KR20130029024A

KR20130029024A - Steel excellent in toughness of base metal and weld-heat affected zone and method for manufacturing the same

Info

Publication number: KR20130029024A
Application number: KR1020120101036A
Authority: KR
Inventors: 데쯔시 데우라; 다까시 스기따니; 히로끼 오오따; 요시또미 오까자끼; 히데노리 나꼬
Original assignee: 가부시키가이샤 고베 세이코쇼
Priority date: 2011-09-13
Filing date: 2012-09-12
Publication date: 2013-03-21
Also published as: JP5883257B2; CN102994872A; CN102994872B; BE1020801A3; JP2013060631A

Abstract

PURPOSE: Steel with a base material and a HAZ(Heat-Affected Zone) of excellent toughness and a method for manufacturing the same are provided to obtain the improvement of the ductility of the base material and the HAZ at high precision. CONSTITUTION: Steel with a base material and a HAZ of excellent toughness comprises 0.03-0.16% of C, 0.25% or less of Si(exclusive of 0%), 1-2.0% of Mn, 0.03% or less of P(exclusive of 0%), 0.015% or less of S(exclusive of 0%), 0.05% or less of Al(exclusive of 0%), 0.010-0.08% of T, 0.0005-0.010% of Ca, 0.0020-0.020% of N, and the rest of iron and inevitable impurities. [Reference numerals] (AA) HAZ toughness; (BB) Z value

Description

STEEL EXCELLENT IN TOUGHNESS OF BASE METAL AND WELD-HEAT AFFECTED ZONE AND METHOD FOR MANUFACTURING THE SAME}

BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to steel materials used in welding structures such as bridges, high-rise buildings, ships, and the like, and the site affected by heat when welding the toughness and high heat input of the base metal (hereinafter referred to as "welding heat affected zone" or "HAZ"). It is related with the technique to improve the toughness of the case).

With the increase in the size of the welded structure, welding of thick steel sheets having a plate thickness of 50 mm or more is inevitable. For this reason, the high heat input welding of 50 kJ / mm or more is aimed at the viewpoint of the improvement of welding construction efficiency. However, when the heat input welding is performed, the HAZ is heated to a high temperature austenite region and then slowly cooled, so that the structure of the HAZ (particularly near the bond portion of the HAZ) becomes coarse and the toughness of the portion tends to deteriorate. have. It is a problem to ensure good toughness in the HAZ (hereinafter sometimes referred to as "HAZ toughness").

The deterioration prevention technology of HAZ toughness at the time of high heat input welding is variously proposed. As a representative example of such a technique, for example, Patent Documents 1 to 4 disperse and deposit fine TiN in steel, thereby suppressing coarsening of austenite particles generated in HAZ when a high heat input welding is performed, and thus HAZ toughness is suppressed. Steel materials with reduced deterioration have been proposed. However, in these techniques, when the weld metal is at a high temperature of 1400 ° C. or higher, in the portion (bond portion) of the HAZ, which is particularly close to the weld metal, the TiN dissolves due to heat received during welding, thereby sufficiently deteriorating the HAZ toughness. There is a problem that cannot be suppressed.

In addition, Patent Literature 5 proposes a technique for improving the HAZ toughness by suppressing the generation of coarse TiN having a particle diameter exceeding 0.1 μm by optimizing the number density of fine TiN having a particle size of 0.01 to 0.1 μm. However, it was found that even when the number density of the fine TiN was optimized, sufficient HAZ toughness could not be secured.

On the other hand, the present applicant actively contains Nb in the TiN inclusions present in the welded steel and controls the Ti / Nb ratio, so that the number of inclusions having a particle diameter of 0.01 to 0.25 µm is 1.0 × 10 ⁴ per ¹ mm 2. By the above, the technique which ensures HAZ toughness in a wide heat input range is proposed (for example, patent document 6). However, even with this technique, the loss of solid solution of TiN due to the heat received during welding cannot be avoided, and the HAZ toughness may be deteriorated in some cases.

By the way, in addition to HAZ toughness, the steel used for a welded structure also requires that the toughness (base metal toughness) of the steel itself is good as a basic characteristic. Therefore, this applicant proposes to the patent document 7 the thick steel plate which improved both the base material toughness and HAZ toughness. In this technique, the base steel sheet and the HAZ toughness provide a thick steel sheet by controlling the number density in accordance with the size of the Ti-containing nitride contained in the steel sheet and appropriately controlling the area ratio of the island-like martensite. However, in the above method, since the number density for each size is measured by microscopic observation, it is not necessarily a method with high accuracy, and there is a possibility that a variation occurs in the characteristic.

Japanese Patent Publication No. 55-26164 Japanese Patent Publication No. 2003-166017 Japanese Patent Laid-Open No. 2003-213366 Japanese Patent Laid-Open No. 2001-20031 Japanese Patent Laid-Open No. 2001-98340 Japanese Patent Laid-Open No. 2004-218010 Japanese Patent Laid-Open No. 2010-95781

This invention is made | formed in view of the above circumstances, and the objective is to provide the steel material excellent in both a base material toughness and HAZ toughness, and its manufacturing method.

The steel material which is excellent in the toughness of the base material and the weld heat-affected zone which concerns on the said subject can be solved by C: 0.03 to 0.16% (mean of mass%. It is the same for the following components), Si: 0.25 % Or less (including 0%), Mn: 1 to 2.0%, P: 0.03% or less (0%), S: 0.015% or less (0%), Al: 0.05% or less (0% is not included), Ti: 0.010% to 0.08%, Ca: 0.0005% to 0.010%, and N: 0.0020% to 0.020%, and the remainder is a steel material composed of iron and unavoidable impurities.

And as Ti containing inclusions exceeding 2.0 micrometers in the total Ti amount Q contained in steel materials, Ti amount contained in steel materials is 0.010% or less (it does not contain 0%), and is Ti containing inclusions exceeding 0.1 micrometer. It has the point that the ratio R / Q of the value R which subtracted Ti amount contained in steel materials from total Ti amount Q, and the total Ti amount Q contained in steel materials is 0.30-0.70.

In the present invention, the Ti-containing inclusion means a precipitate containing at least Ti, and a nitride forming element (for example, Nb, Zr, TiN) or a part of Ti (about 50% or less by atomic ratio) Means inclusions containing Ti, such as Ti containing oxide, besides Ti containing nitrides, such as complex nitride substituted with V etc.). As the Ti-containing oxide, not only a Ti oxide (for example, TiO ₂ ) but also a part of Ti (about 50% or less in atomic ratio) is used for other oxide-forming elements (for example, Si, Mn, Al, Ca, Zr, REM and the like) is also intended to include a composite oxide.

The steel is also another element,

(a) Ni: 1.5% or less (without 0%), Cu: 1.5% or less (without 0%), Cr: 1.5% or less (without 0%), and Mo: 1.5% or less One or more elements selected from the group consisting of (not containing 0%),

(b) Nb: 0.10% or less (does not contain 0%), and / or V: 0.1% or less (does not contain 0%),

(c) B: 0.005% or less (does not contain 0%),

(d) Zr: 0.02% or less (does not contain 0%), and / or REM: 0.02% or less (does not contain 0%), etc. may be included.

The steel is an Al ₂ O ₃ -containing inclusion (specifically, Al ₂ O included in the steel by flotation of the inclusions contained in the molten steel after the steel, so that Ti, N and Si satisfies the following formula 1) It can manufacture by casting after controlling the number of the inclusions containing 80 mass% or more of ₃ to 10 or less (including 0) per 1 mm <2>. In following formula 1, [] expresses content (mass%) of each element in steel.

[Formula 1]

However, when Si = 0 mass%, steel is melted so that Ti and Ni may satisfy following formula (2).

[Formula 2]

According to the present invention, for the Ti-containing inclusions in the steel, the number density for each size is not controlled by microscopic observation as in the prior art, but a coarse size larger than 2.0 µm in the total amount of Ti Q included in the steel. While the amount of Ti contained in the steel as Ti-containing inclusions is reduced as much as possible, the amount of Ti contained in the steel as Ti-containing inclusions having a size exceeding 0.1 μm is quantified by electrolytic extraction, and the amount of Ti is determined from the total Ti amount Q. Since the value R which subtracted and the ratio R / Q of the total Ti amount Q contained in steel materials is controlled suitably, the toughness improvement of a base material and HAZ can be implement | achieved with high precision.

1 is the total Ti amount Q (total Ti amount contained in the sample) prescribed in the present invention, the Ti-containing inclusions having a size exceeding 2.0 μm, the amount of Ti contained in the steel, the size exceeding 0.1 μm, and 2.0 μm The schematic diagram explaining the concept of the amount of Ti contained in steel materials, and the amount of solid solution Ti amount R (The amount of Ti contained in steel materials as Ti containing inclusions whose size is 0.1 micrometer or less is included) as Ti containing inclusions which are the following.
2 is a graph showing the relationship between the value (Z value) of [Ti] × [N] × [Si] and the HAZ toughness.

MEANS TO SOLVE THE PROBLEM In order to improve both the base material toughness and HAZ toughness of steel materials, the present inventors only control a number density according to the magnitude | size of Ti containing inclusions determined based on microscopic observation conventionally, The observation field area is only 300. Since it was small about 2 mm <2>, in view of the fact that a precision was low and a variation generate | occur | produced, it examined, in order to provide the method with a higher precision instead of this. At that time, by the combination of filtration separation by a plurality of membrane filters (hereinafter sometimes referred to simply as filters) having different electrolytic extractions and scales (meshes), the total amount of Ti in the steel is sifted for each size, and Considering whether the method of quantifying Ti amount (mass concentration) cannot be used properly, it has been repeated.

As a result,

(A) After the electrolytic extraction with a predetermined electrolyte solution, when the Ti passed through the filter having a scale of 0.1 μm to solid solution Ti, the amount of solid solution Ti has a great influence on the improvement of the base metal toughness and HAZ toughness of the steel,

(B) The amount of solid solution Ti is not controlled as an absolute value, and the desired characteristics are not controlled unless it is controlled by the balance with the total amount of Ti contained in the steel (specifically, the ratio of the amount of solid solution Ti to the total amount of Ti). Not exerted,

(C) Furthermore, in order to effectively exhibit the desired characteristics, it is not enough to simply control the ratio of the amount of solid solution Ti, so that the size of not passing through the filter having a scale of 2.0 μm (remaining on the filter) is more than 2.0 μm. It is also important to appropriately control the amount of Ti contained in the Ti-containing inclusions,

(D) Therefore, in order to improve the base material toughness and HAZ toughness of steel materials, it is contained in steel materials as the ratio of the solid solution Ti amount with respect to the total amount of Ti computed by the said method, and Ti containing inclusions whose size exceeds 2.0 micrometers. It was discovered that it is very important to control Ti amount appropriately, and this invention was completed.

In the present specification, the amount of Ti (in the present invention, this is called solid solution Ti) passing through a filter having a scale of 0.1 μm after Q and electrolytic extraction (details will be described later) is determined by the total amount of Ti contained in the sample. Naming. The total Ti amount Q is a value quantified by ICP emission analysis after electrolytic extraction. The quantity R of the said solid solution Ti means the quantity of Ti which passed the filter when it filtered using the filter whose scale is 0.1 micrometer after performing an electrolytic extraction. Among the filters currently available, the smallest scale is 0.1 µm, and in the present invention, passing through the minimum diameter filter was considered to be "employed Ti" even if it existed as Ti-containing inclusions, for example.

In addition, the amount R of solid solution Ti determined as described above does not directly measure the amount of solid solution Ti and does not pass through the filter (remains on the filter) using a filter having a scale of 0.1 µm after the electrolytic extraction. Is indirectly calculated by quantifying the amount of Ti contained in the Ti-containing inclusion having a content of more than 0.1 µm by ICP emission analysis and subtracting it from the total amount of Ti Q contained in the steel. This is because it is difficult to directly quantitatively analyze the amount of solid solution Ti.

In the present specification, the term "Ti-containing inclusions larger than 2.0 µm" means not passing through the filter (remaining on the filter) when the scale is filtered using a filter having a thickness of 2.0 µm after the electrolytic extraction. Conventionally, it has been known that coarse Ti-containing inclusions adversely affect HAZ toughness and the like. According to the results of the present inventors, coarse Ti-containing inclusions having a size of more than 2.0 μm, which is distinguished by the above method, among coarse Ti-containing inclusions, are particularly known. As a result, it has been found that the amount of inclusions has a great adverse effect on the toughness of the base metal and the HAZ toughness, in particular, the amount of the coarse Ti-containing inclusions is properly controlled.

First, the total amount of Ti contained in the steel of this invention is demonstrated. In the present invention, the steel material is dissolved by the electrolytic extraction method, the extraction residue after the electrolytic extraction is filtered by a membrane filter having a smallest scale of 0.1 µm among commercially available ones, and the extraction residue obtained by filtration is melted for each filter to obtain ICP. Ti amount is measured by luminescence analysis. According to this method, the total Ti amount Q contained in steel materials passes Ti amount contained in steel materials as Ti containing interference | inclusion exceeding 0.1 micrometer in size which does not pass through a filter, as shown in FIG. It is expressed as the total amount of the solid solution Ti amount including the Ti amount contained in the Ti-containing inclusion.

And the present inventors examined, and the ratio R / Q of the value R which subtracted Ti amount contained in steel materials from total Ti amount Q as Ti containing inclusions whose size exceeds 0.1 micrometer, and the total Ti amount Q contained in steel materials By adjusting to the range of 0.30 to 0.70, it became clear that the base material toughness and HAZ toughness of steel materials can be improved. This is illustrated in the examples herein.

That is, the steel grade N shown in Table 1 of the Example, and the steel grade d shown in Table 2 are steel materials with substantially the same composition, and the amount of Ti contained in steel materials as coarse Ti containing inclusions exceeding 2.0 micrometers is also almost the same. However, while steel grade d is inferior in base metal toughness and HAZ toughness, steel type N is improved in both base metal toughness and HAZ toughness. Similarly, even though the composition and the amount of Ti are almost the same for the steel grade E shown in Table 1 and the steel grade b shown in Table 2 below, the steel grade b is deteriorated in the base material toughness and the HAZ toughness, whereas the steel type E is the base material. Both toughness and HAZ toughness are improved.

For this reason, the steel grade d has a high ratio of solid solution Ti to the added Ti amount (total Ti content in the steel), and the steel grade b has a low ratio of solid solution Ti to the added Ti amount. In addition, it is estimated that this amount of solid solution contributes greatly to the improvement of the base metal toughness and the HAZ toughness.

Based on these results, it was clarified that the ratio R / Q of the amount of solid solution Ti to the total amount of Ti contained in the steel influences the toughness of the base metal and the HAZ.

By the way, only by controlling the ratio R / Q of the solid solution Ti amount with respect to the total Ti amount contained in steel materials to an appropriate range, it became clear that the base material toughness and HAZ toughness improvement effect are inadequate.

That is, the present inventors further examined that, among the Ti-containing inclusions having a size exceeding 0.1 μm, the Ti content included in the steel as Ti-containing inclusions having a size exceeding 2.0 μm also affects the base material toughness and the HAZ toughness. It turned out. This is also demonstrated by the Example mentioned later. For example, steel grade L shown in Table 1 of the Example, and steel grades a and e shown in Table 2 are steel materials of almost the same composition, and the said ratio R / Q is all controlled in the range of 0.30-0.70, but steel grade a Since e has a large amount of Ti contained in the coarse Ti-containing inclusion having a size exceeding 2.0 µm, both the base metal toughness and the HAZ toughness deteriorate. On the other hand, since the amount of Ti contained in the coarse Ti containing inclusions whose size exceeds 2.0 micrometers is reduced to the range prescribed | regulated by this invention, steel grade L becomes favorable both a base material toughness and HAZ toughness.

Thus, it can be seen that by reducing the amount of Ti contained in the steel as a coarse Ti-containing inclusion having a size exceeding 2.0 µm, both the base metal toughness and the HAZ toughness can be improved. This reason is not known in detail, but it is considered as follows. In order to improve HAZ toughness, since the refinement of the spherical γ particle size by fine Ti-containing inclusions (for example, TiN precipitates) is effective, much Ti is required for that purpose. However, when Ti contained in the steel is present as coarse Ti-containing inclusions having a size exceeding 2.0 µm, the number of fine Ti-containing inclusions (for example, TiN precipitates) is insufficient, resulting in miniaturization of the old? Particle size. Not only can this be realized, but the coarse Ti containing inclusion itself becomes a starting point of destruction, and it is thought that both the base material toughness and the HAZ toughness are reduced.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated concretely.

<The ratio R / Q of the value R obtained by subtracting the Ti amount included in the steel material from the total Ti amount Q as the Ti-containing inclusions having a size exceeding 0.1 μm, and the total Ti amount Q included in the steel is 0.30 to 0.70.

In the present invention, the ratio of the amount of solid solution Ti to the total amount of Ti contained in the steel (hereinafter, may be referred to as a solid solution Ti ratio) is set to 0.30 to 0.70. When the solid-solution Ti ratio is less than 0.30, the Oswald growth of the TiN particles becomes remarkable during heat treatment or welding, so that Ti-containing nitrides tend to coarsen, and fine Ti-containing nitrides effective for improving HAZ toughness and base metal toughness. The production amount of can not be secured. Therefore, since metal structure cannot be refined at the time of welding, base material toughness and HAZ toughness fall. Therefore, the solid solution Ti ratio is 0.30 or more, preferably 0.35 or more, and more preferably 0.40 or more. However, when the solid solution Ti ratio exceeds 0.70, the amount of solid solution Ti increases too much, resulting in coarsening of the metamorphic structure generated from the old? Grain boundary, resulting in a decrease in base metal toughness and HAZ toughness. Therefore, the solid solution Ti ratio is 0.70 or less, preferably 0.65 or less, and more preferably 0.60 or less.

The solid solution Ti ratio may be represented by a value R obtained by subtracting the Ti amount included in the steel as a Ti-containing inclusion having a size exceeding 0.1 μm from the total Ti amount Q and the ratio R / Q of the total Ti amount Q included in the steel. (See FIG. 1). That is, the value R means the total amount of Ti actually dissolved in the steel material and the amount of Ti contained in the ultrafine inclusions that passed through the filter having a scale of 0.1 µm, and in the present invention, the value R is included in the ultrafine inclusions. Ti amount is regarded as solid solution Ti.

Regarding the amount of Ti contained in the steel as a coarse Ti-containing inclusion having a size exceeding 2.0 μm>

In this invention, the amount of Ti contained in steel materials as coarse Ti containing inclusions whose size exceeds 2.0 micrometers is made into 0.010% or less (0% is not included). When the Ti amount exceeds 0.010%, coarse Ti-containing inclusions that become the starting point of fracture increase, which causes a decrease in base metal toughness and HAZ toughness. The smaller the amount of Ti is, the more it is preferably 0.0080% or less, and more preferably 0.0050% or less.

The amount of Ti contained in steel as coarse Ti containing inclusions exceeding 2.0 micrometers means the amount of Ti contained in Ti containing inclusions extracted by electrolytic extraction from steel materials and not passing the filter whose scale is 2.0 micrometers. Doing. The above Ti-containing inclusions are intended to include all of Ti-containing inclusions, and are meant to include Ti-containing oxides, Ti-containing carbides, composite compounds thereof, and the like, in addition to nitrides containing Ti. In the present invention, as will be described later, the residue extracted by the electrolytic extraction method is melted, and the amount of Ti is measured by the ICP emission spectrometry. Therefore, all compositions contained in the steel as Ti-containing inclusions exceeding 2.0 µm in size are used. With respect to the inclusions, the total amount of Ti amount can be measured. And base metal toughness and HAZ toughness can be improved as the total amount of Ti contained in the coarse inclusion whose size exceeds 2.0 micrometers is 0.010% or less with respect to steel materials.

In addition, the magnitude | size of the coarse Ti containing inclusions extracted from steel materials shall exceed 2.0 micrometers in size. It is because the influence of toughness by the difference of Ti amount was hardly seen about Ti containing inclusion whose size is 2.0 micrometers or less.

As described above, the steel of the present invention is a Ti-containing inclusion having a size exceeding 2.0 μm, and the amount of Ti contained in the steel is 0.010% or less, and the solid solution Ti ratio (ratio / Q) is 0.30 to 0.70. There is a characteristic in point.

Next, the component composition of the steel material of this invention is demonstrated.

[C: 0.03 to 0.16%]

C is an indispensable element in order to secure strength, and when C amount is less than 0.03%, strength cannot be secured. Therefore, C amount is 0.03% or more, Preferably it is 0.04% or more, More preferably, it is 0.05% or more. However, when the amount of C becomes excessive, a lot of hard island martensite (MA) is produced, which degrades the base metal toughness and the HAZ toughness. Therefore, the amount of C needs to be 0.16% or less, preferably 0.12% or less, and more preferably 0.10% or less.

[Si: 0.25% or less (including 0%)]

Si is an element useful for securing strength by solid solution strengthening, but when the amount of Si is excessive, hard island-like martensite (MA) is generated, and coarse Ti-containing inclusions are generated to form substrate toughness and HAZ toughness. This is deteriorated. Therefore, Si amount is 0.25% or less, Preferably it is 0.2% or less, More preferably, it is 0.1% or less, More preferably, you may be 0.08% or less. It is preferable to contain 0.01% or more of Si amount, More preferably, it is 0.02% or more, More preferably, it is 0.03% or more.

[Mn: 1 to 2.0%]

Mn is an element useful in securing strength and needs to be contained 1% or more. Mn amount becomes like this. Preferably it is 1.2% or more, More preferably, it is 1.4% or more. However, when Mn is excessively contained in excess of 2.0%, the strength rises excessively and the base metal toughness and the HAZ toughness deteriorate. Therefore, Mn amount is 2.0% or less, Preferably it is 1.8% or less, More preferably, you may be 1.7% or less.

[P: 0.03% or less (does not include 0%)]

Since P is an unavoidable impurity element and tends to cause grain boundary breakage and adversely affects both base metal toughness and HAZ toughness, the amount is preferably as small as possible. Therefore, P amount needs to be suppressed to 0.03% or less, Preferably it is 0.02% or less, More preferably, you may be 0.01% or less. However, it is industrially difficult to make the amount of P in steel into 0%, and about 0.003% is contained normally.

[S: 0.015% or less (does not include 0%)]

Since S is an unavoidable impurity element and causes grain boundary destruction by grain boundary segregation and deterioration of base metal toughness by coarse sulfide, the amount is preferably as small as possible. Therefore, the amount of S needs to be suppressed to 0.015% or less, Preferably it is 0.010% or less, More preferably, it is 0.008% or less, More preferably, you may be 0.005% or less. However, it is industrially difficult to make S amount in steel into 0%, and about 0.0001% is contained normally.

[Al: 0.05% or less (does not contain 0%)]

Al acts as a deoxidizer, but when excessively contained, Al ₂ O ₃ -containing inclusions are formed in the steel, which causes coarse Ti-containing inclusions to be generated, thereby degrading base metal toughness and HAZ toughness. Therefore, it is necessary to suppress Al amount to 0.05% or less, Preferably it is 0.040% or less, More preferably, it is 0.030% or less. The minimum of Al amount is 0.0003%, for example.

[Ti: 0.010 to 0.08%]

Ti is an element that reacts with N to form nitrides and to refine metal structures to improve base material toughness. Therefore, it is necessary to contain Ti 0.010% or more, Preferably it is 0.012% or more, More preferably, it is 0.015% or more. However, when excessively contained, many coarse Ti containing inclusions generate | occur | produce, and base metal toughness and HAZ toughness deteriorate. Therefore, Ti amount is 0.08% or less, Preferably it is 0.07% or less, More preferably, it is 0.06% or less, More preferably, you may be 0.05% or less.

[Ca: 0.0005 to 0.010%]

Ca is an element which suppresses crystallization of coarse Ti-containing inclusions and improves base metal toughness and HAZ toughness. Therefore, Ca needs to be contained 0.0005% or more, preferably 0.0008% or more, and more preferably 0.001% or more. However, when Ca amount becomes excess, coarse Ca containing oxide will produce | generate and base metal toughness will deteriorate. Therefore, Ca amount is 0.010% or less, Preferably it is 0.008% or less, More preferably, it is 0.006% or less.

[N: 0.0020 to 0.020%]

N is an element which forms Ti containing nitride, prevents coarsening of austenite particles by the pinning effect, refines the structure, and improves base metal toughness and HAZ toughness. In addition, the Ti-containing nitride also has an effect of promoting ferrite transformation in the particles, and contributes to improving the base material toughness and HAZ toughness by miniaturizing the structure. In order to exhibit such an effect, N amount should be 0.0020% or more, Preferably it is 0.0030% or more, More preferably, it is necessary to set it as 0.0040% or more. However, when the amount of N becomes excessive, the amount of solid solution N increases, strain aging occurs, and the base metal toughness and the HAZ toughness deteriorate. Therefore, N amount is 0.020% or less, Preferably it is 0.018% or less, More preferably, you may be 0.016% or less.

The basic component composition of the steel of this invention is as above, and remainder is iron and an unavoidable impurity. As an unavoidable impurity, mixing of elements (for example, Sn, As, Pb, etc.) mixed together according to the situation of raw materials, materials, manufacturing facilities, and the like is allowed. Moreover, it is also effective to contain the following element actively, and the characteristic of a steel material further improves according to the kind of component to contain.

[Ni: 1.5% or less (does not contain 0%), Cu: 1.5% or less (does not contain 0%), Cr: 1.5% or less (does not contain 0%), and Mo: 1.5% or less (0 One or more elements selected from the group consisting of%;

Ni, Cu, Cr, and Mo are all elements which act effectively to increase the strength of the steel, and the effect thereof increases as the content thereof increases. Do. Ni, Cu, Cr, and Mo are more preferably any element, 0.10% or more. However, when the content of these elements becomes excessive, excessive increase in strength results, and the base metal toughness and the HAZ toughness deteriorate. Therefore, it is preferable to suppress all the said elements to 1.5% or less. Ni, Cu, Cr, and Mo are more preferably 1.2% or less, and still more preferably 1% or less.

[Nb: 0.10% or less (does not include 0%), and / or V: 0.1% or less (does not contain 0%)]

Nb and V are elements which precipitate as carbonitrides and suppress the coarsening of the austenite particles to improve the base material toughness. In order to effectively exhibit such an effect, it is preferable to contain Nb 0.002% or more, More preferably, it is 0.005% or more, More preferably, it is 0.010% or more. However, when Nb amount becomes excessive, carbonitride will coarsen and base material toughness will deteriorate on the contrary. Therefore, Nb amount is preferably 0.10% or less, more preferably 0.08% or less, still more preferably 0.06% or less, particularly preferably 0.04% or less. In addition, it is preferable to contain V 0.002% or more, More preferably, it is 0.005% or more. However, excessive amount of V causes precipitation of coarse carbonitride, and the base metal toughness deteriorates. Therefore, it is preferable to make V amount into 0.1% or less, More preferably, it is 0.08% or less.

[B: 0.005% or less (does not include 0%)]

B is an element effective in suppressing formation of coarse grain boundary ferrite and improving base material toughness and HAZ toughness. Although such an effect increases as its content increases, it is preferable to contain 0.0005% or more in order to exhibit such an effect effectively. B amount is more preferably 0.0010% or more, and still more preferably 0.0013% or more. However, when B amount becomes excessive, BN will precipitate at an austenite grain boundary, and base metal toughness and HAZ toughness will deteriorate. Therefore, the amount of B is preferably made 0.005% or less, more preferably 0.004% or less, and still more preferably 0.003% or less.

[Zr: 0.02% or less (does not include 0%), and / or REM: 0.02% or less (does not contain 0%)]

Zr and REM (rare earth elements) are elements that contribute to improving HAZ toughness by miniaturizing oxides. Although such an effect increases as its content increases, in order to exhibit such an effect effectively, it is preferable to contain all 0.0001% or more. Zr and REM are more preferably both 0.0005% and more. However, when it contains excessively, since an oxide will coarsen and deteriorate base material toughness and HAZ toughness, it is preferable to restrain it to 0.02% or less. Zr and REM are more preferably 0.018% or less, and still more preferably 0.015% or less.

In the present invention, REM means a lanthanoid element (15 elements from La to Lu) and Sc (scandium) and Y (yttrium). Among these elements, it is preferable to contain at least one element selected from the group consisting of La, Ce and Y, and more preferably La and / or Ce.

Next, the method of manufacturing the steel material of this invention is demonstrated. As described above, in order to reduce the amount of Ti contained in the steel as a coarse Ti-containing inclusion having a size exceeding 2.0 µm to a predetermined amount or less, and to control the solid-solution Ti ratio in the steel to be in a predetermined range, Ti, N and After dissolving the steel so that Si satisfies the following formula 1, the number of Al ₂ O ₃ containing inclusions contained in the steel is not more than 10 per 1 mm 2 (including 0) by floating separation of inclusions contained in molten steel. What is necessary is just to control, and to cast. In following formula 1, [] expresses content (mass%) of each element in steel.

[Formula 1]

[Formula 2]

The reasons for each requirement are as follows.

In dissolving steel, it is necessary to adjust components so that Ti, N, and Si satisfy following formula (1). In the following formula 1, [Ti] x [N] on the left side represents the product of the allowable solubility of Ti and N. When this value exceeds a certain value, coarse Ti containing a size exceeding 2.0 µm during casting is contained. It was found that inclusions were produced. In addition, when the present inventors examined, it became clear that this permissible solubility product is influenced by the amount of Si in steel. In other words, the value of [Ti] × [N] changes depending on the Si concentration in the steel, and as the amount of Si in the steel increases, the value of [Ti] × [N] decreases, resulting in coarse Ti-containing inclusions. It was found to be suppressed. Therefore, in order to control Ti amount and solid-solution Ti ratio contained in steel as a coarse Ti containing inclusion suitably, it is necessary to adjust components so that Ti, N, and Si amount in steel may satisfy | fill the relationship of following formula (1). Equation 1 below is an expression set by the present inventors repeatedly. The following formula 1 can be modified as in the following formula 1a, and the components may be adjusted so as to satisfy this formula 1a. When the value of the left side of following formula 1a is made into Z value, Z value becomes like this. Preferably it is 5x10 <^-6> or less, More preferably, it is 1x10 <^-6> or less.

[Formula 1]

[Formula 1a]

In addition, what is necessary is just to melt | dissolve steel so that Ti and N may satisfy following formula (2) in solventing steel, when Si content in steel is 0 mass%, without adding Si. The following formula 2 calculates by substituting Si = 0.01 mass% (the minimum value of Si in the Example mentioned later) in said Formula 1.

[Formula 2]

After the solvent, it is necessary to cast after controlling the number of Al ₂ O ₃ -containing inclusions contained in the steel to 10 or less (including 0) per 1 mm ₂ by floating separation of inclusions contained in molten steel. Al ₂ O ₃ inclusions contained is, in the invention, and means that Al ₂ O ₃ inclusions containing Al ₂ O ₃ containing at least 80% by weight. Ti-containing inclusions are generally Al ₂ O ₃ But to crystallization by an oxide, such as a core is known, the steel of the chemical composition as in the present invention, by the Al ₂ O ₃ crystallizes into the nucleus-containing inclusions is considered that forming the inclusions containing Ti. This is commonly referred to as heterogeneous nucleation. Thus, reducing the number density of Al ₂ O ₃ inclusions contained in the molten steel, and the size can be reduced the amount of Ti contained in the steel as inclusions containing Ti exceeding 2.0㎛, besides the employment Ti ratio to an appropriate range Can be controlled.

The Al ₂ O ₃ containing the number density of inclusions, at most 10 per visual field observation area 1㎟ (includes 0). When the number density exceeds 10 / mm 2, the Ti-containing inclusions are coarsened, so that the amount of solid solution Ti cannot be secured, the solid solution Ti ratio is less than 0.3, and the base metal toughness and HAZ toughness deteriorate. The number density is preferably 8.0 pieces / mm 2 or less, and more preferably 6.0 pieces / mm 2 or less.

The number density of the Al ₂ O ₃ containing inclusions can be adjusted by floating separation of the inclusions (mainly oxide-based inclusions) contained in the molten steel from the molten steel. As a method of floating separation of inclusions, oxides are agglomerated and coalesced using, for example, a refining device by gas stirring such as LF (Ladle Furnace), or a reflux vacuum degassing refining device such as RHr (Ruhrstahl Hausen). It is desirable to promote floating separation of the ₂ O ₃ containing oxide. When the flow rate of the reflux gas is 100 to 200 Nm ³ / hour using the RH type degassing refiner, for example, 5 minutes is the time from the addition of Al to the molten steel until the reflux is stopped. It is preferable to set it as above, More preferably, it is 10 minutes or more. It preferred because it can be reduced by extending the refluxing time, Al ₂ O ₃ containing the number density of inclusions, however, since the productivity is decreased and the upper limit is 90 minutes.

What is necessary is just to cast according to a conventional method, and to hot-roll (hot-roll cold-rolling as needed) after the floating matter isolate | separated in molten steel is separated. Specifically, rolling is performed with the cooling time at 1400 to 1500 ° C at the time of casting within 600 seconds, the heating conditions before rolling to 1050 to 1200 ° C x 2 to 5 hours, and the finish rolling end temperature of 750 ° C or higher. What is necessary is just to perform cooling after completion | finish of rolling with an average cooling rate of 2-15 degreeC / sec, and cooling stop temperature as 300-500 degreeC.

The form of the steel material of this invention is not specifically limited, For example, it is used as a thick steel plate. A thick steel plate generally means that plate | board thickness is 3.0 mm or more as defined by JIS. This thick steel sheet can be used, for example, as a material for structures such as bridges, high-rise buildings, ships, and the like. The thick steel sheet is excellent in base metal toughness and HAZ toughness in small to medium heat welding as well as large heat input welding. The steel material of the present invention exhibits good HAZ toughness even when a large heat input welding with a heat input amount of 50 kJ / mm or more is performed with respect to a steel plate having a sheet thickness of 50 mm or more, for example, and therefore, it is preferable to apply it to a steel plate having such a thickness. However, plate | board thickness is not limited to 50 mm or more, and does not exclude the application to the steel plate used below.

Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not restrict | limited by the following Example of course, Of course, it carries out by changing suitably in the range which may be suitable for the meaning mentioned above. Possible, they are all included in the technical scope of the present invention.

Example

The steel of the component composition shown in Table 1 and Table 2 (the remainder is iron and unavoidable impurities) is dissolved, and the inclusions contained in the molten steel are separated from the molten steel and then cast to form a slab (cross section is 150 mm × 250 mm). Thereafter, hot rolling was performed to obtain a hot rolled steel sheet having a plate thickness of 80 mm.

In the hot rolling, the cooling time at 1400 to 1500 ° C at the time of casting is rolled within 600 seconds, the pre-rolling heating condition is 1100 ° C x 3 hours, the finish rolling end temperature is 780 ° C or higher, and the rolling is completed. The cooling to 450 ° C was performed at an average cooling rate of 6 ° C / sec and the cooling stop temperature of 450 ° C.

In Table 1 below, REM was added in the form of a misch metal containing about 50% La and about 25% Ce. In addition, in following Table 1 and Table 2, "-" has shown that it did not contain an element.

Based on Ti amount, N amount, and Si amount shown in following Table 1, Table 2, the value (Z value) of [Ti] x [N] x [Si] is computed, and a result is shown in following Table 3. In addition, about the steel grade A of the following Table 1, the value of [Ti] x [N] was computed, and the calculation result was shown in the column of Z value. In addition, in following Table 3, "(alpha) ^E- ( ^beta) " means "(alpha) * 10 <-( ^beta) ".

The inclusions contained in the molten steel were separated from the molten steel by changing the time (reflux time) from the addition of Al to the stop of the reflux gas after the flow rate of the reflux gas at RH was 100 to 200 Nm ³ / hour. . Table 3 shows the reflux time. In addition, No. 29 and No. 31 shown in following Table 3 are the examples which cast the inclusions contained in the said molten steel without floating separation from molten steel.

After the inclusions were separated from the molten steel by flotation, the number density of the Al ₂ O ₃ containing inclusions in the molten steel was investigated in the following order before casting.

[Number Density of Al ₂ O ₃ -containing Inclusions]

Molten steel was extract | collected from the continuous casting tundish using the cup-shaped sampler (inner diameter about 35 mm x height about 50 mm), and it solidified by air cooling. The steel obtained by solidification was taken out from the cup-shaped sampler, it cut | disconnected to the horizontal plane of about 10 mm position from the bottom of a sample, and the cut surface was grind | polished, and this was made into the sample for inclusion observation. The sample for observation of the inclusions was observed with an EPMA (Electron Probe X-ray Micro Analyzer; electron probe X-ray microanalyzer, `` JXA-8500F '' manufactured by Nippon Denshi Corporation). At the same time, the component composition of this particle was quantitatively analyzed. Observation conditions are acceleration voltage of 20 kV, sample current of 0.01 μA, observation field of view, the center of the polishing surface is 1 to 5 cm 2, the number of analytical particles is 100 or more, and the component composition of the particles is energy dispersive characteristic X. Semiquantitative analysis was performed by a line detector (EDS). The element to be analyzed was Al, Mn, Si, Ti, Zr, Ca, La, Ce, O, and after all the detected element concentrations were converted to oxides and normalized, the Al ₂ O ₃ concentrations were obtained. Of the detected total inclusions, and the inclusions containing Al ₂ O ₃ less than 80% by mass as Al ₂ O ₃ inclusions contained. In terms of the number of Al ₂ O ₃ inclusions containing, per 1㎟ was determined by the number density. The number density of Al ₂ O ₃ containing inclusions is shown in Table 3 below.

Next, about the hot rolled sheet steel manufactured as mentioned above, by the following point,

(a) Ti content contained in steel as coarse Ti containing inclusions whose size exceeds 2.0 micrometers among the total Ti contained in steel materials,

(b) the ratio R / Q (titer Ti ratio) of the value R obtained by subtracting the Ti amount included in the steel as a Ti-containing inclusion having a size exceeding 0.1 μm from the total Ti amount Q, and the total Ti amount Q included in the steel;

(c) base material toughness,

(d) HAZ toughness at the time of welding a base material was measured.

These results are shown in Table 3 below.

[(a) Ti content contained in steel as coarse Ti containing inclusions whose size exceeds 2.0 micrometers among the total Ti contained in steel materials. "

The test piece (15 mm long x 15 mm wide x 5 mm long) is cut out of each hot rolled steel sheet so that the shaft passes through the depth t / 4 position (t is the plate thickness) from the surface of the hot rolled steel sheet, and 2% triethanolamine-1% tetramethylammonium The chloride-methanol solution was used as an electrolytic solution, and electrolytic extraction was performed at a current of 500 A / m 2 or less at normal temperature. After electrolytic extraction, the extraction residue was filtered with a membrane filter having a scale of 2.0 mu m.

Subsequently, the extraction residue (inclusion exceeding 2.0 micrometers in size) which remained on the filter by filtration was put into the filter case platinum crucible, and it heated and inhaled with the gas burner. Then, the alkali flux (mixture of sodium carbonate and sodium tetraborate) was added, and it heated again by the gas burner and melted the extraction residue. Subsequently, 18 volume% hydrochloric acid was added, the melt was liquefied, transferred to a measuring flask, pure water was added, and the volume thereof was made up to 50 ml to obtain an analyte. Ti concentration in this analyte was measured by ICP emission spectrometry, and the amount of Ti contained in the steel as Ti-containing inclusions having a size exceeding 2.0 µm was measured. The measurement results are shown in Table 3 below.

In general, the larger the coarse inclusions, the smaller the number of residues remaining in the steel. Therefore, it is difficult to accurately determine the amount of Ti present as the coarse inclusions by microscopic observation of the polishing sample, which is a common method for inclusion inclusion investigation. In this analysis method which combined the filtration by a filter, since the whole quantity of Ti contained in steel materials and the inclusion exceeding 2.0 micrometers can be measured, there is little measurement error and it can measure with high precision. .

[(b) Ratio R / Q (Employment Ti Ratio)]

In the above (a), instead of using a membrane filter having a scale of 2.0 mu m, the filter is filtered using a membrane filter having a scale of 0.1 mu m, and Ti contained in the steel as Ti-containing inclusions having a size exceeding 0.1 mu m. The amount was measured.

In addition, in the said (a), the test piece of the same size is cut out separately from the position close to the test piece cut out from each hot rolled sheet steel, and all the said test pieces are melt | dissolved according to JIS G1258-1 "acid decomposition-potassium disulfate melting method," Ti concentration in this solution was measured by ICP emission spectrometry, and the total Ti amount Q contained in the steel was measured.

Next, the value R obtained by subtracting the amount of Ti contained in the steel material from the total Ti amount Q as a Ti-containing inclusion having a size exceeding 0.1 μm was obtained. This corresponds to the amount of dissolved Ti. The ratio R / Q of this value R and total Ti amount Q contained in steel materials was calculated | required. The obtained ratio R / Q is shown in Table 3 below.

[(c) Base material toughness]

From the surface of each hot-rolled steel sheet, the Charpy impact test piece (No. 4 test specimen of JIS Z2201) was taken in the rolling direction from the depth t / 4 position (t is the sheet thickness), and the Charpy impact test was performed at -60 ° C based on JIS Z2242. It carried out and measured absorption energy (vE- ₆₀ ). At this time, absorption energy (vE- ₆₀ ) was measured about three test pieces, and the minimum value was calculated | required. The lowest value of vE- ₆₀ evaluated 100J or more as the base material toughness.

[(d) HAZ Toughness when Welded Base Material]

From the depth t / 4 position (t is plate thickness) from the surface of each hot-rolled steel sheet, a Charpy impact test piece (No. 4 test specimen of JIS Z2201) was taken in the rolling direction, and a hot cycle test simulating high heat input welding was performed to perform hot rolling. The HAZ toughness at the time of welding the steel plate (base material) was evaluated. At this time, in the heat cycle test, after heating the said test piece to 1400 degreeC and holding for 60 second, the temperature range of 800-500 degreeC was cooled over 500 second, and the heat input amount of weld heat equivalent to 55 kJ was given. The Charpy impact test was done at -40 degreeC based on JISZ2242, and absorbed energy (vE- ₄₀ ) was measured. At this time, absorption energy (vE- ₄₀ ) was measured about three test pieces, and the minimum value was calculated | required. The lowest value of vE- ₄₀ was 100J or more, and it evaluated that HAZ toughness was excellent.

In addition, in Fig. 2, the relationship between the value (Z value) of [Ti] × [N] × [Si] and the HAZ toughness is shown graphically. In Fig. 2, the results of Inventive Examples (Nos. 1 to 25) shown in Table 3 below are taken as ◇, and the result of Z values out of the range recommended in the present invention among the Comparative Examples (Nos. 26, 28, 30, and 31). ) Is indicated by ■. From FIG. 2, there is a correlation between the Z value and the HAZ toughness, and when the Z value is suppressed to 1.0E-05 (1.0 × 10 ⁻⁵ ) or less, or does not contain Si (steel grade A in Table 1) It is understood that the HAZ toughness can be improved by suppressing the Z value to 1.0E-03 (1.0 × 10 ⁻³ ) or less.

From the following Tables 1-3, it can consider as follows. Nos. 1 to 25 are examples satisfying the requirements specified in the present invention, the composition of the components is appropriately adjusted, and the amount of Ti contained in the steel as coarse Ti-containing inclusions is suppressed to 0.010% or less. Since an appropriate amount of Ti is dissolved in the steel, it can be seen that a steel sheet having good base material toughness and HAZ toughness is obtained.

In contrast, Nos. 26 to 46 are examples that deviate from any of the requirements defined in the present invention, and at least one of the base metal toughness and the HAZ toughness is deteriorated. In detail, it is as follows.

No. 26 and No. 30 are examples in which [Ti] × [N] × [Si] exceeds 1.0 × 10 ⁻⁵ , and the amount of Ti contained in the steel as Ti-containing inclusions exceeding 2.0 μm in size. Since it is excessive, base metal toughness and HAZ toughness deteriorate. No. 27 and No. 29 are examples where the ratio R / Q is out of a predetermined range, and the amount of solid solution Ti in the steel is too large. Therefore, base metal toughness and HAZ toughness deteriorate. No. 28 and No. 31 are examples in which [Ti] × [N] × [Si] exceeds 1.0 × 10 ⁻⁵ , and the amount of Ti contained in the steel as Ti-containing inclusions exceeding 2.0 μm in size. Since it becomes excess and the amount of solid solution Ti in steel materials is too small, it is an example in which ratio R / Q is less than the predetermined range. Therefore, base metal toughness and HAZ toughness deteriorate.

No. 32-39 and 41-44 are all the examples which do not satisfy | fill the component composition prescribed | regulated by this invention. No. 32 is one in which the C content in the steel sheet exceeds the range specified by the present invention, and the base metal toughness and the HAZ toughness deteriorate. The deterioration of the base material toughness and the HAZ toughness is considered to be due to an increase in the amount of hard island martensite (MA) produced. No. 33 is that the Si content in the steel sheet exceeds the range specified in the present invention, and the base metal toughness and the HAZ toughness deteriorate. The deterioration of the base material toughness and the HAZ toughness is considered to be due to an increase in the amount of hard island martensite (MA) produced. No. 34 exceeds the range prescribed | regulated by this invention in the Mn content of a steel plate, and since the intensity | strength of the steel plate became high too much, base material toughness and HAZ toughness deteriorate.

No. 35 is that the P content in the steel sheet exceeds the range specified in the present invention, and the base metal toughness and the HAZ toughness deteriorate. No. 36 is one in which the S content in the steel sheet exceeds the range specified in the present invention, and the HAZ toughness is good, but the base metal toughness is deteriorated. No. 37 is the Al content in the steel plate exceeding the range prescribed | regulated by this invention, and base material toughness and HAZ toughness deteriorate. No. 38 is a thing whose Ti content in a steel plate does not fall within the range prescribed | regulated by this invention, but HAZ toughness is favorable, but base material toughness deteriorates. No. 39 is that the Ti content in the steel sheet exceeds the range specified in the present invention, and the base metal toughness and the HAZ toughness deteriorate.

No. 40 is a reference example, the content of Nb added as a selection element exceeds the range specified in the present invention, and the base metal toughness deteriorates.

No. 41 is one in which the Ca content in the steel sheet does not fall within the range specified in the present invention, and the base metal toughness and the HAZ toughness deteriorate. No. 42 is one in which the Ca content in the steel sheet exceeds the range specified in the present invention, and the base metal toughness and the HAZ toughness deteriorate. No. 43 is a thing whose N content in a steel plate does not fall within the range prescribed | regulated by this invention, and base metal toughness and HAZ toughness deteriorate. No. 44 is an example where N content in a steel plate exceeds the range prescribed | regulated by this invention, ratio R / Q is less than the predetermined range, and it is an example with too little solid solution Ti in steel materials. Therefore, the base material toughness and the HAZ toughness deteriorate.

No. 45 and No. 46 are reference examples, and the content of Ni or Cu added as a selection element exceeds the range specified in the present invention, and the base metal toughness and the HAZ toughness deteriorate.

In addition, No. 28, 29, 31, and 41, since the time (reflux time) from the addition of Al to stopping the reflux gas in RH is too short, the inclusions contained in the molten steel are sufficiently separated from the molten steel. It is not. Therefore, base metal toughness and HAZ toughness deteriorate.

Claims

Ingredients in the river
C: 0.03 to 0.16% (mean of mass%. The same applies to the following components),
Si: 0.25% or less (including 0%),
Mn: 1 to 2.0%,
P: 0.03% or less (does not contain 0%),
S: 0.015% or less (does not contain 0%),
Al: 0.05% or less (does not contain 0%),
Ti: 0.010 to 0.08%,
Ca: 0.0005 to 0.010% and
N: 0.0020 to 0.020%,
The remainder is a steel material consisting of iron and inevitable impurities,
The amount of Ti contained in steel as Ti containing inclusions exceeding 2.0 micrometers is 0.010% or less (it does not contain 0%), and
When the total amount of Ti contained in the steel is Q, and the value obtained by subtracting the amount of Ti contained in the steel material from the above Q as Ti-containing inclusions exceeding 0.1 µm is set to R, the ratio R / Q of R and Q is 0.30. Steel material excellent in the toughness of a base material and a welded heat affected zone, characterized by being from 0.7 to 0.70.

The steel according to claim 1, wherein the steel further contains at least one group of the following groups (a) to (d) as another element.
(a) Ni: 1.5% or less (without 0%), Cu: 1.5% or less (without 0%), Cr: 1.5% or less (without 0%), and Mo: 1.5% or less 1 or more types chosen from the group which consists of (it does not contain 0%),
(b) at least one selected from the group consisting of Nb: 0.10% or less (does not contain 0%) and V: 0.1% or less (does not contain 0%),
(c) B: 0.005% or less (does not contain 0%)
(d) one or more selected from the group consisting of Zr: 0.02% or less (does not contain 0%) and REM: 0.02% or less (does not contain 0%).

It is a method of manufacturing the steel materials of Claim 1 or 2.
Ti, N, and Si melt the steel so as to satisfy the following Equation 1, and then separate the inclusions in the molten steel to separate the number of Al ₂ O ₃ -containing inclusions included in the steel by 10 or less per 1 mm 2 (0 It comprises a) and then cast, the method of producing a steel material excellent in the toughness of the base material and the weld heat affected zone.
[Formula 1]

In Formula 1, [] shows content (mass%) of each element in steel.
However, when Si = 0 mass%, steel is melted so that Ti and Ni may satisfy following formula (2).
[Formula 2]