JP2013060631A - Steel material excellent in toughness of basic material and weld-heat affected zone, and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel material excellent in toughness of both a basic material and an HAZ (Heat Affected Zone), and manufacturing method therefor.SOLUTION: The steel material has the component compositions in the steel, composed, by mass, 0.03-0.16% C, ≤0.25% (including 0%) Si, 1-2.0% Mn, ≤0.03% (excluding 0%) P, ≤0.015% (excluding 0%) S, ≤0.05% (excluding 0%) Al, 0.010-0.08% Ti, 0.0005-0.010% Ca and 0.020-0.020% N and the balance Fe with inevitable impurities, and in the total Ti content (Q) contained in the steel material, Ti contented in the steel material as the Ti-contained inclusion exceeding 2.0 μm size, is ≤0.010% (excluding 0%) and further, a ratio (R/Q) of a value (R) which deducts Ti content contained in the steel material as the Ti-contained inclusion exceeding 0.1 μm size from the total Ti content (Q), to the total Ti content (Q), is 0.30-0.70.

Description

  The present invention relates to a steel material used for a welded structure such as a bridge, a high-rise building, a ship, etc., toughness of a base material and a part affected by heat when large heat input welding (hereinafter referred to as “welding heat affected zone” or “ It may be referred to as “HAZ”).

  With the increase in size of welded structures, it is inevitable to weld thick steel plates having a thickness of 50 mm or more. For this reason, from the viewpoint of improving welding construction efficiency, large heat input welding of 50 kJ / mm or more is directed. However, when high heat input welding is performed, the HAZ is heated to a high temperature austenite region and then gradually cooled, so the structure of the HAZ (particularly, near the bond portion of the HAZ) becomes coarse and the toughness of that portion deteriorates. There is a problem that it is easy. It has been a problem to ensure such toughness in HAZ (hereinafter sometimes referred to as “HAZ toughness”).

  Various HAZ toughness deterioration prevention techniques during high heat input welding have been proposed so far. As a typical example of such a technique, for example, in Patent Documents 1 to 4, fine TiN is dispersed and precipitated in a steel material, thereby suppressing the austenite grain coarsening that occurs in HAZ when high heat input welding is performed. Steel materials that suppress degradation of HAZ toughness have been proposed. However, in these techniques, when the weld metal becomes a high temperature of 1400 ° C. or higher, the TiN dissolves and disappears due to the heat received at the time of welding, particularly in the portion close to the weld metal (bond portion) of the HAZ, and the HAZ toughness There is a problem that deterioration of the film cannot be sufficiently suppressed.

  Patent Document 5 discloses that HAZ toughness is suppressed by optimizing the number density of fine TiN having a particle size of 0.01 to 0.1 μm to suppress the formation of coarse TiN having a particle size exceeding 0.1 μm. Techniques for improving the above have been proposed. However, it has been found that even if the number density of fine TiN is optimized, sufficient HAZ toughness cannot be ensured.

On the other hand, the present applicant actively contains Nb in the TiN inclusions present in the welding steel, controls the Ti / Nb ratio, and includes inclusions having a particle size of 0.01 to 0.25 μm. A technique for securing HAZ toughness in a wide heat input range by setting the number of slabs to 1.0 × 10 ⁴ or more per 1 mm ² is proposed (for example, Patent Document 6). However, even with this technique, it is unavoidable that TiN dissolves and disappears due to heat received during welding, and the HAZ toughness may deteriorate.

  Incidentally, steel materials used for welded structures are required to have good toughness (base material toughness) as a basic characteristic in addition to HAZ toughness. Therefore, the present applicant has proposed a thick steel plate in which both the base metal toughness and the HAZ toughness are improved in Patent Document 7. In this technique, the number density is controlled in accordance with the size of the Ti-containing nitride contained in the steel sheet, and the area ratio of the island-like martensite is appropriately controlled, so that the base metal toughness and the HAZ toughness are excellent. We provide thick steel plates. However, in the above method, since the number density for each size is measured by microscopic observation, the method is not necessarily a highly accurate method, and there is a possibility that characteristics may vary.

Japanese Patent Publication No.55-26164 Japanese Patent Laid-Open No. 2003-166017 JP 2003-213366 A Japanese Patent Laid-Open No. 2001-20031 JP 2001-98340 A JP 2004-2181010 A JP 2010-95781 A

  The present invention has been made paying attention to the above circumstances, and an object thereof is to provide a steel material excellent in both base metal toughness and HAZ toughness, and a method for producing the same.

  The steel material excellent in toughness of the base material and the weld heat-affected zone according to the present invention that has solved the above-mentioned problems means that the component in steel is C: 0.03 to 0.16% (meaning mass%. Same for ingredients), Si: 0.25% or less (including 0%), Mn: 1 to 2.0%, P: 0.03% or less (not including 0%), S: 0.015% Below (not including 0%), Al: not more than 0.05% (not including 0%), Ti: 0.010 to 0.08%, Ca: 0.0005 to 0.010% and N: 0.00. It is a steel material containing 0020 to 0.020%, the balance being iron and inevitable impurities.

  And among the total Ti amount Q contained in the steel material, the Ti amount contained in the steel material as a Ti-containing inclusion exceeding 2.0 μm is 0.010% or less (excluding 0%) and exceeds 0.1 μm. The gist is that the ratio R / Q between the value R obtained by subtracting the Ti amount contained in the steel material from the total Ti amount Q as the Ti-containing inclusions and the total Ti amount Q contained in the steel material is 0.30 to 0.70. have.

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

The steel material, as another element,
(A) Ni: 1.5% or less (not including 0%), Cu: 1.5% or less (not including 0%), Cr: 1.5% or less (not including 0%), and Mo: One or more elements selected from the group consisting of 1.5% or less (excluding 0%),
(B) Nb: 0.10% or less (not including 0%), and / or V: 0.1% or less (not including 0%),
(C) B: 0.005% or less (excluding 0%),
(D) Zr: 0.02% or less (not including 0%), and / or REM: 0.02% or less (not including 0%),
Etc. may be included.

The steel material contains Al ₂ O ₃ contained in steel by fusing and separating inclusions contained in the molten steel after melting the steel so that Ti, N, and Si satisfy the following formula (1) It can be manufactured by controlling the number of inclusions (specifically, inclusions containing 80% by mass or more of Al ₂ O ₃ ) to 10 or less (including 0) per 1 mm ² . In the following formula (1), [] represents the content (% by mass) of each element in the steel.
[Ti] × [N] ≦ (1 × 10 ⁻⁵ ) / [Si] (1)
However, when Si = 0% by mass, the steel is melted so that Ti and Ni satisfy the following formula (2).
[Ti] × [N] ≦ 1 × 10 ⁻³ (2)

  According to the present invention, the Ti-containing inclusions in the steel material do not control the number density for each size by microscopic observation as in the past, but the size is 2.0 μm out of the total Ti amount Q contained in the steel material. In addition to reducing the amount of Ti contained in steel as coarse Ti-containing inclusions exceeding as much as possible, the amount of Ti contained in steel as Ti-containing inclusions whose size exceeds 0.1 μm is quantified by electrolytic extraction, and this Ti amount Since the ratio R / Q between the value R obtained by subtracting from the total Ti amount Q and the total Ti amount Q contained in the steel material is appropriately controlled, the toughness of the base material and the HAZ can be improved more accurately.

FIG. 1 shows the total Ti amount Q defined in the present invention (total Ti amount contained in the sample), the Ti amount contained in the steel as a Ti-containing inclusion having a size exceeding 2.0 μm, and the size exceeding 0.1 μm. The concept of Ti amount contained in steel materials as Ti-containing inclusions of 2.0 μm or less and solid solution Ti amount R (including Ti amount contained in steel materials as Ti-containing inclusions having a size of 0.1 μm or less) will be described. It is a schematic diagram. FIG. 2 is a graph showing the relationship between the value (Z value) of [Ti] × [N] × [Si] and the HAZ toughness.

In order to improve both the base metal toughness and the HAZ toughness of the steel, the present inventors have controlled the number density according to the size of the Ti-containing inclusions determined based on microscopic observation as in the prior art. In view of the fact that the observation visual field area is as small as about 300 mm ² , the accuracy is low, and the characteristics vary, and studies have been conducted to provide a more accurate method instead of this. In this case, the total amount of Ti in the steel material is sieved by size by combination with electrolytic extraction and filtration with a plurality of membrane filters (hereinafter, simply referred to as filters) with different mesh openings (mesh). Thus, it has been studied by considering whether the method of quantifying the amount of Ti (mass concentration) of each size can be successfully used.

as a result,
(A) After electrolytic extraction with a predetermined electrolytic solution, when Ti that has passed through a filter having a mesh size of 0.1 μm is used as solute Ti, the amount of solute Ti has a significant effect on the improvement of the base metal toughness and HAZ toughness of the steel. What it is doing,
(B) This solid solution Ti amount is not controlled as an absolute value, but must be controlled by a balance with the total Ti amount contained in the steel material (specifically, the ratio of the solid solution Ti amount to the total Ti amount). The desired properties are not exhibited,
(C) Furthermore, in order to effectively exhibit the desired characteristics, it is not sufficient to control the ratio of the solid solution Ti amount, and the size that does not pass through a filter having a mesh size of 2.0 μm (remains on the filter) It is also important to appropriately control the amount of Ti contained in inclusions containing Ti exceeding 2.0 μm,
(D) Therefore, in order to improve the base metal toughness and the HAZ toughness of the steel material, the ratio of the solid solution Ti amount to the total Ti amount calculated by the above method and the steel material as a Ti-containing inclusion whose size exceeds 2.0 μm. It is extremely important to appropriately control the amount of Ti contained in
The present invention has been completed.

  In this specification, the total amount of Ti contained in the sample is Q, and after electrolytic extraction (details will be described later), Ti that passes through a filter having an opening of 0.1 μm (in the present invention, this is referred to as solid solution Ti). Is called R. The total Ti amount Q is a value determined by ICP emission analysis after electrolytic extraction. The amount R of the solid solution Ti means the amount of Ti that has passed through the filter when filtered using a filter having an opening of 0.1 μm after the electrolytic extraction. The filter with the smallest mesh size among the filters currently on the market is 0.1 μm. In the present invention, the filter that passes through this minimum diameter filter is “solid solution Ti even if it exists as a Ti-containing inclusion. "

  The amount R of solute Ti determined as described above does not directly measure the amount of solute Ti, and does not pass through the filter using a filter having an opening of 0.1 μm after the electrolytic extraction. Indirectly calculated by quantifying the amount of Ti contained in Ti-containing inclusions with a size exceeding 0.1 μm (remaining on the filter) by ICP emission analysis, and subtracting this from the total Ti amount Q contained in the steel. doing. This is because it is difficult to directly analyze the amount of solute Ti directly.

  In this specification, “inclusions containing Ti exceeding 2.0 μm” means that after performing the above-described electrolytic extraction and filtering using a filter having an opening of 2.0 μm, the filter does not pass through the filter (on the filter). Stuff). Conventionally, it has been known that coarse Ti-containing inclusions have an adverse effect on HAZ toughness and the like, but according to the results of the study by the present inventors, among the coarse Ti-containing inclusions, in particular, by the above method. As a result of the fact that the amount of coarse Ti-containing inclusions whose size to be separated exceeds 2.0 μm has been found to have a particularly bad influence on the base metal toughness and HAZ toughness, in particular, It is up to the amount to be properly controlled.

  First, the total amount of Ti contained in the steel material of the present invention will be described. In the present invention, steel material is dissolved by electrolytic extraction, and the extraction residue after electrolytic extraction is filtered through a 0.1 μm membrane filter having the smallest mesh size among the commercially available extractions. The residue is melted together with the filter, and the Ti content is measured by ICP emission spectrometry. According to this method, the total Ti amount Q contained in the steel material passes through the filter with the Ti amount contained in the steel material as a Ti-containing inclusion whose size that does not pass the filter exceeds 0.1 μm, as shown in FIG. It is expressed as the total amount with the solid solution Ti amount including the Ti amount contained in the Ti-containing inclusions.

  And when the present inventors examined, the value R which subtracted the Ti amount contained in steel materials from the total Ti amount Q as Ti containing inclusions whose size exceeds 0.1 μm, and the total Ti amount Q contained in the steel material It was revealed that the base metal toughness and the HAZ toughness of the steel material can be improved by adjusting the ratio R / Q in the range of 0.30 to 0.70. This is demonstrated in the examples herein.

  That is, the steel type N shown in Table 1 of the example and the steel type d shown in Table 2 have almost the same composition, and the amount of Ti contained in the steel material as coarse Ti-containing inclusions exceeding 2.0 μm is almost the same. It is a steel material. However, steel grade d has reduced base metal toughness and HAZ toughness, whereas steel grade N has improved both base metal toughness and HAZ toughness. Similarly, the steel type b shown in Table 1 below and the steel type b shown in Table 2 below have substantially the same composition and the same Ti content, but the steel type b has reduced base metal toughness and HAZ toughness. On the other hand, Steel E can improve both the base metal toughness and the HAZ toughness.

  When this reason was examined, steel type d has a high proportion of the solid solution Ti amount in the added Ti amount (total Ti amount contained in the steel material), and steel type b has a proportion of the solid solution Ti amount in the added Ti amount. Therefore, it was speculated that this solid solution Ti amount contributed greatly to the improvement of the base metal toughness and the HAZ toughness.

  Further examination based on these results revealed that the ratio R / Q of the solid solution Ti amount to the total Ti amount contained in the steel material affects the base metal toughness and the HAZ toughness.

  However, it has also been clarified that the effect of improving the base metal toughness and the HAZ toughness is insufficient only by controlling the ratio R / Q of the solute Ti amount to the total Ti amount contained in the steel material within an appropriate range.

  That is, when the present inventors further examined, among Ti-containing inclusions having a size exceeding 0.1 μm, the amount of Ti contained in the steel material as a Ti-containing inclusion having a size exceeding 2.0 μm is also determined by the base material toughness and HAZ. It was found to affect toughness. This is also demonstrated in the examples described later. For example, the steel types L shown in Table 1 of the Examples and the steel types a and e shown in Table 2 are steel materials having substantially the same composition, and the ratio R / Q is in the range of 0.30 to 0.70. Although controlled, the steel types a and e have a large amount of Ti contained in coarse Ti-containing inclusions having a size exceeding 2.0 μm, so both the base metal toughness and the HAZ toughness are deteriorated. On the other hand, in the steel type L, since the amount of Ti contained in coarse Ti-containing inclusions having a size exceeding 2.0 μm is reduced to the range specified in the present invention, both the base metal toughness and the HAZ toughness are improved. ing.

  Thus, it can be seen that both the base metal toughness and the HAZ toughness can be improved by reducing the amount of Ti contained in the steel material as a coarse Ti-containing inclusion having a size exceeding 2.0 μm. The reason for this is unknown in detail, but is considered as follows. In order to improve the HAZ toughness, it is effective to refine the old γ grain size with fine Ti-containing inclusions (for example, TiN precipitates), and a large amount of Ti is required for this purpose. However, when Ti contained in the steel material exists 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, and the old γ grains It is considered that not only the diameter reduction cannot be realized, but also the coarse Ti-containing inclusion itself becomes a fracture starting point, and both the base metal toughness and the HAZ toughness are lowered.

  Hereinafter, the present invention will be specifically described.

<< The ratio R / Q of the value R obtained by subtracting the Ti amount contained in the steel material from the total Ti amount Q as the Ti-containing inclusions whose size exceeds 0.1 μm and the total Ti amount Q contained in the steel material is 0.30. 0.70 >>
In the present invention, the ratio of the solute Ti amount to the total Ti amount contained in the steel material (hereinafter sometimes referred to as the solute Ti ratio) is set to 0.30 to 0.70. When the solute Ti ratio is less than 0.30, the Ostwald growth of TiN particles becomes prominent during heat treatment and welding, so that Ti-containing nitrides are likely to be coarsened and effective in improving HAZ toughness and base metal toughness. The amount of fine Ti-containing nitride produced cannot be secured. Accordingly, since the metal structure cannot be refined during welding, the base metal toughness and the HAZ toughness are lowered. Therefore, the solid solution Ti ratio is 0.30 or more, preferably 0.35 or more, more preferably 0.40 or more. However, if the solid solution Ti ratio exceeds 0.70 and becomes excessive, the amount of solid solution Ti becomes too large, so that the transformation structure generated from the old γ grain boundary becomes coarse, and the base metal toughness and the HAZ toughness are lowered. Therefore, the solid solution Ti ratio is 0.70 or less, preferably 0.65 or less, more preferably 0.60 or less.

  The solute Ti ratio is a ratio R / R between the value R obtained by subtracting the Ti amount contained in the steel material from the total Ti amount Q as the Ti-containing inclusion having a size exceeding 0.1 μm and the total Ti amount Q contained in the steel material. Q can be indicated (see FIG. 1). That is, the above-mentioned value R means the total amount of Ti amount actually dissolved in the steel material and Ti amount contained in ultrafine inclusions having passed through a filter having an opening of 0.1 μm. In the present invention, the amount of Ti contained in the ultrafine inclusion is regarded as solute Ti.

<< About the amount of Ti contained in steel as coarse Ti-containing inclusions whose size exceeds 2.0 μm >>
In the present invention, the amount of Ti contained in the steel material as coarse Ti-containing inclusions having a size exceeding 2.0 μm is set to 0.010% or less (excluding 0%). When this Ti amount exceeds 0.010%, coarse Ti-containing inclusions that become the starting point of fracture increase, which causes a decrease in the base metal toughness and HAZ toughness. The amount of Ti should be small, 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 μm is extracted from the steel by an electrolytic extraction method and contained in Ti-containing inclusions that do not pass through a filter having an opening of 2.0 μm. It means the amount of Ti. The above Ti-containing inclusions include all inclusions containing Ti in the inclusions, in addition to nitrides containing Ti, oxides containing Ti, carbides containing Ti, or a composite thereof It is meant to include compounds and the like. In the present invention, as will be described later, the residue extracted by the electrolytic extraction method is melted, and the Ti amount is measured by the ICP emission analysis method, so that it is included in the steel material as a Ti-containing inclusion having a size exceeding 2.0 μm. The total amount of Ti can be measured for inclusions of any composition. And if the sum total of Ti amount contained in the coarse inclusion whose size exceeds 2.0 micrometers is 0.010% or less with respect to steel materials, base material toughness and HAZ toughness can be improved.

  The size of the coarse Ti-containing inclusions extracted from the steel material is assumed to exceed 2.0 μm. This is because, for Ti-containing inclusions having a size of 2.0 μm or less, the influence of toughness due to the difference in Ti amount was hardly observed.

  As described above, the steel material of the present invention has a Ti content of 0.010% or less as a Ti-containing inclusion having a size exceeding 2.0 μm and a solid solution Ti ratio (ratio R / Q) of 0.30. It is characterized in that it is set to ~ 0.70.

  Next, the component composition of the steel material of the present invention will be described.

[C: 0.03-0.16%]
C is an element that is indispensable for securing strength. If the amount of C is less than 0.03%, strength cannot be secured. Therefore, the C content is 0.03% or more, preferably 0.04% or more, more preferably 0.05% or more. However, when the amount of C is excessive, a large amount of hard island martensite (MA) is generated, which deteriorates the base metal toughness and the HAZ toughness. Therefore, the C content needs to be suppressed to 0.16% or less, preferably 0.12% or less, 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 martensite (MA) is generated and coarse Ti-containing inclusions are generated. As a result, the base metal toughness and the HAZ toughness deteriorate. Accordingly, the Si content is 0.25% or less, preferably 0.2% or less, more preferably 0.1% or less, and still more preferably 0.08% or less. The Si content is preferably 0.01% or more, more preferably 0.02% or more, and still more preferably 0.03% or more.

[Mn: 1 to 2.0%]
Mn is an element useful for ensuring strength, and needs to be contained by 1% or more. The amount of Mn is preferably 1.2% or more, more preferably 1.4% or more. However, when Mn is contained excessively exceeding 2.0%, the strength is excessively increased and the base metal toughness and the HAZ toughness are deteriorated. Therefore, the Mn content is 2.0% or less, preferably 1.8% or less, more preferably 1.7% or less.

[P: 0.03% or less (excluding 0%)]
P is an unavoidable impurity element and easily causes grain boundary fracture. Therefore, P is preferably as small as possible because it adversely affects both the base metal toughness and the HAZ toughness. Therefore, the amount of P needs to be suppressed to 0.03% or less, preferably 0.02% or less, more preferably 0.01% or less. However, it is industrially difficult to reduce the amount of P in steel to 0%, and usually about 0.003% is contained.

[S: 0.015% or less (excluding 0%)]
S is an inevitable impurity element, and causes grain boundary fracture due to grain boundary segregation and deterioration of base metal toughness due to coarse sulfides, and therefore the amount is preferably as small as possible. Therefore, the amount of S must be suppressed to 0.015% or less, preferably 0.010% or less, more preferably 0.008% or less, and still more preferably 0.005% or less. However, it is industrially difficult to reduce the amount of S in steel to 0%, and usually about 0.0001% is contained.

[Al: 0.05% or less (excluding 0%)]
Al acts as a deoxidizer, but if it is contained in excess, it will produce Al ₂ O ₃ containing inclusions in the steel and cause coarse Ti containing inclusions, which will degrade the base metal toughness and HAZ toughness. It is an element to be made. Accordingly, the Al content must be 0.05% or less, preferably 0.040% or less, more preferably 0.030% or less. The lower limit of the Al amount is, for example, 0.0003%.

[Ti: 0.010 to 0.08%]
Ti is an element that reacts with N to form a nitride, refines the metal structure, and improves the base material toughness. Therefore, Ti needs to be contained in an amount of 0.010% or more, preferably 0.012% or more, more preferably 0.015% or more. However, when it contains excessively, many coarse Ti containing inclusions will produce | generate and will deteriorate a base material toughness and HAZ toughness. Therefore, the Ti content is 0.08% or less, preferably 0.07% or less, more preferably 0.06% or less, and still more preferably 0.05% or less.

[Ca: 0.0005 to 0.010%]
Ca is an element that suppresses the crystallization of coarse Ti-containing inclusions and improves the base metal toughness and the HAZ toughness. Therefore, Ca needs to be contained in an amount of 0.0005% or more, preferably 0.0008% or more, more preferably 0.001% or more. However, when the amount of Ca becomes excessive, coarse Ca-containing oxides are generated and the base metal toughness is deteriorated. Therefore, the Ca content is 0.010% or less, preferably 0.008% or less, more preferably 0.006% or less.

[N: 0.0020 to 0.020%]
N is an element that forms Ti-containing nitrides, prevents austenite grains from coarsening by the pinning effect, refines the structure, and improves the base metal toughness and HAZ toughness. The Ti-containing nitride also has an action of promoting intragranular ferrite transformation, contributing to refinement of the structure and improvement of the base metal toughness and the HAZ toughness. In order to exert such an effect, the N amount needs to be 0.0020% or more, preferably 0.0030% or more, more preferably 0.0040% or more. However, when the amount of N becomes excessive, the amount of dissolved N increases, strain aging occurs, and the base metal toughness and the HAZ toughness deteriorate. Therefore, the N content is 0.020% or less, preferably 0.018% or less, more preferably 0.016% or less.

  The basic component composition of the steel material of the present invention is as described above, and the balance is iron and inevitable impurities. As inevitable impurities, mixing of elements (for example, Sn, As, Pb, etc.) brought in depending on the status of raw materials, materials, manufacturing equipment, etc. is allowed. Moreover, it is also effective to further contain the following elements, and the characteristics of the steel material are further improved according to the types of components contained.

[Ni: 1.5% or less (not including 0%), Cu: 1.5% or less (not including 0%), Cr: 1.5% or less (not including 0%), and Mo: 1. One or more elements selected from the group consisting of 5% or less (excluding 0%)]
Ni, Cu, Cr, and Mo are all elements that effectively act to increase the strength of steel materials, and the effect thereof increases as the content thereof increases. Is also preferably contained in an amount of 0.05% or more. Ni, Cu, Cr, and Mo are more preferably 0.10% or more for any element. However, when the content of these elements is excessive, the strength is excessively increased, and the base metal toughness and the HAZ toughness are deteriorated. Therefore, it is preferable to suppress the above elements to 1.5% or less. Ni, Cu, Cr and Mo are more preferably 1.2% or less, still more preferably 1% or less.

[Nb: 0.10% or less (not including 0%) and / or V: 0.1% or less (not including 0%)]
Nb and V are elements that precipitate as carbonitride and improve the base metal toughness by suppressing the coarsening of austenite grains. In order to effectively exhibit such effects, Nb is preferably contained in an amount of 0.002% or more, more preferably 0.005% or more, and further preferably 0.010% or more. However, when the amount of Nb becomes excessive, the carbonitride becomes coarse and the base material toughness deteriorates. Therefore, the Nb content is preferably 0.10% or less, more preferably 0.08% or less, still more preferably 0.06% or less, and particularly preferably 0.04% or less. Moreover, it is preferable to make V contain 0.002% or more, More preferably, it is 0.005% or more. However, when the amount of V is excessive, coarse carbonitride is precipitated, and the base metal toughness is deteriorated. Therefore, the V amount is preferably 0.1% or less, more preferably 0.08% or less.

[B: 0.005% or less (excluding 0%)]
B is an element effective in suppressing the formation of coarse grain boundary ferrite and improving the base metal toughness and the HAZ toughness. Although such an effect increases as the content increases, in order to effectively exhibit such an effect, it is preferable to contain 0.0005% or more. The amount of B is more preferably 0.0010% or more, and further preferably 0.0013% or more. However, when the amount of B becomes excessive, BN precipitates at the austenite grain boundary, and the base metal toughness and the HAZ toughness deteriorate. Accordingly, the B content is preferably 0.005% or less, more preferably 0.004% or less, and still more preferably 0.003% or less.

[Zr: 0.02% or less (excluding 0%) and / or REM: 0.02% or less (excluding 0%)]
Zr and REM (rare earth elements) are elements that contribute to refinement of oxides to improve HAZ toughness. Such an effect increases as the content thereof increases. However, in order to effectively exhibit such an effect, it is preferable that the content is 0.0001% or more. Both Zr and REM are more preferably 0.0005% or more. However, if excessively contained, the oxide becomes coarse and deteriorates the base metal toughness and the HAZ toughness. Therefore, it is preferable to suppress both 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), 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, more preferably La and / or Ce.

Next, a method for producing the steel material of the present invention will be described. As described above, the amount of Ti contained in the steel material as a coarse Ti-containing inclusion having a size exceeding 2.0 μm is reduced to a predetermined amount or less, and the solid solution Ti ratio in the steel material is controlled to be within a predetermined range. In which Ti, N, and Si melt the steel so that the following formula (1) is satisfied, and then the inclusions contained in the molten steel are levitated and separated, thereby including Al ₂ O ₃ -containing inclusions contained in the steel. The casting may be performed after controlling the number of objects to 10 or less (including 0) per 1 mm ² . In the following formula (1), [] represents the content (% by mass) of each element in the steel.
[Ti] × [N] ≦ (1 × 10 ⁻⁵ ) / [Si] (1)
However, when Si = 0% by mass, the steel is melted so that Ti and Ni satisfy the following formula (2).
[Ti] × [N] ≦ 1 × 10 ⁻³ (2)

  The reasons for specifying each requirement are as follows.

<< Balance of Ti, N, and Si >>
In melting steel, it is necessary to adjust the components so that Ti, N, and Si satisfy the following formula (1). In the following formula (1), [Ti] × [N] on the left side indicates an allowable solubility product of Ti and N, and if this value exceeds a certain value, the size exceeds 2.0 μm during casting. It was found that Ti-containing inclusions were produced. Further, as a result of studies by the present inventors, it has been clarified that this allowable solubility product is affected by the amount of Si in the steel. That is, the value of [Ti] × [N] varies depending on the Si concentration in the steel, and as the amount of Si in the steel increases, the value of [Ti] × [N] decreases and the coarse Ti-containing inclusions. Production was found to be suppressed. Therefore, in order to appropriately control the amount of Ti contained in the steel material as coarse Ti-containing inclusions and the solid solution Ti ratio, the amounts of Ti, N, and Si in the steel satisfy the relationship of the following formula (1). It is necessary to adjust the components as follows. The following formula (1) is a formula set by the present inventors by repeating various experiments. The following formula (1) can be transformed as the following formula (1a), and the components may be adjusted so as to satisfy the formula (1a). When the value on the left side of the following formula (1a) is the Z value, the Z value is preferably 5 × 10 ⁻⁶ or less, more preferably 1 × 10 ⁻⁶ or less.
[Ti] × [N] ≦ (1 × 10 ⁻⁵ ) / [Si] (1)
[Ti] × [N] × [Si] ≦ 1 × 10 ⁻⁵ (1a)

In addition, when Si is not added and the Si content in the steel is 0% by mass, when melting the steel, the steel is melted so that Ti and N satisfy the following formula (2). That's fine. The following formula (2) is calculated by substituting Si = 0.01% by mass (the lowest limit value of Si in Examples described later) into the above formula (1).
[Ti] × [N] ≦ 1 × 10 ⁻³ (2)

<< Floating separation of inclusions >>
After melting, casting is performed after controlling the number of inclusions containing Al ₂ O ₃ contained in the steel to 10 or less (including 0) per 1 mm ² by floating and separating inclusions contained in the molten steel. There is a need. The Al ₂ O ₃ containing inclusions, in the present invention means a Al ₂ O ₃ containing inclusions containing Al ₂ O ₃ less than 80 wt%. Ti-containing inclusions are generally known to crystallize using oxides such as Al ₂ O ₃ as nuclei, but in steels having a component composition such as the present invention, Al ₂ O _3- containing inclusions are crystallized. It is considered that Ti-containing inclusions are formed as nucleation. This is generally called heterogeneous nucleation. Therefore, if the number density of Al ₂ O ₃ -containing inclusions in molten steel is reduced, the amount of Ti contained in the steel material as Ti-containing inclusions whose size exceeds 2.0 μm can be reduced, and the solid solution Ti ratio is within an appropriate range. Can be controlled.

The number density of the inclusions containing Al ₂ O ₃ is 10 or less (including 0) per 1 mm ^{2 of} the observation visual field area. When the number density exceeds 10 pieces / mm ² , the Ti-containing inclusions become coarse, 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 the HAZ toughness deteriorate. . The number density is preferably 8.0 pieces / mm ² or less, more preferably 6.0 pieces / mm ² or less.

The number density of the Al ₂ O ₃ -containing inclusions can be adjusted by levitating and separating the inclusions (mainly oxide inclusions) contained in the molten steel from the molten steel. Examples of the method for floating and separating inclusions include agglomeration and coalescence of oxides using a refining apparatus using gas stirring such as LF (Ladle Furnace) or a reflux type vacuum degassing refining apparatus such as RH (Ruhrstahl Hausen). It is preferable to promote floating separation of the Al ₂ O ₃ -containing oxide. When the reflux gas flow rate is set to 100 to 200 Nm ³ / hour using an RH degassing refining apparatus, for example, the time (reflux time) from when Al is added to the molten steel until the reflux is stopped is 5 minutes or more. Preferably, it is 10 minutes or more. Although the number density of Al ₂ O ₃ containing inclusions can be reduced by increasing the reflux time, it is preferable, but the upper limit is about 90 minutes because the productivity is reduced.

  After the inclusions contained in the molten steel are floated and separated, they may be cast according to a conventional method and hot rolled (cold rolled as necessary). Specifically, the rolling is carried out at a cooling time of 1400 to 1500 ° C. during casting within 600 seconds, the pre-rolling heating conditions of 1050 to 1200 ° C. × 2 to 5 hours, and the finish rolling end temperature of 750 ° C. or higher. Cooling after completion may be performed at an average cooling rate of 2 to 15 ° C./second and a cooling stop temperature of 300 to 500 ° C.

  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 refers to one having a plate thickness of 3.0 mm or more, as defined by JIS. This steel plate can be used as a material for structures such as bridges, high-rise buildings, ships, etc., and has excellent base metal toughness and HAZ toughness not only in small to medium heat input welding but also in large heat input welding. Become. Since the steel material of the present invention exhibits good HAZ toughness even when a large heat input welding with a heat input of 50 kJ / mm or more is performed on a steel plate having a thickness of 50 mm or more, for example, Although it is a preferable aspect to apply to a steel plate having such a thickness, the plate thickness is not limited to 50 mm or more, and application to a steel plate having a thickness less than that is not excluded.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  Steels having the composition shown in Tables 1 and 2 (the balance is iron and inevitable impurities) are melted, and inclusions contained in the molten steel are floated and separated from the molten steel, and then cast into a slab (the cross-sectional shape is 150 mm × 250 mm), and then hot-rolled to obtain a hot-rolled steel sheet having a thickness of 80 mm.

  Hot rolling is performed by setting the cooling time at 1400 to 1500 ° C. during casting within 600 seconds, heating conditions before rolling as 1100 ° C. × 3 hours, finishing rolling finish temperature of 780 ° C. or higher, and after completion of rolling, Cooling to 0 ° C. was performed at an average cooling rate of 6 ° C./second and a 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 Tables 1 and 2 below, “-” indicates that no element is contained.

Based on the Ti amount, the N amount, and the Si amount shown in Tables 1 and 2 below, a value (Z value) of [Ti] × [N] × [Si] is calculated, and the results are shown in Table 3 below. For steel type A in Table 1 below, a value of [Ti] × [N] was calculated, and the calculation result is shown in the Z value column. In Table 3 below, “αE-β” means “α × 10 ^−β ”.

Inclusions contained in the molten steel are separated from the molten steel by changing the flow rate of the reflux gas at RH to 100 to 200 Nm ³ / hour and changing the time (reflux time) from when Al is added until the reflux gas is stopped. did. Table 3 below shows the reflux time. In addition, No. shown in Table 3 below. 29 and No. No. 31 is an example in which the inclusions contained in the molten steel are cast without being floated and separated from the molten steel.

After floating and separating the inclusions from the molten steel, before casting, the number density of the inclusions containing Al ₂ O ₃ in the molten steel was examined by the following procedure.

[Number density of inclusions containing Al ₂ O ₃ ]
Using a cup-shaped sampler (inner diameter: about 35 mm × height: about 50 mm), molten steel was collected from the tundish for continuous casting and solidified by air cooling. The steel obtained by solidification was taken out from the cup-shaped sampler, cut with a horizontal plane at a position of about 10 mm from the bottom of the sample, the cut surface was polished, and this was used as an inclusion observation sample. The inclusion observation sample was observed with EPMA (Electron Probe X-ray Micro Analyzer; electron probe X-ray microanalyzer, “JXA-8500F” manufactured by JEOL Ltd.), and the equivalent circle diameter was 0.2 μm or more. While measuring the number, the component composition of the particles was quantitatively analyzed. The observation conditions are an acceleration voltage of 20 kV, a sample current of 0.01 μA, an observation field of view of 1 to 5 cm ² centered on the center of the polished surface, and the number of analysis particles of 100 or more. Semi-quantitative analysis was performed with a line detector (EDS). The analysis target elements were Al, Mn, Si, Ti, Zr, Ca, La, Ce, and O. The detected element concentrations were all converted into oxides and normalized, and then the Al ₂ O ₃ concentration was obtained. . Among the detected total inclusions, the inclusions containing Al ₂ O ₃ more than 80 wt% was Al ₂ O ₃ containing inclusions. The number density of Al ₂ O ₃ containing inclusions was calculated per 1 mm ² to determine the number density. Table 3 shows the number density of inclusions containing Al ₂ O ₃ .

Next, for the hot rolled steel sheet manufactured as described above, in the following manner,
(A) Of the total amount of Ti contained in the steel material, the amount of Ti contained in the steel material as a coarse Ti-containing inclusion having a size exceeding 2.0 μm,
(B) Ratio R / Q (solid solution) of the value R obtained by subtracting the Ti amount contained in the steel material from the total Ti amount Q as the Ti-containing inclusion having a size exceeding 0.1 μm and the total Ti amount Q contained in the steel material Ti ratio),
(C) base material toughness,
(D) The HAZ toughness when the base material was welded was measured. These results are shown in Table 3 below.

[(A) Ti amount contained in steel material as coarse Ti-containing inclusions having a size exceeding 2.0 μm out of total Ti amount contained in steel material]
A test piece (length 15 mm × width 15 mm × length 5 mm) is cut from 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% triethanol. Electrolytic extraction was performed at room temperature with an electric current of 500 A / m ² or less using an amine-1% tetramethylammonium chloride-methanol solution as an electrolytic solution. After electrolytic extraction, the extraction residue was filtered through a membrane filter having an opening of 2.0 μm.

  Next, the extraction residue (inclusions having a size exceeding 2.0 μm) remaining on the filter after filtration was put in a platinum crucible together with the filter, and heated with a gas burner to be incinerated. Next, an alkali flux (a mixture of sodium carbonate and sodium tetraborate) was added and heated again with a gas burner to melt the extraction residue. Next, 18 vol% hydrochloric acid was added to make the melt into a solution, and then the solution was transferred to a measuring flask, and pure water was added to make up to 50 ml to obtain an analysis solution. The Ti concentration in the analysis solution was measured by ICP emission analysis, and the amount of Ti contained in the steel material as a Ti-containing inclusion having a size exceeding 2.0 μm was measured. The measurement results are shown in Table 3 below.

  In general, the larger the inclusions in the steel material, the smaller the number of residual inclusions in the steel material. Therefore, it is possible to accurately grasp the amount of Ti present as coarse inclusions in a microscopic observation of a polished sample, which is a conventional method for investigation of inclusions. Although it was difficult, with this analysis method that combines electrolytic extraction and filtration with a membrane filter, it is possible to measure the total amount of Ti contained in inclusions with a size exceeding 2.0 μm. It can be measured with little accuracy.

[(B) ratio R / Q (solid solution Ti ratio)]
In the above (a), instead of using a membrane filter having an aperture of 2.0 μm, filtration is performed using a membrane filter having an aperture of 0.1 μm, and Ti-containing intervening having a size exceeding 0.1 μm. The amount of Ti contained in the steel as a product was measured.

  Moreover, in said (a), the test piece of the same magnitude | size is cut out separately from the position close | similar to the test piece cut out from each hot-rolled steel plate, and all this test piece is JIS G1258-1 "acid decomposition-potassium disulfate melting method. The Ti concentration in this solution was measured by ICP emission spectrometry, and the total Ti amount Q contained in the steel material was measured.

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

[(C) Base material toughness]
A Charpy impact test piece (No. 4 test piece of JIS Z2201) is taken in the rolling direction from a depth t / 4 position (t is the plate thickness) from the surface of each hot-rolled steel sheet, and at −60 ° C. based on JIS Z2242. A Charpy impact test was performed and the absorbed energy (vE- ₆₀ ) was measured. At this time, the absorbed energy (vE- ₆₀ ) was measured for the three test pieces, and the minimum value was obtained. Those having a minimum value of vE- ₆₀ of 100 J or more were evaluated as being excellent in base material toughness.

[(D) HAZ toughness when base metal is welded]
A thermal cycle simulating large heat input welding by collecting Charpy impact test pieces (No. 4 test piece of JIS Z2201) in the rolling direction from the depth t / 4 position (t is the plate thickness) from the surface of each hot-rolled steel plate. A test was conducted to evaluate the HAZ toughness when a hot-rolled steel sheet (base material) was welded. At this time, in the heat cycle test, the test piece is heated to 1400 ° C. and held for 60 seconds, and then the temperature range of 800 to 500 ° C. is cooled over 500 seconds, whereby the heat input corresponding to the welding heat input is 55 kJ. Gave. Based on JIS Z2242, a Charpy impact test was performed at −40 ° C., and the absorbed energy (vE ₋₄₀ ) was measured. At this time, the absorbed energy (vE- ₄₀ ) was measured for the three test pieces, and the minimum value was obtained. Those having a minimum value of vE- ₄₀ of 100 J or more were evaluated as having excellent HAZ toughness.

FIG. 2 is a graph showing the relationship between the value (Z value) of [Ti] × [N] × [Si] and the HAZ toughness. In FIG. 2, the results of the invention examples (Nos. 1 to 25) shown in Table 3 below are indicated by ◇, and among the comparative examples, the results (Nos. 26, 28, and 30) in which the Z value is outside the range recommended by the present invention. , 31) are 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 when Si is not contained (Table It can be seen that the HAZ toughness can be improved by suppressing the steel type A) Z value to 1.0E-03 (1.0 × 10 ⁻³ ) or less.

  From the following Tables 1 to 3, it can be considered as follows. No. 1 to 25 are examples that satisfy the requirements defined in the present invention, the component composition is appropriately adjusted, and the amount of Ti contained in the steel material as coarse Ti-containing inclusions is suppressed to 0.010% or less. It can be seen that a steel sheet having good base metal toughness and HAZ toughness is obtained because an appropriate amount of Ti is dissolved in the steel material.

  In contrast, no. Nos. 26 to 46 are examples that deviate from any requirement defined in the present invention, and at least one of the base material toughness and the HAZ toughness is inferior. Details are as follows.

No. 26 and no. 30 is an example in which [Ti] × [N] × [Si] exceeds 1.0 × 10 ⁻⁵ , and the amount of Ti contained in the steel material is excessive as a Ti-containing inclusion whose size exceeds 2.0 μm. Therefore, the base material toughness and the HAZ toughness are deteriorated. No. 27 and No. No. 29 is an example in which the ratio R / Q is out of the predetermined range, and the amount of solute Ti in the steel material is too large. Therefore, the base material toughness and the HAZ toughness are deteriorated. No. 28 and No. 31 is an example in which [Ti] × [N] × [Si] exceeds 1.0 × 10 ⁻⁵ , and the amount of Ti contained in the steel material is excessive as a Ti-containing inclusion whose size exceeds 2.0 μm. This is an example in which the ratio R / Q is below a predetermined range because the amount of dissolved Ti in the steel material is too small. Therefore, the base material toughness and the HAZ toughness are deteriorated.

  No. 32 to 39 and 41 to 44 are examples that do not satisfy the component composition defined in the present invention. No. No. 32 is that in which the C content in the steel sheet exceeds the range specified in the present invention, and the base metal toughness and the HAZ toughness are deteriorated. It is considered that the deterioration of the base metal toughness and the HAZ toughness is due to an increase in the amount of hard island martensite (MA) produced. No. In No. 33, the Si content in the steel sheet exceeds the range specified in the present invention, and the base metal toughness and the HAZ toughness are deteriorated. It is considered that the deterioration of the base metal toughness and the HAZ toughness is due to an increase in the amount of hard island martensite (MA) produced. No. In No. 34, the Mn content in the steel sheet exceeds the range specified in the present invention, and the strength of the steel sheet becomes too high, so that the base metal toughness and the HAZ toughness are deteriorated.

  No. No. 35 is such 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 are deteriorated. No. No. 36 is that 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. In No. 37, the Al content in the steel sheet exceeds the range specified in the present invention, and the base metal toughness and the HAZ toughness are deteriorated. No. No. 38 is one in which the Ti content in the steel sheet is less than the range specified in the present invention, and the HAZ toughness is good, but the base metal toughness is deteriorated. No. In No. 39, the Ti content in the steel sheet exceeds the range specified in the present invention, and the base metal toughness and the HAZ toughness are deteriorated.

  No. Reference numeral 40 is a reference example, in which the content of Nb added as a selective element exceeds the range specified in the present invention, and the base material toughness is deteriorated.

  No. No. 41 is one in which the Ca content in the steel sheet is less than the range specified in the present invention, and the base metal toughness and the HAZ toughness are deteriorated. No. No. 42 is such that the Ca content in the steel sheet exceeds the range specified in the present invention, and the base metal toughness and the HAZ toughness are deteriorated. No. No. 43 is the one in which the N content in the steel sheet is less than the range defined in the present invention, and the base metal toughness and the HAZ toughness are deteriorated. No. No. 44 is an example in which the N content in the steel sheet exceeds the range specified in the present invention, the ratio R / Q is below the predetermined range, and the amount of solute Ti in the steel material is too small. Accordingly, the base metal toughness and the HAZ toughness are deteriorated.

  No. 45 and no. 46 is a reference example in which the content of Ni or Cu added as a selective element exceeds the range specified in the present invention, and the base metal toughness and the HAZ toughness are deteriorated.

  No. 28, 29, 31, and 41 are not sufficiently floated and separated from the molten steel because inclusions contained in the molten steel are too short since the time (refluxing time) from when Al is added in RH until the reflux gas is stopped is too short. . Therefore, base material toughness and HAZ toughness are deteriorated.

Claims (6)

Components in steel are C: 0.03 to 0.16% (meaning 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 (excluding 0%),
S: 0.015% or less (excluding 0%),
Al: 0.05% or less (excluding 0%),
Ti: 0.010 to 0.08%,
Ca: 0.0005 to 0.010% and N: 0.0020 to 0.020%,
The balance is steel made of iron and inevitable impurities,
Of the total Ti amount Q contained in the steel material,
The amount of Ti contained in steel as Ti-containing inclusions exceeding 2.0 μm is 0.010% or less (excluding 0%), and the amount of Ti contained in steel as Ti-containing inclusions exceeding 0.1 μm The ratio R / Q between the value R subtracted from the Ti amount Q and the total Ti amount Q contained in the steel material is 0.30 to 0.70, and is excellent in the toughness of the base material and the weld heat affected zone Steel material. The steel material is still another element,
Ni: 1.5% or less (excluding 0%),
Cu: 1.5% or less (excluding 0%),
The steel material according to claim 1, comprising at least one element selected from the group consisting of Cr: 1.5% or less (not including 0%) and Mo: 1.5% or less (not including 0%). . The steel material is still another element,
The steel material according to claim 1 or 2, wherein Nb: not more than 0.10% (not including 0%) and / or V: not more than 0.1% (not including 0%). The steel material is still another element,
B: 0.005% or less (0% is not included) The steel material in any one of Claims 1-3. The steel material is still another element,
Zr: 0.02% or less (excluding 0%) and / or REM: 0.02% or less (not including 0%) The steel material according to any one of claims 1 to 4. A method for producing the steel material according to any one of claims 1 to 5,
After the steel is melted so that Ti, N, and Si satisfy the following formula (1), the inclusions contained in the steel are levitated and separated, whereby the number of inclusions containing Al ₂ O ₃ contained in the steel A base material and a method for producing a steel material excellent in toughness of a weld heat affected zone, wherein the steel is cast after being controlled to 10 or less (including 0) per mm ² .
[Ti] × [N] ≦ (1 × 10 ⁻⁵ ) / [Si] (1)
In formula (1), [] represents content (mass%) of each element in steel.
However, when Si = 0% by mass, the steel is melted so that Ti and Ni satisfy the following formula (2).
[Ti] × [N] ≦ 1 × 10 ⁻³ (2)

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