JP4556770B2 - Carburizing steel and method for producing the same - Google Patents

Description

  The present invention relates to carburizing steel and a method for producing the same, and is particularly suitable for application to steel for automobiles and steel for construction machinery.

  Crankshafts and gears of automobiles are required to have excellent fatigue characteristics, wear resistance, and pitting resistance. Chromium steel, chrome molybdenum steel, nickel chrome molybdenum steel, etc. with a C content of around 0.2 mass% are in the desired shape. It is manufactured by carburizing or carbonitriding after molding.

In the above carburizing and carbonitriding processes, if heat treatment distortion occurs during quenching, the tooth shape will be incorrect in the case of gears, and finishing and polishing processes will be required, and in the case of shaft parts, bending will occur. It is necessary to correct this, and in either case, productivity is hindered and costs increase.
It is said that such heat treatment strain is caused by coarsening of austenite grains during carburizing treatment, resulting in unstable hardenability, and non-uniform stress due to expansion during martensitic transformation. Yes.

Patent Document 1 and Patent Document 2 propose that the thermal history of steel and the amounts of Al, Nb, and N are adjusted to suppress the generation of coarse grains by the pinning effect of Al and Nb nitride. This technique has a problem in terms of stability of the effect of suppressing the generation of coarse particles.
Patent Document 3 and Patent Document 4 include the contents of nitrides, carbides, carbonitride-forming elements such as Al, Nb, Ti, etc., the size, distribution density, bainite structure fraction, ferrite of each precipitate. Although the above problems are solved by controlling the band score and rolling conditions, it is practically difficult to control these many parameters in the actual operation of manufacturing various dimensions and shapes by rolling.

As a solution to the above problem, Patent Document 5 discloses that the content of C, Ti, and Mo in steel is controlled within the range of the following formula (I) to make the carburized component a ferrite single phase structure, and ferrite It has been proposed to prevent the generation of coarse particles during carburizing treatment and reduce heat treatment strain by dispersing fine precipitates with a particle size of less than 10 nm in the phase.
0.5 ≤ (C / 12) / {(Ti / 48) + (Mo / 96)} ≤ 1.5 --- (I)
However, the technique described in Patent Document 5 cannot sufficiently secure the strength and toughness of the interior other than the carburized layer, and leaves room for improvement to be applied to the crankshaft and gears described above. It was.

JP 58-45354 A JP 61-261427 A Japanese Patent Laid-Open No. 11-50191 JP 11-335777 A Japanese Patent Laid-Open No. 2OO3-321731

  The present invention was developed in view of the above-mentioned actual situation, while ensuring the internal strength and toughness other than the carburized layer, carburizing steel that can effectively achieve grain refinement during carburizing, The object is to propose together with its advantageous manufacturing method.

As a result of intensive studies on the internal structure other than the carburized layer in order to solve the above problems, the inventors have obtained the following knowledge.
a) In order to improve the strength and strength of the internal structure other than the carburized layer after the carburizing treatment, it is advantageous that the steel structure is a structure in which ferrite and pearlite and / or bainite are mixed.
b) In order to obtain the above internal structure, it is necessary to increase the C content to about 0.1 to 0.4 mass%.
c) When the C content is high, in order to precipitate fine carbides, it is necessary to increase the amount of Ti and Mo that form carbides and form fine precipitates, as is clear from the above formula (I). However, when the C content is increased to make a structure in which ferrite and pearlite and / or bainite are mixed, sufficient fine precipitates can be obtained even if the components are adjusted to satisfy the above formula (I). Cannot be obtained.

  Therefore, as a result of intensive studies to solve the above problems, the inventors have made it difficult to effectively produce fine precipitates useful for preventing the generation of coarse grains during carburizing treatment. In particular, the inventors have newly found out that it is important to strictly control the amounts of S and N mixed as impurities, and have completed the present invention.

That is, the gist configuration of the present invention is as follows.
1. % By mass
C: 0.1 to 0.4%
Si: 0.5% or less,
Mn: 2.0% or less,
Al: 0.1% or less,
Ti: 0.1-0.5%
Mo: 0.05-1.0%
S: 0.005% or less and N: 0.004% or less are contained within the range satisfying the following formula (1), the balance is composed of Fe and inevitable impurities, and the steel structure is ferrite, pearlite and / or bainite. For carburizing, characterized in that the ferrite has a structure fraction of 50% or more and fine precipitates having a particle size of less than 20 nm are dispersed in the ferrite phase at 1 × 10 ³ particles / μm ³ or more. steel.
Record
0.3 ≤ (C / 12) / [{Ti- (48S / 32)-(48N / 14)} / 48+ (Mo / 96)] ≤ 2.0 --- (1)

2. The composition of the steel is further mass%,
Cr: 2.0% or less,
Nb: 0.1% or less,
It contains one or more selected from V: 0.15% or less and W: 1.5% or less, and satisfies the following formula (1) ′ instead of the above formula (1) The carburizing steel according to 1.
Record
0.3 ≤ (C / 12) / [{Ti- (48S / 32)-(48N / 14)} / 48+ (Mo / 96) + (Cr / 52) + (Nb / 93)
+ (V / 51) + (W / 184)] ≦ 2.0 --- (1) '

3. % By mass
C: 0.1 to 0.4%
Si: 0.5% or less,
Mn: 2.0% or less,
Al: 0.1% or less,
Ti: 0.1-0.5%
Mo: 0.05-1.0%
S: 0.005% or less and N: 0.004% or less in a range satisfying the following formula (1), with the balance being hot after heating a steel material having a composition of Fe and inevitable impurities to 1100 ° C or higher A method for producing carburized steel, characterized in that, when cooled after processing, the cooling rate in the temperature range of 800 to 500 ° C during hot processing and in the cooling process is 1.0 ° C / s or less.
Record
0.3 ≤ (C / 12) / [{Ti- (48S / 32)-(48N / 14)} / 48+ (Mo / 96)] ≤ 2.0 --- (1)

4). The composition of the steel material is further mass%,
Cr: 2.0% or less,
Nb: 0.1% or less,
It contains one or more selected from V: 0.15% or less and W: 1.5% or less, and satisfies the following formula (1) ′ instead of the above formula (1) 3. A method for producing carburizing steel according to 3.
Record
0.3 ≤ (C / 12) / [{Ti- (48S / 32)-(48N / 14)} / 48+ (Mo / 96) + (Cr / 52) + (Nb / 93)
+ (V / 51) + (W / 184)] ≦ 2.0 --- (1) '

  ADVANTAGE OF THE INVENTION According to this invention, the steel for carburization which can achieve refinement | miniaturization of a crystal grain at the time of a carburizing process can be obtained, after ensuring intensity | strength and toughness about the inside of steel materials other than a carburized layer.

Hereinafter, the steel structure, component composition, and production conditions of the carburizing steel of the present invention will be specifically described.
Steel structure The carburizing steel according to the present invention has a steel structure in which ferrite and pearlite and / or bainite are mixed, and the ferrite structure fraction is 50% or more of the entire structure.
The fact that the steel structure before carburizing treatment is a structure in which pearlite and / or bainite are not mixed means that the same structure is obtained after carburizing. Strength and toughness cannot be ensured, and it is difficult to reduce the weight of components such as gears and crankshafts.
In addition, when the ferrite structure fraction is less than 50%, as will be described later, desired fine precipitates cannot be precipitated, and coarsening of crystal grains during carburization cannot be prevented.
From the above, the steel structure was ferrite, pearlite, and / or bainite, and the structure fraction of the ferrite was limited to 50% or more.

Here, the structure fraction of ferrite is preferably 50 to 95%, while the structure fraction of pearlite and / or bainite is preferably 5 to 50%. Further, from the viewpoint of ensuring cold workability, the ferrite structure fraction is preferably 70 to 95%, and the pearlite and / or bainite structure fraction is preferably 5 to 30%. In addition, as the steel structure, martensite and retained austenite may be mixed, but there is no problem if the mixed amount is 10% or less.
In addition, each structure fraction can be calculated | required by an area ratio by cross-sectional structure | tissue observation (200 times optical microscope structure | tissue observation).

Further, in the present invention, it is necessary that fine precipitates having a particle size of less than 20 nm are dispersed in the ferrite structure at 1 × 10 ³ particles / μm ³ or more. Due to the presence of precipitates having a particle size of less than 20 nm, grain growth is effectively prevented by the pinning effect of the crystal grain boundaries during the carburizing process. When the number of precipitates having a particle size of less than 20 nm is less than 1 × 10 ³ / μm ³ , the pinning effect described above is insufficient and the effect of suppressing grain growth cannot be obtained.
The number of fine precipitates is determined by the following method. An electron microscope sample is prepared by an electropolishing method using a twin jet method and observed at an acceleration voltage of 200 kV. At that time, the crystal orientation of the parent phase is controlled so that the fine precipitates have a measurable contrast with respect to the parent phase, and the focus is shifted from the normal focus so that the precipitates are not counted and observed by the defocus method. did.
Further, the thickness of the sample in the region where the precipitate particles are measured is calculated by measuring the elastic scattering peak intensity and the inelastic scattering peak intensity using electron energy loss spectroscopy.
By this method, the number of particles and the sample thickness can be measured in the same region. The number of particles and the particle diameter are measured at four locations of a 0.5 × 0.5 μm region of the sample, and the precipitates distributed per 1 μm ² are calculated as the number for each particle diameter.
In the present invention, the number of precipitates having a particle size of less than 2 Onm determined in this manner is defined in a range of 1 × 10 ³ / μm ³ or more. The number of precipitates less than 20 nm is more preferably 1 × 10 ⁴ / μm ³ or more.

Ingredient composition In order to secure the characteristics required for mechanical structural parts such as gears and crankshafts that are used for the carburizing steel of the present invention, the composition of the present invention is limited as follows. . Hereinafter, the content (%) of each component means mass%.

C: 0.1-0.4%
C is added to obtain the required strength. However, if the C content is less than 0.1%, the required strength cannot be ensured. On the other hand, if it exceeds 0.4%, it becomes hard and it becomes difficult to ensure cold workability, but also the core toughness after carburizing. Therefore, the C content is limited to a range of 0.1 to 0.4%. More preferably, it is 0.2 to 0.4% of range.

Si: 0.5% or less
Si is added to improve the strength and ductility, but when the content exceeds 0.5%, not only the effect is saturated, but also the deformation resistance during cold working increases and the workability decreases. Si amount is limited to 0.5% or less.

Mn: 2.0% or less
Mn is added to improve strength and hardenability. However, if the content exceeds 2.0%, the effect is not only saturated, but also deformation resistance during cold working increases and workability decreases. , Mn is limited to 2.0% or less.

Al: 0.1% or less
Al is useful as a deoxidizer and also has the effect of improving strength and ductility, so 0.1% is added as the upper limit.

Ti: 0.1-0.5%
Ti is an element useful for improving the pinning effect by finely depositing Ti-based carbides and precipitates containing Ti-Mo-based carbides together with Mo. However, if the content is less than 0.1%, the amount of precipitates is too small to obtain the pinning effect necessary for suppressing coarse grains, while if it exceeds 0.5%, the precipitates become coarse and the above-mentioned pinning effect is reduced. Ti is limited to a range of 0.1 to 0.5%.

Mo: 0.05-1.0%
Mo is an element useful for improving the pinning effect by finely depositing Mo-based carbides and precipitates containing Ti-Mo-based carbides together with Ti. However, if the content is less than 0.05%, the amount of precipitates is too small to obtain the pinning effect necessary for suppressing coarse grains. On the other hand, if the content exceeds 1.0%, not only the bainite phase but also the martensite phase is formed. Therefore, Mo is limited to a range of 0.05 to 1.0%.

S: 0.005% or less S does not contribute to the pinning effect in the high Ti steel as in the present invention and does not contribute to the pinning effect, but rather in the presence of coarse TiS, precipitation sites of Ti and Mo-based carbides. Thus, the amount of fine precipitates effective for the pinning effect is reduced. Here, when the S content exceeds 0.005%, the above-described adverse effects are remarkable, so it is important to limit S to 0.005% or less.

N: 0.004% or less N also forms coarse TiN in the high Ti steel as in the present invention, which not only contributes to the pinning effect but rather precipitates sites of Ti and Mo-based carbides when coarse TiN is present. Thus, the amount of fine precipitates effective for the pinning effect is reduced. Here, when N is contained in excess of 0.004%, the above-described adverse effects are remarkable. Therefore, it is important to limit N to 0.004% or less.

The preferred composition range of the basic component of the present invention has been described above. However, in the present invention, it is not sufficient that each element simply satisfies the above range. For C, Ti, Mo, S and N, It is important to satisfy the requirement of formula (1).
0.3 ≤ (C / 12) / [{Ti- (48S / 32)-(48N / 14)} / 48+ (Mo / 96)] ≤ 2.0 --- (1)
The above formula affects the size and number of precipitates. When each element is contained within the range satisfying this formula, if it is manufactured according to the manufacturing method described later, the particle size is less than 20 nm. A desired structure in which precipitates are dispersed at 1 × 10 ³ particles / μm ³ or more can be obtained.

Furthermore, in the present invention, the following elements can be appropriately contained.
One or more selected from Cr ≤ 2.0%, Nb ≤ 0.1%, V ≤ 0.15%, W ≤ 1.5%
Cr is not only effective for improving strength and toughness, but is also a useful element that forms fine carbides with Ti. However, if its content exceeds 2.0%, it causes an increase in hardness and cold forgeability. Since it deteriorates, it should be 2.0% or less.
Nb contributes to the strength increase by forming fine precipitates together with Ti. In addition, there is an effect of improving the ductility by refining the structure and adjusting the crystal grains, but if it exceeds 0.1%, it becomes excessively fine and the ductility is lowered, so the content is made 0.1% or less.
V contributes to an increase in strength by forming fine precipitates together with Ti. Moreover, although there exists an effect which improves a ductility by refine | miniaturizing a structure | tissue and adjusting the grain size of a crystal grain, when it contains exceeding 0.15%, since it refines | miniaturizes excessively and a ductility falls, it is 0.15% or less.
W forms a fine precipitate together with Ti and contributes to an increase in strength. In addition, there is an effect of improving the ductility by refining the structure and adjusting the grain size, but if it exceeds 1.5%, it becomes excessively fine and the ductility is lowered, so the content is made 1.5% or less.

Further, when adding one or more selected from Cr, Nb, V, and W, it is necessary to satisfy the requirement of the following formula (1) ′ for the same reason as the above formula (1). There is.
0.3 ≤ (C / 12) / [{Ti- (48S / 32)-(48N / 14)} / 48+ (Mo / 96) + (Cr / 52) + (Nb / 93)
+ (V / 51) + (W / 184)] ≦ 2.0 --- (1) '

  The balance other than the above elements is Fe and inevitable impurities.

Next, the manufacturing conditions of the present invention will be described.
After heating the steel material with the preferred composition described above to 1100 ° C or higher, and then hot working at a finishing temperature of 800 ° C or higher and 1000 ° C or lower, the temperature range from 800 to 500 ° C is 1.0 ° C / s or lower. Need to cool at speed. Hereinafter, the reason why each processing condition is limited as described above will be described.

Heating temperature: 1100 ° C. or higher In the present invention, carbides are sufficiently dissolved during heating before hot working, and fine precipitates are deposited during hot working and after cooling. At this time, if the heating temperature is less than 1100 ° C., carbides cannot be sufficiently dissolved in the matrix, so that coarse carbides are likely to be generated after hot working, and generation of coarse particles during carburization cannot be suppressed. . Therefore, it is necessary to set the heating temperature to 1100 ° C. or higher during hot working. Preferably it is 1150 degreeC or more, More preferably, it is 1200 degreeC or more.

Cooling rate in the temperature range of 800 to 500 ° C: 1.0 ° C / s or less During the hot working and the subsequent cooling process, if the cooling rate in the temperature range of 800 to 500 ° C exceeds 1.0 ° C / s, fine precipitates are sufficient Does not precipitate. Furthermore, the ferrite structure fraction is reduced, and not only is coarsening easily generated during carburizing, but the hardness of the rolled material is increased and cold forgeability is deteriorated. Therefore, the cooling rate is limited to 1.0 ° C./s or less. Preferably it is 0.5 ° C./s or less, more preferably 0.3 ° C./s or less.
In addition, as a method for reducing the cooling rate, there is a method in which a heat insulation cover or a heat insulation cover with a heat source is installed behind the rolling or forging line, thereby gradually cooling.

If the finishing temperature during hot working is less than 800 ° C., a band structure is likely to be formed, and mixed grains are formed during reverse transformation of carburizing heating, causing coarse grains. On the other hand, when the finishing temperature exceeds 1000 ° C., the work material becomes hard and cold forgeability deteriorates. For the above reasons, the finishing temperature during hot working is preferably 800 to 1000 ° C.
Here, hot working refers to hot working immediately before carburizing treatment. When hot working is not performed on steel bars, carburizing treatment is performed after forming into a part shape by cold working or cutting. The above heating temperature, finishing process conditions, and subsequent cooling conditions may be applied during hot rolling at the time of manufacturing the steel bar. In addition, when a hot forging process is performed in forming a part shape, the above heating temperature, finishing process conditions, and subsequent cooling conditions may be applied during hot forging.

Steels with various compositions shown in Table 1 are melted in a 100 kg vacuum melting furnace, forged to 150 mm square, welded to a dummy billet, and hot-rolled with various heating temperatures and finishes shown in Table 2. Steel bars having a diameter of 20 to 35 mm were manufactured under conditions of temperature and cooling rate.
A sample for observing fine precipitates was collected from the steel bar thus obtained, and the size and amount of the precipitates were determined. Similarly, a sample for microstructural observation was collected and subjected to structural observation to determine the ferrite area ratio.

The number of fine precipitates and the ratio of the total precipitates were obtained by the following method.
An electron microscope sample is prepared by an electropolishing method using a twin jet method and observed at an acceleration voltage of 200 kV. At that time, the crystal orientation of the parent phase is controlled so that the fine precipitates have a measurable contrast with respect to the parent phase, and the focus is shifted from the normal focus so that the precipitates are not counted and observed by the defocus method. Went. Moreover, the thickness of the sample in the region where the precipitate particles were measured was calculated by measuring the elastic scattering peak intensity and the inelastic scattering peak intensity using electron energy loss spectroscopy. By this method, the number of particles and the sample thickness can be measured in the same region. The number of particles and the particle size were measured at four locations of a 0.5 × 0.5 μm region of the sample, and the precipitates distributed per 1 μm ³ were calculated for each particle size, and the amount of precipitates having a particle size of less than 20 nm was determined.

  Moreover, the ferrite area ratio was calculated | required by observing the cross-sectional structure | tissue of a sample using a 200 times optical microscope.

  Furthermore, an upsetting test piece of 8 mmφ × 12 mm was made from the rolled steel bar, and after upsetting at a rolling reduction of 70%, carburization simulation was performed. The conditions for carburizing simulation are heating to 950-125 ° C. for 3 hours and water cooling. Thereafter, the cut surface was polished and etched, and the prior austenite grain size was observed to determine the coarse grain generation temperature (crystal grain coarsening temperature). Since the carburizing process is normally performed in the temperature range of 900 to 950 ° C., it was determined that those having a coarsening temperature of 950 ° C. or lower are inferior in crystal grain coarsening characteristics. The prior austenite grain size was measured according to JIS G O551, measured 10 times at 400 times, and if any coarse grain having a grain size number of 5 or less was present, it was determined that coarse grains were generated.

  Furthermore, a 2 mm U-notch No. 3 test piece for Charpy impact test specified in JIS Z 2202 was created from the obtained steel bar, and Charpy impact test was conducted at a test temperature of 20 ° C in accordance with JIS Z 2242. Was measured.

Nos. 1, 2, 4, 5, 6, 8, 9, 11, 12, 13, 14, 15, 24, 25 and 28 are invention examples, all of which have a ferrite structure fraction of 50% after hot rolling. As described above, precipitates of less than 20 nm were contained at 1 × 10 ³ pieces / μm ³ or more. As a result, in each case, an excellent characteristic that the coarsening temperature during the carburizing simulation experiment was 1025 ° C or higher was obtained. Also, an excellent value of absorbed energy of 70 J / cm ² or more was obtained.
In steel a (No. 1) and steel c (No. 5), the steel bar after rolling was subjected to spheroidizing annealing at 740 ° C. for 5 hours, as in the above, the upsetting ratio: 70 % Carburizing simulation was performed after cold forging, and no coarse grains were observed even at 1025 ° C.
Thus, the steel of the present invention had excellent coarse grain prevention characteristics even when passing through the annealing process before cold forging.

On the other hand, in each of Nos. 3, 7, and 10, the hot rolling conditions were outside the appropriate range of the present invention, so either or both of the ferrite structure fraction and the number of precipitates satisfied the requirements of the present invention. As a result, the coarse grain generation temperature is 950 ° C. or less, which is inferior in the coarsening prevention characteristics.
Further, No. 16 contains a large amount of S exceeding the upper limit of the present invention, and coarse precipitates are formed, so that fine precipitates are reduced and the coarsening prevention property is poor.
No. 17 contains a large amount of N in excess of the upper limit of the present invention, and coarse precipitates are formed, so that fine precipitates are reduced and the coarsening prevention properties are poor.
In No. 18, since the Ti amount is less than the appropriate range of the present invention, fine precipitates are reduced and the coarsening prevention property is inferior.
In No. 19, since the amount of Ti is too much larger than the proper range of the present invention, coarse precipitates are formed. As a result, fine precipitates are reduced and the coarsening prevention property is poor.
Since No. 20 does not contain Mo, fine precipitates are reduced and the coarsening prevention property is inferior.
In No. 21, since the amount of Mo is larger than the appropriate range of the present invention, the ferrite structure ratio is less than 50%, and the coarsening prevention property is inferior.
In No. 22, since the A value is smaller than the appropriate range specified in the present invention, coarse precipitates are formed, fine precipitates are reduced, and the coarsening prevention characteristics are deteriorated.
In No. 23, since the A value is larger than the appropriate range specified in the present invention, coarse precipitates are also formed, the fine precipitates are reduced, and the coarsening prevention characteristics are deteriorated.
In No. 26, since the heating temperature was much lower than the lower limit of the present invention, fine precipitates were reduced and the coarsening prevention property was inferior. Further, the absorbed energy was 10 J / cm ² and the toughness was inferior.
In No. 27, since the heating temperature was lower than the lower limit of the present invention, the amount of fine precipitates and the coarsening prevention characteristic were satisfied, but the absorbed energy was inferior to 30 J / cm ² and toughness.

Claims (4)

% By mass
C: 0.1 to 0.4%
Si: 0.5% or less,
Mn: 2.0% or less,
Al: 0.1% or less,
Ti: 0.1-0.5%
Mo: 0.05-1.0%
S: 0.005% or less and N: 0.004% or less are contained within the range satisfying the following formula (1), the balance is composed of Fe and inevitable impurities, and the steel structure is ferrite, pearlite and / or bainite. For carburizing, characterized in that the ferrite has a structure fraction of 50% or more and fine precipitates having a particle size of less than 20 nm are dispersed in the ferrite phase at 1 × 10 ³ particles / μm ³ or more. steel.
Record
0.3 ≤ (C / 12) / [{Ti- (48S / 32)-(48N / 14)} / 48+ (Mo / 96)] ≤ 2.0 --- (1) The composition of the steel is further mass%,
Cr: 2.0% or less,
Nb: 0.1% or less,
It contains one or more selected from V: 0.15% or less and W: 1.5% or less, and satisfies the following formula (1) ′ in place of the formula (1) Item 2. The carburizing steel according to Item 1.
Record
0.3 ≤ (C / 12) / [{Ti- (48S / 32)-(48N / 14)} / 48+ (Mo / 96) + (Cr / 52) + (Nb / 93)
+ (V / 51) + (W / 184)] ≦ 2.0 --- (1) ' % By mass
C: 0.1 to 0.4%
Si: 0.5% or less,
Mn: 2.0% or less,
Al: 0.1% or less,
Ti: 0.1-0.5%
Mo: 0.05-1.0%
S: 0.005% or less and N: 0.004% or less in a range satisfying the following formula (1), with the balance being hot after heating a steel material having a composition of Fe and inevitable impurities to 1100 ° C or higher A method for producing carburized steel, characterized in that, when cooled after processing, the cooling rate in the temperature range of 800 to 500 ° C during hot processing and in the cooling process is 1.0 ° C / s or less.
Record
0.3 ≤ (C / 12) / [{Ti- (48S / 32)-(48N / 14)} / 48+ (Mo / 96)] ≤ 2.0 --- (1) The composition of the steel material is further mass%,
Cr: 2.0% or less,
Nb: 0.1% or less,
It contains one or more selected from V: 0.15% or less and W: 1.5% or less, and satisfies the following formula (1) ′ in place of the formula (1) Item 4. A method for producing carburizing steel according to Item 3.
Record
0.3 ≤ (C / 12) / [{Ti- (48S / 32)-(48N / 14)} / 48+ (Mo / 96) + (Cr / 52) + (Nb / 93)
+ (V / 51) + (W / 184)] ≦ 2.0 --- (1) '