CN114086054A

CN114086054A - High-hardenability quenched and tempered steel, round steel and manufacturing method thereof

Info

Publication number: CN114086054A
Application number: CN202010857901.8A
Authority: CN
Inventors: 黄瑞; 黄宗泽; 赵四新
Original assignee: Baoshan Iron and Steel Co Ltd
Current assignee: Baoshan Iron and Steel Co Ltd
Priority date: 2020-08-24
Filing date: 2020-08-24
Publication date: 2022-02-25

Abstract

The invention discloses high-hardenability quenched and tempered steel which contains the following chemical elements in percentage by mass besides Fe: c: 0.35-0.40 wt.%, Si: 0.15-0.35 wt.%, Mn: 0.90-1.05 wt.%, Cr: 0.90-1.00 wt.%, Cu: 0.10-0.20 wt.%, Ni: 0.10-0.20 wt.%, Al: 0.020-0.045 wt.%, Ti: 0.03-0.05 wt.%, B: 0.0010-0.0035 wt.% and N is less than or equal to 0.0080 wt.%. In addition, the invention also discloses round steel made of the high-hardenability quenched and tempered steel. In addition, the invention also discloses a manufacturing method of the round steel, which comprises the following steps: (1) smelting; (2) casting; (3) rolling: the initial rolling temperature is controlled to 1050-. The high-hardenability quenched and tempered steel has high hardenability, keeps good comprehensive mechanical properties, has wide applicability and good use prospect and value.

Description

High-hardenability quenched and tempered steel, round steel and manufacturing method thereof

Technical Field

The invention relates to a steel product, a round steel and a manufacturing method thereof, in particular to a quenched and tempered steel, a round steel and a manufacturing method thereof.

Background

The working conditions of large engineering machinery are quite complex, and the large engineering machinery often needs to work in quite harsh environment. Therefore, the market places quite high demands on the comprehensive mechanical properties of the materials of the parts of the crawler traveling system of the engineering machinery. Among them, boron-containing steels are widely used for such parts with their overall excellent combination of properties.

At present, the boron-containing steel which is most commercially used is manganese boron steel, the steel has low alloy content, high strength and good toughness, can be suitable for quenched and tempered parts with medium and small sections, and can be used as structural steel for bearing stretching, twisting and shearing actions in the aspects of engineering machinery, automobiles, tractors, machine tools and the like.

However, in recent years, with the continuous development of large-scale and heavy-load development of the mechanical industry in China, the design of engineering mechanical parts is more and more complex, and the cross sections of all parts of the parts are different in size, so that higher requirements on the performances of hardenability, strength, wear resistance and the like of steel are also provided. The traditional manganese boron steel is not enough to meet the requirement, and due to the problem of insufficient hardenability, the traditional manganese boron steel is easy to generate part core hardness deviation in the production process, so that soft spots are locally generated, and defects such as quenching cracking and the like are caused.

For example: chinese patent document with publication number CN108998725A, publication date of 2018, 12 months and 14 days, and entitled 35MnBM steel for track chain track joints and preparation method thereof, discloses a high-hardenability quenched and tempered steel. In the technical solution disclosed in this patent document, the hardenability of the steel material of the example of the invention can only reach J1.5: 53-56HRC, J19: 35-37 HRC.

As can be seen, the conventional high hardenability quenched and tempered steel of the prior art described above has limited hardenability. The complex engineering machinery parts or the oversized parts have high requirement on the hardenability of the part materials, the hardenability of the high-hardenability quenched and tempered steel in the prior art cannot meet the requirement, and in the process of processing and manufacturing the parts, soft spots or cracks are easily generated due to insufficient hardenability.

In order to solve the above problems, it is desired to obtain a high hardenability quenched and tempered steel having higher hardenability and better properties, which can effectively meet the hardenability requirements of medium and large-sized parts.

Disclosure of Invention

One of the objects of the present invention is to provide a high hardenability quenched and tempered steel which is designed by reasonable chemical composition to sufficiently exert the function of alloy elements under the condition of reducing the cost as much as possible, and which is added with elements such as Cr and Ni in a reasonable manner on the basis of manganese boron steel to assist the B element in further improving the hardenability of the steel. The high-hardenability quenched and tempered steel has high hardenability, keeps good comprehensive mechanical properties, has wide applicability and good use prospect and value.

In order to achieve the above object, the present invention provides a high hardenability quenched and tempered steel containing, in addition to Fe, the following chemical elements in mass percent:

C：0.35～0.40wt.％、Si：0.15～0.35wt.％、Mn：0.90～1.05wt.％、Cr：0.90～1.00wt.％、Cu：0.10～0.20wt.％、Ni：0.10～0.20wt.％、Al：0.020～0.045wt.％、Ti：0.03～0.05wt.％、B：0.0010～0.0035wt.％、N≤0.0080wt.％。

further, in the high hardenability quenched and tempered steel of the present invention, the mass percentages of the chemical elements are as follows:

c: 0.35-0.40 wt.%, Si: 0.15-0.35 wt.%, Mn: 0.90-1.05 wt.%, Cr: 0.90-1.00 wt.%, Cu: 0.10-0.20 wt.%, Ni: 0.10-0.20 wt.%, Al: 0.020-0.045 wt.%, Ti: 0.03-0.05 wt.%, B: 0.0010-0.0035 wt.% and N is less than or equal to 0.0080 wt.%; the balance being Fe and other unavoidable impurities.

In the technical scheme of the invention, on the basis of manganese boron steel, the high-hardenability quenched and tempered steel simultaneously improves the content of Mn and Cr elements, adopts the scheme of high manganese and high chromium, further improves the hardenability by adding Mn, Cr, Ni and other elements to assist B, and generates solid solution strengthening, precipitation strengthening and the like so as to improve the strength and the wear resistance of the steel, and finally realizes the ultrahigh hardenability of the steel so as to meet the requirements of the continuously developed engineering machinery industry. The design principle of each chemical element in the invention is specifically as follows:

c: in the high hardenability quenched and tempered steel according to the present invention, the C element may play a role of solid solution strengthening, which may determine the highest quenching hardness (martensitic hardness) of the steel. Meanwhile, the content of C in the steel can also adjust the ductility and the hardenability of the steel. It should be noted that, if the content of C element in the steel is too low, the steel is not easy to harden, and the strength after tempering is insufficient; on the other hand, if the content of C element in the steel is too high, the toughness of the steel becomes insufficient. Based on the above, in order to ensure the ductility and hardenability of the steel, in the high hardenability quenched and tempered steel of the present invention, the mass percentage of the element C is controlled to be 0.35 to 0.40 wt.%.

Si: in the high-hardenability quenched and tempered steel, the Si element in the steel can increase the activity of C in austenite, so that the diffusion speed of C in austenite is increased, ferrite is easy to form, a CCT curve high-temperature transformation area is shifted to the left, and the hardenability and tempering resistance of the steel can be improved. Meanwhile, Si element can be dissolved in alpha phase, so that the solid solution strengthening effect is achieved, the strength of the steel is improved, and the ductility and toughness of the steel are reduced. Based on this, the beneficial effect and adverse effect of Si are comprehensively considered, and the mass percent of Si in the high-hardenability quenched and tempered steel is controlled to be 0.15-0.35 wt.%.

Mn: in the high-hardenability quenched and tempered steel, Mn element can delay the transformation of pearlite and bainite, increase the stability of super-cooled austenite and improve the hardenability of the steel. Mn element can also form a substitutional solid solution in steel, so that a strong solid solution strengthening effect is achieved, the yield strength and the tensile strength of the steel are increased, and the wear resistance is improved. Meanwhile, Mn element also facilitates the formation of MnS inclusions by S element in steel, and MnS may be used to improve machinability. However, when the content of the element S in the steel is high, an excessive amount of the element Mn is combined with the element S to form MnS, so that the content of the element Mn in other forms is reduced, and the other two effects are weakened. Based on this, in order to ensure the hardenability of the steel, in the high hardenability quenched and tempered steel of the present invention, the mass percentage of the Mn element is controlled to be 0.90 to 1.05 wt.%.

Cr: in the high-hardenability quenched and tempered steel, Cr is an important alloy element, and can strongly delay the transformation of pearlite and bainite, increase the stability of super-cooled austenite and improve the hardenability of the steel. In addition, it should be noted that the Cr element is also effective in improving the oxidation resistance and corrosion resistance of the steel. Based on this, in the high hardenability quenched and tempered steel of the present invention, the mass percentage of the Cr element is controlled to be 0.90 to 1.00 wt.%.

Ni: in the high-hardenability quenched and tempered steel, the Ni element can effectively improve the hardenability of the steel, and the Ni element can improve the transformation activation energy of an alpha phase and increase the stability of the undercooled austenite, so that the hardenability of the steel is improved. However, it should be noted that the Ni element is also an expensive element, and it is not desirable to add it in excess, so that the present invention mainly adds Mn and Cr. Based on this, in order to reduce the cost, in the high hardenability quenched and tempered steel of the present invention, the mass percentage of the Ni element is controlled to be 0.10 to 0.20 wt.%.

Cu: in the high hardenability quenched and tempered steel of the present invention, the influence of the Cu element on the hardenability of the steel and the solid solution strengthening effect thereof are similar to those of Ni, and can be used to replace a part of Ni to improve the hardenability of the steel. However, it should be noted that the Cu content in the steel is too high, which causes the steel to be easily cracked during hot working. Based on this, in the high hardenability quenched and tempered steel of the present invention, the mass percentage of the Cu element is controlled to be 0.10 to 0.20 wt.%.

Al: in the high hardenability quenched and tempered steel of the present invention, Al is mainly used as a deoxidizer which can remove O from the steel to form Al₂O₃The non-metallic inclusion is similar, so that the problem that the hardenability of solid solution boron is reduced due to the formation of boron oxide by the combination of an O element and a B element can be effectively avoided, and the hardenability of the steel is ensured. Based on this, in the high hardenability quenched and tempered steel of the present invention, the mass percentage of the Al element is controlled to be 0.020 to 0.045 wt.%.

Ti: in the high-hardenability quenched and tempered steel, Ti element is easy to combine with N element in the steel at a high temperature line to form stable TiN, which can effectively prevent austenite grains from growing in the heating and rolling processes and inhibit the recovery and recrystallization of deformed austenite grains, and meanwhile, Ti element can consume N element in the steel to avoid the combination of B element and N element to form BN, so that the hardenability of solid solution boron is reduced. Based on this, in the high hardenability quenched and tempered steel of the present invention, the mass percentage of Ti element is controlled to be 0.03 to 0.05 wt.%.

B: in the high-hardenability quenched and tempered steel, the element B is a key element for improving the hardenability of the steel, the price of the element B is low, and a trace amount of the element B can generate a good hardenability improvement effect, so that the element B is very favorable compared with scarce and expensive alloy elements such as Ni and Mo. Meanwhile, the addition of B can enable the steel to easily obtain a bainite structure with high strength under a certain cooling condition. Based on this, in the high-hardenability quenched and tempered steel of the present invention, the mass percentage of the element B is controlled to be 0.0010 to 0.0035 wt.%.

In addition, B acts on the hardenability of steel only when it exists as solid-solution boron. Therefore, in the present invention, the content ratio of Al, Ti, and N elements in the steel needs to be strictly controlled to prevent the formation of boron oxide and boron nitride and help solid solution boron to exert the hardenability. It should be noted that in the quenched and tempered steel with high hardenability, the content of the N element in the steel is not too high, the mass percent of the N element can be controlled to be less than or equal to 0.0080 wt.%, and meanwhile, in order to ensure the hardenability of the steel, in the invention, the mass percent of a single chemical element is controlled, and simultaneously, the mass percent of Ti/N can be controlled to be more than or equal to 4, wherein in the above formula, Ti and N respectively represent the mass percent content of the corresponding chemical elements.

Further, in the high hardenability quenched and tempered steel of the present invention, the mass percentage content of each chemical element satisfies: the Ti/N is more than or equal to 4.

Further, in the high hardenability quenched and tempered steel according to the present invention, the content of each impurity element satisfies at least one of the following, among other inevitable impurities: p is less than or equal to 0.025 wt.%, S is less than or equal to 0.025 wt.%, Pb is less than or equal to 0.002 wt.%, As is less than or equal to 0.04 wt.%, Sn is less than or equal to 0.005 wt.%, Sb is less than or equal to 0.008 wt.%, and Ca is less than or equal to 0.0010 wt.%.

In the above technical solutions, P, S, Pb, As, Sn, Sb, and Ca are impurity elements in steel, and the content of the impurity elements in the steel should be reduced As much As possible in order to obtain a quenched and tempered steel with better performance and better quality, As the technical conditions allow.

Further, in the high hardenability quenched and tempered steel according to the present invention, the microstructure thereof is a uniform pearlite + ferrite structure.

Correspondingly, the invention also aims to provide the round steel, which not only has good comprehensive mechanical properties, but also has ultrahigh hardenability, the yield strength is more than or equal to 460MPa, the tensile strength is more than or equal to 790MPa, the elongation is more than or equal to 19%, the reduction of area is more than or equal to 60%, the impact energy Aku at room temperature is more than or equal to 50J, and the hardenability reaches that J1.5 is 54-58 HRC; j40 is 45-49HRC, and can be effectively used for manufacturing engineering machinery parts with medium and large sizes.

In order to achieve the above object, the present invention provides a round steel which is made of the above high hardenability quenched and tempered steel.

Further, in the round steel of the present invention, the diameter thereof is 50 to 140 mm.

Further, in the round steel of the present invention, the performance of the round steel satisfies at least one of the following conditions: the yield strength is more than or equal to 460MPa, the tensile strength is more than or equal to 790MPa, the elongation is more than or equal to 19%, the reduction of area is more than or equal to 60%, the impact energy Aku at room temperature is more than or equal to 50J, and the hardenability reaches that J1.5 is 54-58 HRC; j40 was 45-49 HRC.

In the above-described technical solutions, it should be explained that, in the present invention, regarding the expression of hardenability of the present invention, J1.5 represents hardness at a distance of 1.5mm from the end portion, and accordingly, J40 represents hardness at a distance of 40mm from the end portion.

Another object of the present invention is to provide a method for manufacturing the round bar, which is simple to produce, and which can provide a round bar for construction machinery having a remarkably improved hardenability as compared with conventional steel materials, and which can be suitably used for manufacturing large and medium-sized construction machinery parts.

In order to achieve the above object, the present invention provides a method for manufacturing the round bar, including the steps of:

(1) smelting;

(2) casting;

(3) rolling: the initial rolling temperature is controlled to 1050-;

further, in the manufacturing method of the present invention, in the step (1), the addition sequence of the alloying elements follows the steps of adding Al for deoxidation, then adding Ti for nitrogen fixation, and finally adding B; the temperature of the steel is controlled to be 1640-1660 ℃ during smelting.

In the technical scheme of the invention, the B element is a key element for improving the hardenability of the steel, but the B element only acts on the hardenability of the steel when the B element exists in the form of solid solution boron. Therefore, in the invention, the technical means of adding Al for deoxidation, then adding Ti for nitrogen fixation and finally adding B can be adopted to form more solid solution boron, thereby playing a role in improving the hardenability of the steel.

Further, in the manufacturing method of the present invention, in the step (2), the casting start temperature is controlled to 1530-1560 ℃.

Compared with the prior art, the high-hardenability quenched and tempered steel, the round steel and the manufacturing method thereof have the advantages and beneficial effects as follows:

the high-hardenability quenched and tempered steel can fully play the role of alloy elements through reasonable chemical composition design under the condition of reducing the cost as much as possible. On the basis of manganese boron steel, elements such as Cr and Ni are added to the high-hardenability quenched and tempered steel, so that the hardenability of the steel is further improved by the B element, and the high-hardenability quenched and tempered steel which has high hardenability and maintains good comprehensive mechanical properties is obtained.

The high-hardenability quenched and tempered steel has the advantages of excellent performance, uniform structure, fine crystal grains, convenience for processing parts with large sections and complex sizes, complete quenching after quenching and tempering, long service life, wide applicability, good popularization prospect and application value, and can be used for manufacturing round steel and medium-large-sized engineering mechanical parts.

The round steel prepared from the high-hardenability quenched and tempered steel has ultrahigh hardenability while ensuring excellent comprehensive mechanical properties, wherein the yield strength is more than or equal to 460MPa, the tensile strength is more than or equal to 790MPa, the elongation is more than or equal to 19%, the reduction of area is more than or equal to 60%, the impact energy Aku at room temperature is more than or equal to 50J, and the hardenability reaches that J1.5 is 54-58 HRC; j40 is 45-49HRC, can be effectively suitable for the requirement of engineering machinery industry part manufacturing.

Correspondingly, the manufacturing method provided by the invention is simple in production process, and compared with the existing steel, the round steel for the engineering machinery obtained by the manufacturing method is obviously improved in hardenability, and can be effectively suitable for manufacturing engineering machinery parts with medium and large sizes.

Drawings

Fig. 1 is a metallographic photograph of the round steel of example 1.

Fig. 2 shows hardenability curves for each example of the present case and prior art CN 108998725A.

Detailed Description

The high hardenability quenched and tempered steel, round steel and the manufacturing method thereof according to the present invention will be further explained and illustrated with reference to the drawings and the specific examples, which, however, should not be construed as unduly limiting the technical solution of the present invention.

Examples 1 to 6

The high hardenability quenched and tempered steels of examples 1 to 6 were prepared by the following steps:

(1) smelting according to the chemical components shown in the table 1: smelting by adopting a converter, wherein the tapping temperature is controlled to be 1640-1660 ℃ during smelting; the addition sequence of the alloy elements comprises the steps of adding Al for deoxidation, then adding Ti for nitrogen fixation and finally adding B.

(2) Casting: the casting start temperature was controlled at 1530-1560 ℃.

(3) Rolling: the initial rolling temperature is controlled to 1050-.

It should be noted that the round steels of examples 1 to 6 were all made of the high hardenability quenched and tempered steel of the present invention, and table 1 shows the mass percentages of the chemical elements of the high hardenability quenched and tempered steels of examples 1 to 6.

Table 1 (wt.%, balance Fe and unavoidable impurities other than P, S, Pb, As, Sn, Sb, and Ca)

Numbering	C	Si	Mn	Cr	Cu	Al	Ti	B	Ni	P	S	Pb	As	Sn	Sb	Ca	N	Ti/N
																			Example 1	0.39	0.30	0.98	0.95	0.20	0.045	0.030	0.0016	0.16	0.005	0.002	0.001	0.005	0.005	0.007	0.0001	0.0060	5.0
Example 2	0.37	0.30	0.98	0.94	0.18	0.036	0.030	0.0024	0.19	0.005	0.002	0.001	0.006	0.004	0.007	0.0002	0.0062	4.8
																			Example 3	0.39	0.30	0.98	0.95	0.15	0.033	0.035	0.0014	0.17	0.005	0.002	0.001	0.004	0.005	0.007	0.0002	0.0060	5.8
Example 4	0.35	0.30	0.95	0.93	0.10	0.021	0.032	0.0023	0.10	0.006	0.002	0.001	0.005	0.005	0.007	0.0002	0.0070	4.6
																			Example 5	0.40	0.15	1.04	0.90	0.15	0.027	0.050	0.0011	0.17	0.006	0.003	0.001	0.005	0.004	0.007	0.0002	0.0072	6.9
Example 6	0.38	0.35	0.91	0.99	0.15	0.033	0.042	0.0035	0.15	0.006	0.003	0.001	0.004	0.004	0.007	0.0001	0.0080	5.3

Table 2 lists the specific process parameters for the round bars of examples 1-6 in the above procedure.

Table 2.

The obtained round steels of examples 1 to 6 were sampled and subjected to various relevant performance tests, and the results of the performance tests are shown in tables 3-1 and 3-2, respectively

The mechanical property test results of the round steels of examples 1 to 6 are shown in Table 3-1.

Table 3-1.

Table 3-2.

Note: in the above table, regarding the expression of the hardenability of steel, J1.5 represents the hardness at a distance of 1.5mm from the end portion, J3 represents the hardness at a distance of 3mm from the end portion, J5 represents the hardness at a distance of 5mm from the end portion, and so on, J50 represents the hardness at a distance of 50mm from the end portion. The hardenability test in the technical scheme is carried out according to the national standard GB/T225-2006, and a terminal quenching test method (Jominy test) of steel hardenability.

As can be seen by combining tables 3-1 and 3-2, the round steel of examples 1-6 of the present invention has an ultra-high hardenability while ensuring good comprehensive mechanical properties, and has yield strength of not less than 460MPa, tensile strength of not less than 790MPa, elongation of not less than 19%, reduction of area of not less than 60%, and impact energy Aku of not less than 50J at room temperature, and excellent comprehensive mechanical properties. In addition, the round steels of examples 1 to 6 according to the present invention had hardenability up to J1.5 of 54 to 58 HRC; j40 was 45-49 HRC.

As can be seen from tables 3-2, the round steels in examples 1-6 all have excellent hardenability, the hardenability J1.5 has hardness of 54-58HRC, the J40 has hardness of 45-49HRC, and the hardenability values are all in the required range of the engineering machinery parts, so that the round steels can be used for manufacturing medium and large-sized engineering machinery parts, and have good popularization prospects and application values.

The hardenability is a material characteristic characterized by the depth of the hardening layer of the sample and the hardness distribution under a predetermined condition, that is, the ability to obtain the depth of the hardening layer when the steel is quenched. Therefore, hardenability can be characterized using a hardenability hardness curve, which is also specified in the national standard GB/T225-2006. The difference in hardenability between the present case and the prior art can be clearly seen from fig. 2. As shown in fig. 2, the hardness curves of examples 1 to 6 in this case decreased more slowly, while the hardness of CN108998725A decreased more significantly.

Fig. 1 is a metallographic photograph of the round steel of example 1.

The round steels of examples 1 to 6 of the present invention were all produced using the high hardenability quenched and tempered steel of the present invention. As shown in fig. 1, after the high hardenability quenched and tempered steel of example 1 was rolled to obtain a round steel, it was observed that the pearlite + ferrite structure obtained in example 1 could provide a preliminary structure for the subsequent quenching and tempering heat treatment.

In conclusion, the high-hardenability quenched and tempered steel can fully play the role of alloy elements through reasonable chemical composition design under the condition of reducing the cost as much as possible. On the basis of manganese boron steel, elements such as Cr and Ni are added to the high-hardenability quenched and tempered steel, so that the hardenability of the steel is further improved by the B element, and the high-hardenability quenched and tempered steel which has high hardenability and maintains good comprehensive mechanical properties is obtained.

In addition, the combination of the features in the present application is not limited to the combination described in the claims of the present application or the combination described in the embodiments, and all the features described in the present application may be freely combined or combined in any manner unless contradictory to each other.

It should also be noted that the above-mentioned embodiments are only specific embodiments of the present invention. It is apparent that the present invention is not limited to the above embodiments and similar changes or modifications can be easily made by those skilled in the art from the disclosure of the present invention and shall fall within the scope of the present invention.

Claims

1. A high-hardenability quenched and tempered steel is characterized by comprising the following chemical elements in percentage by mass in addition to Fe:

2. the high hardenability quenched and tempered steel as claimed in claim 1, wherein the mass percentages of the chemical elements are:

3. The high hardenability quenched and tempered steel according to claim 1 or 2, wherein the chemical elements are contained in mass percent as follows: the Ti/N is more than or equal to 4.

4. The high hardenability quenched and tempered steel according to claim 2, wherein, among other inevitable impurities, the content of each impurity element satisfies at least one of: p is less than or equal to 0.025 wt.%, S is less than or equal to 0.025 wt.%, Pb is less than or equal to 0.002 wt.%, As is less than or equal to 0.04 wt.%, Sn is less than or equal to 0.005 wt.%, Sb is less than or equal to 0.008 wt.%, and Ca is less than or equal to 0.0010 wt.%.

5. The high hardenability quenched and tempered steel according to claim 1 or 2, wherein the microstructure is a uniform pearlite + ferrite structure.

6. A round steel produced using the high hardenability quenched and tempered steel as claimed in any one of claims 1 to 5.

7. The round steel as claimed in claim 6, wherein the properties satisfy at least one of: the yield strength is more than or equal to 460MPa, the tensile strength is more than or equal to 790MPa, the elongation is more than or equal to 19%, the reduction of area is more than or equal to 60%, the impact energy Aku at room temperature is more than or equal to 50J, and the hardenability reaches that J1.5 is 54-58 HRC; j40 was 45-49 HRC.

8. Round bar as in claim 6, characterised in that it has a diameter of 50-140 mm.

9. A method of manufacturing round steel as claimed in any one of claims 6 to 8, characterized in that it comprises the steps of:

(1) smelting;

(2) casting;

(3) rolling: the initial rolling temperature is controlled to 1050-.

10. The method according to claim 9, wherein in the step (1), the alloying elements are added in an order of adding Al for deoxidation, then adding Ti for nitrogen fixation, and finally adding B; the temperature of the steel is controlled to be 1640-1660 ℃ during smelting.

11. The manufacturing method as claimed in claim 9, wherein in the step (2), the casting start temperature is controlled to 1530-1560 ℃.