a plate thickness region other than 1/2 part of the plate thickness, the cracks would propagate as they are in an inclined state, and it was also found that a sufficient arresting property improvement effect could not be exhibited. did.

Then, the present inventors further investigated. As a result, in the 1/4 part of the plate thickness, since the crack propagates straight in the direction perpendicular to the external stress, in the 1/4 part of the plate thickness, the {100} plane is at an angle within 15° with respect to the vertical plane It has been found that by setting the area ratio of the region constituting to be 10 to 40%, it is possible to suppress the propagation of cracks inclined by 1/2 part from propagating to parts of the plate thickness other than 1/2 part.

In addition, the present inventors have found that, in the vicinity of the surface, the {110} plane is 15 with respect to the vertical plane at a position of 1 to 5 mm from the surface because the crack propagates obliquely rather than straight in the direction perpendicular to the external stress. It has been found that by setting the area ratio of the region forming an angle within ° to 30 to 60%, propagation of straight cracks in 1/4 part can be suppressed from propagating to the vicinity of the surface.

Based on the knowledge described above, in the steel materials according to the present embodiment, it is preferable that the texture satisfies the conditions of the following (E) to (G).

(E) At a position of 1 to 5 mm from the surface, the area ratio of the region in which the {110} plane forms an angle within 15° with respect to the vertical plane is set to 30 to 60%.

(F) The area ratio of the region where the {100} plane forms an angle within 15° with respect to the vertical plane in the 1/4 part of the plate thickness is 10 to 40%.

(G) In the 1/2 part of the plate thickness, the area ratio of the region where the {110} plane forms an angle within 15° with respect to the vertical plane is set to 40 to 70%.

By satisfying the above (E) to (G), cracks in 1/2 part propagate obliquely, and cracks in 1/4 part propagate straight, and cracks 1 to 5 mm from the surface obliquely propagate. , the crack propagation resistance increases. Thereby, the arrestability can show a sufficient value.

The reason that the area ratio of the region where the {110} plane forms an angle within 15° with respect to the vertical plane at a position of 1 to 5 mm from the surface is 30% or more is the effect of inclining and propagating the crack if it is less than 30% because you can't get

In addition, the reason why the area ratio of the region where the {110} plane makes an angle within 15° with respect to the vertical plane at a position of 1 to 5 mm from the surface is 60% or less is that if it exceeds 60%, 1/4 This is because the arrestability is not sufficiently improved by propagating as it is in an inclined state without receiving resistance.

At a position of 1 to 5 mm from the surface, the area ratio of the region where the {110} plane makes an angle within 15° with respect to the vertical plane is preferably 35 to 55%, more preferably 40 to 50% am.

The reason that the area ratio of the region where the {100} plane forms an angle within 15° with respect to the vertical plane in 1/4 part of the plate thickness is 10% or more is that if it is less than 10%, the effect of propagating cracks straight because you can't get

In addition, the reason that the area ratio of the region where the {100} plane forms an angle within 15° with respect to the vertical plane in 1/4 part of the plate thickness is 40% or less is that if it exceeds 40%, it is 1 more than 1/2 part. This is because the crack propagation of the /4 part becomes dominant, and the arrestability is not sufficiently improved by the straight propagation of the crack.

The area ratio of the region where the {100} plane makes an angle within 15° with respect to the vertical plane in 1/4 part of the plate thickness is preferably 13 to 37%, more preferably 15 to 35% am.

The reason that the area ratio of the region where the {110} plane makes an angle within 15° with respect to the vertical plane in 1/2 part of the plate thickness is 40% or more is the effect of inclining and propagating the crack if it is less than 40% because you can't get

Further, in 1/2 part of the plate thickness, the reason why the area ratio of the region where the {110} plane forms an angle within 15° with respect to the vertical plane is 70% or less is that if it exceeds 70%, the resistance of 1/4 part is reduced. This is because the arrestability is not sufficiently improved by propagating as it is in an inclined state without receiving it.

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

The aggregate structure is measured by the EBSD method.

More specifically, according to the EBSD method, at 1 to 5 mm from the surface, the {110} plane makes an angle within 15° with respect to the vertical plane, and at 1/4 part of the plate thickness, the {100} plane is the vertical plane with respect to the vertical plane. By creating a map of the area where the {110} plane makes an angle within 15° with respect to the vertical plane in the area forming an angle within 15° and 1/2 of the plate thickness, respectively, dividing the total area by the measurement area, Their area ratio can be obtained.

Although the plate|board thickness in particular of the steel materials which concerns on this embodiment is not restrict|limited, When application to a large-sized welded structure is assumed, the range of 50-100 mm is preferable.

Further, the tensile strength TS of the steel according to the present 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 (tensile test) is performed according to JIS Z 2241:2011. The test piece shall be the No. 1B test piece. The test method is the permanent elongation method.

The steel materials according to the present embodiment are excellent in toughness of the heat-affected zone HAZ when welded by high heat input welding. In particular, it is possible to improve the Charpy absorption energy at -40°C.

The HAZ toughness of high heat input welding is evaluated by applying a reproducible heat cycle test simulating high heat input welding, assuming electro gas welding application, to a sample taken from the steel material according to the present embodiment. As a specific reproducible heat cycle condition, a steel material having a plate thickness of 50 mm is approximately 35 kJ/mm welding heat input by electrogas welding, simulating welding in one pass, heating from room temperature to 1400° C., and then heating 1400 It is maintained at ℃ for 5 seconds, and then cooled by controlling the temperature range from 800℃ to 500℃, which is a temperature range affecting intra-particle transformation, at a rate of 1.0℃/sec.

After the heat cycle is applied to the steel material, it is processed into a V-notch test piece, and 3 pieces of each steel material are subjected to a Charpy impact test at a test temperature of -40°C, and the absorbed energy is measured. When the average absorbed energy of the three test pieces is 100 J or more, and the minimum absorbed energy among the three test pieces is 50 J or more, it can be said that the toughness of the weld heat affected zone is excellent. What is necessary is just to create a V-notch test piece according to the V-notch test piece described in JIS Z 2242:2005. In addition, the Charpy impact test may be performed according to JIS Z 2242:2005.

The heat cycle conditions simulate the thermal history when a 50 mm plate thickness steel material is welded with a welding heat input of about 35 kJ/mm, but 35 to 50 kJ/mm for a 50 to 100 mm plate thickness steel material. If it is the HAZ toughness at the time of welding with a welding heat input of a degree, it can be evaluated by the said reproducible heat cycle test.

Moreover, when the steel material which concerns on this embodiment has a predetermined microstructure, and a grain boundary density or a texture exists in the said range, it will be excellent in arrestability. In particular, the arrest toughness value Kca in -10 degreeC can be raised. In the present embodiment, the arrest toughness value Kca- _10° C at -10°C is 6000 N/mm ^1.5 or more, the lead-free transition temperature (NDT temperature) is -60°C or less, and the fracture surface transition temperature (vTrs) is -60°C or less. When all of these are satisfied, it is assumed that the arresting property is excellent.

The evaluation of the arrest toughness value Kca _-10℃ is based on the “Inspection method for the test method for the brittle crack propagation stop toughness value Kca” of K3.12.2-1. do it By the test, the arrest toughness value Kca in -10 degreeC is calculated|required.

In addition, evaluation of a lead-free transition temperature (NDT temperature; Nil-Ductility-Transition Temperature) is calculated|required by performing a test based on the NRL (Naval Research Laboratory) falling weight test method prescribed|regulated to ASTM E208-06. The test piece was set to P-3 type (T: 16 mm, L: 130 mm, W: 50 mm), and it collect|collects up to the position of 16 mm in the plate|board thickness direction so that it may cover the outermost surface of steel materials. The test piece is sampled in the rolling direction (L direction), a weld bead is provided on the outermost surface of the test piece in the L direction, and a notch is provided in the direction (C direction) perpendicular to the rolling direction as a crack starter.

In addition, evaluation of fracture front transition temperature (vTrs) is based on JIS Z 2242:2005, the test piece is made into a V-notch test piece, and the test piece collection position is the position of 1/4 of plate thickness t from the surface of steel material (t/4). part) is collected.

Next, the manufacturing method of the steel materials concerning this embodiment is demonstrated.

In the method for manufacturing steel according to this embodiment, vacuum degassing is performed on molten steel, Zr is added after the dissolved oxygen concentration of the molten steel becomes 0.0050% or less, and B is added after 1.0 to 5.0 minutes from Zr addition. The refining process and the continuous casting process in which the average cooling rate until the surface temperature of the cast slab becomes 1200 ° C. to 900 ° C. is 0.5 ° C./sec or less when continuous casting is performed on the molten steel after the refining process to obtain a slab; , a heating step of heating the slab after the continuous casting step, and a hot rolling step of hot-rolling the slab after the heating step into steel.

In this embodiment, after molten steel is tapped with a ladle in a steelmaking furnace, it is pressure-reduced by a vacuum degassing apparatus. After being tapped with a ladle, while it is conveyed to a vacuum degassing apparatus, you may add an alloy etc. and adjust a component.

In a refining process, after degassing in a vacuum degassing apparatus and adjusting molten steel components except Zr and B, Zr is added to molten steel. Before adding Zr, it is preferable to control the dissolved oxygen concentration in the molten steel to 0.0050% or less. If Zr is added before the dissolved oxygen concentration reaches 0.0050% or less, it becomes difficult to refine the (Zr, B)-containing oxide particles and the number density decreases, and the Al ₂ O ₃ composition of the (Zr, B)-containing oxide particles is reduced. There exists a possibility that it may become impossible to control to 50 mass % or less.

Next, after 1.0 to 5.0 minutes from the addition of Zr, B is added. Thereby, B is segregated around the Zr-containing oxide, B nitride is contained in the Zr-containing oxide, and B nitride can be deposited on the surface layer of the (Zr, B)-containing oxide particles. When the addition timing of B becomes less than 1.0 min or more than 5.0 min from the addition of Zr, the desired (Zr, B)-containing oxide particles cannot be obtained.

The molten steel after the refining process is made into a slab in the continuous casting process. In a continuous casting process, the average cooling rate until the surface temperature of a slab becomes 1200 degreeC to 900 degreeC shall be 0.5 degreeC/sec or less. This makes it possible to the ratio of ZrO ₂ and Al ₂ O ₃ of the separation is conducted, (Zr, B) of the oxide particles containing Al ₂ O ₃ in the oxide containing Zr to less than 50% by weight.

The slab obtained by the continuous casting process is heated by the heating process, it is hot-rolled in a hot rolling process, and becomes steel materials. Although there is no restriction|limiting in particular in the conditions of a heating process and a hot rolling process, It is preferable to set rolling conditions so that the plate|board thickness of a steel material may become the range of 50-100 mm.

However, when making the microstructure and grain boundary density into the above-mentioned range, it is preferable to control a heating process and a hot rolling process as follows.

The heating step is a step that contributes to the control of the structure of the austenite phase by heating the cast slab. When the microstructure and grain boundary density are within a predetermined range, in the heating step, the slab after the continuous casting process is heated so that the maximum temperature of the surface temperature of the slab in the ash furnace in the heating furnace is in the range of 950 to 1150 ° C. When the maximum temperature of the surface temperature of the slab in the ash furnace is less than 950°C, austenitization becomes insufficient, and hardening property decreases because the austenite grains are refined. In this case, the plate thickness is thick and it is difficult to use high-strength steel materials. Moreover, when the maximum temperature of the surface temperature of the slab in a material exceeds 1150 degreeC, the grain boundary density of the microstructure after transformation by quenching is reduced by coarsening austenite grains. Moreover, since the time waiting for the fall of the temperature until the start of rolling arises, productivity becomes low. The range of the maximum temperature of the surface temperature of the cast steel in a preferable material is 1000-1100 degreeC. The maximum temperature of the surface temperature of the slab in the material furnace can be calculated by a heat transfer model from the actually measured atmospheric temperature in the heating furnace.

In a hot rolling process, a rough rolling process, a finish rolling process, and a cooling process are performed one by one.

In the rough rolling step, the slab heated in the heating step is subjected to the recrystallization temperature Trex (°C) or higher and 1050°C or lower at the rolling temperature shown in the following formula (H), and the cumulative reduction ratio (rough rolling) is in the range of 10 to 75%. It is a rolling process as Here, when the slab heated in the heating step is rolled at a rolling temperature of greater than or equal to the recrystallization temperature Trex (°C) and less than or equal to 1050°C represented by the following formula (H), the surface temperature of the slab heated in the heating step is recrystallized. The temperature Trex (°C) or more and 1050°C or less, starting rough rolling, and finishing the rough rolling, the steel material surface temperature is Trex (°C) or more and 1050°C or less. And, rolling with the cumulative reduction ratio (rough rolling) in the range of 10 to 75% means that the plate thickness after rough rolling is subtracted from the plate thickness of the cast slab heated in the heating process, and the slab heated in the heating process Rolling is performed so that the cumulative reduction ratio (rough rolling) divided by the plate thickness is in the range of 10 to 75%. When the rolling temperature of rough rolling exceeds 1050 degreeC, recrystallized austenite grain cannot be made fine also in the subsequent finish rolling. Moreover, when the temperature of rough rolling becomes less than the recrystallization temperature Trex (degreeC), productivity will fall. A preferred rolling temperature is 900 to 1000°C.

The surface temperature of the steel materials at the time of completion|finish of rough rolling may be higher than the surface temperature of the steel materials at the time of the start of rough rolling. This can be considered the effect of generating heat from processing in the steel material by rough rolling, and the heat transfer effect in the sheet thickness direction of the steel material due to the internal temperature of the steel being 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 [Sol.Nb] represented by Formula (I), and when the relationship between Nb content (mass %) in steel is Nb≥[Sol.Nb], [ Let Nb*]=[Sol.Nb], and let [Nb*]=Nb when Nb<[Sol.Nb]. C and N in formula (I) are content (mass %) of C and N contained in steel. T in Formula (I) is the highest temperature (degreeC) of the surface of the slab in the ash furnace in the heating furnace in a heating process.

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

Next, finish rolling is performed with respect to the steel materials after a rough rolling process (finish rolling process). The finish rolling process, the steel material rolled in the rough rolling process, (Ar ₃ -50) ℃ or higher (provided that Ar ₃ is represented by the following formula (J)), recrystallization temperature Trex (℃) shown in the formula (H) At a lower rolling temperature, it is a process of rolling in the range of 45 to 75% of the cumulative reduction ratio (finish rolling). Here, when the steel material after rough rolling is rolled at (Ar ₃ -50) ° C. or higher and the recrystallization temperature Trex (° C.) or lower, the surface temperature of the steel material after rough rolling is (Ar ₃ -50) ° C. or higher, recrystallization The temperature Trex (°C) is lower than the temperature Trex (°C) to start finish rolling, and the steel material surface temperature when finish rolling is finished is set to (Ar ₃ -50)°C or higher and the recrystallization temperature Trex (°C) or lower. In addition, rolling with the cumulative reduction ratio (finish rolling) in the range of 45 to 75% means that the sheet thickness after finish rolling is subtracted from the sheet thickness of the steel rolled by rough rolling, Rolling is carried out 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 is not less than the recrystallization temperature Trex (°C), the non-recrystallized region does not sufficiently enter, the increase in dislocations is suppressed, and a predetermined grain boundary density cannot be obtained. When the finish rolling temperature is lower than (Ar ₃ -50)°C, productivity not only decreases, but also includes a part of deformed ferrite, making it difficult to set the grain boundary density into a desired range. The preferred finish rolling temperature is 760 to 840°C.

The surface temperature of the steel materials at the time of completion|finish of finish rolling may be higher than the surface temperature of the steel materials at the time of the start of finish rolling. This can be considered the effect of work heat generation in the steel materials due to finish rolling, and the heat transfer effect in the sheet thickness direction of the steel materials due to the fact that the internal temperature of the steel materials is higher than the surface temperature of the steel materials.

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

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

When the cumulative reduction ratio at the time of finish rolling is less than 45%, it is difficult to obtain a prescribed grain boundary density due to the accumulation of dislocations, and when it exceeds 75%, productivity decreases. Therefore, the cumulative reduction ratio is set to 45 to 75%. A preferred range of the cumulative reduction ratio is 50 to 70%.

Next, it cools with respect to the steel materials after a finish rolling process (cooling process). In the cooling step, the cooling start temperature is set to (Ar ₃ -100) ° C. or higher (provided that Ar ₃ is represented by the above formula (J)) and less than the recrystallization temperature Trex (° C.) shown in the above formula (H), Cooling stop temperature is made into the range of 0 degreeC or more and 600 degrees C or less, and it cools on the conditions which make the average cooling rate from a cooling start to cooling stop 2-15 degreeC/sec. Let the cooling start temperature, the cooling stop temperature, and the average cooling rate be the temperature in 1/4 part of the plate|board thickness of steel materials. The temperature in 1/4 part of the plate|board thickness of steel materials can be calculated and calculated by the heat transfer model from the actually measured surface temperature.

By making the conditions of a cooling process into the said range, the transformation of the microstructure by quenching is accelerated|stimulated, and a desired microstructure is obtained, while tensile strength TS and yield stress YP become high, restraint property improves.

In addition, when making a microstructure and a texture into the range mentioned above, it is preferable to control a heating process and a hot rolling process as follows.

The heating step is a step that contributes to the control of the structure of the austenite phase by heating the cast slab. When the microstructure and the texture are within the ranges described above, in the heating step, the cast steel after the continuous casting step is heated so that the total thickness average temperature of the cast steel is in the range of 950 to 1200° C. when extracted with a heating furnace. If the average temperature of the total thickness of the cast slab at the time of extraction with a heating furnace is less than 950°C, austenitization becomes insufficient and hardening property decreases due to refining of austenite grains, so the sheet thickness is thick and the strength is high steel material It is difficult to do with In addition, when the average temperature of the total thickness of the slab when extracted with a heating furnace exceeds 1200 ° C, the austenite grains are coarsened, the recrystallization of the austenite grains in the rough rolling process becomes insufficient, and the texture is brought into the desired range. it becomes difficult to do Moreover, since the time waiting for the fall of the temperature until the start of rolling arises, productivity becomes low. A preferred range of heating temperature is 1000 to 1150°C. The average temperature of the total thickness of the cast steel can be calculated from the measured ambient temperature in the heating furnace with a heat transfer model.

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

In the rough rolling step, the slab heated in the heating step is subjected to a rolling temperature of at least the recrystallization temperature Trex (° C.) and 1050° C. or less represented by the following formula (K), and the cumulative reduction ratio (rough rolling) is in the range of 10 to 75%. It is a process of rolling with Here, when the slab heated in the heating step is rolled at a rolling temperature of greater than or equal to the recrystallization temperature Trex (°C) and less than or equal to 1050°C represented by the following formula (K), the surface temperature of the slab heated in the heating step is recrystallized. Temperature Trex (°C) or more and 1050°C or less, starting rough rolling, means that the surface temperature of steel materials when rough rolling is finished shall be Trex(°C) or more and 1050°C or less. And, rolling with the cumulative reduction ratio (rough rolling) in the range of 10 to 75% means that the plate thickness of the cast slab heated in the heating process is subtracted from the plate thickness of the cast slab heated in the heating process, and the plate thickness after rough rolling is subtracted. This means rolling so that the cumulative reduction ratio (rough rolling) divided by the thickness is in the range of 10 to 75%. When the rolling temperature of rough rolling exceeds 1050 degreeC, recrystallized austenite grain cannot be made fine also in the subsequent finish rolling. Moreover, when the temperature of rough rolling becomes less than recrystallization temperature Trex (degreeC), productivity will fall. A preferred rolling temperature is 900 to 1000°C.

The surface temperature of the steel materials at the time of completion|finish of rough rolling may be higher than the surface temperature of the steel materials at the time of the start of rough rolling. This can be considered the effect of heat generation in the steel materials due to rough rolling, and the heat transfer effect in the sheet thickness direction of the steel materials due to the fact that the internal temperature of the steel materials is higher than the surface temperature of the steel materials.

Trex=-91900×[Nb*] ² +9400×[Nb*]+770 … (K)

[Sol.Nb]=(10 ^{(-6770/(T+273)+2.26)} )/(C+12/14×N) … (L)

However, [Nb*] in Formula (K) is [Sol.Nb] represented by Formula (L), and when the relationship between Nb content (mass %) in steel is Nb≥[Sol.Nb], [ Let Nb*]=[Sol.Nb], and let [Nb*]=Nb when Nb<[Sol.Nb]. C and N in Formula (L) are content (mass %) of C and N contained in steel. T in Formula (L) is the total thickness average temperature (degreeC) of the slab at the time of extracting with the heating furnace in a heating process.

In addition, when the cumulative reduction ratio during rough rolling is less than 10%, it is difficult to refine the austenite by recrystallization, and porosity remains, and internal cracking and deterioration of ductility and toughness may occur. In addition, when the cumulative reduction ratio exceeds 75%, the number of passes increases and productivity decreases. A preferable cumulative reduction ratio is 30 to 60%.

Next, primary cooling is performed with respect to the steel materials after rough rolling. In the primary cooling step, the cooling start temperature is set to _{the range of Ar 3} °C or higher and 1050°C or lower shown in the following formula (M), and the cooling stop temperature is 500°C or higher and (Ar ₃ -30)°C or lower (provided that , Ar ₃ is represented by the following formula (M)), and the average cooling rate from the start of cooling to the stop of cooling is cooled under the conditions of 35 to 100° C./sec. By performing primary cooling under this condition, at a position of 1 to 5 mm from the surface of the steel material, the area ratio of the region where the {110} plane forms an angle within 15° with respect to the vertical plane, which is the plane perpendicular to the main rolling direction. can be in the range of 30 to 60%. Let the cooling start temperature, the cooling stop temperature, and the average cooling rate be the temperature in the depth position of 1 mm from the surface of steel materials. The temperature in the depth position of 1 mm from the surface of steel materials is the actually measured surface temperature, and can calculate and compute it with a heat transfer model.

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

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

Next, finish rolling is performed with respect to the steel materials after a primary cooling process. In the finish rolling process, the finish rolling temperature is 750 to 850°C, the number of rolling passes is 4 to 15 passes, the average value of the rolling aspect ratio is 0.5 to 1.0, and the rolling reduction is 45 to 75%. Here, to roll the steel material under the condition that the finish rolling temperature is 750 to 850 ° C. means that the surface temperature of the steel is 750 to 850 ° C. , means 750 to 850 °C. In addition, rolling under the condition that the cumulative reduction ratio (finish rolling) is 45 to 75% means that the sheet thickness of the steel rolled by rough rolling is subtracted from the sheet thickness after finish rolling, and the sheet of steel rolled by rough rolling. It means rolling so that the cumulative reduction ratio (finish rolling) divided by the thickness may be in the range of 45 to 75%.

When the finish rolling temperature exceeds 850°C, the non-recrystallized region is not sufficiently entered, the increase in dislocation is suppressed, and a predetermined texture cannot be obtained. When the finish rolling temperature becomes less than 750° C., not only productivity is lowered but also because it contains processed ferrite, the {110} plane is 15 with respect to the plane perpendicular to the main rolling direction of the steel at a position of 1 to 5 mm from the surface. It may become difficult to make the area ratio of the area|region forming an angle within ° into 60% or less. The preferred finish rolling temperature is 760 to 840°C.

When the number of rolling passes of finish rolling is less than 4 passes _{, it is difficult to make the rolling aspect ratio m j} or less into 1, and when it exceeds 15 passes, productivity decreases. A preferable number of passes is 5 to 13 passes.

The rolling aspect ratio m _j is calculated|required by following formula (N). In addition, the average value of the rolling shape ratio _j m is a mean value of all rolling shape ratio m _j in the rolling passes.

m _j =2{R(H _j-1 -H _j )} ^1/2 /(H _j-1 +H _j ) … (N)

In formula (N), j is the number of rolling passes, m _j is the aspect ratio of the j-th pass, R is the roll radius (mm), and H _j represents the plate thickness (mm) after the j-pass.

The rolling aspect ratio m _j is an index indicating which strain component is imparted to steel materials by rolling. When the aspect ratio is small, a shear strain component is applied, and when the aspect ratio is large, a large compressive strain component is provided. The change in the deformation component due to this aspect ratio change has a particularly large influence on the formation of the texture of a quarter of the plate thickness, and the range is set as described above.

The reason why the average value of the rolling aspect ratio m _j is 0.5 to 1.0 is as follows. When the average value of the rolling aspect ratio m _j is less than 0.5, the shear deformation of rolling becomes dominant, and a {100} aggregate structure thereby develops, and in 1/4 part of the sheet thickness, the surface perpendicular to the main rolling direction of the steel material This is because it is difficult to set the area ratio of the region where the {100} plane forms an angle within 15° to 40% or less. In addition, when _{the average value of the rolling aspect ratio m j} is more than 1.0, the compression deformation of rolling becomes dominant, and a {110} aggregate structure is developed by this, so that in a quarter part of the sheet thickness, the {100} plane is a vertical plane This is because it is difficult to make the area ratio of the region forming an angle within 15° to 10% or more. A preferable range of the average value of the aspect ratio m _{j is 0.6 to 0.9.}

When the cumulative reduction ratio is less than 45%, it is difficult to develop a prescribed texture due to the accumulation of strain, and when it is more than 75%, the productivity decreases. Therefore, the cumulative reduction ratio is set to 45 to 75%. A preferred range of the cumulative reduction ratio is 50 to 70%.

Next, secondary cooling is performed with respect to the steel materials after a finish rolling process (secondary cooling process). In the secondary cooling process, the cooling start temperature is set to (Ar ₃ -100)°C or higher (provided that Ar ₃ is represented by the above formula (M)), and is less than the recrystallization temperature Trex (°C) shown in the above formula (K). Then, the cooling stop temperature is set to be in the range of 0°C or higher and 600°C or lower, and the average cooling rate from the start of cooling to the stop of cooling is 2 to 15°C/sec, and cooling is performed. Let the cooling start temperature, the cooling stop temperature, and the average cooling rate be the temperature in the 1/4 position of the thickness direction of steel materials. The temperature at the 1/4 position in the thickness direction of steel materials can be calculated and calculated with a heat transfer model from the actually measured surface temperature.

By setting the conditions of the secondary cooling step in the above range, the transformation of the microstructure by quenching is accelerated, and the desired microstructure is obtained, thereby increasing the tensile strength TS and the yield stress YP, and improving the arrestability. do.

In the manufacturing method of the steel materials which concerns on this embodiment, the steel materials after hot rolling may be left to cool, or may be quenched by rapid cooling. Moreover, you may perform a tempering process after quenching by rapid cooling.

However, when the heating step and the hot rolling step for obtaining a predetermined grain boundary density or texture are controlled as described above, it is preferable to perform a tempering step of heating in the range of 350 to 650° C. after the hot rolling step. . By performing a tempering process, the dislocation density which became excessively high by rolling can be reduced.

Example

Next, examples of the present invention will be described. However, the conditions in the examples are examples of conditions employed in order to confirm the practicability and effects of the present invention, and the present invention is limited to these examples of conditions. no. Various conditions can be employ|adopted for this invention, as long as the objective of this invention is achieved, without deviating from the summary of this invention.

(Example 1)

The molten iron extracted from the smelting furnace was desulfurized in the molten iron preliminary treatment, and after de-P and de-C treatment was carried out in a converter-type refining vessel, it was taken in a ladle. At the time of steel taping, an alloying element was added, and cover slag for thermal insulation was added.

In the refining process, the molten steel in a ladle was pressure-reduced by the RH vacuum degassing apparatus. In the solvent, a molten steel sample was suitably collected, used for analysis, and a molten steel component was obtained. The molten steel temperature was changed from 1560°C to 1610°C. In the first half of the RH treatment, an alloy except for Zr and B was added to adjust the components, and vacuum degassing was performed to adjust the dissolved oxygen concentration. The dissolved oxygen concentration was measured using an oxygen concentration probe. Then, Zr was added, and after 0.7 to 5.4 minutes, B was further added. And in order to mix uniformly, the reflux process was performed. However, for steel AR, B was added 2.4 minutes before Zr addition. For this reason, in Table 2B, the addition time difference of Zr and B of steel AR was described as "-2.4".

After processing in the RH vacuum degassing apparatus, a slab having a thickness of 250 mm was obtained as a semi-finished product by a continuous casting method. In continuous casting, the average cooling rate until the surface temperature of the cast slab was 1200°C to 900°C was 0.1 to 0.7°C/sec. After that, it was processed to a thickness of 50 to 100 mm by a hot rolling process, and steel materials were manufactured.

Tables 1A to 1D show the chemical composition and carbon equivalent of the steel. Tables 2A and 2B show the dissolved oxygen concentration at the time of Zr addition, the time from Zr addition to B addition, and the average cooling rate until the surface temperature of the slab at the time of continuous casting becomes from 1200°C to 900°C. In Table 3A and Table 3B, Insol.Zr content, Sol.Zr content, B _F , the number density of (Zr, B) containing oxide particles, and Charpy absorbed energy are shown. Underlining in Tables 1C, 1D, 2B and 3B indicates that the values are outside the scope of the present invention.

The equivalent circle diameter and number density of the (Zr, B)-containing oxide particles were measured by observing the mirror-polished steel surface with a scanning electron microscope (SEM). Specifically, by SEM, the number of oxide particles containing (Zr, B) having a circle equivalent diameter of 0.5 µm or more in a range of 10 mm × 10 mm (100 mm 2 ) is measured, and divided by the area of the observed field of view. The number density was measured. Particles to be measured for number density have an equivalent circle diameter of 0.5 µm or more, and by quantitative analysis by an energy dispersive X-ray analyzer (EDX) attached to SEM, Zr of 5.0 mass% or more and 0.1 mass% or more comprises a B or more and 1.0 mass% of O, in addition, the particles confirmed that the composition of the Al ₂ O ₃ less than 50% by weight.

The contents of Insol.Zr and Sol.Zr were measured by an electrolytic extraction residue analysis method. In the electrolytic extraction residue analysis method, the mother phase was dissolved by electrolysis of steel in a non-aqueous solvent (acetylacetone-methanol solution), and the residue (precipitates and inclusions) was extracted and separated with a filter having a pore diameter of 0.2 µm. After separation, the amount of Zr contained in the solution was defined as Sol.Zr content, and the amount of Zr contained in the residue was defined as Insol.Zr content.

B _F was calculated|required by said Formula (C1) and Formula (C2).

Next, the test piece for a thermal cycle test was extract|collected from the steel material. A thermal cycle was applied to this test piece. As a specific thermal cycle condition, after heating from room temperature to 1400°C, it is held at 1400°C for 5 seconds, and then, the temperature range from 800°C to 500°C, which is a temperature range affecting intra-particle transformation, is 1.0°C/sec. It was cooled by controlling the speed.

Three V-notch test pieces were taken each from the steel material after giving a thermal cycle, the Charpy impact test was _{done at -40} degreeC, and absorbed energy (vE-40) was measured. The V-notch test piece was created according to the V-notch test piece described in JIS Z 2242:2005. In addition, the Charpy impact test was performed based on JIS Z 2242:2005.

As shown in Table 1A, Table 1B, Table 2A, and Table 3A, the steels A to V, which are examples of the present invention, all had an average of 100 J or more, and the minimum absorbed energy among the three specimens showed an absorbed energy of 50 J or more, and excellent toughness had a

On the other hand, as shown in Table 1C, Table 1D, Table 2B, and Table 3B, the comparative examples steels W to Z, AA to AF, and AI to AN had chemical compositions outside the ranges specified in the present invention, so they were all tough. This was getting worse.

Further, the steels AG to AH, AO to AS met the chemical composition range of the present invention, but the manufacturing conditions did not satisfy the present invention. Therefore, steel is AG does not satisfy the present invention the content Sol.Zr, river AH did not satisfy the invention is Insol.Zr content B and _F present. In the steels AO to AS, the number density of oxide particles containing (Zr, B) did not satisfy the scope of the present invention. As a result, the HAZ toughness deteriorated.

[Table 1A]

[Table 1B]

[Table 1C]

[Table 1D]

[Table 2A]

[Table 2B]

[Table 3A]

[Table 3B]

(Example 2)

The molten iron extracted from the blast furnace was desulfurized in the molten iron preliminary treatment, and after de-P and de-C treatment with a converter-type refining vessel, it was taken in a ladle. At the time of steel taping, an alloying element was added, and cover slag for thermal insulation was added.

In the refining process, the molten steel in a ladle was pressure-reduced by the RH vacuum degassing apparatus. In the solvent, a molten steel sample was suitably collected, used for analysis, and a molten steel component was obtained. The molten steel temperature was changed from 1560°C to 1610°C. In the first half of the RH treatment, an alloy except for Zr and B was added to adjust the components, and vacuum degassing was performed to adjust the dissolved oxygen concentration. The dissolved oxygen concentration was measured using an oxygen concentration probe. Then, Zr was added, and after the elapse of 0.8 to 5.3 minutes, B was further added. And in order to mix uniformly, the reflux process was performed. However, in No. 151, B was added 2.2 minutes before Zr addition. For this reason, in Table 2C, No. The time difference between addition of Zr and B in 151 was described as "-2.2".

After processing in the RH vacuum degassing apparatus, the average cooling rate until the surface temperature of the cast slab reached from 1200°C to 900°C was 0.1 to 0.7°C/sec in continuous casting by the continuous casting method. Then, a slab having a thickness of 251 to 372 mm was obtained as a semi-finished product. Thereafter, the steel was manufactured by processing it to a thickness of 50 to 100 mm by a hot rolling process.

Tables 4A to 4D show the chemical composition and carbon equivalent of the steel. Tables 5A to 5C show the dissolved oxygen concentration at the time of Zr addition, the time from Zr addition to B addition, and the average cooling rate until the surface temperature of the slab at the time of continuous casting becomes from 1200°C to 900°C. To Tables 5A to 5C, the conditions of the heating step and the hot rolling step are shown. In Tables 6A to 6C, Insol.Zr content, Sol.Zr content, B _F , number density of (Zr, B)-containing oxide particles, evaluation results of microstructure, grain boundary density, tensile strength TS, yield stress YP, Charpy Absorbed energy and arrest toughness value Kca at -10°C, NDT temperature, and vTrs are shown. The underlined portions in Tables 4C, 4D, 5B, 5C, 6B, and 6C indicate outside the scope of the present invention.

The equivalent circle diameter and number density of the (Zr, B)-containing oxide particles were measured in the same manner as in Example 1.

The contents of Insol.Zr and Sol.Zr were measured in the same manner as in Example 1. B _F was calculated|required by said Formula (C1) and Formula (C2).

Next, a test piece for a thermal cycle test was taken from the steel, and in the same manner as in Example 1, a Charpy impact test was performed at -40°C, and the absorbed energy (vE _-40 ) was measured.

The arresting property was evaluated in accordance with the "Inspection Procedures for the Test Method for Brittle Crack Propagation Stop Toughness Value Kca" of NK Classification Society Steel Wire Rules Inspection Rules K-Part K3.12.2-1. (2018). By the test, the arrest toughness value Kca in -10 degreeC was calculated|required.

Moreover, as evaluation of arrestability, the non-flammable transition temperature (NDT temperature; Nil-Ductility-Transition Temperature) was calculated|required. The NDT temperature was calculated|required by performing a test based on the NRL (Naval Research Laboratory) falling weight test method prescribed|regulated to ASTM E208-06. The test piece was set to P-3 type (T: 16 mm, L: 130 mm, W: 50 mm), it was made to cover the outermost surface of steel materials, and it extract|collected up to the position of 16 mm in the plate|board thickness direction. The test piece was sampled in the rolling direction (L direction), a weld bead was provided on the outermost surface of the test piece in the L direction, and a notch was provided in the direction (C direction) perpendicular to the rolling direction as a crack starter.

In addition, as evaluation of the arresting property, the fracture front transition temperature (vTrs) was calculated|required. Evaluation of vTrs was based on JIS Z 2242:2005, and the test piece was made into a V-notch test piece, and the test piece sampling position was extract|collected so that 1/4 part of the plate|board thickness of steel materials might be included.

Arrest toughness value Kca at -10°C When -10°C is 6000N/mm 1.5 or more, lead-free transition temperature (NDT temperature) is -60°C or less, and fracture front transition temperature (vTrs) is -60°C or less was judged to have excellent arrestability.

Evaluation of tensile strength TS and yield stress YP was performed according to JIS Z 2241:2011. The test piece was set as the No. 1B test piece. The test method was made into the permanent extension method. Tensile strength TS judged that preferable strength was obtained that 510-720 MPa and yield stress YP of 390-650 MPa were obtained.

As shown in Table 4A to Table 4D, Table 5A, and Table 6A, the inventive example No. All of 101 to 125 had excellent HAZ toughness and arrestability, and also had excellent mechanical properties.

On the other hand, as shown in Table 4C, Table 4D, and Table 5B, Table 5C, Table 6B, and Table 6C, the Comparative Example No. Since the chemical compositions of 126 to 136, 140 to 142, and 144 to 146 were outside the range specified in the present invention, the HAZ toughness was low.

Also, No. 138, 139, 148 to 152, the chemical composition met the component range of the present invention, but the manufacturing condition did not satisfy the condition of the present invention. For that reason, No. 138, Sol.Zr content does not satisfy the present invention, No. 139 did not satisfy the invention is Insol.Zr content B and _F present. No. In 148 to 152, the number density of (Zr, B)-containing oxide particles did not satisfy the range of the present invention.

No. 137, No. 143 had excellent HAZ toughness, but insufficient restraint property, because the chemical composition met the component range of the present invention, but was outside the preferred range. No. 153, 155 to 159, 162, 163, and 166 did not satisfy the preferable range of grain boundary density. No. 154, 160, 161, 164, 165, and 167 did not satisfy the preferable range of microstructure. As a result, although the HAZ toughness was excellent, neither the arresting property nor the mechanical property was in a preferable range.

[Table 4A]

[Table 4B]

[Table 4C]

[Table 4D]

[Table 5A]

[Table 5B]

[Table 5C]

[Table 6A]

[Table 6B]

[Table 6C]

(Example 3)

Using the same slab as in Example 2, it was processed to a thickness of 50 to 100 mm by a hot rolling process to prepare a steel material. That is, the chemical composition and carbon equivalent of the steel are as shown in Tables 4A to 4D.

Tables 7A to 7C show the conditions of the heating process, the rough rolling process, the primary cooling process, the finish rolling process, the secondary cooling process, and the tempering process. In addition, in Tables 8A to 8C, Insol.Zr content, Sol.Zr content, B _F , evaluation result of microstructure, evaluation result of texture, number density of oxide particles containing (Zr, B), Charpy absorbed energy, Tensile strength TS, yield stress YP, the arrest toughness value Kca in -10 degreeC, NDT temperature, and vTrs are shown. The underlined portions in Tables 7A to 8C indicate outside the scope of the present invention.

The equivalent circle diameter and number density of the (Zr, B)-containing oxide particles were measured in the same manner as in Examples 1 and 2.

The contents of Insol.Zr and Sol.Zr were measured in the same manner as in Examples 1 and 2.

B _F was calculated|required by said Formula (C1) and Formula (C2).

Next, a test piece for a thermal cycle test was taken from the steel, and in the same manner as in Examples 1 and 2, a Charpy impact test was performed at -40°C, and the absorbed energy (vE _-40 ) was measured.

The arresting property was evaluated in the same manner as in Example 2.

Evaluation of tensile strength TS and yield stress YP was performed by the method similar to Example 2.

As shown in Tables 4A to 4D, and Tables 7A to 8C, the inventive No. All of 201 to 225 had excellent HAZ toughness and arrestability, and also excellent mechanical properties.

On the other hand, the comparative example No. 226 to 236, 240 to 242, and 244 to 246 had low HAZ toughness because their chemical composition was outside the range specified in the present invention.

Also, No. 238, 239, 248 to 252, the chemical composition met the component range of the present invention, but the manufacturing condition did not satisfy the condition of the present invention. For that reason, No. 238, the Sol.Zr content does not satisfy the scope of the present invention, No. 239, B _F did not satisfy the scope of the present invention. No. 248 to 252, the number density of (Zr, B) containing oxide particles did not satisfy the range of the present invention.

No. Chemical compositions of 237, 243, and 247 were within the range of the present invention, but were outside the preferred range. No. 253, 265 to 270, 272, 274, and 277 are the area ratios of the region where the {110} plane forms an angle within 15° with respect to the vertical plane, which is the plane perpendicular to the main rolling direction at 1/2 part of the plate thickness. out of the desired range. No. In 254 and 276, the microstructure was out of the preferable range. No. In 255 to 264 and 271, the area ratio of the region where the {110} plane makes an angle within 15° with respect to the vertical plane, which is the plane perpendicular to the main rolling direction at a position of 1 to 5 mm from the surface, is outside the preferable range. it happened No. As for 273, since the average cooling rate in secondary cooling was too high, the microstructure was out of the preferable range. Also, No. In 275, since the cooling start temperature in secondary cooling was too high, the microstructure was out of the preferable range.

Therefore, in these examples, although the HAZ toughness was excellent, the arrestability and mechanical properties were not in the preferable range.

[Table 7A]

[Table 7B]

[Table 7C]

[Table 8A]

[Table 8B]

[Table 8C]

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

Claims (8)

The chemical composition, in mass %,
C: 0.040 to 0.160%;
Si: 0.01 to 0.50%,
Mn: 0.70 to 2.50%,
P: 0.030% or less;
S: 0.008% or less;
Al: 0.010% or less;
Sum of content of Ca, Mg and REM: 0.0005% or less;
N: 0.0010 to 0.0080%,
O: 0.0005 to 0.0040%,
Ti: 0.003 to 0.024%,
Zr: 0.0007 to 0.0050%;
B: 0.0003 to 0.0040%;
Cu: 0 to 1.00%,
Ni: 0 to 2.50%,
Cr: 0 to 1.00%,
Mo: 0 to 0.50%,
Nb: 0 to 0.050%,
V: 0 to 0.150%,
W: 0 to 1.00%,
Sn: 0 to 0.50%,
Balance: consisting 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 0.0030% or less,
Among the (Zr, B)-containing oxide particles containing 5.0 mass% or more of Zr, 0.1 mass% or more of B, and 1.0 mass% or more of O, (Zr, B) containing oxide particles having a circle equivalent diameter of 0.5 µm or more, Al ₂ O _{3 The} steel materials whose number density of the (Zr, B) containing oxide particle whose composition is 50 mass % or less is 5-300 pieces/mm<2>.
B _F '=B-[N-{Ti-(O-Insol.Zr×(32/91.224))×(95.734/48)}×(14/47.867)]×(10.811/14) … (One)
For B _F '>B, B _F =B, for 0≤B _F ' _{≤B B F} =B _F ', for B _F '<0 B _F =0 … (2)
However, in Formula (1) and Formula (2), N, Ti, O, and B are content by mass % of N, Ti, O, and B contained in steel, Insol.Zr is acid-insoluble Zr It is content by mass %. According to claim 1,
The chemical composition is, in mass%,
Cu: 0.10 to 1.00%,
Ni: 0.10 to 2.50%,
Cr: 0.10 to 1.00%,
Mo: 0.01 to 0.50%,
Nb: 0.003 to 0.050%;
V: 0.010 to 0.150%,
W: 0.01 to 1.00%, and
Sn: 0.03 to 0.50%
A steel material containing one or two or more selected from the group consisting of. 3. The method of claim 1 or 2,
The chemical composition contains Nb: 0.003 to 0.050% by mass%,
The B _F is 0.0020% or less,
The carbon equivalent Ceq represented by the following formula (3) is 0.30% to 0.55%,
It contains 5 to 70% ferrite by area ratio, 30% or more bainite by area ratio, 0 to 15% pearlite by area ratio, and 0 to 5% martensite-austenite mixed structure by area ratio. have a microstructure,
The grain boundary density at a position of 1 to 5 mm from the surface is 500 to 1100 mm/mm 2 ,
The grain boundary density at the position of 1/4 part of the plate thickness is 400 to 1000 mm / mm 2 ,
The steel material whose grain boundary density in the position of 1/2 part of plate|board thickness is 300-900 mm/mm<2>.
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15 … (3)
C, Mn, Cr, Mo, V, Cu, and Ni in Formula (3) are content (mass %) of each element contained in steel, When the said element is not contained, 0 is substituted. 3. The method of claim 1 or 2,
The chemical composition contains Nb: 0.003 to 0.050% by mass%,
The B _F is 0.0020% or less,
The carbon equivalent Ceq represented by the following formula (4) is 0.30% to 0.55%,
It contains 5 to 70% ferrite by area ratio, 30% or more bainite by area ratio, 0 to 15% pearlite by area ratio, and 0 to 5% martensite-austenite mixed structure by area ratio. have a microstructure,
At a position of 1 to 5 mm from the surface of a vertical plane that is a plane perpendicular to the main rolling direction, the area ratio of the region where the {110} plane forms an angle within 15° with respect to the vertical plane is 30 to 60%,
In 1/4 part of the plate thickness of the vertical plane, the area ratio of the area where the {100} plane forms an angle within 15° with respect to the vertical plane is 10 to 40%,
In 1/2 part of the plate thickness of the vertical plane, the area ratio of the region where the {110} plane makes an angle within 15° with respect to the vertical plane is 40 to 70%, steel.
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15 … (4)
C, Mn, Cr, Mo, V, Cu, and Ni in Formula (4) are content (mass %) of each element contained in steel, When the said element is not contained, 0 is substituted. It is a method for manufacturing the steel according to claim 1 or 2,
A refining step of vacuum degassing the molten steel, adding Zr after the dissolved oxygen concentration of the molten steel becomes 0.0050% or less, and adding B after 1.0 to 5.0 minutes from Zr addition;
A continuous casting process in which the average cooling rate until the surface temperature of the slab is 1200°C to 900°C is 0.5°C/sec or less when continuous casting is performed on the molten steel after the refining step to obtain a slab;
A heating step of heating the slab after the continuous casting step;
A hot rolling process of hot rolling the cast slab after the heating process into a steel material;
A method for manufacturing steel, characterized in that it is provided. 6. The method of claim 5,
A method for manufacturing the steel according to claim 3,
In the heating step, heating is performed so that the maximum temperature of the surface temperature of the cast steel in the ash furnace in the heating furnace is in the range of 950 to 1150°C,
The hot rolling process is a process of sequentially performing a rough rolling process, a finish rolling process, and a cooling process,
In the rough rolling step, the slab heated in the heating step is rolled at a rolling temperature of at least the recrystallization temperature Trex (°C) and 1050° C. or lower shown in the following formula (5) at a cumulative reduction ratio of 10 to 75%,
When Ar ₃ is expressed by the following formula (6), in the finish rolling step, the finish rolling temperature is set to (Ar ₃ -50)°C or higher and the recrystallization temperature Trex (°C) or lower, and the cumulative reduction ratio is 45 to 75% rolled under the conditions of
In the cooling step, the cooling start temperature is set to (Ar ₃ -100)°C or higher and lower than the recrystallization temperature Trex (°C), and the cooling stop temperature is set to 0°C or higher and 600°C or lower, from the start of cooling Cooling under the condition that the average cooling rate to stop cooling is 2 to 15 ° C / sec.
A method for manufacturing steel, characterized in that.
Trex(℃)=-91900×[Nb*] ² +9400×[Nb*]+770 … (5)
Ar ₃ (℃)=910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo … (6)
[Sol.Nb]=(10 ^{(-6770/(T+273)+2.26)} )/(C+12/14×N) … (7)
However, [Nb*] in Formula (5) is [Sol.Nb] represented by Formula (7), and when the relationship between Nb content (mass %) in steel is Nb≥[Sol.Nb], [ Let Nb*]=[Sol.Nb], let [Nb*]=Nb when Nb<[Sol.Nb],
The element symbols in formulas (6) to (7) are the content by mass% of each element contained in steel, and when the element is not contained, 0 is substituted;
T in the formula (7) is the temperature in unit °C of the slab at the time of extraction of the slab in the heating step. 6. The method of claim 5,
A method for manufacturing the steel according to claim 4,
In the heating step, heating is performed so that the average temperature of the total thickness of the cast slab when extracted in a heating furnace is in the range of 950 to 1200 ° C,
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 the recrystallization temperature Trex (°C) and not more than 1050°C shown in the following formula (8), at a cumulative reduction ratio of 10 to 75%,
When Ar ₃ is represented by the following formula (9), in the primary cooling step, the cooling start temperature is in _{the range of Ar 3} ° C. or higher and 1050 ° C. or lower, and the cooling stop temperature is 500 ° C. or higher, (Ar ₃ -30) ° C. or less, and the average cooling rate from the start of cooling to the stop of cooling is cooled under the conditions of 35 to 100 ° C/sec,
In the finish rolling process, the finish rolling temperature is 750 to 850 ° C., the number of rolling passes is 4 to 15 passes, the average value of the rolling aspect ratio is 0.5 to 1.0, and the cumulative rolling reduction is 45 to 75%.
In the secondary cooling step, the cooling start temperature is set to (Ar ₃ -100) ° C. or higher, the recrystallization temperature Trex (° C.) shown in the following formula (8) is set to be less than the range, and the cooling stop temperature is 0 ° C. or higher, 600 ℃ or less, and cooling under the condition that the average cooling rate from the start of cooling to the stop of cooling is 2 to 15 °C/sec.
A method for manufacturing steel, characterized in that.
Trex=-91900×[Nb*] ² +9400×[Nb*]+770 … (8)
Ar ₃ (℃)=910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo … (9)
[Sol.Nb]=(10 ^{(-6770/(T+273)+2.26)} )/(C+12/14×N) … (10)
However, [Nb*] in Formula (8) is [Nb] when the relationship between [Sol.Nb] represented by Formula (10) and Nb content (mass %) in steel is Nb≥[Sol.Nb] Let *]=[Sol.Nb], and when Nb<[Sol.Nb], let [Nb*]=Nb,
The element symbols in formulas (9) to (10) are the content by mass% of each element contained in steel, and when the element is not contained, 0 is substituted;
T in the formula (10) is the temperature in unit °C of the slab at the time of extraction of the slab in the heating step. 8. The method of claim 6 or 7,
After the hot rolling step, a tempering step of heating the steel to a range of 350 to 650 ° C. is provided, characterized in that the manufacturing method of steel materials.