CN113862566A

CN113862566A - Flywheel rotor and preparation method thereof

Info

Publication number: CN113862566A
Application number: CN202111098163.4A
Authority: CN
Inventors: 顾金才; 董瀚; 徐文斌; 胡春东; 于广文; 郭军霞; 满廷慧
Original assignee: Zhangjiagang Guangda Special Material Co ltd
Current assignee: Zhangjiagang Guangda Special Material Co ltd
Priority date: 2021-09-18
Filing date: 2021-09-18
Publication date: 2021-12-31

Abstract

The application discloses a flywheel rotor and a manufacturing method thereof, and relates to the technical field of rotor energy storage. The flywheel rotor is made of high-strength steel, the high-strength steel is subjected to electric arc furnace smelting, external refining, electroslag remelting, forging, normalizing, quenching and tempering control, the yield strength of 1150MPa-1700MPa and the room-temperature V-shaped notch impact power of more than 27J are achieved, and the method is suitable for manufacturing an energy storage disc type flywheel rotor with the outer diameter size of 600-2000mm and the maximum height of less than or equal to 600mm or a cylindrical flywheel rotor with the diameter of less than or equal to 1000 mm.

Description

Flywheel rotor and preparation method thereof

Technical Field

The application relates to the technical field of energy storage systems, in particular to a flywheel rotor and a preparation method thereof.

Background

The flywheel energy storage technology is an advanced physical energy storage technology, and energy is stored by a flywheel rotor which rotates at a high speed under the vacuum magnetic suspension condition. The flywheel body is composed of a high-speed rotating flywheel and a coaxial motor/generator, and charging (energy storage process, from electric energy to kinetic energy) and discharging (energy release process, from kinetic energy to electric energy) are realized through the conversion of the rotation of the flywheel and the action of the motor/generator.

Compared with the problems of limited charging and discharging times, serious environmental pollution, high working temperature requirement and the like in the chemical battery energy storage technology, the flywheel energy storage technology has the unique advantages of rapid charging and discharging, suitability for high-power frequent charging and discharging, high power, high efficiency, long service life (20 years), wide working temperature range, small occupied area, greenness, no pollution, high reliability and the like, solves the problem which cannot be solved by other energy storage technologies, and is very suitable for energy storage of the wind power photovoltaic industry.

The flywheel stores energy during high speed rotation and is also subjected to centrifugal forces. The basic formula of the stored energy and the centrifugal force of the flywheel is shown in the formulas (1) and (2):

F＝m·r·w² (2)

in the formula: m-rotor mass, J-rotor moment of inertia, r-rotor radius of gyration, ω -angular velocity, E-kinetic energy of the flywheel, F-centrifugal force.

Therefore, the stored energy of the flywheel can be improved by two modes of increasing the mass of the flywheel so as to increase the rotational inertia and increasing the rotation speed of the rotor. However, the higher the rotational speed, the greater the corresponding centrifugal force. If the internal stress caused by centrifugal force during operation exceeds the tensile strength (limit allowable stress) of the material, the flywheel rotor will be damaged, so that the rotation speed of the flywheel cannot be infinitely increased. Therefore, the use of materials with higher strength, especially higher specific strength (Rm/rho), is significant for improving the energy storage capacity and safety of the flywheel rotor. Various rotor materials such as high-strength steel, titanium alloy, aluminum alloy, carbon fiber composite material, glass fiber, kevlar fiber, and the like have been tried. Cost analysis shows that carbon fiber composite materials, glass fibers, Kevlar fibers and other materials have higher specific strength than steel, but the manufacturing cost is high. Although the specific strength of the steel is not high, the strength which can be obtained by unit cost is the highest, and the cost advantage is remarkable.

Research and simulation calculation find that the internal stress borne by the center of the flywheel rotating at high speed is the largest (mainly tensile stress), and the internal stress gradually decreases with the increase of the radius. For a solid cylindrical flywheel made of an isotropic material, simulation calculation shows that the internal stress at the center of the rotor is 1.5-3 times of the internal stress at the edge part when the rotor rotates at a high speed. The strength, toughness and structure of the central part of the flywheel rotor are seen to be the key factors influencing the fatigue life and safety of the rotor.

The important factor restricting the popularization and development of the energy storage flywheel technology at present is the cost, especially the large-size flywheel rotor made of isotropic materials. The steel rotor has certain practical application and popularization due to the advantages of cost and high reliability. However, in practice, it is found that due to the problem of hardenability, the core of the large-size steel rotor cannot obtain the same structure as that of the edge part, the core strength is obviously lower than that of the edge part, and the fatigue life of the flywheel rotor is seriously influenced. The steel rotor is made of low-alloy high-strength steel (such as AISI4140 and AISI4340), which has insufficient hardenability, cannot obtain a microstructure satisfying strength and toughness requirements in the core of a large-size rotor, and is not suitable for manufacturing the large-size rotor.

In contrast, the chinese patent publication CN 1247139a provides a flywheel rotor made of maraging steel with maximum allowable stress of 2400MPa, and the maraging steel is alloyed by precious alloy elements of Ni, Co, Mo, and Ti, has very good hardenability and high strength, and can meet the high stress characteristics of the central part of the rotor, but the total content of the precious elements reaches more than 30%, the cost is very high, and the large-scale popularization cannot be achieved.

The Chinese patent with the publication number of CN 106715935B provides a large-size solid steel rotor which is made of 300M steel and has the yield strength of more than 900MPa, the outer diameter of the rotor is within the range of 36 inches to 72 inches (914 mm to 1828mm), and the maximum thickness of 14 inches (356 mm). The carbon content of the 300M steel is as high as 0.40-0.44%, and the steel is alloyed by noble elements such as Cr, Ni, Mo, V and the like, although the composition design can improve the strength and the hardenability, the toughness is inevitably reduced, and the cost is increased, therefore, the patent requires that the vacuum arc remelting is carried out to ensure the toughness, but the manufacturing cost of the flywheel rotor is further increased.

Disclosure of Invention

The application aims to provide a flywheel rotor and a preparation method thereof, and the problem that cost and hardenability cannot be considered when the flywheel rotor is made of steel materials in the prior art is solved.

In order to achieve the above purpose, the embodiments of the present application adopt the following technical solutions: a flywheel rotor is made of high-strength steel; the yield strength of the high-strength steel is 1150MPa-1700MPa, and the impact energy of the V-shaped notch at room temperature is more than 27J.

In the technical scheme, the flywheel rotor with the surface hardness equivalent to the core hardness can be prepared by adopting the high-strength steel with the yield strength of 1150-1700 MPa and the room-temperature V-shaped notch impact energy of more than 27J, the low cost and the high hardenability are both considered, the manufacturing requirement of the large-size flywheel rotor can be met, and the mass production is facilitated.

Further, according to the embodiment of the application, the flywheel rotor is a disc type flywheel rotor with the outer diameter ranging from 600mm to 2000mm and the maximum height less than or equal to 600 mm.

Further, according to the embodiment of the application, the flywheel rotor is a cylindrical flywheel rotor with the diameter being less than or equal to 1000 mm.

Further, according to the embodiment of the application, the chemical composition of the high-strength steel comprises the following components in percentage by mass:

c: 0.10-0.40%, Si: 1.20-2.80%, Mn: 2.00-6.00%, Cr: 0.50-1.50%, Mo: 0.15-0.50%, less than 0.002%, less than 0.005%, less than 0.50% Ni, less than or equal to 0.10% V, less than or equal to 0.005% B, and the balance Fe and inevitable impurity elements.

In order to achieve the purpose, the embodiment of the application also discloses a flywheel rotor which is made of high-strength steel; the high-strength steel is smelted in an electric arc furnace, refined outside the furnace, remelted by electroslag, and subjected to controlled forging, normalizing, quenching and tempering.

In the technical scheme, the flywheel rotor is made of high-strength steel through electric arc furnace smelting, external refining, electroslag remelting and forging, normalizing, quenching and tempering controlling, so that stable structures of lath martensite, a small amount of bainite and residual austenite can be obtained, the edge of the flywheel rotor is consistent with the core structure, the strength is consistent, the hardness is equivalent, the process flow is simple and clear, and the flywheel rotor is suitable for industrial mass production.

Further, according to the embodiment of the application, the high-strength steel has yield strength of 1150MPa-1700 MPa.

Further, according to the embodiment of the application, the high-strength steel has a room-temperature V-notch impact energy of more than 27J.

In order to achieve the above object, an embodiment of the present application further discloses a method for manufacturing a flywheel rotor, including the following steps:

smelting, namely smelting the steel materials subjected to batching by adopting an electric arc furnace, refining outside the furnace and electroslag remelting to form steel ingots;

forging, wherein the steel ingot is subjected to forging hot working to obtain a steel billet;

performing heat treatment, namely normalizing, quenching and tempering the steel billet to obtain a rotor blank;

and machining, namely machining the rotor blank to obtain the flywheel rotor.

In the technical scheme, the flywheel rotor is made of high-strength steel through electric arc furnace smelting, external refining, electroslag remelting and forging, normalizing, quenching and tempering controlling, stable structures of lath martensite, a small amount of bainite and residual austenite can be obtained, the edge of the flywheel rotor is consistent with the core structure, the strength is consistent, the hardness is equivalent, the process flow is simple and clear, and the flywheel rotor is suitable for industrial mass production.

Further, according to the embodiment of the application, after the smelting step, the steel ingot is subjected to homogenization treatment.

Further, according to the embodiment of the application, the chemical components of the steel material comprise the following components in percentage by mass:

Compared with the prior art, the method has the following beneficial effects:

(1) by adopting the high-strength steel with the yield strength of 1150-1700 MPa and the room-temperature V-shaped notch impact energy of more than 27J, the flywheel rotor with the surface hardness equivalent to the core hardness can be prepared, the cost is low, the hardenability is high, the manufacturing requirement of a large-size flywheel rotor can be met, and the mass production is facilitated;

(2) the flywheel rotor is made of high-strength steel through electric arc furnace smelting, external refining, electroslag remelting and forging, normalizing, quenching and tempering controlling, stable structures of lath martensite, a small amount of bainite and residual austenite can be obtained, the edge of the flywheel rotor is consistent with the core structure, the strength is consistent, the hardness is equivalent, the process flow is simple and clear, and the flywheel rotor is suitable for industrial batch production.

Drawings

The present application is further described below with reference to the drawings and examples.

FIG. 1 is a flow chart of a method of manufacturing a flywheel rotor of the present application.

FIG. 2 is a full section stiffness profile of the flywheel rotor after dissection in example 1.

FIG. 3 shows the microstructure of the core of the flywheel rotor after heat treatment in example 1.

FIG. 4 is a surface microstructure of a flywheel rotor after heat treatment in example 1

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention clear and fully described, embodiments of the present invention are further described in detail below with reference to the accompanying drawings. It is to be understood that the specific embodiments described herein are merely illustrative of some embodiments of the invention and are not limiting of the invention, and that all other embodiments obtained by those of ordinary skill in the art without the exercise of inventive faculty are within the scope of the invention.

In the description of the present invention, it should be noted that the terms "center", "middle", "upper", "lower", "left", "right", "inner", "outer", "top", "bottom", "side", "vertical", "horizontal", and the like indicate orientations or positional relationships based on those shown in the drawings, and are only for convenience of description and simplicity of description, but do not indicate or imply that the referred device or element must have a specific orientation, be constructed in a specific orientation, and be operated, and thus, should not be construed as limiting the present invention. Furthermore, the terms "a," "an," "first," "second," "third," "fourth," "fifth," and "sixth" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance.

In the description of the present invention, it should be noted that, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, e.g., as meaning either a fixed connection, a removable connection, or an integral connection; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art.

For the purposes of simplicity and explanation, the principles of the embodiments are described by referring mainly to examples. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the embodiments. It will be apparent, however, to one skilled in the art that the embodiments may be practiced without these specific details. In some instances, well-known methods and structures have not been described in detail so as not to unnecessarily obscure the embodiments. In addition, all embodiments may be used in combination with each other.

Manufacture of flywheel rotor

The application discloses a flywheel rotor, and a manufacturing process of the flywheel rotor is shown in figure 1. As shown in fig. 1, the flywheel rotor described in the present application is manufactured by the following steps:

smelting: smelting the steel material subjected to batching in an electric arc furnace, refining outside the furnace and electroslag remelting to form a steel ingot;

forging: forging and hot-working the steel ingot to obtain a steel billet;

and (3) heat treatment: normalizing, quenching and tempering the steel billet to obtain a rotor blank;

machining: and machining the rotor blank to obtain the flywheel rotor.

In the technical scheme, the rotor blank processed by heat treatment is high-strength steel, has the yield strength of 1150-1700 MPa and the room-temperature V-shaped notch impact energy of more than 27J, and can be used for manufacturing a disc flywheel rotor with the outer diameter range of 600-2000mm and the maximum height of less than or equal to 600mm or a cylindrical flywheel rotor with the diameter of less than or equal to 1000 mm.

Further, the chemical components of the high-strength steel comprise the following components in percentage by mass: c: 0.10-0.40%, Si: 1.20-2.80%, Mn: 2.00-6.00%, Cr: 0.50-1.50%, Mo: 0.15-0.50%, less than 0.002%, less than 0.005%, less than 0.50% Ni, less than or equal to 0.10% V, less than or equal to 0.005% B, and the balance Fe and inevitable impurity elements.

Further, homogenization treatment can be performed on the steel ingot obtained by smelting. The homogenization treatment refers to the heat preservation of the steel ingot after heating, wherein the heat preservation temperature is 1170-1210 ℃, preferably 1180-1200 ℃, and the heat preservation time is 5-12 hours, preferably 8-10 hours. This application alleviates dendrite segregation, the banded segregation that produces among the solidification process through homogenization treatment.

Furthermore, in the forging step, a multi-fire hot forging process is preferred, the open forging temperature is controlled to be 1100-1150 ℃, and the finish forging temperature is controlled to be more than 850 ℃. The total forging deformation ratio is more than 6. The forging process can improve the internal stress and strain distribution of the blank, refine crystal grains and improve strength and toughness.

Further, in the heat treatment step, the forged and annealed blank is subjected to normalizing treatment to achieve uniform structure, refined crystal grains and stress relief. And then quenching and tempering the blank to obtain the energy storage flywheel rotor blank with high strength and high toughness. Specifically, the temperature of the normalizing treatment is preferably 880-920 ℃, the heat preservation time is 2-5 hours, and the air cooling is carried out. The quenching temperature is preferably 850-900 ℃, the heat preservation time is 2-6 hours, and the air cooling is carried out. The tempering temperature is preferably 250-300 ℃, the temperature is kept for 5-8 hours after heat penetration, and air cooling is carried out.

According to the technical scheme, the flywheel rotor is made of high-strength steel through electric arc furnace smelting, external refining, electroslag remelting and forging, normalizing, quenching and tempering controlling, stable structures of lath martensite, a small amount of bainite and residual austenite can be obtained, the edge of the flywheel rotor is consistent with the core structure, the strength is consistent, the hardness is equivalent, the process flow is simple and clear, and the flywheel rotor is suitable for industrial batch production.

The technical effects described above will be described below with reference to example 1 and comparative example 1.

Example 1

The steel for the energy storage flywheel rotor is prepared by adopting electric arc furnace smelting, external refining and electroslag remelting, and comprises the following chemical components in percentage by mass: 0.32% of C, 1.55% of Si, 3.10% of Mn, 1.0% of Cr, 0.25% of Mo, 0.25% of Ni, 0.08% of V, 0.0005% of B, and the balance of Fe and unavoidable impurities. The homogenization temperature of the steel ingot is 1200 ℃, and the heat preservation time is 8 hours. The steel ingot heating temperature is 1200 ℃, the initial forging temperature is 1150 ℃, the final forging temperature is 880 ℃, and the forging ratio is 8. Normalizing at 900 deg.C, holding for 5 hr, and air cooling. Quenching temperature is 850 ℃, heat preservation time is 5 hours, and air cooling is carried out. Tempering at 250 deg.c for 8 hr, and air cooling.

The outer diameter of the flywheel rotor is 940mm, the height of the flywheel rotor is 420mm, and the flywheel rotor is sampled at the edge and the center of the rotor respectively.

The actual rotor of example 1 was dissected, and the hardness was measured at different positions in the diameter direction and the height direction, and the actual hardness distribution was as shown in fig. 2. As can be seen from FIG. 2, the hardness value of the whole section of the rotor is distributed between HBW456-471, the surface hardness is equivalent to the core hardness, and the rotor made of the steel of the invention has good hardenability.

The structure of the rotor of example 1 after heat treatment of the core and the rim is shown in fig. 3 and 4. As can be seen from fig. 3 and 4, the core (fig. 3) and the surface (fig. 4) both have a structure of lath martensite, a small amount of bainite, and retained austenite after heat treatment, and the rotor surface is identical to the core structure.

Comparative example 1

300M steel prepared by electric arc furnace smelting, external refining and electroslag remelting processes is used as rotor steel, and comprises the following chemical components in percentage by mass: 0.40% of C, 1.70% of Si, 0.75% of Mn, 0.85% of Cr, 1.85% of Ni, 0.45% of Mo, 0.07% of V, 0.005% of P, 0.002% of S, and the balance of Fe and unavoidable impurities. The homogenization temperature of the steel ingot is 1200 ℃, and the heat preservation time is 8 hours. The steel ingot heating temperature is 1200 ℃, the initial forging temperature is 1150 ℃, the final forging temperature is 880 ℃, and the forging ratio is 8. Normalizing at 925 deg.C, holding for 5 hr, and air cooling. Quenching temperature is 870 ℃, heat preservation time is 5 hours, and air cooling is carried out. Tempering at 300 deg.c for 6 hr, and air cooling. The external diameter of the flywheel rotor is 1100mm, the height is 300mm, and the sampling is respectively carried out at the center and the edge of the rotor.

The results of the performance test of the above examples and comparative examples without passing the site sampling are shown in Table 1.

TABLE 1

In Table 1, the tensile test standard is GB/T228, and the impact test standard is GB/T229, K_ICThe test standard adopted GB/T4161.

As can be seen from Table 1, the large-sized rotor made of the high-strength steel of the present invention has good hardenability, the tensile strength and yield strength of the core are equivalent to those of the edge, and the impact toughness and fracture toughness of the core and the edge are also equivalent. Compared with the 300M steel rotor in the comparative example, the tensile strength and the yield strength of the core of the rotor are far higher than those of the core of the comparative example, and the strength of the edge of the comparative example is achieved.

The implementation results show that the steel realizes the strength equivalent to that of 300M steel on the premise of lower carbon content and no increase of cost, has good plasticity and toughness, can meet the manufacturing requirements of large-size flywheel rotors, and is beneficial to batch production.

Chemical composition of high-strength steel

The present application uses high strength steel for the manufacture of a flywheel rotor, which, as can be seen from the above, has chemical compositions by mass, comprising the following: c: 0.10-0.40%, Si: 1.20-2.80%, Mn: 2.00-6.00%, Cr: 0.50-1.50%, Mo: 0.15-0.50%, less than 0.002%, less than 0.005%, less than 0.50% Ni, less than or equal to 0.10% V, less than or equal to 0.005% B, and the balance Fe and inevitable impurity elements.

In contrast, the high strength is obtained by using Fe-C alloy with medium carbon content as a matrix, the tempering resistance and the strength are improved by Si and a small amount of Cr, Ni, Mo and V, and the toughness is improved. Specifically, 1.20-2.80% of Si is added into a medium-carbon Fe-C martensite matrix to inhibit cementite precipitation so as to improve tempering resistance, 2.00-6.00% of Mn improves hardenability without increasing cost, and a small amount of Cr, Ni, Mo and V are used for coaction to further improve hardenability and tempering resistance, delay tempering softening, stabilize carbide and refine grains. The high-strength steel disclosed by the application is suitable for obtaining stable structures of lath martensite, a small amount of bainite and residual austenite by adopting a heat treatment process of quenching and low-temperature tempering, and has good comprehensive mechanical properties.

Specifically, the ratio of the elements is as follows:

c, carbon C: c is the most important element for improving strength and hardenability, but too high C content impairs toughness and aggravates the formation of C-Mn segregation band, which is detrimental to toughness. The C content of the invention is 0.10-0.40%, preferably 0.28-0.35%.

Manganese Mn: mn obviously improves hardenability without obviously increasing cost. Mn reduces Ms points, refines the microstructure substructure of the matrix and improves the toughness of the matrix, and the Mn also improves the strength through solid solution strengthening without reducing the toughness. Too high Mn content will cause severe C-Mn segregation, and the Mn content of the present invention is 2.00 to 6.00%, preferably 2.50 to 3.80%.

Silicon Si: si inhibits cementite Fe3C from being separated out, improves the stability of super-cooled austenite and avoids the decomposition of austenite, thereby improving the tempering resistance of steel and achieving ultrahigh strength and high plasticity. There is a suitable range of Si content, and Si is too high and strength is rather decreased. The Si content of the present invention ranges from 1.20 to 2.80%, preferably from 1.40 to 2.00%.

Chromium Cr: cr strongly improves hardenability, Cr and Mo act together to improve strength and ensure high toughness, and the Cr content of the invention ranges from 0.50 to 1.50%, preferably from 0.60 to 1.20%, in view of cost.

Molybdenum Mo: mo obviously improves hardenability, and the capability of improving hardenability is better than that of Mn. Mo can delay pearlite transformation and ensure that all parts of the whole section of the large-size rotor obtain uniform structures. Mo and Cr can stabilize carbide, delay tempering softening and refine crystal grains. Mo is expensive, and the Mo content of the invention is 0.15-0.50%, preferably 0.20-0.28%.

Nickel Ni and vanadium V: ni improves hardenability, lowers Ms point, increases the forming tendency of retained austenite, lowers ductile-brittle transition temperature, and improves low-temperature toughness. V also improves hardenability and forms carbide refined grains. In the present invention, only a small amount of Ni and V is added in consideration of cost.

Other elements in the alloy such as boron, calcium and the like are common deoxidizers in the smelting process of the alloy.

The other elements in the rotor steel are iron, and impurity elements in the alloy must be controlled to ensure good toughness, such as [ O ] < 0.002%, [ N ] < 0.005%, P < 0.010%, and S < 0.002%.

Properties of high strength steel

The high-strength steel adopted by the application has yield strength of 1150-1700 MPa and room-temperature V-shaped notch impact energy of more than 27J. The surface hardness of the flywheel rotor prepared by adopting the high strength is equivalent to the core hardness, and the low cost and the high hardenability are both considered, so that the manufacturing requirement of the large-size flywheel rotor can be met, and the mass production is facilitated. The yield strength of the core of the flywheel rotor prepared by the method can reach 1150MPa or above, the flywheel rotor has room-temperature V-shaped notch impact energy of more than 27J, and the hardness is far higher than that of the flywheel rotor prepared by the prior art.

Although the illustrative embodiments of the present application have been described above to enable those skilled in the art to understand the present application, the present application is not limited to the scope of the embodiments, and it is to be understood that all modifications that come within the spirit and scope of the application, as defined by the appended claims, are desired to be protected by the present application.

Claims

1. A flywheel rotor characterized by:

is made of high-strength steel;

the yield strength of the high-strength steel is 1150MPa-1700MPa, and the impact energy of the V-shaped notch at room temperature is more than 27J.

2. A flywheel rotor as claimed in claim 1, wherein the flywheel rotor is a disc flywheel rotor having an outer diameter in the range 600mm to 2000mm and a maximum height of 600mm or less.

3. The flywheel rotor as claimed in claim 1, wherein the flywheel rotor is a cylindrical flywheel rotor having a diameter of 1000mm or less.

4. A flywheel rotor as claimed in claim 1, wherein the chemical composition of the high strength steel comprises, by mass:

5. A flywheel rotor characterized by:

is made of high-strength steel;

the high-strength steel is smelted in an electric arc furnace, refined outside the furnace, remelted by electroslag, and subjected to controlled forging, normalizing, quenching and tempering.

6. A flywheel rotor according to claim 5 wherein the high strength steel has a yield strength of 1150-1700 MPa.

7. A flywheel rotor according to claim 5 wherein the high strength steel has a room temperature V-notch work of impact of greater than 27J.

8. A method of manufacturing a flywheel rotor, comprising the steps of:

and machining, namely machining the rotor blank to obtain the flywheel rotor.

9. A flywheel rotor according to claim 8, wherein the ingot is homogenised after the smelting step.

10. A flywheel rotor according to claim 8, wherein the chemical composition of the steel material comprises, by mass: