KR101892412B1

KR101892412B1 - Steel

Info

Publication number: KR101892412B1
Application number: KR1020177036196A
Authority: KR
Inventors: 요시아키 신타쿠; 아츠시 오카야마
Original assignee: 신닛테츠스미킨 카부시키카이샤
Priority date: 2016-04-19
Filing date: 2017-04-18
Publication date: 2018-08-27
Also published as: KR20180002875A; WO2017183630A1; JPWO2017183630A1; CN107709599A; JP6245417B1; CN107709599B

Abstract

This steel material has a chemical composition in mass% of C: 0.01 to 0.20%, Si: 0.02 to 0.70%, Mn: 0.30 to 2.50%, Ti: 0.003 to 0.024%, B: 0.0010 to 0.0050% 0.0090%, O: 0.0010 to 0.0050%, Insol. Z: 0.0005 to 0.0100%, P: 0.050% or less, S: 0.0080% or less, Al: 0.005% or less, Sol. Zr: not more than 0.0010%, the total content of Ca and REM: not more than 0.0005%, the _balance being Fe and impurities, and B _asBN = (N- (Ti- (O- and (95.734 / 48)) × ( 14 / 47.867)) × (10.811 / 14) represented by B _asBN more than 0.0005 and in addition, B _F = B _F is represented as 0.0005 BB _asBN, the circle-equivalent diameter 0.5㎛ Or more of the Zr-containing oxide.

Description

Steel

The present invention relates to a steel material, and more particularly to a steel material excellent in toughness of a weld heat affected zone (hereinafter sometimes referred to as " HAZ ").

The present application claims priority based on Japanese Patent Application No. 2016-083595, filed on April 19, 2016, the contents of which are incorporated herein by reference.

Examples of the use of the steel include ships, high-rise buildings, other buildings, bridges, offshore structures, LNG storage tanks and other large tanks, and welded structures such as line pipes. BACKGROUND ART [0002] In recent years, the size of a welded structure has progressed due to the increase in the weight of the container wire and the increase in the height of the building structure. Along with this, it is required to thicken the plate thickness and to increase the strength of the steel. Further, in the above-described welded structure, it is necessary to further secure the safety and reliability of the welded portion, and it has been a problem to improve the toughness of the weld heat affected zone (hereinafter also referred to as " HAZ toughness ") .

In addition, the cost of welding construction over the entire drying cost of the welded structure is large, and it is required to perform high efficiency welding in order to reduce the cost. Concretely, it is effective to reduce the number of welding passes by carrying out welding with a large number of rows. However, in the case of performing the welding of the substitution heat, the structure of the HAZ of the steel becomes coarse, and deterioration of toughness can not be avoided.

It has been known that crystal grain size of austenite (γ), transformation texture, hardness of HAZ, coarse hard phase and the like have a great influence on HAZ toughness of high tensile strength steel sheet and various measures for improving HAZ toughness have been proposed . Among them, the improvement of the HAZ toughness is most effective in the miniaturization of the HAZ structure, and a number of methods for making the HAZ structure finer by utilizing the inclusions have been proposed.

In order to miniaturize the HAZ structure using inclusions, there are a method of suppressing the growth of the crystal grains by the pinning effect of the inclusions and a method of forming ferrite in the austenite grains surrounded by the inclusions as a nucleus Particle transformation) to make the structure finer. With respect to texture refinement by intracellular transformation, a technique of using nitride such as TiN, sulfide such as MnS, or an oxide chemically stable even at high temperature has been proposed as a ferrite formation site.

For example, Patent Document 1 discloses a method in which Ti and Zr are simultaneously added to molten steel which does not substantially contain Al to produce a composite oxide of fine Ti and Zr, and the composite oxide of Ti and Zr produces a weld heat effect A method of finely minifying the tissue of the part has been proposed. In Patent Document 1, it is described that the composite oxide of fine Ti and Zr becomes a nucleus of transformation in the particle to generate fine ferrite in a radial direction.

Also, in Patent Document 2, a method of improving HAZ toughness by inclusions including REM and Zr has been proposed.

Patent Document 3 proposes a method of dispersing an oxide containing Ti as a main component and a complex precipitate of TiN, MnS and BN in a steel substantially containing no Al. In Patent Document 3, according to this method, it is disclosed that in addition to the in-particle transformation by the Ti oxide, the generation of ferrite from the grain boundary is inhibited by B and the HAZ toughness is improved.

Patent Document 4 discloses a method for improving the toughness by suppressing the softening of the HAZ by making the HAZ finer by the pinning effect by TiN and the in-particle transformation by BN and by improving the ignitability by B Has been proposed.

In the techniques of Patent Documents 1 to 4, when the heat input is small, a constant HAZ toughness improving effect is obtained. However, in order to increase the efficiency of welding, it has been difficult to stably improve the HAZ toughness of the steel material when performing large heat input welding with an incident heat exceeding 40 kJ / mm. The reason for this is that inclusions such as oxides tend to agglomerate in the molten steel and are difficult to uniformly disperse in the steel. However, since the inclusions are altered by being exposed at a high temperature for a long time at the time of large heat welding, It is thought that it is difficult to do.

Japanese Unexamined Patent Application Publication No. 1-159356 Japanese Patent Application Laid-Open No. 2008-291347 Japanese Unexamined Patent Application Publication No. 3-162522 Japanese Patent Laid-Open No. 2007-177327

In view of such circumstances, the present invention aims to provide a steel having excellent toughness, particularly excellent toughness in HAZ of heat input welding of 40 kJ / mm or more heat input.

The inventors of the present invention have made extensive studies with attention paid to Zr-containing oxides (including complex oxides containing Zr and Ti, hereinafter the same) and B nitrides as in-particle ferrite formation sites for microstructure in HAZ . As a result, new knowledge of the following (A) to (F) was obtained.

(A) When the number density of the Zr-containing oxide having a circle equivalent diameter of 0.5 탆 or more is 50 / mm 2 or more, a large amount of ferrite in the grain is finely generated in the HAZ and HAZ toughness is improved.

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

(C) Addition of Zr, Ti and B precipitates B nitrides in the steel with the Zr-containing oxide as a nucleus. The Zr-containing oxide in which the B-nitride is precipitated functions more effectively as a ferrite formation site in the particle. In order to obtain this effect, it is necessary to set the content (mass%) of B to be B nitride to 0.0005% or more.

(B) a content (mass%) of B (B _asBN ) to be a B nitride based on the molecular weight of ZrO ₂ , Ti ₂ O ₃ , TiN, and BN to inhibit the formation of Ti nitride and (D) . Concretely, when the value of the following formula <1> is 0.0005 or more, the effect of improving the HAZ toughness by the B nitride is obtained.

Here, N, Ti and O in the equation are the content (mass%) of each element (N, Ti, O) contained in the steel. And Zr is the content (mass%) of the acid-insoluble Zr.

(E) Solid solution B is segregated at the old austenite grain boundaries and contributes to improvement of HAZ toughness by suppressing the generation of coarse grain boundary ferrite. For this reason, it is necessary to secure the amount of B for obtaining solid solution B in addition to the amount of B precipitated as the above-mentioned B nitride. Concretely, when B _F represented by the following formula <2> is 0.0005 or more, a predetermined amount of solid solution B is secured, and grain boundary ferrite suppressing effect is obtained.

Here, B in the formula is the B content (mass%) contained in the steel, and B _asBN is a value obtained from the formula <1>.

(F) If Al, which functions as a steel element, is excessively contained in steel, the formation of Zr and Ti oxides is inhibited. It is important to limit the Al content to 0.005 mass% or less in order to secure the dissolved oxygen amount in the molten steel and to produce the Zr-containing oxide in the steel. In addition, it is important to limit the total deoxidization power of elements such as Ca and REM to 0.0005 mass% or less in total.

The present invention has been completed on the basis of the above knowledge, and its gist is as follows.

(1) A steel material according to one embodiment of the present invention has a chemical composition of 0.01 to 0.20% of C, 0.02 to 0.70% of Si, 0.30 to 2.50% of Mn, 0.003 to 0.024% of Ti, 0.003 to 0.024% of Ti, : 0.0010 to 0.0050%, N: 0.0010 to 0.0090%, O: 0.0010 to 0.0050%, Insol. Z: 0.0005 to 0.0100%, P: 0.050% or less, S: 0.0080% or less, Al: 0.005% or less, Sol. 0.005% or less, Zr: 0.0010% or less, Ca and REM content: 0.0005% or less, the _balance being Fe and an impurity, B _asBN represented by the following formula < B _F is more than 0.0005, has a Zr oxide containing not less than the equivalent circle diameter of more than 50 0.5㎛ / ㎟.

Note that N, Ti, O and B in the formulas are the contents by mass% of N, Ti, O, and B contained in the steel. Zr is the content by mass% of acid-insoluble Zr, and Sol. Zr is the content by mass% of acid soluble Zr.

(2) The steel material according to the above item (1) is characterized in that the above chemical composition further contains, by mass%, Cu: 1.50% or less, Ni: 3.00% or less, Cr: 1.00% % Or less and Nb: 0.035% or less.

(3) The steel material according to the above (1) or (2) preferably has one or two or more selected from the group consisting of W, 1.00% or less and Sn: 0.50% .

According to this aspect of the present invention, it is possible to provide a steel having excellent toughness in the HAZ of large heat input welding. By using this steel material, high-efficiency welding is possible, and the construction cost of the welded structure can be drastically reduced. It is also possible to improve the safety and reliability of the welded structure.

It is known that Ti oxide or B nitride is dispersed in a weld metal or HAZ and has an effect of refining the structure. On the other hand, Zr is not an element added to steel in general, and research done in the past on the effect of Zr addition is very limited.

Particularly, there has never been studied how B nitrides further precipitated in the Zr-containing oxide affect the refinement of the HAZ structure of the steel and the improvement of the HAZ toughness.

The present inventors paid attention to Zr-containing oxides and B-nitrides as ferrite-generating sites in grains for HAZ structure refinement, and studied extensively. As a result, new knowledge of the following (a) to (f) was obtained.

(a) The present inventors have studied a method of actually improving the HAZ toughness by dispersing a Zr-containing oxide in the steel. As a result, when the Zr-containing oxide having a circle equivalent diameter of 0.5 mu m or more is dispersed at 50 / mm < 2 > or more, a larger amount of ferrite in the particle is generated finely and in comparison with the case of dispersing the Ti single- , It has been found that HAZ toughness can be improved through microfabrication of the tissue.

(b) In order to obtain a predetermined number or more of Zr-containing oxides contributing to the miniaturization of the HAZ structure, it is necessary to set the Zr content to a certain amount or more. On the other hand, not all of Zr in the steel forms an oxide, and some Zr remains in the steel without forming an oxide. Zr (Sol. Zr) which does not form this oxide remarkably deteriorates the toughness of the steel itself as well as the HAZ. Therefore, in order to secure the toughness of the steel itself as well as the HAZ, It is necessary to reduce Zr. Sol. The lower the Zr, the tougher tends to be improved. In order to obtain a steel having excellent HAZ toughness, Sol. It is important to limit the Zr to 0.0010 mass% or less. In order to further improve HAZ toughness, Sol. It is preferable to limit Zr to 0.0003 mass% or less.

(c) In the steel in which the Zr-containing oxide was dispersed, it was found that inclusions that function as a ferrite formation site and inclusions that do not function as a formation site exist even if the number of inclusions increases.

Further, the present inventors have studied various elements in order to more effectively promote ferrite generation. As a result, it has been found that when B is contained in a certain amount or more, B nitride is precipitated with Zr-containing oxide as a nucleus during casting, hot rolling or welding, and this complex precipitate further functions more effectively as a ferrite formation site in the grain I found out.

That is, the Zr-containing oxide which was difficult to function alone as a ferrite formation site in the particle by the B nitride also becomes a ferrite formation site, contributing to the miniaturization of the HAZ structure more efficiently. In order to obtain these effects, it is necessary to set the index ( _BasBN ) of the B content (mass%) for depositing B nitride to 0.0005 or more and to set the B content to B _asBN or more.

(d) However, in addition to B, Ti acts as a nitride-forming element in the steel. Therefore, in order to efficiently deposit B nitride, it is necessary to suppress the formation of Ti nitride. The inventors of the present invention clarified the formation mechanism of inclusions including oxides and nitrides and examined to clarify conditions for producing B nitrides.

In molten steel containing Ti, Zr and B, Zr having a higher deoxidizing power than Ti is preferentially oxidized, and the remaining oxygen and Ti are combined to form a composite oxide of Zr and Ti. Next, Ti remaining without forming an oxide bonds with nitrogen to form a nitride. Next, it is considered that nitrogen remaining unbound with Ti forms a B nitride.

It is considered that Zr is formed of ZrO ₂ , Ti is Ti ₂ O ₃ and TiN, and B is formed of BN. Thus, based on the atomic weight or molecular weight thereof, B ( _asBN ) (% By mass) of the composition of the present invention can be obtained. When this value is 0.0005 or more and the value of the B content is B _{asBN or} more, it is considered that the HAZ toughness improving effect by the B nitride is obtained.

A fine Zr-containing oxide (a composite oxide mainly containing Zr and Ti) is dispersed in a steel material obtained by hot rolling the billet having the component satisfying the above formula <1>. Further, B nitrides are additionally precipitated in some Zr-containing oxides.

The B nitrides are reusable when they are heated to a temperature range of more than 1200 ° C at the time of welding, but the Zr-containing oxides are stably present even when heated to 1400 ° C. Thus, during the heating of the weld, the B nitride is solidified and the solid solution B is localized around the Zr-containing oxide. It is considered that the solid solution B is precipitated as a B nitride having an oxide as a nucleus in the cooling process after welding.

(e) Hardening B segregated at the old austenite grain boundaries of steel suppresses generation of coarse grain boundary ferrite at the time of welding, thereby improving HAZ toughness. For this reason, it is necessary to contain a sufficient amount of B in order to ensure the B content to be precipitated as the B nitride and to secure the solid solution B as well.

In order to sufficiently precipitate the B nitride, it is necessary to set the B content to B _asBN or more (BB _asBN ? 0). However, in order to obtain the grain boundary ferrite suppressing effect, the B content is increased, It is necessary to set the solubility B (B _F ) to 0.0005 or more (BB _asBN ? 0.0005).

Here, B in the equation is the content (mass%) of B contained in the steel, and B _asBN is a value obtained from the formula <1>.

(f) On the other hand, Al acts as a strong acid element in the steel, and if it is contained in a large amount in steel, it inhibits the formation of Zr and Ti oxides. It is important to limit the Al content to 0.005 mass% or less in order to secure the dissolved oxygen amount in the molten steel and to produce the Zr-containing oxide in the steel. More preferably, the content of Al is limited to 0.003 mass% or less. It is important to limit the total amount of deoxidation elements stronger than Al to 0.0005 mass% or less, such as Ca and REM.

In the steel material satisfying these conditions, a predetermined number or more of Zr-containing oxides of a predetermined size are produced. Most of the Zr-containing oxide is a complex oxide containing Zr and Ti, and B nitride is precipitated with the oxide as a nucleus. It has been found that when the heat treatment is actually performed on this steel material, the oxide particles effectively function as a ferrite generating site in the HAZ to improve the HAZ toughness through miniaturization of the HAZ structure.

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

First, the reason for limiting the chemical composition of the steel material according to the present embodiment will be described. In the following description, "%" in the description of each element means "% by mass".

(C: 0.01 to 0.20%)

C is a necessary element to secure strength. If the C content is less than 0.01%, the strength required as a general structural member can not be secured. Therefore, the lower limit of the C content is set to 0.01%. The lower limit of the C content is preferably 0.03%. On the other hand, if the C content exceeds 0.20%, it becomes difficult to secure toughness for both the base material and the HAZ. Therefore, the upper limit of the C content is set to 0.20%. The preferred upper limit is 0.15%.

(Si: 0.02 to 0.70%)

Si is an element contributing to the increase in the strength of the steel by increasing the quenching of the steel. In order to obtain this effect, the lower limit of the Si content is set to 0.02%. Preferably, the lower limit of the Si content is set to 0.05%. On the other hand, Si has a high reactivity with oxygen and has a deoxidizing action, thus affecting the formation of a Zr-containing oxide. If the Si content exceeds 0.70%, the composition of the oxide is changed, and the HAZ toughness is not achieved and the HAZ toughness is lowered. Therefore, the upper limit of the Si content is set to 0.70%. The upper limit of the Si content is more preferably 0.50%, and still more preferably 0.40%.

(Mn: 0.30 to 2.50%)

Mn has an effect of increasing the quenching of the steel, and is an element effective for securing strength and toughness. If the Mn content is less than 0.30%, strength and toughness as structural members can not be obtained due to lack of uniformity. Therefore, the lower limit of the Mn content is set to 0.30%. The lower limit of the Mn content is set to 0.60%. On the other hand, if the Mn content exceeds 2.50%, the toughness of the central segregation portion is lowered due to the Mn segregation at the time of solidification, and the quenching becomes too high, so that the hardness of both the base material and the HAZ increases and the toughness deteriorates. Therefore, the upper limit of the Mn content is set to 2.50%. The preferred upper limit is 2.00%.

(Ti: 0.003 to 0.024%)

Ti is an element that forms a complex oxide together with Zr. This composite oxide functions as a ferrite generating site in the grain in the HAZ and contributes to miniaturization of the HAZ structure. In order to obtain this effect, the lower limit of the Ti content is set to 0.003%. The lower limit of the Ti content is preferably 0.005%. On the other hand, Ti produces nitride. When a large amount of the Ti nitride is produced, the amount of the B nitride to be produced is suppressed, and the desired effect can not be obtained in the present embodiment. Also, excessive Ti forms TiC, deteriorating toughness of the base material and HAZ. Therefore, the upper limit of the Ti content is set to 0.024%. The preferred upper limit is 0.020%.

(B: 0.0010 to 0.0050%)

B is an element which bonds with nitrogen in the steel to form a B nitride around the Zr-containing oxide. By setting the B content to 0.0010% or more and satisfying predetermined conditions for B _asBN and B _F , which will be described later, the ferrite generation _ability in the HAZ is enhanced, contributing to improvement in toughness through microstructure. Further, solid solution B segregates in the austenite grain boundary, thereby suppressing coarse grain boundary ferrite generation. Therefore, the lower limit of the B content is set to 0.0010%. In order to further improve the HAZ toughness, the lower limit of the B content is preferably 0.0015%. On the other hand, when the B content is excessive, not only the effect of increasing the strength is saturated but also the toughness is deteriorated in both the base material and the HAZ. Therefore, the upper limit of the B content is set to 0.0050%. The preferred upper limit of the B content is 0.0030%.

(N: 0.0010 to 0.0090%)

N is an element necessary for forming B nitride in combination with B in the steel. To obtain this effect, the lower limit of the N content is set to 0.0010%. The lower limit of the N content is preferably 0.0020%. On the other hand, if the N content is excessive, the toughness of the base material and the HAZ deteriorates. Therefore, the upper limit of the N content is set to 0.0090%. The preferred upper limit is 0.0060%.

(O: 0.0010 to 0.0050%)

O (oxygen) is an indispensable element for the formation of the Zr-containing oxide. Therefore, the lower limit of the O content is set to 0.0010%. The preferable lower limit of the O content is 0.0015%. On the other hand, when the content of O is excessive, oxides are excessively produced, the cleanliness of the steel is lowered, and ductility such as base material toughness, elongation and drawing is deteriorated. Therefore, the upper limit of the O content is set to 0.0050%. The preferred upper limit is 0.0040%.

(Insol. Zr: 0.0005 to 0.0100%)

Insol. Zr represents acid-insoluble Zr, that is, Zr present in the steel as an oxide. Insol. Zr alone forms an oxide, or forms a composite oxide together with Ti. This oxide functions as a ferrite generating site in the grain in the HAZ, contributing to the miniaturization of the HAZ structure.

In order to obtain the above effect, Insol. It is necessary to set the lower limit of the Zr (acid insoluble Zr content) to 0.0005%. The lower limit is preferably 0.0010%. Meanwhile, Insol. When Zr is excessive, oxides are produced in a large amount in the steel, and HAZ toughness is deteriorated. Therefore, Insol. The upper limit of Zr is 0.0100%. The preferred upper limit is 0.0075%.

(P: 0.050% or less)

P is an impurity that is inevitably present in the steel. The P content is preferably as small as possible, but if the P content exceeds 0.050%, P is segregated at the austenite grain boundary, and the toughness remarkably decreases. Further, P segregated at grain boundaries causes high-temperature cracks at the time of welding. Therefore, the P content is limited to 0.0050% or less. Preferably, it is 0.030% or less. The smaller the P content, the better, so that the lower limit is not particularly specified, but may be 0.001% or more from the viewpoint of the production cost.

(S: 0.0080% or less)

S is an element that is inevitably present in the steel as an impurity. When the S content exceeds 0.0080%, a large amount of elongated MnS is generated in the central segregation portion, and the toughness and ductility of the base material and the HAZ deteriorate. Therefore, the S content is limited to 0.0080% or less. And preferably 0.0050% or less. The smaller the S content is, the better, so that the lower limit is not specifically defined, but may be 0.0001% or more from the viewpoint of the production cost.

(Al: 0.005% or less)

Al is an element that is actively added as a deoxidizing element in general. However, since Al predominantly reacts with oxygen, if the content is excessive, the formation of the desired Zr-containing oxide becomes insufficient and the effective ferrite producing sites in the HAZ decrease.

When the Al content is excessive, formation of alumina (Al ₂ O ₃ ) inclusions on the coarse clusters is promoted, and toughness of the base material and the HAZ is deteriorated. Therefore, it is preferable that the content of Al is reduced as much as possible. The permissible Al content is 0.005% or less, preferably 0.003% or less.

(Total of Ca and REM: 0.0005% or less)

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

(Sol. Zr: 0.0010% or less)

Sol. Zr represents the acid soluble Zr, that is, Zr dissolved in the steel. Sol. When the content of Zr is increased, the HAZ toughness remarkably deteriorates. Therefore, the content thereof is limited to 0.0010% or less. Sol. Zr is preferably as small as possible, so the lower limit is not specifically defined and may be 0%.

The above-mentioned Insol. Zr and Sol. Zr can be measured by electrolytic extraction residue analysis. The electrolytic extraction residue analysis method is a method of dissolving a hull by electrolysis in a nonaqueous solvent (acetylacetone-methanol solution or the like) and extracting a residue (precipitate or inclusion) with a filter having a pore diameter of 0.2 μm and separating. After separation, the amount of Zr contained in the solution is Sol. Zr, and the amount of Zr contained in the residue is insol. Zr.

Sol. Zr and Insol. The sum of Zr is the Zr content contained in the steel. The lower limit of the Zr content is calculated by the following formula: Insol. Like Zr, it is 0.0005%, preferably 0.0010%. The upper limit of the Zr content is as follows: Insol. The upper bound of Zr and Sol. Zr, that is, 0.0110%, preferably 0.0075%.

The steel material according to the present embodiment is based on that each of the above elements is contained and the remainder is made of Fe and impurities. The impurity means a component which is incorporated from a raw material such as ore and scrap when manufacturing a steel material industrially or other factors, and is allowed within a range not adversely affecting the characteristics.

However, as for P and S among the impurities, it is necessary to limit the upper limit as described above. Further, it is preferable that Al, Ca and REM act as a strong acid element in the steel to inhibit the generation of oxides by Zr or Ti, so that it is preferable to reduce them as much as possible.

The steel material according to the present embodiment may contain one or more selected from the group consisting of Cu, Ni, Cr, Mo, V, and Nb in the range described below for the purpose of further increasing the strength, . For the purpose of enhancing the corrosion resistance, one kind or two kinds selected from the group consisting of W and Sn may be added in the range described later.

(Cu: 1.50% or less)

Cu is an element having an effect of improving the strength and corrosion resistance of steel. In order to obtain these effects, the Cu content is preferably 0.10% or more. More preferably, it is 0.20% or more. On the other hand, even when Cu is added in an amount exceeding 1.50%, improvement in performance corresponding to an increase in alloy cost is not observed, which may cause surface cracks of the steel material. Therefore, the Cu content is set to 1.50% or less even when it is contained. The Cu content is preferably 1.00% or less, more preferably 0.70% or less, still more preferably 0.50% or less.

(Ni: 3.00% or less)

Ni is an element having an effect of improving the strength of a steel. Further, Ni is an element having an effect of increasing the toughness of a steel matrix (raw paper) in a solid state. In order to obtain these effects, the Ni content is preferably 0.10% or more. On the other hand, even when Ni is contained in an amount exceeding 3.00%, improvement of characteristics corresponding to an increase in alloy cost can not be obtained. Therefore, even when contained, the Ni content is set to 3.00% or less. Preferably, the Ni content is 2.00% or less, more preferably 1.00% or less.

(Cr: 1.00% or less)

Cr is an element useful for improving the strength by increasing the quenching. Cr is also an element for increasing the corrosion resistance. In order to obtain these effects, the Cr content is preferably 0.10% or more. On the other hand, even when Cr is contained in an amount exceeding 1.00%, not only the effect of improving the corrosion resistance is saturated but also the HAZ is hardened and the toughness is sometimes deteriorated. Therefore, even in the case of inclusion, the Cr content is set to 1.00% or less. Preferably, the Cr content is 0.50% or less.

(Mo: 1.00% or less)

Mo is an element having an effect of improving the strength and toughness of the base material. In order to obtain this effect, the Mo content is preferably 0.05% or more. On the other hand, when the Mo content exceeds 1.00%, the hardness of the HAZ increases, and the HAZ toughness may deteriorate in some cases. Therefore, even in the case of containing Mo, the Mo content is set to 1.00% or less. Preferably, the Mo content is 0.50% or less, more preferably 0.30% or less.

(V: 0.100% or less)

V is an element having an effect of improving the strength of the base material mainly by precipitation of carbonitride during tempering. In order to obtain this effect, the V content is preferably 0.010% or more. On the other hand, if the V content exceeds 0.100%, not only the effect is saturated but also the hardness is increased and the toughness is sometimes deteriorated. Therefore, the V content is made 0.100% or less even when it is contained. Preferably, the V content is 0.050% or less.

(Nb: 0.035% or less)

Nb is an element that improves the strength and toughness of the base material due to atomization and deposition of carbide. In order to obtain these effects, the Nb content is preferably 0.005% or more. On the other hand, if the Nb content exceeds 0.035%, not only the above effect is saturated but also the HAZ toughness is lowered. Therefore, even when it is contained, the content of Nb is set to 0.035% or less. Preferably, the Nb content is 0.025% or less.

(W: 1.00% or less)

W is dissolved and adsorbed to rust in the form of oxygen acid ion WO ₄ ^- to inhibit the permeation of chloride ions in the green layer, thereby improving the corrosion resistance. In order to obtain this effect, the W content is preferably 0.01% or more. On the other hand, if the W content exceeds 1.00%, not only the effect is saturated but also the base material and HAZ toughness are lowered in some cases. Therefore, even in the case of incorporation, the W content is set to 1.00% or less. Preferably, the W content is 0.75% or less.

(Sn: 0.50% or less)

Sn is an element having an action of inhibiting corrosion by inhibiting the action of an inhibitor in an acidic chloride solution by dissolving as Sn ²⁺ . Further, Sn has an action of suppressing the anode dissolution reaction of the steel and improving the corrosion resistance. In order to obtain these effects, the Sn content is preferably 0.03% or more. On the other hand, if Sn is contained in an amount exceeding 0.50%, the effect is saturated and rolling cracks of the steel sheet are liable to occur. Therefore, even when Sn is contained, its content is made 0.50% or less.

The steel material according to the present embodiment can be obtained by controlling the content of each element as described above, and _thereafter , the B _asBN expressed by the following formula <1> is 0.0005 or more and the B _F represented by the following formula <2> It is necessary. The reason for each will be described below.

Here, N, Ti, O and B in the formula (1) are contents (mass%) of N, Ti, O and B contained in the steel, respectively. And Zr is the content (mass%) of the acid-insoluble Zr.

As described above, in the steel material according to the present embodiment, the B nitride is precipitated in the surface layer of the Zr-containing oxide, so that generation of ferrite in the particles during cooling after welding is more effectively promoted than in the case of the Zr- Improves HAZ toughness.

In order to obtain these effects, it is necessary to set the index of the B content contributing to the precipitation of B nitride to B _asBN or more after the B _asBN represented by the above formula <1> is _set to 0.0005 or more. More preferably, the B _asBN is 0.0010 or more. On the other hand, when the B _asBN is more than 0.0030, not only the above effect is saturated but also surface cracks tend to occur during casting. Therefore, the upper limit of the preferable _AsBN is 0.0030 or less.

When B _asBN is 0.0005 or more, a certain number or more of BN is formed even if Ti is contained in the steel. Since Ti is an element preferentially binding to N more than B, in this case, Ti in the steel exists as Ti oxide or Ti nitride, and solid Ti does not exist in the steel.

Further, in the steel material according to the present embodiment, grain formation due to generation of ferrite in the grain is made finer, and coarse grain boundary ferrite generation by the solid solution B segregated in the austenite grain boundary is suppressed, and HAZ toughness is improved.

In order to obtain this effect, it is necessary to set the B content present as solid solution B, that is, B _F represented by the above formula <2> to 0.0005 or more. More preferably, it is 0.0007 or more. As described above, since B? B _asBN , B _F does not exceed the B content (B in the formula <2>). On the other hand, if B _F exceeds 0.0020, the above effect is saturated, and the quenching of the steel becomes excessive, which causes generation of low-temperature cracks in the welded portion. Therefore, the upper limit of the more preferable B _F is 0.0020.

Next, the oxide contained in the steel material according to the present embodiment will be described.

The steel material according to the present embodiment has a Zr-containing oxide having a circle equivalent diameter of 0.5 mu m or more at 50 / mm < 2 > or more.

In the steel material according to the present embodiment, B nitride is precipitated with the Zr-containing oxide as nuclei to form a composite inclusion. This composite inclusion becomes a ferrite formation site in the particle at the time of cooling after welding. The Zr-containing oxide is mainly composed of an oxide containing Zr and Ti, but when the oxide is a precipitate nucleus of B nitride, it is preferable that the Zr concentration in the oxide is equal to or higher than the Ti concentration.

This effect is obtained when the circle-equivalent diameter of the Zr-containing oxide (the diameter of a circle having the same area as the observed cross-sectional area of the oxide) is 0.5 탆 or more. In order for the oxide to function as a ferrite generating site in the particle, the upper limit is not limited because the circle equivalent diameter is preferably larger. However, when the circle-equivalent diameter becomes larger, the number density of oxides becomes relatively small, and the possibility that the coarse oxide itself acts as a starting point of fracture increases. Therefore, the circle equivalent diameter of the Zr-containing oxide is preferably 10.0 占 퐉 or less.

It is essential that at least one Zr-containing oxide is dispersed in the austenite grains when heated at the time of welding as a condition for acting as a site for producing ferrite in the grain. Therefore, it is necessary that the oxides of the above-mentioned sizes are dispersed at a number density of 50 / mm 2 or more. The larger the number density of oxides is, the more preferable is that the ferrite formation sites are increased, but the effect is saturated even if the number exceeds 500 pieces / mm 2.

The circle equivalent diameter and the number density of the Zr-containing oxide can be measured by observing the surface of the mirror-polished steel material with a scanning electron microscope (SEM). Specifically, the number of Zr-containing oxide particles having a circle equivalent diameter of 0.5 탆 or more was measured by SEM in a range of 10 mm × 10 mm (100 mm 2) or more and divided by the area of the observed field of view, . Photographs taken by SEM may be used. The particles to be subjected to the measurement of the number density of oxide can be at least particles in which Zr and O are detected by qualitative analysis by an energy dispersive X-ray analyzer (EDX) attached to the SEM.

The steel material according to the present embodiment can be obtained, for example, by melting a molten steel by a known method such as a converter or an electric furnace, forming a steel material such as a slab or billet by a known method such as a continuous casting method, Rolling may be performed. The molten steel may be subjected to treatment such as ladle refining or vacuum degassing. The steel material after casting or ingot may be directly subjected to hot rolling. After hot rolling, heat treatment or cold working can be carried out.

However, in the steel material according to the present embodiment, when the molten steel is dissolved, the O 2 activity amount in the molten steel is analyzed, and the amount of Zr added is adjusted in accordance with the dissolved O amount to obtain "Insol. Zr and Sol. Quot; Zr " For example, if the amount of dissolved O in the molten steel is 0.0025 mass%, about 7 g of Zr per 100 kg of molten steel is added to obtain Insol. Zr, Sol. Zr can satisfy the desired content range.

Further, if the time from the addition of Zr to the casting is made longer than usual, there is a fear that the oxides coagulate and coarsen and an oxide having a desired number density may not be obtained. Therefore, the time until casting is preferably set to 60 minutes or less Do.

Example

Next, the embodiments of the present invention will be described, but the conditions in the embodiments are examples of conditions employed to confirm the feasibility and effect of the present invention, and the present invention is not limited to this one conditional example . The present invention can adopt various conditions as long as the objects of the present invention are achieved without departing from the gist of the present invention.

A high-frequency induction furnace using a refractory material of magnesia substrate as refractory material was used to dissolve electrolytic iron or industrial pure iron having a low Al content as a base material. Thereafter, granular carbon was added so as to have a predetermined concentration and maintained for a predetermined time under a reduced pressure inert gas atmosphere while induction heating. Here, the pressure was less than 1 Torr, the argon gas was used at 90% or more of the residual gas, and the holding time was about 10 minutes. Further, the molten steel temperature is generally set at 1600 to 1650 DEG C at which the melting of steel occurs.

Thereafter, the pressure was changed from 100 Torr to an inert gas atmosphere at about normal pressure, and the necessary alloy components were adjusted. Then, casting was performed through a trough in a mold commonly used at the beginning of 50 to 150 kg of steel. "Insol. Zr, Sol. Zr "was performed by analyzing the O activity amount in the molten steel and adjusting the amount of Zr in accordance with the dissolved O amount. For example, if the amount of dissolved O in the molten steel is 0.0025 mass%, about 7 g of Zr per 100 kg of molten steel is added to obtain Insol. Zr, Sol. Zr could be obtained satisfying the desired content range.

Further, a steel sheet having a thickness of 30 mm was obtained by forging and hot rolling, and this steel sheet was used as a test steel.

First, the insol of this steel. Zr and Sol. Zr was dissolved by electrolysis in a non-aqueous solvent (acetylacetone-methanol solution or the like), and the mother phase was dissolved and measured by electrolytic extract residue analysis. The residue (precipitate or inclusion) was extracted with a filter having a pore diameter of 0.2 mu m. After the separation, the amount of Zr (Sol. Zr content) contained in the solution and the amount of Zr (insol. Zr content) contained in the residue were measured by chemical analysis.

Further, the number density of the Zr-containing oxide having a circle-equivalent diameter of 0.5 mu m or more was measured by observation using an SEM. At that time, the observation surface was a mirror surface polished steel surface. In addition, EDX was used to confirm the composition of the particles. As a result of observation, in the inventive example, at least 90% of Zr-containing oxides having a circle equivalent diameter of 0.5 탆 or more were oxides containing Zr and Ti.

Next, a test piece for thermal cycle test was taken from the steel material. The test piece was provided with a heat cycle in which 40 kJ / mm of heat input (welding heat welding) was reproduced. Specific heat cycle conditions include heating to 1400 占 폚 at room temperature, holding at 1400 占 폚 for 10 seconds, and thereafter the temperature range from 800 占 폚 to 500 占 폚, which is a temperature range related to the transformation in the grain, at a rate of 1.0 占 폚 / And cooled.

Three Charpy test pieces of JIS No. 4 were taken from the steel material subjected to thermal cycling, and Charpy test was performed at -40 캜 to measure the absorbed energy (vE- ₄₀ ). The Charpy test was conducted in accordance with JIS Z 2242.

Table 1 and Table 2 show the chemical composition of the test steel. Table 3 shows the number density of the Zr-containing oxide having a circle equivalent diameter of 0.5 탆 or more and the Charpy test result, respectively. In either example, since Ca and REM were not added, the total content thereof was 0.0005% or less.

As shown in Table 2, all of Nos. 1 to 30 of the present invention had excellent toughness of 100 J or more on average. On the other hand, in the comparative examples No. x1 to x18, the toughness was deteriorated because the chemical composition was out of the range specified in the present invention. No. x13 satisfied the component range of the present invention. However, after the addition of Zr, the time until the casting was made longer than the other examples, the oxide coagulated and coarsened, and the number density of the oxides required for the ferrite formation site decreased. As a result, the toughness deteriorated.

The steel material according to the present invention has excellent toughness particularly in the HAZ of large heat input welding. By using this steel material, high-efficiency welding is possible, and the construction cost of the welded structure can be drastically reduced. It is also possible to improve the safety and reliability of the welded structure. Therefore, the present invention is very remarkable in industrial contribution.

Claims

Chemical composition, in% by mass,
C: 0.01 to 0.20%
0.02 to 0.70% Si,
Mn: 0.30 to 2.50%
Ti: 0.003 to 0.024%,
B: 0.0010 to 0.0050%,
N: 0.0010 to 0.0090%,
O: 0.0010 to 0.0050%,
Insol. Zr: 0.0005 to 0.0100%,
P: 0.050% or less,
S: 0.0080% or less,
Al: 0.005% or less,
Sol. Zr: 0.0010% or less,
Ca and REM: 0.0005% or less,
The remainder is composed of Fe and impurities,
A B _asBN represented by the following formula <1> is 0.0005 or more, a B _F represented by the following formula <2> is 0.0005 or more,
And a Zr-containing oxide having a circle-equivalent diameter of 0.5 mu m or more at 50 / mm < 2 > or more.

The method according to claim 1,
Wherein the chemical composition further comprises, by mass%
Cu: 1.50% or less,
Ni: 3.00% or less,
Cr: not more than 1.00%
Mo: 1.00% or less,
V: 0.100% or less, and
And Nb: 0.035% or less, based on the total weight of the steel material.

3. The method according to claim 1 or 2,
Wherein the chemical composition further comprises, by mass%
W: not more than 1.00%, and
And Sn: 0.50% or less.