US20240084432A1

US20240084432A1 - Method for heat-treating boron steel, and boron steel with high strength and good toughness, and use thereof

Info

Publication number: US20240084432A1
Application number: US17/908,264
Authority: US
Inventors: Yulin JU; Xiaonong Cheng; Zhizhong Yuan; Shun Guo; Rui Luo; Fuyang Cao; Hao Huang
Original assignee: Jiangsu University
Current assignee: Jiangsu University
Priority date: 2021-07-07
Filing date: 2021-11-03
Publication date: 2024-03-14
Also published as: WO2023279602A1; CN113529009A

Abstract

The disclosure provides a method for heat-treating a boron steel, which includes: carburizing a boron steel on a surface thereof to obtain a carburized boron steel; austenitizing the carburized boron steel at a temperature of 885-895° C. for 25-35 min, to obtain an austenitized boron steel; sequentially subjecting the austenitized boron steel to an oil quenching and a tempering, wherein the oil quenching is performed at a temperature of 55-65° C. for 29-31 min.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit and priority of Chinese Patent Application No. 202110764926.8, entitled “Method for heat-treating boron steel, and boron steel with high strength and good toughness and use thereof” filed on Jul. 7, 2021, the disclosure of which is incorporated by reference herein in its entirety as part of the present application.

TECHNICAL FIELD

The present disclosure relates to the technical field of heat-treated boron steel, in particular to a method for heat-treating a boron steel, and a boron steel with high strength and good toughness and use thereof.

BACKGROUND ART

Agriculture plays a significant and fundamental role in the improvement of industrialization in China, and its productivity and competitiveness are directly related to the stability and durability of the economic development in China. The advancement and development of agricultural mechanization and automation have greatly improved the agricultural productivity and competitiveness, which also results in a sustainable, rapid and healthy development of the agricultural economy. Knives for agricultural machinery, such as tips of ploughs, shovels of ploughs, blades of rotary tillers, are key components of agricultural machinery, all of which ascertain the safe and excellent operation of agricultural machinery within the service cycle.
High-speed ploughs are the most widely-used farming tools in the world. In terms of promoting the agricultural machinery power and farming speed, higher requirements are imposed on the performance and service life of materials for tips of high-speed ploughs. At present, the tips of high-speed ploughs in China have been mainly made of boron steels, which is obtained by subjecting the boron steel to a heat treatment process as follows: austenitizing the boron steel at 910° C. for 10 minutes, then water quenching, and finally low-temperature tempering at 200° C. for 2 hours. The obtained heat-treated boron steel has a hardness of 47-52 HRC. However, there are two major problems in the microstructure: (1) coarse prior austenite grains and lath martensite structures have been observed after water quenching for this grade of steel, as shown in FIG. 1 ; (2) a dual-phase structure of ferrite and lath martensite appears, if the cooling time and temperature cannot be properly controlled during water quenching, as shown in FIG. 2 . These two microstructural issues would result in the decreased hardness and wear resistance of boron steel, which would directly lead to a shorter service life of the material for tips of ploughs.
Therefore, it is necessary to improve the heat treatment process of the boron steel to further promote the hardness and wear resistance of the this steel.

SUMMARY

The present disclosure is intended to provide a method for heat-treating a boron steel, a boron steel with high strength and good toughness and use thereof. The boron steel after the innovative heat treatment schedule according to the present disclosure exhibits increased hardness and wear resistance.
In order to achieve the above-mentioned objects, the present disclosure provides the following technical solutions.
The present disclosure provides a method for heat-treating a boron steel, comprising steps of

- (1) carburizing the boron steel on a surface thereof, to obtain a carburized boron steel;
- (2) austenitizing the carburized boron steel obtained in step (1) at a temperature of 885-895° C. for 25-35 min, to obtain an austenitized boron steel; and
- (3) sequentially subjecting the austenitized boron steel obtained in step (2) to an oil quenching and a tempering,
- wherein the oil quenching is performed at a temperature of 55-65° C. for 29-31 minutes,

In some embodiments, the boron steel in step (1) has a chemical composition comprising 0.30-0.36 wt % of C, 1.20-1.50 wt % of Mn, less than 0.40 wt % of Si, 0.0008-0.005 wt % of B, 0.30-0.60 wt % of Cr, ≤0.025 wt % of P, ≤0.015 wt % of S, ≥0.015 wt % of Al, 0.020-0.050 wt % of Ti, and a balance of iron.
In some embodiments, carburizing the boron steel on the surface thereof in step (1) comprises a soaking period, a carburazing period, and a diffusion period.
In some embodiments, the soaking period is performed at a temperature of 915-925° C. for 25-35 min, and a carbon potential for the soaking period is in the range of 0.9-1.1 wt %.
In some embodiments, the soaking period is performed at 920° C. for 30 min, and the carbon potential for the soaking period is 1.0 wt %.
In some embodiments, the carburazing period is performed at a temperature of 915-925° C. for 595-605 min, and a carbon potential for the carburazing period is in the range of 1.05-1.25 wt %.
In some embodiments, the carburazing period is performed at 920° C. for 600 min, and the carbon potential for the carburazing period is 1.15 wt %.
In some embodiments, the diffusion period is performed at a temperature of 915-925° C. for 345-355 min, and a carbon potential for the diffusion period is in the range of 0.95-1.15 wt %.
In some embodiments, the diffusion period is performed at 920° C. for 350 min, and the carbon potential for the diffusion period is 1.05 wt %.
In some embodiments, in step (1) the carburized boron steel comprises a carburized layer with a thickness of 1-2 mm.
In some embodiments, austenitizing the carburized boron steel in step (2) is performed at a temperature of 888-892° C. for 28-32 min.
In some embodiments, austenitizing the carburized boron steel is performed at 890° C. for 30 min.
In some embodiments, the oil quenching in step (3) is performed at a temperature of 58-62° C. for 30 minutes.
In some embodiments, the tempering in step (3) is performed at a temperature of 160-170° C. for 115-125 min.
In some embodiments, the tempering is performed at 165° C. for 118-122 min.
The present disclosure also provides the boron steel with high strength and good toughness prepared by the method for heat-treating a boron steel as described in the above technical solutions.
The present disclosure also provides the use of the boron steel with high strength and good toughness as described in the above technical solutions in a tip of a high-speed plough.
The present disclosure provides a method for heat-treating a boron steel, which comprises the following steps: carburizing a boron steel on a surface thereof to obtain a carburized boron steel; austenitizing the carburized boron steel at a temperature of 885-895° C. for 25-35 min, to obtain an austenitized boron steel; sequentially subjecting the austenitized boron steel to an oil quenching and a tempering process, wherein the oil quenching is performed at a temperature of 55-65° C. for 29-31 minutes. In the method according to the present disclosure, the heat treatment of the boron steel is improved and optimized. First of all, the boron steel is carburized on a surface thereof, which enables that the surface hardness of the boron steel is improved, further leading to a high wear resistance of the boron steel. Secondly, the carburized boron steel sequentially experiences the austenization, oil quenching and tempering, where the process parameters of the austenitizing and oil quenching are adjusted to ensure that quenching crackings and deformations would not occur in the carburized boron steel, thereby further increasing the hardness and wear resistance of the boron steel. As shown in the experimental results, the average surface hardness for the boron steel with a carburized layer after being heat-treated by the method according to the present disclosure is in the range of 58.3-59 HRC, and the central hardness is in the range of 51-53 HRC.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the microstructure of lath martensite obtained by water quenching in a boron steel.

FIG. 2 shows a dual-phase structure of ferrite and lath martensite caused by improper control of cooling time and temperature in the process of water quenching the boron steel.

FIG. 3 shows the microstructure of the surface layer of the boron steel with high strength and good toughness prepared in Example 1.

FIG. 4 shows the microstructure of the core of the boron steel with high strength and good toughness prepared in Example 1.

FIG. 5 shows a hardness curve of the boron steel with high strength and good toughness prepared in Example 2 from the surface layer to the core as a function of the depth.

FIG. 6 shows the microstructure of the surface layer of the boron steel with high strength and good toughness prepared in Example 2.

FIG. 7 shows the microstructure at the subsurface (at a distance of 3-4 mm from the surface layer) of the boron steel with high strength and good toughness prepared in Example 2.

FIG. 8 shows the microstructure of the core of the boron steel with high strength and good toughness prepared in Example 2.

FIG. 9 shows the microstructure of the surface layer of the boron steel with high strength and good toughness prepared in Example 3.

FIG. 10 shows the microstructure of the core of the boron steel with high strength and good toughness prepared in Example 3.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure provides a method for heat-treating a boron steel, comprising the following steps:

- (1) carburizing the boron steel on a surface thereof, to obtain a carburized boron steel;
- (2) austenitizing the carburized boron steel obtained in step (1) at a temperature of 885-895° C. for 25-35 min, to obtain an austenitized boron steel; and
- (3) sequentially subjecting the austenitized boron steel obtained in step (2) to an oil quenching and a tempering,
- wherein the oil quenching is performed at a temperature of 55-65° C. for 29-31 minutes.

The method for heat-treating the boron steel according to the present disclosure is suitable for a forged boron steel that has not been heat-treated. In the present disclosure, there is no special limitation on the forging operation of the boron steel, and a forging operation well known to those skilled in the art may be used. In some embodiments of the present disclosure, the boron steel is 33MnCrB5. In some embodiments, the 33MnCrB5 has a chemical composition comprising 0.30-0.36 wt % of C, 1.20-1.50 wt % of Mn, less than 0.40 wt % of Si, 0.0008-0.005 wt % of B, 0.30-0.60 wt % of Cr, ≤0.025 wt % of P, ≤0.015 wt % of S, ≥0.015 wt % of Al, 0.020-0.050 wt % of Ti, and a balance of iron.
In the present disclosure, the boron steel is carburized on a surface thereof, to obtain the carburized boron steel. In the present disclosure, the surface hardness of the boron steel is improved by carburizing the boron steel on the surface thereof, thereby promoting the wear resistance.
In some embodiments of the present disclosure, carburizing the boron steel on the surface thereof comprises a soaking period, a carburazing period, and a diffusion period. In the present disclosure, three stages of the soaking period, the carburazing period, and the diffusion period are set in the process of carburizing the boron steel, which enables that carbon atoms diffuse into the boron steel more uniformly from the atmosphere, thereby further enhancing the wear resistance of the boron steel.
In some embodiments of the present disclosure, the soaking period is performed at a temperature of 915-925° C., and preferably 920° C. In some embodiments, the soaking period is performed for 25-35 min, and preferably 30 min. In some embodiments, a carbon potential for the soaking period is in the range of 0.9-1.1 wt %, and preferably 1.0 wt %. In the present disclosure, during the soaking period, a carburizing medium is decomposed into activated carbon atoms and the activated carbon atoms are adsorbed onto the surface of the boron steel, so that the carbon concentration in the surface layer of austenite increases, and carbon diffuses from the surface to the center of the boron steel. Due to the carbon potential difference between the environment and the surface layer of the boron steel, carbon atoms are continuously supplemented into the surface layer, which promotes the diffusion and homogenization of carbon atoms, thereby further improving the wear resistance.
In some embodiments of the present disclosure, the carburazing period is performed at a temperature of 915-925° C., and preferably 920° C. In some embodiments, the carburazing period is performed for 595-605 min, and preferably 600 min. In some embodiments, a carbon potential for the carburazing period is in the range of 1.05-1.25 wt %, and preferably 1.15 wt %. In the present disclosure, in the carburazing period, a “carbon pool” with high carbon content could be established in the surface layer, and a carbon potential higher than the expected carbon concentration of the finished product is set, forming a steeper concentration gradient, such that there is sufficient carbon diffusing to the center of the boron steel during the diffusion period, and thus a carburized layer with a certain concentration gradient is formed, thereby enhancing the surface hardness of the workpiece and improving the wear resistance. By controlling the process parameters of the carburazing period, the wear resistance could be further improved.
In some embodiments of the present disclosure, the diffusion period is performed at a temperature of 915-925° C., and preferably 920° C. In some embodiments, the diffusion period is performed for 345-355 min, and preferably 350 min. In some embodiments, a carbon potential for the diffusion period is in the range of 0.95-1.15 wt %, and preferably 1.05 wt %. In the present disclosure, the diffusion period enables that carbon fully diffuses into the interior of the carbon steel, thereby forming a carburized layer. By controlling the process parameters of the diffusion period, the wear resistance could be further improved.
In some embodiments of the present disclosure, the carburized boron steel comprises a carburized layer with a thickness of 1-2 mm, preferably 1.5 mm. In the present disclosure, the wear resistance could be further improved by controlling the thickness of the carburized layer.
In the present disclosure, after the carburized boron steel is obtained, the carburized boron steel is austenitized to obtain an austenitized boron steel.
In the present disclosure, austenitizing the carburized boron steel is performed at a temperature of 885-895° C., preferably 888-892° C., and more preferably 890° C. In the present disclosure, austenitizing the carburized boron steel is performed for 25-35 min, preferably 28-32 min, and more preferably 30 min. In the present disclosure, there is no special limitation on the rate of heating during austenization, and rates well known to those skilled in the art may be used. In the present disclosure, the process parameters of austenitizing is controlled to ensure that quenching crackings and deformations do not occur in the carburized boron steel, and to avoid the coarse grains of the matrix caused by excessively high temperature.
In the present disclosure, after the austenitized boron steel is obtained, the austenitized boron steel is sequentially subjected to an oil quenching and a tempering. In the present disclosure, by oil quenching, the appearance of ferrite and lath martensite dual-phase structure (which is caused by improper control of cooling time and temperature in the process of water quenching) could be avoided, thereby improving the strength of austenitized boron steel.
In the present disclosure, the oil quenching is performed at a temperature of 55-65° C., preferably 58-62° C., and more preferably 60° C. In some embodiments, the oil quenching is performed for 29-31 min, and preferably 30 min. In the present disclosure, process parameters of the oil quenching are controlled to ensure that quenching crackings and deformations do not occur in the carburized boron steel.
In some embodiments of the present disclosure, the tempering is performed at a temperature of 160-170° C., and preferably 165° C. In some embodiments, the tempering is performed for 115-125 min, preferably 118-122 min, and more preferably 120 min. In the present disclosure, by controlling process parameters of the tempering, residual stress could be eliminated, thereby further improving hardness and wear resistance.
In the method according to the present disclosure, the heat treatment of the boron steel is improved and optimized. Firstly, the boron steel is carburized on a surface thereof, and the surface hardness of the boron steel is improved by carburizing, thereby enhancing the wear resistance of the boron steel. Secondly, the carburized boron steel is sequentially subjected to an austenitizing, an oil quenching, and a tempering, and process parameters of the austenitizing and the oil quenching are adjusted to ensure that quenching crackings and deformations would not occur in the carburized boron steel, thereby further improving the hardness and wear resistance of the boron steel.
In the present disclosure, by carburizing the boron steel on a surface thereof, high surface hardness, good wear resistance, and a good combination of high strength and good toughness in the center could be achieved in the boron steel, thereby providing a hard-outer and tough-inner “sandwich” structure, with low manufacturing cost, which meets both use demand and economic strength of users.
The present disclosure also provides a boron steel with high strength and good toughness prepared by the method for heat-treating as described in the above technical solutions. In some embodiments of the present disclosure, the microstructure of a surface layer of the boron steel with high strength and good toughness comprises carbide particles and tempered martensite, and the central microstructure comprises tempered martensite. The boron steel according to the present disclosure has high surface hardness, good wear resistance, a good combination of high strength and good toughness in the center, and is of a hard-outer and tough-inner “sandwich” structure.
The present disclosure also provides use of the boron steel with high strength and good toughness as described in the above technical solutions in a tip of a high-speed plough.
In the present disclosure, there is no special limitation on the use of the boron steel with high strength and good toughness in the tip of a high-speed plough, and the use operation of the boron steel in the tip of a high-speed plough well known to those skilled in the art may be used.
The technical solutions according to the present disclosure will be clearly and completely described below in conjunction with examples of the present disclosure. Obviously, the described examples are only parts of, but not all examples of the present disclosure. Based on the examples of the present disclosure, all other examples obtained by those of ordinary skill in the art without creative labor shall fall within the scope of the present disclosure.

Example 1

33MnCrB5 was heat-treated according to a method consisting of procedures below.

- (1) A forged 33MnCrB5 was carburized on a surface thereof, obtaining a carburized boron steel with a carburized layer 1.0 mm in thickness, wherein the chemical composition of the 33MnCrB5 was as follows: 0.33 wt % of C, 1.40 wt % of Mn, 0.30 wt % of Si, 0.0035 wt % of B, 0.45 wt % of Cr, 0.012 wt % of P, 0.010 wt % of S, 0.015 wt % of Al, 0.035 wt % of Ti, and a balance of iron.

The carburizing on the surface consisted of a soaking period, a carburazing period, and a diffusion period. The soaking period was performed at 915° C. for 25 min, with a carbon potential of 0.9 wt %. The carburazing period was performed at 915° C. for 595 min, with a carbon potential of 1.05 wt %. The diffusion period was performed at 915° C. for 345 min, with a carbon potential of 0.95 wt %.

- (2) The carburized boron steel obtained in step (1) was austenitized at 885° C. for 25 minutes, obtaining an austenitized boron steel.
- (3) The austenitized boron steel obtained in step (2) was sequentially subjected to an oil quenching and a tempering, wherein the oil quenching was performed at 55° C. for 29 min, and the tempering was performed at 160° C. for 115 min.

The microstructure of the boron steel with high strength and good toughness prepared in Example 1 is shown in FIGS. 3 and 4 , in which, FIG. 3 shows the microstructure of the surface layer of the boron steel with high strength and good toughness prepared in Example 1, and FIG. 4 shows the microstructure of the core of boron steel with high strength and good toughness prepared in Example 1.
As seen in FIGS. 3 and 4 , the microstructure of the surface layer of the boron steel with high strength and good toughness is tempered martensite and a small amount of retained austenite, and the microstructure of the core is tempered martensite, and is free from carburized layers.
The boron steel prepared in Example 1 was subjected to a performance test. With a thickness of 1 mm, the carburized layer has an average hardness of 58.5 HRC; the hardness of the core is in the range of 51-52 HRC (obtained by testing different positions of the core of the boron steel).

Example 2

33MnCrB5 was heat-treated according to a method consisting of procedures below.

- (1) A forged 33MnCrB5 was carburized on a surface thereof, obtaining a carburized boron steel with a carburized layer 1.5 mm in thickness, wherein the chemical composition of the 33MnCrB5 was as follows: 0.33 wt % of C, 1.40 wt % of Mn, 0.30 wt % of Si, 0.0035 wt % of B, 0.45 wt % of Cr, 0.012 wt % of P, 0.010 wt % of S, 0.015 wt % of Al, 0.035 wt % of Ti, and a balance of iron.

The carburizing on the surface consisted of a soaking period, a carburazing period, and a diffusion period. The soaking period was performed at 920° C. for 30 min, with a carbon potential of 1.0 wt %. The carburazing period was performed at 920° C. for 600 min, with a carbon potential of 1.15 wt %. The diffusion period was performed at 920° C. for 350 min, with a carbon potential of 1.05 wt %.

- (2) The carburized boron steel obtained in step (1) was austenitized at 890° C. for 30 minutes, obtaining an austenitized boron steel.
- (3) The austenitized boron steel obtained in step (2) was sequentially subjected to an oil quenching and a tempering, wherein the oil quenching was performed at 60° C. for 30 min, and the tempering was performed at 165° C. for 120 min.

The hardness curve of the boron steel with high strength and good toughness prepared in Example 2 from the surface layer to the core as a function of depth is shown in FIG. 5 . As seen from FIG. 5 , with a thickness of 1.5 mm, the carburized layer has an average hardness of 59 HRC; the hardness of the core is in the range of 52-53 HRC (obtained by testing different positions of the core of the boron steel).
The microstructure of the boron steel with high strength and good toughness prepared in Example 2 is shown in FIGS. 6-8 , in which, FIG. 6 shows the microstructure of the surface layer of the boron steel with high strength and good toughness prepared in Example 2; FIG. 7 shows the microstructure of the subsurface (at a distance of 3-4 mm from the surface) of the boron steel with high strength and good toughness prepared in Example 2; FIG. 8 shows the microstructure of the core of the boron steel with high strength and good toughness prepared in Example 2.
As seen in FIGS. 6-8 , the microstructure of the surface layer of the boron steel with high strength and good toughness is tempered martensite and a small amount of retained austenite, and the amount of retained austenite is reduced compared with that of Example 1. The microstructure of the core is tempered martensite, and free from carburized layers.

Example 3

33MnCrB5 was heat-treated according to a method consisting of procedures below.

- (1) A forged 33MnCrB5 was carburized on a surface thereof, obtaining a carburized boron steel with a carburized layer 2.0 mm in thickness, wherein the chemical composition of the 33MnCrB5 was as follows: 0.33 wt % of C, 1.40 wt % of Mn, 0.30 wt % of Si, 0.0035 wt % of B, 0.45 wt % of Cr, 0.012 wt % of P, 0.010 wt % of S, 0.015 wt % of Al, 0.035 wt % of Ti, and a balance of iron.

The carburizing on the surface consisted of a soaking period, a carburazing period, and a diffusion period. The soaking period was performed at 925° C. for 35 min, with a carbon potential of 1.1 wt %. The carburazing period was performed at 925° C. for 605 min, with a carbon potential of 1.25 wt %. The diffusion period was performed at 925° C. for 355 min, with a carbon potential of 1.15 wt %.

- (2) The carburized boron steel obtained in step (1) was austenitized at 895° C. for 35 minutes, obtaining an austenitized boron steel.
- (3) The austenitized boron steel obtained in step (2) was sequentially subjected to an oil quenching and a tempering, wherein the oil quenching was performed at 65° C. for 31 min, and the tempering was performed at 170° C. for 125 min.

The microstructure of the boron steel with high strength and good toughness prepared in Example 3 is shown in FIGS. 9 and 10 , in which FIG. 9 shows the microstructure of the surface layer of the boron steel with high strength and good toughness prepared in Example 3; FIG. 10 shows the microstructure of the core of the boron steel with high strength and good toughness prepared in Example 3.
As seen in FIGS. 9 and 10 , the microstructure of the surface layer of the boron steel with high strength and good toughness is tempered martensite and a small amount of retained austenite, and the amount of retained austenite is increased compared with that of Example 2. The microstructure of the core is tempered martensite, and free from carburized layers.
The boron steel prepared in Example 3 was subjected to a performance test. With a thickness of 2 mm, the carburized layer has an average hardness of 58.3 HRC, and the hardness of the core is in the range of 51-52 HRC (obtained by testing different positions of the core of the boron steel).
It can be seen from the above examples that the boron steel after being heat-treated by the method according to the present disclosure exhibits improved hardness and wear resistance.
The descriptions of the above embodiments are used to obtain a deeper understanding of the present disclosure. It should be pointed out that for those skilled in the art, without departing from the principle of the present disclosure, several improvements and modifications could also be made to the present disclosure, and these improvements and modifications also fall within the scope of the claims of the present disclosure. Various modifications to these embodiments will be obvious to those skilled in the art, and the generic principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present disclosure. Thus, the present disclosure is not intended to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for heat-treating a boron steel, comprising steps of

(1) carburizing the boron steel on a surface thereof, to obtain a carburized boron steel;

(2) austenitizing the carburized boron steel obtained in step (1) at a temperature of 885-895° C. for 25-35 min, to obtain an austenitized boron steel; and

(3) sequentially subjecting the austenitized boron steel obtained in step (2) to an oil quenching and a tempering,

wherein the oil quenching is performed at a temperature of 55-65° C. for 29-31 minutes.

2. The method for heat-treating the boron steel as claimed in claim 1, wherein the boron steel in step (1) has a chemical composition comprising 0.30-0.36 wt % of C, 1.20-1.50 wt % of Mn, less than 0.40 wt % of Si, 0.0008-0.005 wt % of B, 0.30-0.60 wt % of Cr, ≤0.025 wt % of P, ≤0.015 wt % of S, ≥0.015 wt % of Al, 0.020-0.050 wt % of Ti, and a balance of iron.

3. The method for heat-treating the boron steel as claimed in claim 1, wherein carburizing the boron steel on the surface thereof in step (1) comprises a soaking period, a carburazing period, and a diffusion period.

4. The method for heat-treating the boron steel as claimed in claim 3, wherein the soaking period is performed at a temperature of 915-925° C. for 25-35 min, and a carbon potential for the soaking period is in the range of 0.9-1.1 wt %.

5. The method for heat-treating the boron steel as claimed in claim 4, wherein the soaking period is performed at 920° C. for 30 min, and the carbon potential for the soaking period is 1.0 wt %.

6. The method for heat-treating the boron steel as claimed in claim 3, wherein the carburazing period is performed at a temperature of 915-925° C. for 595-605 min, and a carbon potential for the carburazing period is in the range of 1.05-1.25 wt %.

7. The method for heat-treating the boron steel as claimed in claim 6, wherein the carburazing period is performed at 920° C. for 600 min, and the carbon potential for the carburazing period is 1.15 wt %.

8. The method for heat-treating the boron steel as claimed in claim 3, wherein the diffusion period is performed at a temperature of 915-925° C. for 345-355 min, and a carbon potential for the diffusion period is in the range of 0.95-1.15 wt %.

9. The method for heat-treating the boron steel as claimed in claim 8, wherein the diffusion period is performed at 920° C. for 350 min, and the carbon potential for the diffusion period is 1.05 wt %.

10. The method for heat-treating the boron steel as claimed in claim 1, wherein in step (1) the carburized boron steel comprises a carburized layer with a thickness of 1-2 mm.

11. The method for heat-treating the boron steel as claimed in claim 1, wherein austenitizing the carburized boron steel in step (2) is performed at a temperature of 888-892° C. for 28-32 min.

12. The method for heat-treating the boron steel as claimed in claim 11, wherein austenitizing the carburized boron steel is performed at 890° C. for 30 min.

13. The method for heat-treating the boron steel as claimed in claim 1, wherein the oil quenching in step (3) is performed at a temperature of 58-62° C. for 30 minutes.

14. The method for heat-treating the boron steel as claimed in claim 1, wherein the tempering in step (3) is performed at a temperature of 160-170° C. for 115-125 min.

15. The method for heat-treating the boron steel as claimed in claim 14, wherein the tempering is performed at 165° C. for 118-122 min.

16. A boron steel with high strength and good toughness prepared by the method for heat-treating the boron steel as claimed in claim 1.

17. (canceled)