WO2022124587A1 - High strength ferritic stainless steel having improved corrosion resistance at welded portion - Google Patents

Abstract

The present specification discloses a ferritic stainless steel having improved ridging resistance. According to an embodiment, the disclosed ferritic stainless steel having improved ridging resistance comprises: at most 0.01 wt% (exclusive of 0) of C; 1.0 to 3.0 wt% of Si; at most 0.5 wt% (exclusive of 0) of Mn; 0.05 to 0.2 wt% of P; 13 to 21 wt% of Cr; 0.1 to 0.5 wt% of Ti; and at most 0.01 wt% (exclusive of 0) of N, with the balance being Fe and other unavoidable impurities.

Description

High-strength ferritic stainless steel with improved corrosion resistance in welds and manufacturing method therefor

The present invention relates to a ferritic stainless steel and a method for manufacturing the same, and more particularly, to a high-strength ferritic stainless steel having improved corrosion resistance at welds.

In general, stainless steel is classified according to its chemical composition or metal structure. According to the metal structure, stainless steel is classified into austenitic (300 series), ferritic (400 series), martensitic, and ideal.

Among them, ferritic stainless steel is a steel with high price competitiveness compared to austenitic stainless steel because it contains less expensive alloying elements. Ferritic stainless steel has good surface gloss, drawability and oxidation resistance, and is widely used in kitchenware, building exterior materials, home appliances, and electronic parts.

However, when the ferritic stainless steel is heated to a temperature of 400°C or higher or exposed to a temperature atmosphere of 400°C or higher for a long time, intergranular corrosion occurs. The main cause of intergranular corrosion is Cr-carbide formed at the grain boundary by the reaction of carbon (C) contained in stainless steel and chromium (Cr), which is the main alloying element of stainless steel. is known to be caused by

Meanwhile, STS 430 among ferritic stainless steels is a ferritic stainless steel in which 300 to 800 ppm of C and N are added to a composition containing 16 to 18% of chromium (Cr). STS 430 has an elongation of about 30% and has been applied to parts such as refrigerator doors. However, in the case of STS 430, there is a problem that the C content is high, so it is difficult to use by welding, and the yield strength is low as 300 MPa or less.

In the case of STS 430J1L among ferritic stainless steels, 100 ppm or less of C and N were added to a composition containing about 18 to 20% Cr, respectively, so that it could be used by welding. However, STS 430J1L has a problem that, although Cu and Nb are added for the purpose of improving strength, it still has a low yield strength of 300 MPa or less and a high manufacturing cost.

It is an object of the present invention to provide a high-strength ferritic stainless steel having improved corrosion resistance in welds by optimizing the alloy composition range of the ferritic stainless steel in order to solve the above problems.

High-strength ferritic stainless steel with improved corrosion resistance at the weld according to an embodiment of the present invention is, by weight, C: 0.01% or less (excluding 0), Si: 1.0 to 3.0% or less, Mn: 0.5% or less (excluding 0) ), P: 0.05 to 0.2%, Cr: 13 to 21%, Ti: 0.1 to 0.5%, N: 0.01% or less (excluding 0) remaining Fe and other unavoidable impurities.

In addition, the high-strength ferritic stainless steel with improved corrosion resistance at the weld according to an embodiment of the present invention may satisfy the yield strength index of 1.50 or more, which is expressed by the following formula (1).

Formula (1): Si+6P

(Here, Si and P mean the content (wt%) of each element)

In addition, the high-strength ferritic stainless steel with improved corrosion resistance at the weld according to an embodiment of the present invention may satisfy the following Equation (2), an elongation index of 3.00 or less.

Formula (2): Si+10P

(Here, Si and P mean the content (wt%) of each element)

In addition, the high-strength ferritic stainless steel with improved corrosion resistance at the weld according to an embodiment of the present invention may satisfy the intergranular corrosion resistance index of the weld, expressed by the following formula (3), of 6.0 or more.

Equation (3): Ti/(C+N)

(Here, C, N and Ti mean the content (wt%) of each element)

According to an embodiment of the present invention, it is possible to provide a high-strength ferritic stainless steel having improved corrosion resistance in the weld zone by optimizing the alloy composition range and component relation of the alloying elements Si and P.

1 is a graph showing the change in yield strength according to the addition of P to the Fe-13Cr-1.0Si alloy composition.

2 is a graph showing the change in yield strength according to the addition of Si to the Fe-13Cr-0.05P alloy composition.

3 is a graph showing the change in yield strength of ferritic stainless steel according to the yield strength index (Si+6P).

4 is a graph showing the change in the elongation of the ferritic stainless steel according to the elongation index (Si+10P).

High-strength ferritic stainless steel with improved corrosion resistance at the weld according to an embodiment of the present invention is, by weight, C: 0.01% or less (excluding 0), Si: 1.0 to 3.0% or less, Mn: 0.5% or less (excluding 0) ), P: 0.05 to 0.2%, Cr: 13 to 21%, Ti: 0.1 to 0.5%, N: 0.01% or less (excluding 0) remaining Fe and other unavoidable impurities.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following describes preferred embodiments of the present invention. However, the embodiment of the present invention may be modified in various other forms, and the technical idea of the present invention is not limited to the embodiment described below. In addition, the embodiments of the present invention are provided in order to more completely explain the present invention to those of ordinary skill in the art.

The terms used in this application are only used to describe specific examples. Therefore, for example, a singular expression includes a plural expression unless the context clearly requires it to be singular. In addition, terms such as "comprises" or "including" as used in the present application are used to clearly indicate that the features, steps, functions, components, or combinations thereof described in the specification exist, and other features It should be noted that it is not intended to preliminarily exclude the existence of elements, steps, functions, components, or combinations thereof.

Meanwhile, unless otherwise defined, all terms used herein should be regarded as having the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Accordingly, unless explicitly defined herein, specific terms should not be construed in an unduly idealistic or formal sense. For example, a singular expression herein includes a plural expression unless the context clearly dictates otherwise.

In addition, in this specification, "about", "substantially", etc. are used in or close to the numerical value when manufacturing and material tolerances inherent in the stated meaning are presented, and are used in a precise sense to aid the understanding of the present invention. or absolute figures are used to prevent unreasonable use by unconscionable infringers of the mentioned disclosure.

High-strength ferritic stainless steel with improved corrosion resistance in the weld zone according to the present invention is, by weight, C: 0.01% or less (excluding 0), Si: 1.0 to 3.0% or less, Mn: 0.5% or less (excluding 0), P: 0.05 to 0.2%, Cr: 13 to 21%, Ti: 0.1 to 0.5%, N: 0.01% or less (excluding 0) The remaining Fe and other unavoidable impurities are included.

Hereinafter, the reason for limiting the component range of each alloy element will be described.

The content of carbon (C) is 0.01% or less (excluding 0).

C is a low-cost austenite stabilizing element, and effectively suppresses the formation of a delta (?)-ferrite phase. In addition, C is an interstitial element and improves the yield strength of steel by a solid solution strengthening effect. However, if the content of C is excessive, chromium carbide (Cr ₂₃ C ₆ ) is generated when used for a long time at 600° C. or less after welding, intergranular corrosion occurs, and the elongation, toughness, etc. of the steel are reduced. Therefore, in the present invention, the content of C is to be controlled to 0.01% or less (excluding 0).

The content of silicon (Si) is 1.0 to 3.0%.

Si is a component added as a deoxidizer in the steelmaking step, and when a certain amount is added, it has the effect of improving yield strength and corrosion resistance. In addition, Si is a ferrite phase forming element and increases the stability of the ferrite phase, improves the pitting dislocation, and increases the oxidation resistance. Therefore, in the present invention, Si is added by 1.0% or more. However, when the content of Si is excessive, elongation and toughness are lowered, so the Si content is controlled to 3.0% in the present invention.

The content of manganese (Mn) is 0.5% or less (excluding 0).

Mn forms inclusions (MnS), thereby reducing hot workability, ductility, and toughness of the steel. However, since excessive reduction of Mn increases the refining cost, in the present invention, the Mn content is controlled to 0.5% or less.

The content of chromium (Cr) is 13 to 21%.

Cr is an element that improves corrosion resistance by forming a passivation film in an oxidizing environment, and 13% or more is added to secure corrosion resistance. However, when the content of Cr is excessive, delta (δ) ferrite formation in the slab is promoted, and elongation and impact toughness are reduced. Therefore, in the present invention, the upper limit of the Cr content is to be limited to 21%.

The content of phosphorus (P) is 0.05 to 0.2%.

In the conventional case, P is a harmful element that segregates at grain boundaries to reduce corrosion resistance and hot workability, and its content is limited to 0.04% or less. However, when P is added, it not only has a solid solution strengthening effect, but also forms precipitates such as Fe ₂ P, Cr ₂ P, and Ti ₂ P to improve the yield strength of stainless steel due to the precipitation strengthening effect. Using this point, in the present invention, the P content is added by 0.05% or more to secure high strength of 400 MPa or more. However, when the content of P exceeds 0.2%, there is a problem in that the elongation value falls to less than 25%. Therefore, in the present invention, in order to secure high strength and workability, the content of P is to be controlled to 0.05 to 0.2%.

The content of nitrogen (N) is 0.01% or less (excluding 0 silver).

Like C, N is an interstitial element and improves the yield strength of steel by the solid solution strengthening effect. However, if the content of N is excessive, carbonitride is generated when used for a long time at 600° C. or less after welding, intergranular corrosion occurs, and the elongation, toughness, etc. of the steel are reduced. Therefore, in the present invention, the content of N is to be controlled to 0.01% or less (excluding 0).

The content of titanium (Ti) is 0.1 to 0.5%.

Ti is an element effective in reducing the amount of solid solution C and solid solution N in steel and securing corrosion resistance of steel by preferentially combining with interstitial elements such as C and N to form precipitates (carbonitrides). However, when the content is excessive, the toughness is reduced, the titanium-based inclusions increase, and surface defects such as scabs occur, and the nozzle clogging phenomenon may occur during playing. Therefore, in the present invention, it is intended to control the content of titanium to 0.1 to 0.5%.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to any person skilled in the art in the manufacturing process, all details thereof are not specifically mentioned in the present specification.

The present inventors obtained the following knowledge through specific studies on P and Si, which are alloying elements that affect the yield strength characteristics of ferritic stainless steel, in order to improve the strength of ferritic stainless steel.

It was found that the value of the yield strength of ferritic stainless steel increases in proportion to the yield strength index expressed as (Si+6P). In addition, it was found that the elongation value of the ferritic stainless steel decreases in proportion to the elongation index expressed as (Si+10P).

Referring to FIG. 2 , it can be seen that the yield strength increases linearly as the yield strength index increases. In addition, referring to FIG. 2 , when the yield strength index is 1.50 or more, it is possible to secure a yield strength of 400 MPa or more.

Referring to FIG. 3 , it can be seen that the elongation value decreases linearly as the elongation index decreases. In addition, referring to FIG. 3 , when the elongation index is 3.00 or less, an elongation of 25% or more may be secured.

Accordingly, the high-strength ferritic stainless steel having improved corrosion resistance at the weld according to an embodiment of the present invention may have a yield strength index of 1.50 or more.

In addition, the high-strength ferritic stainless steel with improved corrosion resistance at the weld according to an embodiment of the present invention may have an elongation index of 3.00 or less.

In general, stainless steel is a steel with improved corrosion resistance by the Cr component containing 11% or more. However, depending on the environment of use, the corrosion resistance of stainless steel is deteriorated when the Cr component is locally lowered to 11% or less, and this area is said to be sensitized.

Ferritic stainless steels have lower carbon solubility than austenitic stainless steels, so Cr carbonitrides are more easily generated at the grain boundaries when subjected to heat, thereby forming a Cr-depleted layer around the grain boundaries, and thus grain boundary sensitization is easy to occur.

In particular, when welding ferritic stainless steel, Cr carbide is precipitated at grain boundaries in the heat affected zone (Heat Affected Zone) heated to the sensitized temperature range by welding heat input. Therefore, the ferritic stainless steel has a problem of corrosion due to the occurrence of sensitization phenomenon due to the lack of chromium having a lower chromium content than the matrix effective for corrosion resistance near the grain boundary during welding.

As a result of examining various control conditions to solve the intergranular corrosion problem at the weld, it was found that the intergranular corrosion resistance index value at the weld where intergranular corrosion starts to occur is different depending on the content of the alloying element P.

The intergranular corrosion resistance index of the weld is expressed by the following formula (3).

Equation (3): Ti/(C+N)

(Here, C, N and Ti mean the content (wt%) of each element)

Depending on the content of the alloying element P in the ferritic stainless steel, the intergranular corrosion resistance index value of the weld zone where intergranular corrosion occurs at the weld zone is different.

If the content of the alloying element P in the ferritic stainless steel is less than 0.05% by weight, intergranular corrosion does not occur when the intergranular corrosion resistance index of the weld zone satisfies 12 or more. On the other hand, when the content of the alloy element P in the ferritic stainless steel is 0.05% or more by weight, intergranular corrosion does not occur when the intergranular corrosion resistance index of the weld zone is 6.0 or more.

Therefore, when the content of P is 0.05% (wt%) or more, it is possible to improve the intergranular corrosion resistance of the weld zone while reducing the addition amount of Ti, which is an expensive alloying element.

Accordingly, in the high-strength ferritic stainless steel with improved corrosion resistance at the weld according to an embodiment of the present invention, the content of P is 0.05% or more by weight, and the intergranular corrosion resistance index of the weld can satisfy 6.0 or more.

As described above, by optimizing the composition ratio and component relation of the alloy elements, it is possible to secure higher strength compared to STS 430. When it is used in the field of home appliances such as a door of a refrigerator by using high strength characteristics, the thickness of the material can be reduced and dent characteristics can be improved. In addition, by adjusting the content of the alloying element P, it is possible to improve the intergranular corrosion resistance of the weld zone while reducing the content of Ti, which is an expensive stabilizing element.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

{Example}

For the various alloy component ranges shown in Table 1 below, 120 mm thick ingots were prepared by dissolving in a 50 kg vacuum melting facility. The prepared ingot was hot-rolled at 1100 to 1200° C. to a thickness of 3.0 mm. After that, the hot-rolled material was annealed and pickled, cold-rolled to 0.6 mm, and then cold-rolled annealing and pickling were performed.

division	alloy composition
	C	Si	Mn	P	Cr	Ti	N
Example 1	0.006	1.50	0.30	0.070	13	0.12	0.007
Example 2	0.008	2.00	0.48	0.069	13.5	0.12	0.007
Example 3	0.004	1.68	0.30	0.085	15.5	0.14	0.005
Example 4	0.005	1.25	0.20	0.095	16.5	0.15	0.005
Example 5	0.006	1.19	0.20	0.160	16.5	0.10	0.006
Example 6	0.008	2.01	0.20	0.050	18.5	0.12	0.005
Example 7	0.008	1.35	0.20	0.070	18.5	0.19	0.006
Example 8	0.005	1.22	0.20	0.089	19.5	0.21	0.006
Example 9	0.006	1.20	0.21	0.185	20.5	0.25	0.006
Comparative Example 1	0.05	0.30	0.20	0.020	16.5	0.01	0.035
Comparative Example 2	0.008	1.40	0.28	0.020	11.5	0.22	0.005
Comparative Example 3	0.007	1.18	0.19	0.019	17.5	0.31	0.005
Comparative Example 4	0.008	0.60	0.48	0.022	13.5	0.12	0.007
Comparative Example 5	0.003	0.75	0.30	0.021	14.5	0.15	0.019
Comparative Example 6	0.007	0.95	0.48	0.022	16.5	0.09	0.007
Comparative Example 7	0.008	0.85	0.30	0.021	17.5	0.13	0.005
Comparative Example 8	0.006	0.90	0.48	0.045	19.5	0.10	0.007
Comparative Example 9	0.008	2.60	0.30	0.300	13.5	0.22	0.006
Comparative Example 10	0.009	2.90	0.25	0.120	14.5	0.19	0.008
Comparative Example 11	0.008	1.85	0.25	0.200	15.5	0.20	0.008
Comparative Example 12	0.009	2.90	0.25	0.120	14.5	0.19	0.008
Comparative Example 13	0.008	1.83	0.25	0.150	15.5	0.09	0.008
Comparative Example 14	0.006	1.45	0.40	0.250	16.5	0.05	0.009

Next, a test piece of the cold-rolled material was prepared in a direction perpendicular to the rolling direction, and a JIS 13B tensile test was performed. Table 2 below shows the yield strength index values and elongation index values of Examples and Comparative Examples, and the yield strength values and elongation values measured through the tensile test.

division	Yield strength index	Yield strength (MPa)	elongation index	Elongation (%)
Example 1	1.92	437.0	2.20	27.4
Example 2	2.41	481.1	2.69	25.9
Example 3	2.19	462.4	2.53	26.4
Example 4	1.82	429.6	2.20	27.4
Example 5	2.15	462.8	2.79	25.6
Example 6	2.31	471.0	2.51	26.5
Example 7	1.77	423.2	2.05	27.9
Example 8	1.75	423.1	2.11	27.7
Example 9	2.31	479.1	3.05	24.8
Comparative Example 1	0.42	299.0	0.50	32.5
Comparative Example 2	1.52	398.0	1.60	29.2
Comparative Example 3	1.29	377.6	1.37	29.9
Comparative Example 4	0.73	327.2	0.82	31.5
Comparative Example 5	0.88	340.1	0.96	31.1
Comparative Example 6	1.08	358.7	1.17	30.5
Comparative Example 7	0.98	349.1	1.06	30.8
Comparative Example 8	1.17	368.0	1.35	30.0
Comparative Example 9	4.40	674.0	5.60	17.2
Comparative Example 10	3.62	593.0	4.10	21.7
Comparative Example 11	3.05	546.5	3.85	22.5
Comparative Example 12	3.62	593.0	4.10	21.7
Comparative Example 13	2.73	514.7	3.33	24.0
Comparative Example 14	2.95	540.5	3.95	22.2

Referring to Table 2, Examples 1 to 9 have a yield strength index of 1.5 or more, and an elongation index of 3.0 or less. Accordingly, Examples 1 to 9 had a high yield strength of 400 MPa or more, and an elongation value of 25% or more was derived. Therefore, in Examples 1 to 9, by securing high strength, it is possible to reduce the thickness of the material, and at the same time, suitable processing is possible.

In Comparative Examples 1 to 8, the elongation index value satisfies 3.0 or less. Accordingly, in Comparative Examples 1 to 8, elongation values of 25% or more were derived. However, Comparative Examples 1 to 8 had a yield strength index value of less than 1.5, and thus, had a low yield strength value of less than 400 MPa.

Comparative Examples 9 to 14 had a yield strength value of 1.5 or more, and thus a yield strength value of 400 MPa or more was derived. However, Comparative Examples 9 to 14 had an elongation index value greater than 3.0, resulting in an elongation of less than 25%.

In addition, the sensitization characteristics and intergranular corrosion characteristics were evaluated by measuring the intergranular corrosion test of the weld zone with respect to the cold rolled material.

As a welding method, TIG (Tungsten inert gas) welding was performed in parallel. For TIG welding, a DC type welding machine (maximum welding current 350A) was used, and bead-on-plate was used. Welding conditions were a welding current of 110A, welding speed of 0.32m/min, tungsten electrode diameter of 2.5mm, arc length of 1.5mm, and shielding gas Ar (15L/min).

The intergranular corrosion of the weld was evaluated by the Modified-Strauss test method under the condition that sensitization treatment was not performed on the TIG welding test piece. Modified-Strauss test method is a method to evaluate intergranular corrosion by placing Cu balls on the bottom of 6% CuSO4 + 0.5% H2SO4 aqueous solution, immersing them in a boiling aqueous solution for 24 hours, and then performing a U-bend test.

Table 3 below shows whether the P content of Examples and Comparative Examples is 0.05% or more by weight, the intergranular corrosion resistance index of the weld zone, and whether the intergranular corrosion of the weld zone occurs.

division	P content (wt%)	Intergranular corrosion resistance index of welds	Whether or not intergranular corrosion occurs in the weld area
Example 1	≥0.05	9.2	X
Example 2	≥0.05	8.0	X
Example 3	≥0.05	15.6	X
Example 4	≥0.05	15.0	X
Example 5	≥0.05	8.3	X
Example 6	≥0.05	9.2	X
Example 7	≥0.05	13.6	X
Example 8	≥0.05	19.1	X
Example 9	≥0.05	20.8	X
Comparative Example 1	< 0.05	0.1	O
Comparative Example 2	< 0.05	16.9	X
Comparative Example 3	< 0.05	25.8	X
Comparative Example 4	< 0.05	8.0	O
Comparative Example 5	< 0.05	7.0	O
Comparative Example 6	< 0.05	6.4	O
Comparative Example 7	< 0.05	9.6	O
Comparative Example 8	< 0.05	7.7	O
Comparative Example 9	≥0.05	15.7	X
Comparative Example 10	≥0.05	11.2	X
Comparative Example 11	≥0.05	12.5	X
Comparative Example 12	≥0.05	11.2	X
Comparative Example 13	≥0.05	5.6	O
Comparative Example 14	≥0.05	3.3	O

Referring to Table 3, Examples 1 to 9 have a P content of 0.05% or more, and a weld intergranular corrosion resistance index of 6 or more. Accordingly, Examples 1 to 9 improved the intergranular corrosion resistance of the weld zone.

In Comparative Examples 1 to 8, P was added in an amount of less than 0.05% (wt%), and in Comparative Example 1, the intergranular corrosion resistance index of the weld zone was 0.1 and had a value of less than 12, and intergranular corrosion of the weld zone occurred. In addition, in Comparative Examples 4 to 8, the intergranular corrosion resistance index of the weld portion had a value of less than 12, and intergranular corrosion of the weld portion occurred.

On the other hand, Comparative Example 2 had a weld resistance index value of 16.9, which is 12 or more, so that intergranular corrosion of the weld zone did not occur. In addition, Comparative Example 3 also had a weld resistance index value of 25.8, which is 12 or more, so that intergranular corrosion of the weld area did not occur.

In Comparative Examples 9 to 14, P was added in an amount of 0.05% (wt%) or more, but in Comparative Examples 9 to 12, the intergranular corrosion resistance index of the weld intergranular corrosion resistance index had a value of 6 or more, and intergranular corrosion of the weld area did not occur. didn't

On the other hand, in Comparative Example 13, the intergranular corrosion resistance index of the weld zone had a value of 5.6, which is less than 6, and intergranular corrosion of the weld zone occurred. In Comparative Example 14, the intergranular corrosion resistance index of the weld zone had a value of 3.3, which is less than 6, and intergranular corrosion of the weld zone occurred.

According to the revised embodiment, by optimally controlling the alloy composition and the compositional formula, it is possible to secure high strength and at the same time to enable proper processing, and to secure improved corrosion resistance in which intergranular corrosion does not occur in the welded part.

In the foregoing, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and those of ordinary skill in the art may not depart from the concept and scope of the claims described below. It will be appreciated that various modifications and variations are possible.

According to an example of the present invention, by optimizing the alloy composition range of the ferritic stainless steel, it is possible to provide a high-strength ferritic stainless steel with improved corrosion resistance at the weld.

Claims (4)

1. By weight%, C: 0.01% or less (excluding 0), Si: 1.0 to 3.0% or less, Mn: 0.5% or less (excluding 0), P: 0.05 to 0.2%, Cr: 13 to 21%, Ti: 0.1 to 0.5%, N: 0.01% or less (excluding 0) High-strength ferritic stainless steel with improved weld corrosion resistance, including the remaining Fe and other unavoidable impurities.
2. The method according to claim 1,

   High-strength ferritic stainless steel with improved corrosion resistance in the weld, which is expressed by the following formula (1), and has a yield strength index of 1.50 or more.

   Formula (1): Si+6P

   (Here, Si and P mean the content (wt%) of each element)
3. The method according to claim 1,

   A high-strength ferritic stainless steel with improved corrosion resistance at welds, which is expressed by the following formula (2), and has an elongation index of 3.00 or less.

   Formula (2): Si+10P

   (Here, Si and P mean the content (wt%) of each element)
4. The method according to claim 1,

   A high-strength ferritic stainless steel with improved corrosion resistance at the weld, having a weld intergranular corrosion resistance index of 6.0 or more, expressed by the following formula (3).

   Equation (3): Ti/(C+N)

   (Here, C, N and Ti mean the content (wt%) of each element)

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