GB2612440A

GB2612440A - Method for preparing non-quenched and tempered steel for large-specification direct cutting

Info

Publication number: GB2612440A
Application number: GB2213835.8A
Authority: GB
Inventors: Gao Huayao; Liu Donglin; Jiang Hongliang; Du Yan; Zhou Zhan; Zhu Kangning; Zhang Chunxiang
Original assignee: Jiangsu Yonggang Group Co Ltd
Current assignee: Jiangsu Yonggang Group Co Ltd
Priority date: 2021-04-27
Filing date: 2022-03-14
Publication date: 2023-05-03
Anticipated expiration: 2042-03-14
Also published as: CN113134510B; GB202213835D0; CN113134510A; WO2022227891A1; GB2612440B

Abstract

A method for preparing non-quenched and tempered steel for large-specification direct cutting. The steel components are: 0.40-0.48％ C, 0.20-0.35％ Si, 0.80-1.30％ Mn, P≤0.020％, S≤0.035％, 0.10-0.20％ Cr, 0.05％-0.13％ V, 0.010％-0.020％ Ti, Ni≤0.025％, Mo≤0.015％, Al≤0.030％, Cu≤0.2％, 130-200ppm N, H≤2.0ppm, O≤20ppm, and the remainder is Fe and inevitable impurities. The method omits commonly used Nb while not increasing the silicon content. By means of optimizing the composition and using semi-continuous rolling technology to achieve a controlled rolling and controlled cooling process, non-quenched and tempered steel for large-specification direct cutting which has an ideal structure and comprehensive mechanical properties is finally obtained.

Description

METHOD FOR PREPARING NON-QUENCHED AND TEMPERED STEEL FOR LARGE-SPECIFICATION DIRECT CUTTING

TECHNICAL FIELD

The present disclosure belongs to the technical field of non-quenched and tempered steels, and in particular relates to a manufacturing method of a large-sized non-quenched and tempered steel for direct cutting.

BACKGROUND

In the field of engineering machinery, important components are mainly made from quenched and tempered steels. However, the production of quenched and tempered steels involves a long production cycle and a large energy consumption, and a workpiece is easily deformed or cracked during quenching, resulting in large product quality fluctuation, high manufacturing cost, and heavy environmental pollution. At present, the field of engineering machinery often involves the mass production of large-sized components, but the current CO development and application of large-scale cutting of non-quenched and tempered steels cannot meet the needs of engineering machinery. As the environmental protection, processing cost, CJ processing cycle, and material stability have attracted more and more attention, it is urgent to use a non-quenched and tempered steel instead of a quenched and tempered steel in the direct cutting to produce medium-sized and large-sized round bar products such as tie rods of injection molding machines and piston rods of mechanical cylinders in the mechanical processing industry. However, as the specifications of non-quenched and tempered hot-rolled round steels gradually increase, an edge and a core of a round steel section are much different in strength, toughness, microstructure, and grain size, which makes it prone to the mixed crystal phenomenon.

At present, materials used for injection molding machines on the market are mainly quenched and tempered 42CrMo and 414514 structural alloy steels. Since a quenching and tempering process of components increases the energy consumption, pollutes the environment, makes steels undergo defects such as oxidation, decarburization, deformation, and cracking, it is urgent to develop new non-quenched and tempered steels to gradually replace the quenched and tempered steels such as 42CrMo, 4145H, and 40Cr.

Through investigation, it is found that only small-sized non-quenched and tempered steels (diameter: < 60 mm) have been reported, and due to the difficulty in producing large-sized round bars, large-sized non-quenched and tempered steels have rarely been reported. Therefore, it is urgent to design a large-sized non-quenched and tempered steel with excellent performance for direct cutting.

SUMMARY

In order to overcome the technical defects in the prior art that the existing large-sized non-quenched and tempered steels for direct cutting (diameter: 75 mm to 140 mm) have mechanical performance and machinability that fluctuate greatly and thus can hardly meet the current production requirements, the present disclosure provides a medium-sized or large-sized non-quenched and tempered steel with a high strength and toughness for direct cutting, where the steel composition is optimized, and a semi-continuous rolling technique is used to realize controlled rolling/controlled cooling, such that the large-sized non-quenched and tempered steel for direct cutting has excellent mechanical performance and machinability.

In order to achieve the above objective, the present disclosure first provides a manufacturing method of a large-sized non-quenched and tempered steel for direct cutting. The large-sized non-quenched and tempered steel for direct cutting has high strength, high toughness, and excellent machinability. The manufacturing method adopts a semi-continuous rolling technique to realize controlled rolling/controlled cooling, and includes the following steps: CO (1) mixing raw materials according to a predetermined ratio to obtain a resulting mixture, smelting and casting the resulting mixture to obtain a steel billet, and heating the steel billet in a CJ continuous heating furnace to obtain a heated steel billet, where in a high-temperature section of the continuous heating furnace, the steel billet is heated for 2 h to 3 h at 1,180°C to 1,240°C for soaking; (2) subjecting the heated steel billet to descaling with high-pressure water to remove an iron oxide scale on a surface of the heated steel billet, and subjecting the steel billet after the descaling to 7 to 11-pass rolling in a cogging mill to obtain an intermediate rectangular billet, where the rolling is performed at 990°C to 1,120°C in the cogging mill and a deformation amount in each of the first 4 passes is higher than or equal to 17%; (3) subjecting the intermediate rectangular billet after the rolling in the step (2) to a holding treatment to 850°C to 920°C for 4 min to 6 min; (4) subjecting the intermediate rectangular billet after the holding treatment to 5 to 9-pass rolling in a continuous rolling mill to obtain a round steel bar, where the rolling is performed at a finishing rolling temperature of 780°C to 880°C and the round steel bar has a diameter of 100 mm to 140 mm, (5) subjecting the round steel bar after the rolling in the step (4) to through-water cooling to 325°C to 414°C after exiting the continuous rolling mill to obtain a cooled round steel bar; (6) slow cooling: placing the cooled round steel bar in a traverse marshalling system to move to a sawing roller table for sawing during which a self-tempering temperature on a surface of the round steel bar is 571°C to 590°C, then slowly cooling in a pit on a cooling bed to a temperature of lower than or equal to 200°C, finishing, and straightening to obtain a required steel, which is tested and stored; where the large-sized non-quenched and tempered steel for direct cutting includes the following components in weight percentage: C: 0.40% to 0.48%; Si: 0.20% to 0.35%; Mn: 0.80% to 1.30%; P: less than or equal to 0.020%; S: less than or equal to 0.035%; Cr: 0.10% to 0.20%; V: 0.05% to 0.13%; Ti: 0.010% to 0.020%; Ni: less than or equal to 0.025%; Mo: less than or equal to 0.015%; Al: less than or equal to 0.030%; Cu: less than or equal to 0.2%; N: 130 ppm to 200 ppm; H: less than or equal to 2.0 ppm; 0: less than or equal to 20 ppm; and Fe and inevitable impurities: the balance.

Preferably, in the step (1), the soaking is performed for a heating time of 2 h to 3 h. Preferably, in the step (2), the descaling with the high-pressure water is performed at a pressure of 20 MPa to 30 MPa.

Preferably, in the step (2), the deformation amount in each of the first 4 passes is higher than or equal to 20%.

CO Preferably, in the step (5), the through-water cooling is specifically as follows: the through-water cooling is performed in 3 to 5 sections, and a valve opening is controlled at 4% to CJ 30% in each of the sections; a water flow rate is adjusted by controlling a valve opening of a through-water cooling device to control an intensity of the through-water cooling for the round steel bar; and a strong cooling and a weak cooling are performed alternately, and a difference between valve openings in two adjacent ones of the sections is not less than 5%.

Composition design: The mechanical performance and machinability of the existing medium-sized and large-sized non-quenched and tempered steel products for direct cutting fluctuate greatly and are difficult to meet the requirements. An alloy composition design is optimized to manufacture a round steel bar, and the round steel bar has excellent mechanical performance and machinability and can meet the design requirements for tie rods of injection molding machines. Silicon mainly exists in a solid solution form in steel, and mainly plays a role of solid solution strengthening, which can significantly increase a volume fraction of a ferrite and strengthen a ferrite structure. Increasing the silicon content can cause a left shift of a pearlite C curve, increase an activity of carbon, promote the diffusion of carbon in an austenite, and increase the precipitation of a carbide. The conventional non-quenched and tempered steels have a silicon content of less than 0.50%, but are small-sized non-quenched and tempered steels and contain Nb to improve the performances. However, the present disclosure optimizes the alloy composition design and omits the commonly used Nb without increasing the silicon content to obtain a medium-sized or large-sized non-quenched and tempered steel for direct cutting, which has excellent strength, toughness, hardness, and machinability.

Advantages and technical effects of the present disclosure: The present disclosure optimizes the alloy composition design, and omits the commonly used Nb without increasing the silicon content, and reduces other alloying elements, which reduces the raw material cost. A semi-continuous rolling technique is used to produce a medium-sized or large-sized non-quenched and tempered steel, the controlled rolling is performed by large deformation-cogging and holding treatment, the controlled cooling is performed by a composite cooling system with multiple through-water cooling pipe sections, and the strong cooling and weak cooling are performed alternately to achieve continuous low-temperature rolling, such that the ideal structure and comprehensive mechanical performance are finally obtained.

In the present disclosure, a cooling rate of a ferritic-pearlitic non-quenched and tempered steel after rolling is controlled, and a cooling rate in a range of 800°C to 500°C can refine ferrite grains, increase a pearlite proportion, and reduce the interlamellar spacing. The large CO deformation amount in each of the first 4 passes in the cogging mill allows the as-cast structure of a steel billet to be fully crushed and the austenite grains to be refined, and the continuous CJ rolling technique adopts low-temperature rolling and controlled cooling to prevent the recrystallization of austenite grains, which facilitates the subsequent phase transition to obtain finer ferrite and pearlite structures. Through the two major process technologies of low-temperature rolling and controlled cooling, the present disclosure realizes the high strength and toughness of the medium-sized or large-sized non-quenched and tempered steel for direct cutting, while playing the roles of microalloy precipitation and dispersion strengthening.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. I is a metallographic image of the steel manufactured in Example 1.

FIG 2 is an image illustrating a grain size of the steel manufactured in Example I

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure will be described in detail below with reference to examples, but the present disclosure is not limited to these examples.

Example 1:

In this example, a round steel was provided, which had a diameter of 135 mm and included the following chemical components in mass percentage: C: 0.45%; Si: 0.29%; Mn: 1.21%; P: 0.013%; S: 0.005%; Cr: 0.14%; V: 0.07%; Ti:0.016%; Ni: 0.013%; Mo: 0.005%; Al 0.011%; Cu: 0.21%; N: 159 ppm; H: 0.9 ppm 0: 15 ppm and Fe and inevitable impurities: the balance.

(1) The raw materials were mixed according to the above chemical components, the resulting mixture was smelted and casted to obtain a steel billet, and the steel billet was heated in a continuous heating furnace to obtain the heated steel billet, where in a high-temperature section of the continuous heating furnace, the steel billet was heated for 3 h at 1,180°C for soaking, and in the above temperature range, alloying elements were fully solutionized and the structure was completely austeni ti zed.

(2) The steel billet was subjected to descaling with high-pressure water at a pressure of 25 Mpa on a roller table to remove an iron oxide scale on the steel billet to prevent defects such as dimples and pits on a surface of a finished round steel, and the steel billet obtained after the descaling was directly placed in a reciprocating rolling mill (a cogging mill) and subjected to 11-pass rolling at a temperature controlled at 1,005°C to obtain an intermediate billet with a size of 200 mm * 220 mm, where a deformation amount in each of the first four passes was higher than or equal to 17%.

CO (3) The intermediate billet exited the cogging mill and was subjected to a holding treatment with air cooling on a front roller table in a continuous rolling mill to 870°C for 5.5 min, and a head and a tail of the intermediate billet were cut off (4) The surface temperature of the intermediate rectangular billet obtained after the holding treatment was reduced to 835°C, then the intermediate rectangular billet entered the continuous rolling mill, and then subjected to 5-pass rolling at a finishing rolling temperature of about 780°C to obtain a 140 mm round steel bar.

(5) The round steel bar obtained after the continuous rolling in the step (4) exited the continuous rolling mill and was immediately subjected to through-water cooling in a water cooling system (through-water cooling pipe) in which four through-water cooling pipe sections were adopted for the through-water cooling. The valve opening was controlled at 25% in the first through-water cooling pipe section, the valve opening was controlled at 10% in the second through-water cooling pipe section, the valve opening was controlled at 20% in the third through-water cooling pipe section, and the valve opening was controlled at 5% in the fourth through-water cooling pipe section. After the through-water cooling was completed, a surface of the round steel bar was cooled to about 325°C, and it took 1 min for the through-water cooling from entering the first through-water cooling pipe section to exiting the fourth through-water cooling pipe section.

(6) The round steel bar was placed in a traverse marshalling system and moved to a sawing roller table for sawing, during which a self-tempering temperature on a surface of the round steel bar was 571°C, and then placed on a cooling bed, slowly cooled in a slow cooling pit, finished, straightened, tested, and stored.

Example 2:

In this example, a round steel was provided, which had a diameter of 100 mm and included the following chemical components in mass percentage: C: 0.41%; Si: 0.30%; Mn: 1.20%; P: 0.010%; S: 0.002%; Cr: 0.16%; V: 0.07%; Ti: 0.014%; Ni: 0.019%; Mo: 0.006%; Al: 0.012%; Cu: 0.21%; N: 143 ppm; H: 1.1 ppm; 0: 10 ppm; and Fe and inevitable impurities: the balance.

(1) The raw materials were mixed according to the above chemical components, the resulting mixture was smelted and casted to obtain a steel billet, and the steel billet was heated in a continuous heating furnace to obtain the heated steel billet, where in a high-temperature section of the continuous heating furnace, the steel billet was heated for 1 h at 1,160°C for soaking, and in the above temperature range, alloying elements were fully solutionized and the structure was completely austenitized.

(2) The steel billet was subjected to descaling with high-pressure water at a pressure of 25 CO Mpa on a roller table to remove an iron oxide scale on the steel billet, and the steel billet obtained after the descaling was directly placed in a reciprocating rolling mill and subjected to CJ 11-pass rolling at a temperature controlled at 995°C to obtain an intermediate billet with a size of mm * 180 mm, where a deformation amount in each of the first four passes was higher than or equal to 20%.

(3) The intermediate billet exited the cogging mill and was subjected to a holding treatment with air cooling on a front roller table in a continuous rolling mill to 865°C for 5 min, and a head and a tail of the intermediate billet were cut off (4) The surface temperature of the intermediate rectangular billet obtained after the holding treatment was reduced to 840°C, then the intermediate rectangular billet entered the continuous rolling mill, and then subjected to 7-pass rolling at a finishing rolling temperature of about 775°C to obtain a 100 mm round steel bar.

(5) The round steel bar obtained after the continuous rolling in the step (4) exited the continuous rolling mill and was immediately subjected to through-water cooling in a water cooling system (through-water cooling pipe) in which four through-water cooling pipe sections were adopted for the through-water cooling. The valve opening was controlled at 25% in the first through-water cooling pipe section, the valve opening was controlled at 10% in the second through-water cooling pipe section, the valve opening was controlled at 20% in the third through-water cooling pipe section, and the valve opening was controlled at 10% in the fourth through-water cooling pipe section. After the through-water cooling was completed, a surface of the round steel bar was cooled to about 386°C, and it took 0.7 min for the through-water cooling from entering the first through-water cooling pipe section to exiting the fourth through-water cooling pipe section.

(6) The round steel bar was placed in a traverse marshalling system and moved to a sawing roller table for sawing, during which a self-tempering temperature on a surface of the round steel bar was 585°C, and then placed on a cooling bed, slowly cooled in a slow cooling pit, finished, straightened, tested, and stored.

Example 3:

In this example, a round steel was provided, which had a diameter of 75 mm and included the following chemical components in mass percentage: C: 0.42%; Si: 0.29%; Mn: 1.20%; P: 0.011%; S: 0.004%; Cr: 0.15%; V: 0.07%; Ti: 0.015%; Ni: 0.017%; Mo: 0.004%; Al: 0.012%; Cu: 0.20%; N: 149 ppm; El: L2 ppm; 0: 14 ppm; and Fe and inevitable impurities: the balance.

(1) The raw materials were mixed according to the above chemical components, the resulting mixture was smelted and casted to obtain a steel billet, and the steel billet was heated in CO a continuous heating furnace to obtain the heated steel billet, where in a high-temperature section of the continuous heating furnace, the steel billet was heated for 3 h at 1,240°C for soaking, and C\I in the above temperature range, alloying elements were fully solutionized and the structure was completely austeni ti zed.

(2) The steel billet was subjected to descaling with high-pressure water at a pressure of 20 Mpa on a roller table to remove an iron oxide scale on the steel billet, and the steel billet obtained after the descaling was directly placed in a reciprocating rolling mill and subjected to 11-pass rolling at a temperature controlled at 990°C to obtain an intermediate billet with a size of 150 mm * 180 mm, where a deformation amount in each of the first four passes was higher than or equal to 20%.

(3) The intermediate billet exited the cogging mill and was subjected to a holding treatment with air cooling on a front roller table in a continuous rolling mill to 870°C for 5 min, and a head and a tail of the intermediate billet were cut off (4) The surface temperature of the intermediate rectangular billet obtained after the holding treatment was reduced to 850°C, then the intermediate rectangular billet entered the continuous rolling mill, and then subjected to 9-pass rolling at a finishing rolling temperature of about 775°C to obtain a 75 mm round steel bar.

(5) The round steel bar obtained after the continuous rolling in the step (4) exited the continuous rolling mill and was immediately subjected to through-water cooling in a water cooling system (through-water cooling pipe) in which three through-water cooling pipe sections were adopted for the through-water cooling. The valve opening was controlled at 20% in the first through-water cooling pipe section, the valve opening was controlled at 10% in the second through-water cooling pipe section, and the valve opening was controlled at 4% in the third through-water cooling pipe section. After the through-water cooling was completed, a surface of the round steel bar was cooled to about 414°C, and it took 0.5 min for the through-water cooling from entering the first through-water cooling pipe section to exiting the third through-water cooling pipe section.

(6) The round steel bar was placed in a traverse marshalling system and moved to a sawing roller table for sawing, during which a self-tempering temperature on a surface of the round steel bar was 590°C, and then placed on a cooling bed, slowly cooled in a slow cooling pit, finished, straightened, tested, and stored.

Examples 1 to 3 mainly provide manufacturing methods of non-quenched and tempered steels for direct cutting which replace the ordinary quenched and tempered 40Cr and 45 steels, where the steel bar is subjected to through-water cooling in 3 to 5 sections, and the strong cooling-weak cooling-strong cooling combination is adopted for composite cooling, and after the CO continuous rolling, the temperature of the steel bar is relatively high, and thus the strong cooling is adopted to rapidly cool the steel bar, due to the heat transfer from a high-temperature region to a low-temperature region, the heat at the core of the steel bar is gradually transferred to the surface of the steel bar. In order to make the heat transferred as much as possible from the core of the steel bar to the surface of the steel bar while avoiding a thermal stress caused by a large temperature gradient of the steel bar, the weak cooling is performed after the strong cooling to allow more time for heat transfer from the core during the cooling, and after the weak cooling, the surface temperature increases and thus the strong cooling is performed once again to lower the temperature on the surface of the steel bar, and such strong cooling-weak cooling-strong cooling is repeated, such that the core temperature and the surface temperature finally tend to be consistent, thereby ensuring the uniform structure and mechanical performance of the round steel.

The present disclosure designs a forming process, and adopts molten iron and scrap steel as raw materials (molten iron: 82% to 85%, and scrap steel: the balance). In a specific operation process, continuous multiple procedures are involved, for example, after a first billet enters a continuous rolling procedure, a second billet can enter a cogging rolling procedure, and after the second billet enters the continuous rolling procedure, a third billet can enter the cogging rolling procedure, and so on. This continuous production process meets the process requirements of rolling with intermediate holding treatment from a continuous casting billet to an intermediate billet, compensates the time waste caused by holding treatment, ensures that the production CO C\I capacity of the rolling with the holding treatment is close to the production capacity of the normal rolling, and improves the production efficiency of the rolling with the holding treatment.

For the non-quenched and tempered steel manufactured in Example 1, a metaIlographic image of a core at a magnification of 500 is shown in FIG. 1, from which a ferrite and a pearlite can be seen. The actual grain size (as shown in FIG. 2, at a magnification of 100) is graded as Grade 9 to 10 according to the GB/T 6394 standard, the grain size is uniform and small, and a grade difference between the core and the edge is not larger than 1.5 grades. The mechanical performance is uniform and fluctuates in a small range from the core to the edge, which meets the general machining requirements, and the hardness difference between the core and the edge is smaller than or equal to 30 HBW, which can effectively avoid adverse effects on tool processing when the hardness changes greatly.

Mechanical performance data of Examples 1 to 3 are shown in Table 1 below.

Table 1 Mechanical nerformance data of Exam les 1 to 3 Example Yield Tensile Elongation Reduction. Impact Surface strength strength in area value hardness (MPa) (MPa) (%) (%) (AKu/J) (HBW) 1# 715 931 20 55 76 316 2# 740 938 20 53 72 311 3# 723 940 21 53 81 326 From the yield strength, tensile strength, elongation, impact value, and surface hardness data in Table 1, it can be seen that the yield strength is no less than 740 MPa and the tensile strength is no less than 930 MPa, which achieves both high strength and excellent plastic toughness. In the present disclosure, a medium-sized or large-sized non-quenched and tempered steel for direct cutting with excellent strength, toughness, hardness, and machinability is manufactured The present disclosure optimizes the alloy composition design and omits the commonly used Nb without increasing the silicon content, which reduces the raw material cost in combination with the optimization of microalloying elements such as V and Ti, and the addition of microalloys causes the formation and precipitation of the complex carbide with a small carbide precipitation size and a wide precipitation temperature range, which plays a pinning role on austenite grains, and can effectively prevent the growth of austenite grains during the heating, delay the recrystallization during rolling, and improve the toughness of a material, such that the strength and toughness of the steel can be greatly improved.

In combination with the semi-continuous rolling and controlled rolling/controlled cooling techniques, the large deformation amount in each of the first 4 passes in the cogging mill allows the as-cast structure of a steel billet to be fully crushed and the austenite grains to be refined. The continuous rolling technique adopts low-temperature rolling to prevent the recrystallization of austenite grains, which facilitates the subsequent phase transition to obtain finer ferrite and pearlite structures. It can be seen from the metallographic image and grain size image of the present disclosure that the present disclosure achieves high strength and toughness for the large-sized non-quenched and tempered steel for direct cutting.

It should be noted that the above examples are merely intended to illustrate the present disclosure, rather than to limit the technical solutions described in the present disclosure. Therefore, although this specification describes the present disclosure in detail with reference to the above-mentioned examples, those of ordinary skill in the art should understand that a modification or equivalent replacement can still be made to the present disclosure, and all technical solutions and improvements made without departing from the spirit and scope of the present disclosure should be covered by the scope of the claims of the present disclosure.

CO

Claims

CLAIMSWHAT IS CLAIMED IS: I. A manufacturing method of a large-sized non-quenched and tempered steel for direct cutting, comprising the following steps: (1) mixing raw materials according to a predetermined ratio to obtain a resulting mixture, smelting and casting the resulting mixture to obtain a steel billet, and heating the steel billet in a continuous heating furnace to obtain a heated steel billet, wherein in a high-temperature section of the continuous heating furnace, the steel billet is heated for 2 h to 3 h at 1,180°C to 1,240°C for soaking; (2) subjecting the heated steel billet to descal ng with high-pressure water to remove an iron oxide scale on a surface of the heated steel billet, and subjecting the steel billet after the descaling to 7 to 11-pass rolling in a cogging mill to obtain an intermediate rectangular billet, wherein the rolling is performed at 990°C to 1,120°C in the cogging mill and a deformation amount in each of the first 4 passes is higher than or equal to 17%; (3) subjecting the intermediate rectangular billet obtained after the rolling in the step (2) to a holding treatment to 850°C to 920°C for 4 min to 6 min; (4) subjecting the intermediate rectangular billet after the holding treatment to 5 to 9-pass rolling in a continuous rolling mill to obtain a round steel bar, wherein the rolling is performed at a finishing rolling temperature of 780°C to 880°C and the round steel bar has a diameter of 100 mm to 140 mm; (5) subjecting the round steel bar after the rolling in the step (4) to through-water cooling to 325°C to 414°C after exiting the continuous rolling mill to obtain a cooled round steel bar, wherein the through-water cooling is as follows: the through-water cooling is performed in 3 to 5 sections, and a valve opening is controlled at 4% to 30% in each of the sections; a water flow rate is adjusted by controlling a valve opening of a through-water cooling device to control an intensity of the through-water cooling for the round steel bar; and a strong cooling and a weak cooling are performed alternately, and a difference between valve openings in two adjacent ones of the sections is not less than 5%; and (6) slow cooling: placing the cooled round steel bar in a traverse marshalling system to move to a sawing roller table for sawing during which a self-tempering temperature on a surface of the round steel bar is 571°C to 590°C, then slowly cooling in a pit on a cooling bed to a temperature of lower than or equal to 200°C, finishing, and straightening to obtain a required steel, which is tested and stored; wherein the large-sized non-quenched and tempered steel for direct cutting comprises the following components in weight percentage: C: 0.40% to 0.48%; Si: 0.20% to 0.35%; Mn: 0.80% to 1.30%; P: less than or equal to 0.020%; 5: less than or equal to 0.035%; Cr: 0.10% to 0.20%; V: 0.05% to 0.13%; Ti: 0.010% to 0.020%; Ni: less than or equal to 0.025%; Mo: less than or equal to 0.015%; Al: less than or equal to 0.030%; Cu: less than or equal to 0.2%; N: 130 ppm to 200 ppm; FL less than or equal to 2.0 ppm; 0: less than or equal to 20 ppm; and Fe and inevitable impurities: the balance.
2. The manufacturing method of the large-sized non-quenched and tempered steel for direct cutting according to claim 1, wherein in the step (1), the soaking is performed for a heating time of 2 h to 3 h.
3. The manufacturing method of the large-sized non-quenched and tempered steel for direct cutting according to claim I, wherein in the step (2), the descaling with the high-pressure water is performed at a pressure of 20 MPa to 30 MPa.
4. The manufacturing method of the large-sized non-quenched and tempered steel for direct cutting according to claim 1, wherein in the step (2), the deformation amount in each of the first 4 passes is higher than or equal to 20%.