CN108220770B

CN108220770B - Smelting preparation method of boron-added high-carbon pure steel in vacuum induction furnace

Info

Publication number: CN108220770B
Application number: CN201711348432.1A
Authority: CN
Inventors: 潘孝定; 罗俊义; 王德洪; 叶超; 王璞
Original assignee: Advanced Technology and Materials Co Ltd
Current assignee: Advanced Technology and Materials Co Ltd
Priority date: 2017-12-15
Filing date: 2017-12-15
Publication date: 2020-06-23
Anticipated expiration: 2037-12-15
Also published as: CN108220770A

Abstract

The invention belongs to the technical field of special steel and smelting thereof, and relates to a method for preparing boron-added high-carbon pure steel by smelting in a vacuum induction furnace. The boron-added high-carbon pure steel comprises the following components in percentage by mass: 0.67-0.75% of C, 0.15-0.37% of Si, 0.30-0.80% of Mn, less than or equal to 0.012% of P, less than or equal to 0.006% of S, 0.20-0.50% of Cr, less than or equal to 0.30% of Mo, less than or equal to 0.30% of V, 0.002-0.100% of B, and the balance of Fe and inevitable impurities; the preparation method comprises the following steps: smelting, continuous casting and the like in a vacuum induction furnace. The method can greatly improve the mechanical property of the steel, the tensile strength Rm of the product is more than or equal to 1450MPa, the yield strength R0.2 is more than or equal to 1350MPa, the plate is used for sintering the mother plate of the saw blade, the plastic deformation damage is small, the service life of the steel is prolonged, and the production cost of the steel is low.

Description

Smelting preparation method of boron-added high-carbon pure steel in vacuum induction furnace

Technical Field

The invention belongs to the technical field of special steel and smelting thereof, and relates to a method for preparing boron-added high-carbon pure steel by smelting in a vacuum induction furnace.

Background

At present, when a steel plate is used for sintering a plate such as a saw blade, the steel plate needs to be heated to a certain temperature (about 750 ℃) in the processing process, the use environment is severe, and the material needs to bear strong tensile force and fatigue failure. At present, if the carbon steel plate is used, the sintered plate is easy to generate plastic deformation and has short service life; the use of high strength alloy steel plates is costly. Although the high-quality carbon structural steel No. 70 steel is suitable for hot working at high temperature (about 750 ℃), the contents of harmful elements and inclusions in the components are high, and the mechanical properties are limited.

Therefore, there is a need to develop a high strength pure carbon steel with high carbon content, micro-alloying, low content of harmful elements and inclusions, strong mechanical properties of the product, and relatively low cost, which can be used in hot working at high temperature.

Disclosure of Invention

The invention provides a method for smelting high-carbon pure steel with boron in a vacuum induction furnace.

The invention is realized by the following technical scheme:

the smelting preparation method of the boron-added high-carbon pure steel in the vacuum induction furnace comprises the following components in percentage by mass: 0.67-0.75% of C, 0.15-0.37% of Si, 0.30-0.80% of Mn, less than or equal to 0.012% of P, less than or equal to 0.006% of S, 0.20-0.50% of Cr, less than or equal to 0.30% of Mo, less than or equal to 0.30% of V, 0.002-0.100% of B, and the balance of Fe and inevitable impurities; the preparation method comprises the following steps:

smelting in a vacuum induction furnace:

firstly, smelting the smelting ingredients in a vacuum induction furnace, refining, alloying and tapping to obtain alloy molten steel;

and (3) continuous casting treatment:

and carrying out continuous casting treatment on the alloy molten steel to obtain the boron-added high-carbon pure steel.

In a preferred embodiment, in the vacuum induction furnace smelting step, the smelting raw materials are industrial pure iron, a carburant, a Cr source, a Mo source and a V source; preferably, the amount of the carburant is 0.7-0.8% of the mass of the industrial pure iron.

In a preferred embodiment, in the vacuum induction furnace smelting step, the vacuum degree is less than or equal to 20Pa, preferably 1-20Pa, in the melting treatment.

In a preferred embodiment, in the vacuum induction furnace smelting step, the temperature at which the refining treatment is started is 1580-1600 ℃.

In a preferred embodiment, in the smelting step in the vacuum induction furnace, the refining treatment is carried out at a vacuum degree of less than or equal to 5Pa, preferably 1-5Pa, at a temperature of 1580-.

In a preferred embodiment, in the vacuum induction furnace smelting step, the alloying treatment is performed under an argon atmosphere, and preferably, the argon pressure is 4 to 6 KPa; in the alloying treatment, a Si source, a B source and a Mn source are sequentially added into the vacuum induction furnace; preferably, the Si source is a B-Fe alloy, the B source is a B-Fe alloy, and the Mn source is electrolytic Mn.

In a preferred embodiment, in the smelting step of the vacuum induction furnace, the tapping temperature is 1540-1560 ℃.

In a preferred embodiment, the Cr source is elemental Cr, the Mo source is elemental Mo, and the V source is a V-Fe alloy.

As a preferred embodiment, the boron-added high carbon pure steel consists of the following components: 0.67-0.75% of C, 0.15-0.3% of Si, 0.3-0.6% of Mn, less than or equal to 0.012% of P, less than or equal to 0.006% of S, 0.25-0.40% of Cr, 0.010-0.025% of Mo, 0.20-0.30% of V, 0.002-0.01% of B, and the balance of Fe and inevitable impurities.

In a preferred embodiment, the tensile strength Rm of the boron-added high-carbon pure steel is not less than 1450MPa, and the yield strength R0.2 is not less than 1350 MPa.

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

1. on the basis of the high-quality carbon structural steel 70# steel prepared by the method, the content of harmful element S, P, O, N is further reduced, the content of inclusions is reduced, molten steel is purified, and the mechanical properties of the steel can be greatly improved.

2. The mechanical properties of the product prepared by the invention are greatly improved compared with 70# steel and high-carbon pure steel 72A, and the performance of the steel obtained by smelting, continuous casting treatment, forging, hot rolling, cold rolling and heat treatment in a vacuum induction furnace is detected, and the results are as follows: the tensile strength Rm is more than or equal to 1450 MPa; the yield strength R0.2 is more than or equal to 1350MPa, the plate is used for sintering the saw blade mother plate, the plastic deformation damage is small, the service life is prolonged, and in addition, the steel has low production cost.

3. The chemical components of the invention are reasonably proportioned, and the product quality is improved together: the addition of Cr effectively improves the toughness of steel; the addition of a small amount of V can effectively control the form and distribution of crystal grains in the steel and improve the mechanical property, strength and plasticity of the steel; the addition of B can improve the hardenability and save a large amount of noble alloy elements.

4. The invention adopts the vacuum induction furnace which is not used by people in the field of pure steel preparation as the main equipment to prepare the high-strength carbon pure steel with excellent mechanical properties in all aspects, and the obtained product has good stability and high qualification rate. In the prior art, in order to control the production cost, the vacuum induction furnace is mainly applied to experiments performed in a research and development stage, and is not basically applied to actual production. The vacuum induction furnace is applied to the production process of the high-strength carbon pure steel, the quality of the prepared pure steel product is excellent, and the defect of overhigh production cost can be overcome through the profit obtained by the product.

5. The mechanical properties of the high-strength carbon pure steel are further improved by the reasonable proportion of the components and the synergistic effect of the components and the steps and parameters of the preparation method.

Detailed Description

The invention will be further illustrated with reference to the following specific examples. It should be understood that these examples are only for the purpose of the present invention and are not intended to limit the scope of the present invention. It should be understood that various changes and modifications can be made by those skilled in the art after reading the disclosure of the present invention, and equivalents fall within the scope of the appended claims.

The invention provides a preparation method of the boron-added high-carbon pure steel (namely the high-strength carbon pure steel), which is a production process taking a vacuum induction furnace as main equipment and prepares a product by using raw materials such as industrial pure iron and the like as furnace raw materials. The preparation method comprises the following steps:

step one, batching: taking industrial pure iron, a carburant (the proportion of C is 0.7-0.8%, namely after the carburant is added, the mass percentage of C in the carburant in the industrial pure iron is 0.7-0.8%), and taking Cr strips, Mo strips and V-Fe alloy as smelting ingredients.

Step two: smelting in a vacuum induction furnace:

feeding the smelting ingredients into a vacuum induction furnace, and feeding electricity to melt the ingredients under the condition that the vacuum degree is less than or equal to 20Pa (preferably 1-20 Pa);

when the temperature of the molten steel reaches 1580-;

after refining, argon is filled to make the pressure in the furnace reach 4-6KPa (such as 4.2KPa, 4.5KPa, 5KPa, 5.2KPa, 5.5KPa, 5.8KPa), and then alloying treatment is carried out: sequentially adding Si iron, B-Fe alloy and electrolytic Mn into a furnace; tapping when the temperature of the molten steel is reduced to 1540-.

Step three, continuous casting treatment:

carrying out continuous casting treatment on the alloy molten steel to obtain a boron-added high-carbon pure steel casting blank;

the preparation method of the invention can also comprise the following steps: forging and rolling the boron-added high-carbon pure steel billet and performing heat treatment to obtain a finished product steel, wherein the forging and rolling process and the heat treatment can adopt a conventional process or a process used in the embodiment of the invention.

The high-strength carbon pure steel prepared by the preparation method disclosed by the invention comprises the following components in percentage by mass: 0.67-0.75% of C, 0.15-0.37% of Si, 0.30-0.80% of Mn, less than or equal to 0.012% of P, less than or equal to 0.006% of S, 0.20-0.50% of Cr, less than or equal to 0.30% of Mo, less than or equal to 0.30% of V, 0.002-0.100% of B, and the balance of Fe and inevitable impurities.

Illustratively, the mass percentage of C may be any of 0.67%, 0.68%, 0.70%, 0.72%, 0.75%, or a range between any two; the Si content may be any one of 0.15%, 0.20%, 0.25%, 0.30%, 0.35%, 0.37% or a range between any two of these; the Mn may be contained in an amount of 0.30%, 0.40%, 0.45%, 0.50%, 0.60%, 0.70%, 0.80% by mass or in a range between any two of them; the mass percentage of P may be 0.003%, 0.005%, 0.008%, 0.010%, 0.012%, or a range therebetween; the mass percentage of S may be any of 0.002%, 0.003%, 0.004%, 0.005%, 0.006%, or a range between any two; the Cr may be contained in an amount of 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, 0.45%, 0.50% by mass or in a range between any two of these amounts; the mass percentage of Mo may be any value of 0.05%, 0.08%, 0.1%, 0.2%, 0.25%, 0.3%, or a range between any two; the mass percentage of V may be any value of 0.05%, 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, or a range between any two; the mass percentage of B may be any value or a range between any two of 0.002%, 0.004% 0.005%, 0.01%, 0.02%, 0.05%, 0.08%, and 0.1%.

Wherein, the content of each component is preferably as follows: 0.67-0.75% of C, 0.15-0.3% of Si, 0.3-0.6% of Mn, less than or equal to 0.012% of P, less than or equal to 0.006% of S, 0.25-0.40% of Cr, 0.010-0.025% of Mo, 0.20-0.30% of V and 0.002-0.01% of B.

The chemical component proportions have synergistic effect, the product quality is improved together, and the effects of the components in the steel and the selection reasons of the content of the components are as follows.

C: 0.67 to 0.75%. Carbon is an indispensable element in steel, the gamma phase region of carbon is enlarged in the steel, the carbon is a composition element of high-strength carbide, the strengthening effect of the carbon in the steel is that the components and the structure of the carbide formed by the carbon are closely related, the carbon content in the steel is increased, the yield point and the tensile strength are increased, but the plasticity and the impact property are reduced, so the C content is selected to be 0.67-0.75% by comprehensively considering the performance of the steel.

Si: 0.15 to 0.37%. In the steel-making process, silicon is added as a reducing agent and a deoxidizing agent, and the co-alloying of silicon and molybdenum has an obvious effect of improving the high-temperature oxidation resistance of steel.

Mn: 0.30-0.80%. In the steel-making process, manganese is a good deoxidizer and desulfurizer, and the content of manganese in the invention is selected to be 0.3-0.8%, so that the steel has enough toughness, higher strength and hardness, the quenching property of the steel is improved, and the hot workability of the steel is improved.

Mo is less than or equal to 0.30 percent, molybdenum is refractory metal, the melting point is high (2625 ℃), molybdenum is an alloy element for reducing a gamma-Fe phase region and expanding a α -Fe phase region and is a strong carbide forming element when being dissolved in base metal, molybdenum is an alloying element of steel, the strength, particularly the high-temperature strength and the toughness of the steel can be improved, and the wear resistance, the hardenability, the weldability and the heat resistance of the steel are improved.

0.20 to 0.50 percent of Cr. The addition of Cr can refine the pearlite sheet spacing and improve the hardenability of steel, thereby effectively improving the toughness of steel; the pearlite transformation involves a proportion of chromium entering the cementite and ferrite phases, thereby improving strength properties and altering workability.

V is less than or equal to 0.30 percent. The growth of austenite in steel can be effectively hindered by adding a small amount of vanadium, so that the refinement of crystal grains is realized; vanadium can improve the nucleation driving force of cementite, and cementite crystal grains rapidly and independently nucleate at different positions, so that cementite is distributed in a dispersed manner; the addition of vanadium reduces the thickness of the grain boundary cementite and distributes along the grain boundary dispersedly, and reduces the size of austenite grains, thereby improving the mechanical property of steel; vanadium produces precipitation strengthening in steel, and refines pearlite lamellar spacing in high-carbon steel, and improves the strength and plasticity of the steel.

0.002-0.100% of B, and strong hardenability improvement capability of B; a great deal of precious alloy elements can be saved by only needing a little boron.

Compared with the prior art, the product performance of the invention is as follows:

1. in the prior art, the chemical components of the steel are similar to those of the steel, such as 65#, 70#, 75#, 65Mn, 70Mn and the like, the chemical components are shown in table 1, and the detailed information in table 1 is derived from GB/T699-2015 high-quality carbon structural steel.

Table 1: GB/T699-containing 2015 high-quality carbon structural steel

The mechanical properties of the above structural steels are shown in table 2.

Table 2: mechanical properties of high-quality carbon structural steel (GB/T699-

3. Compared with the steel grades of the above grades, the mechanical property of the product of the invention is obviously enhanced: the tensile strength Rm is more than or equal to 1450MPa, the yield strength R0.2 is more than or equal to 1350MPa, and the elongation is more than 6 percent; the plate is used for sintering a saw blade mother plate, the plastic deformation and damage are small, and the service life of the saw blade mother plate is prolonged.

Example 1

In this example, 25Kg of vacuum induction furnace was used for smelting and casting 25Kg of shell ingot.

The high-strength carbon pure steel of the embodiment comprises the following components in percentage by mass:

C	Mn	Si	P	S	Cr	Mo	V	B
									0.67	0.731	0.289	0.005	0.0006	0.328	0.001	0.18	0.0075

the balance being Fe and unavoidable impurities.

The preparation method of this example includes the following steps:

(1) preparing materials: the industrial pure iron, the carburant (C ratio is 0.7-0.8%), the Cr strips, the Mo strips and the V-Fe alloy are used as smelting ingredients.

(2) Smelting in a vacuum induction furnace:

a) melting: feeding the smelting ingredients into a vacuum induction furnace, and feeding electricity to melt the ingredients under the vacuum degree of 15 Pa;

b) refining: when the temperature reaches 1590 ℃ and the vacuum degree is 3Pa, refining is started, the refining time is 45min, and the temperature is 1620 ℃, so that refined molten steel is obtained;

c) alloying: and after refining, filling argon to ensure that the pressure in the furnace reaches 4-6KPa for alloying treatment: sequentially adding Si iron, B-Fe alloy and electrolytic Mn into a furnace; and tapping when the temperature of the molten steel is reduced to 1550 ℃ to obtain alloy molten steel.

(3) Continuous casting: continuously casting alloy molten steel to obtain 25Kg of shell ingot;

(4) forging and rolling: forging the 25Kg cannonball ingot into a blank with the thickness of 30mm and the width of 80mm at the forging temperature of 900-1150 ℃, and air-cooling to the normal temperature; then carrying out hot rolling treatment, wherein the hot rolling starting temperature is 1150 ℃, rolling the steel strip into strip steel with the thickness of 4.0mm and the width of 90mm, and keeping the temperature of heat-preservation cotton to normal temperature; then cold rolling the steel strip into strip steel with the thickness of 1.5mm and the width of 90 mm; keeping the temperature at 830 ℃ for 2 hours and carrying out isothermal oil quenching.

The mechanical properties of the product obtained in this example are significantly enhanced: the tensile strength Rm is 1487MPa, the yield strength R0.2 is 1366MPa, and the elongation is 8.2%.

Example 2:

C	Mn	Si	P	S	Cr	Mo	V	B
									0.71	0.747	0.289	0.004	0.0004	0.341	0.001	0.13	0.0091

the balance being Fe and unavoidable impurities.

In the preparation method of this example, the operations and parameters were the same as those of example 1 except that the amounts of the elements added in steps (1) and (2) -c) were different.

The mechanical properties of the product obtained in this example are significantly enhanced: the tensile strength Rm is 1453MPa, the yield strength R0.2 is 1404MPa, and the elongation is 8.6%.

Example 3:

C	Mn	Si	P	S	Cr	Mo	V	B
									0.69	0.755	0.295	0.005	0.0003	0.333	0.001	0.20	0.0126

the balance being Fe and unavoidable impurities.

The mechanical properties of the product obtained in this example are significantly enhanced: the tensile strength Rm is 1470MPa, the yield strength R0.2 is 1378MPa, and the elongation is 8.6%.

Comparative example 1

The high-strength carbon pure steel of the comparative example consists of the following components in percentage by mass:

C	Mn	Si	P	S	Cr	Mo	V	B
									0.67	0.731	0.289	0.005	0.0006	0.328	0.001	0.18	0

the balance being Fe and unavoidable impurities.

The process parameters were the same as in example 1, except that no BFe alloy was added in steps (2) to c).

The mechanical properties of the strip steel product obtained in this example were: the tensile strength Rm was 1200MPa, the yield strength R0.2 was 1150MPa, and the elongation was 10.5%, from which it was seen that the mechanical properties of the product to which the B element was not added were degraded.

Comparative example 2

The high-strength pure carbon steel of the comparative example comprises the following components in percentage by mass:

C	Mn	Si	P	S	Cr	Mo	V	B
									0.67	0.731	0.289	0.005	0.0006	0.328	0.001	0.18	0.3

the balance being Fe and unavoidable impurities.

The process parameters were the same as those of example 1 except that the amount of the (2) -c) B-Fe alloy was increased.

The mechanical properties of the strip obtained in this example were: the tensile strength Rm is 1550MPa, the yield strength R0.2 is 1400MPa, and the elongation is 1.5%. Brittle fracture occurs, and therefore, it is critical to control the range of the B element in the preparation of pure steel, and if the content of B is too high, the mechanical properties of the product are reduced.

Example 4

C	Mn	Si	P	S	Cr	Mo	V	B
									0.75	0.30	0.15	0.012	0.006	0.50	0.001	0.30	0.010

the balance being Fe and unavoidable impurities.

The preparation method of this example includes the following steps:

(2) Smelting in a vacuum induction furnace:

a) melting: feeding the smelting ingredients into a vacuum induction furnace, and feeding electricity to melt the ingredients under the vacuum degree of 5 Pa;

b) refining: when the temperature reaches 1580 ℃ and the vacuum degree is 1Pa, refining is started, the refining time is 40min, and the temperature is 1590 ℃, so that refined molten steel is obtained;

c) alloying: and after refining, filling argon to ensure that the pressure in the furnace reaches 4-6KPa for alloying treatment: sequentially adding Si iron, B-Fe alloy and electrolytic Mn into a furnace; and tapping when the temperature of the molten steel is reduced to 1540 ℃ to obtain the alloy molten steel.

(3) Continuous casting: and continuously casting the alloy molten steel to obtain 25Kg of shell ingot.

The mechanical properties of the product obtained in this example are significantly enhanced: the tensile strength Rm is 1470MPa, the yield strength R0.2 is 1360MPa, and the elongation is 8.5%.

Example 5

C	Mn	Si	P	S	Cr	Mo	V	B
									0.70	0.75	0.30	0.012	0.006	0.20	0.001	0.25	0.050

the balance being Fe and unavoidable impurities.

The preparation method of this example includes the following steps:

(2) Smelting in a vacuum induction furnace:

a) melting: feeding the smelting ingredients into a vacuum induction furnace, and feeding electricity to melt the ingredients under the vacuum degree of 20 Pa;

b) refining: when the temperature reaches 1600 ℃ and the vacuum degree is 5Pa, starting refining for 50min at 1640 ℃, and obtaining refined molten steel;

c) alloying: and after refining, filling argon to ensure that the pressure in the furnace reaches 4-6KPa for alloying treatment: sequentially adding Si iron, B-Fe alloy and electrolytic Mn into a furnace; and when the temperature of the molten steel is reduced to 1560 ℃, tapping to obtain the alloy molten steel.

The mechanical properties of the product obtained in this example are significantly enhanced: the tensile strength Rm is 1480MPa, the yield strength R0.2 is 1365MPa, and the elongation is 7.8%.

Example 6

C	Mn	Si	P	S	Cr	Mo	V	B
									0.72	0.80	0.37	0.012	0.006	0.30	0.001	0.10	0.030

the balance being Fe and unavoidable impurities.

The preparation method of this example includes the following steps:

(2) Smelting in a vacuum induction furnace:

a) melting: feeding the smelting ingredients into a vacuum induction furnace, and feeding electricity to melt the ingredients under the vacuum degree of 10 Pa;

b) refining: when the temperature reaches 1585 ℃ and the vacuum degree is 2Pa, refining is started, the refining time is 42min, and the temperature is 1610 ℃ to obtain refined molten steel;

c) alloying: and after refining, filling argon to ensure that the pressure in the furnace reaches 4-6KPa for alloying treatment: sequentially adding Si iron, B-Fe alloy and electrolytic Mn into a furnace; and tapping when the temperature of the molten steel is reduced to 1545 ℃ to obtain alloy molten steel.

The mechanical properties of the product obtained in this example are significantly enhanced: tensile strength Rm is 1575MPa, yield strength R0.2 is 1372MPa, and elongation is 8.2%.

Claims

1. A method for smelting and preparing boron-added high-carbon pure steel for a sintered saw blade mother plate in a vacuum induction furnace is characterized by comprising the following steps of: the boron-added high-carbon pure steel comprises the following components in percentage by mass: 0.67-0.75% of C, 0.15-0.37% of Si, 0.30-0.80% of Mn, less than or equal to 0.012% of P, less than or equal to 0.006% of S, more than or equal to 0.20% and less than 0.50% of Cr, 0.010-0.30% of Mo0.0075-0.1% of B, and the balance of Fe and inevitable impurities;

the preparation method comprises the following steps:

smelting in a vacuum induction furnace:

the smelting ingredients are industrial pure iron, a carburant, a Cr source, a Mo source and a V source; the Cr source is a Cr simple substance, the Mo source is a Mo simple substance, and the V source is a V-Fe alloy;

in the alloying treatment, a Si source, a B source and a Mn source are sequentially added into the vacuum induction furnace; the Si source is B-Fe alloy, the B source is B-Fe alloy, and the Mn source is electrolytic Mn;

and (3) continuous casting treatment:

2. The method of claim 1, wherein:

the dosage of the recarburizing agent is 0.7-0.8% of the mass of the industrial pure iron.

3. The production method according to claim 1 or 2, characterized in that:

in the smelting step of the vacuum induction furnace, the vacuum degree is less than or equal to 20Pa in the melting treatment.

4. The method according to claim 3, wherein:

in the smelting step of the vacuum induction furnace, the vacuum degree is 1-20Pa in the melting treatment.

5. The method of claim 1, wherein: in the smelting step of the vacuum induction furnace, the temperature when the refining treatment is started is 1580-1600 ℃.

6. The method of claim 1, wherein: in the smelting step of the vacuum induction furnace, the vacuum degree is less than or equal to 5Pa, the temperature is 1580-.

7. The method according to claim 6, wherein: in the smelting step of the vacuum induction furnace, the vacuum degree is 1-5Pa and the time is 40-50min in the refining treatment.

8. The method of claim 1, wherein:

in the smelting step of the vacuum induction furnace, the alloying treatment is carried out in an argon atmosphere, and the pressure of the argon is 4-6 kPa.

9. The method of claim 1, wherein:

in the smelting step of the vacuum induction furnace, the tapping temperature is 1540-1560 ℃.

10. The method of claim 1, wherein: the boron-added high-carbon pure steel comprises the following components: 0.67-0.75% of C, 0.15-0.3% of Si, 0.3-0.6% of Mn, less than or equal to 0.012% of P, less than or equal to 0.006% of S, 0.25-0.40% of Cr, 0.010-0.025% of Mo, 0.20-0.30% of V, 0.0075-0.1% of B, and the balance of Fe and inevitable impurities.