Title:
Cu-Ni-Si system copper alloy for electronic materials
Kind Code:
A1


Abstract:
The present invention provides Corson alloy having remarkably improved properties, in particular, by bringing out the effect of added Cr more efficiently. A copper alloy for electronic materials comprising 2.5-4.5% by mass of Ni; 0.50-1.2% by mass of Si; 0.0030-0.2% by mass of Cr; balance Cu and inevitable impurities, wherein the weight ratio of Ni to Si is in the range of 3 to 7 and the content of carbon is 50 ppm by mass or less is provided.



Inventors:
Era, Naohiko (Ibaraki-ken, JP)
Fukamachi, Kazuhiko (Ibaraki-ken, JP)
Kuwagaki, Hiroshi (Ibaraki-ken, JP)
Application Number:
11/654864
Publication Date:
07/24/2008
Filing Date:
01/18/2007
Assignee:
Nippon Mining & Metals Co., Ltd. (Tokyo, JP)
Primary Class:
International Classes:
C22C9/06
View Patent Images:



Primary Examiner:
IP, SIKYIN
Attorney, Agent or Firm:
Faegre Drinker Biddle & Reath LLP (Phili) (PHILADELPHIA, PA, US)
Claims:
What is claimed is:

1. A copper alloy for electronic materials comprising 2.5-4.5% by mass of Ni; 0.50-1.2% by mass of Si; 0.0030-0.2% by mass of Cr; balance Cu and inevitable impurities, wherein the weight ratio of Ni to Si is in the range of 3 to 7 and the content of carbon is 50 ppm by mass or less.

2. The copper alloy according to claim 1, further comprising one or more element selected from the group consisting of Mg, Mn, Sn and Ag in a total amount of up to 0.5% by mass.

3. The copper alloy according to claims 1, further comprising one or more element selected from the group consisting of Zn, P, As, Sb, Be, B, Ti, Zr, Al, Co and Fe in a total amount of up to 2.0% by mass.

4. A copper alloy product made of the copper alloy according to any one of claims 1 to 3.

5. An electronic component using the copper alloy according to any one of claims 1 to 3.

Description:

FIELD OF THE INVENTION

The present invention relates to age-hardening type copper alloys, and, in particular, to age-hardening copper alloys suitable for the use in various electronic components.

BACKGROUND OF THE INVENTION

Copper alloys for electronic materials used in electronic components such as a leadframe, a connector, a pin, a terminal, a relay and a switch are fundamentally required to have both high strength and high electric conductivity (or thermal conductivity). In accordance with the recent trend toward high integration, miniaturization and thinning of electronic components, copper alloys used in electronic components need to meet a higher standard.

In terms of high strength and high electric conductivity, in recent years, an increasing amount of age-hardening type copper alloys has been used instead of conventional solid-solution hardening type copper alloys such as phosphor bronze and brass. Age-hardening type copper alloys are produced through an aging treatment of supersaturated solid solution after a solution treatment. The aging treatment develops a uniform dispersion of fine precipitates in the alloys, resulting in improved strength. It also decreases the amount of dissolved elements in the alloys, resulting in improved electric conductivity. Accordingly, age-hardening type copper alloys can possess high mechanical properties such as strength and spring properties as well as high electric and thermal conductivity.

Among the age-hardening type copper alloys, Cu—Ni—Si system copper alloy, which is generally called Corson alloy, is a typical copper alloy that possesses high electric conductivity and strength as well as good stress-relaxation properties and bendability. Cu—Ni—Si system copper alloy is thus one of the alloys on which much research has been done in copper products industry. In this copper alloy, strength and electric conductivity can be improved by precipitating fine particles of Ni—Si intermetallic compound in the copper matrix.

The precipitated particles of the Ni—Si intermetallic compound generally have stoichiometric composition. Japanese Unexamined Patent Application Publication No. 2001-207229 teaches that electric conductivity can be improved by bringing the mass ratio of Ni to Si in the alloy close to that of Ni2Si (atomic weight of Ni×2: atomic weight of Si×1), that is, by adjusting the weight ratio Ni/Si to 3-7.

To Cu—Ni—Si system copper alloy, Cr is sometimes added. Japanese Patent No. 2862942 discloses a method for heat treatment of Corson alloy comprising 1.5-4.0% by mass of Ni; 0.35-1.0% by mass of Si; optionally 0.05-1.0% by mass of at least one metal selected from the group consisting of Zr, Cr and Sn; balance Cu and inevitable impurities, the method comprising heating or cooling the alloy such that tensile and thermal strain is 1×10−4 or less in a temperature range of 400-800 C degree. It is stated that the method can prevent ingot cracking during the heat treatment.

Japanese Patent No. 3049137 discloses a high-strength copper alloy having improved bendability comprising 2-5% by mass of Ni; 0.5-1.5% by mass of Si; 0.1-2% by mass of Zn; 0.01-0.1% by mass of Mn; 0.001-0.1% by mass of Cr; 0.001-0.15% by mass of Al; 0.05-2% by mass of Co; balance Cu and inevitable impurities, wherein the content of S, which is one of the inevitable impurities, is controlled to 15 ppm or less. It is stated that Cr is an element that reinforces the grain boundaries of ingot, improving hot workability. It is also stated that Cr over 0.1% by mass may cause oxidization of molten metal and worsen casting properties. According to JP'137, the alloy is melted and cast under a charcoal covering in a cryptol furnace in the atmosphere.

SUMMARY OF THE INVENTION

Though the properties of the alloy may be improved by controlling the weight ratio of Ni/Si within a range of 3-7 as disclosed in JP'229, it was difficult to achieve high strength while maintaining relatively high electric conductivity. An increase in Ni and Si concentrations by weight may lead to higher strength but also to lower electric conductivity. In addition, it may deteriorate hot workability, decreasing yield factor and therefore cost efficiency. It is has been found difficult to remarkably improve the properties of the alloy even if the amount of Ni and Si is adjusted to strictly control the composition.

JP'942 fails to disclose or suggest any effect of Cr added to the alloy. According to the method disclosed in JP'942, temperature control will become difficult if ingot is upsized. It is therefore more desirable to preventingot cracking by improving the properties of Cu—Ni—Si system alloy based on an inherent function that an alloying element renders rather than by temperature control.

JP'137 teaches that Cr can enhance hot workability but does not teach any other function of Cr. It also teaches the concentration conditions that can bring out the effect of added Cr on the properties of the alloy but does not teach any other condition.

Therefore, it is an object of the present invention to provide Corson alloy having remarkably improved properties, in particular, by bringing out the effect of added Cr more efficiently.

The present inventors have conducted extensive research in order to solve the above problems and determined that Cr has a noticeable influence on the improvement in strength and electric conductivity of Corson alloy under a certain condition. In particular, the present inventors have noticed the relationship between Cr and carbon and determined that controlling the content of carbon in Corson alloy can enhance the effect of Cr.

The present invention has been completed based on the above findings. An aspect in accordance with the present invention provides a copper alloy for electronic materials comprising 2.5-4.5% by mass of Ni; 0.50-1.2% by mass of Si; 0.0030-0.2% by mass of Cr; balance Cu and inevitable impurities, wherein the weight ratio of Ni to Si is in the range of 3 to 7 and the content of carbon is 50 ppm by mass or less.

In accordance with one embodiment of the present invention, the copper alloy may further comprise one or more element selected from the group consisting of Mg, Mn, Sn and Ag in a total amount of up to 0.5% by mass.

In accordance with another embodiment of the present invention, the copper alloy may further comprise one or more element selected from the group consisting of Zn, P, As, Sb, Be, B, Ti, Zr, Al, Co and Fe in a total amount of up to 2.0% by mass.

Another aspect in accordance with the present invention provides a copper alloy product made of the copper alloy of the present invention.

A further aspect in accordance with the present invention provides an electronic component using the copper alloy of the present invention.

Corson copper alloy for electronic materials according to the present invention may possess remarkably improved strength and electric conductivity because the effect of added Cr, one of the alloying elements, is brought out more efficiently.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the influence of the carbon content.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Added Amount of Ni and Si

By conducting a proper heat treatment, Ni and Si may form nickel silicide intermetallic compounds such as Ni2Si, contributing to an increase in strength without deteriorating electric conductivity. As mentioned above, the weight ratio of Si to Ni should be closer to the stoichiometric composition, and is preferably 3 to 7, more preferably 3.5 to 5.0.

However, when the added amount of Si is less than 0.5% by mass of the alloy, desired strength may not be obtained even if Ni/Si ratio is within the above range. On the contrary, when the added amount of Si is more than 1.2% by mass of the alloy, strength may be improved but electric conductivity is significantly decreased and hot workability is undesirably deteriorated due to the formation of liquid phase in segregation portions. Hence, the added amount of Si is 0.5 to 1.2% by mass, preferably 0.5 to 0.8% by mass of the alloy.

The added amount of Ni may be defined so as to satisfy the above preferable ratio of Ni to Si in accordance with the added amount of Si. For example, the added amount of Ni may be 2.5 to 4.5% by mass, preferably 3.2 to 4.2% by mass, more preferably 3.5 to 4.0% by mass of the alloy to keep the balance with Si.

Added Amount of Cr

In conventional Cu—Ni—Si system alloy, an increase in Ni and Si concentrations will increase the total number of precipitated particles that contribute to age hardening, resulting in an increase of strength. The increase in Ni and Si will also lead to an increase in the amount of solid solution that does not contribute to age hardening, resulting in a decrease of electric conductivity. Eventually, the peak strength will be improved by age hardening but electric conductivity at the peak strength will be decreased. However, if Cr is added in an amount of 0.003 to 0.2% by mass, preferably 0.01 to 0.1% by mass to the Cu—Ni—Si system alloy, the alloy may have higher electric conductivity without deteriorating the strength than before the addition of Cr. The addition of Cr will also increase hot workability, improving yield factor.

Cr precipitates preferentially at grain boundaries at the cooling stage of casting process, reinforcing the grain boundaries, preventing the generation of cracking during hot working and therefore suppressing the decrease of yield factor. In addition, Cr can readily precipitate as chromium silicides such as Cr3Si, which are compounds with Si, through suitable heat treatment. In particular, Cr can precipitate as Cr3Si, etc. with dissolved Si that has not precipitated during the fabrication process of the alloy in combination of solution treatment, cold rolling, and aging treatment. Cr can therefore suppress the decrease in electric conductivity caused by dissolved Si, resulting in improved electric conductivity without deteriorating strength.

Though Cr precipitated at grain boundaries during casting process is redissolved by solution treatment, etc., it precipitates again as silicides during the subsequent aging treatment. In conventional Cu—Ni—Si system alloy, among the added Si, Si that has not precipitated during the aging treatment remains dissolved in the matrix and suppresses an increase in electric conductivity. However, by adding Cr, which is a silicide-forming element, and further precipitating silicides, the amount of dissolved Si can be reduced compared with conventional Cu—Ni—Si alloy, resulting in an increase in electric conductivity without deteriorating strength.

It should be noted that when the content of Cr is less than 0.003% by mass, the effect of Cr may not be sufficiently exhibited. When the content of Cr is more than 0.2% by mass, Cr may be likely to form coarse inclusions that do not contribute to an increase in strength during hot rolling. In addition, as Cr—Si based precipitates have lower ability of work hardening, an excess amount of Cr may increase the number of Cr—Si compounds that do not contribute to an increase in strength, undesirably deteriorating workability and platability.

The Carbon Content

Ni—Si system alloy is usually melted and cast in the reducing atmosphere to prohibit oxidization of Si, which is an active metal. When the melting and casting process is conducted in the air, carbon-containing materials such as charcoal and carbon flux are often used to cover molten metal. As a result, the cast alloy may include a relatively high amount of C as an impurity.

Cr has high carbide-forming ability in the molten metal. The formation of carbide may decrease the amount of Cr precipitated at grain boundaries during solidification, reducing the effect of grain boundaries reinforcement and suppressing the improvement in yield factor. Once Cr-based carbides are formed, they are not likely to dissolve in solution treatment. Cr-based carbides not only decrease the amount of Cr that precipitates during aging treatment but also deteriorate bendability and platability, greatly worsening the final properties.

The present inventors have found that strict control of carbon content in the melting and casting process is necessary since a very small amount of C greatly affects the improvement in Cu—Ni—Si system alloy properties by the addition of Cr. It has been also determined that hot workability may not be virtually deteriorated and chromium silicides such as Cr3Si that contributes to an increase in electric conductivity may not be virtually suppressed, if the content of carbon is 50 ppm by mass or less.

The method for controlling the content of carbon may include, but not limited to, removing oil adherent to raw material, stirring after melting raw material, adjusting the amount of charcoal covering, covering the surface of molten metal with inert gases such as Ar not with charcoal for the prevention of oxidization of active metals, and vacuum melting. The content of carbon may be controlled to 50 ppm by mass or less, 40 ppm by mass or less, 30 ppm by mass or less, or 25 ppm by mass or less. Cu—Ni—Si system alloy according to the present invention may contain 10 to 30 ppm by mass of carbon, for example.

In this regard, JP patent No. 3049137 remains silent as to what will happen when the amount of Cr that precipitates at grain boundaries is drastically decreased due to the formation of Cr-carbides and -oxides.

Mg, Mn, Sn and Ag

The addition of one or more element selected from the group consisting of Mg, Mn, Sn and Ag in the total amount of up to 0.5% by mass to the Cu—Ni—Si system alloy according to the present invention may improve stress-relaxation properties, etc. without significantly deteriorating strength and electric conductivity. When the added amount is less than 0.01% by mass, a sufficient effect cannot be obtained. When the added amount is more than 0.5% by mass, manufacturability such as castability and hot workability, and electric conductivity of the product will be deteriorated. Therefore, they are preferably added in the total amount of 0.01 to 0.5% by mass.

Other Elements to be Added

The proper addition of Zn, P, As, Sb, Be, B, Ti, Zr, Al, Co and Fe may exhibit various effects. For example, they, complementary to one another, may improve strength and electric conductivity, as well as bendability, platability and manufacturability such as hot workability through refinement of cast structure. Therefore, one or more of these elements may be added up to 2.0% by mass in total to the Cu—Ni—Si system alloy of the present invention in accordance with the required properties. When the added amount is less than 0.001% by mass in total, a desired effect cannot be obtained. When the added amount is more than 2.0% by mass in total, electric conductivity and manufacturability will be deteriorated significantly. Therefore, they are preferably added in the total amount of 0.001 to 2.0% by mass, more preferably 0.01 to 1.0% by mass.

Incidentally, other elements not specifically mentioned herein may be added to the Cu—Ni—Si system alloy of the present invention in an amount that does not have adverse effect on the alloy properties.

The method of manufacturing the alloy according to the present invention will be explained below. The Cu—Ni—Si system alloy according to the present invention may be manufactured by any conventional manufacturing methods for Cu—Ni—Si system alloy except that the carbon content is controlled. So, those skilled in the art could choose the optimal manufacturing method in accordance with the composition and required properties. Though it may not be necessary to provide detailed explanation, a general procedure for manufacturing the alloy will be provided for the illustrative purpose only.

Raw materials such as electrolytic copper, Ni, Si and Cr are introduced into an atmospheric melting furnace to obtain a molten metal having a desired composition. The molten metal is cast into an ingot. The carbon content is controlled by adjusting oil content of the introduced raw materials, adjusting the amount of charcoal coating, introducing inert gases, and stirring the molten metal, etc. Hot rolling is then conducted. Cold rolling and heat treatment are repeated to produce into a strip or foil having desired thickness and properties. Heat treatment includes solution treatment and aging treatment. The solution treatment comprises heating the alloy at 700-1000 degrees C. to dissolve Ni—Si based compounds and Cr—Si based compounds into Cu matrix and also to recrystallize Cu matix. The hot rolling sometimes serves as solution treatment. The aging treatment comprises heating the alloy at 350-550 degrees C. for one hour or more to precipitate fine particles of the Ni—Si based compounds and Cr—Si based compounds having been dissolved through the solution treatment. The aging treatment increases strength and electric conductivity of the alloy. Cold rolling may be conducted before and/or after the aging treatment in order to obtain higher strength. In case where the cold rolling is conducted after the aging treatment, stress relief annealing (lower temperature annealing) may follow the cold rolling.

The Cu—Ni—Si alloy in accordance with an embodiment of the present invention may have a 0.2% yield strength of 780 MPa or greater and an electric conductivity of 45% IACS or greater, a 0.2% yield strength of 860 MPa or greater and an electric conductivity of 43% IACS or greater, or a 0.2% yield strength of 890 MPa or greater and an electric conductivity of 40% IACS or greater.

The Cu—Ni—Si alloy of the present invention may be processed into various copper alloy products such as sheet, strip, pipe, rod and wire. The Cu—Ni—Si alloy of the present invention may also be used in electronic components such as a leadframe, a connector, a pin, a terminal, a relay and a switch, as well as foil used in secondary batteries, for which high strength and high electric conductivity (or thermal conductivity) are required.

EXAMPLES

The invention and its advantages will be understood more readily with reference to the following examples; however these examples are intended to illustrate the invention and are not to be construed to limit the scope of the invention.

The copper alloys used in Examples, as listed in Table 1, have compositions of various contents of Ni, Si and Cr, and some of them also contain elements selected from Mg, Mn, Sn, Ag, Ti, Fe, B and Co. The copper alloys used in Comparative Examples are Cu—Ni—Si system alloys having parameters outside the range of the present invention.

After melting at 1,300 degrees C. in a high-frequency vacuum melting furnace, casting was carried out to form copper alloy ingots having a thickness of 30 mm of various compositions as listed in table 1. During this process, the carbon content was controlled by adjusting oil content of the introduced raw materials, adjusting the amount of charcoal coating, introducing inert gases, and stirring the molten metal, etc. Each ingot was then heated at 1,000 degrees C. and was hot rolled into a sheet of 10 mm thickness, followed by rapid cooling. The sheet was then scalped into 8 mm thickness for the removal of scale from the surface before it was cold rolled into 0.2 mm thickness. Subsequently, the obtained sheet was subjected to solution treatment for 120 seconds at 850-1,000 degrees C. depending on the added amounts of Ni and Cr, immediately followed by water-cooling. After that, the sheet was cold rolled into 0.1 mm thickness and finally subjected to aging treatment in the inert atmosphere at 400-550 degrees C. for 1-12 hours depending on the added amounts to obtain a specimen.

Strength and Electric conductivity were evaluated on each specimen obtained through the above-mentioned method. Strength was evaluated with 0.2% yield strength (YS; MPa) measured by a tensile test performed in a direction parallel to the rolling direction. Electric conductivity (EC; % IACS) was evaluated with volume resistivity using a double bridge. Bendability was evaluated as follows. Each specimen was bent 90 degrees using a W-shaped die under the condition such that the ratio of bend radius to specimen thickness is equal to 1. The bent surface was observed using an optical microscope. When the cracking was not observed, the specimen was regarded as having practical use and evaluated as good. When the cracking was observed, the specimen was evaluated as bad.

The content of carbon was quantitatively analyzed by high frequency melting-infrared absorption method after the specimen was subjected to high frequency combustion using a LECO CS-400 instrument.

TABLE 1
composition (mass %)cracking duringYSECC
NiSiCrMg, Mn, Sn, Agothershot rolling(MPa)(% IACS)bendability(ppm)
claimedmin2.50.50.003
rangemax4.51.20.20.52.050
Example12.70.60.005ND75048good16
22.70.60.05ND75547good23
32.70.60.1ND77046good38
42.70.60.050.2 MgND78046good23
52.70.60.050.1 MnND78045good23
62.70.60.050.3 SnND78045good23
72.70.60.050.3 Sn, 0.1 AgND78045good23
84.00.90.005ND86043good18
94.00.90.05ND87042good25
104.00.90.1ND87042good40
114.00.90.17ND87542good44
124.00.90.050.2 MgND89041good25
134.00.90.050.1 MnND89041good25
144.00.90.050.3 SnND89041good25
154.00.90.050.3 Sn, 0.1AgND89041good25
164.00.90.10.03 Ti,ND89040good40
0.03 Fe
174.51.000.005ND93038good18
184.51.000.17ND94042good43
194.51.000.050.005 BND94037good28
204.51.000.10.1 CoND96037good41
Compara-12.70.6slight cracking75040good2
tive24.00.9slight cracking86037good3
Example32.70.60.0005slight cracking75041good16
44.00.90.0005slight cracking86038good19
52.70.60.0005unable to evaluate due to cracking53
64.00.90.0005unable to evaluate due to cracking56
74.00.90.05slight cracking86036bad65
84.00.90.1slight cracking86036bad78
92.70.60.3slight cracking76046bad40
104.00.90.3slight cracking86042bad43
114.00.90.3slight cracking86035bad85
124.00.90.050.6 Mgcracking due to casting surface degradation15
134.00.90.050.8 Mnslight cracking90021bad15

Cu—Ni—Si System Alloy Containing 2.7% by Mass of Ni and 0.6% by Mass of Si

Examples 1 to 7, and Comparative Examples 1, 3, 5 and 9 have commonalities in that they contain 2.7% by mass of Ni and 0.6% by mass of Si. As can be seen from Examples 1 to 3, an increase in the Cr content has enhanced YS while suppressing the decrease in EC. As can be seen from Examples 4 to 7, YS has been further improved by addition of Mg, Mn, Sn and Ag.

On the contrary, in Comparative Example 1, the amount of dissolved Si increased as it did not contain Cr, resulting in lower EC. Slight cracking was occurred during the hot rolling.

Though Comparative Example 3 contained Cr, the content was not enough and the desired effect was not obtained. As a result, the amount of dissolved Si was still too much and EC was decreased. Slight cracking was occurred during the hot rolling.

In Comparative Example 5, the content of Cr was insufficient as in Comparative Example 3. Moreover, the content of C was excessive. As a result, large cracking was occurred during the hot rolling, making the subsequent evaluation impossible.

In Comparative Example 9, coarse Cr particles were generated due to the excess content of Cr. As a result, slight cracking was occurred during the hot rolling and bendability was also bad.

Cu—Ni—Si System Alloy Containing 4.0% by Mass of Ni and 0.9% by Mass of Si

Examples 8 to 16, and Comparative Examples 2, 4, 6 to 8 and 10 to 13 have commonalities in that they contain 4.0% by mass of Ni and 0.9% by mass of Si. As can be seen from Examples 8 to 11, an increase in the Cr content has enhanced YS while suppressing the decrease in EC. They contained more Ni and Si than Examples 1 to 7, resulting in higher YS and lower EC.

As can be seen from Examples 12 to 15, YS has been further improved by addition of Mg, Mn, Sn and Ag. In Example 16, Ti and Fe were added as other additives. Again, It can be seen YS was improved.

On the contrary, in Comparative Example 2, the amount of dissolved Si increased as it did not contain Cr, resulting in lower EC. Slight cracking was occurred during the hot rolling.

Though Comparative Example 4 contained Cr, the content was not enough and the desired effect was not obtained. As a result, the amount of dissolved Si was still too much and EC was decreased. Slight cracking was occurred during the hot rolling.

In Comparative Example 6, the content of Cr was insufficient as in Comparative Example 4. Moreover, the content of C was excessive. As a result, large cracking was occurred during the hot rolling, making the subsequent evaluation impossible.

In Comparative Examples 7 and 8, chromium carbides were generated due to the excess content of C while the generation of chromium silicides was suppressed. As a result, the amount of dissolved Si was increased and EC was decreased. Slight cracking was occurred during the hot rolling. Bendability was also bad.

In Comparative Example 10, coarse Cr particles were generated due to the excess content of Cr. As a result, slight cracking was occurred during the hot rolling. Bendability was also bad.

In Comparative Example 11, the Cr content was excessive as in Comparative Example 10. The C content was also excessive. The excess content of C caused chromium carbides to form while suppressing the generation of chromium silicides. As a result, the amount of dissolved Si was increased and EC was decreased. Bendability was also bad.

In Comparative Examples 12 and 13, Mg and Mn were added excessively. In Comparative Example 12, where Mg was added excessively, cracking caused by fault of casting surface made the subsequent evaluations impossible. In Comparative Example 13, where Mn was added excessively, slight cracking was occurred during the hot rolling. EC and Bendability were also bad Cu—Ni—Si system alloy containing 4.5% by mass of Ni and 1.0% by mass of Si

Examples 17 to 20 contained 4.5% by mass of Ni and 1.0% by mass of Si. They contained more Ni and Si than Examples 1 to 16, resulting in higher YS and lower EC.

The Influence of the Carbon Content

FIG. 1 is a plot of YS (the vertical axis) versus EC (horizontal axis) for Examples 9 and 10 in which the carbon content was within the claimed range, and Comparative Examples 7 and 8 in which the carbon content was outside the claimed range. The contents of Ni, Si and Cr were within the claimed range for all these Examples and Comparative Examples. Examples 9 contained the same amount of Ni, Si and Cr, with Comparative Examples 7. Examples 10 contained the same amount of Ni, Si and Cr with Comparative Examples 8. It can be seen that the difference of the carbon content, which was only 40 ppm by mass, generated significant differences in YS and EC.